Grind Before or after Heat Treat?; Lessons Learned

K

Kurt Krueger

Well-Known Member
Last year I sold a knife to a guy who wanted to give it to his brother as a best-man's gift for his wedding. He loved it, field dressed and butchered a bear and 3 deer with it. Last winter, I asked to see it to evaluate how it was holding up. I was dismayed to find that the edge looked like he'd been cutting music wire and rocks with it. I honed it back up and put a steeper angle on the edge, but something just wasn't right.

I'd always heat treated first and ground second, except on this blade. The steel is AEBL and I heat treat with the blades in a foil pouch to prevent oxygen contact. At first, I though that maybe more oxygen had been trapped in the pouch due to the contour of the primary grinds. The more I thought about it, the less I believed that was the case. Then it occurred to me that the quench plates would have only made good contact with the full thickness portions of the blade and that heat in the ground sections would have to be conducted out through the nearest point of contact with the quench plates, slowing the cooling rate near the edge.

In order to prove my theory, I'd need to do hardness testing on his knife, so I remade the knife for him and asked for it back in exchange. Sure enough, the spine is hard and the edge is softer, and by quite a bit. The picture shows hardness of 60 on the spine and as low as 51 near the edge,,, and I suspect the edge itself is softer than that. I realize that testing on non-parallel surfaces will alter the results, but in this case I think this proves my conclusion.

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Now, this would be a moot point with and oil quenching steel, but makes a great deal of difference in this case. So, lesson learned, heat treat first for oxygen sensitive stainlesses unless you have a quenching method that is less geometry sensitive than quench plates.

-Kurt
 
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My question is how did you get the edges on the bevel to be perfectly flat 'n perpendicular with the diamond? I've never been able to Rc test on the edge due to that problem

Then it occurred to me that the quench plates would have only made good contact with the full thickness portions of the blade and that heat in the ground sections would have to be conducted out through the nearest point of contact with the quench plates, slowing the cooling rate near the edge.
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As to that issue, as long as the temperature is below 1100⁰F in less than 2 minutes. As long as the edge it below that temp in 2 minutes, you should get full hardness - per Sandvik engineers. AEB-L is VERY similar to 13C26 Sandvik steel. If you wish to be sure you're getting below 1100⁰F quickly, spray water on quench plates.
 
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Your conclusion is flawed. EVERY single AEB-L knife I have sent to Peter's for heat treating (with the exception of 2), have been ground pre HT. They came back at 62HRC. Yes, the test was performed on the ricasso, square to the diamond penetrator, but was 62HRC throughout the blade.
 
Ken, short answer is, I didn't, and acknowledge that not being perpendicular to the surface will affect the results, but for gross measurements, I believe the measurements do reflect the observed behavior of the cutting edge. It's definitely soft.

Stuart, I agree, Peter's heat treat is very good, but I doubt they're plate quenching. That's my point, any other method of quenching a pre-ground blade, be it in liquid or air jet would have yielded a properly hardened blade at the cutting edge. This blade in particular measured 61.5 about 1.5" back from the ricasso (beneath the scales when finished). That portion of the blade was properly plate quenched, it's the bevels that weren't in contact with the quench plates that cooled more slowly.

-Kurt
 
samuraistuart said:
Your conclusion is flawed. EVERY single AEB-L knife I have sent to Peter's for heat treating (with the exception of 2), have been ground pre HT. They came back at 62HRC. Yes, the test was performed on the ricasso, square to the diamond penetrator, but was 62HRC throughout the blade.
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OK, you're saying the 62 Rc was on the edge as well as ricasso - how did you test the edge? I agree, I think the edge is hard as well as the ricasso. Remember, as long as the edge gets below 1100⁰F in 2 minutes or less then it should harden just fine. I'd guess in plate quenching the edge will be below 1100⁰F will before 2 minutes.

Kurt, have you done the brass rod test on the edge? If so, did the "curl" stay curled? OR - did it go back straight? OR - did it chip out?
 
