Page 74 - Mechanical Behavior of Materials

Section 3.3 Irons and Steels 75

Table 3.5 Summary of the AISI–SAE Designations for Common Carbon and
Low-Alloy Steels
Designation 1 Approx. Alloy Content, % Designation Approx. Alloy Content, %
Carbon steels Nickel–molybdenum steels
10XX Plain carbon 46XX Ni 0.85 or 1.82; Mo 0.25
11XX Resulfurized 48XX Ni 3.50; Mo 0.25
12XX Resulfurized and
rephosphorized
15XX Mn 1.00 to 1.65
Manganese steels Chromium steels
13XX Mn 1.75 50XX(X) Cr 0.27 to 0.65
51XX(X) Cr 0.80 to 1.05
52XXX Cr 1.45
Molybdenum steels Chromium–vanadium steels
40XX Mo 0.25 61XX Cr 0.6 to 0.95; V 0.15
44XX Mo 0.40 or 0.52
Chromium–molybdenum steels Silicon–manganese steels
41XX Cr 0.50 to 0.95; 92XX Si 1.40 or 2.00;
Mo 0.12 to 0.30 Mn 0.70 to 0.87;
Cr 0 or 0.70
Nickel–chromium–molybdenum steels Boron steels 2
43XX Ni 1.82; Cr 0.50 YYBXX B 0.0005 to 0.003
or 0.80; Mo 0.25
47XX Ni 1.45; Cr 0.45;
Mo 0.20 or 0.35
81XX Ni 0.30; Cr 0.40; Mo 0.12
86XX Ni 0.55; Cr 0.50; Mo 0.20
87XX Ni 0.55; Cr 0.50; Mo 0.25
94XX Ni 0.45; Cr 0.40; Mo 0.12
1
Notes: Replace “XX” or “XXX” with carbon content in hundredths of a percent, such as AISI
2
1045 having 0.45% C, or 52100 having 1.00% C. Replace “YY” with any two digits from
earlier in table to indicate the additional alloy content.
of carbon present exceeds the 2% that can be held in solid solution at elevated temperature, and in
most cast irons the excess is present in the form of graphite.
Gray iron contains graphite in the form of flakes, as seen in Fig. 3.7 (left). These flakes easily
develop into cracks under tensile stress, so that gray iron is relatively weak and brittle in tension. In
compression, the strength and ductility are both considerably higher than for tension. Ductile iron,
also called nodular iron, contains graphite in the more nearly spherical form of nodules, as seen in
Fig. 3.7 (right). This is achieved by careful control of impurities and by adding small amounts of
magnesium or other elements that aid in nodule formation. As a result of the different form of the
graphite, ductile iron has considerably greater strength and ductility in tension than gray iron.
White iron is formed by rapid cooling of a melt that would otherwise form gray iron. The
excess carbon is in the form of a multiphase network involving large amounts of iron carbide,