Blade Steel Rehardening

Spyderco does pretty well with their target numbers and hitting the high notes. This was a problem with the zdp mule, it was too hard, and some blades were breaking.

Someone had their bunch of Chinese mules tested, and the 9Cr18Mo blades were all right at 59 Rc. That's pretty humbling when your limited production all-American super alloyed particle metallurgy blade is softer, which has happened to some of us.
 
cotdt said:
As a knife modder/tinkerer, I believe the biggest mod you can do to a knife is to sharpen it. Right after that is rehardening the steel.

If anyone is interested, I can reharden your blade for free. Any steel, any attainable hardness. But I should note that there may be some risk involved, depending on the steel and desired hardness. It's like turbocharging a car engine.

Edit: Added warning about risk involved.
Click to expand...

What blades have you done so far? Any testimonials?
 
SteelSnob said:
What blades have you done so far? Any testimonials?
Click to expand...

Yeah good question, I've rehardened S30V, 13C26, and CPM M4, and heat treated other steels. The jury is still out as I need to carry the knives for 6 months before developing any strong opinions about them. Here are some initial impressions.

I got eye-popping results doing 1084 and 1095 using Alvin Johnson's heat treat formula for ~64 rc, and used them as EDC knives. Even at 9° per side, they cut solid copper wires while my 154CM chipped at a thicker 11° per side. However, they took just as long to sharpen. The plain carbon steels kept that razor-sharp edge for much longer than the 154CM, even though in theory 154CM should be more wear-resistant. Maybe the 154CM would hold a semi-sharp edge for longer than 1095, but this aspect does not interest me.

My CPM M4 was heat treated to 65 rc. Sharpened blade to 9° per side and did not notice any chipping carving hardwood. I used it to scribe grind lines onto steel blade blanks, cut pretty deep and tip remained sharp. Dropped knife onto concrete floor from 5 ft, only ~1 mm of the tip broke off, fixed it in about 10 minutes. Gets wicked sharp, I can't even whittle hair with it because it cuts the hair in half as soon as it touches. Got some burring when sharpening, a sign of good ductility even at this hardness. I might do a passaround of this knife so people can see what this wondersteel can do, and compare findings as usage patterns differ.

I also did an S30V paring knife to 62 rc thanks to some advice from Larrin and Phil Wilson. I opted to use the high temper as previous experiments from another knifemaker using the low temper had some chipping. I believe the chipping problems when S30V first came out were due to retained austenite since Crucible recommended the low temper and cryogenic treating was not always used. Over time the retained austenite converts to untempered martensite which gives very brittle edges. S30V is by no means a brittle steel, its carbide volume is actually lower than other steels in its class.

Did a straight razor with AEB-L razor steel, 62 rc, 7° per side. Have been shaving with it for the past week without stropping, and it's just as sharp as on the first shave. AEB-L is more like carbon steel than other stainless steels, as it has very little carbide volume. It's basically 52100 but with 11-12% free chromium instead of the tougher/harder martensite. Like my other choices, this steel can held a thin edge well (no surprise). Takes a mirror polish easily and is easy to sharpen. It doesn't have the massive burring during sharpening that it does at lower hardness.

I haven't had any bad results yet except for one warped blade during a water quench. There was about 1.5mm of warp that I didn't notice until later, so I didn't straighten it out while it was still hot.
 
Last edited:
hardheart said:
Spyderco does pretty well with their target numbers and hitting the high notes.
Click to expand...

I love the Mules. Spyderco's heat treat is more geared toward performance, and their S30V is very good too. And who else hardens CPM D2 to nearly 63 rc!? Using Spydercos was one of the things that originally got me interested in harder heat treated knives.
 
Awesome that you got this going, hurricane Gustav took out part the shed and set me back behind square one before I could even get started. Lost hardware, quench oil, and floor space.
 
hardheart said:
Awesome that you got this going, hurricane Gustav took out part the shed and set me back behind square one before I could even get started. Lost hardware, quench oil, and floor space.
Click to expand...

Good thing we don't have hurricanes in California! I can deal with the 20 second Earthquakes that everyone seems to be so afraid of. Thanks for the kind words. I'm curious where this leads.
 
