Critical Heat Transfer Coefficient Test Method for Determining Gas Quenching Steel Hardenability

Gas quenching is developing rapidly with the intent to replace liquid quenching to reduce distortion and residual stress. Medium and high hardenability steels are needed for gas quenching, since the quenching power is lower compared to liquid quenching. The Jominy end quench test and Grossmann test, designed for liquid quenching steel hardenability, are not suitable for gas quenching. In order to determine gas quenching steel hardenability, a new test method is required. The critical heat transfer coefficient (HTC) is defined as the HTC required to form 50% martensite at the center of a steel alloy rod with 25.4mm (1 inch) diameter. The critical HTC is the analog to the Grossmann critical diameter. The details of the experimental approaches and test methodology were discussed. The gas quenching hardenability of AISI 4140 steel is proved to be better than AISI 52100 steel by using critical HTC test.