Study On Microstructure Transformation In Austenitic Stainless Steel Weldments Serviced In Ultrahigh Temperature

With the development of scientific technology, there is a growing concern for the problem of how the materials of steel structure can be serviced under the condition of ultrahigh temperature. In addition, the service conditions are usually about to approach the service limit of the steel structure, which requires the higher performance of steel structure. Since austenitic stainless steels have a fine mechanical properties and the capability of the scaling resistance, that is why, it is widely used for the working environment around 1000癈. However, under the higher temperature, nickel based heat resisting steels are often used. A great concern both nationwide and worldwide has been showed for the study of austenitic stainless steels and its welded joints, and a lot of research projects in this area has been carried out. But currently, the research work on austenitic stainless steels service under the temperature over 1000C,especially around 1150癈 and 1200癈 is very limited.This study focuses on the inner cover of a big rotary annealing furnace (ROF) of silicon steel in a domestic plant. The temperature under this working condition is 1200C, which is the threshold for the use of austenitic stainless steels . This research work has an emphasis on the transformations of the weldments microstructure by the means of the scanning electron microscope, and the transmission electron microscope, both before and after the service of the inner cover of the austenitic stainless steels under the ultrahigh temperature. Its service behavior under the ultrahigh temperature status is also analyzed. This study provides reliable information for the industrial production and a valuable reference for further theoretical investigations.The paper consists of seven chapters. Chapter 1 introduces the background and the significance of the project. Chapter 2 describes the composition design requirement for austenitic stainless heat resisting steels,and problems about ultrahigh temperature furnace arising from austenitic stainless steels. It further focuses on a new conceptual design for furnace using austenitic stainless steels. Chapter 3 describe the characteristics of solidification of austenitic stainless steels and their crystal configurations. Chapter 4 describes existing studies of fully austenitic weld metal. It mainly discusses the eutectic carbides among the austenitic stainless steels and its affection to hot cracks. Chapter 5 describes materials for experiment and experimental procedures, which focuses on comparing the different effects of two etchant. Chapter 6 analyses the test result and explains why the welded joints' scaling resistance is better than that of the base metal. Chapter 7 is the conclusions of the paper, and put forward the questions which should be investigated in the future according to the work in this paper.