MICROSTRUCTURE AND MECHANICAL PROPERTIES OF THE WELDING JOINT OF A NEW CORROSION- RESISTING NICKEL-BASED ALLOY AND 304 AUSTENITIC STAINLESS STEEL

Abstract

With the fast development of industry, a serious global problem, pollution, becomes more apparent. A large number of wastewater is discharged, causing the environment pollution. Supercritical water oxidation(SCWO) becomes the most effective method to treat the wastewater within recent years, but the material used in the equipment plays a key role in restricting the application of the SCWO process. Currently, during the SCWO wastewater treatment process, 304 austenitic stainless steel, alloy 625, P91 and P92 steels are the mainly preheater and reactor materials. In order to reduce the serious corrosion and improve economic efficiency of the materials for this process, a new corrosion resistant Ni-based alloy(called X-2# alloy) has been developed with an aim of replacing the previous ones. In particular, it is highly important to the related behavior of this new alloy welding with the original SCWO. Therefore, the microstructure and mechanical properties of the welding joint of the new alloy and304 austenitic stainless steel with manual argon arc welding were investigated. The microstructure and fracture morphologies of the welding joint were analyzed through OM, SEM and EDS, and the detailed analysis of the micro- hardness, tensile strength and other mechanical properties were performed. The results demonstrated that the parent material with the typical 40~65 mm grains size is helpful for dissimilar steel welding, and the microstructurein fusion zone of X-2# side does not show welding defects. However, some ferrites are further formed near the fusion zone of 304 stainless steel sides. There are Cr-rich and Ni-poor distributions in the ferrites. The grain grows seriously in both the areas near the remelt zone and 304 stainless steel side of heat affected zones(HAZs), which affect heavily the performance of welding joint. In addition, the results also uncover that the Vickers-hardness is the minimum in the HAZ. At room temperature, the fracture location of the tensile tests of X-2#/304 is in the welding seam, whereas at 500 ℃ the corresponding position is in the 304 matrix. Due to the strengthening effects of Al, W and Mo elements, the high temperature mechanical properties of X-2# alloy have been found to be even better than those of the 304 austenitic stainless steel.