Introduction to History Dependent Materials

CHAPTER 6 87 61 Introduction to History Dependent Materials Engineering materials are available with a wide range of useful properties and characteristics Some of these are inherent to the particular material but many others can be varied by controlling the manner of production and the details of processing Metals are classic examples of such historydependent materials The final properties are clearly affected by their past processing his tory The particular details of the smelting and refining process control the resulting purity and the type and nature of any influ ential contaminants The solidification process imparts struc tural features that might be transmitted to the final product Preliminary operations such as the rolling of sheet or plate often impart directional variations to properties and their impact should be considered during subsequent processing and use Thus although it is easy to take the attitude that metals come from warehouses it is important to recognize that aspects of their prior processing can significantly influence further opera tions as well as the final properties of a product The breadth of this book does not permit full coverage of the processes and methods involved in the production of engineering metals but certain aspects will be presented because of their role in affect ing subsequent performance 62 Ferrous Metals In this chapter we will introduce the major ferrous ironbased metals and alloys summarized in Figure 61 These materials made possible the Industrial Revolution 150 years ago and they continue to be the backbone of modern civilization We see them everywhere in our livesin the buildings where we work the cars we drive the homes in which we live the cans we open and the appliances that enhance our standard of living Numer ous varieties have been developed over the years to meet the specific needs of various industries These developments and improvements have continued with recent decades seeing the introduction of a number of new varieties and even classes of ferrous metals According to the American Iron and Steel Insti tute the number of available grades of steel has doubled since 2000 The newer steels are stronger than ever rolled thinner Ferrous Metals and Alloys Cast Irons PlainCarbon Steels Alloy Steels Gray Irons LowCarbon LowAlloy Steels Malleable Iron MediumCarbon HSLA Steels Ductile Iron HighCarbon Advanced HighStrength Steels Compacted Graphite Iron Maraging Steels Microalloyed Steels Austempered Ductile Iron Stainless Steels White Iron Tool Steels Ferrous Metal Alloys FIGURE 61 Classification of common ferrous metals and alloys 88 CHAPTER 6 Ferrous Metals and Alloys easier to shape and more corrosion resistant As a result steel still accounts for more than half of the metal used in an average vehicle in North America In addition all steel is recyclable and this recycling does not involve any loss in material quality In fact more steel is recycled each year than all other materials combined including aluminum glass plastic and paper Because steel is magnetic it is easily separated and recovered from demol ished buildings junked automobiles and discarded appli ances The overall recycling rate for steel is approximately 88925 for automobiles 90 for appliances and 72 for steel packaging Structural beams and plates are the most recycled products at 975 Twothirds of all new steel is produced from old steel and each ton of recycled steel saves more than 4000 pounds of raw materials including 1400 pounds of coal and 74 of the energy required to make new steel 63 Iron For centuries iron has been the most important of the engi neering metals Although iron is the fourth most plentiful element in the earths crust it is rarely found in the metallic state Instead it occurs in a variety of mineral compounds known as ores the most attractive of which are iron oxides coupled with companion impurities To produce metallic iron the ores are processed in a manner that breaks the iron oxygen bonds chemical reducing reactions Ore limestone coke carbon and air are continuously introduced into spe cifically designed furnaces and molten metal is periodically withdrawn Within the furnace other oxides that were impurities in the original ore will also be reduced All of the phosphorus and most of the manganese will enter the molten iron Oxides of silicon and sulfur compounds are partially reduced and these elements also become part of the resulting metal Other contaminant elements such as calcium magnesium and alu minum are collected in the limestonebased slag and are largely removed from the system The resulting pig iron tends to have roughly the following composition Carbon 3045 Manganese 01525 Phosphorus 0120 Silicon 1030 Sulfur 00501 Although the bulk of molten pig iron is further processed into steel a small portion is cast directly into final shape and is classified as cast iron Most commercial cast irons however are produced by recycling scrap iron and steel with the pos sible addition of some newly produced pig iron Cast iron was introduced in Chapter 4 and the various types of cast iron will be discussed later in this chapter The conversion of this material to cast products will be developed when we present the casting processes in Chapters 13 through 15 64 Steel Steel is an extremely useful engineering material offering strength rigidity and durability From a manufacturing perspec tive its formability joinability and paintability as well as repair ability are all attractive characteristics For the past 20 years steel has accounted for about 55 of the weight of a typical pas senger car Although the automotive and construction industries are indeed the major consumers of steel the material is also used extensively in containers appliances and machinery and pro vides much of the infrastructure of such industries as oil and gas The manufacture of steel is essentially an oxidation pro cess that decreases the amount of carbon silicon manganese phosphorus and sulfur in a molten mixture of pig iron andor steel scrap In 1856 the KellyBessemer process opened up the industry by enabling the manufacture of commercial quantities of steel The openhearth process surpassed the Bessemer pro cess in tonnage produced in 1908 and was producing more than 90 of all steel in 1960 Most of our commercial steels are cur rently produced by a variety of oxygen and electric arc furnaces In many of the steelmaking processes air or oxygen passes over or through the molten metal to drive a variety of exother mic refining reactions Carbon oxidizes to form gaseous CO or CO2 which then exits the melt Other elements such as silicon and phosphorus are similarly oxidized and being lighter than the metal rise to be collected in a removable slag At the same time however oxygen and other elements from the reaction gases dissolve in the molten metal and might later become a cause for concern Solidification Concerns Regardless of the method by which the steel is made it must undergo a change from liquid to solid before it can become a usable product Prior to solidification however we want to remove as much contamination as possible The molten metal is first poured from the steelmaking furnaces into containment vessels known as ladles Historically the ladles simply served as transfer and pouring containers but they now provide a site for additional processing Ladle metallurgy refers to a variety of processes designed to provide final purification and to fine tune both the chemistry and temperature of the melt Alloy additions can be made carbon can be further reduced and dis solved gases can be reduced or removed Stirring degassing and reheating can all be performed The liquid can then be poured into molds to produce finish shape steel castings or solidified into a form suitable for further processing In most cases some form of continuous casting produces the feedstock material for subsequent forging or roll ing operations The molten metal is usually introduced into the continuous caster through a bottompouring process such as the one shown schematically in Figure 62 By extracting the metal from the bottom of the ladle slag and floating matter are not transferred and a cleaner product results Figure 63a illus trates a typical continuous caster in which molten metal flows from a ladle through a tundish into a bottomless watercooled mold usually made of copper Cooling is controlled to ensure that the outside has solidified before the metal exits the mold 64 Steel 89 Closely spaced rolls support the emerging product and direct water sprays complete the solidification The newly solidified metal can then be cut to desired length or because the cast solid is still hot it can be bent and fed horizontally through a short reheat furnace or directly to a rolling operation By varying the size and shape of the mold products can be cast with a vari ety of cross sections with names such as slab bloom billet and strand Figure 63b depicts the simultaneous casting of multiple strands Compared to the casting of discrete ingots continuous casting offers significant reduction in cost energy and scrap In addition the products have improved surfaces more uniform chemical composition and fewer oxide inclusions Deoxidation and Degasification During the steelmaking process large amounts of oxygen can become dissolved in the molten metal During the subsequent cooling and solidification the solubility levels decrease signifi cantly as shown in Figure 64 The excess oxygen can no longer be held within the material and frequently links with carbon to produce carbon monoxide gas This gas might then escape through the liquid or might become trapped to produce pores within the solid ranging from small dispersed voids to large Refractory sleeves Lever for pouring Graphite stopper Graphite pouring hole FIGURE 62 Diagram of a bottompouring ladle Ladle Water spray headers Tundish Mold Mold discharge rack Vertical guideroll rack b a Pinch rolls Bending cluster Curved guide rack Slab straightener Reheat furnace Sizing mill Torch cutoff FIGURE 63 a Schematic representation of the continuous casting process for producing billets slabs and bars b Simultaneous continuous casting of multiple strands Reproduced with permission from Pen ton Media 90 CHAPTERE Ferrous Metals and Alloys PlainCarbon Steel Although theoretically an alloy of only iron and carbon commer Liquid cial steel actually contains manganese phosphorus sulfur and 5 oo silicon in significant and detectable amounts When these four 3 Solidification additional elements are present in their normal percentages o and no minimum amount is specified for any other constitu oO oO Solid ent the product is referred to as plaincarbon steel Strength is Melting point primarily a function of carbon content increasing with increas ing carbon as shown in Table 61 Unfortunately the ductility Temperature toughness and weldability of plaincarbon steels decrease as the carbon content is increased and hardenability is quite low 7 In addition the