Welding metallurgy of Cobalt alloys..Day 7 of Down and Dirty Metallurgy


Welcome to Day 7 of your crash course on down and dirty welding metallurgy.

Welding metallurgy of Cobalt alloys

Cobalt alloys are used where resistance to heat and oxidation is a big factor like in the aircraft and aerospace industries.

If you watched the YouTube video above, you noticed the hot section of the engine colored red. It runs red hot and it takes a special alloy to withstand that kind of heat and stress from pressure and vibration.

Cobalt alloys like Haynes 188, and L605 are used in the hot sections of gas turbine engines because they retain acceptable strength and hardness even at red hot temperatures.

Cobalt alloys usually contain some combination of cobalt, nickel, chromium, and tungsten. And are very heat and abrasive resistant.

Here comes the broken record again:

Remember on day one when I talked about how metals harden? And that carbon and low alloy steels, cast iron, and some 400 series stainless steels harden by heating and then quick cooling? And that practically every other metal actually got softer by heating and then quick cooling? And that you needed to understand that before going any further?

Well it is day 7 of the course now and we are talking about cobalt alloys.

Are you getting it?

If you are, then you already could guess the answer to this question: If I heat a cobalt alloy red hot and then drop it in a bucket of ice water, will it harden? The answer is …….NOPE

Cobalt alloys are very easily welded. In fact some of them are much easier to weld than certain grades of stainless steel.

One down and Dirty welding metallurgy tip is that copper fixtures should not come into contact with cobalt alloys. Copper pickup from tooling or fixturing will cause cracking. That includes brass or bronze tooling. If you need the heat sink, then Aluminum is good...or you can have the fixture nickel plated if you can make that happen.

Another welding metallurgy tip is to use low heat...just enough to get the job done. More heat will cause centerline cracking and just add more residual stress to the weld area.

Satellite 6b is a cobalt alloy and is used routinely in manufacturing where resistance to abrasion is important. Processing machinery that is subjected to repetitive friction is often cladded with satellite 6b. Like all cobalt alloys, It is readily weldable and can even be easily welded to cold rolled steel, stainless steel or pretty much any other steel provide the correct rod is used.

I would recommend either inconel 625 or Hastelloy W for welding Stellite 6b to other steels for non structural and non critical applications.

The good news about welding cobalt alloys is that they flow like honey and weld clean and pretty just like 304 stainless.

The bad news is that cobalt alloys cost an arm and a leg.

like about 5 grand for a .032" 3'x6' sheet of L605.

Yes I said $5,000

So you kind of want to do the job right the first time.

Thanks for "gutting it out" and completing your 7 days of down and dirty welding metallurgy .

I hope you got something out of it.

Would you do me a favor and weigh in on " Down and Dirty Metallurgy" ? Was it helpful? Was it too simplified? I could use some feedback

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Uli Zimmermann 
Dear guys, i just finished reading the down and dirty metallurgy section and i liked it very much. Since engineers and especially welding engineers who …

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