Vacuum hardening of stainless steel vernier calipers

Abstract

This project is being carried out for ENGINEERS INSTRUMENTS LTD., which manufactures stainless steel Vernier Calipers. In this project, Vacuum with Inert Gas Quenching was investigated Hardening as an alternative heat treatment process for the Induction Heating process being applied at present. A total of 12 tests were performed, in which the Heat Treatment Parameters, Hardness, Geometric Variations, Metallography and Corrosion were studied. The variables during the heat treatment were the Purity of Nitrogen used, Positioning of the charge inside the furnace, Orientation of the charge, Cycle Times, as well as Tempering times and temperatures. The results showed that the material is easily hardened with gas quenching, and a hardness of 57-60HRc was reached. On tempering for 30 minutes at 200°c, this may be reduced to 54HRc. The charge should be placed centrally inside the heating element to ensure that this hardness is reached. Dimensional changes exceeded the tolerance limits set by the BS standards, and these changes were not consistent. Hence the components cannot be pressed prior to the heat treatment process. Bending also occurred. The Flat position gave the worst results. The material did not require a pre-heat period or a long soaking time because of the thin sections involved. Metallurgical investigation of the material was carried out on both the Induction Heated and the Vacuum Hardened components. Corrosion seemed to attack the Vacuum Hardened pieces more readily. A study on Vacuum Furnace Technology is also presented.