properties of ordinary carbon steels are dimin a Solubility of gas in a metal as a function of temperature ished by both high and low temperatures loss of strength and showing significant decrease upon solidification embrittlement respectively and they are subject to corrosion in most environments blowholes Although these pores can often be welded shut dur Plaincarbon steels are generally classified into three sub based on thei tent Lowcarbon steels h ing subsequent hot forming some might not be fully closed and es than 030 carbon pon voecees ood ene canbe others might not weld on closure Cracks and internal voids can P et y then persist into a finished product strengthened by cold work and weldability Their structures are Porosity problems can often be avoided by either removing usually ferrite and pearlite and the material is generally used as segs as it fi the hotformi dformi i the oxygen prior to solidification or by making sure it does not PP Comes Tom Ene NOW TOMMINE OF CO dforming processes or in reemerge as a gas Aluminum ferromanganese or ferrosilicon the aswelded condition Mediumcarbon steels have between 020 and 0509 b th b hed to f can be added to molten steel to provide a material whose affin an S carbon and ney can be quenc ed to form ity for oxygen is higher than that of carbon Dissolved oxygen martensite or bainite if the section size is small and a severe yas ae t bri hi Th t bal f ti then reacts with these deoxidation additions to produce solid water or brine quencn Is used The best ba ance oF properties metal oxides that are either removed from the molten metal is obtained at these carbon levels where the high toughness when they float to the top or become dispersed throughout the and ductility of the lowcarbon material is in good compromise structure with the strength and hardness that come with higher carbon Deoxidation additions can effectively tie up dissolved oxy contents These steels are extremely popular and find numerous gen but small amounts of other gases such as hydrogen and mechanical applications Highcarbon steels have more than 0509 Tough f bilit low but h nitrogen can also have deleterious effects on the performance ot carbon Tous ness and Severe ty are Tow but harcness of steels This is particularly important for alloy steels because re ED 4 the solubility of these gases tends to increase with alloy addi tensite but hardenability is still poor Quench cracking is often tions such as vanadium niobium and chromium Alterna a problem when the material is pushed to its limit Figure 65 tive degassing processes have been devised that reduce the depicts the characteristic properties of low medium and high amounts of all dissolved gases In vacuum degassing a stream carbon steels using a balance of properties that shows the off of molten metal is passed through a vacuum during pouring By setting characteristics of strength and hardness and ductility on d tough creating a large amount of exposed surface the vacuum is able anc sougnness to extract most of the dissolved gas Compared to other engineering materials the plain car In the consumable electrode remelting processes an bon steels offer high strength and high stiffness coupled with ne ble toughness Unfortunately they al t easil already solidified metal electrode is progressively remelted with reasonable Tougnness Unfortunately they a So rust easily and the molten droplets passing through a vacuum If the melting generally require some form of surface protection such as paint is done by an electric arc the process is known as vacuum arc galvanizing or other coating Because the plain carbon steels remelting VAR If induction heating is used to melt the elec are generally the lowestcost steel material they are often given trode the rocess becomes vacuum induction melting VIM first consideration for many applications Their limitations how P 8 ever might become restrictive When improved performance is Both are highly effective in removing dissolved gases but they are unable to remove any nonmetallic impurities that might be present in the metal Effect of Carbon on the Strength of Annealed Both gas and impurities can be removed by the elec TABLE 61 PlainCarbon Steels troslag remelting ESR process A solid electrode is again mi r melted and recast using an electric current but the entire Minimum Tensile Strength remelting is conducted under a blanket of molten flux Non UT Mpa LS metallic impurities float and are collected in the flux and the 1020 020 414 60 progressive freezing permits easy escape for the rejected gas 1030 030 448 65 The result is a newly solidified metal structure with much 1040 040 517 75 improved quality This process is simply a largescale version 1050 050 621 90 of the electroslag welding process that will be discussed in CCSocoeeew Chapter 38 Data are from ASTM Specification A732 64 Steel 91 t t t t t t Compared to carbon steels the alloy steels o o o o o o can be considerably stronger For a specified level on Cc on Cc on Cc or 8 2 3 2 3 of strength the ductility and toughness tend to be Zz Zz Z higher For a specified level of ductility or tough z 3 z 3 z 8 ness the strength is higher In essence there is S S S an improvement in the overall combination of e e e properties Mechanical properties at both low and 5 5 3 high temperatures can be increased but all these a 2 a 2 a 2 improvements come with an increase in cost a b c Weldability machinability and formability gener Lowcarbon steel Mediumcarbon steel Highcarbon steel ally decline Selection of an alloy steel still begins with identifying the proper carbon content Table 62 A comparison of lowcarbon mediumcarbon and highcarbon shows the effect of carbon on the strength steels in terms of their relative balance of properties a Lowcarbon has excellent of quenchedandtempered alloy steels The ductility and fracture resistance but lower strength b Mediumcarbon has balanced strength values are significantly higher than those properties c Highcarbon has high strength and hardness at the expense of ductility of Table 61 reflecting the difference between the and fracture resistance annealed and quenchedandtempered micro structures The 4130 steel has about 12 total required these steels can often be upgraded by the addition of alloying elements 4330 has 30 and 8630 has about 13 yet all have the same quenchedandtempered one or more alloying elements tensile strength Strength and hardness depend primarily on carbon content The primary role of an alloy addition there Alloy Steels fore is usually to increase hardenability but other effects such as modified toughness or machinability are also pos The differentiation between plain carbon and alloy steel is often sible The most common hardenabilityenhancing elements somewhat arbitrary Both contain carbon manganese and usu in order of decreasing effectiveness are manganese molyb ally silicon Copper and boron are possible additions to both denum chromium silicon and nickel Boron is an extremely classes Steels containing more than 165 manganese 060 powerful hardenability agent Only a few thousandths of a per silicon or 060 copper are usually designated as alloy steels cent are sufficient to produce a significant effect in lowcarbon Also a steel is considered to be an alloy steel ifadefinite or mini Steels but the results diminish rapidly with increasing carbon mum amount of other alloying element is specified The most content Because no carbide formation or ferrite strengthening common alloy elements are chromium nickel molybdenum accompanies the addition improved machinability and cold vanadium tungsten cobalt boron and copper as well as man forming characteristics might favor the use of boron in place ganese silicon phosphorus and sulfur in amounts greater than of other hardenability additions Small amounts of vanadium are normally present If the steel contains less than 8 of total CaN also be quite effective but the response drops off as the alloy addition it is considered to be a lowalloy steel Steels quantity is increased with more than 8 alloying elements are highalloy steels Table 63 summarizes the primary effects of the common In general alloying elements are added to steels in small alloying elements in steel A working knowledge of this infor percentages usually less than 5 to improve strength or hard mation might be useful in selecting an alloy steel to meet a enability or in much larger amounts often up to 20 to pro given set of requirements Alloying elements are often used duce special properties such as corrosion resistance or stability in combination however resulting in the immense variety of at high or low temperatures Additions of manganese silicon alloy steels that are commercially available To provide some or aluminum can be made during the steelmaking process to degree of simplification a classification system has been remove dissolved oxygen from the melt Manganese silicon nickel and copper add strength by forming solid solutions in fer rite Chromium vanadium molybdenum tungsten and other elements increase strength by forming dispersed secondphase Effect of Carbon on the Strength of Quenched carbides Nickel and copper can be added in small amounts to ee eos andTempered Alloy Steels improve corrosion resistance Nickel has been shown to impart a increased toughness and impact resistance and molybdenum helps resist embrittlement Zirconium cerium and calcium can Type of Steel Carbon Content whe LS also promote increased toughness by controlling the shape of 4130 030 1030 150 inclusions Machinability can be enhanced through the forma 4330 030 1030 150 tion of manganese sulfides or by additions of lead bismuth 8630 030 1030 150 selenium or tellurium Still other additions canbe usedtopro 4j49 040 41241 42180 vide ferrite or austenite grainsize control Phosphorus and sul 4340 040 441241 180 fur as well as hydrogen oxygen and nitrogen are considered to SS qq be undesirable elements Data from ASTM Specification A732 92 CHAPTERG6 Ferrous Metals and Alloys TABLE 63 Principal Effects of Alloying Elements in Steel Aluminum 095130 Alloying element in nitriding steels Bismuth Improves machinability Boron 00010003 Powerful hardenability agent Chromium 052 Increase of hardenability 418 Corrosion resistance Copper 0104 Corrosion resistance Lead Improved machinability Manganese 025040 Combines with sulfur to prevent brittleness 1 Increases hardenability by lowering transformation points and causing transformations to be sluggish Molybdenum 025 Stable carbides inhibits grain growth Nickel 25 Toughener 1220 Corrosion resistance Silicon 0207 Increases strength and hardenability 2 Spring steels Higher percentages Improves magnetic properties Sulfur 008015 Freemachining properties Titanium Fixes carbon in inert particles Reduces martensitic hardness in chromium steels Tungsten Hardness at high temperatures Vanadium 015 Stable carbides increases strength while retaining ductility promotes fine grain structure developed and has achieved general acceptance in a variety When hardenability is a major requirement one might of