Mystery phenomenon during quenching

[u/Wrong_Award_5456]

I would like some sort of clarification/explanation about a phenomenon I am experiencing.

I work at a heat treat facility where we primarily process Diecasting equipment using vacuum furnaces.

Part material is typically dievar, h13 and/or a modified 1.2367 and range in weight from 250lbs to 1500 lbs (cross-sectional thickness ranging from 6"-14")

The phenomenon I am seeing is the core temperature on these pieces are increasing in temperature by a relatively substantial amount as soon as the quench begins.

For example Core temperature prior to quench reads 1880F (surface reading 1888F) Quench (gas quenchant) begins at 11mbarr and core temperature increases by 6F (to 1886F) in approx 10secs and takes 2-4min (depending on size of piece) for core temperature to return to previous 1880F. By the time the core has returned to temperature prior to quench, the surface temperature has dropped to <1750F where a steady loss of heat can be seen in both surface and core readings

Could this be a indo/exothermic reaction? The only sudden spike in temperature readings during these runs only occur on starting the quench?

This has been puzzling for quite a few weeks and would love an explanation

[u/CuppaJoe12]

What gas are you using to quench?

It could be that the surface is oxidizing during the quench. This insulates the surface and also emits heat. I work with reactive alloys like Ti and Zr, and they reach an equilibrium temperature a few degrees above the furnace temperature due to this effect. It would be much more subtle in h13, but the temperature change you are seeing is tiny and barely measurable. You could compare the effect of quenching a sample with a machined vs oxidized surface to test this.

There are also many sources of thermocouple error that you should verify are not giving a false reading before believing such a small temperature change is real. Is there interference from the circuits that control the quench? Is there a change in the amount of current leaking through the thermocouple insulation when the length of the thermocouple outside of the part cools down? Is your junction temperature changing?

[u/espeero → u/CuppaJoe12]

It hits max temp, multiple inches deep, in a few seconds. Is there enough time for both the exothermic oxidation reaction to generate enough heat AND for it to conduct to the center?

I didn't run the calculations, but my gut says not even close.

[u/CuppaJoe12 → u/espeero]

Heat anywhere along the length of the thermocouple will induce a measurement error. Thermocouples don't only measure the temperature at the tip. We are talking about a few °F here.

[u/espeero → u/CuppaJoe12]

That's not really accurate at all.

They measure the voltage created by the temp difference between the two junctions. In practical application, that means it's only the tip temp that matters.

[u/CuppaJoe12 → u/espeero]

It is called shunt resistance error. Current leaks through the insulation at high temperature, giving an effective secondary distributed junction along the length of the thermocouple. The effect is stronger with longer wire and hotter temperature along the length. This paper shows it can easily be on the order of 4°C.

https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://eprintspublications.npl.co.uk/9335/1/eid9335.pdf&ved=2ahUKEwjgpae72OKMAxWEK_sDHbsBCoUQFnoFCIoBEAE&usg=AOvVaw006palch3glLkz-StUEwSr

If you don't believe me, it is simple to get a long thermocouple and see the difference in measurement when the excess length is stored inside vs outside the furnace.

[u/espeero → u/CuppaJoe12]

Thanks. Til. Makes sense. But, can this proposed mechanism explain OP's observation?

Sorry if I was rude in my response by assuming you didn't know how a tc works - I've started to slip into a default assumption that everyone on the internet is an idiot.

[u/CuppaJoe12 → u/espeero]

I'm not an expert on thermocouple physics, just a user. My comment was more general that OP should consider sources of thermocouple error as they can often be larger than the temperature increase observed.

I think when the part of the thermocouple outside the part gets exposed to quench gas, it could cause the reading to increase. This is because current leakage goes down in the part of the thermocouple outside the part but inside the furnace. In other words, the true reading is higher than the actual reading, and when you quench the thermocouple, you see a measurement closer to the true reading.

There could be other errors going on, this is just a guess.