Effect of Cryogenic Treatment on Microstructure and Wear Characteristics of AISI M35 HSS

Cryogenic treatment has been widely acknowledged as a means of improving wear resistance of tool materials. A Comparative study on conventionally heat treated and cryogenic treated AISI M35 grade high-speed steel specimens has been presented in this paper. Specimens initially subjected to conventional heat treatment at austenitizing temperature of 1200 ̊C were subsequently subjected to shallow cryogenic treatment at -84 ̊C for 8 hours and deep cryogenic treatment at –195 ̊C for 24 hours followed by double tempering at 200˚C. Presence of retained austenite was studied at the end of each of the above treatment using XRD analyzer. An estimated 19% retained austenite present at the end of conventional heat treatment was reduced to 5% at the end of shallow cryogenic treatment, while deep cryogenic treatment practically removed all traces of austenite in the sample. Changes in the microstructure were studied using SEM. Fine precipitates of carbides of size 0.3-0.5µ were observed in cryogenically treated samples. Variation in mechanical properties such as toughness and hardness has been studied. There was no change in toughness due to cryogenic treatment and it corroborates well with the results of fractography. Wear characteristics were studied using pin on disc wear tester. The operative modes and mechanisms of wear have been identified as severe delaminative and mild oxidative from the morphology of worn surface of pin. The results unambiguously confirm enhancement in hardness and wear resistance of cryogenically treated specimens.