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Thermal Cycling


Stewart Light
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I was rewatching some of Andrew Jordan's YouTube clips the other day and it got to a section with thermal cycling.

 

It starts at 1min 20secs

 

I didn't realise that this part was such a rapid process. It's obviously something I've missed before, thinking that it was meant to be done as hardening three times.

 

Is there anything more to know about it than what Andrew describes? I have done a wee search on here but can't see anything on my initial look.

Edited by Stew
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Well, that's not what I've been taught as a thermal cycle. What works for me is to heat the blade to non-magnetic (1650 F for carbon steels) and allow it to air cool to under the 900 F knee in the HT curve. My experience is that reduces grain size and set's you up for hardening.

 

I don't know quite what he is accomplishing with what he is doing. He is nowhere near 800 F (though he might be at 800 C).

 

It is not what I understand thermocycling to be, but I'm willing to be enlightened. Anyone?

 

Geoff

Edited by Geoff Keyes

"The worst day smithing is better than the best day working for someone else."

 

I said that.

 

If a thing is worth doing, it is worth doing badly.

- - -G. K. Chesterton

 

So, just for the record: the fact that it does work still should not be taken as definitive proof that you are not crazy.

 

Grant Sarver

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I think I might understand his quench thing..

though it does seem a little silly when using oil.

 

I think I get basically what he is doing, and I'm going by what he was saying about trying to the the structures in the steel shocked into shrinking, then bounce back.. I think that he heats it up to critical, does an interupted quench, puts it back into the forge to confuse the steel :P whips it back out once the steel is starting to raise in temp again, dunks it at what would be a lower temperature than the first.. and so on until its cool. I think his proccess might be related to the idea of using increasingly cool temperatures with your three normalizations that most of us do.

 

I don't know, im likely wrong.. its 1am here :/

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As I was taught, thermal cycle can be quite a complex phenomenon. The center of the whole process is the convertion of pearlite into austenite, which is anyway an allothropic change. Simply put, the steel recristallizes during this process, forming entirely different grains, and the fineness of the new grain structure depends on several parameters.

One of this is the speed of the tempreature rising when the temp passes the A3 line.

Another, more important factor is the soaking temp. The higher the soaking temp is, the coarser the grain will get in a certain time. Alloy content has naturrally a dramatic impact on this too, vanadium for example being a good grain refiner.

When austenite is formed, it can be finer than the starting grain structure, and the result can be a finer grain at room temp too. Several of this cycle can help to refine the grain structure, if carried out properly, but the more beneficial effect of this is the well known stress relief.

I have great respect for this gentleman sawn in the video, but keep in mind, that there isn't something like shrinking grains. If the grains shrank, the entire workpiece would shrink, which would be, let's just say, miserable :huh: The method saw on youtube can be good for this cycle, as the important part of the process is forming austenite and then pearlite, and then austenite and pearlite again... But there is also the rather annoying retained austenite thingy, but that is a completely another story ^_^

Despite my poor english, I hope I could help you out on this.

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There's some pinned info at the top of the metallurgy forum that might help a little.

 

Take care, Craig

 

Craig,

Thank you for suggesting this thread.as a resource.. ..when it was first written I did not know enough to understand it...I am getting closer..this particular post is most relevant to the topic posted by Stew

( http://www.bladesmithsforum.com/index.php?showtopic=11873&view=findpost&p=106814 ). If the author of this post is still reading posts...thank you!

 

The post below has a little Banana blade made from wrought iron which had the carbon level brought up ( blister steel ).....during my many attempts to get hamon ( about 9 times) I was in effect doing what Mr. Jordan was demonstrating on the video. I do not have time to look at the grain size but if someone wants to have a look and is equipped and qualified to measure it , I will slice off a piece. I was quenching in water not oil. Niko and Greg were warning me about the grain size getting too fine and affecting the hamon and the hardening...I am getting it now.

( http://www.bladesmithsforum.com/index.php?showtopic=14726&view=findpost&p=144229 )

 

Stew...thanks for bringing up the topic and quuestioning what you are being told.

Jan

Edited by Jan Ysselstein
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  • 2 weeks later...

I'm going to disagree with the reasoning behind the technique if not the technique itself. "Squeezing" the steel together to reduce the grain? Sounds like liquid edge packing to me. :)

 

What causes a reduction in grain size in simple carbon steels is indeed done with thermal cycling, but not like this. What you need to do is to bring the steel to a temperature just above critical for that steel. Critical temperature is the point at which carbon goes into solution within the matrix and can be seen by eye through the phenomena of decalesence/recalesence. If you heat a piece of steel up above critical and watch it cool in dim light, you will see a point when the steel momentarily brightens and it looks like a shadow passes through it. This minor change in brightness is the exothermic reaction of carbon falling back out of solution. You can also see this on a rising heat as the carbon goes into solution, though this is harder to catch. In order to reduce grain size, you need to heat to the point where carbon has gone into solution, but not much higher. As temperature increases grain colonies grow. So, you need to stop the heat just above critical. This is what we call normalizing.

 

"Shocking" the steel like this won't have a positive effect on the grain unless your temperatures are right. If your temps are right, there's no need to "shock" the steel, just let it cool in still air.

 

-d

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