Examinando por Autor "Bulzak, Tomasz"

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  • Miniatura
    Ítem
    Effect of warm forming process parameters on 42CrMo4 skew rolled bar mechanical properties and microstructure
    (Springer Science and Business Media Deutschland GmbH, 2024-04) Murillo Marrodán, Alberto; Bulzak, Tomasz; García Gil, Eduardo; Derazkola, Hamed Aghajani; Majerski, Krzysztof; Tomczak, Janusz; Pater, Zbigniew
    Skew rolling is a manufacturing process in which two or three rolls are used to reduce the diameter or modify the shape of a cylindrical workpiece, which is used to manufacture mechanical components such as shafts, rods or balls. Hot conditions are used to overcome limitations related to material ductility, residual stress and machine capacity. In this paper, the warm skew rolling (WSR) process of 42CrMo4 rods is modeled by the finite element method. The effects of forming parameters, namely initial temperature and roll rotational velocity, on the material strain rate, thermal properties, microstructure and hardness were analyzed. Simulation results were validated by experimental process data, while hardness tests and SEM-EBSD microscopy were used to assess mechanical properties and microstructure, respectively. The WSR resulting microstructure is different from the normalized ferritic–pearlitic initial one. The degree of spheroidization (DoS) of cementite increases with temperature. The maximum DoS of 86.5% occurs at the initial temperature of 750 °C, leading to the highest material softening. Rolling from lower temperatures favors grain fragmentation and the achievement of incomplete spheroidization, which, in combination with the highest proportion of high-angle boundaries, contributes to a higher hardness of the rods with respect to those rolled at higher temperatures. The highest reduction in hardness takes place at 750 °C and 30 rpm, leading to 209.4 HV1 (30.7% reduction) and 194.1 HV1 (35.7% reduction) in the near-surface and internal regions, respectively. The driving factor is the transformation of cementite precipitates into a spheroidal form characterized by the greatest degree of dispersion.
  • Miniatura
    Ítem
    Internal crack formation in cross wedge rolling: fundamentals and rolling methods
    (Elsevier Ltd, 2022-06-18) Bulzak, Tomasz ; Pater, Zbigniew; Tomczak, Janusz; Wójcik, Łukasz; Murillo Marrodán, Alberto
    Material fracture is a serious problem in cross wedge rolling (CWR). Finding new solutions for material fracture prevention in CWR processes presents a technological challenge for researchers these days. This paper presents a new approach to the problem of material fracture, which investigates a not previously undertaken aspect concerning the influence of the cross wedge rolling method on the development of internal cracks. CWR rolling methods in which two tools are used (two flat wedges, two roll wedges, roll wedge-concave segment, two concave wedges) were analysed. In addition, the effect of the number of moving tools on material fracture in CWR is investigated. Material fracture is predicted by a novel hybrid fracture criterion. In this paper, FE simulations were used to determine the value of the hybrid fracture criterion. Experimental studies were used to validate the built numerical model. The conducted research has shown that with the increase in the ovalization of the cross-section of the rolled forging, which depends on the rolling method and the number of moving tools, the degree of damage to the forging increases. The highest degree of damage occurs during the flat wedge rolling method, in which only one wedge is moving. The lowest degree of damage occurs when rolling with two moving concave wedges. The most important conclusion of the study is that changing the rolling scheme from the least favourable to the most favourable one allows to reduce the degree of forgings damage twice.