Steels_ Metallurgy and Applications, Third Edition

314 Steels:MetallurgyandApplications

Table 4.4

Property Base+ Base+ Base + 0.37% Base + 0.33%

0.015% N 0.045% N Nb, 0.013% N Nb, 0.043% N

TS (N/mm 2) 936 1055 981 1084

0.2% PS (N/ram 2) 772 871 815 962

Elongation 4~/A% 22.8 22.4 18.1 19.1

Charpy V-J at RT 94 47 104 52

The action of niobium in promoting tempering resistance is different to that of

the other elements discussed above in that it intensifies the secondary hardening

reaction by increasing the lattice parameter of the precipitate relative to that of the

ferrite matrix. Thus the effect can be superimposed on other tempering retarding

reactions, involving the stabilization of precipitates, in order to gain additional

benefit. This is demonstrated in the tensile data given in Table 4.4 for 12% Cr

2.5% Ni 1.5% Mo 0.3% V steels, tempered 650~ 1 h. These data also indicate

that nitrogen produces a powerful strengthening effect, albeit with a significant

loss in toughness.

As indicated earlier, the carbon content of 12% Cr steels can be increased

to high levels in order to promote higher hardness levels but at the expense

of toughness and weldability. Notable examples are cutlery steels containing

12% Cr and 0.3% C and stainless razor steels containing 12% Cr and 0.6% C.

In the latter steel, substantial amounts of carbon remain out of solution as M23C6

carbides after solution treatment at 1050~ so that the full martensitic hardness

associated with 0.6% C is not realized. However, the presence of carbides in the

microstructure improves the abrasion resistance of such steels.

Ferritic stainless steels

Ferritic stainless steels can contain up to 30% Cr with additions of other elements

such as molybdenum for improved pitting corrosion resistance and titanium or

niobium for improved resistance to intergranular corrosion. These forms of corro-

sion will be described in a later section. However, the bulk of the requirement

for ferdtic stainless steels is satisfied by two major grades, namely Type 430

and 434. As shown in Table 4.2, Type 430 is a 17% Cr steel and at the normal

solution treatment temperature of 9500C, this steel generally contains a propor-

tion of austenite which transforms to martensite on cooling to room temperature.

However, on tempering at 750~ the martensite breaks down to ferrite and

carbide, giving a microstructure which is essentially fully ferritic.

The corrosion resistance of stainless steels in chloride environments is

improved substantially with the addition of molybdenum, and Type 434 (17% Cr,

1% Mo) is the most common grade of this type within the ferritic range.

As illustrated later, the corrosion resistance of stainless steels can be seriously

impaired by the precipitation of chromium carbides at the grain boundaries. One

method of overcoming this problem is to add elements such as titanium and