Heat Treatment And Properties Evaluation Of Deep-sea Oil And Gas Well Head Materials

The development of offshore oil and gas is gradually developing into the deep sea area,and the underwater wellhead system plays an important role as a platform hub.At present,the wellhead forging materials used in our country basically rely on imports.Based on AISI 8630(G1)steel,microalloying 8630(G2)steel is independently designed in order to realize the domestic production of such wellhead materials.The mechanical properties and corrosion resistance of G1 and G2 steels are comparatively studied in this article.The heat treatment system was formulated by the orthogonal test method.Mechanical properties such as tensile and low-temperature toughness were tested;the electrochemical behavior of the test steel in the simulated seawater solution and the corrosion resistance of the immersion and salt spray test were evaluated,which was used to optimize the heat treatment process.The test steel treated by the best heat treatment process was subjected to stress corrosion test under H₂S environment.The test results show that the forged structure of G1 and G2 steel is mainly upper bainite and the microstructures of G1 and G2 steels after tempering are tempered martensite.TEM results show that spherical and long-chain carbides are distributed on the slats,which is M₂₃C₆ and Fe₃C,respectively.The carbide content of G2 steel is higher than that of G1 steel.The conditions of normalizing temperature of 870?,quenching temperature of 840?,tempering temperature of 580?,and tempering time of 4 h are the optimal heat treatment process for G1 steel,whose strong plastic product can reach 18307 and low temperature impact energy at-18?is 207 J.The most favorable conditions for G2 steel are 890?,820?,580?,6 h,whose strong plastic product and low temperature impact energy at-18?are 16986,109 J,respectively.Electrochemical experiments show that G1 and G2 steels after heat treatment in different processes show similar electrochemical behavior in the same electrolyte solution.Corrosion resistance decreases in the order of 3.5%Na Cl,simulated oilfield production fluid,3.5%Na Cl+0.5%CH₃COOH.The 3.5%Na Cl solution immersion indicates that the corrosion products formed on the surface of the test steel will protect the material from Cl-erosion.Corrosion products are mainly composed of Fe₃O₄ and Fe OOH.The 3.5%Na Cl+0.5%CH₃COOH solution will cause the corrosion resistance of G1 and G2 steel to drop sharply,and the flowing aqueous solution will significantly accelerate the corrosion rate.The neutral salt spray test shows that the corrosion rate of G1 steel is higher than that of G2 steel,and the main corrosion products are Fe OOH,Fe₂O₃ and Fe(OH)₃.After the optimal heat treatment process,the sample was subjected to SCC test in H₂S environment.The G1 steel did not break for 720 hours,whose fracture morphology still showed ductile fracture mode after stretching and the penetration thickness of S element was only 2?m.The G1 steel showed higher corrosion resistance.The cracks on the surface of G2 steel present the characteristics of stepped hydrogen-induced cracking.In the H₂S environment,the hydrogen generated by ionization is captured by the hydrogen trap and continuously accumulates to form hydrogen bubbling or hydrogen-induced cracks.The Fe S corrosion film prevents the formation of oxide film and increases the possibility of anode dissolution,which interacts with stress to improve the SCC sensitivity of G2 steel.