Study On The Properties Of N80 Steel By QPQ Treatment

N80 steel is one of the most widely used steel in oil instrument. Half of the oil pipe is made up of N80 Steel. As N80 steel works in high temperature, high pressure and high salinity oil field water corrosive environment, billions economic lose as a result of corrosion cracking every year. Recent years, major domestic steel plants stepped up in the study of composition, process, and protective means of N80 steel. In order to enhance the applicability of N80 steel in oil field environment, realizes the industrial application of QPQ technology in oil field equipment, it is very important to study mechanical properties and electrochemical corrosion resistance of QPQ treated N80 steel, and to optimize the QPQ technology for N80 steel.In this paper, both mechanical properties and corrosion resistance of N80 steel were investigated according to American Petroleum Institute (API) standard tensile experiments and electrochemical corrosion tests. The results show that, (1) the mechanical properties of the N80 steel after QPQ treatment at 650 ? match the API standard requirement. The ferrite coarsening and surface hardening is evident after QPQ treatment. Both the yield strength and tensile strength vary less than 5% compared with that of the untreated samples. The initiation of micro cracks in loose layer of QPQ treatment should be responsible for the decrease of elongation on average by 5% compared with the untreated samples. (2) Compared with galvanized, chrome plating, gas nitriding, salt bath composite processing samples and untreated N80 steel sample, the QPQ treatment sample at 650 ? possess the highest corrosion potential, the smallest of the corrosion current density and impedance values is the largest. The corrosion performance of QPQ technology processing sample and gas nitriding treatment one degrade because of loose layer, while the weak of the electrochemical corrosion resistance of the galvanized and chromium plating attribute to the dissolution of coating corrosion products. (3)As polarization curve in the passivation comes into being, QPQ process of N80 steel corrosion resistant performance is the best when salt bath nitriding at 620 ? for 2.5 h and salt bath oxidation at 350 ? for 15 min, salt bath C (CNO-) in 0.24?0.28 mol/L.