Effect of Martempering on Thermal Stress and Strain

Summary

Martempering is well established as a method by which the stresses and strains generated during the quenching of a steel component may be controlled. In this process the component is held at an intermediate temperature in a salt bath subsequent to the initial cooling from the austenitization temperature, but before the final quench to ambient. Doubts have been raised about the fundamental assumptions on which the process is based, since the absence of a vapour-blanket stage in the initial cool in the salt bath produces very high surface heat-transfer coefficients at the start of the process, when the material is very soft. It has been the purpose of the present investigation to verify these assumptions by the use of a mathematical model of the stress-generation process. Modification of the process to include an oil quench from the salt bath was not beneficial, as the stresses and strains produced at the end of the quench were very similar to those produced by a conventional oil quench from the austenitization temperature. The calculated residual stress and strain distributions were checked against the corresponding experimental values. Agreement was reasonably good, except for a small but uniform discrepancy in the case of the strains.