Investigation On The Austenitic Welding Material With Low Cr And High Mn Contents

Dissimilar metal welds (DMWS) between the austenitic stainless steel and Cr-Mo pearlitic heat-resistant steels are widely used in petrochemical industry and pressure vessels servicing at elevated temperature. In addition, the dissimilar joints are also used in the steam tube of boiler of power plants. Normally, the filler metals used to weld Cr-Mo steels are austenitic welding materials. From the incestigation on the earlier failure of dissimilar joints, it indicates that two factors affect the earlier failure of the dissimilar joints: one is thermal stress caused by the difference of thermal expand coefficient between the weld metal and Cr-Mo steel based metal; another is the carbon migration resulted from the carbon chemical potential gradient. Carbon migration across the weld interface occurred in the fusion zone when the dissimilar joints service at high temperature. Carbon migration induces the formation of carbon-enriched layer in the austenitic weld metal and carbon-denuded layer in the Cr-Mo steels base metals. The increase of hardness in the carbon-enriched layer is harmful to the toughness of the weld metal. The carbon-denuded layer causes the loss of creep strength and low tensile in the Cr-Mo steels adjacent to the weld interface. The distinct difference in properties across a narrow region in fusion zone is responsible for the earlier failure of dissimilar welds serving at high temperature.The objective of the current work is to developing new welding materials to substitute the previous Cr-Ni austenitic welding materials to join the Cr-Mo steels, which was part of the project "D&P of low Cr austenitic welding materials" supported by the National "Ninth Five-year Plan" key project of China under contract No. 96-120-49-01.The research had passed the Identified Conference hold by National Ministry of Education. In addition, two patents were authorized in August of 2002Basing on the investigation on failure mechanisms, influencing factors and previous study on the project, we successfully design and develop new low Cr austenitic steel weld materials. The research includes: alloy design, smelting, forging and hot rolling, the preparation of welding wire and welding electrodes, the property testing of the new austenitic steels and their dissimilar joints. In addition, the changing of the width of carbon-enriched layer and the distribution of the alloy elements around the weld interface are studied. Innovative results obtained in this paper are listed as followed:1 .In the design of alloy composition, comparing with the composition of the previous austenitic filler metals, low content of Cr is selected to reduce the chemical potential gradient, and then, the "uphill diffusion of carbon" in fusion zone is weakened. The high content of Mn element is applied to get stable austenitic microstructure and reduce the contents of Cr and Ni in the austenitic steel. Thermal expand coefficient of new weld materials matches the Cr-Mo steel base metal by adjusting the equivalences of Cr and Ni elements. By adding Mo, Ti andSi elements, the properties and microstructure are improved.2. We developed a new kind of welding electrode with alkali coating. The electrodes are available for both AC and DC arc welder. It can also effectiviely overcome the weakness happened during the welding process welded using previous austenitic welding electrodes, such as the desquamation of the coating and the rubefaction of the coating can be overcome effectively.3. The technical research and microstructure investigation show that the new welding materials have better cold working and hot machining properties and excellent weldability than E309-15 and E309-16 electrodes. The mechanical properties and oxidization-resistant of new welding materials are superior to those of Cr-Mo steel. The coefficient of thermal expansion of the austenitic steels is match with the base metal. The mechanical properties of the dissimilar joints are similar to the base metal.4. Comparing with the E309-15/Cr-Mo dissimilar joints, differences are obs...