Effect of Heat Treatment Parameters in Plasma Arc Surface Hardening of AISI 4340 Steel

Abstract

Flame hardening has been traditionally used for selective surface hardening of steel. This technique frequently resulting in imprecise harden area and part distortion when overheated. Focused heating source such as plasma arc can be an alternative to overcome this problem. In this work, plasma arc is scanned at the 4340 steel surface to improve the hardness. The variable parameters investigated were at scanning speed and operating current. Four types of surfaces were observed after they are scanned with plasma arc. They are fully-melted, partially-melted, non-melted continuous and non-melted intermittent where each type of surface has different roughness value. This work found that scanning speed and operating current has significantly influence the type of surface and roughness values. Analysis on non-melted surface gives the maximum depth of hardened layer of about 187 μm as well as hardness values of about 990 HV50. It is also observed that the depth of hardened layer and hardness value is significantly decreasing with increase in scanning speed or the decrease in operating current. Microstructure examination on hardened layer revealed that the increase of hardness is due to formation of fine grain martensitie structure.