Surface modifications of H13 steel to improve the durability of forging dies

Abstract

Two plasma-based techniques, namely, pulsed laser deposition (PLD) and active screen plasma nitriding (ASPN) have been used to improve the surface structure and properties of a H13 steel which is used as a die in metal forming processes. In present study characterization was first carried out on a failed die to reveal the origin of defects and also to examine the wear mechanisms which dominate when the die is in operation. Different types of wear mechanisms, such as oxidation and delamination dominate wear of the gas nitrided H13 steel. Oxidation wear involves the formation of an oxide layer and its removal by spallation giving rise to the formation of wear debris. Delamination wear involves the initiation of subsurface (or surface) cracks of the compound layer, crack propagation and formation of wear particles. The second part of the project focused on investigating the nitriding mechanism in active screen plasma nitriding (ASPN) and the effects of axial magnetic fields on the microstructure and surface as well as cross-sectional hardness. This study confirms that mass transfer is taking place from the active screen, resulting in the deposition or formation of nano-particles on the steel substrate. However, there were no major effect of this deposition of nano particles on the nitriding response of the materials studied. The nitrogen transfer mechanism in ASPN is based on the implantation of energetic nitrogen species generated at the active screen. Also, the surface hardness and cross-sectional hardness of ASPN showed a significant improvement following the use of magnetic field around the screen. The third project, PLD fabrication of coatings, confirms that the deposition conditions influenced the microstructure of both TiN and V2O5 coatings. The microstructure of the TiN and V2O5 coatings were also found to be important in controlling their modes of deformation under contact loading. Porous microstructures were found to lead to catastrophic failure under load. Different modes of cracking, such as columnar and inclined cracking, as well as shear steps at the coating/substrate interface, were observed. Thicker coatings were seen to contain more equiaxed grains, so less columnar shearing occurred and inclined cracks were found to be more dominant.