Tapping Thread in Welded Steel

Bugger! I had the mig there too. Didn't have the gas bottle set up so I figured the stick welder would do just as well. I was more worried about the flux than about hard spots.

What do you reckon happens? I was wondering about hitting the area with the oxy to see if I could soften up the hard spots. Mind you, a bit late for my poor 16mm tap!

Did you find a really notchy tap cutting action?
Then as TonyS say's ...slag inclusions ...they rip the cutting teeth off the tap as you try to pass thru them.

Cheers Woody

"An expert is a man who has made all the mistakes which can be made, in a narrow field."

Ah-ha. Slag inclusions. I did my best to keep them out but there's no doubt that there'd be some. Too small a hole to work in to keep them all out. It's definitely a notchy cutting action.

Oh well, start again. Sadder but wiser!

I'm not an expert in metallurgy or welding but wouldn't the mild steel be effectively heat treated and become harder as a consequence of welding? Perhaps if you anneal the weld by continuing to heat it and cause it to cool very slowly you can lessen the effect.

Even mild, low carbon steel will harden if quenched rapidly. To anneal it, you need to find a gas torch, heat the weldment to about cherry red, then gradually withdraw the torch to cool it slowly. It should then be soft enough to tap without murdering your tool.

If the material that you were welding to was medium or high carbon steel you will get hard brittle area in weld. If you had tapped it bigger , maybe 30mm you could then insert sleeve with 30mm od and 16mm id thread.

Hmmm, that's where I started, thinking I had hard spots from the welding, but the comment about including flux seems to make a lot of sense. I didn't quench anything and because I was working slowly with a small rod, I'd have thought there would have been a fairly gentle cooling. I welded a bit, then chipped it clean with a punch and hammer, then welded a bit more until the hole was filled in. I then smoothed off the face with a flap wheel and left it to cool.

I'm a bit limited in what I can do as this hole is on the side of the clutch housing on a tractor. The housing is made with a "lump" on the inside to give the depth to the hole. I considered drilling it larger so that I could fill the hole with a bit of round bar, or simply tap for a bigger bolt, but was worried I'd be weakening the attachment point too much.

Most weld material feels "sticky" when tapped. A bit like somewhere between copper and water pipe. It sort of tears rather that cuts.

What I'd suggest is a product called Trefolex - or - alternately, the 'lard' that is used on pipe threading.

BUT you still need to beware of 'inclusions' - so it's tap a bit, feel for bind-up or dead stops - back off, to break the 'chip' (read curl), let lube 'back wipe', and tap a bit more.

Every two turns or so back right out and clean and re-lube, and if you can blow the hole out with air, do so. Tiny spatter balls and slag 'crystals' are your enemy.

Hope this helps

When it is necessary to repair a thread in underground mining equipment especially flameproof boxes the process is to drill out the hole to a larger size and mig weld it up.

In this application the repair is then drilled and bored before tapping to a 6F standard. The threads are repaired to a very high standard (probably much higher than any new thread on implements and are tested with Go / No gauges. The thread in your application is absolutely repairable, by correct techniques. There are some completely correct answers above, which I have marked with GAs. The threaded sleeve method is very effective, one that is also legal for flamefroof equipment. Any methods cleared for the most demanding applications will certainly do your job well.

The threaded insert can be scotch locked or welded in place and made as a boss if desired. That would be my favoured method if clearance is available. Drilling the large hole can be a bit scary if using a normal twist drill. How I get around the problem is to convert the drill into a counterbore bit on the grinder. First make a decent round hole then grind the core of the bit to fit leaving the normal cutting tip intact and shallow flat cutting edges for the outer portion. Using this type of tool it is easy to drill large holes with a hand held drill in situ.

The method you used is unfortunately probably the worst way, is guarantees inclusions. Slag is hard and can be used as abrasive grit.

Thanks for that. You have to remember that I was very challenged as to metal available. Drilling out would have been a risky proceeding with failure requiring the breaking of the tractor to access the inside of the housing.

I think the answers you have GA'd are on the money. If I'd used the mig, all would have been well. I'd noted those two for GA'ing myself but I think it good to let the thread run it's course first.

On the positive side, I tried a bolt in the hole today and to my surprise it went in fine. My tap is scrap metal but if the bolt does up, it's mission accomplished. Ultimately, all this bolt does is connect the loader to the tractor. If it does up tight and stays that way, it probably won't be touched for twenty years.

Good news for me as I was not looking forward to gouging it out with the gas axe!

Pleased to hear it is working. What could now save you a lot of future grief, is to follow the lead of Cameco Cane Harvesters and Araldite the bolt, or preferably a stud in place.

From your description it sounds like the gas axe would not have been a great idea. For real mongrel jobs, never forget, the air die grinder is your friend. Fine burrs are great and stones can cut out the nodules of hard stuff provided of course you clean up the grit before tapping or drilling.

I love what die grinders do but whenever I use one, no matter how good my goggles are, some little bit of metal always seems to find it's way into my eye.

My grinder resides on a shelf with a big sign"Absolute emergency use only"!

You don't say what the parent metal is your welding, that has the hole needing repair. It almost sounds like a casting to me. If it is a casting of gray iron and you did not use the proper filler rod you may have to bore the material out and start over. I would suggest boring to a size that a plug can be inserted in and welded in to place using the correct filler materials to join. Not an easy task if it is gray iron, but can be done.

To be honest, I don't know what the parent metal is. I know it can be welded as this is my second one of these tractors and the first one had a broken lug on it that I welded back into place some ten years ago and it's still there.

Same sort of stuff as most older gear boxes and such like are made of. You can generally weld them quite happily.

I guess it's a little late but do you know about Helicoils?

Yep, I know about helicoils. This wasn't a stripped thread though, it was a flogged out hole, way oversize!

I do not know if you have enough thread on your bolt, stud, but if all else fails can you weld a nut (or two) to the far end of your "hole" ?

Can't get at the far end without breaking the tractor in half. Probably wouldn't do the job regardless as the load is basically sheer so the bolt needs sideways support right where the bracket meets the tractor. Sideways support 40mm in wouldn't cut the mustard, although, if I combined it with the araldite suggested above, it could be a winner. Araldite to support and locate, nut to handle the tension. I'll save that one for the next desperate situation!

I nearly forgot to come back to this one with the end of the story. It was almost definitely slag inclusion. Interestingly, deeper down the thread cut OK. I suspect this is due to me being paranoid about flux when I started welding. Initially I was welding down a hole. I'd only put a little spot of weld and then chip it clean. As I got further out, the hole became a big oval and I was able to proceed faster, doing a run on either side before stopping to chip. This was the area where I included the slag and made a headache for myself.

The end of the story is that, although it damaged my tap, the thread cut. A bolt screwed in and was lock tighted and torqued up. Hopefully never to come out again. Thank you to all who contributed.