Steels_ Metallurgy and Applications, Third Edition

Engineering steels 261

9 0.2% proof strength, 790-830 N/mm 2
9 tensile strength, 1000-1150 N/mm 2
9 elongation, 17-19%
9 reduction of area, 27-35%

These steels also contain substantial amounts of cobalt and molybdenum and
smaller additions of titanium which promote age-hardening reactions when the
solution-treated steels are aged at temperatures of about 480~ In the 18%
Ni-Co-Mo-Ti grades, the main precipitation-strengthening phases are Ni3Mo
(orthorhombic) and a complex sigma phase (tetragonal), based on FeTi. 3~ As
illustrated in Figure 3.38, the presence of cobalt intensifies the hardening effect
of molybdenum but no ageing effects are produced in high-cobalt-molybdenum-
free compositions. 31

Commercial grades

Although a large number of maraging grades has been developed, including
compositions containing 20% and 25% Ni, the main commercial grades are based
on the five composition ranges shown in Table 3.16. In this table, the numbers
ascribed to the various grades relate to the nominal proof strength values in
units of N/mm 2. However, these grades are also designated according to their

52

48

44

40

36

32

28

24

7Co + Me

Me

Mamged martensite

aged at 800 to 900~

28 [- ~ 9 Annealed

martensite

24T ~1 I I I I I,, I I .... l , I I ,,I I I

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Molybdenum (%)

Figure 3.38 Effect of molybdenum and cobalt on hardening response in maraging steels
(After Decker et al. 31)