4140 & tig

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Hi All,I have trouble welding 4140 steel. The current part is .110 thick x .375 wide. I'm joining two pieces and have beveled the edges. I may mention the weld must be dressed down to the parent material. The edges of the puddle, the parent material bubbles, instead of forming a stable puddle. This causes undercutting at the bead edges and pits in the finished weld. I'm guessing this is caused by high carbon content.I'm using 15 CFMs of pure argon, a gas lens with #8 cup and 1/16 tungsten. I have preheated the material to around 400 degrees. What am I doing wrong?   I appreciate any suggestions.
Reply:

Originally Posted by TinkerII

Hi All,I have trouble welding 4140 steel. The current part is .110 thick x .375 wide. I'm joining two pieces and have beveled the edges. I may mention the weld must be dressed down to the parent material. The edges of the puddle, the parent material bubbles, instead of forming a stable puddle. This causes undercutting at the bead edges and pits in the finished weld. I'm guessing this is caused by high carbon content.I'm using 15 CFMs of pure argon, a gas lens with #8 cup and 1/16 tungsten. I have preheated the material to around 400 degrees. What am I doing wrong?   I appreciate any suggestions.
Reply:I should have known. Miller Syncrowave 250 @ 50 amps.  1/16 dia 2% Ceritated tungsten ground 60 deg. 3/8" Stick out with #8 gas lens cup. Torch angle of 5-10 degrees. ER70S-2 .046 filler.  Thanks Oscar
Reply:I have welded 4140.  The metal does not bubble. That usually happens either due to some form of contamination (oil for example) or lack of gas coverage or a combination of both.  I would also bump up the gas coverage as already recommended.  And your amperage is way too low.  I would set the machine at 150 and use the Pedal.
Reply:Do you think a standard cup would be better than the gas lens?
Reply:

Originally Posted by TinkerII

Do you think a standard cup would be better than the gas lens?
Reply:The bubbling will be contamination, oil or grease is likely but if there's any sort of mill scale left on it you'll be fighting that as well. That's the case for pretty much every alloy steel or tool steel I've ever welded. 15CFH on a #8 should be about right with a fairly tight stickout, could bump it up a couple CFH and see but I'd be more inclined to think it's contamination from the joint. Was it tacked together already when you preheated it? Could still have something trapped in the joint. An included angle of 60° on your tungsten is extremely blunt in my opinion, I would do a 30° taper and knock the end off to leave a 0.010-0.020" flat on the end if you're worried about inclusions, otherwise leave it sharp. Excessively blunt tungsten tends to lead to poor arc stability, and on tool/alloy steels the elements added to improve machinability (manganese, silicon and sometimes sulfur) all lower the melting point of the steel. They make the puddle want to swirl which tends to lead to severe undercutting, as shovelon said you want to make the puddle nice and fat with wire additions to "cool" it and fill in the undercut. You also probably need to move at a brisk pace with a 400°F preheat, if you go slow it'll just build more heat and make things even harder to control. I would think 50 amps works but you probably have to move slower than you should, usually the 1 amp per thousands of material thickness gets you close enough so if it were me I'd set it to 110amps to start and see what that was like. 1/16th ceriated might not last too long at that amperage but it shouldn't blow apart for what sounds like a pretty small weld. 2% thoriated is the work horse that you can thrash without mercy for a reason, that's what I would run as well.What are you using as filler? 0.045" dia wire can work but you really need to shove it in the puddle which isn't easy since you should be moving at a good pace. If .045" is all you have then multiple small stringers are better than trying to go slow and pound in lots of wire at once. If you feel like you're welding at a very comfortable pace it's probably still too slow, just pay attention to the interpass temperature if you end up doing stringers.