13 Myths about Heat Treating Knives

[Larrin]

Serrated edges have poorer cutting ability but superior edge retention when slicing.

Edit: data is here - https://knifesteelnerds.com/2018/11/26/ ... etention2/

Larrin, " serrated edges having poorer cutting ability when slicing" is so contrary to my (admittedly total layman and non scientific!!) in-use-findings, that I may ask:
Could it be that I find Spydercos serrated edges to be even better slicers than their plain edges not because they are serrated, but because SE is ground to a much acuter angle (due to the chisel grind)?

Acute angle certainly helps with improving cutting ability.

[Bill1170]

Another factor in slicing ability that doesn’t show up on a CATRA test is the greater ease in hand due to the serrations locking the blade into the media being cut. Preventing a plain edge from slipping out of heavy corrugated cardboard takes a noticeable effort. This effort is much reduced with FFG SE because the scallops and points help hold the blade in the cut as it proceeds.

[zhyla]

Larrin, " serrated edges having poorer cutting ability when slicing" is so contrary to my (admittedly total layman and non scientific!!) in-use-findings, that I may ask:
Could it be that I find Spydercos serrated edges to be even better slicers than their plain edges not because they are serrated, but because SE is ground to a much acuter angle (due to the chisel grind)?

If I understand the article he linked, I think the summary is SE is more effective at cutting even when worn.

Chisel grinds aren’t necessarily more acute than a PE, it just depends on what angle the maker chooses to grind.

[Wartstein]

Larrin, " serrated edges having poorer cutting ability when slicing" is so contrary to my (admittedly total layman and non scientific!!) in-use-findings, that I may ask:
Could it be that I find Spydercos serrated edges to be even better slicers than their plain edges not because they are serrated, but because SE is ground to a much acuter angle (due to the chisel grind)?

If I understand the article he linked, I think the summary is SE is more effective at cutting even when worn.

Chisel grinds aren’t necessarily more acute than a PE, it just depends on what angle the maker chooses to grind.

That's true of course. :)
But I was referring specifically to the chisel grind on Spydercos SE knives, those are always more acute than the V grinds on their PE knives as far as I know (so for example around 20 inklusive instead of 20 per side).

[vivi]

Thanks for this post! I've gotten about half way through and I agree with everything you've said so far.

From the article you linked Wartstein to:

There were also a few comparisons between a serrated and a “plain” or non-serrated edge. The serrated edges are typically at lower edge angles to improve their cutting ability relative to plain edges. The serrations mean the edge wears much more slowly than plain edges; the CATRA test is often performed with 200 cuts rather than the standard 60 because the edge wear is so much slower. The initial cutting ability of a serrated edge is significantly lower than a plain edge but after a sufficient number of cuts the serrated edge keeps going while the plain edge has dulled. Below I have a comparison with the same steel, one with a plain edge at 26° and the serrated with a 22° edge angle.

Why was the starting cutting ability of the serrated knife so much lower than the plain edge? Did they start with different sharpness levels? Or did it simply do a poor job cutting the cards on the catra test?

[Cambertree]

Vivi, my guess would be that perhaps the high points between the serration scallops cause more drag when being sliced into thick, binding media. At least, compared to a PE blade when both are freshly sharpened. Then, as the graph shows, SE takes over as the PE blade wears.

Just a guess, based on my own observations and use.

[emanuel]

I think you know about one blade forum member BluntCut (I think he is also here) and his crystal weaving foundation heat treat (or something like that). What are your thoughts on that? He claimed that he could HT some steel higher then anybody else on the planet (well, maybe not these exact words but close) and his HT include some swirling and slicing in quench oil to get the microstructure.

I think he's a great bladesmith, and his heat treatment is top notch and he can push for a nice balance of properties, particularly hardness (I own two knives heat treated by him), but it's nothing revolutionary, a 65-66hrc s110v knife by him still has the same properties and limits as another one at 65 done by some Russian guy that uses plate quench, and arguably the same crystalline structure/grain size etc if done right. His heat treatment is no different than what Big Brown Bear ends up with. Respectable and very good, but chemistry and physics don't change, they're the same for everyone no matter what steps you do during heat treatment. There's only so much you can influence with a heating curve and quenching/tempering.

Great article, always a joy to read. Cheers!
Question for Larrin: How stable is martensite over time, at what we would call normal environment temperature (20-40 degrees celsius)? Is it's metastability just a scientific naming to be more accurate (in relation to truly stable states) and its actually stable for geological time frames, or does it switch to austenite over, quote marks, "short" periods of time?

[Larrin]

Great article, always a joy to read. Cheers!
Question for Larrin: How stable is martensite over time, at what we would call normal environment temperature (20-40 degrees celsius)? Is it's metastability just a scientific naming to be more accurate (in relation to truly stable states) and its actually stable for geological time frames, or does it switch to austenite over, quote marks, "short" periods of time?

Martensite decomposes to ferrite given enough time and temperature. We force this to occur all the time through tempering. A knife might be tempered at 400°F twice for 2 hours each time. The higher the tempering temperature the lower the hardness and the closer it gets to ferrite. Tempering is controlled by both time and temperature but because time is on a "log scale" the change with further increases in time beyond 2-6 hours is rather slow. This is represented by the Hollomon-Jaffe equation: https://en.wikipedia.org/wiki/Hollomon% ... _parameter

To start reducing hardness below the original target, we would need to temper beyond the original target. Re-tempering at 300°F for 2 hours would not appreciably affect a 400°F tempered knife, for example. So if we calculate the time required at room temperature to be equivalent to a 400°F 4 hour temper I calculate about a billion years. To have a more significant change, like an equivalent to a 450°F temper, I calculate 46 billion years.

[Chinook3]

Very informative as usual! Thank you very much.

[Mike Blue]

... So if we calculate the time required at room temperature to be equivalent to a 400°F 4 hour temper I calculate about a billion years. To have a more significant change, like an equivalent to a 450°F temper, I calculate 46 billion years.

How about the effects of time on retained austenite converting into untempered martensite? There are legends out there for old blades changing hardness over time. Over periods of hundreds of years.

[Larrin]

How about the effects of time on retained austenite converting into untempered martensite? There are legends out there for old blades changing hardness over time. Over periods of hundreds of years.

The retained austenite is presumably very low in old blades to begin with. For one thing, they are often not even quenched to martensite. More in the 40s Rc and pearlitic. I would need to see some rock-solid explanations for how they knew there was retained austenite hundreds of years ago but not today.

One other factor: there are two different transformations that can occur with retained austenite, decomposition and destabilization. Decomposition leads to "ferrite + carbides" or some type of bainite, not martensite. Destabilization usually occurs at higher temperatures and leads to the formation of martensite upon cooling. Decomposition is more common with low alloy steels with typical tempering temperatures. Which is what we would expect in old swords.

[Tucson Tom]

I really enjoyed this article and think I learned more from it than from any of your articles, though I would do well to backtrack and reread some things. No doubt this says more about me than your writing, or maybe this article brought it down to a level I can wrap my head around. Anyway, thanks a lot.

[curlyhairedboy]

Great article, Larrin. A lot of these myths seem to arise out of not realizing just how many factors go into steel.