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Effect of Various Heat Treatments

Posted by me2 
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me2
Effect of Various Heat Treatments
December 01, 2014 01:04AM
I've been doing a bit of reading on various forums and was wondering something. Given that the differences in steels can be overshadowed by user applied changes in geometry, how much difference could be seen going from a simple procedure in heat treatment to a complicated one that theoretically improves performance? How much difference could be expected between the standard heat treatment for say 3V and the complicated procedure proposed/discussed on BFC some time back?
Re: Effect of Various Heat Treatments
December 01, 2014 01:36AM
Quote
me2
[...] How much difference could be expected between the standard heat treatment for say 3V and the complicated procedure proposed/discussed on BFC some time back?

Are we talking about :

-oil or plate quench vs air
-immediate cryo vs snap temper (or no cryo)
-multiple cycles of #2 vs one (or none)

As far as I know there is no hard data and the best I have seen are conjectures based on HRC differences which are used to infer martensite percentages vs retained austenite and pearlite/bainite forming in the slower quench. Thus the difference is at most 2-4 HRC points lost - which is then usually given up in a higher temperature draw anyway, unless they are actually running 62+ HRC 3V.

In short I would argue it is more optimal, but I would be very surprised if the differences was that dramatic you could pick it out without multiple runs to normalize out the expected random scatter. Now this is assuming the "standard" method is achieving something sensible. If we are talking about what Roman was referring to where people were getting in the low to mid fifties with air quenching then likely that could be much easier to notice as it is 5-8 HRC points of a difference alone.
me2
Re: Effect of Various Heat Treatments
December 01, 2014 02:59AM
Oh, yea. I forgot to mention that part. This all assumes there is some sort of sensible heat treatment as the base line to start. If you quench 1095 in used motor oil, then yes, a complicated procedure that uses a faster quench can definately be noticable. I mainly thinking of the very complicated procedures some use that they claim give double or even triple the edge holding, or other properties, from their older "inferior" procedure.

This question mainly comes from me wondering about the difference between optimal and practical. Say one were doing home heat treatment with an oven on something relatively simple, say A2. 1750 Austenize, air cool, double temper at 360-380 for 61-62 HRc (my specified procedure for my A2 blade at the local shop). Now, how much, if any, difference would I notice with the more optimal procedure of 1750 F austenize, slow oil quench, into dry ice with no snap temper, then double temper at 375-400 for roughly the same hardness range, assuming the dry ice gave a 1-2 point bump in hardness, with water quenching between tempers. Thats a couple of extra steps. Will it yeild a difference measurable by CATRA when differences in my sharpening (because I want to sharpen it and not have CATRA do it) will be 10% or more? If CATRA sharpens it and the differences from that source are only 1%-3%, would I see the difference then? Obviously, the only way to know is try it, but I think you all will get my point.

To complicate things even further, how about if I do go to the extra steps and get the RA down to 1%-5%, but don't really do anything else different in terms of knife shape and geometry? People often say "all else being equal" when asking these comparison questions, but that usually isn't the case, nor should it be IMO.
Re: Effect of Various Heat Treatments
December 01, 2014 04:32AM
IMO, if you are going to test hardness/heat treatment differences, everything else about the knives should be the same. the way i see it, you make two identical knives except for "X", then test them in an identical manner, if there are any performance differences, "X" is the cause.

whatever you call the concept of just changing one variable at a time, seems to be the hardest concept to teach at the office. you are forging and the forged part is too long. there are 3 places you can adjust to decrease length. the best operators adjust one at a time until the part is in spec. the rest change all 3 at once then begin a tail chase as now the part is too short.
scott
Re: Effect of Various Heat Treatments
December 01, 2014 09:14AM
It was already posted elsewhere on this forum by Collin, but here is a video of Nathan Carothers.
video: [www.youtube.com]

The 2 3v blades that he compares are same hardness (HRC 60), one is the standard Peters HT, one is the tweaked HT done by Peters along the lines presented by Roman on the HF thread. The difference seems to be clearly observable.

Edit to add: the great thing about the video is the knives are as identical in geometry as possible, being that the blades are CNC milled and for the finishing par Nathan uses a nice jig that helps him get very consistent and symmetrical angles..



Edited 1 time(s). Last edit at 12/01/2014 09:17AM by Bogdan M..
Re: Effect of Various Heat Treatments
December 01, 2014 09:50PM
I am not sure the difference is to that extent. In the description it notes that the altered HT doesn't cause a loss in toughness while it produces (the expected) increase in apex stability.
Re: Effect of Various Heat Treatments
December 01, 2014 11:21PM
I think it's clearer here, in response to my question:
[www.youtube.com]

Basically fast quench and avoiding the secondary hardening carbide precipitation yelds same hardness, but with less RA and smaller carbide volume. Third effect is better corrosion resistance, noted in the description of this video but not tested.
Re: Effect of Various Heat Treatments
December 01, 2014 11:22PM
Coincidence as I was going ask a similar question regarding the influence of heat treat, I will ask it here as it seems appropriate.

