Strength evaluation of Ni-base alloy overlay welding tubes for the coal-fired boiler furnace

Recently in coal-fired boiler of Japan, a water wall tube damage such as fireside corrosion and circumferential cracking in a reducing atmosphere due to low NOx environment has become a serious issue. And the improvement of the corrosion resistance of the furnace wall is required. In order to apply the Ni-based Alloy 622 overlay welding material to the furnace wall, corrosion test, fatigue test, and long-term aging test have been conducted. And an overlay welded panel has been inserted into the actual furnace wall. This report describes these test results of the Alloy 622 overlay welding material tube. Alloy 622 overlay weld metal demonstrates good corrosion resistance to H2S gases in a high temperature and low NOx environment. It was found that the corrosion rate of Alloy 622 is one tenth (1/10) that of 1.25 % Cr bare tube in corrosion testing. From the results of the high-temperature bending fatigue test, it is considered that the Alloy 622 overlay weld metal has enough durability for actual action stress due to the rapid temperature change. The Alloy 622 overlay welded panel also showed no evidence of corrosion after seven years of service. All of the field test results clearly demonstrate that the Alloy 622 has excellent against fireside corrosion and circumferential cracking in supercritical pressure boilers. © 2017 Japan Welding Society. All rights reserved.