Influence of Intermediate Cryogenic Treatment on the Microstructural Transformation and Shift in Wear Mechanism in AISI D2 Steel

Abstract

AISI D2 steels are an interesting family of tool steels that generally have applications in knife blades, shaper blades, and forging and punching dies due to its optimum toughness, lower wear rate, and relatively high hardness. The present research work aims to design a new heat treatment cycle consisting of single tempering followed by cryogenic treatment and soft tempering to improve the wear properties of these steels. Specimens of AISI D2 tool steel were subjected to three different heat treatment cycles: hardened and double tempered at 450 °C (HTT450), hardened and double tempered at 450 °C followed by cryotreatment and soft tempering at 250 °C (HTT450CT250), and hardened and single tempered at 450 °C followed by cryotreatment and soft tempering at 250 °C (HT450CT250). The microstructures of the samples were characterized for hardness, carbide density, impact energy, wear loss, wear surfaces, and residual stresses. The influence of these measured parameters on the wear behavior was determined using a pin-on-disc wear testing machine. The results confirmed that HT450CT250 is the optimized heat treatment cycle out of the three cycles studied, which showed enhanced wear resistance index, minimum compressive residual stresses, and optimum impact toughness due to increased volume fraction and finer distribution of carbides in the tempered martensite structure. It is further revealed that there is a change in morphology of carbides from globular to needle-shaped, resulting in wear transition, which is well corroborated by scanning electron microscopy studies and wear surface analysis.