Surface degradation of nitrided hot work tool steels under repeated impact-sliding contacts: Effect of compound layer

Wear behaviour of quenched and tempered (QT) hot work tool steels (Uddeholm QRO90) was investigated against SAE 52100 grade bearing steel balls after gas nitriding using a dedicated laboratory scale impact-sliding wear test rig. Gas nitriding was employed in a fluidised bed reactor under two alternative regimes: i. High Temperature Nitriding (HTN) carried out at 510 degrees C and ii. Low Temperature Nitriding (LTN) carried out at <= 400 degrees C. The HTN process resulted in the formation of -2 mu m thick external compound layer, whereas the LTN processed steels were free of any surface compound layer formation. After the impact-sliding wear tests employed at room temperature (RT), the prevailing wear mechanisms of the examined steels were assessed as tribo-oxidation and fatigue wear. The testing at 600 ?C induced different wear mechanisms for the HTN and the LTN steels. While tribo-oxidation and fatigue wear were preserved for the HTN steel, plastic deformation dominated the wear that progressed on the compound layer free surface of the LTN steel. Impact-sliding wear testing at 600 degrees C showed that the wear rate of LTN > HTN steels, as opposed to the wear rate at RT where wear rate of HTN > LTN.