Welding steel on stainless with 309 rods

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There is no big question here, i've always use 309 rods with SMAW or the 309 filler rod with TIG to weld stainless to mild steel, i've always been told to use that, but today i've had to use it again and i was wondering WHY this is the only rod to use ?309 seem to be only another type of stainless as 304 or 316, so what is special with the 309 ones ?and what happen if i try to weld steel on stainless with 308L rods for example ?Calculator > Bevel Square
Reply:OK you asked for it...When welding carbon steel with a stainless steel, you will get carbon diffusion to the side with the higher chromium content (stainless steel). That will create a decarburized  in the ferritic steel HAZ, and at the same time a carburized zone on the stainless steel side. The decarburized ferritic steel zone will have lower mechanical properties and the carburized stainless steel will cause sensitization of the stainless steel. You can't do much for the first but you can do a lot for the second.A great tool to check all possible problems is Shaeffler's diagram. This diagram gives the composition based on chromium and nickel equivalents. When used correctly and taking into account the dilution of both base metals and the filler metal you can predict the composition of the weld metal.When welding dissimilar steels you aim for a very specific region in Shaeffler's diagram, outside of this region you can get problems like martensitic cracking, hot cracking, brittleness after heat treatment, high temperature brittleness etc.309L has a higher Cr and Ni content (and also Creq and Nieq) compared to 308L, which when diluted with the carbon steel gives a weld metal composition in that sweet spot.Finally 309L is not the only filler metal for this application. There are a lot of alternatives but 309 is the cheapest that will do the job.Last edited by taz00; 05-10-2013 at 06:50 PM.
Reply:If you use 308 rod to weld mild steel to stainless steel, the weld itself will be subjuct to rusting due to the mild steel mixing with the weld metal.If you use 309, to weld dissimilar metals, the weld itself will not rust due to the extra alloys in the 309 filler rod.JasonLincoln Idealarc 250 stick/tigThermal Dynamics Cutmaster 52Miller Bobcat 250Torchmate CNC tableThermal Arc Hefty 2Ironworkers Local 720
Reply:Thanks, Jason, for a very succinct English translation of the previous post.  A few weldersA lot of hammersA whole lot of C-clamps
Reply:Originally Posted by taz00Finally 309L is not the only filler metal for this application. There are a lot of alternatives but 309 is the cheapest that will do the job.
Reply:307 , 312 and various nickel alloys
Reply:We use inconel for all our carbon to stainless welds.