ARC MAGAZINE

Welding Stainless to Carbon Steel

Many times fabricators are faced with the task of joining dissimilar metals. In situations like this, maintenance welders, auto shop technicians and home hobbyists have to decide which filler metal and welding process to use to produce a quality weld.

One such application is connecting a 304L stainless exhaust tube to a 1018 carbon steel flange.

When selecting filler wire for welding dissimilar metals, such as stainless to mild steel, fabricators must understand that the weld deposit is a mixture of three metals – the two metals being joined, as well as the filler metal that joins them.

Using typical carbon steel and stainless filler wires are not appropriate for welding the dissimilar stainless and carbon steel.

The chemistry of 304L stainless steel contains about 18% chromium and 8% nickel, which makes it a ductile material that is not hardenable by heat treatment. Two pieces of 304L stainless are typically successfully welded with 308L filler metal. But using 308L filler metal to join carbon to stainless is not a good idea, because the diluted weld deposit could also be sensitive to solidification cracking and embrittlement.

Similarly, by improperly using a carbon steel filler metal, the weld deposit chemistry would be a mixture of the carbon steel flange, the carbon steel filler wire and 304L tube. The chemistry of the weld would not be a ductile stainless steel but possibly chrome nickel steel, very hard and brittle.

There are a few considerations in choosing which stainless steel filler is right to join 304L stainless exhaust tube to a 1018 carbon steel flange. The highly alloyed 309L, 310 and 312 have been successfully used for joining various stainless alloys to each other and to carbon and alloy steel for years. 312 stainless is popular for welding dissimilar and other problem steels, but it is not suitable for the elevated service temperatures encountered in exhaust systems. The higher levels of ferrite can lead to the development of a brittle microstructure in the weld metal and premature failure.

310 is another product that can work, but its chemistry makes a fully austenitic weld that could be susceptible to solidification cracking. Typically, 309L would give the best results and is often chosen for this application.

Process selection is another consideration. Shielded metal arc welding (stick) and flux-cored arc welding are ruled out because of the thin wall thickness of the tube and potential burn through. Gas metal arc (MIG) welding works well for welding stainless exhaust systems on a regular basis but may be cost prohibitive for occasional users, who would have to consider the cost of a wire coil and special shielding gas needed to weld stainless.

To join 304L stainless and 1018 carbon, we chose the gas tungsten arc (TIG) welding process with argon gas.

Sometimes purging stainless thin wall tubing is necessary to achieve acceptable results with some welds, especially full penetration butt welds. For fillet andlap joints, purging can sometimes be avoided.