Strange steel characteristics, any ideas?

BloodrootBlades

BloodrootBlades

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Nov 29, 2011
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Hello all, this is one of my first posts in this forum, though I have greatly appreciated many of the discussions found here! I'm a knifemaker in NE Georgia that mostly makes kitchen knives out of recycled materials (gutsy, and perhaps stupid, I know ;-). I get away with it by testing every blade for a variety of characteristics and by doing my homework as best I can. Please don't try to dissuade me from using recycled steels, I am well aware of the dangers/impracticality of using recycled steel and have used a variety of them for 16 years and have personal reasons for doing so.

I use a lot of different steels, some much more frequently than others. I have a heat treat oven and a propane forge and use both for heat treatment. Honestly I prefer the forge for its flexibility when hardening a variety of different steels and get good results (small grain size, toughness, evenness of hardness, max hardness for the steel, etc) on many of the simple 10xx steels, including hyperutectoid varieties, 5160, 9260, and others- believe it or not for fun I've even used a forge to successfully heat treat 440c, but I do not recommend it! I use the oven for 52100, 440C, and other steels that are touchy about soak times.

I have a Rockwell hardness tester that I keep well-calibrated. I test every blade and iteratively temper in a home oven until each piece is to the range I want for each particular piece. Tests are done on 400 grit polished steel and in multiple locations on the blade to ensure I get a good read and to test for evenness. I also use a blade of known hardness that's very consistent as a scratch-tester, do tip-break tests on brick, cement, and twisting in a piece of oak end-grain and generally keep an eye out for "strangeness" during HT).

Which brings me to my quandary. I often use integral-tooth old large lumbermill circular saw blades for chef's knives. They're almost all different steels, but they're big enough to get 30ish knives out of and they're almost always a good blade steel when HT'd right, so they're worth the testing effort. I have two blades that I've cut into a bit that have some weird properties. First, they are sensitive to hardening temp, a tad too high and they only harden to the mid 50's HRC, just right (watch them transform) with a reasonable soak and they harden to HRC 63-64. At about 300F temper (2 hrs) they come out to a very consistent HRC 61.5 +- the 1 point tolerance of my machine. Now that's great, I can hang with that if everything else checks out, but here's the weird part, it's REALLY tough and ductile at that hardness (great in some ways, but it lacks much resilience or strength- I can correct warpage on hammer and anvil (even in very thin sections like tips <1mm), it is possible but very difficult to break in a vice (good grain structure though due to careful subcritical cycling prior to quench) and grinds like butter, but it is not resilient like 5160. AND 9260 (read pretty much exactly 5160) at HRC 60.5 can scratch it, though it can actually scratch back a little bit (weird enough huh?). The steel acts like it is at HRC 56-57, but the tester is dead-on accurate on other types of steel and on it's calibration block, (which is right at HRC 60.7, so not way off one direction or another). I just finished a blade out of it and sharpened it to approx. 15 degree 50/50 bevels to 8000 grit- VERY thin cross section (think Japanese style chef knife). I roughly and aggressively chopped through a 2x4 with no rolling or edge deformation, but it SEEMS LIKE IT SHOULD given how ductile it is. I then tried a piece of seasoned and very hard 1" thick white oak board, chopping hard crossgrain, which caused an almost imperceptible deformation after 20+ hits, so it seems to be supporting the edge relatively well for how soft it seems.

I've also had issues with some sawblade steels being too ductile to support a wafer thin edge unless they're up at HRC 63 or greater (one of my sawblades gets to HRC 65-66). They're not chippy at HRC 64 hardness either. 1095 is much more rigid at that hardness, though MUCH more brittle.

Is it possible to have steel be hard, but act soft/too ductile?

Anyone else have this or similar issues? Any ideas?

Thanks,

~Luke
 
Your question is a little confusing to me and I don't understand some of your statements but I'll take a shot at it...

Some steels like S7 have very low abrasion resistance and do not hold an edge very well even when at an identical hardness as an abrasion resistant grade such as D2.

Some steels, particularly high chrome steels, develop a lot of stabilized retained austenite if austenitized above their recommended window which would explain why some steels you've worked showed lower hardness when overheated. These steels can also show relatively good tempered hardness but suffer poor edge stability including rolling due to RA. The RA is a very ductile element in an otherwise hard matrix which could show some of what you're seeing.

And lastly, be aware that for all intents and purposes all steels are equally rigid. The modulus of elasticity of all steels are approximately the same regardless of alloy or heat treat. The only differences will be seen outside of their elastic limits and in different cross sections.
 
'Old large lumbermill blade' can be almost anything. Though I had some connection with a mill I never learned what steels were used. One interesting thing about those blades is that they are 'tuned' with a hammer and anvil , cold, .That's a unique skill used to get the sawblade to run true at speed. Where the art takes over from the science !!!
 
Nathan,

Your answer was exactly what I was interested in, sorry for the confusing post, I was trying to be thorough, but probably just wrote too much. I may try a cryo treatment with the steel to see if that helps convert the retained austenite or try side-by side comparisons of the forge-treated with the oven treated at a pretty low average austenitizing temp . and see what comes of it. Thanks for your time!

~Luke
 
Mete: that was very interesting, had heard of it and forgot. Wonder if anyone still working now can do it?
 
Reminds me of the old guys who would true a boat drive shaft by pinging it with a brass hammer.....while it was turning.
 
Ed Fowler said:
Mete: that was very interesting, had heard of it and forgot. Wonder if anyone still working now can do it?
Click to expand...

All circular blades have to be hammered into a dish to prevent wobble. The amount of dish was set to the RPM of the blade so that they will be straight (by centrifical force) at a given speed giving strength to the blade. Any mill still using circular blades will still have someone to hammer the blades. There are still hardwood mills in my area that run circular blades but they aren't as common as they were years ago as many have gone to band saws. I understand that large band saw blades are hammered as well but have never been around this being done to see the process. [I once met a man who did this at the mill but didn't get a full explaination of the how & why.]

Gary
 
A gentleman who has a set up in his truck to test various critical oil field metal parts, he has been doing it for years and is much in demand.

He said it was all harmonics and that with a few modifications he felt the could test knife blades and graph the results. I don't know how serious he was, but I ordered some books and hope to explore the subject more.
 
Just following up-

Retained austenite was not the problem, but decarb (I should have known, as it is a eutectic steel). It explains why the edge would stand up to testing but the hardness tester was giving me variable readings. Also the steel is just THAT tough that at HRC 61.5 I can hammer it to straighten without cracking it, even in thin sections. I was able, in the right light and with etching to see that the decarb was deeper than I expected (I forge thin and grind just enough to clean it up, so I wasn't grinding all the way through the decarb in areas) Grinding deeper eliminated the problem and I got very consistent hardness readings. Strange that this particular sawblade decarbs so badly compared to other steels. My comparison of forge treating to oven treating showed that the forge treatment was actually better as the decarb was not as severe from the long soak in uncontrolled atmospheres. I think the decarb was from the rusty surface layer of the sawblade, not from the forging per-se as the atmospheres were controlled in the forge.

~Luke
 
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