Analysis of the evolutions of the local mechanical properties through the carbon-enriched sub-surface of a cemented steel bar using image analysis

摘要

Some pieces made of carbon steel may be easily modified about their surface and sub-surface microstructure by carbon enrichment and inwards diffusion at high temperature, for example by applying the cementation technique. After this operation and cooling from the austenitic domain down to room temperature these pieces are usually characterized by an outer microstructure containing hard phases and compound (pro-eutectoid cementite and pearlite) while the bulk's microstructure has become the initial one (ferritic or ferritic-pearlitic) since not reached by the added carbon atoms. Thus the improved wear resistance of the outer part of the piece is not accompanied by any loss of ductility, impact toughness or resistance against cracks propagation. For a more complete knowledge of the evolution of the overall mechanical behaviour from surface to the piece's centre it is possible to use Nital etching (colouring pearlite in grey and letting bright both free cementite and ferrite) and image analysis to quantify the microstructure evolution and then the new carbon distribution. That is what was undertaken here for a cemented carbon steel bar, with in parallel a study of the evolution of mechanical properties of several carbon steels in order to get laws of variation, on the carbon content, of hardness, ultimate tensile strength and rupture strain. The combination of the two works led to specify the local values of the three above properties and then their evolution through the cemented bar under study, mechanical data which can be of importance for further mechanical modelization calculations.