Steels_ Metallurgy and Applications, Third Edition

330 Steels: Metallurgy and Applications

Although clearly beneficial in the formation of passive films and in suppressing

pitting corrosion, both chromium and molybdenum appear to have a variable

effect on the SCC behaviour of stainless steels. These effects may well be linked

to austenite stability and a reduction in stacking fault energy which promotes

crack propagation.

Temperature is a very important parameter in the SCC behaviour of stainless

steels and is rarely observed at temperatures below about 60"C.

High-alloy stainless steels

As indicated at the beginning of this chapter, most of the popular grades of

stainless steel were formulated in the 1920s and 1930s and have undergone little

change to the present day. However, in the intervening period, a large amount of

research effort has been devoted to the development of new grades but relatively

few of these compositions have enjoyed major commercial exploitation.

High.alloy austenitic/duplex grades

From the basic Type 304 (18% Cr, 9% Ni) composition, a number of develop-

merits have taken place which were designed to improve a particular property.

These include:

1. The addition of up to 0.5% N to improve the strength (solid solution strength-
   ening) or to compensate for reductions in the nickel content (e.g. Types 201
   and 202).


2. The addition of elements such as aluminium, titanium or phosphorus to
   produce precipitation-strengthening reactions.


3. The inclusion of high levels of nickel and molybdenum and moderate levels of
   copper and nitrogen to improve the resistance to stress corrosion or reducing
   acids.

The properties of steels in the first category were discussed briefly on p. 316.
In contrast to the situation in low-carbon strip and structural grades, very little
use has been made of Hiproof-type compositions which contain about 0.2% N
and provide 0.2% PS values of about 330 N/mm 2 (cf. 250 N/mm 2 in standard
grades). Having overcome the welding problems that Were encountered when
these steels were first introduced, it is surprising that they are not yet being used
in large quantities. Whereas the 200 series have enjoyed commercial success in
the United States, they have always been regarded as inferior substitutes to the
traditional 300 steels in the UK and, as stated earlier in the text, they also present
problems in production.
The metallurgy of precipitation-strengthened austenitic stainless steels was the
subject of detailed research work in the 1950s and 1960s. 11 Depending upon the
precipitation system and heat treatment, such steels can develop 0.2% PS values
up to 750 N/mm 2, i.e. three times that produced in Type 304 in the solution-
treated condition. However, such grades were produced in very limited quantities,
probably for the following reasons: