Designers of Boiler Superheater/Reheater pendants incorporate the favorable mechanical properties of stainless steel within these sections of the boiler that combine high heat and high gas flows. However, due to the high material costs, these sections are ultimately welded to more common low alloy steels. These resulting dissimilar metal welds (DMW’s) have a tendency to suffer service-related deteriorations cracking) over time.
Cracking of dissimilar metal welds is typically attributed to three primary factors. The most significant factor is the difference between the thermal coefficient of expansion of ferrite, representing the low-alloy steel tubing, and austenite, representing the stainless steel weld deposit and tubing. This difference results in a significant temperature induced stress at the weld interface.
The second factor contributing to the degradation of this type of weld is carbon diffusion from the ferrite to the austenite. This diffusion occurs slightly during welding and more extensively during subsequent use in high-temperature service. It leaves a band of carbon depleted low-alloy steel immediately adjacent to the weld interface. This band of low-alloy steel has a much lower resistance to creep than average, and as a result, has a greater propensity for failure by creep.
The third factor affecting the integrity of this type of weld is a difference in oxidation resistance between low-alloy and stainless steels. This difference results in an oxide wedge forming along the outside and inside diameters of the component in question at the interlace between these two materials. These wedges will continue to grow as a result of the difference in the oxidation resistance of the two materials. The oxide wedges reduce the available cross-sectional thickness of the component, and thus, its load bearing capacity.
The creep also effectively reduces the mechanical strength of the component, and its available cross-sectional area, further reducing its load-bearing capacity. These two factors, taken in conjunction with the additional thermally-induced stresses, are sufficient over time to produce failures of dissimilar metal welds subject to elevated temperature service.
Due to the multiple factors that can cause cracking in DMW’s, the initiation points can occur on the outside diameter or the inside diameter. Therefore, both surface inspections involving visual and liquid penetrant examinations as well as subsurface inspections involving ultrasonics or radiography are required to provide an accurate assessment. Thielsch Engineering has developed specialized inspection procedures and calibration standards to determine the locations and severity of DMW cracking.