A study on the grindability of niobium microalloyed forging quality HSLA steels

The grindability of two niobium microalloyed forging quality HSLA steels have been investigated. The microstructure of Steel 1 (0.19 C, 0.23 Nb, 0.71 CE) varied from ferrite+pearlite in the as forged condition (252 VPN) to bainite in the as normalized condition (335 VPN). The microstructure in steel 2 (0.18 C, 0.17 Nb, 0.84 CE) consisted of tempered martensite having a hardness of 640 VPN and 495 VPN, respectively, in the as forged and tempered conditions. The grindability of the steels were assessed in terms of chip formation modes, grinding forces, surface characteristics and surface residual stress in the workpieces. The study revealed that the initial microstructure and the phase transformation characteristics of the steels influenced their grindability. Among the present steels, Steel 1 (as forged) was most amenable to grinding in terms of grinding force and specific energy requirement, but its ground surface finish was poorest. Steel 2 (tempered) developed a good surface finish, but its grindability was poorest in terms of similar parameters. Chip formation mode was also widely different for Steels 1 and 2. The microstructure of the chips varied widely depending upon their thermal history.