Mig welding chrome moly makes it brittle, too much heat. Lot of the older guys that build airplanes welded chrome moly with an acetylene torch, makes welds like a tig if you know how to do it. But they had to heat the joints to keep them from cracking. I acetylene gas weld headers when I build them, had an old air craft mechanic watch me one time and he asked if I used to build airplanes.
MIG Welding 4130 Chrome-Moly
By Galen White, welding engineer, Miller Electric Mfg. Co.
Grade 4130 steel, while containing both chromium and molybdenum as strengthening agents, is considered a heat treatable low alloy (HTLA). Generally referred to as chrome-moly, this HTLA is used largely for aviation, racing and welded tube structure applications.
When welding 4130, preheating to 300°F is strongly recommended by the American Welding Society (AWS) to relieve stresses in the metal. When choosing a wire, most people opt for ER80S-D2 or ER70S-2. ER80S-D2 will provide the most weld strength. The ER70S-2 is easier to find and provides a strong weld, but you’ll be sacrificing some strength by choosing this filler metal over ER80S-D2. When it comes to shielding gas, 75/25 (Ar/CO2) is recommended for most applications and 98/2 (Ar/CO2) for anything over 3/16 in.
Wire Size-Amperage Range- WFS Range Relationships for Short Circuit Transfer on Steel
Wire Size Amperage
Range Wire Feed Speed
Range
.023" 30-90 100-400
.030" 40-145 90-340
.035" 50-180 80-380
.045" 75-250 70-270
Cleanliness is critical when welding 4130. Make sure that all mill scale and oils are removed using mild abrasives and/or acetone. When you strike an arc, keep your heat input low to reduce stresses in the metal.
Post-weld heat treatment of 4130 varies from one application to another. If ductility and toughness are your goal, post-weld heat treatment is recommended up to 1,200°F. If the material you are welding is thinner than .120 in., stress relief through heat treatment is not as critical.
WFS Rule of Thumb:
1 ampere for every .001 thickness
1/8" material=.125=125 Amperes
Wire Burn Off:
.023-3.5" wire/amp - 125 amps=437 IPM
.030 - 2" wire/amp - 125 amps=250 IPM
.035 - 1.6" wire/amp - 125 amps=200 IPM
.045 - 1" wire/amp - 125 amps=125 IPM
Welding 4130 is a lot like welding mild steel and is easy if you know how. These are just some of the reasons 4130 is considered so flexible and is used on everything from airplane engine mounts to bicycle frames
Good information. The old guys running a welding class I attended many years ago made you learn to gas weld before touching a TIG torch. A very good idea even today. Learn how to work your hands before adding the foot (or thumb) control. Plus they had a tight budget and didn't want all the class time used up ruining and grinding tungsten.
I like to use a propane torch to warm a joint before welding but mostly to burn off any contaminates or moisture. You can see the crap come out of it. Preheat on 083 or 058 is debatable. Another technique I've seen recommended is starting a weld at the thin part of an intersecting tube on a notched joint. This is where the minimum heat is needed and working the weld to the corner or the thickest point of the joint serves to preheat that area.
I like ER-70 over 80. Either wire will mix with the base and the resulting weld will a higher strength (tensile) than the wire by itself. 80 will be stronger and harder. I think 70 provides a bit more ductile weld with some give that can help with not cracking the tube in the HAZ over time. The expected use and lifespan of the chassis will effect this decision.
