Steels_ Metallurgy and Applications, Third Edition

316 Steels: Metallurgy and Applications

Table 4.5 Compositions of 200 series steels

C% Si% Mn% Cr% Ni% N%

Type 201 0.1 0.5 6.5 17 4.5 0.25

Type 202 0.1 0.5 8.75 18 5 0.25

An alternative method of preventing the formation of chromium carbide in
stainless steels is to reduce the carbon content to a low level. Thus Type 304L
(0.03% C max.) is a lower carbon variety of Type 304 and is now used
extensively in applications calling for resistance to intergranular attack in the
welded condition. In a similar manner, Type 316L is the lower carbon version
of Type 316.
Type 310 (25% Cr, 20% Ni) represents the most highly alloyed composition in
the popular range of austenitic stainless steels and provides the greatest resistance
to corrosion and oxidation.
During the early 1950s, there was a scarcity of nickel in the United States
and this led to the development of austenitic stainless steels in which some
of the nickel was replaced by alternative austenite-forming elements. The most
successful steels of this type were Types 201 and 202, which have the mean
compositions shown in Table 4.5.
These steels were developed as alternatives to Types 301 (17% Cr, 7% Ni) and
302 (18% Cr, 8% Ni), in which reductions in the nickel content were compen-
sated by large additions of manganese and nitrogen. The addition of 0.25% N
also causes substantial solid solution strengthening and therefore the 200 series
steels have high tensile properties. These steels were used extensively for the
production of railway carriages in the United States but found little application
in the UK. However, they are difficult to produce because of excessive refractory
attack and problems in descaling due to their high manganese contents.
Nitrogen additions of about 0.2% are also made to standard grades such as
Types 304 and 316 in order to generate high proof stress values. In the UK, these
steels are marketed under the Hyproof tradename and provide 0.2 PS values of
about 330 N/mm 2 compared with 250 N/mm 2 in the standard grades (typically
0.04% N).
The strength of austenitic steels can also be increased by warm working, i.e.
by finishing rolling at temperatures below 950~ As illustrated in Figure 4.11,
this results in a major increase in the 0.2% proof stress of standard grades such as
Type 304 and 321, but significantly higher levels of strength can be obtained by
warm working compositions which are substantially solid solution strengthened
with nitrogen. Steels of this type have found limited application in pressure
vessels and also for high-strength concrete reinforcement.

Controlled transformation stainless steels

In the 1950s, interest was generated in an entirely new type of stainless steel
that was austenitic in the as-delivered, solution-treated condition but which
transformed to martensite by means of a simple heat treatment. Such materials