AbstractThe literature relating to the thermal stabilisation of austenite has been reviewed with particular reference to the mechanisms proposed by various workers to account for the phenomenon. The complex interactions between martensite transformation and stress have also been discussed in order to assess the feasibility of a stabilisation mechanism controlled by stress-relaxation.
The kinetics of stabilisation have been studied using an electrical resistance technique in two 0.5% carbon steels containing 19% and 24% nickel, after treatments at temperatures between -100°C and -40%. In addition, the response of the same materials to externally applied stress has been investigated. Stabilisation has been shown to develop extremely rapidly at sub-zero temperatures in both alloys; the degree of stabilisation, Q. increases with increase in ageing time, prior martensite content and ageing temperature above -60°C.
The degree of stabilisation is sensitive to the isothermal transformation to martensite (the 'tail') which immediately follows the initial quench; lower Q values are obtained when this transformation is curtailed. This effect complicates the dependence of Q on ageing temperature and prior martensite content. Short quench treatments produce a C-curve dependence of Q on ageing temperature with a minimum Q value occurring between -80°C and -60°C. The C-curve behaviour can be suppressed in the 19% Ni steel, but not in the 24% Ni steel by extending the quench treatment beyond the time sufficient for the 'tail' transformation.
The stabilisation is thermally reversible and the effects of changes ageing temperature can be predicted from a knowledge of the time dependence of Q at the individual temperatures involved. However, reversibility in the present context does not necessarily require the stabilisation mechanism itself to be reversible.
The degree of stabilisation produced in the 19% Ni alloy is greater than that in the 24% Ni alloy for comparable conditions, but the kinetics of stabilisation are essentially the same in the two alloys, in spite of differences in the martensite habit plane and morphology.
Relaxation of an externally applied stress occurs in Pe-Ni-C alloys at -78°C and is accompanied by transformation to martensite. At least two mechanisms can produce relaxation, one of which involves stress-induced isothermal transformation to martensite. Transformation can be restarted following a stabilising treatment by either an externally applied stress or undercooling; correlation has been revealed between the results of stress relaxation tests and Q values.
None of the previously proposed mechanisms can account for all the features of stabilisation encountered arid the mechanism responsible has not been clearly isolated.
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