I read this statement from Cliff:

'There is more variation among steels in corrosion resistance due to how they are hardened than among the steels, so you actually need to be specific about who is actually using it. ' Found here: [www.edgeucation.com]

Is this saying that the range of corrosion resistance in steel is dependant on how they are hardened?

Or to put it another way, a steel which is considered to be corrosion resistant can only be corrosion resistant if it is correctly hardened, and if it is poorly hardened then it has the capacity to have low corrosion resistance.
Re: Effect of Various Heat Treatments
December 02, 2014 12:28AM
Quote
Reijo

Is this saying that the range of corrosion resistance in steel is dependant on how they are hardened?

Significantly.

The corrosion resistance of a steel is dependent not only on the elements in the steel but in what form they exist. Before the steel is hardened then most of the chromium and molybdenum is in the form of large carbides where it does nothing for corrosion resistance, hence why steel has poor corrosion resistance in that state. When the steel is hardened these carbides have to be dissolved (partially) to put some of the chromium and molybdenum into the martensite. It is critical then in tempering not to let it come back out, hence why low temperature tempering should be used to maximize corrosion resistance.
Re: Effect of Various Heat Treatments
December 02, 2014 12:30AM
Quote
Bogdan M.

Basically fast quench and avoiding the secondary hardening carbide precipitation yelds same hardness, but with less RA and smaller carbide volume. Third effect is better corrosion resistance, noted in the description of this video but not tested.

Yes, Ernest Mayer dared to make that argument in the early 2000's in opposition to Paul Bos who was air hardening ATS-34 with a high temperature tempering. The metallurgy is straightforward but there is a lack of specific data - how much is the strength, wear resistance, toughness, apex stability, etc. changed.
Re: Effect of Various Heat Treatments
December 02, 2014 08:58AM
I haven't understood that before, funny how high Mo (air hardening) steels, much preferred by makers because they are esier to heat treat, are in fact only easy to heat treat in a "bad" way...
Re: Effect of Various Heat Treatments
December 02, 2014 05:39PM
A lot of this comes out of an odd resistance to look at metallurgy. Now to be clear, I find it odd as my background is academics so the first thing I think on when something is unknown to me is just see if someone else knows it, hence literature search. But in the knife industry it goes beyond that to the point that people resist the literature even when it is provided to them with no real argument as to why. When Mayer first argued for low tempering the reason was insanely strong and the metallurgy was rejected in favor of "data" that was at best very poorly collected and at sometimes highly anecdotal. But as time passes and more people just did it and it worked then the hold outs disappeared and even Bos switched, and everyone pretended they always knew.

Roman has always been clear that what he writes about isn't revolutionary, it is mainly known literature results which he applies to various steels in cutlery but even then the physical principles are the same. There is literally 100+ years of research on steels for razor blades and how micro-structure, hardness, carbide volume, etc. influence the ability of a steel to take/hold a fine sharpness so why would you just casually ignore this when it is made available?

But again, it isn't like the knife industry has a claim to doing that alone, look at Homeopathy for example. There is no argument that anyone makes in regards to steels which can match the irrationally used to defend Homeopathy, or argue against Evolution, etc. .
me2
Re: Effect of Various Heat Treatments
December 03, 2014 03:18AM
I don't think I'd go so far as to say air hardening steels that are air hardened are being heat treated in a "bad" way. It's just not optimal for knife use. Which brings me back to the point about optimal and practical. These steels (air hardening) were developed to air harden for specific reasons, and thin edge holding wasn't really one of them. A2 shows some interesting properties, with advantages over O1, based on Alvin's old comparisons. Heat treatment details were not provided on the sources I saw though. Also, steel makers like Sanvik have recommended oil hardening on their stainless steels as far back as I can remember looking, likely because they knew the issues it would avoid in fine edges.
Re: Effect of Various Heat Treatments
December 03, 2014 08:58AM
I only meant suboptimal, of course, hence the quotes. However, there is the pitfall of the target HRC here, the fact that you do your HT and reach 60 HRC might mislead somebody who isn't trying to understand all the details of what he's doing into thinking the HT was a success, while in fact in the steel there is lots of retained austentite and lots of large carbides.
And this is more than a question than a assertion, but I have trouble imagining an end application for which it would be beneficial to have lots of RA + carbides over a complete martensite matrix with an array of finer carbides, even outside of cutlery. The way I'm imagining things, large carbides in a weaker iron matrix will allways have less wear resistance than smaller carbides in a strong matrix, and from what I understand the RA will not improve toughness either.