I will be remembering this. I want to try rehardening some blades. Not sure which ones yet. So all i'd need to do is send return shipping money as well?
 
Can you do folders? I'd really love to see how much better a SAK would be with a harder knife blade. A Cadet with a hard thin blade would be a big surprise to alot of people I think. Heck I have almost doubled the cutting ability just by putting a thinner and convexed edge on mine.
 
I too have an interest in this service you're generously providing. I have spare s30v blade for one of my sebbies that I would be interested in raising the hardness on. I understand that there is always a risk of some warpage developing.
From your experience what are the chances of this occurring in the pivot area of the blade? Would the warp be more prone to just the thinner grind area? I would be more willing to risk some warpage in the grind area as opposed the the pivot area.
Interested in hearing your thoughts.
Thanks,
Scooby
 
codtd,

This sounds like a very generous offer. I assume if we want a folder blade treated, that we should remove it from the handle and send just the blade steel.

Do you have hardness testing gear as well as HT gear?

Bill
 
This is pretty generous, you must be very confident in your ability to re-ht steels.

A higher degree of hardness can be awesome under many applications.

Wouldn't it be alot easier to offer new knives with higher stated hardness levels?

I would think it would be a huge PITA to disassemble and reassemble knives.
 
Hey, I was looking to get a straight razor anyway, might as well ask you to reharden ;)
 
There's couple things you'd need to keep in mind when rehardening. May be you already know that, but some of it was unexpected to me when I went through the process.

Rehardening implies subjecting the blade to high (very high) temperatures. So, whatever coating it has will be gone, and you either have to clean up the blade manually or send to someone for bead blasting or other coating, or polishing. ou can see what the blades look like after rehardening.

Also, regrinding, if you are looking into that, is far easier before rehardening.

Some manufacturers, if not most of them leave rather rough surface under the coating, because it sticks better that way. E.g. my M2 710 blade. After rehardening, once the coating was gone it exposed pretty rough grind lines on the surface. You can see that clearly on the photo attached here.

I've spent 8 hours or so trying to smooth the blade finish, mainly with 120/220 sandpaper. 64.5hrc blade didn't really respond well.
Both Richard J and Tom Krein can comment on those 2 blades too.

So, keep all that in mind when rehardening.

But in the end it is well worth it. The gain in performance that is.
 
Supmonkeyface said:
I will be remembering this. I want to try rehardening some blades. Not sure which ones yet. So all i'd need to do is send return shipping money as well?
Click to expand...

Yeah...

db said:
Can you do folders? I'd really love to see how much better a SAK would be with a harder knife blade. A Cadet with a hard thin blade would be a big surprise to alot of people I think. Heck I have almost doubled the cutting ability just by putting a thinner and convexed edge on mine.
Click to expand...

Yeah I've done folders but I'm not sure what steel they use in the SAKs. If it's 420 I won't be able to do much, 440A I can get to 59 rc. I like your idea though, I've been wanting a high performance SAK myself for a while.

scoobyaz said:
I too have an interest in this service you're generously providing. I have spare s30v blade for one of my sebbies that I would be interested in raising the hardness on. I understand that there is always a risk of some warpage developing.
From your experience what are the chances of this occurring in the pivot area of the blade? Would the warp be more prone to just the thinner grind area? I would be more willing to risk some warpage in the grind area as opposed the the pivot area.
Interested in hearing your thoughts.
Thanks,
Scooby
Click to expand...

Great to see the interest. Warpage doesn't happen with S30V, just 1095 (and even then it was correctable had I noticed it in time). I would love to reharden a Sebenza blade. There's not much risk involved in this one, except a very minor loss in corrosion resistance. This is only in theory, I've never noticed my S30V rusting.

Bill1170 said:
codtd,

This sounds like a very generous offer. I assume if we want a folder blade treated, that we should remove it from the handle and send just the blade steel.

Do you have hardness testing gear as well as HT gear?

Bill
Click to expand...

