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Annealing mild steel


Danijel Gorupec
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Mostly for fun, I would like to try annealing some small steel pieces (say, half inch size or less). Tips?

What would be most easily obtainable tools to do it? What kind of torch should be enough - or should I build a wire heater?

They say: soak it at correct (red-heat?) temperature for some time (seconds, minutes?) and then slowly (how slowly?) cool it down. I guess such small peaces would cool to quickly in air?

Some internet guys also say that you should not heat it too much (orange). I don't understand how can this be a problem?

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  • 2 weeks later...

If anyone interested...

I went to a store and bought a torch - spent more than I expected (cca 70EUR; 75USD - one fuel bottle included). I took the most powerful torch of this type they had. It proved very capable, so maybe I could choose a cheaper one.

torch.jpg.29d83ba3325e98dbafd9476a94f2cad1.jpg

They call the fuel the "MAPP gas". I understand this is some propylene - butane mixture. Hotter than butane, they say.

So, I filled a shallow steel pot with dried crushed stones (angular rock 5-10mm), because I didn't have anything better. Placed steel pieces onto the rock and heated it with the torch. Btw, while heating, the surrounding rock also becomes red-hot (at surface). Interestingly, some gray (granite-like color) rock pieces turned white after this heat treatment.

I kept the metal dark-red-hot for about one minute, and then using pliers transferred it into different pot full of wood-ash (I didn't have anything else, but I read that wood-ash is a good choice). Buried inside the ash the metal cooled slowly enough, I hope.

I annealed following items: a small nail, M5 screw with a nut, a short bright-drawn steel rod, few pieces of transformer lamination (silicon steel)

samples.jpg.3a2ea5eeb21201188edd78f26380fd6f.jpg

 

Results:

To check if I am doing anything good, I compared the annealed nail (sample A1) with an identical non-annealed nail (sample A2). I was surprised how much easier it is to bend the annealed nail. I could bend it with fingers (the smaller bend on the A1 sample is done with fingers only; the larger bends on both samples are made with a help of a vise). Must be I am doing something right.

To check dimensional stability, I annealed a screw with a nut on it (sample D). Unwisely, I choose a coated screw (a bright coating - is this nickel or what?). This coating turned green during heat treatment, so it was less easy to monitor the temperature of the sample. The nut, being smaller, become dark-red-hot much faster than the screw (obviously, there was not that much heat transfer between the nut and the screw).... After the cooling, it was equally easy to turn the nut as before annealing - so I suppose, the dimensional stability is good enough. The nut did not weld itself to the screw.

The bright-drawn mild-steel bar sample (B1) - fi 8mm x 36mm - turned dark (after cooling). It looked more like a hot-drawn or hot-rolled steel. You can see it in comparison with the non-annealed sample B2.

Electrical steel lamination (sample C1) has large surface - so I was afraid it might cool to fast. The sample C2 is not annealed. Not much difference between the two.

 

Magnetic measurements:

I broke the core of a small transformer (old one, low-quality one - there is no insulation between lamination layers) - I arranged it so that I can place metal samples to close magnetic circuit. Look at the picture: the sample B2 is placed into the measurement position (I would also place a weight of non-magnetic material onto the sample during actual measurement).

trafo.jpg.3585acf9e8ed20321e194582c39feb2b.jpg

I am pushing sine-wave current, of constant amplitude, into one coil and then measuring voltage in the other coil. The setup has very bad sensitivity because there is large magnetic reluctance in the gap between sample and transformer core (I guess, larger than the reluctance of the sample itself). Therefore I couldn't measure anything precisely. The results I obtained, are not entirely clear.

With electrical steel, I measure no obvious difference. If there is any difference, the non-annealed electrical steel is better than annealed one. The non-annealed electrical steel sample gives about 5% more measured voltage than annealed sample (but measurement error might be larger than that). In addition, annealed electrical steel does not seem any easier to bend. So, no benefit from annealing electrical steel (or I don't know how to do it properly).

I did however see some improvements with bright-drawn mild steel. The sample B1 produces higher measured voltage than the sample B2. The difference is larger at smaller flux densities (I don't know why - I don't think my sample reached saturation in any case). What surprised me even more is that that the difference is larger at higher frequencies (I would expect otherwise - puzzling).

Excitation current 90mA, 20Hz: Sample B1 -> 7.5V; Sample B2 -> 6V (cca 25% difference in measurement)

Excitation current 90mA, 80Hz: Sample B1 -> 27.5V; Sample B2 -> 20.5V (cca 34% difference in measurement))

Excitation current 300mA, 20Hz: Sample B1 -> 25.5V; Sample B2 -> 23.5V (cca 9% difference in measurement))

Excitation current 300mA, 80Hz, Sample B1 -> 90V, Sample B2 -> 77V (cca 17% difference in measurement))

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