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By experience and file hardness tests. I don't have a RC tester, but I can tell when steel is 62HRC vs 60HRC vs 57HRC. EASILY. I think most can too, who have been doing this for a while, and especially sharpening for a long time.

Pretty sure Peter's uses plates and forced air for AEBL, but don't quote me. Back to Kurt....that's my whole point...you're saying that you didn't think the plates made good contact. For sure I can agree with that. But I cannot agree that the problem is that the bevels were cut FIRST. That wasn't the problem. The problem was poor plate contact.
 
Without going the "right" or "wrong" route, were it me, I would do a couple more similar blades to prove or disprove the theory. When I first saw this thread, I thought of some situations that have occurred with me concerning when things didn't work out on a knife as expected. I always try to replicate the issue, and then go through each factor until I discover the cause. Sometimes it becomes obvious very quickly, and other times it takes several test blades to nail down the cause of the issue. The reality of the situation is that we deal with so many variables from shop to shop, that often times the cause(s) of issues are variables that are unique to our own shop(s).

The take away here is that if the individual knifemaker has an issue, the best thing that can occur is to create a theory as to what happened, and why, then prove or disprove that theory to their personal satisfaction.
 
There is one sure way to find out if your assumption is correct. Harden an unground piece of steel and leave 3-4 inches sticking out of the plates. Drill a hole or otherwise mark the end that will be between the plates so you don't get confused. Do everything else the exact same way and then you'll have a flat and parallel surface to test. I think you'll be surprised at the results. Let us know the results if you try it.
 
Is there a down side to heat-treating first and then grinding? This is what I have started doing with A2. Yes it is slower and probably costs more in belts...but I like the idea of standard thickness material being treated. It seems to me that much of the soak time issues go away with HT first? OR-is not a .020 thick edge much more finicky than a 1/8-1/4 thick edge? Am I incorrect in assuming that I open a bigger sweet-spot in time/temp by heat treating first?

Inquiring minds need to know...LOL!
 
I started grinding them hard after a long phone conversation with Brad at Peter's Heat Treat. I had pre-ground my first couple batches of blades and I was getting some issues with warp. He told me he recommends heat treating first on anything thinner than 1/4. In his words, "If you can can grind it hard, I recommend you do so."

BTW, I don't think Peter's plate quenches unless something has changed. I asked about it and he told me they simply have way too much volume for that. That's why you need to send him blades with holes in them, because that hole is how they hang them.

Please, someone correct me if I'm wrong. I'm going from memory and I don't want to put words in Brad's mouth.
 
John brings up a point... often times we expect that things are done the way WE imagine they should be done, when in reality they are not. I can't image a commercial heat treater who is willing to take the time and effort to actually do a plate quench.

Based on what I've seen in commercial heat treat facilities, they're heat treat methods are very similar to how it's done in production knife facilities..... whatever method is easiest, cheapest, and fastest in order to achieve "acceptable" results is the norm, that's not a dig at any commercial heat treater, it's just the way they do business...... which is the main reason I refuse to send anything that my name goes on, out for heat treating. I have no way of knowing what happened to the item(s) from the time I put them into the mailbox, until they're returned. Speaking only for myself, I am unwilling to place the fate of my reputation in anybody's hands, other than my own. If a person is serious about the quality heat treatment of their blades, I simply can't fathom placing such an important task into a "mass production" environment, even though I understand that some do not have the equipment, or the room for it..... it's just too important to be out sourced. Again....just my mentality.
 
Darrin Sanders said:
There is one sure way to find out if your assumption is correct. Harden an unground piece of steel and leave 3-4 inches sticking out of the plates. Drill a hole or otherwise mark the end that will be between the plates so you don't get confused. Do everything else the exact same way and then you'll have a flat and parallel surface to test. I think you'll be surprised at the results. Let us know the results if you try it.
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Did you mean "leave 3" to 4" sticking out"? OR - perhaps 3/4" sticking out would be more reasonable I think. I agree with Darrin, I think you'll find the answer. There are too many of us grinding post HT to help prevent warping.
 
Sorry for the delay in my response...

