Room Temperature Case Carburizing of a Ni–Cr–Mo Steel Through Shot Peening/Blasting Techniques

Abstract

Controlled shot-peening/blasting is an operation which is used largely in the manufacturing industry. An attempt is made to create a case-hardened surface by the shot peening technique on a Ni–Cr–Mo steel. Steel shots with activated carbon powders were introduced at the surface of the samples for different periods of time, i.e. for 5, 10, 15 and 20 min. The microstructure, chemical composition, and hardness were investigated by optical microscopy, SEM, OES, XRF, XRD and micro-Vickers hardness tester, respectively. An effective diffusion of carbon atoms was found in the samples subjected to shot peening for 10 min. and above. However, the hardness values were found to be non-uniform and showed a maximum hardness of 300 HV0.3 at activated spots. The microstructural studies revealed formation lamellar-shaped cells consisting of nano grains at the shot peened surface. A close observation of these lamellae shows α-ferrite and cementite (Fe3C) formed by diffusion of carbon into iron at room temperature. XRD results confirmed the formation of Cr23C6 and Fe3C by shot peening with activated carbon.