forgot to anneal metal after MIG welding

[wjones43]

looking for some machineing or heilcoil/keen-sert options after a much too quick and hasty welding up of a screw hole on a cast iron brake backing plate. In my haste to repair a stripped out thread-hole ,we MIG welded a 3/8"inch hole shut with steel wire and forgot to correctly anneal the metal in the cooling down process.So subsequently when we attempted to re-drill the 3/8"inch hole I found to my despair that I had created a extremely hard drilling surface and almost impossible hole to re-thread. The only options so far appear to this working class stiff,..are to obtain some really expensive carbide drills and taps or trash the whole brake bracking plate and replace at an additional sum.My question to anyone out there is this , is there a cost-effective way useing heilicoil or keen-sert in this newly drilled and extremely hard hole? Keep in mind that the local consensus here is that nothing will work to cut the threads except a carbide tap.-TIA

[Henro]

Hi,

I'm the last person that should reply, since my background is electrical...but...once hardened due to quick cooling, does that mean hardened forever?

Could you reheat the thing and cool it slowly to soften it?

Just a thought...

[Guest]

I'e been there from the weldor's standpoint...the machinist has to use carbide to re-thread. Annealing will only cause more problems.

[chip hayden]

if it truly is cast iron and you have a martensite problem, you can sometimes drill it with a masonry bit. you didn't say what the original tapped hole size was but even if you are reasonably close to the right diameter for the tap drill, it might be just enough to take out the hard spots.

[wjones43]

..thx all. I am thinking to either re-drill the hole larger to the next size to hopefully get rid of the hard spots and re-weld the hole after inserting a cut off metal rod and properly cooling down everything,correctly over sometime period. Then I am going to try and start the whole process of drilling and tapping over with hopefully better results.Any comments or advice ?-TIA

[Mike Sherman]

If that part is truly cast iron, you are making several mistakes in your welding procedure. These include, but may not be limited to: No preheat, no maintenance of interpass temperature, wrong electrode (actually wrong material altogether) improper post weld heat treatment. If you are welding a brake backing plate made of cast iron without following the proper procedure, failure is imminent. You need a proper preheat (400 to 900 deg. F), maintenance of the preheat temperature (interpass) during the entire welding operation and a proper post weld heat treatment with a proper cool down time. You also need to use a nickle electrode suitable for post weld machining such as something from the ENi group.

[wjones43]

..Thanks Mike.Hope you can excuse this home-brew MIG welder and his sometimes onerous questions ,but what would be a good slow cool down process and what could be ultilized for this 3/8"inch x 8"inch cast-iron plate? We did'nt have excess to a heated oven last time so now I am thinking about useing a pan of thick oil and immerseing the plate.Good or bad idea?-TIA

[Guest]

Originally posted by wjones43
..Thanks Mike.Hope you can excuse this home-brew MIG welder and his sometimes onerous questions ,but what would be a good slow cool down process and what could be ultilized for this 3/8"inch x 8"inch cast-iron plate? We did'nt have excess to a heated oven last time so now I am thinking about useing a pan of thick oil and immerseing the plate.Good or bad idea?-TIA

Bad idea. I don't have a oven in my shop either, but what I do quite sucessfully is to wrap the part in Kaowool or any other flame proof material like glass cloth or fire blanket and leave it there till the next day, or at the very least for five hours.

Most of my stuff won't see heat again, though. Your backing plate will, and it has holes in it, I believe, which is where the cracks will probagate to. Like Mike says, destined for failure.

BTW the oil dip is used for tempering.

[chip hayden]

are you sure this is cast iron? i don't think i have ever seen a backing plate made of cast iron[or any cast product for that matter] maybe a picture would help.

[wjones43]

Sorry I can't send digital pics right now. Inexpensive(..read cheap) foreign digital camera broke at the moment.Thx.all,..Mike,..Rock,..Chip.For all the good comments and sage advice. Will be looking to follow up with all the advice and good comments from here after re-heating + welding 3/8" inch backing plate with Kaowool or other similar material in cool down process. This particular brake backing plate appears to be cast-iron to this novice,but tommorow I will take the part to a local shop for some additional professional input.Right at the moment the economics of re-doing this part makes sense,even with the possibility of heat-related stress cracks down the road.-Again ,thanks to all