industries consider the H grades of AISI steels designated by an H suffix attached to the standard designation The chemistry speci cae fications are somewhat less stringent but the steel must now AISISAE Classification System meet a hardenability standard The hardness values obtained for each location on a Jominy test specimen see Chapter 5 The most common classification scheme for alloy steels is the must fall within a predetermined band for that particular type AISISAE identification system This system which classifies of steel When the AISI designation is followed by an RH suffix alloys by chemistry was started by the Society of Automotive restrictedhardenability an even narrower range of hardness Engineers SAE to provide some standardization for the steels yalyes is imposed used in the automotive industry It was later adopted and Other designation organizations such as the American expanded by the American Iron and Steel Institute AISI andhas society for Testing and Materials ASTM and the US govern been incorporated into the Universal Numbering System thatwas ment MIL and federal have specification systems based more developed to include all engineering metals Both plaincarbon on specific products and applications Acceptance into a given and lowalloy steels are identified by a fourdigit number where classification is generally determined by physical or mechanical the first number indicates the major alloying elements andthe properties rather than the chemistry of the metal ASTM desig second number designates a subgrouping within the major ations are often used when specifying structural steels alloy system These first two digits can be interpreted by con sulting a list such as the one presented in Table 64 The last two digits three if the number contains five digits indicate the Selecting Alloy Steels approximate amount of carbon expressed as points where one point is equal to 001 Thus a 1080 steel would be a plain From the previous discussion it is apparent that two or more carbon steel with 080 carbon Similarly a 4340 steel would be alloying elements can often produce similar effects When prop a MoCrNi alloy with 040 carbon Because of the meanings erly heattreated steels with substantially different chemical associated with the numbers the designation is not read as a compositions can possess almost identical mechanical proper series of single digits such as fourthreefourzero but as a pair ties Figure 66 clearly demonstrates this fact which becomes of doubledigit groupings such as teneighty or fortythreeforty particularly important when one realizes that some alloying Letters can also be incorporated into the designation The elements can be very costly and others might be in short sup letter B between the second and third digits indicates that the ply or difficult to obtain Overspecification has often been base metal has been supplemented by the addition of boron employed to guarantee success despite sloppy manufacturing Similarly an L in this position indicates a lead addition for and heattreatment practice The correct steel however is usu enhanced machinability A letter prefix might also be employed ally the least expensive one that can be consistently processed to designate the process used to produce the steel such as E for to achieve the desired properties This usually involves taking electric furnace advantage of the effects provided by all of the alloy elements 64 Steel 93 When selecting alloy steels it is also important to con sider both use and fabrication For one product it might be permissible to increase the carbon content to obtain greater strength For another application such as the one involving assembly by welding it might be best to keep the carbon content low and use a balanced amount of alloy elements obtaining the desired strength while minimizing the risk of weld cracking Steel selection involves defining the required properties determining the best microstructure to provide those properties strength can be achieved through alloy ing cold work and heat treatment as well as combinations thereof determining the method of part or product manufac ture casting machining metal forming etc and then select ing the steel with the best carbon content and hardenability characteristics to facilitate those processes and achieve the desired goals AISISAE Steel Identification If an individual informs you that he is working with a fourone fourfive steel what information has he provided you a The steel is an alloy steel b The steel contains approximately 045 carbon c The steel is a molybdenumcontaining steel with additional chromium d He is totally unfamiliar with alloy steels and the identifica tion system Correct answerall of the above It is a fortyone fortyfive steel TABLE 64 AISISAE Standard Steel Designations and Associated Chemistries Alloying Elements AISI Number Type Mn Ni Cr Mo V Other 1xxx Carbon steels 10xx Plain carbon 11xx Free cutting S 12xx Free cutting S and P 13xx High manganese 160190 15xx High manganese 2xxx Nickel steels 3550 3xxx Nickelchromium 1035 05175 4xxx Molybdenum 40xx Mo 015030 41xx Mo Cr 040110 008035 43xx Mo Cr Ni 165200 040090 020030 44xx Mo 035060 46xx Mo Ni low 070200 015030 47xx Mo Cr Ni 090120 035055 015040 48xx Mo Ni high 325375 020030 5xxx Chromium 50xx 020060 51xx 070115 6xxx Chromumvanadium 61xx 050110 010015 8xxx Ni Cr Mo 81xx 020040 030055 008015 86xx 040070 040060 015025 87xx 040070 040060 020030 88xx 040070 040060 030040 9xxx Other 92xx High silicon 120220Si 93xx Ni Cr Mo 300350 100140 008015 94xx Ni Cr Mo 030060 030050 008015 94 CHAPTER 6 Ferrous Metals and Alloys HighStrength Steels HSLA Microalloyed and BakeHardenable Among the general categories of alloy steels are 1 the con structional alloys where the desired properties are typically developed by a separate thermal treatment and the specific alloy elements tend to be selected for their effect on hardenabil ity 2 conventional highstrength steels which rely largely on chemical composition to develop the desired properties in a singlephase ferritic microstructure usually in the asrolled or normalized condition and 3 advanced highstrength steels AHSS which are primarily multiphase steels ferrite mar tensite bainite andor retained austenite that provide high strength with unique mechanical properties The constructional alloys are usually purchased by AISISAE identification which effectively specifies chemistry The highstrength designations generally focus on product size and shape and desired prop erties When steels are specified by mechanical properties the supplier or producer is free to adjust the chemistry within lim its and substantial cost savings can result To ensure success however it is important that all of the necessary properties be specified The conventional highstrength steels provide increased strengthtoweight good weldability and acceptable corro sion resistance for only a modest increase in cost compared to the lowcarbon plain carbon steels often referred to as mild steels Because of their higher yield strength up to 690760 MPa or 100110 ksi weight savings of 20 to 30 can often be achieved with no sacrifice to strength or safety They are avail able in a variety of forms including sheet strip plate structural shapes and bars Ductility and hardenability might be some what limited however The increase in strength and the resist ance to martensite formation in a weld zone is obtained by controlling the amounts of carbon 005 to 025 manganese up to 20 and silicon with the addition of small amounts of niobium vanadium titanium or other alloys About 02 cop per can be added to improve corrosion resistance One of the largest groups of conventional highstrength steels the highstrength lowalloy HSLA or microalloyed steels occupy a position between carbon steels and the quenchedandtempered alloy grades in terms of both cost and performance and are being used increasingly as substi tutes for heattreated steels in the manufacture of small to mediumsized discrete parts These low and mediumcarbon steels contain solidsolutionstrengthening alloys such as manganese and silicon along with small amounts 005 to 015 of alloying elements such as niobium vanadium tita nium molybdenum zirconium boron rare earth elements or combinations thereof Many of these additions form alloy carbides nitrides or carbonitrides whose primary effect is to provide grain refinement andor precipitation strength ening Yield strengths between 500 and 750 MPa 70 and 110 ksi can be obtained without heat treatment Weldability can be retained or even improved if the carbon content is simul taneously decreased In essence these steels offer maximum strength with minimum carbon while simultaneously preserv ing weldability machinability and formability Compared to a quenchedandtempered alternative however ductility and toughness are generally somewhat inferior Coldformed HSLA or microalloyed steels require less cold work to achieve a desired level of strength so they tend to have greater residual ductility Hotformed products such as forgings can often be used in the aircooled condition By means of accu rate temperature control and controlledrate cooling directly from the forming operation mechanical properties can be produced that approximate those of quenchedandtempered material Machinability fatigue life and wear resistance can all 260 240 220 200 180 160 140 120 100 Tensile strength 1000 psi 80 120 160 200 240 0 20 40 60 Yield strength Elongation Yield strength 1000 psi Elongation and reduction of area Reduction of area Water quenched SAE 1330 2330 3130 4130 5130 6130 FIGURE 66 Relationships between the mechanical properties of a variety of properly heattreated AISISAE 03 carbon alloy steels Courtesy of ASM International Materials Park OH 64 Steel 95 be superior to those of the heattreated counterparts In appli cations where the properties are adequate microalloyed steels can often provide attractive cost savings Energy savings can be substantial straightening or stress relieving after heat treat ment is no longer necessary and quench cracking is not a prob lem Because of the increase in material strength the size and weight of finished products can often be reduced As a result the cost of a finished product can be reduced by 5 to 25 Bakehardenable steels are lowcarbon steels that are processed in such a way that they are resistant to aging during normal storage but begin to age during sheetmetal forming A subsequent exposure to heat during the paintbaking operation a finishing operation in automotive manufacture completes the aging process and adds an additional 35 to 70 MPa 5 to 10 ksi raising the final yield strength to approximately 275 MPa 40 ksi Because the increase in strength occurs after the forming operation the material offers good formability coupled with improved dent resistance in the final product In addition it allows weight savings to be achieved without compromising the attractive features of steel sheet which include spot weldability good crash energy absorption low cost and full recyclability