| Abstract: | The final heat treatment of austenitic stainless steels of types X 5 CrNi 18 9 (1.4301) and X 2 CrNi 18 10 (1.4306) normally is annealing at 1050°C and subsequent water quenching. The resulting structure is of a metastable fcc-type. Plastic deformation, especially at low temperatures, causes martensitic transformation of these metastable structures. The transformation is accompanied by a substantial flow stress increase. This strengthening mechanism should be used in practice, e.g. to save weight. The deformed structure consists of tetragonal α′-martensite, austenite and hcp ε-martensite. Whereas α′-martensite increases continuously with deformation, the content of ε-martensite reaches a maximum value at about 5% plastic strain at 77 K. The hcp phase is only detectable by means of X-ray analysis, whilst α′-martensite can be determined quantitatively by saturation magnetisation measurement. The flow stress increase during low temperature deformation of metastable austenitic stainless steels is based on normal work-hardening by dislocation accumulation, in addition to a distinct amount of work-hardening due to martensitic transformation. Analysis of the work-hardening behaviour in the range of stable deformation (T > MD) can be used to predict the amount of normal work-hardening when deformation is performed in the instable temperature regime. Separation of the flow stress contributions according to the procedure described above enables the possible savings in weight to be predicted when using cryogenically stretched instable austenitic steels in comparison with stable grades deformed under the same conditions. |
