Mig welding 4130

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OK!! I've only been welding for about 6 months. I havea MM175 machine. I'm currently working on a Rally Car project. I need to weld strips of 4130(14-16 gauge) to reinforce the tunnel of my car and add some gussets to the roll cage.  Is it possible to mig weld 4130(chromoly)? If so whats the best wire to use? Thanks......
Reply:It can be mig welded.  But do not use 4130 filler unless you plan to heat treat afterwards.  Otherwise you'll have a brittle joint.  4130 is considered a heat treatable low alloy steel with relatively high carbon content.  Rapid cooling, as with mig, produces a tendancy to form martensite with carbide precipitates in the weld zone, which promotes hot cracking.  Using 4130 filler would just add even more carbon and make the problem worse.  To reduce the cracking tendancy, you could use a mild steel filler and  some preheat and postheat, around 200- 400 deg F.  For this reason O/A is probably better than mig for 4130 since it heats and cools more slowly.If you want to skip all preheat and postheat BS, then I'd mig it using 309 or 310 stainless and argon or argon/2-5% O2.  You'll suffer some joint strength loss, but it shouldn't have any cracking problems and should be plenty strong for this application.