P110 grade oil casing is an important component in petroleum drilling. The original 25CrMnMo oil casing steel manufacturing, in order to reduce production costs, improve market competitiveness, by adjusting the ratio of Mn/C, and with a small amount of V to replace the alloy elements Mo and Cr, the development of a new type of low cost 25MnV oil casing. The casing when inserted in borehole, the borehole separated strata, preventing wellbore collapse and ensure the drilling mud circulation, for drilling, drilling due to different geological conditions, underground complex stress state, the service conditions, which have higher requirements on the strength and toughness, the technical requirements for o b more than 862MPa. Sigma s=758 ~ 965MPa, delta > 11%, AK = 20 ~ 41Jcm-2. As a senior product wells tube, P110 grade oil casing after quenching and tempering heat treatment to achieve good comprehensive mechanical properties, therefore, choose the reasonable heat treatment process system is the fundamental guarantee to improve the quality of products. However, in the production, it is found that the impact toughness of the oil casing is low and unstable, which is not enough to guarantee the design requirement, and the impact rate is even lower than 15% to 20%. This paper adopts the Intercritical Hardening Process Improvement, the process test and product evaluation, and achieved good results.
    On casing sampling test its hardness is 29 ~ 31HRC, chemical composition (mass fraction,%) of 0.28C, 0.22Si, 1.42Mn, 0.09V, 0.12Ni, 0.14Cr, 0.013P, 0.010S, sampling by metallographic examination for uniform cut tempered sorbite structure conforms to the standards of APISPEC5CT. 25MnV is an alloy structural steel, so there is a tendency of high temperature temper brittleness. Presumably the oil casing steel after quenching and tempering causes low and unstable impact toughness after tempering embrittlement and corrosion caused by the cold, but the existing literature on the mechanism analysis of 25MnV steel high temperature temper brittleness is less reported. This paper has designed a experimental scheme to show that the high temperature temper embrittlement of 25MnV steel: take the same material to do a batch of the same standard impact test sample is 910 DEG C, c * * 30min+600 60min the same quenching and tempering after impact test, the results show that the temperature of 600 DEG C after tempering water cooling, air cooling and furnace cooling impact toughness were 21 ~ 23, 14 ~ 16 and 9 ~ 11J/cm2, high temperature tempering process in 25MnV steel after quenching and tempering when slowly after through 450 to 650 DEG C brittle Development Zone, will produce high temperature temper brittleness obviously, resulting in significantly lower impact toughness. In order to eliminate the high temperature temper brittleness of 25MnV steel and solve the problem of low and unstable impact toughness of oil casing, the method of rapid cooling after tempering is usually adopted.
     Heat treatment by high temperature tempering 910 DEG C * 30min+600 c * 60min improved intercritical quenching + tempering temperature is 800 DEG C C * 30min+600 * 60min. The 25MnV steel oil casing pipe with the improved process was tested by random sampling, the tensile strength was 926MPa, the yield strength was 822MPa, and the impact toughness was 25.6J. Cm-2, which basically eliminated the phenomenon of ductile instability. The results of application in production show that 25MnV steel casing by sub temperature quenching and tempering was obtained (M+F) duplex microstructure, while ensuring the material deformation resistance and abrasion resistance, can significantly improve the toughness, strength and toughness are organic, and achieved good application effect.