Preventing warpage in heat treat

[Jason Cutter]

Let me be very specific, fellows. I'm talking about stock removal of high carbon tool steels - annealed O-1 precision ground stock.

I heat evenly the entire blade section to critical, allowing the torch to blast the blade from both sides equally, and then edge quench 1/2-way up the width of the blade. I hold for 15-20 count, the red heat in the spine has disappeared by now and I plunge in and cool the rest of the blade.

I'm finding that this step results in a proportion of warpage left or right. It occurs more in thinner stock - 1/8inch and thinner and with thinner grinds. Sometimes its very severe.

My solutions so far ? I use thicker stock for most knives (appropriate) and leave far more material on with the initial rough grind (which I take to 180grit) and regrind to shape fully hardened and tempered. I've been accepting that warpage is a fact of life, because I'm very pleased with the performance of my blades - single quenched, triple tempered, given that they are not forged.

Any suggestions on how to deal with this ?

Thanks in advance. Cheerio.

[Mike Hull]

When you grind, make a pass or two, then flip the blade and do the other side equally. Quench in between. Don't let it get too hot. Trying to get a grind on one side done, then doing the other, creates stress in the steel. You will notice it at heat treat.
I know it feels good when you have a nice groove going, to just want to keep grinding that side, but try to fight temptation. It really doesn't take any longer to do it this way, it just seems that way.
One other thing, are you heating your quench oil? Should be about 125-145 degrees F, before quenching.

[Jason Cutter]

That makes sense. I've recently started doing all grinding without gloves, which means I no longer get the steel hotter than I can handle with bare fingers. My dunking bucket is well-used. I might try and take that idea a bit further and grind progressively alternating both sides, even if its not getting hot.

I've noticed it happens more with thinner stock steel, and I am wanting to get into the kitchen knife market, where really thin blades are a boon. I'm getting 1 in 4 warpage with 1/8inch stock and nearly 100% warpage with each piece thinner than 1/8inch. I've resorted to only grinding in the initial 45degree bevel before heat treat and doing all grinding "neat" like Harry Morseth used to do after hardening and tempering.

I guess I just would like to know - is this a fact of life ?
Do I need to work on my straightening techniques ?
Am I just getting something badly wrong ?

Cheers.

[Jerry Hossom]

All my blades warp. Paul Bos straightens them. While it is still warm after quench and before tempering, the steel is usually somewhat maleable and can be straightened by hand or by leaning on it a little.

You might note if they are all warping in one direction, and see what you're doing differently on that side - like maybe it's the side where you start grinding and are a little heavy handed.

[Micky Wise]

Hey Jason, normallize the blade before quinching. Bring it up to critical and let cool back to black. Do this three times and quinch on the fourth heat. Try it I think you'll like it. It will also improve the quality of the blade.

[Jason Cutter]

I was reading Jerry's post and went back to the workshop to have a look at my kaowool forging chamber. I noticed that 95% of all warpages occured to the right. The forge actually has the nozzle of the high-powered gas burner coming from the LEFT side. I wonder if its possible the blast from the burner is enough to force the blade into a bend ?

I do flip the blade over from side to side in order to heat it up evenly, but this may be a factor.

Next 5 blades, I'm going to do a combo of what you guys have suggested.
Mike's idea - grind evenly (alternating) both sides
Jerry's - go slow / even when grinding and maybe try flipping the blade more often in the forge.
Micky's - triple normalise, then harden on the last (4th) heat.

Thanks guys. great stuff.

[OutWest]

Uneven heating or cooling has been the biggest factor in warpage for me, both stainless and plain steels.

Everything that warped was due to my not paying attention to even heating or dropping the blade before quench. Uneven grinding doesn't seem to be a factor but everything is normalized during HT.

[Jason Cutter]

I edge quench a fully heated blade. Could that be enough stresses and "uneven" cooling to account for the warpage ? Although I also get warpage with full quenches.

With multiple edge quenches (triple quench), which I don't do much of anymore, there is that arching effect that occurs with each progressive quench. For that to happen, I'm presuming that there must be an amazing amount of expansion / contraction.

Cheers.

[John Frankl]

Lots of good replies so far.

Jason, for what you are doing, warpage is absolutely NOT something unavoidable. Yes, uneven heating will cause warpage. Normalize three times after grinding, twice from critical to black (that's completely black in a dark/shaded area) and on the third trip to critical, let the blade cool to room temp. You must also cool with the blade suspended in still air. Laying a blade down to cool when normalizing or after quenching will warp it due to the rapid cooling of one side (the side in contact with whatever you have laid it on). Still air is important too because a breeze from one direction will have the same effect. So, triple normalize, rotate the blade evenly in the forge while coming up to critical, quench in preheated oil and I bet you have zero problems.

