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Welding Dissimilar Metals Using 309 Stainless Steel

Q: We commonly build and weld carbon steel parts with a standard ER 70S-6 wire using a 98 percent argon/2 percent carbon dioxide shielding gas. Recently we've begun manufacturing parts that combine carbon steel, ferritic stainless steel, and other grades of stainless steel (SS) using a 309LSi grade of wire with GMAW. Our welders are struggling with weld fusion on the ferritic-to-carbon-steel joints—the welds don't seem to be fusing with the carbon steel. Any suggestions?

A: This problem is not uncommon when welding stainless steel to carbon steel. In many cases it can be resolved simply by removing any mill scale or surface impurities from the base materials. Keep in mind that the surface of stainless steel has a tough oxide layer that makes it passive to environmental attack. This oxide can sometimes cause welding difficulties.

Because you specifically mentioned welding ferritic to carbon steel, we will focus on the potential problems welding these dissimilar metals.

In most cases when welding dissimilar metals or, more specifically, carbon to SS, we recommend you use a 309 filler metal because of its higher ferrite content. This higher ferrite content can minimize weld dilution and prevent weld cracking. The 309LSi has a low carbon content and a higher silicon content, hence the "LSi" in the designator. The lower carbon content is ideal for applications that have a risk of intergranular corrosion cracking. The higher silicon content serves as a deoxidizer and helps remove weld impurities and increase weld puddle fluidity.

Because you are welding carbon steel to SS, be some weld dilution from the sides of the joint will occur. The small amount of base material dilution in the weld metal will provide a better match to each of those respective base materials. You should not have difficulties welding on the ferritic side of the joint as stainless-to-stainless welding should fuse relatively well. If there are problems, check to ensure the welding machine is set up correctly with good work lead connections.

The 309LSi weld issues on the carbon steel may be the result of contamination. Try cleaning the steel with an approved cleaner to remove any grease, oil, or paint. Grinding the mill scale ½ inch back from the weld joint may also help.

Last, the 98 percent argon/2 percent CO2 may not be aggressive enough to allow for proper weld bead wetting on the carbon steel side of the joint. Generally, you want to keep the CO2 low to minimize carbon pickup in the weld joint and prevent the welds from oxidizing too much. However, since there is already cross-contamination from welding on the carbon steel, using a higher-CO2 mix, such as 90/10 or even 85/1, may provide the necessary oxidizing action to allow the weld to tie in properly. 


This article originally appeared in The WELDER magazine.
It is reprinted here with permission of the Fabricators & Manufacturers Association, Intl.

Posted in Filler Metals , Tagged with Gas, MIG, Steel