Research On Low Temperature Mechanical Properties Of Pre-tensioned Austenitic Stainless Steel (S30408) Combination Welded Joints

Natural gas consumption is growing rapidly across the globe,and LNG is one of the major ways of natural gas storage and transportation.As the main equipment for LNG storage and transportation,the demand for cryogenic vessels increases rapidly.Austenitic stainless steel has been widely used in the manufacture of cryogenic vessels due to its good toughness and ductility at room temperature and low temperature,corrosion resistance and weldability and so on.It has become an important trend in the development of cryogenic vessels to reduce the weight volume ratio and achieve the goal of lightweight.For austenitic stainless steel(ASS)cryogenic vessel,lightweight can be effectively realized by using the cold-stretching technique and considering the strength of the material under low temperature and improvement in design,which have attracted widespread attention,and have been incorporated into relevant standards in the EU and the USA.With the decrease of temperature,the mechanical properties of austenitic stainless steel changes significantly.Sufficient data of low temperature mechanical properties of materials is the basis for the operation safety as well as utilizing the higher strength at low temperature of ASS in design.As a relatively weak part in the vessel structure,the performance of the welded joint directly affects the safety of the vessel.And while the welded joint is subjected to significant deformation in cold-stretching process,its performance is worth paying more attention to.Therefore,it is of great importance to study the mechanical properties of austenitic stainless steel base metal and welded joints in cryogenic environment.On the support of National Key Research Program(Number:2016YFC080190),a study on low temperature mechanical properties of cold-stretched austenitic stainless steel base metal and welded joints is carried out.The main contents of this thesis are the following parts:(1)Tensile tests of a domestic grade austenitic stainless steel S30408 were carried out from room temperature to low temperature(77K).Low temperature tensile properties of as-received and pre-tensioned S30408 ASS were both studied.And The mechanism of the low temperature yield plateau were studied from microscopic aspect.The results show that as temperature decreases,the yield and tensile strength both increases while the yield tensile ratio and elongation after fracture decreases,and the ReL is higher than Rp1..;for the pre-tensioned material,the yield strength and yield tensile ratio increases while the elongation after fracture decreases,and the tensile strength stays almost invariant,ReL lies between Rp1.0 and Rp2.0 and showed a better ability in constraining plastic deformation;occurrence of the low temperature yield plateau is highly correlated with the martensite transformation.(2)Experiments of austenitic stainless steel PAW+GTAW combination weld with different heat input were carried out.Effect of different heat inputs on the microstructure,micro hardness,tensile and impact properties were studied.The results show that as the heat input increases,the austenite dendrite become more directional,the micro hardness increases and then decreases,the tensile strength increases slightly while the impact ductility decreases.(3)Inhomogeneous deformation of the S30408 ASS combination welded joints in 9%pre-tension process and its influence on the microstructure,micro hardness as well as tensile properties and impact toughness under were also investigated.The results show that after pre-tension,the WM deformed much less than HAZ and BM;the micro hardness decreases in WM while increases in HAZ and BM;the low temperature yield strength increases slightly while the tensile strength increases significantly;impact ductility decreases in both WM and HAZ while it decreases more in the HAZ.