Temper Embrittlement

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Temper Embrittlement

Postby John Harper » Mon Jun 19, 2017 1:43 am

Hi,

I have a question for the metallurgists, or anyone well versed in this subject, and it is this:

Assume that the end of a quench hardened bar of steel is heated to red heat, above its re-crystallization temperature, and allowed to slowly cool so as to soften it. Now we will have a hardness gradient from annealed to fully hardened due to the temperature gradient that brought this about. Now what I would like to know is whether along the heat affected zone we have martensite temper embrittled and a temper embrittled zones.

To me the relevance of the answer to this question is in regards to knife tangs softened at one end for peening, edge only quenches, and also edges overheated by the knife-maker during mechanical sharpening.

Cheers
John
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Re: Temper Embrittlement

Postby Stefano Z. » Tue Jun 20, 2017 9:12 am

As far as i know, the TME would significantly affect the steel when its temperature range gets applied for at least an hour.
This would rule out TME issues for edge quench and tang softening.
Of course overtempering the edge with the grinder would ruin your blade regardless TME considerations.
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Re: Temper Embrittlement

Postby Doug Lester » Tue Jun 20, 2017 11:01 am

One thing that we need to know here is what alloy are you using. Also I don't quite follow the process. I understand that you, at least theoretically, are austenizing the bar of steel and allowing it to cool to normalize it. What I don't understand is why that bar would have a hardness gradient. Is this a flat bar or are you talking about a forged or ground knife blade? A quenchant that has a adequate speed for the alloy should give you martinsite steel unless you are using a very shallow hardening steel in which case a flat bar would only have marensite on the corners and a blade would only have martensite out to a given thickness, around 1/8", depending on the grain size.

I'm also a little unclear as to where the heat is coming from that might cause embrittlement. Is this heat coming from a tempering process? If it is you usually don't run into this problem until you get up to about 500°-600°F. and that is only if you are using a more complex alloy.

If you want to prepare the end of a through tang for peening wrap a wet piece of cloth or leather around the upper tang to serve as a heat sink and only get the very end of the tang red.

Edge quenching doesn't effect tempering.

When grinding the edge of a hardened blade hold the blade in your bare hands. The blade will become too hot to hold long before you have to worry about drawing your temper. Also keep a bucket of water handy to cool the blade off in every 2-3 passes,

Doug
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Re: Temper Embrittlement

Postby Stuart Davenport » Tue Jun 20, 2017 12:36 pm

I think this is what John is asking (forgive me if I have this wrong!) I'll be specific on some things just for clarity sake....

Take a round bar of O1 tool steel and harden the entire piece to 65HRC+. Then "anneal" one end by taking it past critical, say 1500F, and letting it cool in still air. There will be a hardness differential from "soft" on the annealed end to "hard" on the other end. Is there TME somewhere along that bar?

I think "NO" (not a metallurgist!), that there was no "significant" time at TME temperature for the embrittlement to occur. TME occurs when tempering hardened martensite in that 500-600F range occurs, and this tempering involves time. In this case, the martensite is re-transformed to austenite on one end and some distance inwards, cooling to pearlite and possible bainitic structures as well (alloy dependent), and is not being held at 500-600F. That's just my WAOHEG. (wild a** opinionated halfway educated guess)
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Re: Temper Embrittlement

Postby John Harper » Tue Jun 20, 2017 11:41 pm

Hi,

Thank you all for your inputs and Stuart summed up my question well.

We know that tempering is a diffusion process and that temperature and time play roles, but nevertheless we also know that tempering can take place very quickly, as in an overheated edge whilst sharpening, and the self tempering that occurs with edge only quenches.

The chipping of some heat affected edges after factory sharpening and which is gradually eliminated with repeated sharpening is also a puzzle to me and was wondering if temper embrittlement could be playing a part.

Would be nice to hear from a metallurgist.

Cheers
John
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Re: Temper Embrittlement

Postby Andre Grobler » Thu Jun 22, 2017 4:24 pm

those burned edges are often from temperatures far above tempering... so those can be tempering - if overheating was mild enough... but when most people notice edge degradation from grinding, it is from even higher temperatures at the very edge.
Most of our's not to reason why, but to control heat or die... ;-)
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Re: Temper Embrittlement

Postby John Harper » Thu Jun 22, 2017 10:12 pm

Andre,

Thanks for your thoughts, and as you say often, but not always; The temperature reached at the edge can be anything from room temperature to red heat.

But come to think of it, I imagine that an air hardening steel could get heated to the point that the edge is momentarily austenitized and then the heat sink effect of the rest of the blade and the air quenches it, and thus the tendency to chip until the HAZ is sharpened away.

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John
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Re: Temper Embrittlement

Postby Stefano Z. » Fri Jun 23, 2017 6:28 am

Yes the chipping of an overheated edge from grinding is actually due to local re-austenitizing and quenching, hence the brittlessness of that untempered martensite.
I have seen nice micrographs of that fresh martensite surface layer you get due to grinding without coolant.

It could be allucinations, but i swear i actually smelled burnt steel by sharpening without coolant on the whetstones!!! Of course i always use coolant since then
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Re: Temper Embrittlement

Postby John Harper » Fri Jun 23, 2017 8:20 am

Hi Stefano,
I have seen nice micrographs of that fresh martensite surface layer you get due to grinding without coolant.


How was the fresh martensite distinguishable from the rest of the martensite?

Cheers
John
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Re: Temper Embrittlement

Postby Stefano Z. » Mon Jul 10, 2017 5:11 am

Sorry for the late reply.
Micrograph of the sample section, shows the difference through etching. The "fresh" martensite (alpha martensite) tipcally appears whiter, due to the absence of tempering carbide precipitates that occour with tempering. The extra fine tempering carbides scatter the light giving a darker etch.
That's a basic explanation, but there are a lot of different microscopy sample preparation procedures, different etchant mixtures... and different features can be enhanched to be captured in the image.
In the images i have seen, the fresh martensite was a white external layer.

I don't remember the specific image i have seen, but you can find a similar situation here:
https://www.google.it/url?sa=t&rct=j&q= ... iA&cad=rja
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