Effect of vacuum annealing and nitrogen alloying on gas atomised M4 high speed steel powder

In the present study the authors have used X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) to assess how surface oxides limit the gas nitriding depth of gas atomised M4 high speed steel powder and compacts. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) have been used for phase identification. In model experiments XPS and AES analyses of vacuum annealed powder were performed in an interconnected furnace, limiting reoxidation. Sintering cycles with and without vacuum annealing treatment were also evaluated. Generally, the authors found that an increased vacuum annealing treatment time decreased the amount of residual oxygen, which improved densification. AES and XPS analyses of the model experiments showed that the vacuum annealing time increased the absorption of nitrogen. In the sintered compacts, SEM, AES and XRD analysis as well as Thermo-Calc simulations showed that similar amounts of nitrogen were tied to vanadium carbonitrides. An AES comparison between the model and sintering experiments showed that the nitrogen absorption had the Sieverts' law dependence once the surface oxide had been removed.