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Author Topic: Cracks in spine  (Read 314 times)
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Will
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« on: December 30, 2016, 02:08:49 PM »

Not sure if this is where to ask or not, if not please move to the appropriate sub forum.

I just finished forging and thermal cycling a pair of knives from 52100 a couple of days ago.  While grinding and dressing up in preparation for heat treat I noticed a couple of shallow cracks on the spines, towards the points, about an 1" to 1 1/2" back from the point.  They were only about 3/16" to 1/4" deep, I went ahead and broke the blades to see just how bad.

Anyway, here's the heat treat recipe I've been using.  It's been a while since I forged 52100, but I didn't do anything different than I've done before.  Steel is from Rex Walther, or at least I'm 99% sure it was, I do have a few billets from large Timken roller bears on hand when I want to try something weird.

Anyway, forged between 1625 deg. F and 1650 max, when finished forging to shape, left a bit thick, both because of wanting to protect the steel and because 52100 is a bear to move compared to something like 1084, 3 edge quenches in type A oil from critical, held for 30 seconds.  3 Normalizing cycles, first at high end of critical, 2nd just at critical, 3rd below critical.  Then to the oven, 1625, air cool.  Heat to 1600, quench in oil, heat to 1350, quench, heat to 1250, quench, heat to 900 and air cool.  Grind to shape.  This is where I noticed the cracks.

I don't normally do much grinding while thermal cycling, just a quick truing up of the profile.  I'm starting to think I may have left a couple of deep scratches on the spine and since I was edge quenching it put too much stress on it.  But I normally make sure all the scratches are running lengthwise on the spine and edge, and no stress obvious risers on the sides.

Thoughts?  This is the first time in years I've had a failure with 52100, and two in a row from the same bar of steel forged at the same time leads me to thinking there is something I screwed up.    

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Ed Fowler
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« Reply #1 on: January 01, 2017, 10:34:03 AM »

When I first started forging I saw some cracks in the spine of blades. Looking back I feel these happened because of several factors: 1) I was forging too hot,   2) my Quench was too fast or 3) the steel was of poor quality. These blades cracked in 1956 so my memory may be a little hazy.

The blades could also have been stressed by forging too cold, or they could have been too hot when quenching.
All I can do is ques.

I do try to quench blades while hardening at the exact same temp each time, about 29 degrees over critical, with the spine below critical temp. I can see no reason to ramp the quench temp down. Maybe you have a reason, it so I would like to hear it.
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Will
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« Reply #2 on: January 01, 2017, 11:27:45 AM »

You mention too fast a quench, I'm using type A quench, but since re working my smithy I've started using a smaller quench tank and during the post forging quenches, and during the thermal cycling in the oven, the oil got hot enough to flame up several times.  I do not believe I've ever had that issue before.  I'm now thinking the oil was hot enough to speed up the quench enough to cause the cracks.  It's possible the blades could have been too hot going into the quench, but I'm very careful when working 52100 about over heating.  One other possibility that comes to mind is I did not do any normalizing heats while forging, only after forging did I normalize.  Normally when forging 52100 I'll throw 3-4 normalizing heats in while forging the bars down, and another 3-4 while forging to shape.  I found out the hard way this is necessary while forging 4140 for tongs and top tools and such if you didn't want it full of cracks.  That and the oil being hotter than normal is the only things I can think of that would be different from the last time I worked 52100.

Ramping the quench temp down is an idea I got from you, Ed.  It's not hardening per say, it's thermal cycling prior to hardening, which is just above critical on each quench.  So far since doing 3 reducing heat quenches and a 900 deg. air cool prior to triple quenching I have had zero issue with warpage.

I will say, even though I did something wrong, the grain at the breaks was very, very fine, so something right is going on as well.  The one thing about 52100, it'll let you know if your messing up.
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Ed Fowler
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« Reply #3 on: January 02, 2017, 12:33:45 PM »

"Ramping the quench temp down is an idea I got from you, Ed.  It's not hardening per say, it's thermal cycling prior to hardening, which is just above critical on each quench.  So far since doing 3 reducing heat quenches and a 900 deg. air cool prior to triple quenching I have had zero issue with warpage."

Must have been an error in communication. I do note that it takes less time for each normalizing cycle, this is after the post forging quenches and before annealing. That may be where I did not describe it well.

You do need enough volume of oil to handle the heat load of the blade. I have seen oil flare up when quenching large blades, but never had one crack. Would like to watch you work a blade and maybe then we could figure it out.
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Will
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« Reply #4 on: January 02, 2017, 10:33:12 PM »

Well, I thought I got the idea from you, but I may have picked it up somewhere else.  Before I had been doing 3 post forging quenches, 3 annealing cycles of reducing heats, then a 1525 deg normalizing cycle in the oven, followed by a 1200 deg. stress reducing cycle.  Then grind and heat treat.

