Acm and non-magnetic...Mr Cashen?

D

Dana Hackney

Joined
Nov 1, 2005
Messages
174
Hi folks,

I guess this question is directed mostly towards Kevin as it was his articles I've been reading regarding forging, as well as all the others about the different states of steel. I've also been pouring over Prof. Verhoeven's (200 pg) write up on heat treating (sloooow, but interesting, going!).

In your forging article, last paragraph, you mentioned that you heat the blade (after forging) to above Acm 2-3 times (with a quench for further grain refinement). My question is: can you use non-magnetic interchangeably with Acm? That is, to determine if I'm above Acm (in the austenite phase?) am I safe by using a magnet and when it goes non-magnetic (plus a little time)?

The more I read, the more I scare myself! And, the more conserned I've become about forging at the right temps and not taking too long or too many heats!

Thanks in advance,
Dana Hackney
Monument, CO
ABS aprentice
 
I believe that Mr. Cashen will tell you, if you are relating to quench heat, to go above non-magnetic for the quench. As far as I know, most steels require 50o, to 75o, above, with soak time, to get a proper quench. The triple heats you mention, are for the most part, stress relievers. The soak time varies from steel to steel, and is intended to allow the carbon and other additives time to homogenize before the quench. With simple steels, this can be just a few minutes, but with more advanced steels, it can be upwards of fifteen, or twenty minutes, to get the maximum benefit.
 
for the information. The more I read about soak times the more I can see the benefit of high temp salts.

Best regards,
Dana
 
No doubt that salts are the best way to go, but I have experimented with soak times beyond twenty minutes using PBC powder coating on the blades with no loss of carbon that I can detect.
 
There are a number of things going on during heat treatment ,it's not so easy as just the Curie point. The Acm is taken from the iron cabon diagram but that's just iron and carbon .When you add other elements things change. In addition that diagram is an 'equilbrium' diagram , something that doesn' take into account various dynamics.
 
Dana, I am very pleased to see that many others decided to give input here, this is not a criticism of your title, just me feeling uncomfortable with whole threads addressed exclusively to me possibly robbing you of others valuable input. I don’t know where this unshakable myth among bladesmiths of the magnet being foolproof came from, but it is exactly that- a myth. For one, there is a vast number of fools who have managed to totally botch what the magnet is telling us, so obviously it wasn’t very “proof” from them (relax fools, I was one of you for a while).

I believe this myth, like a majority of them in this business, was born from the fact that it worked with the simple steels that smiths worked with in previous centuries. If you have a simple iron-carbon alloy, Ac1 is around 1335F. the critical temperature that you would want could be much lower than with modern alloyed steel, making the Currie point of 1414F. a very nice number for heat treating.

The best way to explain it that I, and some of the experts I have mentally crunched this issue with, have is that the Currie point is a function of the spin of the electrons in the iron atom, this is typically set at 1414F. The problem is that Acm is well above this temperature even for simple steels, but when you add alloying that locks up carbon that temperature goes even higher than is possible to guess from looking at the Fe/Fe3C equilibrium diagram. The only time you want to totally exceed Acm is when you want total solution with less concern about grain size, the rest of the time you like to get close without going over, heck if it was easy all these folks for all these years wouldn’t have devoted so much to nailing it.

What so many bladesmiths just can’t seem to figure out is that there is no such thing as a “one size fits all” heat treatment anymore. In centuries past, when there were only slight variations on simple iron carbon themes for steel, set points in standard “fool-proof” recipes worked, but we are in a new era and even a slight addition of chromium moves all the points and corresponding numbers around. Each steel has its own required heat treatment and quaint little ticks like the magnet no longer apply. I would be nice if the “Heat Treaters Guide” was just a 2 page pamphlet, but it is a tome of hundreds of pages for a reason.

I would stress that I do multiple cycles in my normalizing, before even annealing or grinding, for the purpose refinement and equalization of structure and stress/strain.
 
I realized that, there was an important facet of this topic that I neglected I the last post, and that is how the magnet can entirely lie to us when the Currie point does start to move around on us! The only time one can somewhat count on the Currie point telling us something reliable about a set temperature is on the way up. It can measure Ac2 but one rarely sees Ar2 discussed because on cooling things get all screwy. Since it appears that the electron spin responsible for ferromagnetism (yes there are other forms of magnetisms) is tied to the allotropic atomic configuration (sorry for that mouthful, I can explain if folks wish), suppressing the shift from FCC to BCC will also inhibit the return of magnetism.

Those on this forum that were at my lecture at Ashokan will remember me demonstrating this, but I would encourage folks to try it themselves to really grasp the facts. Take a piece of your favorite knife steel and heat it to nonmagnetic and then just let it air cool while continually checking it with the magnet, then ask yourself if you would ever want to quench at any of the colors the steel was glowing at before it regain magnetism.

Now for a real mind scramble, and something that should make you look differently at you friend the magnet forever, Heat the steel to nonmagnetic and quench it to around 500F and then interrupt the quench and check it with the magnet. You will wonder if the steel will ever be magnetic again! 400F will go by and the magnet will not stick. 350F, nothing! Finally as enough martensite forms to alter the ferromagnetic qualities, the magnet will start to regain its senses and when you reach room temperature things will get back to normal.

For determining carbon solution in austenite, on the way up the magnet is shakey at best, on the way down it is all but useless.
 
For determining carbon solution in austenite, on the way up the magnet is shakey at best, on the way down it is all but useless.
Click to expand...


dang,,,,,,



thanks for the laymans version Kevin,
I enjoy reading your posts,


jm
 
Then you can heat it to 2600 F and it will become magnetic again !!!!....It never ceases to amaze me that Kevin can take my 1 paragraph response and expand it into a book !!
 
I have company here now and we all just got a good laugh at mete's post. This is because, good humor has a basis in fact, and I always enjoy facts over opinion, and I would be a fool to argue with mete on this one, as it is a fact that I may be the most verbose forumite around:D

It is metes example of Delta iron being magnetic well above the gamma iron range that has never allowed me to totally disassociate the atomic arrangement from magnetism, since delta iron is also BCC.

Hey mete… I did that in one sentence! :D And not a single metaphor or simile ;) .
 
for the valued and much appreciated input. I'll be cutting and pasting the entire post into a file for future reference. Off to get a good set of tables,
a pyrometer and (eventually) a salt pot.

Thanks again folks!

Dana Hackney
Monument, CO
ABS Aprentice
 
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