Edited 1 time(s). Last edit at 12/03/2014 09:09AM by Bogdan M..
Re: Effect of Various Heat Treatments
December 03, 2014 12:42PM
Quote
Bogdan M.
... the RA will not improve toughness either.

Retained austenite will improve toughness in the steel on a macroscopic level, it will decrease it on a microscopic level. On a large scale, in big pieces, the retained austenite will absorb loads in impacts and actually transform to other phases and thus it will show higher numbers in charpy tests and other impact loading.

However on a microscopic scale, on the edge, the very low strength will not only indent/deform easier, it will be more prone to carbide tear out. There is also an argument, for which I have seen no actual data, but it makes sense, that retained austenite can transform to martensite in the edge during work which isn't tempered and which will then fracture easier than if the blade was tempered martensite.

Note as well that in steel which isn't knives, in very complicated shapes, air cooling is much less likely to produce cracking and warping as because it is a slower cooling, the piece has the ability to reach the same temperature much more evenly. In a harsh quench, the temperature gradients can be severe and therefore so will be the transformation gradients.

Essentially it takes time for the heat to travel through the material and equalize the temperature. It is possible to cool the piece so severe this can't happen. Just put a frying pan against a scalding hot element, the bottom of the pan will get very hot and the inside will lag behind. In comparison if you heat it up much slower, the two surfaces will stay closer together temperature wise.
Re: Effect of Various Heat Treatments
December 03, 2014 05:04PM
Quote
CliffStamp

Retained austenite will improve toughness in the steel on a macroscopic level...

As a decent reference :

"Retained austenite in the structure of 70MnCrMoV9-2-4-2 steel after hardening increased its fracture toughness
proportionally to its content."

Ref : "Effect of retained austenite on the fracture toughness of tempered tool steel", A. Kokosza*, J. Pacyna

-link : [www.researchgate.net]
Re: Effect of Various Heat Treatments
December 04, 2014 07:36AM
Quote
CliffStamp



However on a microscopic scale, on the edge, the very low strength will not only indent/deform easier, it will be more prone to carbide tear out. There is also an argument, for which I have seen no actual data, but it makes sense, that retained austenite can transform to martensite in the edge during work which isn't tempered and which will then fracture easier than if the blade was tempered martensite.

.

I think it would be interesting to see that argument for the retained austenite transforming into martensite tested. I use to hear it argued quite often. But some actual experiments done based on the hypothesis would be nice.

Re: Effect of Various Heat Treatments
December 04, 2014 09:00AM
Cool info, thanks Cliff. Makes sense, given austentite is soft it will absorb shocks by deforming, and sometimes transforming to one of the stable phases. However, a steel with a lot of RA must have pretty unreliable fatigue characteristics, I imagine? Is leaving some RA really a strategy for improving tool toughness?
I imagine the superior dimensional stability and low likelihood of warpage or cracking as a big advantage in an industrial production environmen, that coupled with the less complicated HT can result in big cost savings.
Re: Effect of Various Heat Treatments
December 04, 2014 11:10AM
Yeah I read that RA can give the higher toughness but in normal applications i.e. more than one impact the virgin martensite from transformation can result in lower toughness

At the edge that might be interesting for a self serrating edge

Re: air hardening,
knife blades are not usually very complex shapes but warping is typically such a problem that processes are resorted to in order to keep blades straight, such as Grinding in the hardened state or plates being heat treated along with blades... or not getting steel hot enough to reach full hardness, or quenching slower to reduce thermal shock, or bending blades back in fixtures... many air hardening steels warp less, the manufacturers publish charts on the distortion

BTW I recall a case study of L6 parts failure, simple threaded plug for a pressure vessel (muzzleloader)... they were cracking in the quench and failing in use. Could have been a number of factors, but an air hardening steel probably would not have cracked in the quench (my personal hypothesis is that the oil was too fast for that particular batch of L6 which can sometimes be air hardened and sometimes needs medium oil depending on source)
Re: Effect of Various Heat Treatments
March 13, 2015 12:53AM
[www.youtube.com]

Here's some more from Nathan Carothers, this time with A2. I really like him and his approach.

_______________________________________________________________________________________________

Always in search of a good choppa'
Re: Effect of Various Heat Treatments
March 13, 2015 08:30AM
Nice video, thanks Collin. I would just be curious to understand why he has moved from 3V to A2 for this one, otherwise cutting performance is nice. For a field knife I would like a little more blade, to help a bit more with batoning and chopping, but I'm sure this knife will sell as easily as his pigsticker designs, at least smiling smiley
Re: Effect of Various Heat Treatments
March 13, 2015 10:24AM
Bogdan,

This was a collaboration with a fellow over at Bladeforums: [www.bladeforums.com]
I don't know how much of the initial cost that fellow had to offset, so maybe the A2 was just a economic choice. Also, this was more or less that fellow's design.

