Dissimilar Metal Weld (DMW) Cracking | Materials And Corrosion Control

Damage Description

·         Important factors include the type of filler metal used to join the materials, heating & cooling rate, metal temperature, time at temperature, weld geometry and thermal cycling.

·         In environments that promote liquid ash corrosion, weld cracking problems may be accelerated by stress-assisted corrosion.

·         Thermal cycling aggravates the problem.

·         Poor geometry of the weld, excessive undercut, and other stress intensification factors will aggravate crack formation.

·         The temperature at which carbon diffusion becomes a concern is above 800°F to 950°F (427°C to 510°C) for carbon steels and low alloy steels.

·         Ferritic/austenitic joints can generate significant thermal expansion/thermal fatigue stresses at temperatures greater than 510°F (260°C).

Affected Materials

Control Methodology

·         If 300 Series Stainless Steel welding electrodes are used, the dissimilar metal weld should be located in a low temperature region.

·         In steam generating equipment, the weld at the high temperature end should be made in the penthouse or header enclosure, out of the heat transfer zone.

·         Install a pup piece that has an intermediate thermal expansion coefficient between the two materials to be joined.

Monitoring Techniques

·         Environmental cracking will also result in surface breaking cracks initiating on the ID surface exposed to the corrosive environment, which can be detected using Ultrasonic Testing methods.

Inspection Frequency

KPIs

·         % DMWs inspected and declared crack-free

Reference Resources (Standards/GIs/BPs)