The influence of visco-elastic processes on the generation of thermal stresses in quenched low alloy steel plates.

The visco-elastic effect in quenched specimens of 835M30 steel was described quantitatively by the use of the standard linear solid, which allowed the calculation of the thermal stress and strain during quenching in water, oil and in a martempering situation. The introduction of viscous flow into the calculation produced a marked improvement in the degree of agreement between the calculated and experimental residual stresses after an oil quench, although this was accompanied by a small reduction in the corresponding agreement in the case of water quench. The use of isothermal stress relaxation data in a continuous cooling situation was dealt with by the use of several models that cover a range of situations that varied from progressive hardening during the quench to one where recovery was predominant at all times. The use of martempering was of limited value, although an air cool following an intermediate salt bath treatment above the Ms temperature prevented the generation of thermal stresses during the formation of martensite. The use of an oil quench following the salt bath treatment was of no value, and none of the combinations of heat treatment possible during martempering significantly reduced the distortions to a lower level than was obtained by a conventional oil quench from 850°C. The mathematical model was also extended to include the effect of transformation plasticity, viz. the effect of an applied stress on the volume change that accompanied a phase transformation, as a consequence of heterogeneous plastic flow. This led to an excellent level of agreement between calculation and experiment in the case of residual stresses: it also produced adeterioration in the case of residual strains. Although some further work is required, the mathematical model has been developed sufficiently for it to be of practical value.