Large utility-type steam generators inevitably contain a large number of pressure part welds that join components fabricated from different alloys.
The welds made between austenitic stainless steel tubing and the lower-alloyed ferritic grades of tubing (T11, T22) deserve special mention because of the early failures that developed in some of these dissimilar metal welds (DMWs) soon after their introduction in superheater and reheater assemblies. Prior to the mid-1970s, many DMWs were fabricated either as standard fusion welds using an austenitic stainless filler metal, such as TP308, or as induction pressure welds, in which the tubes were fused directly to each other without the addition of filler metal. Some of these welds failed after less than 40,000 hours of operation, with the earliest failures being associated with DMWs that operated “hot” in units that cycled heavily and were subjected to bending stresses during operation.
After the mid-1970s, and in response to extensive research carried out by EPRI and other organizations, an increasing number of DMWs in superheater and reheater tubes were fabricated as fusion welds using nickel-based filler metals, such as the INCO A, INCO 82, INCO 182, etc. The technical rationale for the switch to the nickel-based filler metals was the improved compatibility in thermal-physical properties with the lower alloyed materials (typically Grades 11 or 22). It was hoped this would reduce the complex stresses that developed in the lower alloyed material near the dissimilar metal fusion boundary during temperature cycling and that were believed to be an important factor in the early deterioration of the original DMWs.
Other factors that have been shown to influence DMW life are the average temperature of operation and secondary stresses acting on the weld. Subsequent experience has confirmed the relative longevity of the DMWs made with nickel-based filler metals, although that same experience has proven that even these welds will fail at the dissimilar metal interface with the lower alloyed material after prolonged service under certain conditions of stress and temperature. Because of the different metallurgies at the dissimilar metal interface when comparing DMWs made with austenitic stainless filler metal (or the induction pressure type DMWs) and DMWs made with nickel-based filler metal, the evolution of damage in the two types of DMW differs slightly.
For DMWs to low alloy steels (T11, T22), creep and creep-fatigue are the primary damage mechanisms responsible for the failure of DMWs in superheater and reheater tubing, with creep being the dominant component in the damage evolution process. Creep damage is exacerbated by carbon migration that occurs from the low Cr base metal to the high Cr filler metal, resulting in a creep-weak region adjacent to the fusion line. As such, the most important influences on weld life will be the average operating temperature and the magnitude of the stresses acting on the weld. The stresses include primary stresses (i.e., hoop stress and dead weight load), secondary stresses (e.g., bending stresses caused by malfunction of tube supports or by restraint of thermal expansion), and so-called “self stresses” generated along the dissimilar metal interface by the difference in thermal expansion between the austenitic weld metal and ferritic base metal.
Fusion welds made with austenitic stainless filler metal typically contain damage in the form of creep-induced cavitation and micro-fissuring at grain boundaries in the ferritic material adjacent to the dissimilar metal fusion boundary. Fusion welds made with nickel-based filler metals most often develop damage as cavitation associated with large blocky carbides preferentially aligned along the fusion boundary. The large blocky carbides have been designated “Type I” carbides to distinguish them from the diffuse array of smaller carbides – the “Type II” carbides – that form in DMWs made from both austenitic stainless steel and nickel-based filler metals.
Less commonly, DMWs have failed by a separate damage mechanism involving the initiation and growth of an oxidation-fatigue notch from the toe of the weld on the ferritic side of the joint. These oxide notches, which are fairly common in DMWs and generally have a minimal impact on weld life, can become the life-limiting factor if the tube is relatively thin-walled and if it is subjected to a relatively high bending load.
Ferritic side of superheater or reheater tube welds that join austenitic stainless material to lower-alloyed ferritic material.
The following article summarizes several case studies of damage in DMWs between low alloy steels and stainless steels.
A future article will provide insight into DMWs that involve the Grade 91 alloy, such as welds between P91 piping and 1CrMoV valves or where P91 is welded to stainless steel flow meters. There are some differences between DMW failures in bainitic steels like Grades 11 and 22, and in martensitic steels like Grade 91, so look for that article in a future edition.