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About 50 years ago, when starting to do my master's thesis in physical metallurgy, the local Research Assistant taught me how to build a furnace: take a tube of sintered aluminum oxide, wrap coiled heating element wire around it, and hold it in position by smearing refractory cement around it, filling all the gaps inside and between the coils. For my doctorate at a different university, exactly the same procedure. Ditto, when I went to do research at a third university in a different country. The materials used for thermal insulation were different, however: ceramic wool (Kaowool) on the first two, hollow aluminum oxide spheres on the third.Originally Posted by DIYSwede
The most important instruction, however, was on the spacing of the wire: closely spaced near the ends, and much wider near the middle of the alumina tube, to compensate for the extra heat losses that occur at the end of the tube. Even with this, the temperature profile over the length of the tube was pretty pathetic - the zone within which the temperature stayed the same within a degree or two was only a few centimeters long. So, we extended the zone by inserting a piece of metal, utilizing the metal's higher thermal conductivity to extend the uniform zone.
Unfortunately, at the time this was done, there was no internet, and nobody thought of putting up videos on how to construct a furnace. Who would have cared, when the Research Assistant knew how to do it, and where to source the materials? That was part of his job, and he was paid to do just that.
What I learned only much later (when I learned why CORTEN steel can be left to rust without ruining it, and what the Japanese did to use it without paying royalties to its American inventor) is that the refractory cement not only held the heating coil in place, it also protected it partially from oxygen, and from mechanical shock, extending its life indefinitely. Only if we allowed the furnace to heat beyond its design temperature (1100oC) would it fail, even with daily use.
I rather have a furnace that does not need fixing, and to heat treat carbon steel I do not need a furnace that goes hotter than about 1000oC, or, for stainless steel / HSS, above 1100oC . But if you want to melt glass or sinter ceramics, you do have to routinely work above 1200oC in an atmosphere containing oxygen and nitrogen. And that means that your heating elements will not last, unless you go to fancy stuff like silicon soaked silicon carbide rods, which, if I remember correctly, are good for up to 1400oC .
This depends on the kind of steel anthonyget is going to use for his knifemaking. If he follows the most prominent choice on the internet, that would be SAE 1080. Assuming his SAE 1080 is continuously cast and thus contains about 0.05% aluminum, his choice of quenching temperatures would be from 850oC to 1100 oC or even 1150oC. If he were Japanese and would want to use tamahagane (an aluminum-free, unalloyed high carbon steel), his upper temperature tolerance would drop to 950oC. Both of these steels need to be tempered at 200 oC. The temperature tolerance applicable here is only a few degrees, if you want to make everything optimal. Going for optimal tolerance is applicable only if anthonyget were to manufacture knives on an industrial scale, as it is about maximizing toughness (=resistance to breakage at the same thickness). If quantities are small, the safest approach to avoid breakage is to slightly increase the blade's thickness, since the force needed to break the blade is proportional to the fourth power of its thickness. Since toughness is not only dependent on tempering temperature, other contributors to toughness like oxygen or sulphur content will overshadow the effects of temperature on toughness, and annealing temperature tolerance is firstly chosen to maintain hardness, and that means 200-250 oC or even 200-300oC - assuming that the quenching temperature, and time at that temperature, has been low enough not to lead to an increase in grain size, and high enough to dissolve all iron carbide into austenite.
If I were in his place, I would use SAE 1060 (most ready source for DIYers: leaf springs from the scrap yard), assume that it does contain the required aluminum, quench from somewhere between 900-1000oC and skip tempering at all, as the supposed benefits of tempering on toughness are seriously constrained by the steel's inclusion content (mainly in the form of silicon oxides / manganese sulfide mixed phases, and in aluminum bearing steel, calcium aluminum oxides, too. Occasionally, you'll also find copper (from an electric motor that was sneaked into the scrap from which the steel was melted) or tin (from too many beverage cans in the scrap) limiting the toughness of the steel.
If anthonyget were to use a steel with a higher alloy content( =Stainless or HSS), he would have to choose a higher value for the minimum quenching temperature, since his objective is to dissolve not only iron carbide, but also chromium, vanadium and tungsten carbide, if those elements had been added to the steel. But his upper temperature limit would remain the same at 1100-1150 oC, since he must keep the aluminum nitride precipitated, or have massive grain growth with its concomitant drop in toughness. He will most likely notice this toughness drop by fracturing his knife when quenching. If not then, then soon thereafter, as this toughness loss is not averted by tempering.
On alloy steels, tempering cannot be avoided, since the objective here is to re-precipitate the chromium, vanadium or tungsten carbides, as they are the source of the improved properties of alloy steel. And that means not only the tempering temperature, but also the rate of heating must be controlled so that precipitates may be obtained with the right size, the right quantity, embedded in a matrix that does not change over time. And the latter means that tempering needs to be repeated several times until the matrix has a stable composition.
You don't believe in stabilising? This is what is behind JKeetonKnives "Straightening Plates For The Knifemaker".
And lastly, these temperature tolerances are applicable over the whole length of the knife, excluding, maybe, the tang. And we will have to assume that the blade is 200mm long.
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