"In-situ" Observation Of Corrosion Process In The Fusion Boundary In Dissimilar Steel Welding Joints And Prediction Of Carbon Diffusion

Along with the development of modern industry, new technologies, dissimilar steel welding is widely used in variant engineering fields. As a main part of workpiece, the performance and technics of dissimilar steel welding joints have a key impact on the life-span of workpiece. The fusion boundary is a transition zone which is totally different from base metal and weld metal in microstructure and components. It has a key role in the performance of the joints. Because the working environment of dissimilar welding joints is complicated, mostly in high temperature, high pressure and corrosion environment. Therefore, the research of corrosion resistance of dissimilar welding joints is far more important.Carbon diffusion in service is a main factor of the performance of dissimilar welding joints. We provide a new method to the research of life-span in high temperature by simulating the situation of workpiece in service through our mathematic model.This dissertation consists of five chapters. Chaper one is an introduction, in which the origin and supporter of the project, significance. The development and present research in the world about the corrosion of dissimilar welding joints are reviewed. We use high resolution electron microscope to evaluate the corrosion resistance of the fusion boundary of dissimilar welding joints. Compared to the traditional macroscopically methods, it is a new one to analysis the corrosion resistance from microscopically. We also propose to the method for prediction of the carbon migration process in dissimilar steel welded joints during heat treatment or high temperature service based on grey modeling assisted artificial neural network (ANN).In chapter two, the welding materials and experiment methods are presented. In chapter three, our original idea of "In-Situ" method to the corrosion of dissimilar welding joints has been introduced. We use this method to observe the characteristics of Cr5Mo+A302 and 2.25Cr-1Mo+Cr24Nil3L welding joints in corrosion processprecisely and systematically. The main reasons of the corrosion resistance of the fusion boundary have been discussed. In chapter four, the method of grey modeling assisted artificial neural network to the prediction of carbon diffusion has been presented. It has been shown that the calculated prediction curve using the present method exhibits a good match with the actual measured curve from the workpiece which have been served in high temperature for a long term.Chapter five is the conclusion of all the research mentioned in this dissertation.