Dang Warpage!

Sando

Sando

Knife Maker
Joined
Jul 4, 2002
Messages
1,148
I just built a large kitchen knife in ATS-34. Carefully straighted before grinding and recheck again after.

I used 180 degree oil quench (so I can straighten easier.) But when lowering into the oil the blade slipped and the first 1/3 from the tip went in first. Fummbled around a little then got the rest in.

When it came out there was warpage. tried to straighten it. Didn't look too bad. 300 degree snap temper for an hour. Recheck - didn't see any warp, but didn't look real close. Then dry ice cryo (I like to try everything). After 18 hours. pulled it out and warp city!!! dang, dang, dang!

I'm hoping I can stress relieve, re-heat, re-straighten after quench. Possible?

Or should I anneal ( a hard process for me to get done)?

Steve
 
Restraighten after quench ?? All attempts should be made to eliminate the causes of warpage. If your blade warps it means that there is a difference of some kind between one side and the other. It may be forging -try to stike equally on each side of the blade.It may me hardening - are the temps the same on each side ? It may be more decarb on one side than the other.Quenching - quenching flat will guarantee warping ,quench edge down or point down only, but you dropped the blade in the quench that may have been the problem.Apparently no one uses a circulating pump it the oil but some method should be used to circulate even if it's moving the blade back and forth (edge to spine)..... Since it's warped you should stress relieve, straighten(you could do this hot),harden and hopefully it will be straight . Check every part of your procedure to eliminate sources of warping . Think straight !!!!
 
Thanks mete,

Knew I could count on you.

I did a second blade in the same heat cycle. No problem. 1/16" but hadn't been ground yet. Staight as an arrow.

I'll try and straighten after the stress relief. I need to look that up, but I believe the cycle is simplier than annealing.



FWIW my trouble with annealing is I don't have 220 in the barn yet. So I'm using the generator. What a bunch of noise! The anneal process is going to take 10+ hours.

RE: Restraighten after quench ??

As I was lead to believe. Immediately after the quench, before it hits room temp, the steel is in a 'plastic' state. It can get straighten out at that point. That's why I tried the oil. Makes sure it doesn't get to room temp before I want it to.

It is flexible right after the quench, and I believe I had it straight. But the dry ice stage really set the shape in. Maybe I didn't let it rest long enough after the snap temper and dry ice. It was kinda warm when I put it in.

???

Steve
 
It looks to me that you could straighten the blade with the three point squeeze method in a vice. Ive done it and it works Great. What really sucks is a twist. Those go in "the pile". Good luck Steve
 
Steve, I would guess that if there is a plastic state it would be during heat treat when the steel is in solid solution (neither a liquid or a solid). In fact I have been meaning to ask Mete about that: Is there a plastic state while austenitizing?

When stress relieving and when pre-heating ramp slowly to those temperatures. The steel will expand at those temperatures and it is wise to allow it to equalize as it ramps.

However, that may not have been why you got a warp this time.

RL
 
Mark,

RE: What really sucks is a twist.

I never said I liked you did I? Being one that enjoys shooting the messenger, watch your back....

Dang again and again. Yes It twist too.

I suppose that means I shouldn't even try and save it.

Dang, dang, dang.

Roger, I suppose it was one of those little screwups that make for big problems. I think my process is fine, it was the quench. Well, maybe I should go back to the plates.

Steve
 
warpage on stainless isn't that very uncommon..... maybe Paul B. will chime in here.

Steve, were there any odd discoloration spots on the blade, or was it very homogeneous? Did you have a piece of sacrificial paper , ie, "oxygen sucker", in the packet, and could it have become dislodged and settled close by where the warp is? Did you have it in an envelope and take it out? Next question, was it only in the liquid during cryo, or touching the dry ice? The problem could be nonhomogeneous cryo, localized conversion of retained austenite and crystalline size changes that caused it to pull. There are any number of reasons. It's just really not that uncommon on long, thin blades.

I'd say heat it to about 1400F, take it out hot and try to straighten as best you can, then re-HT, including the stress relief temp hold. If that doesn't do it, you have a curved shopknife.....
 
Fitzo,

>> odd discoloration spots on the blade, or was it very homogeneous?

Ummm I've never had one that was perfectly consistent colored. This one wasnt' either, but nothing that jumped out.

>> Did you have a piece of sacrificial paper , ie, "oxygen sucker", in the packet,

Yes,

>> could it have become dislodged and settled close by where the warp is?

It was setting one the blade in the middle. Which is the warp central.

>> Did you have it in an envelope and take it out?

Yup, removed for quench.

------------------------------------------------------
>> localized conversion of retained austenite and crystalline size changes that caused it to pull.


Wow! Maybe that's it. I didn't dry-ice with acetone. Just dry ice. Didn't see the need. I just set it on one block and another block on top. But the blade, being about 12", over hung on each side. Too boot it's torqued in the center.

I THOUGHT cryo was a minor process (structure-wise) and metal being metal the entire blade would reach temperature pretty soon. Maybe not soon enough.

