Steels_ Metallurgy and Applications, Third Edition

Stainless steels 341

addition of hydrogen and helium to the shielding gas enabled the welding speed

to be increased substantially but, in both cases, the differential in weldability

between the high- and low-sulphur steels was still preserved.

In order to achieve good weld penetration, fabricators may well specify a

minimum of 0.01% S but high levels of sulphur impair the corrosion resistance

of stainless steels. Therefore the sulphur content needs to be controlled to a

narrow range to ensure good weldability on the one hand and the avoidance of

pitting corrosion on the other.

Cold working of stainless steels

The cold-working characteristics of stainless steels are important in relation to:

1. The conversion of hot band to cold-reduced gauges.


2. The production of components by cold forming.

Austenitic stainless steels are characterized by high rates of work hardening

which limit the amount of cold deformation that can be undertaken before an

annealing treatment is required. However, the work-hardening behaviour is influ-

enced markedly by chemical composition and can often dictate the grade of steel

that is used for a particular application.

Role of alloying elements

The austenite to martensite transformation in a material can be discussed in terms

of thermodynamics and this aspect has been covered in detail by Kaufman and

Cohen) 6 In Figure 4.24, FV and F a' represent the chemical free energies of

austenite and martensite which vary with temperature in the manner shown. At

temperature To, the two phases have equal free energies, but above To, austenite

is thermodynamically stable relative to martensite, and below To, martensite is

the more stable phase. On cooling from above To, the austenite to martensite

reaction might be expected to occur at To but, in fact, this reaction does not

occur until the temperature is lowered to Ms, which in iron-base alloys may be of

the order of 2000C below To. At Ms, the austenite has derived sufficient energy

from undercooling to overcome the barriers of nucleation and the inteffacial

strains that inhibit the growth of the bcc nucleus in its foreign fcc environment.

On heating from a low temperature, the reverse reaction at As may also require

superheat and As is generally as far above To as Ms is below. Alloying elements

depress the transformation from austenite to martensite and in very highly

alloyed stainless grades, the theoretical To temperature will be below the absolute

temperature. However, in the leaner alloy grades, To is above and Ms is below

room temperature.

When stress is applied to an austenitic stainless steel during cold working, the

mechanical energy interacts with the thermodynamics of the martensite reaction,

as demonstrated by Patel and Cohen. 27 Thus in Figure 4.25, F y and F ~' repre-

sent the conditions in the unstressed system and Fv' and F ~' are the relative