Influence of Specimen Design on Maximum Heating Rate and Temperature Variation During Induction Heating in an 805L Dilatometer

Commercially, carbon steels are scan induction heated at heating rates on the order of 1,000 ^oCs^-1 for surface hardening. The high precision 805L dilatometer employs induction heating and is often used to study transformation characteristics and prepare test specimens for metallurgical analysis. However, heating the commonly used 4 mm diameter by 10 mm long specimens at rates above 50 ^oCs^-1 results in non-linear heating rates during transformation to austenite and large transient temperature variations along the specimen length. These limitations in heating rate and variances from ideal uniform heating can lead to inaccurate characterization of the transformation behavior compared to commercial induction hardening practices.

In this study it is shown that changing the specimen design to a thin wall tube allows faster heating rates up to 600 ^oCs^-1 or higher and modifies the pattern of temperature variations within the test sample. The response of selected specimen geometries to induction heating in the dilatometer is characterized by modelling and tests using multiple thermocouples are used to verify the models. It is demonstrated that the use of properly designed tubular test specimens can aid in more accurately establishing transformation characteristics during commercial induction hardening.