The stainless steels owe their resistance to corrosion to the presence of chromium. Brearley discovered this fact more or less accidentally in 1913. Today, there is a range of steels from the plain chromium variety to those containing up to six alloying elements in addition to the usual impurities.
The stainless steels owe their
resistance to corrosion to the presence of chromium. Brearley
discovered this fact more or less accidentally in 1913. Today, there
is a range of steels from the plain chromium variety to those
containing up to six alloying elements in addition to the usual
impurities. A simple classification of the steels
1. 12-14 % chromium, iron and
steels, whose mechanical properties are largely dependent
on the carbon content. High strength is combined with considerable
a) Stainless iron,
Stainless steel, (mild, medium and hard)
2. Secondary hardening ,10-12% chromium, 0.12% C, with small additions Mo, V, Nb, Ni; a steel with ultimate stress of 927 MPa is used for gas turbine blades.
3. High chromium steel 17% Cr, 0.15% C, 2.5% Ni (431 S29). It has a higher resistance to corrosion than iron, due to higher chromium content. It is used for pump shafts, valves and fittings subjected to high temperature and high-pressure steam, but is unsuitable for acid conditions.
High carbon, 0.8 C, 16.5 Cr, 0.5 Mo
steel (oil quenched at 1025°C and tempered at 100„aC to give hardness of 700 HB) is used for
stainless ball bearings and instruments such as scalpels.
- 16/18% chromium
rustless iron with low carbon content (430 S15). It has high
resistance to corrosion but low impact and cannot be refined by
heat-treatment alone. Prolonged service at 480°C can cause
embrittlement. It is used for motor car trim.
- 25/30% chromium iron for furnace
parts, resistant to sulphur compounds. Forms sigma phase additions
of Nb and Mo prevent excessive grain growth.
1. Plain 18/8 Austenitic Steels,
2. Soft Austenitic,
3. Decay-proof Steel,
Purpose Austenitic Steels,
5. High Manganese Steel,
7. Precipitation-hardening high tensile
(b) Semi Austenitic
The hardening alloys possess
critical ranges comparable with ordinary carbon steels, and can,
therefore, be hardened, tempered and refined by heat-treatment which
does not depend on recrystallisation after cold working.
The ferritic and normal austenitic
steels, on the other hand, are not amenable to such treatment. Only
cold work with subsequent heat-treatment involving recrystallisation
can be employed to refine large grained material.
Effects of chromium and
It will be readily appreciated that
chromium is the chief alloying element in iron and steel for
inhibiting corrosion. This resistance is not due to the inertness of
the chromium, for it combines with oxygen with extreme rapidity, but
the oxide so formed is very thin and stable, continuous and
impervious to further attack.This property is, fortunately,
conferred upon its solid solution in iron, becoming very marked as
the amount exceeds 12 % in low carbon steels.
Thus, in oxidising environments,
such as nitric acid, the high chromium steel is initially attacked
at the same rate as ordinary plain steel, but it rapidly builds up
an oxide film, known as a self-healing passive-film, which
efficiently protects the underlying metal. This film has actually
been isolated by U. R. Evans. The thickness of the film and its
Cr2O3 content increases with the degree of
In oxidising media any defect in
the film which may arise through abrasion will be quickly repaired
and such steel is quite satisfactory in the atmosphere, but the film
does not offer sufficient permanent resistance to the less oxidising
action of hydrochloric and sulphuric acids, except in very dilute
solutions. Nickel has a low solubility in these acids and thus, with
8 to 10 % of nickel in addition to chromium, the steel is immune
from attack by nitric acid and the resistance to the other acids is
markedly increased. Hence it is very evident why the 18/8 steels
have such extensive uses. Their resistance to particular acids have
been further improved by additions of elements such as molybdenum