Stuart, I think we're in violent agreement here. We agree that poor plate contact is why the entire blade edge didn't get hard. I'm saying that the reason for the poor plate contact is that the bevels, being ground before heat treat, allowed a gap between blade and plate in the area between the spine and the edge. Because the blade is wrapped in a foil pouch when pinched between the plates, even an air jet wouldn't add a significant amount of cooling to portions of the blade not in contact with the plates. As with Peter's, an industrial process doing in some cases, dozens of blades at a time, I have to believe they use furnaces with inert gas environments, negating the need to wrap each blade in foil. That being the case, they have the means to get even cooling on each piece using a variety of methods that I don't have at my disposal, and thus, can accommodate pre-ground blades with fully hardened results. So, the resultant soft edge isn't an artifact of grinding before or after heat treat, it's an artifact of the equipment and methods I have at my disposal for my home-heat-treat. Commercial heat treat is more than capable of doing justice to my pre-ground blades.

In hindsight, maybe my subject title wasn't specific enough.

As it relates to the "feel" of the steel when sharpening, yes, I did feel a difference, and that was a signal I should have heeded. The edge worked faster and the wire edge it turned at each grit level was more difficult to remove than normal (rolled back rather than sheared off). If I'd stepped back from it at the time, I may have headed off the situation I'm addressing now. However, I was on a timeline, and needed to get my client a knife for his wedding.

Ken, since I've received the knife back I have performed a brass rod test, and the fine edge does curl, does not spring back, nor does it chip out. It's definitely soft. Without performing a micro hardness test on a cross section of the blade, there's no way to quantify it. I do have a source that can perform a micro hardness test for me, but at $175/hour, it's not worth the cost to sleuth out the specific hardness. Additionally, I want to use the the knife myself to see if the way I treat my knives results in similar edge damage to that of the original owner. That alone, will teach me something about what I need to plan for in the future.

Good news, the handle and finish held up very well! <-- Glass half full!
 
Oh man, I'd started this reply the other day and just finished it now. I didn't realize the additional attention it had received.

Darrin, the test you recommend reminds me of a lab experiment we did when I was in school, a Jominy Bar Test. It was a round bar, about 3/4" in diameter and about 4" long with a flange at one end. The bar was heated in a furnace and then dropped into a fixture where the flange was supported by a strap of steel, suspending the bar vertically over a water spigot. The water sprayed on just the end of the bar and continued until the sample was cool enough to handle. Hardness tests were then performed along the length of the bar, the further away from the end where the water quenched the sample, the softer the steel got. I don't know what the carbon content of the steel was, but it definitely wasn't a stainless.

I'd love to try the exposed steel test, but fall gets crazy busy for me and don't know when I'll get around to another heat treat session, that and I don't have any more of the steel used for this blade. I will, however, circle back and revive this thread if and when I have a chance to do the test.

I've had good results grinding my blades after heat treat, and am inclined to continue to do so. As John mentioned, it's probably more expensive in belts, and time, but if the results are more consistent, then the time and expense are worth it. Besides, I don't make knives more than 1/4" thick. ;)

Best, Kurt
 
great thread.

A couple comments.
I believe measuring hardness on a ground bevel isn't accurate. I've tested this myself and couldn't get it to work for a typical diamond penetration test. The surfaces have to be parallel. To test this, take a ground blade, heat treat it and test the flats vs the bevels. There will be a significant difference in the readings.

I see lot's of guys grind folder blades after heat threat. Generally, this is to allow a chance to grind out any warps from the quench. I see fewer guys grind fixed blades that are hardened but generally for the same reason, to take out any warps. Some guys just prefer to grind their blades hard. Some abrasives work better grinding hardened material and the feel is different. It's generally a little slower as you have to dip constantly to keep the temp down. It's also feels a little more "slippery" when grinding.

I also see guys grind on a hardened blade and routinely turn the edge and blade tips purple from grinding heat. Purple color indicates +450F and softens things up quite a bit. Experienced guys keep the color out of the blade when grinding. Less experienced guys tend to sand out the color and that just leaves you with a shiny soft blade. I've done it both ways and tend to grind soft, then heat treat.

