Summary of 10 Years of Field Testing Experience with Weld Overlay Materials for Water-wall Applications in Supercritical Boilers

摘要

Weld overlay of the UNS K1157 (ASME SA213-T11) boiler tubes is required in coal fired boilers operating in a low-NO_x combustion mode to prevent excessive corrosion of the pressure tubes. When retrofitting boilers to a low-NO_x combustion mode (to reduce emissions), it was found that corrosion of bare alloy steel tubes increased to an unacceptable rate. The increase in corrosion is due to the presence of reduced sulfur species in the flue gas and deposits created by low-NO_x combustion. Experience has shown that there are a number of weld overlay materials are capable of reducing corrosion to acceptable levels. However, it was also found that circumferential cracking of the boiler tubes and weld overlays is now the limiting factor in boiler tube life. The cracking mechanism is attributed to a corrosion fatigue mechanism. Thermal expansion of the tube surface causes strains that can lead to the observed corrosion fatigue in the combustion environment. While corrosion testing of candidate materials in the laboratory is possible, replicating the conditions responsible for the circumferential cracking (corrosion fatigue) is difficult if not impossible to replicate in a laboratory environment. Therefore, an extensive testing program of candidate materials for use as a weld overlay on water-wall panels in supercritical coal-fired boilers was initiated in 2004. After 10 years of field testing a variation in resistance to circumferential cracking was found among the materials tested. In addition, the method of application of the weld overlay was found to be an important factor. Weld overlay thickness and application process (hand welding versus automatic shop application) appears to be important in the onset of circumferential cracking. One of the most resistant materials in this test program was UNS R20033.