Structure/Property Relationships in Irons and Steels Bruce L

Home , Austenite, Bainite, Carbon steel, Cementite, Gray iron, Ledeburite

Structure/Property Relationships in Irons and Steels Bruce L. Bramfitt, Homer Research Laboratories, Bethlehem Steel Corporation

Basis of Material Selection ...... 153 Role of Microstructure ...... 155 Ferrite ...... 156 Pearlite ...... 158 Ferrite-Pearl ite ...... 160 Bainite ...... 162 Martensite ...... 164 Austenite ...... 169 Ferrite-Cementite ...... 170 Ferrite-Martensite ...... 171 Ferrite-Austenite ...... 171 Graphite ...... 172 Cementite ...... 172

This Section was adapted from Materials 5election and Design, Volume 20, ASM Handbook, 1997, pages 357-382. Additional information can also be found in the Sections on cast irons and steels which immediately follow in this Handbook and by consulting the index.

THE PROPERTIES of irons and steels are structure-sensitive properties, for example, yield in both theoretical and practical terms, with par- linked to the chemical composition, processing strength and hardness. The structure-insensitive ticular focus on the role of microstructure. path, and resulting microstructure of the material; properties, for example, electrical conductivity, this correspondence has been known since the are not discussed in this Section. Processing is a early part of the twentieth century. For a particular means to develop and control microstructure, for Basis of Material Selection iron and steel composition, most properties depend example, hot rolling, quenching, and so forth. In In order to select a material for a particular on microstructure. These properties are called this Section, the role of these factors is described component, the designer must have an intimate

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knowledge of what properties are required. Con- Table I Mechanical properties of selected steels sideration must be given to the environment (corrosive, high temperature, etc.) and how the Tensile Yield Elongation component will be fabricated (welded, bolted, strength strength iaS0muma, ReductionHardness, etc.). Once these property requirements are es- Steel Condition MPa ksi MPa kd tablished the material selection process can be- Carbon steel bar(a) gin. Some of the properties to be considered are: 1006 Hot rolled 295 43 165 24 30 55 86 Colddrawn 330 48 285 41 20 45 95 1008 Hot rolled 305 44 170 24.5 30 55 86 Mechanical properties Other properties/ Colddrawn 340 49 285 41.5 20 45 95 Strength characteristics 1010 Hot rolled 325 47 180 26 28 50 95 Tensile strength (ultimate Formability Cold drawn 365 53 305 44 20 40 105 strength) Dmwability 1012 Hot rolled 330 48 185 26.5 28 50 95 Yield strength Stretchability Colddrawn 370 54 310 45 19 40 105 Compressive strength Bendability 1015 Hot rolled 345 50 190 27.5 28 50 101 Hardness Wear resistance Cold drawn 385 56 325 47 18 40 111 Toughness Abrasion resistance 1016 Hot rolled 380 55 205 30 25 50 110 Notch toughness Galling resistance Cold dmwn 420 61 350 51 18 40 121 Fracture toughness Sliding wear resistance 1017 Hot rolled 365 53 200 29 26 50 105 Ductility Adhesive wear resistance Cold drawn 405 59 340 49 18 40 116 Total elongation Machinability 1018 Hot rolled 400 58 220 32 25 50 116 Reduction in area Weldability Cold drawn 440 64 370 54 15 40 126 Fatigue resistance 1019 Hot rolled 405 59 225 32.5 25 50 116 Cold drawn 455 66 380 55 15 40 131 1020 Hot rolled 380 55 205 30 25 50 l 1 l Cold drawn 420 61 350 51 15 40 121 Table 1 lists mechanical properties of selected steels 1021 Hot rolled 420 61 230 33 24 48 116 in various heat-treated or cold-worked conditions. Colddrawn 470 68 395 57 15 40 131 In the selection process, what is required for 1022 Hot rolled 425 62 235 34 23 47 121 one application may be totally inappropriate for Colddrawn 475 69 400 58 15 40 137 another application. For example, steel beams for 1023 Hot rolled 385 56 215 31 25 50 111 a railway bridge require a totally different set of Cold drawn 425 62 360 52.5 15 40 121 properties than the steel rails that are attached to 1524 Hot rolled 510 74 285 41 20 42 149 Cold drawn 565 82 475 69 12 35 163 the wooden ties on the bridge deck. In designing 1025 Hot rolled 400 58 220 32 25 50 116 the bridge, the steel must have sufficient strength Colddrawn 440 64 370 54 15 40 126 to withstand substantial applied loads. In fact, 1026 Hot rolled 440 64 240 35 24 49 126 the designer will generally select a steel with Colddrawn 490 71 415 60 15 40 143 higher strength than actually required. Also, the 1527 Hot rolled 515 75 285 41 18 40 149 designer knows that the steel must have fracture Colddmwn 570 83 485 70 12 35 163 toughness to resist the growth and propagation of 1030 Hot rolled 470 68 260 37.5 20 42 137 cracks and must be capable of being welded so Cold drawn 525 76 440 64 12 35 149 that structural members can be joined without 1035 Hot rolled 495 72 270 39.5 18 40 143 sacrificing strength and toughness. The steel Colddrawn 550 80 460 67 12 35 163 1536 Hot rolled 570 83 315 45.5 16 40 163 bridge must also be corrosion resistant. This can COlddrawn 635 92 535 77.5 12 35 187 be provided by a protective layer of paint. If 1037 Hot rolled 510 74 280 40.5 18 40 143 painting is not allowed, small amounts of certain Cold drawn 565 82 475 69 12 35 167 alloying elements such as copper and chromium 1038 Hot rolled 515 75 285 41 18 40 149 can be added to the steel to inhibit or reduce Colddrawn 570 83 485 70 12 35 163 corrosion rates. Thus, the steel selected for the 1039 Hot rolled 545 79 300 43.5 16 40 156 bridge would be a high-strength low-alloy Cold drawn 605 88 510 74 12 35 179 (HSLA) structural steel such as ASTM A572, 1040 Hot rolled 525 76 290 42 18 40 149 grade 50 or possibly a weathering steel such as Colddrawn 585 85 490 71 12 35 170 1541 Hot rolled 635 92 350 51 15 40 187 ASTM A588. A t);pical HSLA steel has a ferrite- Cold drawn 705 102.5 600 87 10 30 207 pearlite microstructure as seen in Fig. 1 and is Annealed, cold drawn 650 94 550 80 10 45 184 microalloyed with vanadium and/or niobium for 1042 Hot rolled 550 80 305 44 16 40 163 strengthening. (Microalloying is a term used to Colddrawn 6!5 89 515 75 12 35 179 describe the process of using small additions of Normalized, cold drawn 585 85 505 73 12 45 179 carbonitride forming elements--titanium, vana- 1043 Hot rolled 565 82 310 45 16 40 163 dium, and niobium--to strengthen steels by grain Cold drawn 625 91 530 77 12 35 179 refinement and precipitation hardening.) Normalized, cold drown 600 87 515 75 12 45 179 On the other hand, the steel rails must have 1044 Hot rolled 550 80 305 44 16 40 163 high strength coupled with excellent wear resis- 1045 Hot rolled 565 82 310 45 16 40 163 Colddmwn 625 91 530 77 12 35 179 tance. Modem rail steels consist of a fully pearli- Annealed, cold drawn 585 85 505 73 12 45 170 tic microstructure with a fine pearlite interlamel- 1046 Hot rolled 585 85 325 47 15 40 170 lar spacing, as shown in Fig. 2. Pearlite is unique Cold drawn 650 94 545 79 12 35 187 because it is a lamellar composite consisting of Annealed, cold drawn 620 90 515 75 12 45 179 88% soft, ductile ferrite and 12% hard, brittle 1547 Hot rolled 650 94 360 52 15 30 192 cementite (Fe3C). The hard cementite plates pro- Cold drawn 710 103 605 88 10 28 207 vide excellent wear resistance, especially when Annealed, cold drawn 655 95 585 85 10 35 187 embedded in soft ferrite. Pearlitic steels have 1548 Hot rolled 660 96 365 53 14 33 197 high strength and are fully adequate to support Colddrawn 735 106.5 615 89.5 10 28 217 Annealed, cold drawn 645 93.5 540 78.5 10 35 192 heavy axle loads of modem locomotives and (continued) freight cars. Most of the load is applied in compression. Pearlitic steels also have relatively (a) All values are estimated minimum values; type 1100 series steels are rated on the basis of 0.10% max Si or coarse-grain melt- poor toughness and cannot generally withstand ing practice; the mechanical properties shown are expected minimums for the sizes ranging from 19 to 31.8 mm (0.75 to 1.25 impact loads without failure. The rail steel could in.). (b) Most data are for 25 mm (1 in.) diam bar. Source: Ref 1 not meet the requirements of the bridge builder, Structure/Property Relationships in Irons and Steels / 155

Table I (continued) and the HSLA structural steel could not meet the requirements of the civil engineer who designed the bridge or the rail system. Tensile Yield Elongation strength strength in 50 ram, Reduction Hardness, A similar case can be made for the selection of Steel Condition MPa ksi MPa ksi % ~a area, % HB cast irons. A cast machine housing on a large lathe requires a material with adequate strength, Carbon steel bar(a) (continued) rigidity, and durability to support the applied 1049 Hot rolled 600 87 330 48 15 35 179 load and a certain degree of damping capacity in Cold drawn 670 97 560 81.5 10 30 197 order to rapidly attenuate (dampen) vibrations Annealed, cold drawn 635 92 530 77 10 40 187 from the rotating parts of the lathe. The cast iron 1050 Hot roned 620 90 340 49.5 15 35 179 Cold da'awn 690 100 580 84 10 30 197 jaws of a crusher require a material with substan- Annealed, cold drawn 655 95 550 80 10 40 189 tial wear resistance. For this application, a cast- 1552 Hot rolled 745 108 410 59.5 12 30 217 ing is required because wear-resistant steels are Annealed, cold drawn 675 98 570 83 10 40 193 very difficult to machine. For the machine hous- 1055 Hot rolled 650 94 355 51.5 12 30 192 ing, gray cast iron is selected because it is rela- Annealed, cold drawn 660 96 560 81 10 40 197 tively inexpensive, can be easily cast, and has the 1060 Hot rolled 675 98 370 54 12 30 201 ability to dampen vibrations as a result of the Spheroidized annealed, cold drawn 620 90 485 70 10 45 183 graphite flakes present in its microstructure. 1064 Hot rolled 670 97 370 53.5 12 30 201 These flakes are dispersed throughout the ferrite Spheroidized annealed, cold drawn 615 89 475 69 10 45 183 1065 Hot rolled 690 100 380 55 12 30 207 and pearlite matrix (Fig. 3). The graphite, being a Spheroidized annealed, cold drawn 635 92 490 71 10 45 187 major nonmetallic constituent in the gray iron, 1070 Hot rolled 705 102 385 56 12 30 212 provides a tortuous path for sound to travel Spheroidized annealed, cold drawn 640 93 495 72 10 45 192 through the material. With so many flakes, sound 1074 Hot rolled 725 105 400 58 12 30 217 waves are easily reflected and the sound damp- Spheroidized annealed, cold drawn 650 94 505 73 10 40 192 ened over a relatively short distance. However, 1078 Hot rolled 690 1130 380 55 12 30 207 for the jaw crusher, damping capacity is not a Spheroidized annealed, cold drawn 650 94 500 72.5 10 40 192 requirement. In this case, an alloy white cast iron 1080 Hot rolled 770 112 425 61.5 10 25 229 is selected because of its high hardness and wear Spheroidized annealed, cold drawn 675 98 515 75 10 40 192 1084 Hot rolled 820 119 450 65.5 10 25 241 resistance. The white cast iron microstructure Spheroidized annealed, cold drawn 690 100 530 77 10 40 192 shown in Fig. 4 is graphite free and consists of 1085 Hot rolled 835 121 460 66.5 10 25 248 martensite in a matrix of cementite. Both of these Spheroidized annealed, cold drawn 695 100.5 540 78 10 40 192 constituents are very hard and thus provide the 1086 Hot rolled 770 112 425 61.5 10 25 229 required wear resistance. Thus, in this example Spheroidized aimealed, cold drawn 670 97 510 74 10 40 192 the gray cast iron would not meet the require- 1090 Hot rolled 840 122 460 67 10 25 248 ments for the jaws of a crusher and the white cast Spheroidized annealed, cold drawn 695 101 540 78 10 40 197 iron would not meet the requirements for the 1095 Hot rolled 825 120 455 66 10 25 248 lathe housing. Spheroidized annealed, cold drawn 680 99 525 76 10 40 197 1211 Hot rolled 380 55 230 33 25 45 121 Colddrawn 515 75 400 58 10 35 163 Role of Microstructure 1212 Hot rolled 385 56 230 33.5 25 45 121 Cold drawn 540 78 415 60 10 35 167 In steels and cast irons, the microstructural 1213 Hot rolled 385 56 230 33.5 25 45 121 constituents have the names ferrite, pearlite, Cold drawn 540 78 415 60 10 35 167 bainite, martensite, cementite, and austenite. In 12L14 Hot rolled 395 57 235 34 22 45 121 most all other metallic systems, the constituents Cold drawn 540 78 415 60 10 35 163 are not named, but are simply referred to by a 1108 Hot roUed 345 50 190 27.5 30 50 101 Greek letter (ct, 13, Y, etc.) derived from the loca- Colddrawn 385 56 325 47 20 40 121 1109 Hot rolled 345 50 190 27.5 30 50 101 tion of the constituent on a phase diagram. Fer- Cold drawn 385 56 325 47 20 40 121 rous alloy constituents, on the other hand, have 11i7 Hot roned 425 62 235 34 23 47 121 been widely studied for more than 100 years. In Colddrawn 475 69 400 58 15 40 137 the early days, many of the investigators were 1118 Hot rolled 450 65 250 36 23 47 131 petrographers, mining engineers, and geologists. Colddrawn 495 72 420 61 15 40 143 Because minerals have long been named after 1119 Hot roned 425 62 235 34 23 47 121 their discoverer or place of origin, it was natural Colddrawn 475 69 400 58 15 40 137 to similarly name the constituents in steels and 1132 Hot roUed 570 83 315 45.5 16 40 167 cast irons. Cold drawn 635 92 530 77 12 35 183 ~1137 Hot roiled 605 88 330 48 15 35 179 It can be seen that the four examples described Colddrawn 675 98 565 82 10 30 197 above have very different microstructures: the 1140 Hot rolled 545 79 300 43.5 16 40 156 structural steel has a ferrite plus pearlite micro- Colddrawn 605 88 510 74 12 35 170 structure; the rail steel has a fully pearlitic mi- 1141 Hot roned 650 94 355 51.5 15 35 187 crostructure; the machine housing (lathe) has a Colddrawn 725 105.1 605 88 10 30 212 ferrite plus pearlite matrix with graphite flakes; 1144 Hot rolled 670 97 365 53 15 35 197 and the jaw crusher microstructure contains Colddrawn 745 108 620 90 10 30 217 martensite and cementite. In each case, the mi- 1145 Hot rolled 585 85 325 47 15 40 170 crostructure plays the primary role in providing Colddrawn 650 94 550 80 12 35 187 1146 Hot roUed 585 85 325 47 15 40 170 the properties desired for each application. From Cold drawn 650 94 550 80 12 35 187 these examples, one can see how material proper- 1151 Hot rolled 635 92 350 50.5 15 35 187 ties can be tailored by microstructural manipula- Colddrawn 705 102 595 86 10 30 207 tion or alteration. Knowledge about microstructure is thus paramount in component design and (continued) alloy development. In the paragraphs that follow, each microstructural constituent is described (a) All values are estimated minimum values; type 1100 series steels are rated on the basis of 0.10% max Si or coarse-grain melt- with particular reference to the properties that ing practice; the mechanical properties shown are expected minimums for the sizes ranging from 19 to 31.8 mm (0.75 to 1.25 can be developed by appropriate manipulation of in.). (b) Most data are for 25 mm (1 in.) diam bar. Source: Ref 1 the microstructure through deformation (e.g., hot and cold rolling) and heat treatment. Further de- 156 / Structure/Property Relationships in Irons and Steels

