There are several failure mechanisms that might affect ferritic-austenitic dissimilar metal welds (DMWs) in petrochemical plants and refineries. Examples are cracking due to creep, stress corrosion cracking (SCC), sulphide SSC, thermal fatigue, brittle fracture, pitting corrosion, and hydrogen embrittlement. Of these, creep, SCC, and hydrogen embrittlement are perhaps of greater interest.
Industry has many lessons learned; however, still experiences high consequence failures. This work describes the most common failure mechanisms in dissimilar ferritic-austenitic welds and summarizes a guidance to prepare welding procedures and reduce the likelihood of failures. This guidance is based on a literature review and industry experience.
The metallurgical characteristics of the damage observed in both service and laboratory test samples indicate that creep rupture is the dominant failure mode for Dissimilar Metal Welds (DMW) in some high temperature service conditions. However, it has also been observed that temperature cycling contributes significantly to damage and can cause failure even when primary stress levels are relatively low.
Therefore, a creep-fatigue assessment procedure is required as part of a remaining life calculation. API 579-1/ASME FFS-1 2007 Fitness-For-Service standard includes a compendium of consensus methods for reliable assessment of the structural integrity of equipment containing identified flaws or damage. Part 10 of API 579-1 includes a method for protection against failure from creep-fatigue. In the assessment of DMW, a creep-fatigue interaction equation is provided to evaluate damage caused by thermal mismatch, sustained primary stresses, and cyclic secondary loads [Ref.1].
Failures due to hydrogen embrittlement cracking (HEC) mechanisms are not uncommon and are also described in this paper [Ref. 2].
Finally, a case history of a DMW failure in a steam methane furnace, which is common in the petrochemical industry, is described and shown as an example of a failure mitigation approach.