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cast iron Combined carbon, % Hardness, HB Combined carbon, % Hardness, HB As-cast 0.69 217 0.70 2.55 After quenching from, °C (°F) Ã650 (1200) 0.54 207 0.65 2.50 Ã675 (1250) 0.38 187 0.63 241 Ã705 (1300) 0.09 170 0.59 229 Ã730 (1350) 0.09 143 0.47 217 Ã760 (1400) nil 137 0.45 197 Ã790 (1450) 0.05 143 0.42 207 Ã815 (1500) 0.47 269 0.60 444 Ã845 (1550) 0.59 444 0.69 514 Ã870 (1600) 0.67 477 0.76 601 Note: Specimens were 30.5 mm diam bars, 51 mm long (1.2 in. diam bars, 2 in. long) quenched in oil from temperatures shown.Source: Ref 18 Ordinarily, gray iron is furnace hardened from a temperature of 860 to 870 °C (1575 to 1600 °F). This results in a combined carbon content of about 0.7% and a hardness of about 45 to 52 HRC (415 to 514 HB) in the as-quenched condition. The actual hardness of the martensitic matrix is 62 to 67 HRC, but the presence of graphite causes a lower indicated hardness (Table 14 ). Temperatures much above this are not advisable because the as-quenched hardness will be reduced by retained austenite. Oil is the usual quenching medium for through hardening. Quenching in water may be too drastic and may cause cracking and distortion unless the castings are massive and uniform in cross section. Hot oil and hot salt are sometimes used as quenching media to minimize distortion and quench cracking. Water is often used for quenching with flame or induction hardening if only the outer surface is to be hardened. Hardenability of unalloyed gray iron is about equal to that of low-alloy steel. Hardenability can be measured using the standard end-quench hardenability test employed for steels. The hardenability of cast iron is increased by the addition of chromium, molybdenum, or nickel. Gray iron can be made air hardenable by the addition of the proper amounts of these elements. Some typical data on the hardenability of plain and alloy irons are shown in Table 26 . Compositions of the irons are listed in Table 27 . Table 26 Hardenability data for gray irons quenched from 855 °C (1575 °F) See Table 27 for compositions. Distance from quenched end Hardness, HRC and strength as the normal castings. However, a foundry producing a base iron with 1.7% Si and 3.1% C for a heavy casting would add 0.5 to 0.8% Si to decrease hardness and chill when pouring this iron in light castings. Depending on the strength desired in the final iron, the carbon equivalent of the base iron may vary from approximately 4.4% for weak irons to 3.0% for high-strength irons. The method of producing the base iron will affect mechanical properties and the alloy additions to be made, because factors such as type and percentage of raw materials in the metal charge, amount of superheat, and cooling rate of the iron after pouring all affect the properties. The base iron used for alloying will vary considerably from foundry to foundry, as will the alloying elements selected to give the desired mechanical properties. However, parts produced from different base irons and alloy additions can have the same properties and performance in service. Heat Treatment Gray iron, like steel, can be hardened by rapid cooling or quenching from a suitable elevated temperature. The quenched iron may be tempered by reheating in the range from 150 to 650 °C (300 to 1200 °F) to increase toughness and relieve stresses. The quenching medium may be water, oil, hot salt, or air, depending on composition and section size. Heating may be done in a furnace for hardening throughout the cross section, or it may be localized as by induction or flame so that only the volume heated above the transformation temperature is hardened. In the range of composition of the most commonly used unalloyed gray iron castings, that is, about 1.8 to 2.5% Si and 3.0 to 3.5% TC, the transformation range is about 760 to 845 °C (1400 to 1550 °F). The higher temperature must be exceeded in order to harden the