I forged another blade out, did not do the thermal cycles in the oven yet, but I did a quick grind and no cracks.  This is from the same bar as the cracked blades.  Only thing I did different was to put a couple of normalizing cycles in the mix while forging and used a larger tank with a little more oil.

I'll know more in a couple of days, but I'm thinking it was a combination of too little oil and getting the oil too hot, and maybe needed more stress relief while forging, and maybe doing to many edge quenches. 

I find it kind of funny that a couple of the cracks started right where the quench line runs out on the spine.  Maybe even something as simple as not breaking the edges enough before quenching.

I would love to get up your way and attend one of your courses Ed, but with the economy the way it is, I don't know if that will ever happen. 

Thanks for the insight and help
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« Reply #5 on: January 03, 2017, 09:24:12 AM »

I have tried up to 7 edge quenches with no cracks, there was no significant increase in performance after three quenches.

While forging a blade I may or may not give it a few extra full normalizing quenches, these normalizing quenches are usually the result of an interruption.

Forging cycles are in a very narrow thermal band, 1,625 f. (virtually no slag) on the top end and down until the steel quits moving.

Then three post forging quenches, heat to non magnetic and quench in room temperature Texaco type A like you said for 35 seconds each quench.

Next two flash normalizing heats and then a final full normalizing cycle from non magnetic to to room temperature at 70 f. still air.

Next I anneal at 988 f. in my Paragon, into the paragon at room temp, turn on the oven and let it come up to temp with the blades inside at a rate of 1,000 f an hour. Soak for 200 minutes then let it cool down to room temp in the paragon. then to the freezer and two more annealing cycles.

Nothing needs to happen fast with steel, slowly bring it up to temp during each cycle does not hurt a thing. When hardening the cooling rate of the oil is very significant.

If there is a secret to the High Endurance Performance Blade it is a liberal application of Tincture of Time.
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Will
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« Reply #6 on: January 03, 2017, 10:04:22 AM »

Quote
Nothing needs to happen fast with steel

I think I'd forgotten that, and the steel reminded me.  Thanks for you input Ed, I will be applying it and hopefully no more cracks.

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mreich
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« Reply #7 on: January 03, 2017, 10:10:25 AM »

Sorry, but someone posted while I was writing. I didn't read it, so this was what I had to say.

Anyway, forged between 1625 deg. F and 1650 max, when finished forging to shape, left a bit thick, both because of wanting to protect the steel and because 52100 is a bear to move compared to something like 1084, 3 edge quenches in type A oil from critical, held for 30 seconds.  3 Normalizing cycles, first at high end of critical, 2nd just at critical, 3rd below critical.  Then to the oven, 1625, air cool.  Heat to 1600, quench in oil, heat to 1350, quench, heat to 1250, quench, heat to 900 and air cool.  Grind to shape.  This is where I noticed the cracks.


I notice a couple things, Will.

Did you mean to say you edge quench instead of full quench on your three post forging quenches?

If so, where the cracks close to the place where your edge quench met the spine?

I don't know if heating your quench oil far above spec speeds up the quench. I've never tested that, I just use faster oil. Whenever I quench in a smaller container, I always have cool oil to add to, or replace overheated oil. I know it's damaging to the oil if it gets much above specified temp.

After the 3 full post forging quenches from critical, I flash normalize three times from critical. This is exactly what I remember from the Willow Bow.

From there, normal procedure in my own understanding is cycling the steel down with accurate Full normalizations (not quenches).

Since 52100 does air harden to some degree during normalization, I quench the last heat, in the 800-900 degree range. That alone fully anneals the steel. Normal hardness at this time is around RHC 15. That final quench from black heat makes a significant difference.

IMHO, it sounds like the main issue is too many quenches more than anything. If you're post forge edge quenching, that sounds like defeating the purpose of stabilizing the whole blade to me. 
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Will
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« Reply #8 on: January 03, 2017, 02:15:56 PM »

That makes sense.  I was fully quenching the blades after forging, but only edge quenching on the oven cycles.  I think I'll forgo the quenches from the oven, except for the 800-900 deg. cycle.  The cracks were pretty much right where the edge was quenched coming out of the oven.  It also may be that I was in more of a hurry than I normally am with 52100, I run the oven, quenched the blade to room temp, back in the oven which had cooled, the blade brought the temp the rest of the way down, heat the blade and quench.  All three edge quenches from the oven were done in about an hour.  Seems like I remember doing that a couple days apart before.

Thanks
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