Stable inconel???

I am looking for a stress free inconel. I'm cutting a heater coil with a .025” thick ribbon that is laid out in a maze like pattern. I just got done with a 136hr cut of inconel 600. It looked great except one .5" X .5" circular spot that had some stress and scrapped the entire part out for me. Part is approximatley 13" diameter. Called the metallurgist and he said to stress relieve inconel 600 is a waste of time. He recommended inconel X-750. Of course he wants to sell me another chunk, and this stuff is twice the price of the 600. My understanding is 600 is bottom of the barrel inconel. If there is such a thing. Any input from all of you inconel experts?

Inconel is VERY stable, even Inconel 600. I don't think changing to X-750 would yield an appreciable difference. Not knowing what your geometry is, it's hard to guess what's happened, but I would take a close look at the cutting path and see if it can be modified to provide more support throughout the duration of the cut. Also, if you have the ability to leave a few tabs for support (and come back and cut them last) it might be helpful.

That's not what I wanted to hear. I was really hoping the 750 was going to solve all of my problems. I did leave several (24) tabs to attempt to hold things together, as I knew this movement would be a problem. They worked well in all areas except this .5 x .5 spot where everything went crazy. I would love to blame it all on just a bad spot in the material, but before I try another 136hrs, I need to feel a bit more comfortable. My path was such that I attempted to relieve the stress from the inside out constantly putting any areas that would move into positions that had not been cut yet. As I said, all went well except for this .5 x .5 area. I think I will silver solder carbide plate to the top and bottom on my next attempt. I am hoping the carbide will be ridged enough to keep everything in place. Maybe a brass shim in between the carbide and inconel to help the carbide from cracking if things want to move to much. Think it will work??

Sure, but leave the brass out, it will cause more movement than anything. Aircraft applications routinely mate carbide wear surfaces with inconel (via furnace soldering), their thermal expansion coefficients are nearly identical.

Dear EDM AE

Could you clarify your post for me, please?

As I read it you are saying that one should adjust the brazing cycle so that the two materials behave as though they had nearly identical coefficients of expansion.

We typically think of metal as having a coefficient of expansion of about 2.5 to3 times that of metal. I couldn’t find an exact comparison but the closest I could come with a Google search was 13.3 for Inconel 600 and 5.4 for tungsten carbide. (See below.)

We furnace braze so we can use a long, slow exact cycle to compensate for differences in thermal expansion as well as many other reasons.

When we are doing something like tungsten carbide to steel we would use a metal braze alloy sandwich that would be braze alloy on each side of copper. The braze alloy bonds and the copper anneals dead soft and provides a great deal of stress relief for stress caused by thermal expansion. This would be quick brazing with a torch or induction system.

(Inconel 600 - Coefficient of Expansion 13.3 µ m/m °C At 21 - 93°C)
http://www.alloywire.com/inconel_alloy_600.html)

(Tungsten Carbide Cermet 5.4 x 10-6 in./in.*/°F)

Thank you,
Tom

Tom, I'll be happy to oblige. The thermal coefficient of expansion for Inconel alloy 600 is 5.8 x 10-6 per deg F. (your example is in microns per meter per degree C, by the way... same information, but different unit of length AND temperature... of course it looks odd) You are correct that Tungsten Carbide's thermal coefficient of expansion is 5.4 x 10-6. So in reality, the linear change per inch per degree F is .0000004" different between the two materials. (That's 0.4 millionths on an inch) Can't get much closer than that. For all intents and purposes, these ARE nearly identical.

If you wanted to nail it dead-on, Beryllium Carbide has a coefficient of expansion which is 5.8 x 10-6, which is an exact match.

Thank you.

I overlooked the F/C difference in the conversion. I was thinking that all the numbers were relative so that the units of measurement didn’t make a difference. What I overlooked was that one of the units is fixed.

Again thank you for your time and courtesy.

tom