tails about these microstructural constituents can ]'able 1 (continued) be found in Ref 2 to 6. Tensile Yield Elongatba Ferrite strength strength inSOnma, l~lt~tion Hardm~ Steel Condition MPa ksi MPa ksi % ~a area, % lib A wide variety of steels and cast irons fully Low-alloy steels(b) exploit the properties of ferrite. However, only a 1340 Normalized at 870 °C (1600 °F) 834 121 558 81 22.0 63 248 few commercial steels are completely ferritic. An Annealed at 800 °C (1475 °F) 703 102 434 63 25.5 57 207 example of the microstructure of a fully ferritic, 3140 Normalized at 870 °C (1600 oF) 889 129 600 87 19.7 57 262 ultralow carbon steel is shown in Fig. 5. Annealed at 815 °C (1500 °F) 690 100 420 61 24.5 51 197 Ferrite is essentially a solid solution of iron 4130 Normalized at 870 °C (1600 °F) 670 97 435 63 25.5 59.5 197 containing carbon or one or more alloying ele- Annealed at 865 °C (1585 °F) 560 81 460 67 21.5 59.6 217 ments such as silicon, chromium, manganese, Water quenched from 855 °C (1575 °F) 1040 151 979 142 18.1 63.9 302 and nickel. There are two types of solid solu- and tempered at 540 °C (1000 °F) 4140 Normalized at 870 °C (1600 oF) 1020 tions: interstitial and substitutional. In an inter- 148 655 95 17.7 46.8 302 Annealed at 815 °C (1500 °F) 655 95 915 60 25.7 56,9 197 stitial solid solution, elements with small atomic Water quenched from 845 °C ( 1550 °F) 1075 156 986 143 15.5 56,9 311 diameter, for example, carbon and nitrogen, oc- and tempered at 540 °C (1000 °F) cupy specific interstitial sites in the body-cen- 4150 Normalized at 870 °C ( 1600 °F) 1160 168 731 106 11.7 30,8 321 tered cubic (bcc) iron crystalline lattice. These Annealed at 830 °C (1525 °F) 731 106 380 55 20.2 40,2 197 sites are essentially the open spaces between the oil quenched from 830 °C (1525 °F) 1310 190 1215 176 13.5 47.2 375 larger iron atoms. In a substitutional solid solu- and tempered at 540 °C (1000 °F) tion, elements of similar atomic diameter replace 4320 Normalized at 895 °C (1640 oF) 793 115 460 67 20.8 51 235 Annealed at 850 °C (1560 °F) 580 84 425 62 29.0 58 163 or substitute for iron atoms. The two types of 4340 Normalized at 870 °C (1600 oF) 1282 186 862 125 12.2 36.3 363 solid solutions impart different characteristics to Annealed at 810 °C (1490 oF) 745 108 470 68 22.0 50.0 217 ferrite. For example, interstitial elements like Oil quenched from 800 °C (1475 °F) 1207 175 1145 166 14.2 45.9 352 carbon and nitrogen can easily diffuse through and tempered at 540 °C (1000 °F) the open bcc lattice, whereas substitutional ele- 4419 Normalized at 955 °C (1750 oF) 515 75 350 51 32.5 69.4 143 ments like manganese and nickel diffuse with Annealed at 915 °C (1675 °F) 450 65 330 48 31.2 62.8 121 great difficulty. Therefore, an interstitial solid 4620 Normalized at 900 °C (1650 oF) 570 83 365 53 29.0 66.7 174 solution of iron and carbon responds quickly dur- Annealed at 855 °C (1575 oF) 510 74 370 54 31.3 60.3 149 4820 Normalized at 860 °C (1580 oF) 758 110 485 70 24.0 59.2 229 ing heat treatment, whereas substitutional solid Annealed at 815 °C (1500 °F) 685 99 460 67 22.3 58.8 197 solutions behave sluggishly during heat treat- 5140 Normalized at 870 °C (1600 oF) 793 115 470 68 22.7 59.2 229 ment, such as in homogenization. Annealed at 830 °C (1525 °F) 570 83 290 42 28.6 57.3 167 According to the iron-carbon phase diagram Oil quenched from 845 °C (1550 °F) 972 141 841 122 18.5 58.9 293 (Fig. 6a), very little carbon (0.022% C) can dis- and tempered at 540 °C (1000 °F) solve in ferrite (ctFe), even at the eutectoid tem- 5150 Normalized at 870 °C (1600 oF) 869 126 530 77 20.7 58.7 255 perature of 727 °C (1330 °F). (The iron-carbon Annealed at 825 °C (1520 oF) 675 98 360 52 22.0 43.7 197 phase diagram indicates the phase regions that Oil quenched from 830 °C (1525 °F) 1055 159 1000 145 16.4 52.9 311 and tempered at 540 °C (1000 °F) exist over a wide carbon and temperature range. 5160 Normalized at 855 °C (1575 oF) 1025 149 650 94 18.2 50.7 285 The diagram represents equilibrium conditions. Annealed at 815 °C (1495 oF) 724 105 275 40 17.2 30.6 197 Figure 6(b) shows an expanded iron-carbon dia- Oil quenched from 830 °C (1525 °F) 1145 166 1005 146 14.5 45.7 341 gram with both the euteetoid and eutectic re- and tempered at 540 °C (1000 oF) gions.) At room temperature, the solubility is an 6150 Normalized at 870 °C (1600 oF) 938 136 615 89 21.8 61.0 269 order of magnitude less (below 0.005% C). How- Annealed at 815 °C (1500 oF) 670 97 415 60 23.0 48.4 197 ever, even at these small amounts, the addition of Oil quenched from 845 °C (1550 °F) 1200 174 1160 168 14.5 48.2 352 and tempered at 540 °C (1000 oF) carbon to pure iron increases the room-tempera- 8620 Normalized at 915 °C 0675 °F) 635 92 360 52 26.3 59.7 183 ture yield strength of iron by more than five Annealed at 870 °C (1600 oF) 540 78 385 56 31.3 62.1 149 times, as seen in Fig. 7. If the carbon content 8630 Normalized at 870 °C (1600 oF) 650 94 425 62 23.5 53.5 187 exceeds the solubility limit of 0.022%, the car- Annealed at 845 °C (1550 °F) 565 82 370 54 29.0 58.9 156 bon forms another phase called cementite (Fig. Water quenched from 845 °C (1550 °F) 931 135 850 123 18.7 59.6 269 8). Cementite is also a constituent of pearlite, as and tempered at 540 °C (1000 °F) seen in Fig. 9. The role of cementite and pearlite 8650 Normalized at 870 °C (1600) 1025 149 690 100 14 45.0 302 on the mechanical properties of steel is discussed Annealed at 795 °C ( 1465 °F) 715 104 385 56 22.5 46.0 212 oil quenched from 800 °C (1475 °F) 1185 172 1105 160 14.5 49.1 352 below. and tempered at 540 °C ( 1000 °F) The influence of solid-solution elements on the 8740 Normalized at 870 °C (1600 oF) 931 135 605 88 16.0 47.9 269 yield strength of ferrite is shown in Fig. 10. Here Annealed at 815 °C (1500 oF) 696 101 415 60 22.2 46.4 201 one can clearly see the strong effect of carbon on Oil quenched from 830 °C ( 1525 °F) 1225 178 1130 164 16.0 53.0 352 increasing the strength of ferrite. Nitrogen, also and tempered at 540 °C (1000 oF) an interstitial element, has a similar effect. Phos- 9255 Normalized at 900 °C ( 1650 oF) 931 135 580 84 19.7 43.4 269 phorus is also a ferrite strengthener. In fact, there Annealed at 845 °C (1550 oF) 779 113 485 70 21.7 41.1 229 are commercially available steels containing Oil quenched from 885 °C (1625 °F) 1130 164 924 134 16.7 38.3 321 and tempered at 540 °C ( 1000 oF) phosphorus (up to 0.12% P) for strengthening. 9310 Normalized at 890 °C (1630 °F) 910 132 570 83 18.8 58.1 269HRB These steels are the rephosphorized steels (type Annealed at 845 °C (1550 oF) 820 119 450 65 17.3 42.1 241HRB 1211 to 1215 series). Mechanical property data Ferritie stainless steels(b) for these steels can be found in Table 1. In Fig. 10, the substitutional solid solution ele- 405 Annealed bar 483 70 276 40 30 60 150 ments of silicon, copper, manganese, molybde- Cold draw n bar 586 85 483 70 20 60 185 409 Annealed bar 450 65 240 35 25 75HRB num, nickel, aluminum, and chromium are shown 430 Annealed bar 517 75 310 45 30 --65" 155 to have far less effect as ferrite strengtheners (confnued) than the interstitial elements. In fact, chromium, nickel, and aluminum in solid solution have very (a) All values are estimated minimum values; type 1100 series steels are rated on the basis of 0.10% max Si or coarse-grain melt- little influence on the strength of ferrite. ing practice; the mechanical properties shown are expected minimums for the sizes ranging from 19 to 31.8 mm (0.75 to 1.25 In addition to carbon (and other solid-solution in.). (b) Most data are for 25 mm (1 in.) diam bar. Source: Ref I elements), the strength of a ferritic steel is also Structure/Property Relationships in Irons and Steels / 157