I know Nathan has used A2 in other blades...in fact I think the first pig stickers were in A2.

_______________________________________________________________________________________________

Always in search of a good choppa'



Edited 1 time(s). Last edit at 03/13/2015 10:26AM by C Amber.
Re: Effect of Various Heat Treatments
March 13, 2015 12:50PM
Ah, OK, I see, yeah, the difference in cost could be a factor in this context. I know Nathan has worked with D2 too, I just didn't see the reason to go back. My background is computer science, so when you choose an algorithm you usually have to balance many pros and cons, some are good in certauin situations, but not in others, bla, bla. Here, the choice seems much simpler. Performance wise, I see little reason to choose A2 vs 3V in a thin ground and relatively short blade like this. But if you think about the price of a batch, I imagine things looks different when you take into account the price of stock and of processing it.
Re: Effect of Various Heat Treatments
March 13, 2015 05:37PM
for home HT, A2 is way easier, with lower temperature austenitizing and slower quench

Also A2 and D2 have legendary status. Busse himself stated that steels with silly names are nothing compared to A2 which machinists recognize. Oops that was a while ago, just a bit of irony there winking smiley
Re: Effect of Various Heat Treatments
March 14, 2015 02:51PM
Well, 3V can ne air hardened just as well (or bad depending on your viewpoint)
Re: Effect of Various Heat Treatments
March 14, 2015 03:26PM
Yes it can be, however you have to consider ease of hardening. Read for example Harry's problems hardening 3V on HF where he kept ending up with under hardened steel (55-57 HRC) even though he was going beyond the spec-sheet hardening and using plates. He had to step up to frozen plates to stop diffusion from severely limiting the hardness.

But beyond that you have to ask yourself if going to a steel with a lower grindability, lower hardness, greatly increase cost and complexity of hardening is actually of benefit? Unless the edge is failing due to lack of toughness or abrasive wear then the result could be a downgrade in performance which comes at more direct and manufacturing cost.
Re: Effect of Various Heat Treatments
March 14, 2015 07:47PM
Ha I just saw my post, I tried posting from a mobile earlier on, din't go too well, my Nokia kept replacing English with french so I ended up submitting only 25% of my post.

But scratch that I just looked at Latrobe's A2 data sheet, I had no idea there was such a difference in hardenability between the two steels, A2 can hit 63HRC so easily. Still, 57-58 is too low for 3V if you use cryo, but even 3 HRC points is still significant in terms of edge stability. So RTFM it is smiling smiley
Re: Effect of Various Heat Treatments
March 16, 2015 04:10PM
some thoughts by Roman Landes on A2 which would also apply to 3V

You may also consider, if you want to chase the speed devil and you looking for high edge stability, there are better alloys out there.
Especially the high carbon stuff and in addtion to that, you get nearly the same toughness and there is way less husstle to tackle with those, in terms of HT, RA, etc. than going with A2.
If you need more toughness there are also better alloys out there just to name few L6, S- series and those have the same advantages with less effort in HT, always assuming the HT is done right.
Futhermore if you go forging, the low alloy high carbon stuff can be adjusted in its micro structur faily easily compared to A2. While you prone to the factory outlet you bought the A2 because of its high alloy content.
So finally you have to tackle with all the same efforts you put into a stainless but not getting a stainless blade. You getting corrosion resistance close to carbon steel and hardness, edgestability and toughness can be get easier with simple steels and more simple HTs at a higher level.
In the case HT can be done right and you got all the stuff like cryo available A2 makes a good multi purpose steel.
But be aware the more multi, the less purpose you get.
[www.hypefreeblades.com]

the local machine shops here love A2 for one reason, no quench oil tank. they wrap the part in stainless foil, put it in the oven, hit go, when timer dings, move the part to oven two, hit go, when time dings they have HT'd part. our engineers help by giving them specs of Rc 55-61.
scott
Re: Effect of Various Heat Treatments
March 16, 2015 05:02PM
Quote
oldsailorsknives
some thoughts by Roman Landes on A2 which would also apply to 3V

Good points, Scott, and they apply also to 3V, excepting stain resistance. Typically 3V will have all its Chromium and Molybdenum in solution, which makes it probably much more stain resistant than A2.

It's probably also a matter of personal preference, but I find that 8% Cr steels offer an interesting compromise in terms of toughness and stain resistance at a decent strength. At least from a theoretical point of view, because there is not much data on the toughness of AEB-L, Niolox or the like...
Re: Effect of Various Heat Treatments
March 16, 2015 05:40PM
Roman is very strongly focused on corrosion resistance and will often make the argument against HSS for the same reason. Why go through all the trouble to use a HSS when you could have a stainless steel for similar (or less) effort.

However Roman does use non-stainless steels, he uses S series steels for larger knives for example due to the combination of strength/toughness.