Abstract

A series of iron-carbon and iron-nickel-carbon steels have been investigated in order to assess the relative contribution of the following to the strength of martensite : 1. (a) solid solution hardening by carbon, 2. (b) carbon in the form of precipitates or segregates, and 3. (c) the substructure of the martensite, i.e. dislocation tangles or internal twinning. The results for solid solution hardening are consistent with a linear variation of the flow stress with the square root of the carbon content. Precipitation hardening occurs during the quench, or in certain cages during subsequent ageing of the martensite, and the hardening contribution of this precipitation or segregation is found to be comparable with that due to carbon in solution. In the absence of carbon the type of substructure appears to have no appreciable effect on the strength of the martensite. In the presence of carbon, the type of substructure does however affect the strength in that solid solution hardening is more pronounced for twinned martensite and only this type of martensite shows any appreciable hardening during artificial ageing after the quench. The Fleischer theory of solid solution hardening is applied in detail to the movement of a 2 〈111〉 edge and screw dislocations on {110} and {112} slip planes in tetragonal martensite. A model for the passage of slip dislocations through internally twinned martensite is developed from the original suggestion of Sleeswyk and Verbraak and used to account for the difference in solid solution hardening behaviour of lath and twinned martensite. Experimental evidence in support of the proposed model is presented.

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