Table 1 (continued) determined by its grain size according to the Hall-Petch relationship: Tensile Yield Elongation strength strength in 50ram, Reduction Hardness, Gy = Go + kyd -1/2 (Eq 1) Steel Ccmdition MPa ksi MPa ksi % in area, % HB Ferritic stainless steels(b) (continued) 430 (cont'd) Annealed and cold drawn 586 85 483 70 20 65 185 where Oy is the yield strength (in MPa), ~o is a 442 Annealed bar 515 75 310 45 30 50 160 constant, ky is a constant, and d is the grain diame- Annealed at 815 °C (1500 °F) and cold 545 79 427 62 35.5 79 92HRC ter (in mm). worked The grain diameter is a measurement of size of 446 Annealed bar 550 80 345 50 25 45 86HRB the ferrite grains in the microstructure, for exam- Annealed at 815 °C (1500 °F) and cold 607 88 462 67 26 64 96HRB ple, note the grains in the ultralow carbon steel in drawn Fig. 5. Figure 11 shows the Hall-Petch relation- Martensilic stainless steels(b) ship for a low-carbon fully ferritic steel. This 403 Annealed bar 515 75 275 40 35 70 82HRB relationship is extremely important for under- Tempered bar 765 111 585 85 23 67 97HRB standing structure-property relationships in 410 Oil quenched from 980 °C ( 1800 °F); 1085 158 1005 146 13 70 ... steels. Control of grain size through ther- tempered at 540 °C (1000 °F);.16 nun momechanical treatment, heat treatment, and/or (0.625 in.) bar microalloying is vital to the control of strength Oil quenched from 980 °C (1800 °F); 1525 221 1225 178 15 64 45HRB and toughness of most steels. The role of grain tempered at 40 °C (104 °F); 16 mm size is discussed in more detail below. (0.625 in.) bar There is a simple way to stabilize ferrite, 414 Annealed bar 795 115 620 90 20 60 235 Cold drawn bar 895 130 795 115 15 58 270 thereby expanding the region of ferrite in the Oil quenched from 980 °C (1800 °F); 1005 146 800 116 19 58 ... iron-carbon phase diagram, namely by the addi- tempered at 650 °C (1200 oF) tion of alloying elements such as silicon, chro- 420 Annealed bar 655 95 345 50 25 55 195 mium, and molybdenum. These elements are Annealed and cold drawn 760 110 690 100 14 40 228 called ferrite stabilizers because they stabilize 431 Annealed bar 860 125 655 95 20 55 260 ferrite at room temperature through reducing the Annealed and cold drawn 895 130 760 110 15 35 270 amount of y solid solution (austenite) with the 738 107 20 64 ... Oil quenched from 980 °C (1800 °F); 831 121 formation of what is called a y-loop as seen at the tempered at 650 °C (1200 oF) far left in Fig. 12. This iron-chromium phase dia- Oil quenched from 980 °C (1800 °F); 1435 208 1140 166 17 59 45HRC tempered at 40 °C (104 °F) gram shows that ferrite exists up above 12% Cr 440C Annealed bar 760 110 450 65 14 25 97HRB and is stable up to the melting point (liquidus Annealed and cold drawn bar 860 125 690 100 7 20 260 temperature). An important fully ferritic family Hardened and tempered at 315 °C 1970 285 1900 275 2 10 580 of steels is the iron-chromium ferritic stainless (6OO°F) steels. These steels are resistant to corrosion, and Austenitle stainless steels(b) are classified as type 405, 409, 429, 430, 434, 436, 439, 442, 444, and 446 stainless steels. 201 Annealed 760 110 380 55 52 ... 87HRB 50% hard 1035 150 760 ll0 12 ... 32HRC These steels range in chromium content from 11 Full hard 1275 185 965 140 8 ... 41HRC to 30%. Additions of molybdenum, silicon, nio- Extra hard 1550 225 1480 215 1 ... 43HRC bium, aluminum, and titanium provide specific 202 Annealed bar 515 75 275 40 40 ...... properties. Ferritic stainless steels have good Annealed sheet 655 95 310 45 40 ...... ductility (up to 30% total elongation and 60% 50% hard sheet 1030 150 760 110 10 _ ... reduction in area) and formability, but lack 301 Annealed 725 105 275 40 60 70' ... strength at elevated temperatures compared with 655 95 54 61 ... 50% hard 1035 150 austenitic stainless steels. Room-temperature Full hard 1415 205 1330 193 6 ... yield strengths range from 170 to about 440 MPa 302 Annealed strip 620 90 275 40 55 ... 80HRB 25% hard strip 860 125 515 75 12 _ 25HRC (25 to 64 ksi), and room-temperature tensile Annealed bar 585 85 240 35 60 70" 80HRB strengths range from 380 to about 550 MPa (55 303 Annealed bar 620 90 240 35 50 55 160 to 80 ksi). Table 1 lists the mechanical properties Colddrawn 690 100 415 60 40 53 228 of some of the ferritic stainless steels. Type 409 304 Annealed bar 585 85 235 34 60 70 149 stainless steel is widely used for automotive ex- Annealed and cold drawn 690 100 415 60 45 ... 212 haust systems. Type 430 free-machining stainless Cold-drawn high tensile 860 125 655 95 25 ... 275 steel has the best machinability of all stainless 260 38 50 _ 80HRB 305 Annealed sheet 585 85 steels other than that of a low-carbon, free-ma- 205 30 55 65' 150 308 Annealed bar 585 85 chining martensitic stainless steel (type 41.6). 309 Annealed bar 655 95 275 40 45 65 83HRB Another family of steels utilizing a ferrite sta- 310 Annealed sheet 620 90 310 45 45 _ 85HRB Annealed bar 655 95 275 40 45 65' 160 bilizer (y-loop) are the iron-silicon ferritic alloys 314 Annealed bar 689 100 345 50 45 60 180 containing up to about 6.5% Si (carbon-free). 316 Annealed sheet 580 84 290 42 50 _ 79HRB These steels are of commercial importance be- Annealed bar 550 80 240 35 60 70" 149 cause they have excellent magnetic permeability Annealed and cold-drawnbar 620 90 415 60 45 65 190 and low core loss. High-efficiency motors and 317 Annealed sheet 620 90 275 40 45 ... 85HRB transformers are produced from these iron-sili- Annealed bar 585 85 275 40 50 ... 160 con electrical steels (aluminum can also substi- 240 35 45 _ 80HRB 321 Annealed sheet 620 90 tute for silicon in them). Annealed bar 585 85 240 35 55 65' 150 Over the past 20 years or so, a new breed of Annealed and cold-drawnbar 655 95 415 60 40 60 185 very-low-carbon fully ferritic sheet steels has 330 Annealed sheet 550 80 260 38 40 ...... Annealed bar 585 85 290 42 45 ... 80HRB emerged for applications requiring exceptional 347 Annealed sheet 655 95 275 40 45 _ 85HRB formability (see Fig. 5). These are the intersti- Annealed bar 620 90 240 35 50 65" 160 tial-free (IF) steels for which carbon and nitro- (continued) gen are reduced in the steelmaking process to very low levels, and any remaining interstitial (a) All values are estimated minimum values; type 1100 series steels are rated on the basis of 0.10% max Si or coarse-grain melt- carbon or nitrogen is tied up with small amounts ing practice; the mechanical properties shown are expected minimums for the sizes ranging from 19 to 31.8 mm (0.75 to 1.25 of alloying elements (e.g., titanium or niobium) in.). (b) Most data are for 25 mm (1 in.) diam bar. Source: Ref 1 that form preferentially carbides and nitrides. 158/Structure/Property Relationships in Irons and Steels

Table I (continued) pearlite forms. Pearlite is formed by cooling the steel through the eutectoid temperature (the temperature of 727 °C in Fig. 6) by the following qI~mBe Yield Elongation reaction: st~ngth strength inS0mm, ReductionHardness, Sted Cand~laa MPa k~ MPa I~ % in area, % liB Austenilic stainless steels(b) (continued) Austenite ~ cementite + ferrite ffXl2) 347 (eont'd) Annealedandcolddrawnbar 690 100 450 65 40 60 212 384 Annealed wire 1040 °C (1900 °F) 515 75 240 35 55 72 70HRB The cementite and ferrite form as parallel plates Maraging steels(b) called lamellae (Fig. 13). This is essentially a 18Ni(250) Annealed 965 140 655 95 17 75 30 HRC composite microstructure consisting of a very Aged bar 32 mm (1.25 in.) 1844 269 1784 259 11 56.5 51.8 HRC hard carbide phase, cementite, and a very soft and Aged sheet 6 mm (0.25 in.) 1874 272 1832 266 8 40.8 50.6HRC ductile ferrite phase. A fully pearlitic microstruc- 18Ni(300) Annealed 1034 150 758 110 18 72 32HRC ture is formed at the eutectoid composition of Aged bar 32 mm (1.25 in.) 2041 296 2020 293 11.6 55.8 54.7 HRC 0.78% C. As can be seen in Fig. 2 and 13, pearlite Aged sheet 6 mm (0.25 in.) 2169 315 2135 310 7.7 35 55.1HRC forms as colonies where the lamellae are aligned 18Ni(350) Annealed 1140 165 827 120 18 70 35 HRC in the same orientation. The properties of fully Aged bar 32 mm (l.25 in.) 2391 347 2348 341 7.6 33.8 58.4 HRC pearlitic steels are determined by the spacing be- Aged sheet 6 mm (0.25 in.) 2451 356 2395 347 3 15.4 57.7 HRC tween the ferrite-cementite lamellae, a dimension (a) All values are estimated minimumvalues; type 1100 series steels ate rated on the basis of 0.10% max Si or coarse-grain melt- called the interlamellar spacing, X, and the colony ing practice; the mechanical properties shown are expected minimums for the sizes ranging from 19 to 31.8 mm (0.75 to 1.25 size. A simple relationship for yield strength has in.). (b) Most data are for 25 mm (1 in.) diam bar. Some: Ref 1 been developed by Heller (Ref 10) as follows:

fly = -85.9 + 8.3 (X-t/2) (Eq 3) These steels have very low strength, but are used tions, that is, the stacked steel layers in the rotor and stator of the motor. to produce components that are difficult or im- where fly is the 0.2% offset yield strength (in As noted previously, a number of properties possible to form from other steels. Very-low-car- MPa) and X is the interlamellar spacing (in mm). are exploited in fully ferritic steels: bon, fully ferritic steels (0.001% C) are now be- Figure 14 shows Heller's plot of strength versus ing manufactured for automotive components interlamellar spacing for fully pearlitic eutectoid that harden during the paint-curing cycle. These • Iron-silicon steels: Exceptional electrical steels. steels are called bake-hardening steels and have properties It has also been shown by Hyzak and Bernstein controlled amounts of carbon and nitrogen that • Iron-chromium steels: Good corrosion resis- (Ref 11) that strength is related to interlamellar combine with other elements, such as titanium tance spacing, pearlite colony size, and prior-austenite and niobium, during the baking cycle (175 °C, or • Interstitial-free steels: Exceptional forma- grain size, according to the following relation- 350 °F, for 30 min). The process is called aging, bility ship: • Bake-hardening steels: Strengthens during and the strength derives from the precipitation of paint cure cycle titanium/niobium carbonitrides at the elevated • Lamination steels: Good electrical properties YS = 52.3 + 2.18 (~-1/2) -0.4 (de-L'2) -2.88 (d-1/2)(Eq 4) temperature. Another form of very-low-carbon, fully ferritic steel is motor lamination steel. The carbon is re- where YS is the yield strength (in MPa), d e is the moved from these steels by a process known as PearlRe pearlite colony size (in mm), and d is the prior- decarburization. The decarburized (carbon-free) austenite grain size (in mm). From Eq 3 and 4, it ferritic steel has good permeability and suffi- As the carbon content of steel is increased be- can be seen that the steel composition does not ciently low core loss (not as low as the iron-sili- yond the solubility limit (0.02% C) on the iron- have a major influence on the yield strength of a con alloys) to be used for electric motor lamina- carbon binary phase diagram, a constituent called fully pearlitic eutectoid steel. There is some solid-

Fig, 3 Microstructure of a gray cast iron with a ferrite-pearlite matrix. Note the graphite Fig. 4 Microstructure of an alloy white cast iron. White constituent is cementite and the flakes dispersed throughout the matrix. 4% picral etch. 320x. Courtesy of A.O. darker constituent is martensite with some retained austenite. 4% picral etch. Benscoter, Lehigh University 250x. Courtesy ofA.O. Benscoter, Lehigh University Structure/Property Relationships in Irons and Steels/159

steel will typically have a total elongation of austenite grain size. Unfortunately, these three more than 50%, whereas a fully pearlitic steel factors are rather difficult to measure. To deter- (e.g., type 1080) will typically have a total elon- mine interlamellar spacing, a scanning electron gation of about 10% (see Table 1). A low-carbon microscope (SEM), or a transmission electron fully ferritic steel will have a room-temperature microscope (TEM) is needed in order to resolve Charpy V-notch impact energy of about 200 J the spacing, Generally, a magnification of (150 ft. lbf), whereas a fully pearlitic steel will 10,000x is adequate, as seen in Fig. 13. Special have room-temperature impact energy of under statistical procedures have been developed to de- 10 J (7 ft. lbf). The transition temperature (i.e., termine an accurate measurement of the spacing the temperature at which a material changes from (Ref 12). The colony size and especially the ductile fracture to brittle fracture) for a fully prior-austenite grain size are very difficult to pearlitic steel can be approximated from the fol- measure and require a skilled metallographer us- lowing relationship (Ref 11): ing the light microscope or SEM and special etching procedures.

