determining temperature with scale buildup on billet 4

[BMKR]

We are using IR thermometers rated to 1650°C(3002°F) to measure billet temperature leaving a furnace to a forging press. The issue I am having is getting an accurate reading on materials that develop scale quickly upon leaving the furnace, mostly grades of steel(4340, 52100, M50..etc), other materials don't really have this issue ie your Inconels, Ti alloys, and stainless. Is there some way to account for the emmisivity drop between basemetal and the scale buildup. (note: I do not trust the emissivity settings on the IR thermometer)

JC

 

[redpicker]

I find that you will get the best result by leaving the emissivity at 0.85-0.87 and leaving it there. The emissivity of iron oxide does not vary much and in my experience futzing with the emissivity setting is one sure way to get messed-up results.

I suspect your problem is more that the surface of the scale is actually cooler than the underlying steel and that is what is creating the problems. That you will not fix by the e-value, only by getting a clean surface to check or just accepting that your reading will be slightly lower than the actual value.

rp

 

[BMKR]

Thanks RedPicker,

the scale is coming out about 200°F lower than the furnace temperature, (2000°F-->1800°F) I'll have to do some trials but some grades seem to give larger scale "pieces" which throws the thermometer off more.

Thanks again,

JC

 

[Maui]

You could use temple sticks as an independent measure of temperature. These will provide you with a simple method to measure within a fairly narrow range what your surface temperatures actually are with the scale present. Compare these results with your IR readings, and see if the scale really is 200F cooler than the base metal.

Maui

http://www.EngineeringMetallurgy.com

 

[BMKR]

Thanks Maui,

the tricky part is that the distance between the furnace and the press is about 0.5-2.0 seconds maybe a day when production is slowed this would be an option. do you have any additional information on these temple sticks?

JC

 

[btrueblood]

The brand name Maui spoke of is Tempil. You can find them in the McMaster catalog.

Would it be possible to have a small pocket (hohlraum) milled/machined/cast/whatever into a viewable end of the billet? The emissivity of a cavity approaches 1.0 (regardless of surface emissivity) for fairly reasonable depth-to-diameter ratios.

Past that, my only other suggestion would be to kiss the surface of the billet with an abrasive wheel (or sandblast jet) just before measuring.

 

[metengr]

Not sure that temple sticks are useable at 1650 deg F. I would use a contact pyrometer.

 

[btrueblood]

met,

I know they work pretty well at 1650, as we use one here in our shop for case hardening steel (via flame). The "temperature crayons" listed in McMaster are offered up to 2000 F.

 

[redpicker]

They do make Tempil sticks up to 2000F, but in my experience, above 1500 or so, they are much more difficult to use. Instead of melting like the lower temperature crayons do, the higher temperature ones burn, and I have always had trouble determining if the mark left is unburnt crayon or ash. A fast-response thermocouple probe is what I would use if the IR pyrometer was not adequate.

rp

 

[BMKR]

BtrueBlood milling the billet is not advantageous to our forging process, the volumes of what we are inputting and outputting are very important.

I think the tempilstiks are gonna be the way to go. thanks all.