Premature failure of dissimilar metal weld joint at intermediate temperature superheater tube

• Premature failure of EPRI recommended nickel based filler for dissimilar metal welds (DMW) has been investigated to determine the root causes.
• Metallography, mechanical testing and microscopic study allowed the determination of the failure mechanism and its causes.
• The mechanism involved a creep stage I process on the alloy steel side due to improper PWHT leading to undesirable carbides precipitation increasing the hardness.
• The sensitivity of the welding to micro-cracking combined to bending stresses involvement induced a premature failure within short time of operation.

Dissimilar metal weld (DMW) joint between alloyed steel (AS) and stainless steel (SS) failed at one of intermediate temperature superheater (ITSH) tube in steam/power generation plant boiler. The premature failure was detected after a relatively short time of operation (8 years) where the crack propagated circumferentially from AS side through the ITSH tube. Apart from physical examination, microstructural studies based on optical microscopy, SEM and EDX analysis were performed. The results of the investigation point out the limitation of Carbides precipitation at the alloyed steel/welding interface. This is synonym of creep stage I involvement in the failure of ITSH. Improper post-welding operation and bending moment are considered as root causes of the premature failure.