Because of poor ductility/toughness, there are TT = 217.84 - 0.83 (de-1/2) - 2.98(d -1"~) (Eq5) only a few applications for fully pearlitic steels, including railroad rails and wheels and high- strength wire. By far, the largest tonnage applica- where TT is the transition temperature (in °C). tion is for rails. A fully pearlitic rail steel pro- From Eq 5, one can see that both the prior- vides excellent wear resistance for railroad austenite grain size and pearlite colony size con- Fig. 5 Microstructure of a fully ferritic, ultralow carbon trol the transition temperature of a pearlitic steel. wheel/rail contact. Rail life is measured in mil- steel. Marshalls etch + HF, 300x. Courtesy of lions of gross tons (MGT) of travel and current A.O. Benscoter, Lehigh University Unfortunately, the transition temperature of a fully pearlitic steel is always well above room rail life easily exceeds 250 MGT. The wear resis- temperature. This means that at room tempera- tance of pearlite arises from the unique morphol- ture the general fracture mode is cleavage, which ogy of the ferrite-cementite lamellar composite solution strengthening of the ferrite in the lamel- is associated with brittle fracture. Therefore, where a hard constituent is embedded into a soft- lar structure (see Fig. 10). fully pearlitic steels should not be used in appli- ductile constituent. This means that the hard ce- The thickness of the cementite lamellae can cations where toughness is important. Also, pear- mentite plates do not abrade away as easily as the also influence the properties of pearlite. Fine ce- litic steels with carbon contents slightly or mod- rounded cementite particles found in other steel mentite lamellae can be deformed, compared erately higher than the eutectoid composition microstructures, that is, tempered martensite and with coarse lamellae, which tend to crack during (called hypereutectoid steels) have even poorer bainite, which is discussed later. Wear resistance deformation. toughness. of a rail steel is directly proportional to hardness. Although fully pearlitic steels have high From Eq 4 and 5, one can see that for pearlite, This is shown in Fig. 15, which indicates less strength, high hardness, and good wear resis- strength is controlled by interlamellar spacing, weight loss as hardness increases. Also, wear re- tance, they also have poor ductility and tough- colony size, and prior-austenite grain size, and sistance (less weight loss) increases as inter- ness. For example, a low-carbon, fully ferritic toughness is controlled by colony size and prior- lamellar spacing decreases, as shown in Fig. 16.

Carbon, at.% 1 2 3 4 5 6 7 8 9 1180 I I I I I I 1154°C - ~...~ 2125 1140 Fe-C equilibrium (experimental) 2.08 ~ I "'" J., ~1,,/8 °C-'~ 2050 1100 -- Fe-Fe3C equilibrium (experimental) .o"Y 211 -- 1975 1060 • *' Y • .~ -- 1900 1020 -- 1825 980 (~Fe) auatenite u-o -- 1750 940 ¢D AUS tenite + cementite -- 1700 O. 900 ~912 °C , /" E E ~ ~0¢F.) ferrite . ,-~ -- 1625 86O -- 1550 820 %~ 770 °C (Curie temperature) -*°~ .../ -- 1475 780 ...... ~- - -'-~ 0.68 7 I ~ 0.0206 ~ ~, .'°" 738 °C - 1400 740 I 727 °C -- 1325 700 / 0.0218 I I Ferrite + cementite I - 1250 66O I I I I Fe 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 Carbon, wt% Fig. 6(a) Iron-carbon phase diagram showing the austenite (y Fe) and ferrite (ocFe)phase regions and eutectoid composition and temperature. Dotted lines representiron-graphite equilibrium conditions and solid lines represent iron-cementite equilibrium conditions. Only the solid lines are important with respect to steels. Source: Ref 2 160/Structure/Property Relationships in Irons and Steels

Thus, the most important microstructural pa- processes where steel parts are continuously the rod is transformed at a temperature of about rameter for controlling hardness and wear resis- cooled, that is, air cooled, and so forth. 540 °C (1000 °F) by passing it through a lead or tance is the pearlite interlamellar spacing. Fortu- As shown in Fig. 17, the peadite transforma- salt bath at this temperature. This develops a nately, interlamellar spacing is easy to control tion temperature (indicated by the pearlite-start microstructure with a very fine pearlite inter- and is dependent solely on transformation tem- curve, Ps) decreases with increasing cooling rate. lamellar spacing because the transformation perature. The hardness of peaflite increases with decreas- takes place at the nose of the CCT diagram, that Figure 17 shows a continuous cooling transfor- ing transformation temperature. Thus, in order to is, at the lowest possible pearlite transformation mation (CCT) diagram for a typical rail steel. A provide a rail steel with the highest hardness and temperature (see Fig. 17). The rod is then cold CCT diagram is a time versus temperature plot wear resistance, one must cool the rail from the drawn to wire. Because of the very fine inter- showing the regions at which various constitu- austenite at the fastest rate possible to obtain the lamellar spacing, the ferrite and cementite lamel- cnts--ferdte, pearlite, bainite, and martensite-- lowest transformation temperature. This is done lae become aligned along the wire axis during form during the continuous cooling of a steel in practice by a process known as head harden- the deformation process. Also, the fine ccmentite component. Usually several cooling curves are ing, which is simply an accelerated cooling proc- lamella tend to bend and deform as the wire is shown with the associated start and finish trans- ess using forced air or water sprays to achieve elongated during drawing. The resulting wire is the desired cooling rate (Ref 15). Because only formation temperatures of each constituent. one of the strongest commercial products avail- These diagrams should not be confused with iso- the head of the rail contacts the wheel of the able; for example, a commercial 0.1 mm (0.004 thermal transformation (IT or TTT) diagrams, railway car and locomotive, only the head rein.) diam wire can have a tensile strength in the which are derived by rapidly quenching very thin quires the higher hardness and wear resistance. range of 3.0 to 3.3 GPa (439 to 485 ksi), and in specimens to various temperatures, and maintain- Another application for a fully pearlitic steel is special cases a tensile strength as high as 4.8 ing that temperature (isothermal) until the speci- high-strength wire (e.g., piano wire). Again, the mens begin to transform, partially transform, and composite morphology of lamellar ferrite and ce- GPa (696 ksi) can be obtained. These wires are fully transform, at which time they are quenched mentite is exploited, this time during wire draw- used in musical instruments because of the sound to room temperature. An IT diagram does not ing. A fully pearlitic steel rod is heat treated by a quality developed from the high tensile stresses represent the transformation behavior in most process known as patenting. During patenting, applied in stringing a piano and violin and are also used in wire rope cables for suspension bridges.

1~M 3270 Ferrite-Pearlite

1~ 3090 The most common structural steels produced have a mixed ferrite-pearlite microstructure. Their applications include beams for bridges and 2910 high-rise buildings, plates for ships, and reinforcing bars for roadways. These steels are rela- 1 ! 2730 tively inexpensive and are produced in large ton- GFe nages. They also have the advantage of being 1~ 2550 able to be produced with a wide range of properties. The microstructure of typical ferrite-pearlite 2370 steels is shown in Fig. 18. In most ferrite-pearlite steels, the carbon con- 2190 tent and the grain size determine the microstructure and resulting properties. For example, Fig. 19 shows the effect of carbon on tensile and 11 2010 impact properties. The ultimate tensile strength steadily increases with increasing carbon con- lC 1830 ~ tent. This is caused by the increase in the volume fraction of pearlite in the microstructure, which has a strength much higher than that of ferrite. Thus, increasing the volume fraction of pearlite E i~ E 1470 has a profound effect on increasing tensile strength. 7 1290 However, as seen in Fig. 19, the yield strength is relatively unaffected by carbon content, rising rILE[ from about 275 MPa (40 ksi) to about 415 MPa (60 ksi) over the range of carbon content shown. This is because yielding in a ferrite-pearlite steel -~ 930 is controlled by the fcrrite matrix, which is generally considered to be the continuous phase (ma- 4 750

3 570 O3 "~ 35 241 ~:

-'~ 25 ~' ..... 172 I P_~ "N, 30 / 103= Fe 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 0 Carbon, wt% o~ 10 ~ o. 0 0.001 0.002 0.003 0.004 0.005 o o 6 Fig. 6(b) Expanded iron-carbon phase diagram showing both the eutectoid (shown in Fig. 6a) and eutectic regions. Carbon, wt% Dotted lines represent iron-graphite equilibrium conditions and solid lines represent iron-cementite equilibrium conditions. The solid lines at the eutectic are important to white cast irons and the dotted lines are important to gray Fig, 7 Increase in room-temperature yield strength of cast irons. Source: Ref 2 iron with small additions of carbon. Source: Ref 7 Structure/Property Relationships in Irons and Steels / 161

Fig. 8 Photomic.rograph of an annealed low-carbon sheet steel with grain-boundary ce- Fig. 9 Photomicrograph of pearlite (dark constituent) in a low-carbon steel sheet. 2% ni- mentite. 2% nital + 4% picral etch. 1000x tal + 4% picral etch. 1000x trix) in the microstructure. Therefore, pearlite has no effect on yield strength, whereas the yield pact energy versus test temperature, the shelf en- plays only a minor role in yielding behavior. strength in Fig. 19 increases somewhat with car- ergy decreases from about 200 J (150 ft • lbf) for From Fig. 19, one can also see that ductility, as bon content. According to Eq 6, manganese, sili- a 0.11% C steel to about 35 J (25 ft. lbf) for a represented by reduction in area, steadily de- con, and nitrogen have a pronounced effect on 0.80% C steel. Also, the transition temperature creases with increasing carbon content. A steel yield strength, as does grain size. However, in increases from about -50 to 150 °C (-60 to 300 with 0.10% C has a reduction in area of about most ferrite-pearlite steels nitrogen is quite low °F) over this same range of carbon content. The 75%, whereas a steel with 0.70% C has a reduc- (under 0.010%) and thus has minimal effect on effect of carbon is due mainly to its effect on the tion in area of only 25%. Percent total elongation yield strength. In addition, as discussed below, percentage of pearlite in the microstructurc. This would show a similar trend, however, with values nitrogen has a detrimental effect on impact prop- is reflected in the regression equation for transi- much less than percent reduction in area. erties. tion temperature below (Ref 16): Much work has been done to develop empirical The regression equation for tensile strength for equations for ferrite-pearlite steels that relate the same steels is as follows (Ref 16): strength and toughness to microstructural fea- TT = -19 + 44(Si) + 700(N~/2) tures, for example, grain size and percent of + 2.2(P) - 11.5 (d -1/2) (F_.q8) pearlite as well as composition. One such equa- TS = 294,1 + 27.7(Mn) + 83.2(Si) + 3.9(P) + 7.7(d -lt2) tion for ferrite-pearlitc steels under 0.25% C is as (F-4 7) follows (Ref 16): It can be seen in all these relationships that where TS is the tensile strength (in MPa) and P is ferrite grain size is an important parameter in pearlite content (%). Thus, in distinction to yield improving both strength and toughness. It can YS = 53.9 + 32.34 (Mn) + 83.2(Si) also be seen that while pearlite is beneficial for + 354.2(Nf) + 17.4(d-U2) (Eq 6) strength, the percentage of pearlite in the microstructure has an important effect on tensile increasing tensile strength and nitrogen is benefi- strength. cial for increasing yield strength, both are harm- where Mn is the manganese content (%), Si is the Toughness of ferrite-pearlite steels is also an ful to toughness. Therefore, methods to control silicon content (%), Nf is the free nitrogen content important consideration in their use. It has long the grain size of ferrite-pearlite steels have rap- (%), and d is the ferrite grain size (in mm). Equa- been known that the absorbed energy in a Charpy idly evolved over the past 25 years. The two most tion 6 shows that carbon content (percent pearlite) V-notch test is decreased by increasing carbon important methods to control grain size are con- content, as seen in Fig. 20. In this graph of im- trolled rolling and microalloying. In fact, these

