So, a parts order that I was waiting for arrived… BUT… it was unfortunately the wrong stuff. That leaves me with a bit of time on my hands, so I thought I would try an experiment that has been in the back of my mind for a while: comparing stock pole pieces vs home-annealed pieces.
Let me start by saying that this is HOME “annealing” in quotes… i.e. not well controlled. For proper annealing, you would have to slowly raise the steel to it’s curie temperature, hold it there for several hours, and then slowly back off the heat in a controlled manner for several more hours in an oxygen-free environment. That being said, PART of this experiment was to see if ghetto-annealing would have any effect at all (which obviously it did or I would not be writing up this post). The OTHER point of the experiment was to see if there was any improvement in magnetic properties in both A) cold-rolled steel rod stock and B) commercially-purchased 1018 pole pieces (to see if they were annealed after they were cold-worked or not).
The steel was just cold-rolled 1018 bar (1/8” x 1/4” x 1/2”, and the pole pieces were standard 1010 .187” types, salvaged from a donor humbucker. I built a coil of about 2000 turns of magnet wire on a drinking straw that I could slide the pieces in and out of for comparison. For a magnet, I used a small neodymium disk. I tested the pieces against each other to make sure their responses were about the same, and they were indeed.
Next, I filled a stoneware pan with plain clay (non-clumping, plain econo kitty litter… I would have used washed sand, but the local hardware store was out) and baked it in the oven until it was hot all the way through (an hour or so). This was going to be my ‘controlled cooldown’ environment. For heating the steel, I used a plumber’s propane torch and a rack made from an old coat hanger.
The Curie temperature for steel is as high as 1200-1400F, which I had no way of measuring, so I compared the color to a reference photo from an online blacksmithing site. It was a medium-dull red glow, so I moved the torch along the piece and adjusted the distance to match that color as best I could (not very technical, but it did the job). I planned to set the torch at the right distance and leave it running for a while, but I couldn’t find a magic distance, so I just held it as long as I could.
When my arm started tiring out, I began to back away the flame from the piece to slowly cool it off, and when it turned grey I finally buried the steel in the clay to let it slowly cool for a few hours.
Important note: since this was done in ambient air and not an oxygen-free environment, a black oxide layer formed on the pieces, which looks cool, but might not be what you are looking for if you try this at home.
The test rig was just an opa2134 opamp buffer with a small capacitor in parallel with the coil, running into my soundcard at 32-bit float/96kHz.
I actually repeated this twice, because the first time around, I didn’t get a measurable difference, but the second time around I heated the pieces up hotter and longer, and that produced better results.
Here is a sweep of each:
Steel bar (1Meg resistor load):
I also measured each with a gauss meter, and there was an average of 17% increase in magnetic strength an inch above the top of the coil, with the commercial pole pieces showing the bigger improvement (225 uT vs 191 uT). I imagine the effect would be more pronounced if this whole process were to be done… um… properly.
So even though this experiment was a little crunchy and uncontrolled, the general trend was that there WAS a change even with the blowtorch diy method (especially in the commercial pickup pole pieces), which also leads me to believe that the pole pieces were not annealed after being cut and shaped (not surprising), and that it MAY be beneficial to have batches of pieces treated if you are looking for maximal output. I imagine that would be quite expensive… so I for one will continue experimenting with the torch. If it seems to be worthwhile, I’ll probably start looking for a home ceramics kiln to have a little more control over the process.
I am not sure which commercial builders bother with annealing (I know that Bareknuckle does), but there is definitely enough of a change that it is worth considering.