I've been doing a lot of reading here (and You Tubing) and noting discussion of temperatures at various points in the system.
Sometimes this is relation to the outer temperatures of the metalwork, in the case of heat-exchanger stoves - which is after all what we are interested in because this is what radiates (and convects?) the heat into the room. In some cases there has been bowing and distortion of parts (and tops) and even a mention of colour change - to redness!
Anyway, I thought I would post a basic conversion chart for temperatures. Here in the UK, we are now almost wholly used to using Celsius (C) for temperature, especially hot stuff. The only time Fahrenheit (F) is used these days (it seems to me) is when the weather temperature might be mentioned. However, American readers (and others?) prefer still to use F temperatures throughout. That's not a problem of course .... but it's easy to get confused.
Also, when we are talking about the upper hundreds of degrees C, we will be expecting colour changes in steel and beyond a certain point, crystalline and atomic changes in the composition. In the case of extreme temperatures (in steel riser tubes), the effects are even more pronounced, resulting in rapid breakdown.
So then....
2000 F = 1093 C
1900 F = 1038 C - I saw this temp' mentioned in a thread
1800 F = 982 C
1500 F = 815.5 C - saw that in another thread as a suggested optimal
1000 F = 538 C
800 F = 427 C
600 F = 315 C
300 F = 149 C
As regards the steel colour, then this will begin to turn from black heat to dull red (plum red) at about 700 C (1292 F) and certainly by 800 C (1472). It will be more noticeable (and earlier) in a darkened room and this is more accurate. Bright surrounding light can mask true steel colour as it comes up to temperature.
At about 723 C (1333 F) the composition of the steel will begin to change as carbon goes into solution. The steel will at this point start to become non-magnetic. This is probably not an issue from the sake of burnouts anytime too soon and is the basic temperature for annealing steel if allowed to slow cool from this point (about 800C) - which of course burners do. If nothing else, super heated chambers and pipes will be fully annealed and stress-relieved.
Steel however does tend to rust more rapidly if heated to dull red, cooled and left alone. If you keep doing that daily, the rusting isn't getting any better. As an example, take two pieces of thin steel plate, unpainted or otherwise uncoated or unprotected. Polish them a little with a flap disc. Play a propane torch over one of them till it becomes dull red (or just a portion of it dull red). Let it cool and store both pieces in a standard workshop atmosphere (not high summer, not next to the burner). Let them both rust a bit in the atmosphere. Almost certainly the heated piece will rust more than, and before the neutral piece and it will rust most in the heated area. This is not a huge problem as such in burners as the rate of rusting is not that rapid. In any case, thicker steels (3mm must be a minimum) prolong the life to where the rusting eats straight through.
Heating to more than dull red increases this effect. If somehow we get parts up to 1800 degrees F or above we are then in the range of 1000 degrees C. This is the range of 'Cherry Red' colour and now seriously hot. You could harden (tools steels) from this temperature by quenching and it's approaching (and still at when cooling) forging temperature. Beyond this and you are at 'Bright red' (next stage yellow) and getting critically hot.
I doubt anyone will get exchanger tanks bright red (time to worry about combustible surrounds if so) or even rarely cherry red, but dull red (or even just below) seems a possibility. Where you might be getting these upper temperatures is in the rocket tube, at the elbow and heel point. This is the area of the most heat (and air-speed possibly) and in the case of steel (even stainless) there is likely to be bright red metalwork or even possibly yellow heat. This will cause rapid scaling and breakdown. Admittedly the various temperature ranges for stainless steels are pushed up a ways compared to plain carbon steel.
The likelihood of serious metal temperatures and colour change is more likely the thinner the section is. Thicker section will act as a heat soak. Additionally, it will retain heat longer and take longer to cool down. This is easily proved and no doubt already observed by anyone with an acetylene torch. With a welding nozzle try to heat up a small plate of 1mm steel sheet. Very quickly indeed it will go to bright red and even burn right through. Try to heat up an equivalent sized plate of 6mm steel. You will eventually get a red spot (dull, maybe cherry) but it will take a minute or so at least. Try a 10mm plate and you might be there a while. So thicker plate will obviously resist arriving at reddened temperatures by acting as a soak for the worst of it.
However a heat soak is not then heat reflection. It may not pay to have very thick section in the firebox construction, but rather to insulate thinner section (and have false walls for heated air intakes). But in the exchanger walls, thicker section may pay in that it prevents serious distortion (particularly the top plate) and acts as a soak which will then be released into the room. The soak is occurring before the exhaust point.
I haven't over-complicated that I hope, particularly the last bit.
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