Evaluation of experimentally observed asymmetric distributions of hardness, strain and residual stress in cold drawn bars by FEM-simulation

The purpose of this paper is to investigate the correlation of strain and hardness parameters in a drawing process chain of an SAE 1045 steel bar. The present work discusses the application of the experimental and numerical method of analysis, based on hardness measurements, which allows evaluation of the strain distribution and mechanical properties in drawn products. The influence on the strain, hardness and residual stress distributions of cold drawing parameter variations were investigated by numerical simulation. A misalignment between bar and drawing tool was evaluated in the models, as well as variation of friction at different regions of the bar. Compression tests were carried out at different reductions to determine the flow curve and analyze the material behavior during a cold hardening process. The microstructural analysis has shown a potential influence of material segregation in the characteristics of the final products. Simulation results validate the assumption of die misalignment and inhomogeneous lubrication influences on residual stresses profiles after wire drawing. Taking into account effects of misalignment and friction at the same time, in a so-called combination model, a better agreement between numerical simulation and experimental results for cold drawing process was achieved. Keywords: Hardness, Plastic strain deformation, Residual stresse

Experimental Analysis of Residual Stresses in Pre-Straightened SAE 1045 Steel

<div><p>This paper aims at analyzing the effects of the roller pre-straightening of wire-rods on residual stress distributions in SAE 1045 steel bars. The combined drawing process is used in industrial production of bars in order to obtain a good surface quality and improved mechanical properties complying with specifications of the final products. In this process, prior to the drawing step, a roller straightening of the steel wire-rod is essential, because it provides the minimum straightness necessary for drawing. Metallographic analysis and hardness test were done for selected samples after different processing steps. Also, residual stress analysis of pre-straightened wire-rods by X-ray diffraction and neutron diffraction were carried out. The hardness tests show higher values near the surface and lower in the center of the wire-rod. Besides, the residual stresses results show a big inhomogeneity from one peripheral position to another and also in the evaluated cross section.</p></div

Improvement on pitting wear resistance of gears by controlled forging and plasma nitriding

In the present investigation, the use of a bainitic steel for forged gears applications and the suitability of different plasma nitriding treatments is discussed. The gears were forged, machined, ground, polished, and nitrided at 500 °C with three sets of N2–H2 gas mixtures, containing 5, 24, and 76 vol.% N2, to develop a nitrided case of approximately 300 μm depth. Gears were characterized before and after the nitriding concerning the phase composition, microstructure, microhardness, fracture toughness, and residual stresses states. Pitting wear tests were performed on a standard Forschungsstelle für Zahnräder und Getriebebau (FZG) test rig. The nitrided gears with 24 and 76 vol.% N2 formed a biphasic compound layer of ε-Fe2-3(C)N and γ′-Fe4N. As the volume fraction of nitrogen in the gas mixture was decreased, the detected content of γ′-Fe4N in the compound layer increased, but a monophasic compound layer was only reached with 5 vol.% N2. The nitrogen rich gas composition increased the surface hardness and decreased the fracture toughness of the compound layer, as they have more ε-Fe2-3(C)N. The diffusion zone of the different nitrided surfaces showed residual compressive stresses. The best performance was obtained in the nitrided gears with 24 vol.% N2, due to the better combination between the surface hardness, fracture toughness, residual stresses, and compound layer thickness. The nitrided gears with 24 vol.% N2 have ten times improvement over the non-nitrided gears, while the nitrided gears with 5 and 76 vol.% N2 have an improvement of three point seven and five point four times