Structure/Property Relationships in Irons and Steels Bruce L

Copyright © 1998 ASM International® Metals Handbook Desk Edition, Second Edition All rights reserved. J.R. Davis, Editor, p 153-173 www.asminternational.org 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 " "o" - grade 50). 2% nital + 4% picral etch. 200x Fig. :2 Microstructurepearliteinterlamellar°f a typicalspacing.fUllY2%pearlitiCnital + 4%rail steelpicralShowingetch. 500xthe characteristic fine 154/Structure/Property Relationships in Irons and Steels 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 com- pression. 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.

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