Advanced HighStrength Steels AHSS Traditional methods of producing highstrength steel have included adding carbon andor alloy elements followed by heat treatment or coldworking to a high level followed by a partial anneal to restore some ductility As strength increased however ductility and toughness decreased and often became a limiting feature The highstrength lowalloy HSLA and microalloyed steels introduced about 40 years ago used thermomechanical processing to further increase strength but were accompanied by an even further decline in ductility Beginning in the mid 1990s enhanced thermomechanical processing capabilities and controls have led to the development of a variety of new highstrength steels that go collectively by the name advanced highstrength steels AHSS Many were developed for weight savings in automotive applications higher strength enabling reduced size or thickness while preserving or enhancing energy absorption As a result large amounts of lowcarbon and HSLA steels are being replaced by the advanced highstrength steels One group including the dualphase DP and transformation induced plasticity TRIP steels provides greater formability for alreadyexisting levels of strength Another group including complexphase and martensitic varieties provides higher levels of strength while retaining current levels of ductility Because of the improved formability the AHSS materials can often be stamped or formed into more complex parts Parts can often be integrated into single pieces eliminating the cost and time associated with assembly and the higher strength can provide weight reduction accompanied by improved fatigue and crash performance The various types of advanced highstrength steels are pri marily ferritephase soft steels with varying amounts of mar tensite bainite or retained austenite that offer high strength with enhanced ductility Each of the types will be described briefly next Dualphase steels form when we cool material to a tem perature that is above the A1 but below the A3 to form a struc ture that consists of ferrite and highcarbon austenite and then follow with a rapidcool quench During the quench the ferrite remains unaffected while the highcarbon austenite transforms to highcarbon martensite A low or mediumcarbon steel now has a mixed microstructure of a continuous weak ductile ferrite matrix combined with islands of highstrength high hardness highcarbon martensite The dualphase structure offers strengths that are comparable to the conventional high strength materials coupled with improved forming character istics and no loss in weldability The high workhardening rates and excellent elongation lead to a high ultimate tensile strength 590 to 1400 MPa or 85 to 200 ksi coupled with low initial yield strength The high strainrate sensitivity means that the faster the steel is crushed the more energy it absorbsa feature that further enhances the crash resistance of automotive structures Dualphase steels also exhibit the bakehardenable effect a precipitationinduced increase in yield strength when stamping or forming is followed by the elevated temperature of a paint bake oven Whereas the dualphase steels have structures of ferrite and martensite transformationinduced plasticity TRIP steels contain a matrix of ferrite combined with hard mar tensite or bainite and at least five volume percent of retained austenite Because of the hard phases dispersed in the soft ferrite deformation behavior begins much like the dualphase steels At higher strains however the retained austenite trans forms progressively to martensite enabling the high work hardening to persist to greater levels of deformation At lower levels of carbon the austenite transformation begins at lower levels of strain and the extended ductility of the lowercarbon TRIP steels offers significant advantages in operations such as stretchforming and deepdrawing At higher carbon levels the retained austenite is more stable and requires greater strains to induce transformation If the retained austenite can be carried into a finished part subsequent deformation such as a crash can induce transformation The conversion of retained austenite to martensite and the companion high rate of workhardening can then be used to provide excellent energy absorption Tensile strengths range from 590 to 1180 MPa 85 to 170 ksi Complexphase CP steels and martensitic Mart steels offer even higher strengths with useful capacity for deformation and energy absorption The CP steels have a microstructure of ferrite and bainite combined with small amounts of martensite retained austenite and pearlite and are strengthened further by grain refinement created by a fine precipitate of niobium tita nium or vanadium carbides or nitrides Strength ranges from 800 to 1180 MPa 115 to 170 ksi The Mart steels are almost entirely martensite and can have tensile strengths up to 1700 MPa 245 ksi depending on carbon content but elongation and formability are lower than the other types Still other types are making the transition from research into production These include the FerriticBainitic Steels FB also known as StretchFlangeable SF and High Hole Expansion HHE because of the improved stretch formability of sheared edgesTwinningInduced Plasticity Steels TWIP Nano steels and others The FB steels have a microstructure of soft ferrite and hard bainite coupled with grain refinement Formability is excellent and weldability and fatigue properties are both good TWIP steels contain between 17 and 24 manganese mak ing the steel fully austenitic at room temperature Deformation occurs by twinning inside the grains with the newly created twin boundaries providing increased strength and a high rate of strain hardening The result is a high strength more than 1000 MPa or 145 ksi combined with extremely high ductility as high 96 CHAPTER 6 Ferrous Metals and Alloys as 70 elongation Nano steels replace the hard phases that are present in the DP and TRIP steels with an array of ultrafine nanosized precipitates diameters less than 10 nm During sheet forming operations fractures often initiate at the interface between the very soft and very hard phases Because the inter faces are so small in the nano steels these fractures are avoided and formability is increased Hotforming HF steel offers tensile strengths in excess of 1600 MPA 230 ksi with elongation of 48 A manganese boron steel is heated to above 900C 1650F and then rapidly transferred to a press where it is shaped while the structure is austenite the strength is less than 100 MPa 15 ksi and ductility is excellent The part then remains in the watercooled die with a cooling rate in excess of 50Csec where the structure trans forms to martensite in a process known as press hardening Complex shapes can be produced with little or no springback and final strength of 1500 to 1800 MPa 215 to 260 ksi Figure 67 shows the relative strengths and formability elongation of mild steel conventional highstrength steels and the newer AHSS materials Note how the newer AHSS steels increase strength or formability or both Also included is the TWIP steels which occupy a region of very high formability that lies outside of the overlapping band Some useful distinctions between lowstrength steel UTS below 270 MPa or 40 ksi high strength steel and ultrahighstrength steel UTS above 700 MPa or 100 ksi have also been included in this figure Table 65 shows the material content of a North Ameri can light vehicle for 1975 and 2007 with an estimate for 2015 Table 66 looks only at the metal content of the vehicle body and enclosure Note the increased role of the advanced high strength steels Automotive forecasts in the 1960s 1980s and into the 1990s all predicted an increasing role for the lightweight alternative materials such as aluminum magnesium and fiberreinforced polymers and a declining role for steel Recent studies however have shown that the percentage of steel has actually risen over the years This has largely been because of the development of new types of steels that are stronger more energy absorbent and easy to fabricate into thinnergage reducedweight structures Alloy development continues along with research into improved processing methods including induction heating and innovative cooling systems Table 67 provides the mechanical properties of some grades of advanced highstrength automotive sheet steels FreeMachining Steels The increased use of highspeed automated machining has spurred the use and development of several varieties of free machining steels These steels machine readily and form small chips when cut The smaller chips reduce the length of con tact between the chip and cutting tool thereby reducing the Nomenclature of Advanced HighStrength Steels AHSS More than 30 different AHSS steels are projected to be commer cially available between 2015 and 2020 offering enhancements in strength and formability The specific grades are identified by the typedesignating letters Dual phase DP Transformation Induced Plasticity TRIP Complex Phase CP Martensite MS Hot Formed HF and TwinningInduced Plasticity TWIP followed by the minimum yield strength and minimum tensile strength in megapascals where 69 MPa is equivalent to 1 ksi As an example DP 350600 is a dualphase steel with a 350 MPa 507 ksi minimum yield strength and a 600 MPa 87 ksi minimum tensile strength Lowstrength steels 270 MPa Highstrength steels Ultrahighstrength steels 700 MPa Total elongation 70 60 50 40 30 20 10 0 0 300 600 Conventional HSS AHSS 900 1200 1700 Tensile strength MPa MART MILD BH TWIP FIGURE 67 Relative strength and formability elongation of conventional highstrength lowalloy and advanced highstrength steels BH bake hardenable DP dual phase and Mart martensitic Also included is the newer TWIP steels 64 Steel 97 associated friction and heat as well as required power and wear on the cutting tool The formation of small chips also reduces the likelihood of chip entanglement in the machine and makes chip removal much easier On the negative side freemachining steels often carry a cost premium of 15 to 20 over conventional alloys but this increase can be easily recovered through higher machining speeds larger depths of cut and extended tool life Freemachining steels are basically carbon steels that have been modified by an addition of sulfur lead bismuth selenium tellurium or phosphorus plus sulfur to enhance machinabil ity Sulfur combines with manganese to form soft manganese sulfide inclusions These in turn serve as chipbreaking dis continuities within the structure The inclusions also provide a builtin lubricant that prevents formation of a builtup edge on the cutting tool and imparts an improved cutting geometry see Chapter 21 In leaded materials the insoluble lead particles work much the same way The bismuth freemachining steels are an attractive alter native to the previous varieties Bismuth is more environmen tally acceptable compared to lead has a reduced tendency to form stringers and can be more uniformly dispersed because its density is a better match to that of