Weld Cracking on Igniter Tips Between Alloy 625 and 310 Stainless Steel

A power plant reported that several of their natural gas igniter tips used in a coal burning power boiler experienced weld cracking after approximately one year in service. In addition to experiencing probable operating temperatures of 1200-1600 °F (~650-870 °C), the igniters were cycled on and off around 25 times during that year. The cracking occurred at both outer and inner diameter welds made between Alloy 625 and 310 stainless steel materials with a Ni-based filler metal. A cracked igniter tip and its corresponding inner tube were retrieved from one of the power boilers for analysis. Two exemplar igniter tips were also analyzed for comparison.

Several potential causes of failure were explored, the first of which was the use of a Ni-based filler metal with Co and W additions leading to decreased weld metal integrity. The thermal history was also investigated, considering the varying coefficients of thermal expansion of the weldment parts. The design and fabrication of the weld joints were also explored as potential contributions to the eventual failures. The investigation and analysis methods conducted included visual inspection, optical metallography and fractography, SEM fractography, finite element modeling support, and EDS analysis. This presentation will explain how these analyses support the summaries and recommendations developed. It was ultimately determined that cracks had initiated from notches created by undesirable welding features such as incomplete penetration and improper weld bead shape at many of the weld roots.