Evaluation of heat transfer in steel samples immersed in plasma

ABSTRACT

Plasma-assisted processes have several industrial applications such as steel nitriding, sintering, thin film deposition and semiconductor production. Heating rate, heat transfer and temperature play a key role in the properties of materials immersed in a plasma. However, one of the challenges of plasma processing of materials it is to accurately measure the heat transfer and the temperature, particularly in a specific region of the sample, as there may be thermal spikes, raising locally the temperature. Furthermore, there are several parameters such as plasma atmosphere composition, power, pressure and sample composition that can affect diffusion, mass transport and plasma heating rate. Thus, the aim of this study is to evaluate the heating homogeneity of tempered steel samples in argon plasma and compare it to heating in a resistive furnace. For this purpose, microstructure and hardness of plasma tempered samples were compared to samples conventionally tempered in a resistive furnace, so that the equivalent temperature of a solid immersed in a plasma could be determined. In view of the great industrial interest on plasma nitring process, in this study the heating of steel samples immersed in plasma in a typical plasma nitriding chamber was evaluated. For that samples made of SAE 1045 and SAE 8640 steel were quenched and then tempered in plasma. The plasma tempered samples showed a much smaller hardness for treatment at same measured temperature than conventionally tempered samples. This result was related to thermal spikes during plasma heating. A mathematical model to set the equivalent temperature during plasma tempering was proposed. From this model it is expected to develop strategies to optimize plasma parameters, aiming on improving materials properties.

Keywords
plasma heating; heat treatment; heat transfer; microhardness; carbon steel