|Spoluautoři WASIAK Krzysztof, SKOŁEK Emilia, ŚWIĄTNICKI Wiesław|
Carburization, followed by quenching and tempering, is commonly used process to increase hardness and wear resistance of low carbon steels. Such process, besides improving mechanical properties and performance of steel surface, may cause however undesirable side effects such as: increased residual stress in surface layer, distortion of heat treated element or even initiation of cracking. The aim of the work was to reduce those possible risks by developing an alternative heat treatment of steel after carburization. Research data indicate that isothermal holding within the range of temperatures in which bainite is formed results in a reduction of residual stresses and distortion in comparison to quenching and tempering. Austempering of steels with increased content of silicon or aluminium at temperature slightly above the Ms may improve the mechanical properties as compared with quenching and tempering.. This is because the austempering performed at low temperatures range of bainitic transformation may lead to a nano-sized carbide-free bainite formation. Moreover the Si and Al additions hinders or even suppress the cementite precipitation which has a harmful effect on the resistance to cracking. For the purpose of this study two structural steels were chosen: 35CrSiMn5-5-4 containing 1.3 wt.% of silicon and 38CrAlMo6-10 which contains 0.3 wt.% of silicon and 1 wt.% of aluminium. Both steels were subjected to vacuum carburization, then they were austenitized and austempered at temperature close to the temperature Ms of the surface layer. Transmission electron microscope (TEM) observations revealed that after austempering the carburized layer are composed of a nano-sized bainite plates separated by thin films of retained austenite. The results of hardness and wear resistance tests indicate that the austempering is advantageous as compared to the conventional quenching and tempering process.