4-375 I 600 C and N 80 500

+225 80 Si & 400 .--~_m+150 / ~ 300 "~ +75 200 "N. y -- Ni and AI o 0 20 | 100 -75 0 0.5 1.0 1.5 2.0 2.5 3.0 I I I I I I I I I I I I Alloy content, wt% 0 1 2 3 4 5 6 7 8 9 10 11 12 Fig, 10 Influence of solid-solution elements on the Grain diameter (d-l~), mm -1~ changes in yield stress of low-carbon ferritic steels. Source: Ref 5 Fig. 11 Hall-Petch relationship in low-carbon ~mtic steels, souse: Ref 8 162 / Structure/Property Relationships in Irons and Steels

Chromium, at.% acicular morphology and the carbides are dis- 0 10 20 30 40 50 60 70 80 90 100 crete particles. Because of these morphological 20OO I I I i I I I I I differences, bainite has much different property characteristics than pearlite. In general, bainitic 1863 °C steels have high strength coupled with good 1800 toughness, whereas pearlitic steels have high strength with poor toughness. Another difference between baiaite and pearl- 1600 1538 °C 1516 °: ~ ...... ite is the complexity of the bainite morphologies compared with the simple lamellar morphology 21 1400 - 1394 °C of pearlite. The morphologies of bainite are still oo being debated in the literature. For years, since the classic work of Bain and Davenport in the 1200 -~ (~Fe,Cr) 1930s (Ref 18), there were two classifications of (9 ¢:L bainite: upper and lower bainite. This nomencla- E 1000 _(~Fe)//_12. 7 ture was derived from the temperature regions at oc/I which bainite formed during isothermal (constant oc temperature) transformation. Upper bainite formed isothermally in the temperature range of 8001:-- -.7 400 to 550 °C (750 to 1020 °F), and lower bainite formed isothermally in the temperature ~nn I Magnetic "~ • "-----/ ..... (I o I, "* ". range of 250 to 400 °C (480 to 750 °F). Exam- Itransformabon.- ,, : "-.. 475 o C ples of the microstructure of upper and lower / o.'...... =.." ...... ".,.. bainite are shown in Fig. 21. One can see that 400 i r'1 °° I I I I I I I t "'~ both types of bainite have an acicular morphol- 0 10 20 30 40 50 60 70 80 90 100 ogy, with upper bainite being coarser than lower Fe Chromium, wt% Cr bainite. The true morphological differences between the microstructures can only be deter- Fig. 12 Iron-chromium phase diagram. Source: Ref 9 mined by electron microscopy. Transmission electron micrographs of upper and lower baiaite are shown in Fig. 22. In upper bainitc, the iron carbide phase forms at the lath boundaries, methods are used in conjunction to produce in retarding austenite recrystallization, thus al- whereas in lower bainite, the carbide phase forms strong, tough ferrite-pearlite steels. lowing a wide window of rolling temperatures on particular crystallographic habit planes within Controlled rolling is a thermomechanical for controlled rolling. Without retarding recrys- the laths. Because of these differences in mor- treatment in which steel plates are rolled below tallization, as in normal hot rolling, the pancake- phology, upper and lower bainite have different type grains do not form and a fine grain size the recrystailization temperature of aastcnite. mechanical properties. Lower bainite, with a fine cannot be developed. Microalloyed steels are This process results in elongation of the austenite acicular structure and carbides within the laths, used in a wide variety of high tonnage applica- grains. Upon further rolling and subsequent cool- has higher strength and higher toughness than up- ing to room temperature, the austenite-to-ferrite tions including structural steels for the construc- tion industry (bridges, multistory buildings, per bainite with its coarser structure. transformation takes place. The ferrite grains are Because during manufacture most steels un- etc.), reinforcing bar, pipe for gas transmission, restricted in their growth because of the "pan- dergo continuous cooling rather than isothermal and numerous forging applications. cake" austeaite grain morphology. This produces holding, the terms upper and lower baiaite can the fine ferrite grain size required for higher become confusing because "upper" and "lower" strength and toughness. Bainite are no longer an adequate description of mor- Microalloying is the term applied to the addi- phology. Bainite has recently been reclassified tion of small amounts of special alloying ele- Like pearlite, bainitc is a composite of ferrite by its morphology, not by the temperature range ments (vanadium, niobium, or titanium) that aid and cementitc. Unlike pearlite, the ferritc has an in which it forms (Ref 19). For example, a recent classification of bainite yields three distinct types of morphology.

Class 1 (B1): Acicular ferrite associated with intralath (plate) iron carbide, that is, cemcn- tite (replaces the term "lower bainite")

Interlamellar spacing (Sp), nm 300 200 100 80 60 I I f I 900 O. S 8O0 .j¢ ,- ~ 7oo .~>6OO

~.500 0 400 60 80 100 120 140 Reciprocal root of Interlamellar spacing (Sp-1/2), mm -1/2

Fig. 14 Relationship behveen peadite interlamellar spacing and yield strength for eutectoid steels. Fig, 13 SEM micrograph of pearlite showing ferrile and cementite lamellae. 4% picral etch. 10, O00x Source: Ref I0 Structure/Property Relationships in Irons and Steels/163

1.6 1.6 / 1.2 1,2 o~ o o= j J~ 0.8 z 0.8 /~ o i< 0.4 0.4

0 0 2OO 225 250 275 300 325 350 375 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 Brinell hardness, HB Pearlite spacing, pm Fig. 15 Relationship between hardness and wear resistance(weight loss) for rail steels. Fig. 16 Relationship between pearlite interlamellar spacing and wear resistance Source: Ref 13 (weight loss)for rail steels.Source: Ref 13

• Class 2 (B2): Acicular ferrite associated with ties (for example, 0.003%) has a pronounced ef- cept) (in ram), and n is the number of carbides per interlath (plate) particles or films of cementite fect on retarding the ferrite transformation. Thus, mm 2 in the plane of section. and/or austenite (replaces the term "upper in a boron-containing steel (e.g., l/2Mo + B), the With bainitic steels, the lath width of the bainite") ferrite nose in the CCT diagram is pushed to bainite obeys a Hall-Petch relationship as shown • Class 3 (B3): Acicular ferrite associated with a slower cooling rates. Boron retards the nuclea- in Fig. 25. The lath size is directly related to the constituent consisting of discrete islands of tion of ferrite on the austenite grain boundaries austenite grain size and decreases with decreas- austenite and/or martensite and, in doing so, permits bainite to be formed ing bainite transformation temperature. Because (Fig. 23). Whenever boron is added to steel, it of the fine microstructure of bainite, the meas- The bainitic steels have a wide range of me- must be prevented from combining with other urement of lath size and carbide density can only chanical properties depending on the micro- elements such as oxygen and nitrogen. Generally, be done by SEM or TEM. structural morphology and composition; for ex- aluminum and titanium are added first in order to In low-carbon bainitic steels, type B 2 (upper) ample, yield strength can range from 450 to 950 lower the oxygen and nitrogen levels of the steel. bainite has inferior toughness to type B 1 (lower) MPa (65 to 140 ksi), and tensile strength from Even when adequately protected, the effective- bainite. In both cases, strength increases as the 530 to 1200 MPa (75 to 175 ksi). Another aspect ness of boron decreases with increasing carbon transition temperature decreases. In type B 2 (up- content and austenite grain size. of a bainitic steel is that a single composition, per) bainite, the carbides are much coarser than I/2Mo-B steel for example, can yield a bainitic Attempts have been made to quantitatively rein type B 1 (lower) bainite and have a tendency to microstructure over a wide range of transforma- late the microstructural features of bainite to me- crack and initiate cleavage (brittle) fracture. In tion temperatures. The CCT diagram for this chanical properties. One such relationship is (Ref type B l bainite, the small carbides have less ten- steel is shown in Fig. 23. Note that for this steel 22): dency to fracture. One can lower the transition the bainite start (Bs) temperature is almost con- temperature in type B l bainitic steels by provid- stant at 600 °C (1110 °F). This flat transforma- ing a finer austenite grain size through lower- tion region is important because transformation YS = -194 + 17.4(d -1/2) + 15(nl/4) (Eq 10) temperature plays an important role in the devel- temperature thermomechanical treatment and opment of microstructure. A constant transforma- grain refinement. tion temperature permits the development of a where YS is the 0.2% offset yield strength (in Bainitic steels are used in many applications similar microstructure and properties over a wide MPa), d is the bainite lath size (mean linear inter- including pressure vessels, backup rolls, turbine range of cooling rates. This has many advantages in the manufacturing of bainitic steels and is particularly advantageous in thick sections where a wide range in cooling rates is found from the surface to the center of the part. 9oo In designing a bainitic steel with a wide transformation region, it becomes critical that the pearlite and ferrite regions are pushed as far to the right as possible on the CCT diagram; that is, pearlite and ferrite form only at slow cooling 700 rates. Alloying elements such as nickel, chro- ? mium, and molybdenum (and manganese) are se- 600 _I o.n,r .e lected for this purpose. °Clmln ~ ~s ~_ 1 -643 \\1 ~ ~ For low-carbon bainitic steels, the relationship ¢ 500 tz 2 - 600 between transformation temperature and tensile E 0-545_ (%~. 'Bf strength is shown in Fig. 24 (martensite is dis- 400 4 - 500 cussed in the next section). Note the rapid in- 5 - 450 crease in tensile strength as the transformation 300 6 400 temperature decreases. For these steels, a regres- 7 - 352 Ms \ "~ sion equation for tensile strength has been devel- 8 - 300 oped as follows (Ref 21): 200 9 - 253 10 - 225 Martensite 100 11 -189 TS = 246.4 + 1925(C) + 231(Mn + Cr) + 185(Mo) 12 - 50 + 92(W) + 123(Ni) + 62(Cu) + 385(V + 11) (Eq9) 0 I I I I I I 10 lO0 1000 In addition to the elements carbon, nickel, Time, s chromium, molybdenum, vanadium, and so forth, Fig. 17 ^ ccT diagram of a typical rail steel (composition: 0.77% C, 0.95% Mn, 0.22% Si, 0.014% P, 0.017% S, 0.10% it is well known that boron in very small quanti- Cr). Source: Ref 14 164 / Structure/Property Relationships in Irons and Steels

(a) (h)

Fig. 18 Microstructureof typical ferrite-pearlitestructural steelsat two differentcarbon contents. (a) 0.10%C. (b) 0.25%C. 2% nital + 4% picral etch. 200x rotors, die blocks, die-casting molds, nuclear re- by rapid quenching. Most all the conventional The martensite start temperature (Ms) for type actor components, and earthmoving equipment. alloying elements in steel promote hardenability. 4340 is 300 °C (570 °F). Carbon lowers the M s One major advantage of a bainitic steel is that an For example, type 4340 steel shown in Fig. 26 temperature, as shown in Fig. 27, and alloying optimal strength/toughness combination can be has significant levels of carbon, manganese, elements such as carbon, manganese, chromium, produced without expensive heat treatment, for nickel, copper, and molybdenum to promote har- nickel, and molybdenum also lower M s tempera- example, quenching and tempering as in marten- denability. More details about hardenability can ture. Many empirical equations have been devel- sitic steels. be found in Ref 2. oped over the past 50 years relating M s tempera-

Martensite 2O0 160 (271) Martensite is essentially a supersaturated solid Notched impact tests / oo solution of carbon in iron. The amount of carbon 120 in martensite far exceeds that found in solid solu- "~ (217) ~e tion in ferrite. Because of this, the normal body- ,~ If energy centered cubic (bcc) lattice is distorted in order >:, 120 j~ ," Transitiontemperature 80 eE to accommodate the carbon atoms. The distorted (163) lattice becomes body-centered tetragonal (bet). 8 ,== In plain-carbon and low-alloy steels, this super- = 80 40 saturation is generally produced through very (106) rapid cooling from the austenite phase region CO (quenching in water, iced-water, brine, iced- ~ 40 == brine, oil or aqueous polymer solutions) to avoid 0 (54) ~ ~ '------forming ferrite, pearlite, and bainite. Some o highly alloyed steels can form martensite upon 0 -4O air cooling (see the discussion of maraging steels later in this section). Depending on carbon content, martensite in its quenched state can be very ~ 100 hard and brittle, and, because of this brittleness, .cg~ 80 ~ ~'~UItimate:~'~strength I t ~ martensitic steels are usually tempered to restore some ductility and increase toughness. Reference to a CCT diagram shows that =0= Yield strength martensite only forms at high cooling rates in plain-carbon and low-alloy steels. A CCT dia- .¢ 60 ~ gram for type 4340 is shown in Fig. 26, which indicates that martensite forms at cooling rates exceeding about 1000 °C/rain. Most commercial martensitic steels contain deliberate alloying ad- =m Reduction in area ditions intended to suppress the formation of 20 -- Smooth tensile testa I other constituents--that is ferrite, peariite, and 03 bainite--during continuous cooling. This means that these constituents form at slower cooling 0 rates, allowing martensite to form at the faster 0 0.1 0.2 0:3 0.4 0.5 0.6 0.7 0.8 0.9 cooling rates, for example, during oil and water Carbon, wt% quenching. This concept is called hardenability and is essentially the capacity of a steel to harden Fig. 19 Mechanicalproperties of ferrite-pearlitesteels as a functionof carbon content.Source: Ref 2 Structure/Property Relationships in Irons and Steels / 165

