Analysis of failure of dissimilar metal welding joints between furnace tubes and flanges in hydrogen manufacturing unit

In view of the cracking and leaking occurred in high-temperature operation of dissimilar metal welding joints between HK-40 furnace tube and Cr5Mo flange using austenic 407 steel welding rods, the failure was analyzed with optical microscope and scanning electron microscope etc. The results indicated that during a long period of operating at high temperature, carbon diffused and moved from the lean chromium material into the rich one, forming a decarbonized layer at the Cr5Mo base metal side near the molten zone. Therefore, the matrix material softened and the strength decreased due to the lacking of carbon. In addition, owing to the great difference in linear expansion coefficients of the Cr5Mo steel and the austenic weld metal, the inconsistent deformation at high temperature and the concentration at the melting interface, cracks were induced on the decarbonized layer at the Cr5Mo base metal side near the molten zone and extended along with the decarbonized layer to penetrate both inter- and external walls and resulted in leaking.