Steels_ Metallurgy and Applications, Third Edition

Stainless steels 333

160 I-

2(

A J~ ,,,...

:~ 120 -

C ~ J

~ 80 -

.E 10(

>

,1=

~ 40 -

0.01% C

0.006-0.023% N

0.006% C

0.02-0.028% N

0.03% C

0.06-0.033% N

0.05% C

0.009-0.044% N

0 -0

40 80 120 160

Test temperature (~

Figure 4.20 Effect of carbon and nitrogen on the impact properties of 18% Cr 2% Mo
steel (After Hooper et al. 17)

However, the argon-oxygen decarburization (AOD) process has also been used
for the production of these grades. Thus in the mid-1960s major commercial
interest was stimulated in low interstitial ferritic stainless steels which offered
the potential of cheaper alternatives to austenitic grades, particularly in relation
to resistance to stress corrosion.
The influence of carbon and nitrogen on the impact properties of an 18% Cr
2% Mo base steel was investigated by Hooper et al. 17 and the data are shown
in Figure 4.20. From this figure, it is apparent that a reduction in carbon content
to 0.01% produces a marked improvement in the impact transition behaviour,
whereas a change in nitrogen content has little effect. However, the grain size
of these steels was about ASTM 0 and even at low interstitial levels, the impact
transition temperature is still above room temperature. Therefore, in order to
produce attractive impact properties, the grain size must be reduced, i.e. to a
level below ASTM 8.
In addition to a reduction in carbon and nitrogen contents, the new range of
ferritic steels contained higher levels of chromium and additions of molybdenum
and nickel. Streicher Is has reviewed the development of these grades and the
types of composition that have been produced in commercial quantities are shown
in Table 4.10. Although nitrogen has a relatively small effect on the impact
properties, its low solubility in ferritic grades means that chromium nitride forms
very readily and contributes to the susceptibility to intergranular attack. Thus
many of the high-alloy ferritic grades must be stabilized with titanium or niobium,
even when the carbon and nitrogen has been reduced to very low levels.
As illustrated in Table 4.11, most of these grades are resistant to stress corro-
sion cracking in MgCI2 and NaC1 solutions and they also display good resistance
to pitting and crevice corrosion. The inclusion of nickel in steels such as 28% Cr