Temperature, °F tions. The important microstructural units meas- -1 O0 0 100 200 300 400 ured in lath martensite are lath width and packet 250 I I I F I size. A packet is a grouping of laths having a 175 common orientation. Plain-carbon and low-alloy martensitic steels 0.11%C 150 are rarely used in the as-quenched state because 200 of poor ductility. To increase ductility, these martensitic steels are tempered (reheated) to a 125 a=>~ temperature below 650 °C (1200 °F). During ¢= tempering, the carbon that is in supersaturated • 150 g solid solution precipitates on preferred crystal- t~ f 100 lographic planes (usually the octahedral {111} ¢x _E planes) of the martensitic lattice. Because of the preferred orientation, the carbides in a tempered 100 / 0.20% C .." ...... 0.31 YoC - 75 martensite have a characteristic arrangement as seen in Fig. 31. Tempered martensite has similar morphologi- ~ "...... __ 0.41%C ~0.49%C 2060%C- 50 cal features to type B) (lower) bainite. However, 50 ~~...... ~ ."".""" • .- s ~ , • a distinction can be made in terms of the orienta- ,'°1 ,, .-" "'.t..-" --.-'.'...... oO." • tion differences of the carbide precipitates. This ,.: ... s .o.° "~ .." .. " ~'~ 0.80%C 25 can be seen by comparing type B l bainite in Fig. 22 with tempered martensite in Fig. 31. However, 0 • 0 unless the carbide morphology is observed it is -100 -50 0 50 100 150 200 250 very difficult to distinguish between B] bainite Temperature, °C and tempered martensite. The hardness of martensite is determined by its Fig. 20 Effect Of carbon content in ferrite-peadite steels on Charpy V-notch transition temperature and shelf energy. carbon content, as shown in Fig. 32. Martensite Source: Ref 17 attains a maximum hardness of 66 HRC at carbon contents of 0.8 to 1.0%. The reason that the hardness does not monotonically increase with carbon ture to composition. One recent equation by An- to carbon content, as shown in Fig. 27. Lath is that retained austenite is found when the car- drews (Ref 24) is: martensite forms at carbon contents up to about bon content is above about 0.4% (austenite is 0.6%, plate martensite is found at carbon con- much softer than martensite). Figure 33 shows tents greater than 1.0%, and a mixed martensite the increase in volume percent retained austenite M s (°C) = 539 - 423(C) - 30.4(Mn) - 12.1(Cr) microstructure forms for carbon contents be- with increasing carbon content. Yield strength - 17.7(Ni) - 7_5(Mo) (Eq 11) tween 0.6 and 1.0%. An example of lath marten- also increases with increasing carbon content as site is shown in Fig. 28 and plate martensite in seen in Fig. 34. This empirical relationship be- Fig. 29. Generally, plate martensite can be distin- tween the yield strength and carbon content for With sufficient alloy content, the M s tempera- untempered low-carbon martensite is (Ref 25): ture can be below room temperature, which guished from lath martensite by its plate mor- means that the transformation is incomplete and phology with a central mid-fib. Also, plate retained austenite can be present in the steel. martensite may contain numerous microcracks, YS (MPa) = 413 + 17.2 x 10P(C 1/2) (Eq 12) The microstructure of martensitic steels can be as shown in Fig. 30. These form during transfor- generally classed as either lath martensite, plate mation when a growing plate impinges on an ex- martensite, or mixed lath and plate martensite. In isting plate. Because of these microcracks, plate Lath martensite packet size also has an influence plain carbon steels, this classification is related martensite is generally avoided in most applica- on the yield strength, as shown in Fig. 35. The

(a) (b)

Fig. 21 Microstructure of (a) upper bainite and (b) lower bainite in a Cr-Mo-V rotor steel.2% nital + 4% picral etch. 500x 166 / Structure/Property Relationships in Irons and Steels

(a) (b)

Fig, 22 TEM micmgraphs of (a) upper bainite and (b) lower bainite in a Cr-Mo-V rotor steel

linear behavior follows a Hall-Petch type rela- maintain much of the hardness and strength of quenched rod has a hardness of 601 HB. Note tionship of (d-l/2). the quenched martensite and provide a small im- that by tempering at 650 °C (1200 °F), the hard- Most martcnsitic steels are used in the tem- provement in ductility and toughness (Ref 26). ness (see x-axis) decreased to 293 HB; or to less pered condition where the steel is reheated after This treatment can be used for bearings and gears than half the as-quenched hardness. The tensile quenching to a temperature less than the lower that are subjected to compression loading. Tem- strength has decreased from 1960 MPa (285 ksi) critical temperature (Act). Figure 36 shows the pering above 425 °C (796 °F) significantly im- at a 200 °C (400 °F) tempering temperature to decrease in hardness with tempering temperature proves ductility and toughness but at the expense 965 MPa (141 ksi) at a 650 °C (1200 °F) temper- for a number of carbon levels. Plain-carbon or of hardness and strength. The effect of tempering ing temperature. However, the ductility, repre- low-alloy martensitic steels can be tempered in temperature on the tensile properties of a typical sented by total elongation and reduction in area, lower or higher temperature ranges, depending oil-quenched low-alloy steel (type 4340) is on the balance of properties required. Tempering shown in Fig. 37. These data are for a 13.5 mm increases dramatically. The tempering process between 150 and 200 °C (300 and 390 °F) will (0.53 in.) diam rod quenched in oil. The as- can be retarded by the addition of certain alloying elements such as vanadium, molybdenum, manganese, chromium, and silicon. Also, for tempering, temperature is much more important 1000~ 1800 than time at temperature. Ac3 = 930 °C Temper embrittlement is possible during the tempering of alloy and low-alloy steels. This em- 900 Fs 1 ! 0 1600 brittlement occurs when quenched-and-tempered .0o steels are heated in, or slow cooled through the 340 to 565 °C (650 to 1050 °F) temperature ,oo ?-- range. Embrittlement occurs when the embrit- tling elements, antimony, tin, and phosphorus, °o 6oo " concentrate at the austenite grain boundaries and create intergranular segregation that leads to in-

500 - .... -- + tergranular fracture. The element molybdenum ~'E ~lI I "l\ ~ \1 I I \ ~l~""r I I I ~ ~ I I I X I I I II -1800 1200 "'"q'e

~-. 1050 ~'~ 200 ~ ~ 400 " 900 100 200 750 0 32 .== 10 102 103 104 105 ¢/) i.~ 600 ~1 Ferrite + Seconds I ' ' ' ' I ' ' ' ' I ~ ' ' ~ I eel==, peadite 1 10 102 103 450 Martensites Bainites ~'~', ~* "/45 Minutes I ' I ' ' I ' I I I I I ~-'- i i 1- 1 4 10 30 3OO 4OO 500 600 7OO 8O0 Time Hours Transformation temperature, °C Fig. 23 A CCT diagram of a I/2Mo-B steel. Composition: 0.093% C, 0.70% Mn, 0.36% Si, 0.51% Mo, 0.0054% B. Fig. 24 Relationship between transformation tempera- Austenitized at Ac 3 + 30 °C for 12 rain. Bs, bainite start; Bo bainite finish; Fs, ferrite start; Fo ferrite finish. Num- ture and tensile strength of ferrite-pearlite, bain- bers in circles indicate hardness (HV) after cooling to room temperature. Source: Ref 20 itic, and martensitic steels. Source: Ref 5 Structure/Property Relationships in Irons and Steels / 167

1600 870 900 t~ IL / to 750 1400 760 ~ QII) •

" 600 • ° 1200 650

450 15 20 25 Grain size (d-1/2), mm-1/2 1000 540 ~o g Fig. 25 Relationship between bainite lath width (grain size) and yield strength. Source: Ref 5 E 800 425 E

has been shown to be beneficial in preventing temper embrittlement. The large variation in mechanical properties of 600 315 quenched-and-tempered martensitic steels provides the structural designer with a large number of property combinations• Data, like that shown in Fig. 37, are available in the Section "Carbon 400 205 and Alloy Steels" in this Handbook as well as Volume 1 of the ASM Handbook and the ASM Specialty Handbook: Carbon and Alloy Steels. Hardnesses of quenched-and-tempered steels can be estimated by a method established by Grange 200 95 et al. (Ref 27). The general equation for hardness 1 2 5 10 20 50 100 200 500 1000 is: Cooling timel s

26 The CCT diagram for type 4340 steel austenitized at 845 °C (I 550 °F). Source: Ref 23 HV = HV C + AHVMn + AHVp + AHVsi + AHVNi Fig, + AHVcr + AHVMo + AHVv (Eq 13) examples of hardness conversion tables for TS (MPa) =- 42.3 +3.6 HB (Eq 14) where HV is the estimated hardness value (Vick- steels, which can be found in the Section "Glos- ers). sary of Terms and Engineering Data" in this For the above example, a type 4340 quenched- In order to use this relationship, one must de- Handbook), a Brinell hardness of 363 HB equates and-tempered (540 °C, or 1000 °F) steel with a termine the hardness value of carbon (HVc) from to a Vickers hardness of 383 HV. The calculated calculated hardness of 363 HB would have an Fig. 38. For example, if one assumes that a tem- value of 380 HV (in the table above) is very estimated tensile strength from Eq 14 of 1265 pering temperature of 540 °C (1000 °F) is used close to the actual measured value of 383 HV. MPa (183 ksi). From Table 1, this measured ten- and the carbon content of the steel is 0.2% C, the Thus, this method can be used to estimate a spe- sile strength of a type 4340 quenched-and-tem- HV c value after tempering will be 180 HV. Sec- cific hardness value after a quenching-and-tempered (540 °C, or 1000 °F) steel is 1255 MPa ond, the effect of each alloying element must be pering heat treatment for a low-alloy steel. Also, (182 ksi). determined from a figure such as Fig. 39. This as a rough approximation, the derived Brinell It is seen that quenched-and-tempered marten- graph represents a tempering temperature of 540 hardness value can be used to estimate tensile sitic steels provide a wide range of properties. °C (1000 °F). Graphs representing other temper- strength by the following equation (calculated The design engineer can choose from a large ing temperatures can be found in Ref 27. from ASTM E 140 conversion table): number of plain-carbon and low-alloy steels. In To illustrate the use of the Grange et al. method, the same type 4340 steel shown in Fig. 37 is used. The composition of the steel is 0.41% 870 1600 C, 0.67% Mn, 0.023% P, 0.018% S, 0.26% Si, 1.77% Ni, 0.78% Cr, and 0.26% Mo. Assuming a 1400 540 °C (1000 °F) tempering temperature, the es- .8o timated hardness value for carbon is 210 HV. 650 1200 From Fig. 38, the hardness values for each of the ii other alloying elements are: ~o o cff 540 lOOO =d Element Ccetent, % Hardaess,HV 425 ~""~.,,~ 800 Carbon 0.41 210 Q_ Manganese 0.67 38 E Phosphorus 0.023 7 315 ~o ff Silicon 0.26 15 Nickel 1.77 12 205 400 Chromium 0.78 43 Molybdenum 0.26 55 Total hardness 380 95 l%~: ~ ...... Lath .... ?~ Mixea/ ,~ ~:,: Plate D20 0 According to Fig. 37, the hardness value after 0 0.2 0.4 0.6 0.6 1.0 1.2 1.4 1.6 tempering at 540 °C (1000 °F) was 363 HB (see Carbon, wt% Brinell hardness values along x-axis). From the ASTM E 140 conversion table (included in the Fig. 27 Effect of carbon content on M s temperature in steels. Source: Ref 6 168 / Structure/Property Relationships in Irons and Steels