Yeah, or I can take the blade out myself if the person doesn't have a Torx set for blade removal. I send my blades out to be hardness tested, but 90% of the time following datasheets and known heat treat formulas produce very consistent results so I don't bother to do hardness testing on steels I've done before.

More important is using the best temperatures. For example, if A2 is toughest with a 450F temper, but I'm not getting the desired 60.5 rc hardness, using a 400F or 500F temper for adjustment would get me my required hardness but now I lose toughness. In other words, I would pick that 450F temper regardless of the final hardness.

Sparljo said:
Wouldn't it be alot easier to offer new knives with higher stated hardness levels?
Click to expand...

Knives take a long time to make, definitely not easier than heat treating existing knives. I don't know if hard knives are good for do-everything use that most people use knives for. What I do know is that I prefer specialized knives myself. If I wanted to do a knife for skinning, I would make the assumption that this is the only type of cutting the knife would be used for, and optimize it for this specialized task in regards to blade geometry, choice of steel and edge thinness. I don't think this type of knife would sell, because people would just break them prying open paint cans and the maker's reputation would be ruined.

Gator97 said:
There's couple things you'd need to keep in mind when rehardening. May be you already know that, but some of it was unexpected to me when I went through the process.

Rehardening implies subjecting the blade to high (very high) temperatures. So, whatever coating it has will be gone, and you either have to clean up the blade manually or send to someone for bead blasting or other coating, or polishing. ou can see what the blades look like after rehardening.

Also, regrinding, if you are looking into that, is far easier before rehardening.

Some manufacturers, if not most of them leave rather rough surface under the coating, because it sticks better that way. E.g. my M2 710 blade. After rehardening, once the coating was gone it exposed pretty rough grind lines on the surface. You can see that clearly on the photo attached here.

I've spent 8 hours or so trying to smooth the blade finish, mainly with 120/220 sandpaper. 64.5hrc blade didn't really respond well.
Both Richard J and Tom Krein can comment on those 2 blades too.

So, keep all that in mind when rehardening.

But in the end it is well worth it. The gain in performance that is.
Click to expand...

Too true. Blades with coatings have coarse scratches underneath that are nearly impossible to get rid of. Regrinding is probably the best way to get rid of them, and produces a better cutting blade too. Are you planning to mirror polish your 710 blades?
 
knifenut1013 said:
Goes back to the example of the car motor, just because they say your getting 300 hp does not mean you will get it every time. I have S30V in several knives and the HT is different for all of them, even ones from the same company.
Click to expand...

Actually, what car manufcturers advertise as power output is lower limit. You can get more, but you can't get less.
Opposite situation is with speedometers - they can show less, but they can't show more than what the actual speed is.
 
Howdy,

This sounds very interesting. I have a USA made fixed blade Gerber Stag Freeman made out of S30V. I have been disappointed in the edge retention, it seems to be similar to 420C. My guess is it doesn't have a great heat treatment, hence the poor edge retention (lower RC?).

Have you worked with any of the Gerber S30V blades? What do you think the improvement would be?

If I want to move forward with this, what is the next step?
 
Col. Cornelius said:
Howdy,

This sounds very interesting. I have a USA made fixed blade Gerber Stag Freeman made out of S30V. I have been disappointed in the edge retention, it seems to be similar to 420C. My guess is it doesn't have a great heat treatment, hence the poor edge retention (lower RC?).

Have you worked with any of the Gerber S30V blades? What do you think the improvement would be?

If I want to move forward with this, what is the next step?
Click to expand...

Oh yeah I can definitely get S30V to perform. Gerber knives are generally hardened on the softer side, I haven't used their S30V but own some of their knives in some unknown steels. The handles on the Gerber look removable, so you can just send the knife in and I'll re-heat treat it.
 
I've got 2 pairs of possible candidates.

The first is a pair of Ontario Rat-1 folders (AUS-8), and the second is a pair of Gerber Freeman (IIRC) Caping knives (I've seen the blade steel listed as "high carbon steel" and 440A).

I would suggest doing the reheat treatment on one of each of the pairs, then respectively comparing the performance to the untreated ones.

What do you think?

Chris
 
You must log in or register to reply here.
Back
Top