Effect of Surface Hardening by Gas Nitriding on the Fatigue Resistance of Low Alloy Steel EN 1.8509 (41CrAlMo7-10)

In this research, the fatigue resistance of low alloy steel was improved by using surface hardening by gas nitridation at different temperatures, 475 to 610°C, and in a medium containing 70% nitrogen and 30% hydrogen. The results obtained showed a significant improvement in the fatigue resistance of the samples used after the nitridation process, where the highest fatigue resistance was obtained (580 MPa) at the temperature range 475 to 510°C. At the same time, there was a decrease in the fatigue resistance when using high temperatures in the gas nitridation process, which decreased to 400 MPa at 610 °C. In comparison, the value of the fatigue resistance before nitridation was 325 MPa. Through the results of the micro hardness and tensile test, it was found that there was an increase in the hardness tensile strength values on the surface and in general for all groups of samples treated by nitriding, It was also observed that the hardness gradually decreased smoothly as we moved away from the surface towards the depth, as the metal retains good hardness up to a depth of 300 microns. It helps to avoid the sudden and non-gradual transition from a rigid surface to a soft center. Thus, a structure of sufficient thickness is formed from the nitriding layer, coherent with the core. The results of the X-ray diffraction examination also showed the formation of nitrides of various alloying elements, Fe3N, Fe4N, Cr2N, Fe2Al5, Al2N, which have a significant role in increasing the hardness and improving the fatigue resistance.