Volume 73, Issue 5

Nitriding of low-alloy steels is widely used for gears and bearings in aerospace industry. Some highly stressed surfaces require high nitrided depth, which involves long time/high temperature nitriding treatment. This study focused on identifying process parameter effects on hardness and residual stress profiles in a large range of nitriding time and temperature. We demonstrate that core and case can be considered as two materials, softening of which can be predicted by two tempering laws. In addition, a basic relationship was derived from the nitriding depth and a pseudo-diffusion coefficient, allowing prediction of the hardness profile. Residual stress results show that the diffusion controlled effect can also be used to predict residual stress profile affected depth. Likewise, the tempering controlled effect can be used to predict surface and maximum values of the residual stress profile.

Plasma nitrocarburizing of austenitic stainless steel at 420 °C is typically applied to improve the surface hardness and wear resistance without impairing the corrosion resistance of the material. Recently, based on the active screen technology a novel plasma assisted approach has been developed substituting the usually used steel active screen by carbon-fibre reinforced carbon. This new concept avoids the use of carbon containing feed gases and provides highly reactive process gases consisting predominantly of unsaturated hydrocarbons and CN-radicals. The feasibility of this novel approach has been proven by spectroscopic and metallurgical studies in smaller laboratory set-ups but also in industrial scale systems. The present contribution compiles recent achievements in active screen plasma nitrocarburizing of AISI 316L and related materials using a carbon active screen under variations of bias power and process gas composition. The results are compared to plasma nitrocarburizing treatments using an active screen made of steel.

Nitriding and nitrocarburising processes are essential heat treatment processes when a wear resistant surface should be produced during heat treatment. The methods in salt bath, gas or plasma differ considerably in process technology. All three nitriding and nitrocarburising procedures are used for a wide variety of components and applications. Due to the different process technologies, possible sources of error in these heat treatment processes also differ [1]. In the lecture, very different failures and their root causes are presented. Faults in the design, in the material used, in the manufacturing process before and after heat treatment as well as in the operating conditions can lead to considerable damages. Serious errors are also possible in the nitriding and nitrocarburising treatment itself. Particular attention has been paid to heat treatment errors when an unintentional nitriding has occurred.

The production of massive steel-aluminium hybrid components by means of massive (hot) forming following a joining operation requires advanced heat treatment strategies for the adjustment of local material properties with maximum joint strengths. For the given process-step sequence, the challenge is that the heat treatment of the hybrid component can only take place in the joined state and after (hot) forming. Consequently, suitable heat treatment strategies must enable both a T6 heat treatment of the aluminium component and a surface hardening of the steel component. It could be shown that the inductive surface hardening of a steel functional surface in the immediate vicinity of the aluminium component can take place without reducing the bond strength. To achieve the desired hardening result, the use of a field former is required. The bond strength could be increased by up to 20% compared to the friction-welded initial conditions by adjusting the parameters of the T6 heat treatment.

Non-edible vegetable oils of Karanja and neem were used as quench media in the present investigation. The cooling characteristics of quenchants were obtained using Inconel 600 alloy probe and were compared with that of a fast-quenching mineral oil quench medium. Spatiotemporal heat flux was estimated using inverse heat conduction method. Heat removed from the probe during quenching showed higher and faster heat extraction by karanja oil quench medium compared to the mineral oil. Heat transfer characteristics of neem oil were comparable with the mineral oil. Rewetting time and temperature of vegetable oils were found to be higher than that of the mineral oil.