iron during quenching. The proper temperature for hardening depends primarily on silicon content, not carbon content; silicon raises the critical temperature. During the heating of unalloyed gray iron for hardening, graphitization of the matrix frequently begins as the temperature approaches 600 to 650 °C (1100 to 1200 °F) and may be entirely completed at a temperature of 730 to 760 °C (1350 to 1400 °F). This latter range is used for maximum softening. The changes in combined carbon content and hardness that occur upon heating and quenching of both alloyed and unalloyed gray iron are shown in Table 25 . Table 25 Hardness of quenched samples of gray iron Condition Plain cast iron Cr-Ni-Mo cast iron Combined carbon, % Hardness, HB Combined carbon, % Hardness, HB As-cast 0.69 217 0.70 2.55 After quenching from, °C (°F) Ã650 (1200) 0.54 207 0.65 2.50 Ã675 (1250) 0.38 187 0.63 241 Ã705 (1300) 0.09 170 0.59 229 Ã730 (1350) 0.09 143 0.47 217 Ã760 (1400) nil 137 0.45 197 Ã790 (1450) 0.05 143 0.42 207 Ã815 (1500) 0.47 269 0.60 444 Ã845 (1550) 0.59 444 0.69 514 Ã870 (1600) 0.67 477 0.76 601 Note: Specimens were 30.5 mm diam bars, 51 mm long (1.2 in. diam bars, 2 in. long) quenched in oil from temperatures shown.Source: Ref 18 Ordinarily, gray iron is furnace hardened from a temperature of 860 to 870 °C (1575 to 1600 °F). This results in a combined carbon content of about 0.7% and a hardness of about 45 to 52 HRC (415 to 514 HB) in the as-quenched condition. The actual hardness of the martensitic matrix is 62 to 67 HRC, but the presence of graphite causes a lower indicated hardness (Table 14 ). Temperatures much above this are not advisable because the as-quenched hardness will be reduced by retained austenite. Oil is the usual quenching medium for through hardening. Quenching in water may be too drastic and may cause cracking and distortion unless the castings are massive and uniform in cross section. Hot oil and hot salt are sometimes used as quenching media to minimize distortion and quench cracking. Water is often used for quenching with flame or induction hardening if only the outer surface is to be hardened. Hardenability of unalloyed gray iron is about equal to that of low-alloy steel. Hardenability can be measured using the standard end-quench hardenability test employed for steels. The hardenability of cast iron is increased by the addition of chromium, molybdenum, or nickel. Gray iron can be made air hardenable by the addition of the proper amounts of these elements. Some typical data on the hardenability of plain and alloy irons are shown in Table 26 . Compositions of the irons are listed in Table 27 . Table 26 Hardenability data for gray irons quenched from 855 °C (1575 °F) See Table 27 for compositions. Distance from quenched end Hardness, HRC ASM Handbook,Volume 1 Gray Iron 01 Sep 2005 Copyright ASM International. All Rights Reserved. Page 59 mm 1=16 in. increments Plain Iron Mo(A) Mo(B) Ni-Mo Cr-Mo Cr-Ni-Mo 3.2 2 54 56 53 54 56 55 6.4 4 53 56 52 54 55 55 9.5 6 50 56 52 53 56 54 12.7 8 43 54 51 53 55 54 15.9 10 37 52 50 52 55 53 19.0 12 31 51 49 52 54 53 22.2 14 26 51 46 52 54 52 25.4 16 26 49 45 52 54 53 28.6 18 25 46 45 52 53 52 31.8 20 23 46 44 51 50 51 34.9 22 22 45 43 47 50 50 38.1 24 22 43 44 47 49 50 41.3 26 21 43 44 47 47 49 44.4 28 20 40 41 45 47 48 47.6 30 19 39 40 45 44 50 50.8 32 17 39 40 45 41 47 54.0 34 18 36 41 44 38 46 57.2 36 18 40 40 45 36 45 60.3 38 19 38 37 45 34 46 63.5 40 22 38 36 42 35 46 66.7 42 20 35 35 42 32 45 Source: Ref 18 Table 27 Compositions of irons for which hardenability data are given in Table 26 Iron Composition TC CC(a) GC(b) Mn Si Cr Ni Mo P S Plain 3.19 0.69 2.50 0.76 1.70 0.03 ... 0.013 0.216 0.097 Mo(A) 3.22 0.65 2.57 0.75 1.73 0.03 ... 0.47 0.212 0.089 Mo(B) 3.20 0.58 2.62 0.64 1.76 0.005 Trace 0.48 0.187 0.054 Ni-Mo 3.22 0.53 2.69 0.66 2.02 0.02 1.21 0.52 0.114 0.067 Cr-Mo 3.21 0.60 2.61 0.67 2.24 0.50 0.06 0.52 0.114 0.071 Cr-Ni-Mo
