Decarburization: The Silent Enemy

Author:  Prof. Dr. David Kirk, Coventry University, U.K.
Source:  The Shot Peener magazine, Vol 32, Issue 1, Winter 2018
Doc ID:  2018005
Year of Publication:  2018
Abstract:  
INTRODUCTION: A large proportion of metallic engineering components are made from carbon steels. These components may encounter high temperatures during furnace heat treatment and/or hot working. High strengths combined with adequate ductility are achieved by heat-treating carbon steels. This involves heating to a high temperature followed by quenching and then tempering. Hot working includes processes such as rolling and forging. It is at high temperatures in air that “The Silent Enemy” (decarburization) can strike. Carbon can be sucked out of the component, resulting in a severely weakened surface layer. The surface layer is just where we need the component to be at its strongest. Maximum applied stresses and defects acting as stress-raisers occur at the surface. Shot peening is much less effective if applied to a decarburized component. Decarburization would not be a problem, however, if post heat-treatment machining was applied to all sensitive areas. High-strength carbon steel typically contains about 0.8% of carbon. That does not sound like very much but carbon atoms are much lighter than iron atoms with atomic weights of 12 and 58.8 respectively. If we express carbon content in terms of proportion of atoms, we find that the carbon content is about 4%. In other w very twenty-five atoms is a carbon atom.