Steels_ Metallurgy and Applications, Third Edition

262 Steels: Metallurgy and Applications

Table 3.16 Composition ranges for 18% Ni-Co-Mo maraging steels 32

Grade Wrought Cast

18Ni 1400 18Ni 1700 18Ni 1900 18Ni 2400 17Ni 1600

Nominal 0.2% proof stress:
N/ram 2 (MPa) 1400 1700 1900 2400
tonf/in 2 90 110 125 155
1031bf/in 2 200 250 280 350
kgf/mm 2 140 175 195 245
hbar 140 170 190 240
Ni 17-19 17-19 18-19 17-18
Co 8-9 7-8.5 8-9.5 12-13
Mo 3-3.5 4.6-5.1 4.6-5.2 3.5-4
Ti 0.15-0.25 0.3-0.5 0.5-0.8 1.6-2
AI 0.05-0.15 0.05-0.15 0.05-0.15 0.1-0.2
C max. 0.03 0.03 0.03 0.01
Si max. 0.12 0.12 0.12 0.1
Mn max. 0.12 0.12 0.12 0.1
Si + Mn max. 0.2 0.2 0.2 0.2
S max. 0.01 0.01 0.01 0.005
P max. 0.01 0.01 0.01 0.005
Ca added 0.05 0.05 0.05 None
B added 0.003 0.003 0.003 None
Zr added 0.02 0.02 0.02 None
Fe Balance Balance Balance Balance

1600

105

230

165

160

16-17.5

9.5-11

4.4-4.8

0.15-0.45

0.02-0.1

0.03

0.1

0.1

0.2

0.01

0.01

None

None

None

Balance

proof strength values in ksi units, e.g. 18Ni 1700 (N/mm 2) = 18Ni 250 (ksi).
As indicated in Table 3.16, the strength of the wrought grades is increased by
increasing the levels of cobalt, molybdenum and titanium. Whereas the 18Ni
1400, 1700 and 1900 grades are aged for three hours at 480"C, the 18Ni 2400
grade requires three hours at 510*C or longer times at 480"C. The larger amount
of titanium in 18Ni 2400 leads to a greater volume fraction of FeTi sigma phase
precipitates compared with the lower strength grades.
Whereas the carbon content of these grades is restricted to low levels, the
metallurgically active content is virtually zero due to the fixation of carbon by
titanium. Elements such as silicon and manganese are also tightly controlled
in order to promote high toughness levels. Aluminium is added primarily as a
deoxidant, and although larger amounts will supplement the hardening reactions,
this leads to a loss in toughness. Boron and zirconium are added in order to retard
grain boundary precipitation, thereby improving toughness and stress corrosion
resistance. 3~
Maraging steels are generally produced by vacuum melting and sometimes
double vacuum melting (induction plus vacuum arc remelting) is employed. These
procedures are designed to achieve the following:

1. Close control over the main alloying elements and the volatilization of impu-
   rities such as lead and bismuth.


2. Minimization of segregation.