Machining-induced surface hardening due to large wear land

This study experimentally investigated the attainable hardness and hardened depth at the part surfaces in steel machining with large tool wear, 1–2 mm flank wear-land width. Hotrolled AISI 4340 steel bars were turned on a lathe using ceramic inserts with machining forces monitored. Process parameter effects on the depths of the phase-transformed layers and hardened layers were studied with cutting forces and part surface roughness measured and analysed. The phase transformation depth (PTD) and hardened depth generally increase with increasing tool wear, cutting speed, and feed rate; however, they decrease or level off when further increasing the parameters. The maximum hardened depth, about 100 m at a Knoop hardness of 450 HK500, has been repeatedly obtained at 1.65 mm flank wear land, 3m\s cutting speed and 0.025 mm\rev feed rate. It is plausible that machining under aggressive conditions, i.e. a greater amount of tool wear, may be employed for surface hardening of some steel components. The analysis of variance (ANOVA) method was also applied to identify statistically significant factors and interactions between the factors. The ANOVA results of the PTD indicate that cutting speed, flank wear-land width, and feed rate, in that order, are all significant factors. In addition, there is a major interaction between feed rate and cutting speed to the PTD results. Furthermore, the cutting force results imply a strong correlation between the radial cutting forces and the PTD.