Study On Low-Temperature Mechanical Properties And Microstructural Stability Of Different Nitrogen Content Of Austenitic Stainless Steel

High strength, high toughness, lamellar tearing resistance, cold working, and sea water corrosion resistance and other performance indicators are necessary for offshore platform steel, great development space have been brought to austenitic stainless steel because of the use of these cryogenic technology. In this paper, three kinds of nitrogen and low nickel contain austenitic stainless steel are designed: H1,H2 and Z1. SEM, X-ray diffraction, TEM, tensile testing machine and impact testing machine are used to test the low temperature mechanical properties, microstructural stability and cryogenic properties of three steels.For H1 and H2, single austenite phase with twin crystal appeared after solid solution treatment at 1050 °C and heat preservation for 1 h. Solid solution treatment of Z1 is at 1000 °C and heat preservation for 2 h, single austenite phase with a small amount of Crx Oy and Mnx Oy at grain boundary appeared after solid solution treatment,twin crystal also contains in the organization.The result of the low temperature tensile experiment, H1 has the best low temperature mechanical property, H2 after it, Z1 is the lowest. Fractures of H1 and H2 covered with toughening nests, and inclusions at the bottom of toughening nest, the inclusions of H1 are chromium oxides and manganese oxides, and the inclusions of H2 are sulfur and manganese oxide.Results of low temperature impact tests of three steels show that, the impact energy of the tested steels decreased with the decline of the test temperature,ductile-brittle transition occurred in each test. The impact energy of Z1 is higher than that of H1 and H2 at all test temperatures, and the impact energy of H2 is higher than that of H1. The change of nitrogen content had little effect on the ductile-brittle transition temperatures of H1 and H2, but the ductile-brittle transition temperatures of H1 and H2 have obvious improvement compared with Z1.Tensile strength and yield strength of H1 have certainly reduced because of changing the frequency of cryogenic treatment, impact toughness can be improved by changing the frequency but can’t be improved by changing the duration of cryogenictreatment. Impact toughness, tensile strength and yield strength of H2 can’t be improved by changing the frequency and duration of cryogenic treatment, the cryogenic property of H2 which has lower nitrogen content is better.XRD results of three steels indicated that, the organizations of three steels stay still after tensile experiment, impact experiment and cryogenic treatment of H1 and H2. For H1 and H2, no α’ martensitic, ? martensitic and nitride had been found at TEM tests. The three steels have superior stability of microstructure at low temperature.