Fig. 28 Microstructure of a typical lath martensite. 4% picral + HCI. 200x Fig. 29 Microstructure of a typical plate martensite. 4% picral + HCI. 1000x addition to this large list of steels, there are two high-strength stainless steels because they can be 8.5 to 12.5% Co, 4 to 5% Mo, 0.20 to 1.8% Ti, other commercially important categories of fully treated to achieve a yield strength between 550 and 0,10 to 0.15% A1. Because of the high alloy martensitic steels, namely, martensitic stainless MPa (80 ksi) and 1725 MPa (250 ksi), as seen in content, especially the cobalt addition, they are steels and maraging steels. Table 1. On the other hand, ferritic stainless very expensive. Their high strength is developed Like the ferritic stainless steels, martensitic steels, which do not contain carbon, are not con- by austenitizing at 850 °C (1560 °F), followed by stainless steels (e.g., type 403, 410, 414, 416, sidered high-strength steels because their yield air cooling to room temperature to form lath 420, 422, 431, and 440) are high-chromium iron strength range is only 170 to 450 MPa (25 to 64 martensite. However, the martensitic constituent alloys (12 to 18% Cr), but with deliberate addi- ksi). Because of their high strength and hardness, in maraging steels is relatively soft--28 to 35 tions of carbon (0.12 to 1.2% C). These steels coupled with corrosion resistance, martensitic HRC--which is an advantage because the com- use carbon in order to stabilize austenite in iron- stainless steels are used for knives and other ap- ponent can be machined to final form directly chromium alloys (Fig. 12). The expanded region plications requiring a cutting edge as well as upon cooling. The final stage of strengthening is of austenite is called the y-loop. In the Fe-Cr some tool steel applications (for example, molds through an aging process, carried out at 480 °C phase diagram (without C), the y-loop extends to for producing plastic parts). (900 °F) for 3 h. During aging, the hardness in- about 12% Cr (see Fig. 12). With carbon addi- Maraging steels are a separate class of marten- creases to about 51 to 58 HRC depending on the tions, austenite can exist up to 25% Cr. These sitic steels and are considered ultrahigh-strength grade of maraging steel. The aging treatment pro- steels can be heat treated much like those of the steels with yield strength levels as high as 2500 motes the precipitation of a rodlike intermetallic low-alloy steels. However, martensitic stainless MPa (360 ksi), as seen in Table 1. In addition to compound Ni3Mo. These precipitates can only be steels, with such high chromium contents, can extremely high strength, the maraging steels observed at high magnification (e.g., by TEM). form martensite on air cooling, even in thick sec- have excellent ductility and toughness. These The precipitates strengthen the surrounding ma- tions. Martensitic stainless steels are considered very-low carbon steels contain 17.5 to 18% Ni, trix as they form during aging. Full hardening

Fig. 30 Microcracks formed in plate martensite. 4% picral + HCl/sodium metabisulfite Fig. 31 Ttransmission electron micrograph showing carbide morphology in tempered etch. 1000x martensite Structure/Property Relationships in Irons and Steels / 169

900 700 o 65 800 ] M s temperature L 500 >o 700 6o •->¢ 100 3OO E > o =o "I- 600 -.rt~ of "~ 75 100 ¢= 8 500 so ~ t~ Lath marten 'lite,'~~ ~"~1...... ~ -- t~ 03 < "1- E relative volh°/° ---~ ~ ~/ 40 400 40 o 300 25 30 20 p- 20 Retained 7, vol% 200 1o .t ~ I I I 0 0.4 0.8 1.2 1.6 100 J I I I I I Carbon, wt% 0 0.2 0.4 0.6 0.8 1.0 1.2 Carbon, wt% Fig. 33 Effectof carbon content on the volume percent of retained austenite (7) in as-quenched martensite. Source: Ref 4 Fig. 32 Effect of carbon content on the hardness of martensite. Source: Ref 4 alloying elements), their susceptibility to stress- elongation in the annealed condition to about corrosion cracking (certain austenitic steels), 25% elongation after cold working. can be developed, even in very thick sections. their relatively low yield strength, and the fact Some austenitic stainless steels (type 200, 201, Maraging steels are used for die-casting molds that they cannot be strengthened other than by 202, and 205) employ interstitial solid-solution and aluminum hot-forging dies as well as numer- cold working, interstitial solid-solution strength- strengthening with nitrogen addition. Austenite, ous aircraft and missile components. ening, or precipitation hardening. like ferrite, can be strengthened by interstitial The austenitic stainless steels (e.g., type 301, elements such as carbon and nitrogen. However, 302, 303, 304, 305,308, 309, 310, 314, 316, 317, carbon is usually excluded because of the delete- Austenite 321, 330, 347, 348, and 384) generally contain rious effect associated with precipitation of chro- from 6 to 22% Ni to stabilize the austenite at mium carbides on austenite grain boundaries (a Austenite does not exist at room temperature room temperature. They also contain other alloy- process called sensitization). These chromium in plain-carbon and low-alloy steels, other than ing elements, such as chromium (16 to 26%) for carbides deplete the grain-boundary regions of as small amounts of retained austenite that did corrosion resistance, and smaller amounts of chromium, and the denuded boundaries are ex- not transform during rapid cooling. However, in manganese and molybdenum. The widely used tremely susceptible to corrosion. Such steels can certain high-alloy steels, such as the austenitic type 304 stainless steel contains 18 to 20% Cr be desensitized by heating to high temperature to stainless steels and Hadfield austenitic manga- and 8 to 10.5% Ni and is also called 18-8 stain- dissolve the carbides and place the chromium nese steel, austenite is the microstructure. In less steel. From Table 1, the yield strength of back into solution in the austenite. Nitrogen, on these steels, sufficient quantifies of alloying ele- annealed type 304 stainless steel is 290 MPa (40 the other hand, is soluble in austenite and is ments that stabilize austenite at room tempera- ksi), with a tensile strength of about 580 MPa (84 added for strengthening. To prevent nitrogen ture are present (e.g., manganese and nickel). ksi). However, both yield and tensile strength can from forming deleterious nitrides, manganese is The crystal structure of austenite is face-centered be substantially increased by cold working as added to lower the activity of nitrogen in the cubic (fee) as compared to ferrite, which has a shown in Fig. 40 (see Table 1). However, the austenite, as well as to stabilize the austenite. (bcc) lattice. A fcc alloy has certain desirable increase in strength is offset by a substantial de- For example, type 201 stainless steel has compo- characteristics; for example, it has low-tempera- crease in ductility, for example, from about 55% sition ranges of 5.5 to 7.5% Mn, 16 to 18% Cr, ture toughness, excellent weldability, and is non- magnetic. Because of their high alloy content, austenitic steels are usually corrosion resistant. ASTM grain size Disadvantages are their expense (because of the 2 4 6 8 10 12 1200 i i = i i = 120

260 oo -~,~1 1700 a. E ,50 - 220 J~ t300 "~ 800 o-o~ 80 180 "o -lltOO o,° "o

"~ 140 so "~ 600 ~ ~ -r.,

~. 100 o Y t oo 400 ~.~ 40 o 60 I I I I 0 0.10 0.20 0.30 0.40 0 2 4 6 8 10 12 14 16 Carbon content, wt% Lath martensite packet size (d-1/2), mm -1/2 Fig. 34 Relationship between carbon content and the Fig. 35 Relationship between lath martensite packet size (dl and yield strength of Fe-0.2%C (upper line) and Fe-Mn yield strengthof martensite. Source: Ref 4 (lower line) martensites. Source: Ref 2 170 / Structure/Property Relationships in Irons and Steels

Tempering temperature,°F Hardness, HB 200 400 600 800 1000 1200 1400 555 477 415 363 293 70 300 (2070) \

250 \ 60 (172o) 0 ,0,,o N Tensilestrength I1. ~-o.~c % 200 ' ' '\ N n-° 50 ~ ~~~o ~ (13oo) -- Yield point- N N J:: NiX ¢5 = 0=00=0; 150 o} t~ (1030) "l-~=m 40 ~ ~~0.10"0.20 ~/o C~ ~ 70 -- Reductionin area --~ 6O

30 100 4O (690) Elongation ' i ~' 400 600 800 1000 1200 !i 20 (200) (320) (430) (540) (650) 5 Tempering temperature, °F(°C) ILl

\ 10 I I I I I I Fig. 37 Effect of tempering temperature on the me- As- 100 200 300 400 500 600 700 chanical properties of type 4340 steel. Source: Ref 2 quenched Tempering temperature, °C

Fig. 36 Decreasein the hardnessof martensite with tempering temperaturefor various carbon contents. Source: Ref 2 ture (Ac]), the process of spheroidization takes place. Figure 41 shows a fully spheroidized steel microstructure. The microstructure before sphe- 3.5 to 5.5% Ni, and 0.25% N. The other type 2xx tic, these steels can be work hardened to provide roidization is pearlite. During spheroidization, series of steels contain from 0.25 to 0.40% N. higher hardness and wear resistance. A work- Another important austenitic steel is austenitic hardened Hadfield manganese steel has excellent the cementite lamellae of the pearlite must manganese steel. Developed by Sir Robert Had- resistance to abrasive wear under heavy loading. change morphology to form spheroids. The proc- field in the late 1890s, these steels remain Because of this characteristic, these steels are ess is controlled by the diffusion rate of carbon austenitic after water quenching and have consid- ideal for jaw crushers and other crushing and and portions of the lamellae must "pinch-off" erable strength and toughness. A typical Hadfield grinding components in the mining industry. (dissolve) and that dissolved carbon must diffuse manganese steel will contain 10 to 14% Mn, 0.95 Also, Hadfield manganese steels have long been to form a spheroid from the remaining portions to 1.4% C, and 0.3 to 1% Si. Solution annealing used for railway frogs (components used at the of lamellae. This process takes several hours. is necessary to suppress the formation of iron junction point of two railroad lines). Spheroidization takes place in less time when the carbides. The carbon must be in solid solution to starting microstructure is martensite or tempered stabilize the austenite. When completely austeni- martensite. In this process, the spheroidized car- Ferrite-Cementite bides are formed by growth of carbides formed during tempering. 900 When plain-carbon steels are heated to tem- A fully spheroidized structure leads to im- As-quenched peratures just below the lower critical tempera- proved machinability. A steel in its fully sphe- 800 65 As-quenched / ~0~°F I 8O hardiness "~-//I 400°F • Mo '= / °° 70 e• 0= ], / ..,° "1-> 60 "f"" /b'/'' 600 °F..,@ =n- ,6 z~"l"- ~.c. 50 =" ~ 50 / • tw p "O il 45 !° Cr 400 800 °F"°" 40 ~ ~ 4o .S/ • -. ~,,,,, ~o- I .E j o Si ~ 3o 300 ;;~o~ ~.,,,~ ~,-, 1000 OF.o" 30 o 20 200 ~....,~1200 °F=o= 1300 °F lO j f 100 I 0.2 0.4 0.6 0.8 1.0 0 Carbon, % 0.02 0.04 o.o6 0.1 0.2 0.4 0.6 1 2 Element content, % Fig. 38 Relationship between hardness of tempered martensite with carbon content at various tem- Fig. 39 E~ of alloying elements on the retardationor softening during tempering at 540 °C (1 000 °F) relative to iron- pering temperatures.Source: Ref 2 carbon alloys. Source: Ref 2 Strudure/Property Relationships in Irons and Steels / 171

1200 0 ~+, ,+,.. + .,yff/S+~ ++ z9 O,o,:" Q ~ - * o " /

100o /f -- Tensile/~stStrength-- / s ~t" ~ utz , o v # = = o © "x ~, o+ DO . ",-,~', . o o ~ o • ~, .o0. °%0 :':,~, - ,,~. o- ~ ~ mo I rength • . " ," ~. t) • = 600 / / ' (0.2*/.offset) OC~ 06 o o " +' (/)1 (7 * .,~ ,,,,. ~ ====, I=, .',,a¢'~'~ 0 oo o oo v . ,0 I= ~F' = ,~,=p " %

