Ion-nitriding behavior of several tool steels

• Published in: Materials science and engineering 1986-03, Vol.78 (2), p.179-191
• Main Authors: Ozbaysal, K., Inal, O.T., Romig, A.D.
• Format: Article
• Language: English
• Subjects: Applied sciences; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Hardness; Heat treatment; Materials science; Mechanical and acoustical properties of condensed matter; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metals, semimetals and alloys; Metals. Metallurgy; Phase diagrams and microstructures developed by solidification and solid-solid phase transformations; Physics; Production techniques; Specific materials; Thermochemical treatment and diffusion treatment
• ISSN: 0025-5416
• DOI: 10.1016/0025-5416(86)90322-8

The ion-nitriding behavior of several tool steels, H13 hot-worked tool steel, D2 and D3 cold-worked steels and M2 high speed tool steel, has been examined under varying process conditions with microhardness-depth correlations, optical microscopy, X-ray diffraction techniques, transmission electron microscopy and wavelength-dispersive X-ray analyses. The process variables studied include time (2–12 h), temperature (673–793 K) and nitrogen partial pressure (5%–25% nitrogen volume fraction). The highest nitrided hardness was observed in the M2 and H13 steels. The lowest surface hardnesses and lowest case depths were observed in D2 and D3 steels. In general, the behavior of D2 and D3 steels were similar. All four steels showed increasing case depths and decreasing surface hardnesses with increasing ion-nitriding temperatures and times. Both properties were found to be independent of nitrogen partial pressure in the gas mixture for all steels except the M2 steel. The nitriding depth was found to be parabolic with ion-nitriding time. The activation energies for nitriding were calculated for H13 and D3 steels as 19.5 kcal mol −1 and 28.5 kcal mol −1 respectively, corresponding to the slower reaction rates observed for high carbon steels. Optical microscopy showed that only slight differences arise in the bulk structure during ion nitriding. X-ray diffraction analyses showed extensive line broadening of the major martensite peaks and FeCr carbides were seen to convert to chromium nitrides, whereas FeW carbides were stable in the nitrided case. Transmission electron microscopy revealed the existence of modulated structure in the nitrided case of all steels. Decomposition of the martensitic structure and subsequent carbide growth were observed in the core regions. Wavelength-dispersive X-ray analysis revealed that FeCr carbides were converted to chromium nitrides in the regions close to surface, whereas conversion was not complete at larger depths (greater than 80 μm).