When I started doing my own heat treat, the thing that bothered me the most about plate quenching a ground blade was that the blade bevel does not touch the plate. I always shoot compressed air in between the plates as I quench. I feel the heat drawn out of the bevel is fast enough by plate contact on the flat areas combined with the air shooting in between the plates to meet the TTT curve. If the heat foil was bloated around the bevel with no contact to the blade and no air shooting in between the quench plates, I would be concerned about the edge getting hard or not due to insulating effect of the foil pouch. Usually the foil has enough contact with the bevel to help pull heat out with compressed air in addition to the plates.
 
I'll be the first to admit that I have NEVER in my life plate quenched. I stick with carbon steels, the highest alloy I will do in my shop (temp limited kiln) is A2, and it gets oil quenched. When I mess with higher alloy tool and stainless steels, they get sent out. But I have used plates to keep thin 15n20 from warping on me post HT, bevels ground BEFORE the HT and had no issue whatsoever to get a flat aluminum plate to lay dead flat on a flat ground bevel. The edge of the aluminum plate would, of course, have to be right at the edge of the knife. Obviously, right? If your plates went past the edge of the knife, the 2 plates would contact each other, and then yeah....you have a gap. You don't clamp the sandwich together on the ricasso, but rather clamp on the bevels themselves. And what would make that worse.....hollow grinds. NO way you're going to make plate contact on that bevel! (Unless you had plates with a radius that matched your hollow grind!)

And the more I think about it, you guys may be right about Peter's. I have sent a few non beveled AEBL blades to them, and they did have to straighten them, which tells me maybe they DON'T use plates. How did I know they straightened them? Topic for another thread.

As had been stated, if the pearlite nose is beat, regardless if plates are used or not, then that's all that is needed to get the max HRC post quench. If an air cool will drop the temp fast enough to beat the pearlite nose, then you're good to go. And if you've already ground the bevels on an AEBL knife, the edge has much much less "meat" than simply profiled, and will drop temp even faster...thus better ensuring that max HRC. Brad recommends no less than .015" for stainless steels, and he has had success with .010" without bacon warp, from my blades, which initially got me to thinking that he did plate quench. But maybe not.
 
BossDog said:
If the heat foil was bloated around the bevel with no contact to the blade and no air shooting in between the quench plates, I would be concerned about the edge getting hard or not due to insulating effect of the foil pouch.
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Thanks Tracy, that's exactly what I was trying to articulate.

-Kurt
 
Remember, that edge only has to drop below 1100⁰F in 2 minutes or less. AEB-L is almost an air hardening steel, and those aluminum plates are going to draw the heat MUCH faster than air alone does, even when it's only clamping direct contact to the flats of the blade. Your concern was one of the things I expressed to a Sandvik engineer when I first started using Sandvik steels. He says there's no problem the bevel being ground pre-HT - due to the thickness of the edge it's going to cool plenty fast enough. If desired, spray plates with a water mist while cooling. Another point - look at the 100's, if not 1,000s of blades that are ground post HT with no problem of soft edge.

On the "burr" while sharpening - AEB-L is well known for this issue, not a true indication of a soft edge. Now, the brass rod test with the edge curling and staying curled.... that does indicate a problem. I don't think anyone here is saying that blade doesn't have a soft edge, just we don't think it's "necessarily" a problem with grinding post-HT.

Ken H>
 
You may well have a knife with a softer edge, but those hardness tests don't prove that. You'd need to cross section the blade and test spine to edge along the cut. This requires mounting and polishing, and some specialized equipment if you want to test close to the edge.

Testing on sloped surfaces will give values considerably softer than the actual hardness. As the test load is applied, the diamond will skate down the slope, resulting in lower readings. It also can make a very unpleasant sound. It sounds very similar to my boss saying I'd have to buy a new indenter if I did it again.

If cross sectioned and microhardness tested, it would not surprise me to see consistent hardness from spine to edge.
 
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