Microstructural and texture studies of gas nitrided Cr-coated low alloy high carbon steel

Abstract

In the present study, an attempt has been made to develop hard chromium nitride layer on low-alloy high-carbon steel through a two-stage hybrid process comprising electrodeposition of 45-60 μm thick chromium layer followed by gas nitriding in controlled atmosphere (using a gas mixture of NH₃/Ar = 1:3) in the temperature range of 750-950°C for 2-6 h. The coating thus developed consists of three distinct regions - a mixed-nitride (~ 2-30 µm thick) at the top, un-reacted or as-deposited chromium in the middle (15-35 μm) and diffused-carbide layer at the bottom of the coating (5-15 μm) in order of vertical distance from the surface. The top nitride layer records extremely high hardness of ~ 2000 VHN without micro-crack at the corners of the hardness indentation. Detailed orientation distribution function (ODF) analysis shows that the cube component is predominant in samples nitrided at lower nitriding temperature, whereas Goss and rotated Goss texture is prevalent at higher nitriding temperature. The calculated diffusivity of nitrogen in chromium is marginally higher than that reported in the literature.