Hardening Of Steel Through High-Voltage Low-Current Energy Input

A novel approach of high-voltage low-current energy input is applied for hardening of plain carbon eutectoid steel. Initial fine lamellar pearlitic structure disintegrates into four characteristic regions: lamellar pearlite often containing nucleated cementite spheroids (Region-I), fragmented cementite lamella in alpha-ferrite matrix (Region-II), submicroscopic cementite particles/clusters dispersed in alpha-ferrite matrix (Region-III), and supersaturated alpha-ferrite (Region-IV). At a particular applied voltage, structural refinement and matrix supersaturation (evolving martensite) progress concomitantly up to 5 min, followed by a reverse trend of coarsening and degeneration of martensite. The refinement effect and martensite-peak-broadening effect are augmented with increasing voltage up to 75 kV at which the highest hardness (429 HV) of the steel is achieved with treatment duration of 5 min. While primary hardening effect arises from the martensite region of stratified plate morphology (Region-IV), a secondary effect of hardening is resulted from the region containing dispersed submicroscopic cementite particles/clusters in alpha-ferrite matrix (Region-III). In addition, in the specimen exhibiting maximum hardening effect (at 75 kV, 5 min), as a unique feature, nanosized cementite particles also appear in Region-IV being dispersed in martensite matrix so as to provide further effect of dispersion hardening along with martensitic hardening.