What alloy is used to weld stainless steel to carbon steel?

This is one of the most frequent questions we receive.  If you are welding mild steel to stainless the recommended alloy is 309L or 309Lsi.  This is true even if the stainless base material is 304, 308 or even 316.

For joining stainless to mild steel use a 309L filler

A not so common question is whether carbon steel can be welded with 308L.  There are a few reasons why you would want to weld carbon steel with a stainless filler, most of them have to do with not having to change out the wire in robotic applications for short runs of mild steel product.  It makes sense that  308 would be sufficient, but this is not always the case.  A 308 filler would yield a deposit that when mixed with carbon steel would be similar to a 410 stainless deposit.  This would be a weld deposit that may be too brittle for the application.  The recommended filler in this case would also be 309L since when mixed with carbon steel it yields a deposit similar to a 308L deposit.

For more information on welding stainless steels you can read 5 Common Mistakes Made Welding Stainless Steel.