iron Machinability is improved because the heat generated by cutting is sufficient to form a thin film of liquid bismuth that lasts for only fractions of a microsecond Tool life is noticeably extended and the machined product is still weldable The use of freemachining steels is not without compro mise however Ductility and impact properties are somewhat reduced compared to the unmodified steels Weldability can be TABLE 66 Metallic Material Content of the Body and Enclosure of a North American Light Vehicle Material 2007 Percentage Projected 2015 Percentage Mild Steel 546 290 BakeHardenable and Medium HSS 224 235 Conventional HSS 127 102 Advanced HSS 95 348 Aluminum and Magnesium 08 25 Note Data from Drucker Worldwide presentation at the AISI Great Designs in Steel Conference March 2007 TABLE 65 Material Content of a North American Light Vehicle in 1975 and 2007 with a Projection for 2015 Material Content in Pounds 1975 2007 2015 Change from 1975 to 2015 Mild Steel 2180 1748 1314 Down 866 lbs HighStrength Steel 140 334 315 Up 175 lbs Advanced HSS 149 403 Up 403 lbs Other Steels 65 76 77 Up 12 lbs Iron 585 284 244 Down 341 lbs Aluminum 84 327 374 Up 290 lbs Magnesium 9 22 Up 22 lbs Other Metals 120 149 145 Up 25 lbs PlasticComposites 180 340 364 Up 184 lbs Other Materials 546 634 650 Up 104 lbs Total Pounds 3900 4050 3908 Up 8 lbs Note Data from Drucker Worldwide presentation at the AISI Great Designs in Steel Conference March 2007 TABLE 67 Mechanical Properties of Various Grades of Advanced HighStrength Automotive Sheet Steels a ColdRolled AHSS Grades Min Yield Str Min Tensile Str Type MPa ksi MPa ksi Dualphase 250 36 440 64 290 42 490 71 340 49 590 855 550 80 690 100 420 61 780 113 550 80 980 142 TRIP Steels 380 55 590 855 400 58 690 100 420 61 780 113 Martensitic Steels 700 1015 900 1305 860 125 1100 1 1030 1495 1300 1885 1200 174 1500 2175 HotRolled AHSS Grades Min Yield Str Min Tensile Str Type MPa ksi MPa ksi Dualphase 300 435 590 855 380 55 780 113 TRIP Steels 400 58 590 855 450 65 780 113 aData from society of Automotive Engineers document SAE J2745 July 2007 98 CHAPTER 6 Ferrous Metals and Alloys limited if the freemachining component wets weld metal grain boundaries Copperbased braze joints tend to embrittle when used to join bismuth freemachining steels and the machin ing additions reduce the strength of shrinkfit assemblies If these compromises are objectionable other methods might be used to enhance machinability For example the machina bility of steels can be improved by cold working the metal As the strength and hardness of the metal increase the metal loses ductility and subsequent machining produces chips that tear away more readily and fracture into smaller segments Precoated Steel Sheet Traditional sheet metal fabrication involves the shaping of com ponents from bare steel followed by the finishing or coating of these products on a piecebypiece basis In this sequence it is not uncommon for the finishing processes to be the most expensive and timeconsuming stages of manufacture because they involve handling manipulation and possible curing or dry ing as well as adherence to the various EPA environmental and OSHA safety and health requirements An alternative to this procedure is to purchase precoated steel sheet where the steel supplier applies the coating when the material is still in the form of a long continuous strip Clean ing pretreatment coating and curing can all be performed in a continuous manner producing a coating that is uniform in thickness and offers improved adhesion Numerous coatings can be specified including the entire spectrum of dipped and plated metals including aluminum zinc and chromium vinyls paints primers and other polymers or organics Many of these coatings are specially formulated to endure the rigors of sub sequent forming and bending The continuous sheets can also be printed striped or embossed to provide a number of visual effects Extra caution must be exercised during handling and fabrication to prevent damage to the coating but the additional effort and expense are often less than the cost of finishing indi vidual pieces Steels for Electrical and Magnetic Applications Soft magnetic materials can be magnetized by relatively low strength magnetic fields but lose almost all of their magnetism when the applied field is removed They are widely used in prod ucts such as solenoids transformers motors and generators The most common soft magnetic materials are highpurity iron lowcarbon steels ironsilicon electrical steels amorphous fer romagnetic alloys ironnickel alloys and soft ferrites ceramic material In recent years the amorphous metals have shown attrac tive electrical and magnetic properties Because the material has no crystal structure grains or grain boundaries 1 the magnetic domains can move freely in response to magnetic fields 2 the properties are the same in all directions and 3 corrosion resistance is improved The high magnetic strength and low hysteresis losses offer the possibility of smaller lighter weight magnets When used to replace silicon steel in power transformer cores this material has the potential of reducing core losses by as much as 50 To exhibit permanent magnetism materials must remain magnetized when removed from the applied field Although most permanent magnets are ceramic materials or complex metal alloys cobalt alloy steels containing up to 36 cobalt might be specified for electrical equipment where high mag netic densities are required Maraging Steels When superhigh strength is required the maraging grades become a very attractive option These alloys contain between 15 and 25 nickel plus significant amounts of cobalt molyb denum and titaniumall added to a verylowcarbon steel They can be hot worked at elevated temperatures and machined or cold worked in the aircooled condition Yield strengths in excess of 1725 MPa 250 ksi coupled with good residual elonga tion are the result of a lowcarbon tough and ductile ironnickel martensite which is further strengthened by the precipitation of intermetallic compounds during a subsequent age harden ing process Maraging alloys are very useful in applications where ultra high strength and good toughness are important The fracture toughness of maraging steel is considerably higher than that of the conventional highstrength steels They can be welded pro vided the weldment is followed by the full solution and aging treatment As might be expected from the large amount of alloy additions more than 30 and the multistep thermal process ing maraging steels are quite expensive and should be specified only when their outstanding properties are absolutely required Steels for HighTemperature Service As a general rule of thumb plaincarbon steels should not be used at temperatures in excess of about 250C 500F Conven tional alloy steels extend this upper limit to around 350C 650F Numerous application areas however require materials with good strength characteristics corrosion resistance and creep resistance at operating temperatures in excess these values The hightemperature ferrous alloys tend to be lowcarbon materials with less than 01 carbon At their peak operating temperatures 1000hour rupture stresses tend to be quite low however often in the neighborhood of 50 MPa 7 ksi Iron can be a major component of other hightemperature alloys but when the amount is less than 50 the metal is not classified as a ferrous material High strength at high temperature usually requires the more expensive nonferrous materials that will be discussed in Chapter 7 65 Stainless Steels Lowcarbon steel with the addition of 4 to 6 chromium exhibits good resistance to many of the corrosive media encoun tered in the chemical industry This behavior is attributed to the formation of a strongly adherent iron chromium oxide on the surface If more improved corrosion resistance and outstanding appearance are required materials should be specified that use 65 Stainless Steels 99 a superior oxide that forms when the amount of chromium in solution excluding chromium carbides and other forms where the chromium is no longer available to react with oxygen exceeds 12 When damaged this tough adherent transpar ent corrosionresistant oxide which is only 12 nanometers thick actually heals itself provided oxygen is present even in very small amounts Materials that form this superior protective oxide are known as stainless steels or more specifically true stainless steels Several classification schemes have been devised to cat egorize these alloys The American Iron and Steel Institute AISI groups the metals by chemistry and assigns a threedigit number that identifies the basic family and the particular alloy within that family In this text however we group these alloys into microstructural families because it is the basic structure that controls the engineering properties of the metal Table 68 presents the AISI designation scheme for stainless steels and correlates it with the microstructural families Because chromium has a bodycentered cubic structure it tends to stabilize the bodycentered ferrite structure in a steel increasing the temperature range over which ferrite is the stable structure With sufficient chromium and a low level of carbon a corrosionresistant iron alloy can be produced that is ferrite at all temperatures below solidification These alloys are known as the ferritic stainless steels They pos sess rather limited ductility and poor toughness but are read ily weldable No martensite can form in the welds because there is no possibility of forming the FCC austenite structure that can then transform during cooling These alloys can not be heattreated and poor ductility limits the amount of strengthening by cold work The primary source of strength is the bodycenteredcubic BCC crystal structure combined with the effects of solid solution strengthening Characteristic of the BCC metals the ferritic stainless steels exhibit a ductile tobrittle transition as the temperature is reduced The ferritic alloys are the cheapest type of stainless steel however and as such they should be given first consideration when a stainless alloy is required If increased strength is needed the martensitic stainless steels should be considered For these alloys carbon is added and the chromium content is reduced to a level where the material can be austenite FCC at high temperature and fer rite BCC at low Upon heating the carbon will dissolve in the facecenteredcubic austenite which can then be quenched to trap it in the bodycentered martensitic structure The car bon contents can be varied up to 12 to provide a wide range of strengths and hardnesses Caution should be taken how ever to ensure more than 12 chromium remains in solution Slow cools can allow the carbon and chromium to react and form chromium carbides When this occurs the chromium is no longer available to react with oxygen and form the protec tive oxide As a result the martensitic stainless steels can only exhibit good corrosion resistance when in the martensitic con dition when the