John

[Jason Cutter]

Thanks for all the replies guys,

On balance, it sounds like while warpage is reducible through experience and attention to technique, it is perhaps not avoidable and probably expected in some steels to a certain percentage.

Ideas to incorporate which sound good to also improve steel performance is a triple normalising technique prior to quench, even heat, perhaps more slowly, proper design of the forge / heat chamber, and ensuring that whtever you do, do it symetrically, including with grinding and heating up.

Aside from getting Paul Bos to get them straight again, also need to work on my straightening techniques, especially when I work up towards the bigger blades.

Sorry for repeating, but I like to summarise to clarify the main points for myself. Troubleshooting is part of the fun of knifemaking.

Cheers, and thanks again.

[Dan Graves]

Jason, you said in your first letter that you cool blade after a 15 or 20 count. Is the blade warped befor you cool it. I have never heard of cooling that fast after quenching in oil. Not saying it is wrong, just I have not heard that before and makes me think that could have sometheng to do with it.

[Jason Cutter]

Dan, the way I edge quench is in a deep tray of oil. I used to have a adjustable shelf to limit the depth of the edge quench, but I was pretty sure pressing the steel up against it actually resulted in a lot of my early warpages - very, very gross warpages.

I allow the entire blade and about the forward bit of the tang to fully heat up a bit beyond critical, and then quench it into the oil. I dip the first 1/3 of the blade in to fully quench the tip, then rock the edge into the oil so that the oil comes up about 1/2way the width of the blade. I use the tongs, held against the edge of the tray to help keep everything extremely steady.

As this happens, the unquenched spine starts to cool to black. As soon as it cools completely to black, I dunk the entire blade to fully cool it. I've discovered that on O-1 steel, it takes a certain amount of time for the spine to cool to black, which later gives me the best performance in terms of edge quality as well as a visible temper line without etching. On 3/16inch stock, the spine takes between 15 to 20 seconds to cool sufficiently before I dunk the entire blade.

THATS the 15-20 seconds I'm talking about. Of course, it takes a lot longer for the blade to cool properly to the oil temperature. I leave it up to a minute or two before I dare to handle it with bare hands. On thinner stock, the spine cools to the same degree quicker of course and on thicker stock, I leave it longer.

I am trying to get as unhardened a spine as possible as O-1 will air-harden to a small degree and it changes the properties of the spine. On some very thin stock, 3/32inch and less, its impossible to edge quench because the quench will suck the heat right out of the unquenched areas almost as fast, and on hardness testing I can tell no difference.

I do all my carbon steel filework AFTER heat treat and final hand rubbed finish, so that serves as a test of whether or not I got the edge quench (or rather the unquenched spine) correctly done. The hand rubbed finish shows up the temper line quite quickly with even a 240grit finish (I generally use 800 or 1200grit), and will show up even on 180grit grinding belt on the machine.

For these reasons, I'm not willing to give up many aspects of the way I'm doing my heat treat. Thats why I'm trying to research how I can reduce warpage while retaining the essential techniques I've practised hundreds of times to produce those differential quenches and temper lines.

Dan, I hope this explains the process for you. This is my own method and may or may not be correct but it seems to work for me, in some ways at least.

Yes, I am aware that excessively fast quenches are bad for warpage - I'd previously done 1095/1084 in water and had everything from banana blades to explosiding steel. Oil's been far friendlier for me.

Cheers.

[gator68]

Could some of this warpage be due to the fact that you are heating the entire blade, then quenching only the edge? On a japanese sword, with a thick cross section, this cause the whole sword to "warp" -- it curves up. Maybe with the typical knife, the blade is thin but wide, so it can't curve up. Instead, as the edge slowly contracts back, it potato chips the edge. If this were the real problem, then only heating the edge to critical, and quenching it, should minimize the problem.

[Jason Cutter]

Yes, I thought that with that much stress going on, it would be hard to NOT get warpage. But a lot of other knifemakers also do it this way, and it is quite effective to get the performance characteristics.

I think the way you are thinking is similar to what Ed Fowler does - blasts the edge with an oxyacetylene torch, getting just the curve of the edge to critical and then full quenching. He does this 3 times. This works too and from what I understand actually makes an even more rugged temper / hardening line.

I'm not sure, but intuitively, I thought that maybe there would be less warpage with that method, but I've never been able to get it to work for me.

Cheers.

[Dan Graves]

Jason, I agree with gator and your method is one I have not tried this way as I have in the past done this to steel that I didnt know what it was (early years) and had it snap, warp and all kinds of crazy stuff. I cool my steel when it is room temp. than re-harden (total of 3), then triple draw. Will expirement with this. Nice explanation from you.