Synthesizing a class “M” high speed steel on the surface of a plain steel using the plasma transferred arc (PTA) alloying technique: microstructure and wear properties

Abstract

A class M2 high speed steel layer was synthesized from powders on the surface of a plain steel using the plasma transferred arc (PTA) process. This technique is an easy and effective one in producing relatively uniform alloy layers having a thickness of the order of 1.2–1.5 mm and hardness around 750 HV 200 g . The layers have a fine cast microstructure consisting of a dendritic matrix of martensite/residual austenite embedded in a duplex M 6C and M 2C eutectic. Tempering experiments in the temperature range between 200 and 800 °C show that residual austenite decomposes progressively above 400 °C and a fine precipitation of carbides occurs within the matrix, while the M 2C carbides transform to M 6C. Peak hardness of about 1100 HV is obtained after heat treatment at 600 °C for 2 h. Pin-on-disc wear experiments were done on both the untreated and heat treated layers. The wear resistance of the untreated specimens was very low (wear rate of the order of 10 −4 mm 3/m) and was further improved by an order of magnitude for the specimens treated at peak hardness. The friction coefficient was very low, of the order of 0.10, for both the treated and untreated specimens. The dominant wear mechanism in both cases was plastic deformation with the matrix preferentially worn and the carbides supporting the applied load. Oxidation of the wear debris occurred only for the highest applied loads and sliding speeds of the untreated specimens, but it was very slight, so that the wear coefficient remained low, corresponding for all the experiments to the Al 2O 3–carbides tribosystem.