Tuesday, August 20, 2019

Cow Magnets & Phase Changes

This piece sparked by some stories being swapped recently on the Association for Living History Farms and Museums discussion group…

Cow magnets, easily available at my rural farm supply stores around Grey County in Ontario (Canada), have long been part of my blacksmithing tool box.

For those outside the 'Art and Mystery' :
The ideal temperature for quenching (most) cutting edges after forging to shape is what is called the 'critical temperature'. Without going into the science, iron materials change from a rigid solid, into what is technically a 'plastic' at a certain point in their heating. At this point they also stop being magnetic.
Although a skilled smith can judge this by colour and trained eye - the certain way is just to touch a magnet to the hot metal. Just at the point the metal cools enough so a magnet just barely starts to stick? That is your critical temperature.
This certainly a good 'trick' when working in the sun with outdoor demonstrations!

Those long bar magnets many of us used back in public school would be even better (if you could find one).
I recommend my students just get a cow magnet, these run about $5 CDN around here. I've been using the same one for over 30 years for just this purpose.
ok - I see the technical blade makers reading this starting to pout just a wee bit.
It is not that simple :

Noon’s Knives

Note right off the start - the carbon content (even in small % variations) shifts things considerably here!
This is hardly any surprise to the experienced blade smiths reading. Right of the start - there is clearly no ‘one size fits all’ in terms of ‘perfect’ iron / carbon mixture - for ALL cutting edges.  (a)
As it cools to 770 °C (1,418 °F), the Curie point (TC), the iron is a fairly soft metal and becomes ferromagnetic. As iron passes below the Curie temperature, no structural change occurs, but the magnetic properties as the magnetic domains become aligned. (1)
But leave the details here, and accept the generality of the original quote at the start. (Do remember it was intended to a non metalworking audience - and really was about Cow Magnets, not technical blade making!)

Another Cool Thing:

This was demonstrated to a group of us at Lee Sauder’s ‘Smeltfest’ event in 2008 - by Jesus Herandez.
If you take hot bar into a really dark room, then watch it as it cools.
"At the critical / transition point, the bar will flash to a brighter colour = higher temperature, for an instant." Oh, yea, we all said...
As I understood the demonstration:

From 'Iron, Steel and Swords' by Helmut Föll

As the atoms shift from ‘face centred’ crystals / Austenite back to ‘body centred’ crystals / Ferrite, there is a release of energy as they shift over.  (b)

Further Reading?
‘Iron, Steel and Swords’ - Helmut Föll

a) There are a number of past discussions related to this topic on this blog. What the tool is intended for most certainly effects the choice of material. Some combinations of hardness against flexibility, plus other desired qualities (rust resistance) may be possible with modern, exotic (?) alloy types. Often working directly *against* ability to easily forge.

1) ‘Allotropes of Iron’ - Wikipedia

b) Ok, my eyes start to glaze over too.
The best (and by far clearest!) explanation of all this is again ‘Iron, Steel and Swords’ :
Reading Phase Diagrams - part 6.1.2 / 6.1.3

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February 15 - May 15, 2012 : Supported by a Crafts Projects - Creation and Development Grant

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