Welding causing hardened metal?

[InsaneEPP]

I'm fairly new to welding, so I don't have a lot of experience with it. Here's my situation: I've been drilling and tapping holes. Three times I've had the need to redrill the holes because they are slightly off. I welded the holes closed and went to redrill and retap them. Once I had no problems whatsoever, but twice I basically couldn't drill them all the way through. I ended up using a sledge and a punch to finish the hole. I then dressed the hole up with the drill and tried to tap them. They were extremely difficult to tap, and I basically ruined two taps. The resulting threads were not completely cut by the tap, but I could force a bolt into the holes. With all my difficulty, it seems like the metal hardened through the process of welding and cooling too quickly. Is this a correct assumption?? Is there any way to avoid this in the future??

Thanks for any help you can give.

[HuFlungDung]

Don't drill the holes off center!

It would help if you knew the grade of steel you were welding. Standard welding rods and mig wires do not have enough carbon in to harden, so the hardness comes from the carbon and possible alloying with the base metal.
A hardzone is best prevented by preheating the part to about 500 degrees F before immediately welding it. As a further precaution, drill the hole out larger than the tap size and countersink from both sides so you can get down into the hole properly while welding.

Preference might be given to stick welding as it is slower deposit and allows a bit more heat to go into the weld which helps to anneal the weld zone a little bit.

[InsaneEPP]

I was only off by about 1/32", but it was enough to do it over. I am using a wire feed welder (Lincoln 135 Plus) and flux cored solder. I'm not sure what type of steel I have, but I can't change it now! I bought it from my local steel supplier, so I assume it would be a common, cheaper type. The metal I'm using is only 1/8" thick (the hole is 1/4") so penetration isn't a problem.

How do I preheat the metal to 500 degrees F? I don't have a 10' x 4' x 2' oven. Can I just use a propane torch for a minute or two? Do I need to get the whole part to that or just the local area? The first hole was in a piece that weighed 350 pounds or so. I figure it would take more than a propane torch to get that to 500 degrees.

[HuFlungDung]

Well, spot heating is better than none, but you do want to be gentle in the application of heat. A torch would be fine, but don't concentrate the flame at only one spot, or you'll "upset" the metal and cause a permanent warp. Play the flame around the area.

For thin sheet like that, you might also try using a chunk of copper as a backup plate. Heat this too. It should help prevent rapid cooling of the weld. That's really what we're trying to prevent. Don't use a carbon plate backup, as some carbon will mix with the weld pool and create "tool steel" for you.

What type of mig wire is it? The exact grade? Flux cored wire could possibly increase the likelihood of a hardenable alloy forming.

[InsaneEPP]

Well, it wasn't exactly 1/8" sheet. I had 2" square tubing 1/8" thick, so it wouldn't warp much. I also couldn't use a backup sheet since I can't get behind the metal. I'll try heating up the piece a bit before repairing my next hole. I'm sure it will work better than what I've done before. It probably doesn't help that my workshop is probably less than 60 degrees inside. Any hints on how to tell if the metal is >500 degrees? Do I just put my hand on it and see how long it takes before I have to pull it off, or do I do something crazy like buy a thermometer? Thanks for your help!

[HuFlungDung]

You should keep a roast of beef handy for testing the temperature. When it's hot enough to sear the meat, that's 500 degrees

Actually, you can purchase Tempilsticks which come in an assortment of melting points. When the metal gets hot enough to melt a particular stick, then its at least that hot.

[sendkeys]

got any solder around? might use it. to test the temp.

0% TIN 100% LEAD -- 621 Degrees F MELTING POINT
10% TIN 90% LEAD -- 572 Degrees F MELTING POINT
30% TIN 70% LEAD -- 496 Degrees F MELTING POINT
40% TIN 60% LEAD -- 460 Degrees F MELTING POINT
50% TIN 50% LEAD -- 421 Degrees F MELTING POINT
60% TIN 40% LEAD -- 375 Degrees F MELTING POINT
63% TIN 37% LEAD -- 361* Degrees F MELTING POINT

[InsaneEPP]

Thanks guys. You have to love a place where you can ask and learn. I think I'll end up checking the temperature using the roast of beef method. Then I can decide whether I want it rare, medium or well done.

[DareBee]

This doesnt apply (really) to your type of material, but in heavier plate to avoid re-drilling & tapping in weld filler, it is a good idea to bore the hole out oversize (be generous), turn a tight fitting plug and weld it in. Now you have "clean" material to drill in.
In some cases with blind holes and a tight plug just beating (it must be about 0.05 long to start with) it good with your ball peen hammer this peens it into place quite permenantly. In most cases if you throw a surface grind in it you wont know the plug exists.

[ViperTX]

So, I prefer to use the plug and weld method, but you can use the weld puddle method, I would change to the type of welding rod or wire that is used to build up surfaces for machining....off the top of my head I don't remember the numbers...but there are rods that are used to build up chipped gear teeth, grader blades, etc.