Effect of post-boronizing heat treatment on wear behaviour and pit formation in AISI-8620 gear steel

Abstract

Abstract In this study the effect of post-boronizing heat treatment on wear behaviour and pit formation on a starter motor pinion gear, made from AISI-8620 steel, was investigated. This involved using a rotary wear test, where the number of cycles at a fixed load and a fixed rotational speed were varied. Boronizing was performed on starter motor pinion gears (test specimens) in a solid medium consisting of B4C, SiC, KBF4 at a temperature of 900°C for 4 h. Following boronizing, test specimens were quenched in water at room temperature from the boronizing temperature and were subsequently tempered at 180°C for 1, 2 and 3 hours. As part of this investigation, microstructure and microhardness distribution of the boride layer and wear resistance and pitting damage rate of the specimens were examined for each of the tempering heat treatments. Morphology of the boride layer and pit formation were characterized using optical microscopy. Wear and pitting evaluations were carried out using a wear test apparatus made for this purpose, which simulates the pinion and flywheel starter ring gear pair. For each wear test the number of cycles was 25×105, inspected at 5×105–cycle intervals. It was concluded from these examinations that tempering heat treatments following boronizing contribute to the increase of microhardness alongside the surface layer, as well as an increase in wear resistance and pitting damage.