A maior rede de estudos do Brasil

Grátis
1618 pág.
ASM Metals Handbook Volume 01 - Properties and Selection Irons, Steels, and High-Performance Allo

Pré-visualização | Página 41 de 50

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