Abstract: In order to reduce the internal stress of a P110 oil casing during quenching,an optimized cooling process "water-quenching→air-cooling→water-quenching" was proposed.The change and distribution of temperature and stress field of the casing during cooling process were simulated by finite element method.The simulated results show that the maximum temperature difference is 104℃ after water-quenching for 7.5s,and the distribution of cross-section temperature is uniform at the end of air-cooling.When the casing is quenched by water again,the temperature difference is 80℃,which is 24℃ lower than that of 7.5s.For the stress,at the first water-quenching stage,the tangential stress increases from the beginning to 2.5s and then decreases form 2.5 to 5.5s.At 5.5s phase transformation occurs.After that there are both thermal stress and structural stress in the casing,hence the tangential stress increases sharply and peaks at 563MPa at 7.5s.At the air-cooling stage,as the thermal stress decreases and the structural stress disappears,the distribution of tangential stress becomes uniform and levels off at-11 to 27MPa at the end.When the casing is quenched in water again,the tangential stress increases and reaches the peak again at 451MPa at 13.6s,which is 112MPa lower than 563MPa at 7.5s,thereby optimizing the cooling process.