Effect of gas nitriding on the thermal fatigue behavior of martensitic chromium hot-work tool steel

The influence of heat treatment and gas nitriding on the thermal fatigue behavior of martensitic chromium hot-work tool steel was investigated. Thermal fatigue tests were carried out using a special induction heating apparatus, which consisted of induction heating and water spray cooling unit. The process of thermal cycling was simulated using a coupled heat-transfer solid-mechanics finite element model. It was seen that the thermal fatigue resistance was higher in the gas nitrided samples after austenizing at 1020 degrees C than for the gas nitrided samples after austenizing at 1100 degrees C. The thermal fatigue endurance limit was found to be maximum for the samples having a compound layer comprising of the higher phase fraction of gamma' (Fe-4 N1-x). It was also found that, lower the ratio of compound layer thickness to the total diffusion depth, the higher is the fatigue life. These results were influenced by two major effects of nitrided (diffusion) layer. First is the high compressive residual stresses imparted on the surface which tend to mitigate the effect of thermal tensile stress and secondly very high surface hardness, due to the diffusion of nitrogen, which increase the threshold for crack initiation at the surface. (C) 2015 Elsevier B.V. All rights reserved

Effect of Friction Models and Parameters on the Lagrangian Flow Fields in High-Temperature Compression Testing

Friction plays an important role in high-temperature deformation process. Friction affects local displacement field in the tool-workpiece interface region, thus affecting the overall material flow. Under high-temperature compression, macro-indicators like bulge radius and load displacement curves are not sensitive enough to distinguish subtle differences between various friction models. Hence, a new approach to match the experimental Lagrangian flow field with flow field obtained from FE simulation is proposed. For this uniaxial barreling, compression tests at constant temperature were conducted on Gleeble thermo-mechanical simulator. The compression tests were conducted at different strain, strain rate and friction conditions. Finite element simulations employing various friction models and parameters were performed for matching the experimental conditions. Experimental Lagrangian flow fields were obtained from the grain flow lines observed on high-resolution larger area micrographs of the specimen. It was observed that all the investigated friction models provided equally good fit with the macro-experimental indicators (bulge radius and load displacement curves). However, Coulomb friction model was the only friction model that provided the closest fit with the experimentally obtained Lagrangian flow fields. Coulomb friction model provided the best agreement between experimental and numerical simulation for both lubricated and non-lubricated conditions using friction coefficients mu = 0.2993 and mu = 0.3895, respectively