Immersion quenching of steel parts is one of the most widely used processes for achieving hardness in steels. A comprehensive modeling treatment of quenching requires the description of the heat flux as a function of surface temperature, so that the data can be used in numerical calculations. The heat transfer phenomena on the surface of the component being heat treated is quite complex, with film boiling in the temperature regimes of our interest, followed by nucleate boiling. At lower temperatures the heat transfer is more by the convective mode through a boundary layer.

In this paper, the problem of surface heat flux estimation during quenching of alloy steel specimen is treated as an Inverse Heat Conduction Problem (IHCP). During quenching of alloy steel, latent heat is liberated due to phase change. The latent heat liberated during phase transformation is treated as a source term in the formulation of the IHCP.

The austenite decomposition during quenching is modeled by integrating the Fe-C equilibrium diagram and the TTT diagram of the specific grade of steel, for establishing the critical temperatures.

Cylindrical alloy steel specimens of AISI 4140 grade, 20 mm diameter, 50 mm long were instrumented with thermocouples and the cooling curves were recorded during quenching in water as well as during air cooling. The cooling curves near the surface were then used as input data for estimating the surface heat flux by the inverse heat conduction method. The microstructure and hardness at several points across the section of the specimen were obtained by metallurgical examination. The predicted microstructure and hardness were compared with experimental data which were found in good agreement. The cooling curve at the core of the specimen recorded during water quenching was compared with the cooling curve computed by the inverse calculation and the results are reported.