Abstract
Potential-controlled nitriding is an effective technique for enhancing the life of steel molds and dies by improving their surface hardness and toughness against fatigue damage. In this study, the effect of the nitriding potential on the microstructure and fracture toughness of nitrided AISI D2 steels was investigated. The nitrided layers were characterized by microhardness measurements, optical microscopy, and scanning electron microscopy, and their phases were identified by X-ray and electron backscatter diffraction. As the nitriding potential increased to 2.0 atm−1/2, an increase in the surface hardness and fracture toughness was observed with the growth of the compound layer. However, both the surface hardness and the fracture toughness decreased at the higher nitriding potential of 5.0 atm−1/2 owing to the increased porosity in the compound layers, which mainly consist of the ε (Fe2–3N) phase. Additionally, by observing crack growth behavior, the fracture toughness was analyzed considering the material characteristics of the diffusion and compound layers. The fracture toughness was influenced by the location of the initial Palmqvist cracks due to the localized plastic deformation of the diffusion layer and increased crack length due to the porous compound layer.
Highlights
High-carbon, high-chromium AISI D2 steel, known as cold work tool steel, is widely used in the fabrication of punches, shear knives, tools, molds, and dies because of its hardenability and high resistance to wear and softening [1,2]
There are deleterious effects on the mechanical properties of the nitrided layers, namely decreased surface hardness, wear resistance, and toughness [8,11]. This is due to the presence of excess nitrogen and discontinuous precipitation, which result in the coarsening of the nitrides and porosity of the compound layer during nitriding [12]
We investigated the effect of the nitriding potential on the evolution of the nitrided layers, and the fracture toughness by analyzing the crack propagation
Summary
High-carbon, high-chromium AISI D2 steel, known as cold work tool steel, is widely used in the fabrication of punches, shear knives, tools, molds, and dies because of its hardenability and high resistance to wear and softening [1,2] It has been utilized for various manufacturing applications to enhance the working life of components against impact, shear, and fatigue damage [3,4,5]. There are deleterious effects on the mechanical properties of the nitrided layers, namely decreased surface hardness, wear resistance, and toughness [8,11] This is due to the presence of excess nitrogen and discontinuous precipitation, which result in the coarsening of the nitrides and porosity of the compound layer during nitriding [12]. Techniques such as ion-nitriding and gas-controlled nitriding have been developed to guarantee the long service life of the nitrided layer [13,14,15]
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