99 Diseases of Pressure Equipment: Dissimilar Weld Metal (DMW) Cracking and Other Spec Breaks

DMW cracking is another fabrication issue that can and does result in equipment failure. It usually occurs at the weld juncture where carbon steel or low alloy steels are welded to austenitic (300 series) stainless steels in high temperature applications. The large difference in coefficient of expansion of the two steels, sometimes exacerbated by thermal cycling, results in cracking at the toe (HAZ) of the weld joining the two materials. Using an austenitic stainless filler material for the DMW junction also increases the stress intensification on the toe of the weld on the ferritic side of the weldment. This type of cracking is most common when temperatures above 800F (425C) are involved, such as in FCCU reactor/regenerations systems, superheaters, reheaters, fired heaters, and hydroprocess equipment. Use of bolted joints, if possible, or nickel base filler materials helps to avoid the DMW cracking problem.

A related issue involves knowing where all your junctions between materials occur (spec breaks), be they welded, bolted, or other mechanical connections, so that steps can be taken to ensure that corrosive fluids to not come in contact with the wrong side of the junction. Occasionally industry incidents (breaches of containment) occur because corrosive fluid or high temperatures crossed over a spec break into a system that was not designed to handle the more aggressive conditions, i.e. spec break in the wrong place or change in operating conditions from the design conditions. Spec breaks are important information to have on hand during process hazard analyses (PHA’s) so that the team can reaffirm that current and expected operating conditions are not expected to be different than design.

Do you know where your DMW’s and spec breaks are and are you comfortable that their risk of contributing to a failure is adequately low?