Part cracking

Blue Diver · Mon Oct 05, 2015 8:50 pm

Ok, so I recently picked up a Lincoln Squarewave 175pro. I would like to preface this whole topic with the fact that I learned to weld 14 years ago, spent 4 years as a welder and have been out of it for the most part for a decade. I find that I remember just enough to know that I am not doing something right but I'm not sure what it is. I'm basically starting over.

So back to my welder. I pick up this squarewave and am super excited to have the ability to weld aluminum again, work some smaller parts and produce better quality parts for my field of work. Today I picked up a disc brake that I'm using to make a small forge to mess around with some blacksmithing. I needed to weld a 3" steel threaded nipple in the center of the disc for the forced air so I decide to tig it rather than stick since I have my handy dandy new machine. I clean everything to bright shiny metal and make 4 tacks 90 out. My settings are as follows:

3/32" 2% thoriated tungsten
3/32" 70S-2 filler
DCEN 150amps (I was full pedal)
15cfh Argon

About 3/4 of the way through my weld I hear a large pop so I stop and lift my hood. On the opposite side of where I stopped I had a sizeable crack through the weld that led off into the disc and terminated in one of the stud holes. I have obviously introduced some stress to the part and I'm not exactly sure what to do at this point. I think that I was camping out on it too long because I was trying to work on walking the cup which was making me weld really slow while I concentrated and maybe the whole thing got too hot? I'm not sure if there are any other metallurgical issues I should be looking at since the part is a used brake disc. If I were smart I would have gotten a pic or two.

Sorry, if I've missed anything, Ive got kids jumping all over me and it is making it hard to think and type coherently... Any tips would be appreciated!

Oscar · Mon Oct 05, 2015 9:01 pm

most brake rotors are cast iron. If you didn't pre-heat it, that might be it.

Blue Diver · Mon Oct 05, 2015 9:12 pm

Oscar wrote:most brake rotors are cast iron. If you didn't pre-heat it, that might be it.

Really? Well, that would make sense I guess. And no, I didn't preheat.

ex framie · Tue Oct 06, 2015 5:13 am

Blue,
Lots of info out there on welding cast iron.
Youtube and Google will bring up heaps.
New swear words are an option if you dont preheat and slow cool.

exnailpounder · Tue Oct 06, 2015 6:20 am

Blue. My youngest son was interested in TIG welding so I had him try running beads on an old rotor and it cracked, a piece actually broke off of it. Kinda makes you wonder how they are so reliable in their actual capacity.

big gear head · Tue Oct 06, 2015 7:52 am

You can weld steel to cast iron, but it's not easy. You have to grind through the outer skin of the iron and then pre heat to around 500 degrees. Then you have to use nickel N99 rod to weld the two metals together. You need to cover it so that it cools slowly. Some people will burry it in sand after welding. There are other ways to join these metals, such as brazing, but welding them isn't simple.

I've welded steel to cast iron many times. I weld rear end housings for race cars where the steel axle tubes need to be welded to the cast iron center. Don't be surprised at the price of the N99 rods. Last time I bought some they were $99 a pound.

Blue Diver · Tue Oct 06, 2015 9:22 am

Thanks for the advice. I had not realized the disc was cast iron and that makes a whole lot of sense as to why its not working. The cracks will not be a big issue in the purpose I am using the part for but I was trying to get some good practice out of the build.

Another quick question... When walking the cup, can a tapered cup be used or only with a flat ended cup?

Thanks,

Don

DSM8 · Tue Oct 06, 2015 6:02 pm

You could try brazing it now since it already is cracked.
Also preheat and when done welding wrap up on some insulation to it will have a long cool down time.

Just diff ways to deal with cast iron.

Blue Diver · Tue Oct 06, 2015 9:17 pm

DSM8 wrote:You could try brazing it now since it already is cracked.
Also preheat and when done welding wrap up on some insulation to it will have a long cool down time.

Just diff ways to deal with cast iron.

Appreciate it!

Arno · Wed Oct 07, 2015 1:52 am

exnailpounder wrote:Blue. My youngest son was interested in TIG welding so I had him try running beads on an old rotor and it cracked, a piece actually broke off of it. Kinda makes you wonder how they are so reliable in their actual capacity.

A good case of specialised material properties for a certain application.

Brake rotors are usually made from a very high carbon content cast iron which has the effect of creating a very high temperature resistant, form-stable, high-strength and hard-wearing material which is great for this application.

However.. The tradeoff is that the material is quite hard and brittle and will shatter/break on point impacts or when localised heat is applied and the local expansion and contraction makes the material fracture easily from the internal stresses.

Bye, Arno.