Hardening of Steel Through High‐Voltage Low‐Current Energy Input

Abstract

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 α‐ferrite matrix (Region‐II), submicroscopic cementite particles/clusters dispersed in α‐ferrite matrix (Region‐III), and supersaturated α‐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 α‐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.

On ascertaining hardness‐microstructure relationship in annealed steel for varying carbon content

“…This encounters various forms of divorced eutectoid structure, often altogether termed as the 'degenerated pearlite' [13][14][15][16]. Apart from thermal treatment, the feasibility of various solid state phase transformations (including evolution of martensite) in steel has been verified recently with high-voltage low-current electric energy input [17][18][19].…”

Quantitative hardness‐carbon content‐microstructure correlation in normalized plain carbon steel

“…The microstructural refinement in steel has been technologically taken up so far in view of faster cooling rate execution perspective for complete austenitization based heat treatment route [6]. Other approaches such as incomplete austenitization based heat treatment and high voltagelow current electric energy input routes have been also been tried out at laboratory scale in recent years [7][8][9]. In case of complete austenitization based heat treatment route, widely implemented in industry on initial trial at laboratory scale, a faster cooling rate would definitely lower down the actual transformation temperature so as to enhance the degree of undercooling.…”