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.

Stainless Steels- contain sufficient amounts of chromium that they are NOT considered low alloy steels. These require reduced cutting speeds (approx ½), finer feeds, lighter cuts, and sharp carbide tooling. Because the thermal conductivity of stainless steel is 1/3 that of carbon steel, the heat is held in a localized area when welding. This tends to localize the stress. The metal that is hot wants to expand but is hemmed in by the cold adjacent metal. When the material cools and contracts, the cooler metal does not move, with the result that cracks form during the solidification. Methods of dealing with these problems are available.

The corrosion resistance is imparted by the formation of a strong adherent chromium oxide on the surface of the metal. Good resistance to corrosive media encountered in the chemical industry can be obtained by the addition of 4 to 6% chromium to low carbon steel.

AISI classes these with a three digit number for Stainless

200 series = chromium, nickel, manganese (structure is austenitic)

300 series = chromium and nickel (structure is austenitic)

400 series = chromium only (Structure is ferritic or martensitic)

500 series = low chromium (<12%) Martensitic

Types of Stainless Steels

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.  Used for trim moldings and decorative applications and where buffing to a mirror finish is important.

- 25/30% chromium iron for furnace parts, resistant to sulphur compounds. Forms sigma phase additions of Nb and Mo prevent excessive grain growth.

When chromium is added an increase in temperature range is seen by which ferrite is the stable structure. That is, ferrite is at all temperatures below solidification. Has low amount of carbon to chromium ratio; therefore hardening by heat-treating is not done.

Advantages:

_ Readily weldable (no martensite can form at the welds because

chromium retards the bcc martensite.

_ Cheapest of the stainless steels.

_ Magnetic

_ Easiest of all to machine.

Disadvantages :

_ Poor ductility

_ Poor formability because of the bcc crystal structure.

Austenitic steels

Austenitic stainless steels - Best corrosive resistance, but hardenable only by cold working. Not heat treatable, but cold workable.With both nickel and chromium, the fcc austenite is stabilized at room temperature to produce a stainless steel.

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,

Advantages:

_ Best corrosive resistance,

_ Highest of all for strength at high temperatures,

_ Best of all for ductility at low temperatures.

_ Nonmagnetic,

_ Highly resistant to chemical corrosion (except one), mirror polish,

_ Attractive appearance.

_ Formability is outstanding characteristic of the fcc.

_ Strengthen drastically when cold worked

Disadvantages:

_ Corrosive in hydrochloric acid and other halide acids and salts.

_ Most expensive

Water Quench Cold Rolled

Yield Strength 38ksi 117ksi

Tensile Strength 90 ksi 140 ksi

Elongation in 2" 68% 11%

Martensitic Steels

High amount of carbon to chromium ratio therefore can be heat treated. More corrosive resistant than ferrite, but still corrosive. This material can be austenitic (see next type) at high temperatures. At the high temperature, carbon can be dissolved in the fcc austenite, which in turn is quenched to form a bcc martenitic structure. So the steel is austenitized, quenched, then stress relief tempered.

Advantages:

_ Increase in strength

_ more corrosive resistant than Ferritic

_ ability to hold an “edge”

_ good for impact

_ good up to 300 ksi when hardened.

Disadvantages:
May be susceptible to red rust when annealed for machining or fabrication. Cost 1 ½ times more than the Ferritic stainless steels.

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.