Inconel & Heating Advice 1

[raytechinfrared]

Hi Everyone!

I am working on solving a material issue that so far has stumped everyone at my company as well as our vendors and now a third party laboratory. I'm hoping someone on here is up for a challenge.

We manufacture equipment that heats up asphalt surfaces (roads/sidewalks/tennis courts/etc). Our "system" consists of a fuel source (propane), a blower motor and a manifold with an orifice. Fuel and air is mixed and forced through the orifice where it is then ignited. This obviously produces a flame. Instead of heating with just that flame, we clip inconel "grids" over the orifice which creates a combustion chamber of sorts but more importantly provides a surface that will be heated directly by the flame. These grids will glow cherry red within about 30 seconds of being heated. This produces an infrared light wave that then heats the asphalt, etc.

The issue: for 20+ years, the grids reached 1875°F when tested with an infrared thermometer (gun style w/ laser pointer). Within the last few years, we realized a drop in temperature of about 1000°F which is a huge problem.

I narrowed the problem down to the grids themselves through testing that eliminated all other factors. I then set up a number of heating tests to compare some old grids vs. the newer grids. The only variable in these tests were the grids being used. I found that the old grids heated at least 1000°F hotter than the new grids...which is what we had observed in the start. So I sent an old grid and a new grid to a testing facility where they concluded that both grids met the chemical specifications for UNS N06600 (Inconel 600) which is what we have always ordered.

I'm not questioning the lab as I don't have any reason to believe this is not true, but everyone involved is stumped as to why two different samples of one specified material would produce such a difference in temperature when heated.

If anyone has any ideas, we are all ears at this point. To help, I've listed the test report findings below. If anymore information would be helpful, I would be glad to volunteer it! Thank you!

Old Grid (percentages): Carbon .02 / Chromium 14.61 / Copper .01 / Iron 9.42 / Manganese .23 / Nickel 74.88 / Sulfur <.001 / Silicon .12
New Grid (percentages): Carbon .03 / Chromium 16.17 / Copper .03 / Iron 8.18 / Manganese .40 / Nickel 74.78 / Sulfur <.001 / Silicon .08

 

[EdStainless]

I presume that you verified that he material thickness is EXACTLY the same on both?
You have tested by swapping new and old grid assemblies in a new burner unit?

I would bet on surface finish, is one dull and slightly rough with the other one being smooth and shinny?

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P.E. Metallurgy, Plymouth Tube

 

[1gibson]

Sandblast new grid and try again.

 

[raytechinfrared]

I thought about the finish before but figured that was too simple an answer for the problems. Looks like I was wrong...

I don't have any way to sandblast the grid, but I did take a wire brush end on a grinder to it for a few minutes. I was able to get rid of the shiny surface and it is no longer so smooth either.

I ran a test on it and this time it was only about 500°F cooler than it should have been (instead of 1000°F cooler). So I'm thinking maybe that was it after all. I've got an email into our vendor now asking whether we can get it with a different finish.

Thank you for the answers and if I get an update, I'll make sure to post it on here!

 

[Compositepro]

At that temperature you can easily see the temperature of the grid from the color of the glow. A 1000F drop would mean no glow at all. Radiant heaters often use the design you describe. A screen or grid acts as a flame holder where the burning occurs on the screen at heat the screen to a radiant temperature. Malfunctions occur due to poor mixing of the fuel with air or to a gas velocity that is too high or too low for the flame to stay attached to the screen. Too high a velocity pushes the flame front past the screen so the screen does not get hot and radiant. You will just have a blue flame after the screen. Lowering the gas velocity will get the flame closer to the screen so it can get hot enough to glow. Once the screen is glowing, the gas velocity can be increased because hot screen ignites the gas on contact. Too low a gas velocity will cause burning before the screen, and the screen will not glow because the gas has cooled before it reaches the screen.

The thickness of metal or scale on the surface of the metal has a big impact on getting the surface to start glowing. Thin wires or flakes on the surface can reach radiant temperature almost instantaneously.

 

[1gibson]

Try an alternate temperature measurement (even if it is a rough one, there should be a way to pick up a 1000*F difference) because I would bet a lot of the problem is the emissivity of the more polished surface skewing the IR thermometer readings.

 

[EdStainless]

Yes, Gibson is correct. The grid isn't actually 1,000F cooler, but because it is shinny you can 't measure it correctly. It may be somewhat cooler though.
The best mat finish would be from pickling in acid.
Tell them that you want a 2D finish.

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P.E. Metallurgy, Plymouth Tube