chromium is trapped in atomic solution and might be susceptible to red rust when annealed or normalized for ease of machining or fabrication The martensitic stainless steels cost about 1½ times as much as the ferritic alloys with part of the increase being because of the additional heat treat ment which generally consists of an austenitization quench stress relief and temper They are less corrosion resistant than the other varieties are the least weldable of the stainless steels and have a ductiletobrittle transition at low tempera tures The martensitic stainlesses tend to be used in applica tions such as cutlery where strength and hardness are the dominant requirements Nickel being facecentered cubic is an austenite stabilizer and with sufficient amounts of both chromium and nickel and low carbon it is possible to produce a stainless steel in which austenite is the stable structure from elevated to cryogenic tem peratures Known as austenitic stainless steels these alloys can cost two to three times as much as the ferritic variety but here the added expense is attributed to the cost of the nickel and chromium alloys The most widely used austenitic stain less steel Type 304 is also known as 188 because it contains 18 chromium and 8 nickel Manganese and nitrogen are also austenite stabilizers and can be substituted for some of the nickel to produce a lowercost somewhat lowerquality austen itic stainless steel the AISI 200series Austenitic stainless steels are easily identified by their non magnetic characteristic the ferritic and martensitic stainlesses are attracted to a magnet They are highly resistant to corrosion in almost all media except hydrochloric acid and other halide acids and salts and can be polished to a mirror finish thereby combining attractive appearance and corrosion resistance Toughness is excellent particularly at low temperatures Form ability is outstanding because of the low yield strength and high elongation that is characteristic of the FCC crystal structure and these steels strengthen significantly when cold worked The following table shows the response of the popular 304 alloy to a small amount of cold work Water Quench Cold Rolled 15 Yield strength MPa ksi 260 38 805 117 Tensile strength MPA ksi 620 90 965 140 Elongation in 2 in 68 11 Among the stainless varieties the austenitic stainless steels offer the best combination of corrosion resistance and toughness are easily welded and do not embrittle at low temperatures Because they are also some of the costliest they should not be specified where the lessexpensive ferritic or martensitic alloys would be adequate or where a true stain less steel is not required Figure 68 lists some of the popular alloys from each of the three major structural classifications and links them to some associated properties Table 69 summarizes the basic types and the primary mechanism of strengthening A fourth and special class of stainless steel is the precipitationhardening stainless steel These alloys are TABLE 68 AISI Designation Scheme for Stainless Steels Series Alloys Structure 200 Chromium nickel manganese or nitrogen Austenitic 300 Chromium and nickel Austenitic 400 Chromium and possibly carbon Ferritic or martensitic 500 Low chromium 12 and possibly carbon Martensitic 100 CHAPTERG Ferrous Metals and Alloys gage grades are designated by the letters F or Se fol type lowing the threedigit alloy code TT 20 02EO00cwl OrwnwO oS The preceding discussion has focused on Martensitic 410 General purpose the wrought stainless alloys Cast stainless hardenable by heat 420 treatment 440C Hardenable by heat steels have structures and properties that are treatment similar to the wrought grades Most are speci Ferritic 405 fied by properties using standards of ASTM or more corrosion resistant 430 ISO International Organization for Standardi than martensitic but not 446 we spe hardenable by heat zation while chemistries have been classified treatment Hardenable by cold working by the High Alloy Product Group of the Steel Austeniti 501 Founders Society of America The Cseries are Yoest corrosion resistance 202 For elevatedtemperature used primarily to impart corrosion resistance but hardenable only by 301 service and are used in valves pumps and fittings The cold working 302 Hgrades heatresistant have been designed 302B to provide useful properties at elevated temper 310 Modified for welding ature and are used for furnace parts and turbine 316 Superior corrosion components 321 resistance Several potential problems are unique to sss the family of stainless steels Because the pro tective oxide provides the excellent corrosion Different types of stainless steels along with popular alloys within each resistance this feature can be lost whenever type and key properties the amount of chromium in solution drops below 12 A localized depletion of chromium basically martensitic or austenitic types modified by the addi can occur when elevated temperatures allow chromium car tion of alloying elements such as copper aluminum and bides to form along grain boundaries sensitization To titanium that permit the precipitation of hard intermetallic com prevent their formation one can keep the carbon content of pounds at the temperatures used to temper martensite With stainless steels as low as possible usually below 010 Spe the addition of age hardening these materials are capable of cial lowcarbon grades are designated by the letter L after their attaining highstrength properties such as a 1790 MPa 260 ksi threedigit number Another method is to tie up existing car yield strength 1825 MPa 265 ksi tensile strength and a 2 bon with small amounts of stabilizing elements such as tita elongation Because the additional alloys and extra process nium or niobium that have a stronger affinity for carbon than ing make the precipitationhardening alloys some of the most does chromium A letter suffix again designates the modifica expensive stainless steels they should be used only when their tion Rapidly cooling these metals through the carbideforming highstrength feature is absolutely required range of 480 to 900C 900 to 1650F also works to prevent Although the four structures described earlier constitute carbide formation the bulk of stainless steels there are also some additional vari Another problem with highchromium stainless steels is ants Duplex stainless steels contain between 18 and 25 an embrittlement that can occur after long times at elevated chromium 4 and 7 nickel and up to 4 molybdenum and temperatures This is attributed to the formation of a brittle can be water quenched from a hotworking temperature to compound that forms at elevated temperature and coats grain produce a microstructure that is approximately half ferrite and boundaries Known as sigma phase this material then provides half austenite This mixed structure offers good toughness and a brittle crack path through the metal Stainless steels used in a high yield strength coupled with greater resistance to both hightemperature service should be checked periodically to stress corrosion cracking and pitting corrosion than either the detect and monitor sigmaphase formation fullaustenitic or fullferritic grades Stainless steels are difficult to machine because of their workhardening properties and their tendency to seize during cutting To overcome these limitations special freemachining 6 6 Tool Stee Is alloys have been produced within each family Additions of sul fur phosphorus or selenium can raise machinability to approxi mately that of a mediumcarbon steel The freemachining Tool steels are highcarbon highstrength ferrous alloys that have been modified by alloy additions to provide a desired bal ance of strength resistance to deformation toughness abil TABLE 69 Primary Strengthening Mechanism for the ity to absorb shock or impact and wear resistance ability to Various Types of Stainless Steel resist erosion between the tool steel and contact material when properly heattreated Several classification systems have been Ferritic Bee een enna developed some using chemistry as a basis whereas others a employ a hardening method or major mechanical property The Martensitic Phase transformation strengthening een AISI system uses a letter designation to identify basic features Fitcten Gs nr ar cs sene cone renee net such as quenching method primary application special alloy E EE or dominant characteristic Table 610 lists seven basic families 66 Tool Steels 101 of tool steels the corresponding AISI letter and the associated feature or characteristic Individual alloys within the letter grades are then listed numerically to produce a letternumber identification system such as W1 D2 or H13 Waterhardening tool steels Wgrade are essentially highcarbon plaincarbon steels They are the least expensive variety and are used for a wide range of parts that are usually quite small and not subject to severe usage or elevated tem perature Because strength and hardness are functions of the carbon content a wide range of properties can be achieved through composition variation Hardenability is low so these steels must be quenched in water to attain high hardness They can be used only for relatively thin sections if the full depth of hardness is desired They are also rather brittle particularly at higher hardness Typical uses of the various plaincarbon steels are as follows 060075 carbon machine parts chisels setscrews and similar products where medium hardness is required coupled with good toughness and shock resistance 075090 carbon forging dies hammers and sledges 090110 carbon generalpurpose tooling applications that require good balance of wear resistance and toughness such as drills cutters shear blades and other heavyduty cutting edges 110130 carbon small drills lathe tools razor blades and other lightduty applications in which extreme hardness is required without great toughness In applications where improved toughness is required small amounts of manganese silicon and molybdenum are often added Vanadium additions of about 020 are used to form strong stable carbides that retain fine grain size during heat treatment One of the main weaknesses of the plaincarbon tool steels is their loss of hardness at elevated temperature which can occur with prolonged exposure to temperatures higher than 150C 300F When larger parts must be hardened or distortion must be minimized the coldwork tool steels are usually recommended The alloy additions and higher hardenability of the oil or airhardening grades O and A designations respectively enable hardening by less severe quenches Tighter dimensional tolerances can be maintained during heat treatment and the cracking tendency is reduced The high chromium tool steels D designation contain between 10 and 18 chromium and are airhardened offering outstanding deephardening wear resistance Blanking stamping and cold forming dies punches and other tools for large production runs are all common applications for this class Because these steels do not have the alloy content necessary to resist sof tening at