Influence of increased nitriding temperatures on the hardness profile of low-alloy steels

Gaseous nitriding at temperatures of 550, 590 and 630°C (823, 863 and 903 K), was applied for various times to three different steels (16 MnCr 5, 30 CrMoV 9 and 34 CrAlNi 7). In a second stage, two or three of these temperatures were combined to investigate the effects of step-nitriding. Hardness profiles were measured with low-load hardness testing to determine the growth of the case depth after nitriding. Microhardness testing was carried out on selected samples to investigate the hardness profile at the transition from the compound to the diffusion layer, including the transformed austenite layer. Effects, possibilities and limits of an application at higher than usual nitriding temperatures are discussed, emphasizing especially the effects of precipitation ageing and the influence of the porous zone.