Screw hardness question

[ShannonH]

I've noticed in several threads concerns that some screws are "too hard" too use for a pickup, and that a soft screw is better. Could someone please explain? Based on what little I know about metals ( I used to make knives), the magnetics should travel a bit more directly through a heat treated screw vs an annealed screw, but I'm not sure that is would be the kind of difference that would ruin a pickup. Thanks in advance.

Shannon

[David King]

Shannon,
I think the main difference between hardened steel and the soft stuff is a physical change in the way the molecules line up, the result being that hardened steel (Martensite) tends to hold onto any magnetic charge (higher coercivity), whereas the soft steel (Austenite) has a lower coercivity and theoretically makes a more efficient pickup. Annealed Low carbon steel, say 1018, conducts the magnetic field well but once that field passes there's very little of the charge left in the steel. Many nuts and bolts are low carbon but grade 5 and grade 8 bolts are harder and probably higher carbon content to begin with. You can heat treat and quench steel to harden it or you can work-harden it by hammering on it, bending it back and forth or rolling thread into it and forging the end into a screw head.

Then there's permeability, which I think is a metal's ability to conduct a magnetic charge, not sure how that's affected by hardening...
Magnetic permeability of steel is listed as 700µ.
MuMetal is 20,000µ
Pure iron N5 grade is 160,000µ

[David Schwab]

Like David said... the harder the steel (more carbon) the less magnetic it is.

[SK66]

I would consider the difference to be more like the difference between a maple neck (hardened) and a mahogany neck (unhardened). Martensite doesn't have any more carbon than austenite, it's just a more "alligned" molecular structure. Similar to sintered vs cast alnico (anisotropic vs isotropic)
I don't believe a given piece of steel will be any more or less "magnetic" when hardened vs soft (although magnetic metal AT hardening temperature is non-magnetic, it's "magnetism" returns as it cools). I do think there is likely a notable difference in the way the steel will transmit that magnetism though. I believe hardened steel to have greater permeability and that may result in a "harsher" sound. However, blended with weaker magnets may have a desirable benefit.
As explained above, coercivity is steel's ability to be magnetised; however, that is really a steel's remanence characteristic. Coercivity is truly a magnetised material's resistance to demagnetisation. Since the poles are aways in contact with a supplying permanent magnet, I'm not sure if either remanence or coercivity matters.
All that is to say...Hardened has "better" magnetic characteristics, just like Ceramics have "better" magnetic characteristics and dense woods have "better" mechanical characteristics...But in pickups and guitar woods, "better" isn't always better.

[David Schwab]

That makes perfect sense. I forgot about the "alligned" molecular structure thing, and was just thinking of high carbon steel verses lower carbon steel.

[ShannonH]

I've been pondering the carbon content thing. If that were true we'd want screws made out of 1094, or D2. The bigger issue is ferrous content and the alignment of the molecules. The annealed steel offers a less direct path if you will, breaking up the pattern some.

Thanks!

[David King]

The key to steel in pickups is going to be how easy it is for the string to disrupt the field as it moves through the field. Any disruption will result in a signal being generated. More disruption = more signal.

I don't know what this means in terms of hardness or carbon content and their sonic signatures, it may well be irrelevant but you can be sure that anything you change will make a difference that someone, somewhere will claim to hear it.

[ShannonH]

... but you can be sure that anything you change will make a difference that someone, somewhere will claim to hear it.

LOL