O . .++ o,,o 0+,o+ .~l..~,,o.: .o ~, .= .... ~ + • ~ o +° "o. o ~>/2 + .~.,o,'-." • , ."1 ". ~ == ~+)1 ^+L ~- 40 =m o ~:~ ,e-~a ~, ~" 0,//~,,.. ,,- .. oOo..~:" 0~"o o:.- "'+,~" +o~"~'~ ~o'+<3c 200 ~- [] ~ 20 o 2+ :+" ' . oo ,,? a:--,£. 0- v++o • . ¢ ~',,*" o o ~' 0 ~ 0 10 20 30 40 50 60 Cold work. % °e:+ e" ooV+ '~ d'-" o~;~, + " \ Fig. 40 Influence of cold work on mechanical properties of type 304 stainlesssteel. Source:Ref 4 roidized state is in its softest possible condition. Fig, 41 Microstructureof a fullyspheroidized steel.4% picraletch. 1000x Some steels, such as type 1020, are spheroidized before cold forming into tubing because spheroidized steels have excellent formability. 15 to 20% martensite in a matrix of ferrite. The 980X of similar tensile strength. These charac- Ordinary low-carbon, cold-rolled, and an- microstructure of a typical dual-phase steel is teristics are especially important in formability. nealed sheet steels have ferritic microstructures shown in Fig. 42. In most plain-carbon and low- A unique characteristic of a ferrite-martensite with a small amount of grain-boundary cemen- alloy steels, the presence of martensite in the dual-phase steel is its substantial work hardening tite, as shown in Fig. 8. These carbides nucleate microstructure is normally avoided because of capacity. This allows the steel to strengthen and grow on the ferrite grain boundaries during the deleterious effect that martensite has on duc- while being deformed. By proper design of the the annealing process, which takes place in the tility and toughness. However, when the marten- stamping dies, this behavior can be exploited to lower portion of the intercritieal temperature re- site is embedded in a matrix of ferrite, it imparts produce a high-strength component. Most conventional high-strength steels have limited form- gion (i.e,, the region between the A 3 and A 1 tem- desirable characteristics. One desirable charac- ability because their high strength is developed peratures shown in the iron-carbon diagram, Fig. teristic is that dual-phase steels do not exhibit a prior to the forming process. 6). Many modern-day automotive sheet steels are yield point. Figure 43 compares the stress-strain produced with very low carbon levels to avoid behavior of four steels: plain carbon, SAE 950X, these grain-boundary carbides because they de- and SAE 980X, which exhibit a yield point with FerriteoAustenite grade formability. the fourth, a dual-phase steel (GM 980X). This means that the cosmetically unappealing Ltiders High-alloy steels having approximately equal proportions of fcc austenite and bcc ferrite, with Ferrite-Martensite bands that form during the discontinuous yielding (i.e., yield point) are absent in a dual-phase ferrite comprising the matrix, are referred to as duplex stainless steels. The microstructure of a A relatively new family of steels called dual- steel. Also note in Fig. 43 that the dual-phase typical duplex stainless steel is shown in Fig. 44. phase steels consists of a microstracture of about steel has much more elongation than the SAE Although the exact amount of each phase is a function of composition and heat treatment, most alloys are designed to contain about equal amounts of each phase in the annealed condition.

100 ~" (552)(690)80~/ ...... ~',1'IGM 980X'

~v 60 t" ~J ~ / (414) ~'~ / SAE 950X 40 ~ (276) / Plain ca/~n~ 038) 0 I 0 10 20 30 40 Strain in two-inch gage length, %

Fig. 43 Comparisonof the stress-strain curves of three discontinuously yielding sheet steels (plain car- loon, 5AE 950X, and SAE 980X) and a dual-phasesteel (GM 980X). in addition to the differences in yielding behavior, note the higher percentage of uniform elongation in the dual-phase steel compared with the conventional SAE Fig. 42 Microstructureof a typical dual-phasesteel. 2% nital etch. 250x 980X of similar tensile strength. Source:Ref 2 172 / Structure/Property Relationships in Irons and Steels

The duplex structure results in improved stress- corrosion cracking resistance, compared with austenitic stainless steels, and improved toughness and ductility, compared with the ferritic stainless steels. Duplex stainless steels are capable of tensile yield strengths ranging from 400 to 550 MPa (60 to 80 ksi) in the annealed condition, which is approximately twice the strength of either phase alone. The principal alloying elements in duplex stainless steels are chromium and nickel, but nitrogen, molybdenum, copper, silicon, and tung- sten may be added to control structural balance and to impart certain corrosion-resistance characteristics. Four commercial groups of duplex stainless steels, listed in order of increasing corrosion resistance, are:

Fe-23Cr-4Ni-0.1N • Fe-22Cr-5.5Ni-3Mo-0.15N • Fe-25Cr-5Ni-2.5Mo-0.17N-Cu Fig. 44 Microstructure of a typical mill-annealed duplex stainless steel plate showing elongated austenite islands in the • Fe-25Cr-7Ni-3.5Mo-0.25N-W-Cu ferrite matrix. Etched in 15 mL HCI in 100 mL ethyl alcohol. 200x

Because of their excellent corrosion resistance, ferrite-austenite duplex stainless steels have gies as seen in Fig. 45. Type A, because of its ter the matrix microstructure to obtain desired found widespread use in a range of industries, random orientation and distribution, is preferred properties. The matrix can be fully ferritic, fully particularly the oil and gas, petrochemical, pulp in many applications, for example, cylinders of pearlitic, fully martensitic, or fully bainitie, de- and paper, and pollution control industries. They internal combustion engines. The matrix of a pending on composition and heat treatment. The are commonly used in aqueous, chloride-contain- typical gray cast iron is usually pearlite. How- yield strength of typical ductile cast irons ranges ing environments and as replacements for ever, ferrite-pearlite or martensitic micro- from 276 to 621 MPa (46 to 76 ksi), and their austenitic stainless steels that have suffered structures can be developed by special heat treat- tensile strengths range from 414 to 827 MPa (60 stress-corrosion cracking or pitting during ser- ments. As a structural material, gray cast iron is to 120 ksi). Total elongation ranges from about 3 vice. selected for its high compressive strength, which to 18%. Heat treated, austempered ductile irons ranges from 572 to 1293 MPa (83 to 188 ksi), have yield strengths ranging from 505 to 950 although tensile strengths of gray iron range only MPa (80 to 138 ksi), tensile strengths ranging from 860 to 1200 MPa (125 to 174 ksi), and total from 152 to 431 MPa (22 to 63 ksi). Gray cast elongations ranging from 1 to 10%. Uses for due- Graphite irons are used in a wide variety of applications, tile iron include gears, crankshafts, paper-mill including automotive cylinder blocks, cylinder When carbon contents of iron-carbon alloys dryer rolls, valve and pump bodies, steering heads and brake drums, ingot molds, machine exceed about 2%, there is a tendency for graphite knuckles, rocker arms, and various machine com- housings, pipe, pipe fittings, manifolds, compres- to form (see Fe-C diagram in Fig. 6b). This is ponents. especially true in gray cast iron in which graph- sors, and pumps. ite flakes are a predominant microstructural fea- Another form of graphite in cast iron is ture (Fig. 3). Gray cast iron has been used for spheroidal graphite found in ductile cast irons centuries because it melts at a lower temperature (also called nodular cast irons). The micro- Cementite than steel and is easy to cast into various shapes. structure of a typical ductile cast iron is shown in A major microstructural constituent in white Also, the graphite flakes impart good ma- Fig. 46. This form of graphite is produced by a cast iron is cementite. The microstructure of a chinability, acting as chip breakers, and they also process called inoculation, in which a magne- typical white cast iron is shown in Fig. 47. The sium or cerium alloy is thrust into molten cast provide excellent damping capacity. Damping ca- cementite forms by a eutectic reaction during so- iron immediately prior to the casting operation. pacity is important in machines that are subject lidification: to vibration. However, gray cast iron is limited to These elements form intermetallic compounds applications that do not require toughness or duc- that act as a nucleating surface for graphite. With tility, for example, total elongation of less than a spherical morphology, the graphite no longer Liquid ~-~ Cementite + Austeuite (Eq 15) 1%. The flake morphology of the graphite pro- renders the cast iron brittle as do graphite flakes vides for easy crack propagation under applied in gray cast iron. Ductile irons have much higher stress. ductility and toughness than gray iron and thus The eutectic constituent in white cast iron is Gray cast irons usually contain 2.5 to 4% C, 1 expand the use of this type of ferrous alloy. Most called ledeburite and has a two-phase morphology to 3% Si, and 0.1 to 1.2% Mn. The graphite ductile iron castings are used in the as-cast form. shown as the smaller particles in the white matrix flakes can be present in five different morpholo- However, heat treatment can be employed to al- in Fig. 48. The eutectic is shown in the Fe-C

Type A Type B Type C Type D Type E ./T.¢,i! .~>..'

• i:t" ,/ I/ / ; [_,~'~" 72,: ~,~.<.'.,..~ .~ ~.: • / ,'~..,,~, ,, ,,. --'~,1 '~. ' •

Uniform distribution, Rosette grouping, Superimposed flake size, Interdendritic segregation, Interdendritie segregation, random orientation random orientation random orientation random orientation preferred orientation

Fig. 45 Classification of different graphite flake morphology Structure/Property Relationships in Irons and Steels / 173

Fig, 46 Microstructure of a typical ductile (nodular) cast Fig, 47 Microstructure of a typical white cast iron. 4% picral etch. 100x. Courtesy of A.O. Benscoter, Lehigh University iron showing graphite in the form of spheroids. 2% nital etch. 200x. Courtesy of A.O. Benscoter, Lehigh University binary diagram in Fig. 6(b). The austenite in the eutectic (as well as the austenite in the primary phase) transforms to pearlite, ferrite-pearlite, or martensite, depending on cooling rate and composition. Because of the high percentages of cementite, white cast irons are used in applications requiring excellent wear and abrasion resistance. These irons contain high levels of silicon, chromium, nickel, and molybdenum and are termed alloy cast irons. Such applications include steel mill rolls, grinding mills, and jaw crushers for the mining industry. Hardness is the primary mechanical property of white cast iron and ranges from 321 to 400 HB for pearlitic white iron and 400 to 800 HB for alloy (martensitic) white irons.

REFERENCES

1. P.D. Harvey, Ed., Engineering Properties of Steel, American Society for Metals, 1982 2. G. Krauss, Principles of the Heat Treatment of Steel, Fig. 48 Microstructure of the eutectic constituent ledebutite in a typical white cast iron. 4% picral etch. 500x. Courtesy American Society for Metals, 1980 of A.O. Benscoter, Lehigh University 3. R.W.K. Honeycombe, Steels--Microstructure and Properties, American Society for Metals, 1982 4. W.C. Leslie, The Physical Metallurgy of Steels, 12. G.E Vander Voort and A. Ro6sz, Metallography, Vo117 19. B.L. Bramfitt and J.G. Spoer, MetalL Trans. A, Vol McGraw-Hill, 1981 (No. 1), 1984, p 1 21A, 1990, p 817 5. F.B. Picketing, Physical Metallurgy and the Design of 13. H. Iehinose et al., paper 1.3, Proc. First Int. Heavy 20. G.E Vander Voort, Ed., Atlas of Time-Temperature Dia- Steels, Applied Science, 1978 Hauls Railway Conf., Association of American Rail- grams for Irons and Steels, ASM International, 1991, p 6. G. Krauss, Microstruetures, Processing, and Properties roads, 1978, p 1 249 of Steels, Properties and Selection: Irons, Steels, and 14. G.E Vander Voort, Ed.,Atlas of Time-Temperature Dia- 21. W. Steven and A.G. Haynes, J. Iron Steel Inst., Vol 183, High-Performance Alloys, Vol 1, ASM Handbook, grams for Irons and Steels, ASM International, 1991, p 1956, p 349 1990, p 126 570 22. R.W.K. Honeycombe and F.B. Pickering, Metall. 7. E.C. Bain and H.W. Paxton, Alloying Elements in Steel, 15. B.L. Bramfitt, Proc. 32nd Mechanical Working and Trans. A, Vol 3A, 1972, p 1099 2nd ed., American Society for Metals, 1961, p 62 Steel Processing Conference, Vol 28, ISS-AIME, 1990, 23. G.E Vander Voort, Ed., Atlas of~me-Temperature Dia- 8. Microalloying 75, Conference Proceedings (Washing- p 485 grams for Irons and Steels, ASM Intgrantional, 1991, p ton, D.C., Oct 1975), Union Carbide Corp., 1977, p 5 16. F.B. Picketing, Towards Improved Toughness and Duc- 544 9. T.B. Massalski, J.L. Murray, L.H. Bennett, and H. tility, Climax Molybdenum Co., 1971, p 9 24. K.W. Andrews, J. Iron Steel Inst., Vol 203, 1965, p 271 Baker, Ed., Binary Alloy Phase Diagrams, Vol 1, 17. G.J. Roe and B.L. Bramfitt, Notch Toughness of Steels, 25. G.R. Speich and H. Warlimont, J. Iron Steel Inst., Vol American Society for Metals, 1986, p 822 Properties and Selection: Irons, Steels, and High-Per- 206, 1968, p 385 10. W. Haller, 1L Schweitzer, and L. Weber, Can. Metall. formance Alloys, Vol 1, ASM Handbook, ASM Interna- 26. G. Kranss, Z Iron Steel Inst. Jpn., Int., Vol 35 (No. 4), Q., Vol 21 (No. 1), 1982, p 3 tional, 1990, p 739 1995, p 349 11. J.M. Hyzak and I.M. Bernstein, Metall. Trans. A, Vol 18. E.C. Bain, The Sorby Centennial Symposium on the 27. R.A. Grange, C.R. Hrihal, and C.F. Porter, Metall. 7A, 1976, p 1217 History of Metallurgy, TMS-AIME, 1963, p 121 Trans. A, Vol 8A, 1977, p 1775