52100 Heat treat Issues - Cracks

JTknives

JTknives

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Jun 11, 2006
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So the other day I heat treated 4 kitchen knives for a customer who will remain anonymous unless he wants to chime in on this thread. Everything went perfect up to the quenching. All the blades got there normal 3 step grain refining cycles from 1650 to 1450 in 100 deg steps. After this thy looked great, one had a little bow but noting out of the ordinary. The next night I did the actual heat treat on 2 blades. Fired up the Oven to 1475 (this temp gives me the best results with 52100 66-67rc) and let it soak for 1hr (the oven). At this time i also fired up the parks AAA quench tank and set the pid to 120deg. After everything was heated and ready to go I double checked the AAA quench oil with a hand held thermometer.

I then did one blade at a time with a 15min soak. Right out of the oven and straight down into the oil with a slight edge to spine sweep like I always do with blades. The first blade was the one that had a bow and it kept this bow after the quench. So after the quench and still smoking hot (normal hand straightening temp) I slipped it between the quench plates to keep it straight. I then put the other blade in the oven and quenched it the same way. It came out with a slight bow as well so it also went between the quench plates. I grabbed both blades and proceeded to clean the oil and gunk off them to get them ready for tempering. This is when i noticed one blade had a crack in the spine but the other one was OK. Thy both got clamped to the temper plate and got there 400deg temper for 2 hrs twice. Thy settled at 62 rc which was the desired hardness of the customer. I was rather concerned about the cracked blade and was trying to figure out why. The next night I heat treated the other two blades. All 4 blades where identical in profile and thickness. I did these blades exactly like the 2 the night before same temp of oven and oil. these came out straight from the quench so no quench plates needed. I went to clean the oil off and both blades had cracks in the spine.

I don't know what is going on, I have never had any cracking issues with 52100 before. Any time i have gotten a crack its because the blade warped really bad and it snapped while straightening. But even that is rare as I have changed how I treat badly warped blades. So I'm coming to you guys for help as I am left scratching my head and don't know what is going on. And yes I used the correct oil, its hard to mix up AAA with parks 50, for one thing AAA has a smell that's unmistakable and #50 does not smell. I even went out and double checked the oil in the tanks was the correct oil. Here is the pictures of the blades, I was absolutely sick to my stomach to break the news to my rather loyal customer.

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The only 52100 blade Ive seen crack was done in parks and like you said theres really no mistaking parks for AAA. You followed all the correct protocol. That's very weird to be honest. Id never expect that
 
I’m assuming stock removal, not forged. I can’t see where you did anything wrong. ?????
 
I assume it's not recycled 52100? I.e., forged down from bearings or races? Sometimes that material can have issues. Other than that, seems like an anomaly but a hard one to accept.
 
I don't like the idea of clamping oil quench blades like that because I'm afraid they will want to pull so hard on cooling that something like this will happen. I don't know if that's a justifiable fear or not, I know many people do it. I can wait for the second temper.

But two weren't clamped at all? Strange.
 
And at the spine? Weirdest part of all. If the same crack propagated from the edge it would be easy to write up to grinding stress risers and the different cooling rates edge to spine.
 
kuraki said:
I don't like the idea of clamping oil quench blades like that because I'm afraid they will want to pull so hard on cooling that something like this will happen. I don't know if that's a justifiable fear or not, I know many people do it. I can wait for the second temper.

But two weren't clamped at all? Strange.
Click to expand...

Yeah the first two where put in the plates and the last two where not. And thy where cracked by the time I wiped off the oil, still warm. I never heard a tink or anything. I tempered them all any way. I temper by clamping them to a large steel plate in the oven. It keeps them straight and seams to even out the heat along the blade. Also the one that did survive was clamped in the plates.
 
That's the standard recipe that we have used forever.. since Kevin was still active on here and gave it too us. never had an issue at all.
 
Crazy, might help to break/bevel the corners on the spine. Do you know if the material is cut from plate, or is it bar stock?

Hoss
 
i see some really deep scratches, like 36 or 80. any on the spine ? maybe a cause of stress ? also the bevels seem to have a different grit finish than the flats, ( in addition to the really deep scratches) could that cause a stress issue ?
 
They are quench cracks not temper cracks. During temper the stresses are reduced therefore reducing tendency to crack.
Do you really need 1 hour soak?
I would suspect decarb on or near the spine and contour of the spine.
Straightening of hardened steel should never be done less than 400F
 
I said 1hr soak for the oven. I give the oven 1hr at temp to even out befor I put the first blade in. It seams to have less temp swing when the elements turning on and off. The blades only got 15min
 
Bad news, i am sorry to hear that. That is the ghost who always lurks behind our HTs.
In this case it sounds you covered very well all the critical points, if i have to shoot in the dark, then:
- possible uneveness in the microstructure, generated by decarb, in the spine area
- combined with sharp corners at the spine, acting as stress risers
Did you inspect the grain inside? in such cases it is advisable to snap the blades as soon as possible to check for differences in color inside the crack.
Sometimes it shows the crack "embryo" was already there before the quench. Profiling could be harsh sometimes on hypereutectoid steels.
 
Too bad for the cracked 52100 blades and empathize 'sick to my stomach' about it.

* I think * these quench cracks are repeatable for blades with same geometry+cross-section in many steels. Cracks should be mostly silent because it is a separating/slipping/unzipping with continuous cooling. Crack curve pulled toward stress vector direction R - in this case, lower part shape of a left parenthesis '(', ended with net-pulled direction almost straight toward edge. I


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Just something I noticed, especially on the 2nd pic down from the top, there are vertical scratch marks just to the left of your crack that reach the spine. The 3rd and 4th pic even show what looks like maybe vertical scratch lines at the spine, and obvious vertical lines lower in the bevel. I always make sure scratch lines are tip/tang oriented, and at least 120 grit. Not sure if that's what happened here, but maybe something to think about.

As been stated, your 52100 HT looks great. (I bet Mr Cashen would say shorten the soak to 10 minutes, because your concern now is just good solution, and not dissolving spheroidial carbides, since that was done with your 1650F normalizing.)
 
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