elevated temperatures they should not be used for applications that involve prolonged service at temperatures in excess of 250C 500F Shockresisting tool steels S designation offer the high toughness needed for impact applications Low carbon content approximately 05 carbon is usually specified to ensure the necessary toughness with carbideforming alloys providing the necessary abrasion resistance hardenability and hotwork characteristics Applications include parts for pneumatic tool ing chisels punches and shear blades Highspeed tool steels are used for cutting tools and other applications where strength and hardness must be retained at temperatures up to or exceeding redheat about 760C or 1400F One popular member of the tungsten highspeed tool steels T designation is the T1 alloy which contains 07 car bon 18 tungsten 4 chromium and 1 vanadium It offers a balanced combination of shock resistance and abrasion resist ance and is used for a wide variety of cutting applications The molybdenum highspeed steels M designation were developed to reduce the amount of tungsten and chromium required to produce the highspeed properties and M1 has become quite popular for drill bits Hotwork tool steels H designation were developed to provide strength and hardness during prolonged exposure to elevated temperature All employ substantial additions of carbideforming alloys H1 to H19 are chromiumbased alloys with about 50 chromium H20 to H39 are tungstenbased types with 9 to 18 tungsten coupled with 3 to 4 chro mium and H40 to H59 are molybdenumbased The chromium types tend to be less expensive than the tungsten or molybde num alloys Other types of tool steels include 1 the plastic mold steels P designation designed to meet the requirements of zinc die casting and plastic injection molding dies 2 the lowalloy specialpurpose tool steels L designation such as the L6 extreme toughness variety and 3 the carbontungsten type of special purpose tool steels F designation which are water hardening but substantially more wear resistant than the plaincarbon tool steels Most tool steels are wrought materials but some are designed specifically for fabrication by casting Powder metal lurgy processing has also been used to produce special compo sitions that are difficult or impossible to produce by wrought or cast methods or to provide key structural enhancements By subjecting water atomized powders to hotisostatic pressing HIP 100 dense billets can be produced with fine grain size and small uniformly distributed carbide particles These mate rials offer superior wear resistance approaching that of the ceramic carbides combined with useful levels of toughness TABLE 610 Basic Types of Tool Steel and Corresponding AISI Grades Type AISI Grade Significant Characteristic 1 Waterhardening W 2 Coldwork O Oilhardening A Airhardening medium alloy D Highcarbonhighchromium 3 Shockresisting S 4 Highspeed T Tungsten alloy M Molybdenum alloy 5 Hotwork H H1H19 chromium alloy H20H39 tungsten alloy H40H59 molybdenum alloy 6 Plasticmold P 7 Specialpurpose L Low alloy F Carbontungsten 102 CHAPTER 6 Ferrous Metals and Alloys 67 Cast Irons The term cast iron applies to an entire family of metals that are alloys of iron carbon in excess of 20 and silicon 05 to 30 and offer a wide variety of properties Various types of cast iron can be produced depending on the chemical composi tion cooling rate and the type and amount of inoculants that are used Inoculants and inoculation practice will be discussed shortly Each of the types however can be described as hav ing a structure consisting of an ironbased metal matrix such as ferrite pearlite bainite or martensite and a highcarbon sec ond phase that is either graphite pure carbon or iron carbide Fe3C The basic types of cast iron are gray white malleable ductile or nodular austempered ductile compacted graphite and highalloy Types of Cast Iron Gray cast iron is the least expensive and most common vari ety and can be characterized by those features that promote the formation of graphite discussed at the end of Chapter 4 Typical compositions range from 25 to 40 carbon 10 to 30 silicon and 04 to 10 manganese The microstructure consists of threedimensional interconnected graphite flakes which form during the eutectic reaction dispersed in a matrix of ferrite pearlite or other ironbased structure that forms from the cooling of austenite Figure 69 presents a typical section through gray cast iron showing the graphite flakes dispersed throughout the metal matrix Because the graphite flakes have no appreciable strength they act essentially as voids in the structure The pointed edges of the flakes act as preexisting notches or crack initiation sites giving the material its charac teristic brittle nature Because a large portion of any fracture fol lows the graphite flakes the freshly exposed fracture surfaces have a characteristic gray appearance hence the namegray iron and a graphite smudge can usually be obtained if one rubs a finger across the fracture On a more positive note the formation of the lowerdensity graphite reduces the amount of shrinkage that occurs when the liquid goes to solid making pos sible the production of more complex iron castings The size shape and distribution of the graphite flakes have a considerable effect on the overall properties of gray cast iron When maximum strength is desired small uniformly distributed flakes with a minimum amount of intersection are preferred A more effective means of controlling strength however is through control of the metal matrix structure which is in turn controlled by the carbon and silicon contents and the cooling rate of the casting Gray cast iron is normally sold by class with the class number corresponding to the minimum tensile strength in thou sands of pounds per square inch1 Class 20 iron minimum ten sile strength of 20000 psi consists of highcarbon highsilicon metal with a ferrite matrix Higher strengths up to class 40 can be obtained with lower carbon and silicon and a pearlite matrix To go above class 40 alloying is required to provide solid solu tion strengthening and heattreatment practices must be per formed to modify the matrix Gray cast irons can be obtained up through class 80 but regardless of strength the presence of the graphite flakes results in extremely low ductility Gray cast irons offer excellent compressive strength com pressive forces do not promote crack propagation so compres sive strength is typically 254 times tensile strength excellent machinability the graphite flakes act to break up the chips and lubricate contact surfaces good resistance to adhesive wear and galling graphite flakes selflubricate and outstanding sound and vibration damping characteristics graphite flakes absorb transmitted energy Table 611 compares the relative damp ing capacities of various engineering metals and clearly shows the unique characteristic of the highcarbonequivalent high carbon and highsilicon gray cast irons This material is 2025 times better than steel and 250 times better than aluminum High silicon contents promote good corrosion resistance and provide the enhanced fluidity desired for casting operations For these reasons coupled with low cost high thermal conductivity low rate of thermal expansion good stiffness resistance to ther mal fatigue and 100 recyclability gray cast iron is specified for a number of applications including automotive engine blocks heads and cylinder liners transmission housings machine tool bases and large equipment parts that are subjected to com pressive loads and vibrations Weldability however is poor and the material cannot be shaped by deformation White cast iron has all of its excess carbon in the form of iron carbide and receives its name from the white surface that appears when the material is fractured Features promoting its formation are those that favor cementite over graphite a low carbon equivalent 18 to 36 carbon 05 to 19 silicon and 025 to 08 manganese and rapid cooling Because the large amount of iron carbide as much as 50 dominates the microstructure white cast iron is very hard and brittle and finds applications where high abrasion resistance is the overwhelming requirement For these uses it is also com mon to pursue the hard wearresistant martensite structure described in Chapter 5 as the metal matrix In this way both the metal matrix and the highcarbon second phase contribute to the wearresistant characteristics of the material 1 The ASTM class uses numbers reflecting thousands of pounds per square inch while the International Organization for Standardization ISO assigns class num bers using megapascals FIGURE 69 Photomicrograph showing the distribution of graphite flakes in gray cast iron unetched 100 Courtesy Ronald Kohser 67 Cast Irons 103 White cast iron surfaces can also be applied over a base of another material For example mill rolls that require extreme wear resistance can have a white cast iron surface on top of a steel interior By accelerating the cooling rate and controlling chemistry white iron surfaces or regions can be produced in gray iron castings Tapered sections or metal chill bars placed in the molding sand provide the accelerated cooling When regions of white and gray cast iron occur in the same component there is generally a transition zone containing regions of both white and gray irons known as the mottled zone If white cast iron is exposed to an extended heat treatment at temperatures in the range of 900C 1650F the cementite will dissociate into its component elements and some or all of the carbon will be converted into irregularly shaped clusters of graph ite also referred to as clump or popcorn graphite The product known as malleable cast iron has significantly improved ductil ity compared to gray cast iron because the more favorable graph ite shape no longer resembles an internal crack or notch The rapid cooling required to produce the starting white iron struc ture however restricts the size and thickness of malleable iron products such that most weigh less than 5 kilograms 10 pounds Various types of malleable iron can be produced depend ing on the type of heat treatment that is employed If the white iron is heated and held for a prolonged time just below the melt ing point the carbon in the cementite converts to graphite first stage graphitization Subsequent slow cooling through the eutectoid reaction causes the carboncontaining austenite to transform to ferrite and more graphite secondstage graphitiza tion and the resulting product known as ferritic malleable cast iron has a structure of irregular particles of graphite dispersed in a ferrite matrix Figure 610 Typical properties of this mate rial are 10 elongation 240 MPa 35 ksi yield strength 345 MPa 50 ksi tensile strength and excellent impact strength corro sion resistance and machinability The heattreatment times however are quite lengthy often involving more than 100 hours at elevated temperature If the material is cooled more rapidly through the eutectoid transformation the carbon in the austenite does