Stainless steel

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 follow:

Hardenable alloys

1. 12-14 % chromium, iron and steels, whose mechanical properties are largely dependent on the carbon content. High strength is combined with considerable corrosion resistance:
a) Stainless iron,
b) 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.

Ferritic iron

- 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.

Austenitic steels

1. Plain 18/8 Austenitic Steels,
2. Soft Austenitic,
3. Decay-proof Steel,
4. Special Purpose Austenitic Steels,
5. High Manganese Steel,
6. Heat-resisting Steels,
7. Precipitation-hardening high tensile steels.
(a) martensitic,
(b) Semi Austenitic
(c) Austenitic,

Heat-treatment

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 nickel

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 polish.

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 and copper.