Research Of Stainless Steels Used For Heat Transfer In USC Boilers

The materials and manufacturing technology of high pressure steam tubes, superheater tubes and other key components for USC boilers are the development bottleneck of the power equipment manufacturing, which have been attached great attentions in the world. In the past, the standard and production of our boiler steel in the quantities, varieties and specifications, types, and both internal and surface quality can not fully meet the need of USC boilers development in China's power industry. Currently, these steel except a few steel grade basically depend on imports, but the cycle is longer, and the price is more expensive.To achieve domestic production of the critical heat transfer materials in the (ultra) supercritical fossil power units, based on Super304H steel made in Japan Sumitomo, we have studied the development of new austenitic stainless steel of Fe-Cr-Ni-Ti-N series and its key technology. This thesis has gained some stage achievement in the support of technology transformation project in Jiangsu province "Development and Industrialization of Key Pipe Materials for USC Boilers" (BA2007033).New austenitic stainless steel was design based on analysis of the relation of composition, microstructure and properties in the Super304H stainless steel. The main idea were:①Using N alloy instead of C alloy;②Decreasing Nb content;③Lowering the Ni content. Using vacuum induction melting furnace (ZG-0.05) to melt stainless steel Thermal field emission JSM-7001F scanning electron microscope was used to observe the microstructure and fracture surface morphology. Japan Science D/MAX2500PC type X-ray diffraction was used to phase analysis, RDL100 electronic high temperature creep testing machine was used to creep test, and high temperature oxidation test was carried out for stainless steel.Microstructure analysis shows that the second phase in S01 test steel is mainly NbN, and high-temperature 8 ferrite did not appeare due to the lower C content. The corrosion results elucidate that S03 austenitic stainless steel has greater sensitivity of the grain boundary corrosion of corporate pilot production, and S03 steel grain boundary phases (Cr23C6) increase in the analysis of the microstructure after creep. A little high-temperature ferrite appeared, and the main second phase was NbN in S02 stainless steel, which had smaller sensitivity of grain boundary corrosion.The results on the mechanical property tests on this three kinds of austenitic stainless steel show that the tensile strength is higher than the specified value of Sumitomo Super304H steel, especially for S02 steel with 30% higher than the specified value. Meanwhile elongation values at room temperature also satisfy the requirement, consistenting with analytical results. There are some differences between S01 and S03 in the high-temperature short-time tensile strength and steel with Super304H steel, and high temperature tensile strength of S02 steel is higher than the measured value of Super304H steel. Based on tensile and impact fracture analysis, the dimple in the bottom has different morphologies of the second phases, thick pure Nb phase was found. It shows that the improvement in metallurgical quality could be done.By a trial test on the creep performance of austenitic stainless steel at 600℃, the fracture is brittle one, and there is tear ridge along intergranular fracture. A large number of small dimples is in the surface of S01 steel grain, indicating that microscopic holes appear and form slowly the crack source in the grain boundary, while it is found that a considerable number of crystal edges (at the junction of three grains) have the crack in the fracture analysis. Creep rupture is caused by microscopic holes in the grain boundary and cracks in the triple line boundary. The surface of S03 steel grain has no small dimples. The strip second phase NbC appears and obvious tearing ridge around the apparent edge is also found, which show that the crack first appears the edge of the strip second phase. According to creep test, creep rupture strength of S01 steel with lower C content is best.At 700℃, structures of high temperature oxide films of different alloys are different due to different Gibbs free energy of various oxides. During oxidation process at 700℃, Cr2O3 formes preferentially on surfaces of S01 and S03 steel. After oxidateed at 700℃for a long time, the oxide film was mainly composed of Cr2O3 and a small amount of spinel structure FeCr2O4. In S02 steel with high Mn, the mixed oxide spinel structure MnFe2O4 exists in the oxide film. From the oxidation kinetics curve, three kinds of austenitic stainless steel manifested "totally anti-oxidation" under the temperature of 700℃, according to the classification standards of the average oxidation rate.By the use of the TIG welding process with welding current (DC) for the 180A and 210A, S02 and S03 steel were weld respectively with an appropriate increase in welding speed. Cr23C6 is not observed in the HAZ, which indicates elements of C and Cr do not significantly diffuse near the grain boundary. In addition, Super304H flow stress amended JC model has been obtained, after the analysis of the influential factors of flow stress in the constitutive relation of materials.