Effect of Intercritical Quenching on Microstructure and Mechanical Properties of Oil Casing Steel N80

In order to improve the toughness of oil casing steel N80 without the sacrifice of its original high strength, an intercritical quenching treatment was conducted under the temperature determined by a differential scanning calorimetry (DSC) analysis. Effects of intercritical quenching on the microstructure of oil casing steel N80 were characterized by means of optical microscope (OM) and scanning electron microscope (SEM). Tensile strength, reduction of cross-sectional area and microhardness were measured to evaluate the mechanical property of oil casing steel N80 after intercritical quenching treatment. The study results show that the tensile strength and microhardness of intercritical quenched oil casing steel N80 consisting of ferrite (F) and martensite (M) is slightly lower than that of tempered oil casing steel N80 composing of sorbite (S), yet which is still higher than that of full annealled oil casing steel N80 composing of pearlite (P) and a little amount of ferrite (F). In particular, the reduction of cross-sectional area of oil casing steel N80 intercritical quenched at 740 degrees C is higher than those of tempered and full annealled. Additionally, both dimple and cleavage can be found on the impact fracture surface of N80 steel after intercritical quenching at 740 degrees C. The toughness of oil casing steel N80 can be obviously improved by the intercritical quenching treatment at 740 degrees C due to the formation of ferrite (F).