Dissimilar Metal Welds: What to Know Before You Flow
Once upon a time, material cost wasn’t much of an issue. In most industries, a part was replaced with the same material, and often the same material was used throughout an entire system.
Today, it is more common to have three or four materials in a single system. An improved understanding of metallurgy makes this possible. Also, new, advanced materials may be able to provide a better lifetime value. When it comes time to replace a part, plant owners and system designers ask, “Where is this material needed?”, “Where is it not?”, and “Is there another material that would work better?”.
Yet, connecting two different materials in a system can be challenging. If not connected using a mechanical system such as a flange, a common alternative is a dissimilar metal weld (DMW).
If a DMW is necessary, it is a good idea to have a trusted resource to help decide the best approach to complete the weld. Planned and performed correctly, a DMW can meet system design and reliability goals.
Here are a few things to consider when planning for DMWs:
ASME categorizes metals using a “P” number. P1 is standard carbon steel. Each category includes different material grades/groups (low carbon, low chromes, austenitic stainless, high-performance alloys, and duplex). Although the materials are chemically similar, the DMW process will be different.
A DMW will result in a third material that is a combination of the first material, the second material, and the weld wire material. Through a process called “dilution,” this new, third material must also be compatible with design requirements and the service environment.
Heat from the DMW process can alter or even damage a material’s properties. Pre-heat, interpass heat, and post-weld heat-treat temperatures must be considered. For example, sometimes a 309-weld filler material is used to weld austenitic to carbon steel. However, the operating temperature of the system needs to be considered if this temperature will exceed 800 degrees F. The internal stresses will be higher and the graphite phase of the 309-weld material will deteriorate over time. Instead, a higher strength alloy wire such as INCO 112 or 182 should be used.
Many weld procedures exist, but the codes don’t cover every imaginable combination of materials. Even if there is an existing weld procedure, it may not be the right procedure for the specific application. It’s best to evaluate the parameters and develop a custom weld procedure each time.
DMW is best performed in a controlled environment, such as a fabrication shop. In the shop, where a DMW is required, it is typical to add a 6-12” “safe end” that is the same as the field end material. This transition piece allows the final field weld to be completed using compatible materials and minimizes the chance of problems associated with DMWs.
If you recognize that you have an upcoming project involving a DMW process, reach out to a trusted resource to discuss the project and ensure that the proper welding procedures are followed. SEFA’s experienced engineers and welding technicians would be happy to provide advice and guidance concerning DMWs. Contact us to start a conversation about your next project.