I suppose then that the proper dry ice method would be to first cool the acetone. Then get the blade in there without resting directly on the ice.

Another lesson learned. Back to LN. But I don't have a big enough tank to immerse 12" of steel.

Steve

------------------------------------------------
Roger:

I know you're going to ask:

I went from quench to 300 degree oven. After that HRC was 61/62. Then dry ice. HRC was 63/64. So dry ice did have the expected effect. Maybe also the unexpected one.
 
Steve, some just sandwich the blade between two slabs of dry ice. I, PERSONALLY, don't like that. I always made a bath in a tall cooking pot. I suspend the blade in the liquid. After 30 years in the lab, I make the bath backwards, too:

I have acetone in the pot. (You can use methyl alcohol or kerosene, too.)I bust up the dry ice into little chunks. I add small chunks, and it'll boil like mad. Key operative word, here is small chunks. Keep adding chunks slowly until it doesn't boil, then shovel it in, leaving enough space to suspend the blade. Keep dry ice in there. It helps to insulate the pot, too, so the ice lasts longer. Dry ice has gotten really expensive locally. LN is much cheaper, so I recently bought a Dewar from my old company's scientific salvage.

One thing I will assure you, a blade CAN warp in cryo if it doesn't cool evenly, especially if it was warped and straightened post quench.
 
Steve, I believe you were able to transform most of the retained austenite if you got about 64 after cryo.

Regardless of temperatures used, steel will heat or loose heat from that surface closest to the heat source. So, over hangs outside the dry ice will not cool at the same rate as other surfaces. The same theory with ovens. If areas within the chamber are inconsistant in temperature it will be the same for the steel occupying these oven temperature variances. We need to learn our ovens as best we can and plan for what portion of the knife blank should reside at particular areas within our oven. As an example, I prefer the blade portion in that area most consistant and in agreement with the pyrometer while the tang can reside at a higher temperature than my pyrometer indicates the oven chamber to be.

RL
 
Plastic steel ? watch your words Roger. Obviously it's most 'plastic' above the critical temp. You can do things to it if you get it below the pearlite nose and above the Ms but with something like a blade it's going to cool very quickly.....A long time stress relieve will give you a subcritical anneal.....Stainless steels in general will warp more easily than carbon steels because of the lower heat conductivity......plastic immediately after quench , before room temperature ? well, only in that there might be some austenite which you can bend easier than all martensite......Most of the comments in the metallurgy books about warping deal with the problems of complex shapes. It's frustrating to hear so many problems with blades which are the most simple shapes. Again go through EVERY step in you procedures carefully examining them to see if you can temperatures in every part of the blade more uniform.
 
>> It's frustrating to hear so many problems with blades which are the most simple shapes.

Frustrating to hear!? Try holding it in your hands.

;)


Thanks Mete, will do.

Steve
 
Actually, and I never thought it without being caused to think about it, Mete is right. Knife shapes are not as complex as we like to blame them for being. In fact we got it easy compared to tooling and some other shapes. All we got is thin, thinner and curved and on one plane.


RL
 
I need to start out by declaring my ignorance with all things stainless so I cannot address any of those issues. But the topic of plasticity above Ms has been brought up, and this I am very familiar with. One of the great advantages of an interrupted quench (marquenching/martempering) is the ability to straighten any kinks that may have occurred in the initial quenching process. This is easily done with salts (as I do) or with interruption of an oil quench.

Have a good pair of gloves on before the quench, at a point above 400F.(depending upon the alloy), but not too much above, interrupt the cooling process. In oil this will be about right if the sizzling has stopped and there is oil covering the blade but a few wisps of light vapor or smoke coming off. At this point the blade will have avoided the pearlite nose entirely but will not have formed martensite yet, it will be in a stage of very unstable austenite (which means it is also very bendable). Quickly eyeball it straight, using your gloved hands to tweak it. Be very careful as the blade is the same structure as it is at forging temp., so you could easily over bend it (I have even left finger dents at the edge from holding it too tightly when tweaking).

Mete is correct that you will not have all day to do this, but you should have more than enough time to work it straight. The time depends upon the alloy. 1095 blades have to be done real quickly, while I have worked L6 blades for as long as 8 minutes before.

If you still miss some minor warpage, they can also be tweaked at a 400F. tempering temperature, but it will take a lot more force and you need to be a little more careful. If it is a single edge blade, and you still have a kink that you cannot get, put one end in a vice and hold some wet ceramic wool against the edge to protect it while you heat the spine until it is blue. While it is at this temperature it will get very receptive to adjustment again and you can get just about any kink out, and not worry about breaking it unless it cools again while trying.

