Technological Application of the Martensitic Transformation of Some Austenitic Stainless Steels

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 fee-type.Plastic deformation, especially at low temperature, causes martensitic transformation of these metastable structure.The transformation is accompanied by a substantial flow stress increase.This strengthening mechanism should be used in prac tice, e.g. for saving weight /1,2/.The deformed structure consists of tetragonal a'-martensite, austenite and hep c-martensite.Whereas a 1 -martensite increases continuously with deformation, the content of c-martensitc reaches a maximum value at about 5% plastic strain at 77 K.The hep phase is only detectable by means of x-ray analysis whilst a'-martensite can be determined quantitatively by saturation magnetisation measurement.The flow stress increase in low temperature deformation of meta stable austenitic stainless steels is based on normal work-har dening by dislocation accumulation, in addition to a distinct amount of work-hardening due to martensitic transformation.Analysis of the work-hardening behavior in the range of stable deformation (T > M ) can be used to predict the amount of normal work-hardening when deformation is performed in the instable temperature regime.Figure 1 shows the normalized work-hardening coefficient (e) vs. the normalized flow stress (o) for the grade 304 (1.4301) in the temperature range from 293 K to 423 K. Within this range deformation occurs nearly without martensitic transformation.The saturation stresses o in the instable re gime can be evaluated by extrapolating stable values to lower temperatures.On the basis of figure 1 the corresponding flow curves can then be predicted /3/.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.