not form addi tional graphite but is retained in a pearlite or martensite matrix The resulting pearlitic malleable cast iron is characterized by higher strength and lower ductility than its ferritic counterpart Proper ties range from 1 to 4 elongation 310 to 590 MPa 45 to 85 ksi yield strength and 450 to 725 MPa 65 to 105 ksi tensile strength with reduced machinability compared to the ferritic material The modified graphite structure of malleable iron provided quite an improvement in properties compared to gray cast iron However it would be even more attractive if a similar structure could be obtained directly on solidification rather than through a prolonged heat treatment at high elevated temperature If a highcarbonequivalent cast iron is sufficiently low in sulfur either by original chemistry or by desulfurization the addition of certain materials can promote graphite formation and change the morphology shape of the graphite product If ferrosilicon is injected into the melt inoculation it will promote the forma tion of graphite If magnesium in the form of MgFeSi or MgNi alloy is also added just prior to solidification the graphite will form as smoothsurface spheres The latter addition is known as a nodulizer and the product becomes ductile or nodular cast iron One should note that the magnesium nodulizer volatilizes easily Its effectiveness will diminish or be lost with time and the graphite structure will transition to the flake form of gray cast iron This phenomenon is known as fading Subsequent control of cooling can produce a variety of matrix structures with ascast ferrite andor pearlite being the most common Figure 611 and alloying andor heat treatment extending these to include martensite bainite or austenite By controlling the matrix structure properties can be produced that TABLE 611 Relative Damping Capacity of Various Metals Material Damping Capacitya Gray iron high carbon equivalent 100500 Gray iron low carbon equivalent 20100 Ductile iron 520 Malleable iron 815 White iron 24 Steel 4 Aluminum 04 aNatural log of the ratio of successive amplitudes FIGURE 610 Photomicrograph of malleable iron showing the irregular graphite clusters etched to reveal the ferrite matrix 100 Courtesy Ronald Kohser FIGURE 611 Ductile cast iron with a ferrite matrix Note the sphe roidal shape of the graphite 100 Courtesy Ronald Kohser 104 CHAPTER 6 Ferrous Metals and Alloys span a wide range from 2 to 18 elongation 275 to 620 MPa 40 to 90 ksi yield strength and 415 to 825 MPa 60 to 120 ksi ten sile strength The combination of good ductility high strength toughness wear resistance machinability lowmeltingpoint castability and up to a 10 weight reduction compared to steel makes ductile iron an attractive engineering material High siliconmolybdenum ductile irons offer excellent high tempera ture strength and good corrosion resistance Unfortunately the costs of a nodulizer highergrade melting stock better furnaces and the improved process control required for its manufacture combine to place it among the more expensive of the cast irons Austempered ductile iron ADI ductile iron that has undergone a special austempering heat treatment to modify and enhance its properties2 has emerged as a significant engi neering material during the past 30 years It combines the abil ity to cast intricate shapes with strength fatigueresistance and wearresistance properties that are similar to those of heattreated steel Compared to conventional ascast ductile iron it offers nearly double the strength at the same level of ductility Compared to steel it offers reduced cost an 8 to 10 reduction in density so strengthtoweight is excellent 2 The austempering process begins by heating the metal to a temperature between 1500 and 1750 F 815 and 955 C and holding for sufficient time to saturate the austenite with carbon The metal is then rapidly cooled to an austempering tem perature between 450 and 750 F 230 and 400 C where it is held until all crystal structure changes have completed and then cooled to room temperature High austempering temperatures give good toughness and fatigue properties whereas lower austempering temperatures give better strength and wear resistance and enhanced damping capability both because of the graph ite nodules Machinability is generally poorer thermal conduc tivity is lower and there is about a 20 drop in elastic modulus ADI is approximately three times stronger than aluminum more than two times stiffer with better fatigue strength and wear resistance Table 612 compares the typical mechanical prop erties of some malleable cast irons the five grades of ductile cast iron specified in ASTM Standard A536 and the six grades of austempered ductile cast iron that are specified in ASTM Standard A897 Compacted graphite cast iron CGI is also attracting con siderable attention Produced by a method similar to that used to make ductile iron a MgCeTi addition is made compacted graphite iron is characterized by a graphite structure that is intermediate to the flake graphite of gray iron and the nodular graphite of ductile iron and it tends to possess some of the desir able properties and characteristics of each Table 613 shows how the properties of compacted graphite iron bridge the gap between gray and ductile Strength stiffness and ductility are greater than those of gray iron whereas castability machinabil ity thermal conductivity and damping capacity all exceed those of ductile Impact and fatigue properties are good ASTM Specification A842 identifies five grades of com pacted graphite cast iron250 300 350 400 and 450where the numbers correspond to tensile strength in megapascals Areas of application tend to be those where the mechanical properties of gray iron are insufficient and those of ductile iron along with its higher cost are considered to be overkill More specific compacted graphite iron is attractive when the desired TABLE 612 Typical Mechanical Properties of Malleable Ductile and Austempered Ductile Cast Irons Class or Grade Minimum Yield Strength Minimum Tensile Strength Minimum Percentage Elongation Brinell Hardness Number ksi MPa ksi MPa Malleable Irona M3210 32 224 50 345 10 156 max M4504 45 310 65 448 4 163217 M5003 50 345 75 517 3 187241 M5503 55 379 75 517 3 187241 M7002 70 483 90 621 2 229269 M8501 85 586 105 724 1 269302 Ductile Ironb 604018 40 276 60 414 18 149187 654512 45 310 65 448 12 170207 805506 55 379 80 552 6 187248 1007003 70 483 100 689 3 217269 1209002 90 621 120 827 2 240300 Austempered Ductile Ironc 1 70 500 110 750 11 241302 2 90 550 130 900 9 269341 3 110 750 150 1050 7 302375 4 125 850 175 1200 4 341444 5 155 1100 200 1400 2 388477 6 185 1300 230 1600 1 402512 aASTM Specification A602 Also SAE J 158 bASTM Specification A536 cASTM Specification A897 69 The Role of Processing on Cast Properties 105 properties include high strength castability machinability thermal conductivity and thermal shock resistance The Role of Alloys in Cast Irons Because the effects of alloying elements are often the same regardless of the process used to produce the final shape much of what was presented earlier for wrought steel also applies to cast irons When the desired shape is to be made by casting however some alloys can be used to enhance processspecific features such as fluidity and assolidified properties Many cast iron products are used in the ascast condition with the only heat treatment being a stress relief or annealing For these applications the alloy elements are selected for their ability to alter properties by 1 affecting the formation of graph ite or cementite 2 modifying the morphology of the carbonrich phase 3 strengthening the matrix material or 4 enhancing wear resistance through the formation of alloy carbides Nickel for example promotes graphite formation and tends to promote finer graphite structures Chromium retards graphite forma tion and stabilizes cementite These alloys are frequently used together in a ratio of two or three parts of nickel to one part of chromium Between 05 and 10 molybdenum is often added to gray cast iron to impart additional strength form alloy car bides and help to control the size of the graphite flakes Highalloy cast irons have been designed to provide enhanced corrosion resistance andor good elevated tempera ture service Within this family the austenitic gray cast irons which contain about 14 nickel 5 copper and 25 chro mium offer good corrosion resistance to many acids and alka lis at temperatures up to about 800C 1500F The addition of up to 1 molybdenum and 5 silicon to ductile iron greatly enhances high temperature tensile strength and creep strength They are attractive for applications involving temperatures between 650C and 875C 1200F and 1600F 68 Cast Steels If a ferrous casting alloy contains less than about 20 carbon it is considered to be a cast steel Alloys with more than 2 car bon are cast irons Cast steels are generally used whenever a cast iron is not adequate for the application Compared to cast irons the cast steels offer enhanced stiffness toughness and ductility over a wide range of operating temperatures and can be readily welded They are usually heattreated to produce a final quenchedandtempered structure and the alloy additions are selected to provide the desired hardenability and balance of properties The enhanced properties come with a price how ever because the cast steels have a higher melting point more energy to melt and higher cost refractories are necessary lower fluidity leading to increased probability of incomplete die or mold filling and increased shrinkage because graphite is not formed during solidification The diverse applications take advantage of the materials structural strength and its ability to contain pressure resist impacts withstand elevated tempera tures and resist wear 69 The Role of Processing on Cast Properties Although typical properties have been presented for the vari ous types of cast materials it should be noted that the prop erties of all metals are influenced by how they are processed For cast materials properties will often vary with the manner of solidification and cooling Because cast components often have complex geometries the cooling rate can vary from location to location with companion variation in properties To ensure compliance with industry specifications standard geometry test bars are often cast along with manufactured products so the material can be evaluated and quality can be assured independ ent of product geometry Alloy cast irons and cast steels are usually specified by their ASTM designation numbers which relate the materials to their mechanical properties and intended service applications The Society of Automotive Engineers SAE also has specifications for cast steels used in the automotive industry TABLE 613 Typical Properties of Pearlitic Gray Compacted Graphite and Ductile Cast Irons Property Gray CGI Ductile Tensile strength MPa 250 450 750 Elastic modulus GPa 105 145 160 Elongation 0 15 5 Thermal conductivity WmK 48 37 28 Relative damping capacity Gray 1 1 035 022