42CrMo steel, a typical low alloy medium carbon structural steel, is widely used in important structural components that require high strength, plasticity and toughness, such as crane weight-on-wheel, automobile crank shaft, locomotive gear hub, oil drill pipe joints of deep well, fishing tools and so on, for its good harden ability, high temperature strength, good creep resistance and little quenching deformation. The wheels of the polar crane that used in the Chinese third generation nuclear power plant are made of steel 42CrMo. However, cracks and surface peeling normally occur after heat treatment at quenching process of wheel forgings. There is important application background and practical significance to research the effect of the heat treatment process on the microstructures and mechanical properties and the influence of porosity defects inside the forging wheel on the heat treatment process. Large numbers of research work had been focused on segregation and heat treatment process for solving this matter in passing days. This work aims to study the effects of thermal residual stress on porosity defect in a wheel, and explain the reason for cracking and surface feeling in the way of mechanical behavior during quenching. A wheel with surface peeling was analyzed by measuring chemical compositions, macro- and micro-crack observation. Random testing position for mechanical compositions showed that the effects of segregation was small in the wheel. A set of tests and measurements for thermal mechanical properties of 42CrMo steel were conducted from room temperature to 850 ^oC. FEM models containing porosity defects in different sizes and without defects were constructed by Abaqus for studying the residual stresses during quenching in fully coupled temperature-displacement analysis. Simulation results indicate that the porosity defects in wheel cause stress concentration within themselves. The maximum residual stress is not affected by the length of porosity region. The hoop residual stress in porosity region in the wheel due to quenching process is in the highest level and believed to be the driving force of cracks. According to the stress distribution in the wheel, the cracks caused by the hoop residual stress can not propagate out of the defects region too far. While the assumptions of surface peeling of wheel are concluded due to combined influence of the residual stress and external loads when the defects region emerges near the wheel surface border in view of the current simulation.