Structure/Property Relationships in Irons and ... - ASM International

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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 relationship of (d-l/2). Most martcnsitic steels are used in the tempered condition where the steel is reheated after quenching to a temperature less than the lower critical temperature (Act). Figure 36 shows the decrease in hardness with tempering temperature for a number of carbon levels. Plain-carbon or low-alloy martensitic steels can be tempered in lower or higher temperature ranges, depending on the balance of properties required. Tempering between 150 and 200 °C (300 and 390 °F) will maintain much of the hardness and strength of the quenched martensite and provide a small im- provement in ductility and toughness (Ref 26). This treatment can be used for bearings and gears that are subjected to compression loading. Tem- pering above 425 °C (796 °F) significantly im- proves ductility and toughness but at the expense of hardness and strength. The effect of tempering temperature on the tensile properties of a typical oil-quenched low-alloy steel (type 4340) is shown in Fig. 37. These data are for a 13.5 mm (0.53 in.) diam rod quenched in oil. The as- 1 0 0 0 ~ 1800 Ac 3 = 930 °C 900 Fs 1 ! 0 1600 .0o ,oo ?-- ° o 6oo " 500 - . . . . -- + ~'E ~ l I I "l\ ~ \1 I I \ ~l~""r I I I ~ ~ I I I X I I I II -1800 200 ~ ~ 400 100 200 0 32 10 102 103 104 105 Seconds I ' ' ' ' I ' ' ' ' I ~ ' ' ~ I 1 10 102 103 Minutes I ' I ' ' I ' I 1 4 10 30 Time Hours 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. Austenitized at Ac 3 + 30 °C for 12 rain. Bs, bainite start; B o bainite finish; Fs, ferrite start; F o ferrite finish. Num- bers in circles indicate hardness (HV) after cooling to room temperature. Source: Ref 20 quenched rod has a hardness of 601 HB. Note that by tempering at 650 °C (1200 °F), the hardness (see x-axis) decreased to 293 HB; or to less than half the as-quenched hardness. The tensile strength has decreased from 1960 MPa (285 ksi) at a 200 °C (400 °F) tempering temperature to 965 MPa (141 ksi) at a 650 °C (1200 °F) tempering temperature. However, the ductility, repre- sented by total elongation and reduction in area, increases dramatically. The tempering process 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 than time at temperature. Temper embrittlement is possible during the tempering of alloy and low-alloy steels. This embrittlement occurs when quenched-and-tempered steels are heated in, or slow cooled through the 340 to 565 °C (650 to 1050 °F) temperature range. Embrittlement occurs when the embrit- tling elements, antimony, tin, and phosphorus, concentrate at the austenite grain boundaries and create intergranular segregation that leads to intergranular fracture. The element molybdenum 1200 "'"q'e ~-. 1050 ~ ' ~ " 900 .== ¢/) 750 i.~ 600 ~1 eel==, Ferrite + peadite 450 Martensites Bainites ~'~ ', ~* "/45 I I I I ~-'- i i 1- 3OO 4OO 500 600 7OO 8O0 Transformation temperature, °C Fig. 24 Relationship between transformation temperature and tensile strength of ferrite-pearlite, bain- itic, and martensitic steels. Source: Ref 5
t~ IL 900 / to 750 ~ QII) • " 600 • ° 450 15 20 25 Grain size (d-1/2), mm -1/2 Fig. 25 Relationship between bainite lath width (grain size) and yield strength. Source: Ref 5 has been shown to be beneficial in preventing temper embrittlement. The large variation in mechanical properties of quenched-and-tempered martensitic steels pro- vides the structural designer with a large number of property combinations• Data, like that shown in Fig. 37, are available in the Section "Carbon 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 et al. (Ref 27). The general equation for hardness is: HV = HV C + AHVMn + AHVp + AHVsi + AHVNi + AHVcr + AHVMo + AHV v (Eq 13) where HV is the estimated hardness value (Vick- ers). In order to use this relationship, one must de- termine the hardness value of carbon (HVc) from Fig. 38. For example, if one assumes that a tempering temperature of 540 °C (1000 °F) is used and the carbon content of the steel is 0.2% C, the HV c value after tempering will be 180 HV. Sec- ond, the effect of each alloying element must be determined from a figure such as Fig. 39. This graph represents a tempering temperature of 540 °C (1000 °F). Graphs representing other tempering temperatures can be found in Ref 27. 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% 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 540 °C (1000 °F) tempering temperature, the estimated hardness value for carbon is 210 HV. From Fig. 38, the hardness values for each of the other alloying elements are: Element Ccetent, % Hardaess, HV Carbon 0.41 210 Manganese 0.67 38 Phosphorus 0.023 7 Silicon 0.26 15 Nickel 1.77 12 Chromium 0.78 43 Molybdenum 0.26 55 Total hardness 380 According to Fig. 37, the hardness value after tempering at 540 °C (1000 °F) was 363 HB (see Brinell hardness values along x-axis). From the ASTM E 140 conversion table (included in the g 1600 1400 1200 1000 E 800 600 400 Structure/Property Relationships in Irons and Steels / 167 200 1 2 5 10 20 50 100 200 Cooling timel s Fig, 26 The CCT diagram for type 4340 steel austenitized at 845 °C (I 550 °F). Source: Ref 23 examples of hardness conversion tables for steels, which can be found in the Section "Glossary of Terms and Engineering Data" in this Handbook), a Brinell hardness of 363 HB equates to a Vickers hardness of 383 HV. The calculated value of 380 HV (in the table above) is very close to the actual measured value of 383 HV. Thus, this method can be used to estimate a specific hardness value after a quenching-and-tempering heat treatment for a low-alloy steel. Also, as a rough approximation, the derived Brinell hardness value can be used to estimate tensile strength by the following equation (calculated from ASTM .8o E 140 conversion table): 870 760 650 540 o ~ 425 E 315 205 95 500 1000 TS (MPa) =- 42.3 +3.6 HB (Eq 14) For the above example, a type 4340 quenched- and-tempered (540 °C, or 1000 °F) steel with a calculated hardness of 363 HB would have an estimated tensile strength from Eq 14 of 1265 MPa (183 ksi). From Table 1, this measured tensile strength of a type 4340 quenched-and-tempered (540 °C, or 1000 °F) steel is 1255 MPa (182 ksi). It is seen that quenched-and-tempered martensitic steels provide a wide range of properties. The design engineer can choose from a large number of plain-carbon and low-alloy steels. In 870 1600 650 ~ o cff 540 425 ~""~.,,~ 315 205 95 l% ~: ~ ......... Lath .... ?~ Mixea / ,~ ~:,: D20 0 0.2 0.4 0.6 0.6 Carbon, wt% Fig. 27 Effect of carbon content on M s temperature in steels. Source: Ref 6 Plate 0 1.0 1.2 1.4 1.6 1400 1200 ii o lOOO =d 800 Q_ E ~o ff 400
Page 1 and 2: Metals Handbook Desk Edition, Secon
Page 3 and 4: Table I (continued) Tensile Yield s
Page 5 and 6: Table 1 (continued) Tensile strengt
Page 7 and 8: Fig. 5 Microstructure of a fully fe
Page 9 and 10: Fig. 8 Photomic.rograph of an annea
Page 11 and 12: o~ o J~ o Fig. 15 1.6 1.2 0.8 0.4 0
Page 13: Fig. 20 Temperature, °F -1 O0 0 10
Page 17 and 18: "I- ¢= t~ "1- 900 800 700 600 500
Page 19 and 20: 1200 100o / f -- Tensile/~stStrengt
Page 21: Fig, 46 Microstructure of a typical

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