One note about the interrupted oil quench, be careful not to interrupt at too high a temp. If the edge drops below Ms and the spine is still 600F. when you remove the cooling action, that heat is going to bleed down to the edge and start tempering your fresh martensite at 600F. This form of quenching will produce a blade with significantly more impact strength due to the fact that the martensite literally has not time to develop stresses before it has a quick snap temper from the slow cooling process. Many people have noticed a 2-point drop in Rockwell hardness from this method and then wrongly concluded that they had lost overall hardness from it. WRONG! In the big picture you have achieved the same hardness, you just got a jump-start on the tempering process. I must also stress that you must temper after this quench like any other hardening operation. I prefer to use the term "marquench" because too many folks get the idea that they don't have to temper because it is called "martempering".
 
Kevin,

You are using that Park (?) quenching medium, correct, when not using the salts? Something I have wondered about is if the "common" backyard quenching mix, often motor oil and ATF, have sufficient heat removal capacity at 400F to get the blade past the nose fast enough. Or, are the "engineered" mediums more capable of this than "old standbys"?

I have no knowledge on this, so I am quite curious.

Thank you....

BTW, it is nice to hear someone define marquenching as you have. The term has been used lately to define a process of sub-Ms extended hold times, ala the super-Ms bainite quench, but which I think is actually isothermal quenching based on my limited reading.?? Correct my definition if I'm wrong, please.
 
I don't think it is any secret that I am not a proponent of home made secret quench mixes. I do need to say, however, that I have no problem with other folks using whatever works best for them, it is none of my business and we all need to do whatever will get us to the desired finished product. All I have ever had a problem with is folks who try to say that there is no advantage to using proper quenchants over used motor oil, because that is silly.

Now that I have put my disclaimer, lets get to your question. I have worked very little with the "martempering" oils designed to work at the higher temperatures, I have found low temp salts to meet all of my needs very nicely. I do quench 1095 and 1084 in oil, (Park#50) but I heat it only to 140F. and then interrupt the quench. Oils do have to be specially engineered to withstand the kind of heat required in marquenching. First, and foremost, normal oils begin to loose virtually all of their heat extraction capabilities at around 250F. this will result in some skidding through the pearlite nose for anything but oil hardening steels. Secondly most oils have a flash point in the range of 350-400F., adding a 1500F blade to this is just asking for trouble. Quench oils are designed to handle long term exposure to heat and will withstand oxidation and thermal breakdown much better. If I had my choice I would choose ATF over motor oil for simple quenching and just interrupt it at around 450F. Unless one has access to proper martempering oils, learning how to interrupt the quench well is probably the best way to go.

Park offers a martempering oil called thermoquench, that can handle 400f.

Sub-Mf hold times won’t get you much more than retained austenite. Marquenching/martempering is a very handy technique (particularly for swords) and I have been using it for many years now. It could be defined as quenching to a temperature above Ms and holding only long enough for the entire work piece to equalize at temperature. The piece is then allowed to air cool from Ms to Mf for a much less stressful hardening.

Holding the steel at elevated temperatures for extended periods is used in ausquenching/austempering. Here the steel is quenched in a medium that is below 500F. but above Ms, for lower bainite, or between 800F and 500F for upper bainite. If the temp drops below Ms you will get martensite, not bainite. The higher the temp, the shorter the necessary hold time and the softer the bainite. The lower the holding temp the longer the soak time and the harder and more acicular the bainite will be.
 
Kevin,

I am dreaming about getting a vertical oven. Will the high temp salts work for me at the temperatures I heat stainless to (ranging from about 1950 to 2100 F)?? It would be nice to have another one for salt quenching too. A pretty big dream for this little guy.

What brand oven(s) are you using?

Thanks.

Roger
 
Kevin and I have had discussions before about whether it's martempering or marquenching. Metallurgists have used martempering .But that's symantics and the actual procedures vary too. The important thing is to understand quenching stresses.As we go through cooling from just above critical to just above Ms we introduce stresses due to the temperature gradient from the outside to center of the piece.If , once we pass the pearlite nose,we gradually lower the temperature to the Ms ,we minimize these stresses. The second set of stresses occurs as we transform from austenite to martensite primarily due to the difference in volume between the two.. If we do that gradually we minimize these stresses. When we go directly from above critical to the Mf , we are maximizing quenching stresses and these can be very high (ping high).You can develop details for martemper/marquench as Kevin has but before you do that try to perfect all the details of your heat treating.
 
Thanks for the response, Kevin. There has been some discussion on other forums about quenching at ~ 400F and holding for hours, which I perhaps misnamed isothermal quenching. It's in one metallurgy primer I read, too, but not discussed at length.
 
Originally posted by rlinger
Kevin,

I am dreaming about getting a vertical oven. Will the high temp salts work for me at the temperatures I heat stainless to (ranging from about 1950 to 2100 F)?? It would be nice to have another one for salt quenching too. A pretty big dream for this little guy.

What brand oven(s) are you using?

Thanks.

Roger
Click to expand...

The Nu Sal salts that I work with will not handle temps that high, but some of the older formulas on the martket that have other metal salts in them will. I believe some of those use barium chloride and can handle stainless.

I use an Evenheat vertical for the low temp and the annealing, but I will never go back to using electric for high temp now that I have seen the wisdom of a gas fired system.
 
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