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I Forge Iron


Howling dog forge

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When testing burners and forges in expectation of things getting properly hot, I use a type S thermocouple, as I have picked one or two up over the years when decommissioning plant.

Much too fragile/expensive to install permanently (They are Platinum-based and  new replacements would cost around $500 each). They have recrystallized Alumina sheaths 10mm in diameter (about 3/8") and are 500mm, 20" long. The RA sheath is usually considered good to 1600 degC, 2912 degF, but the Type S tables go up to 1768 degC, 3214 degF.

For more regular use, I reach for a handheld Mineral-Insulated type K thermocouple, 24" long and 1/4" diameter below the handle. My preferred one is an Omega KHXL-14U-RSC24, which has the proprietary Super Omegaclad XL sheath, but mostly I use 310-stainless-sheathed examples because I can get them cheaply enough to accept their limited life at welding temperatures.

It may be worth mentioning that the failure mode I see most with the Mineral Insulated probes in forges is failure of the metallic sheath. When it gets hot, an Oxide layer forms. When it cools, the Oxide layer detaches. It does not take many cycles for the sheath to be lost and holes to appear. Oxygen can then get in, the thermocouple wires Oxidize and the thermocouple fails. Types 304 and 316 stainless steels tend to lose the Oxide layer when cycled through about 850 degC, 1562 degF. Type 310 is a 25% Cr, 20% Ni stainless. The extra Chromium helps it to form a stronger Oxide layer and the extra Nickel brings the thermal expansion coefficient of the metal closer to that of the Oxide. As a result, type 310 tends to hang on to its Oxide layer until it is cycled through around 1100 degC, 2012 degF. The Super Omegaclad is claimed to be good to 1335 degF, 2440 degF. In my limited experience, it certainly seems to last better at high temperatures than 310SS. 

I find the 310 sheaths seem to last somewhere in the region of 20 cycles to error reading (over-range for the type K, so 2500 degF-plus) before failing. A good bladesmithing welding temperature seems to be around 1300 degC, 2372 degF so it should be enough for most smiths to learn to recognize the correct temperature range.

For electric HT ovens, I use type N thermocouples, which are "only" good to 1300 degC,  2372 degF (vs 1372 degC, 2500 degF for the type K), but were developed to be much more stable than type K when used at temperatures above 1000 degC, 1832 degF. Again, I use super Omegaclad when budget allows, 310SS when it doesn't.

I tried InfraRed pyrometers in waste-gas burners at work and cannot say I had much success with them. The readings tended to be considerably higher than the Type S thermocouple readings and I felt the thermocouple was more trustworthy.



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Optical pyrometers have many around for many decades and are considered the best way to judge very high temperatures accurately; lots of expense and/or work learning how to use them properly. On the other hand, it generally takes about 2700 F of continual forge temperature to melt steel; if that isn't a fancy enough temperature indicator for someone, perhaps a little overwork and expense would help them appreciate common sense better.

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High yellow should be plenty of heat in the work to suit anyone wanting to weld; that would be about 2300 F. 2500 F is white hot; more than is needed.

Note that what color of incandescence you see depends on the light level in your work area; therefore, you need an area lite burning in tthis area before beginning to judge work temperatures by incandescence.

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Mikey, I am new to this forum and have been exploring . Having found the forge building 101 thread I am totally blown away by the information I have gathered in just a couple of pages. My concept of  how a forge works has been completely changed and a rather large number of questions have been answered . I thank you for the effort 

.This should also extend to frosty the Lucky and all the other contributors on that thread.


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We all just want you to have the information you need; speaking of which I would like to point out that ALL the burner designs recommended in the sticky threads section of this forum will work just dandy in a forge. You don't need to build one of mine to get satisfactory results.

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