Thread: differentially heat treated razors and blade geometry

so as a knifemaker I generally differentially heat treat most blades so I can have an edge that has the maximum working hardness and a spine that gives some toughness to a blade. this usually works out really well.

an added bonus of the differential heat treat is a hardening line, or tempering line depending on how you do it, or a hamon. which can look very plain or very nice.

I did a batch of w-1 blades this weekend and wanted them to have a hardening line just to see how it would look as I have only worked with w1 once before, but someone gave me a bar of it big enough for about 30 razors. I got them forged out, cleaned up, ground, and rough finishes to about 200 grit, then went to set the bevels on a dmt 325, and noticed that the spines were wearing much faster than the edges due to the difference in hardness.

now on a paring knife or edc, or chopper or something, where you aren't depending on the spine as a sharpening guide to maintain geometry, I wouldn't worry about that. but with a razor, where the spine plays a very important role in maintaining the edge geometry, how big of a deal is it if the spine is softer than the edge and will get worn away faster? also how high should you put the hardening line? I would think that if you grind the blade really thin, that as the blade flexes during use and sharpening, that the softer steel will flex differently than the harder steel and may cause a fracture at some point.

so:

is it a big deal?

how high do you put the hardening line?

would you choose a thicker grind for a blade with a hardening line?

could you build the spine thicker {the part of the spine that will wear, not the actual thickness of the spine} to compensate for the faster rate of wear?

and lastly, how is w-1 regarded in razor making? I only work a handful of steels, but like I said I was given this bar, and there is enough of it to make a fair amount of razors. it looks much like 1095, and is available in thicker pieces than 1095 it seems. so should be a good candidate for forged razors.

A lot to address here.

I've been working with 1095. As a shallow hardening steel, the spine inherently ends up softer than the edge. A nital etch revealed this to be true with my razors. I have had no problem with the spine abrading too much.

Experience with honing razors allows one to torque the pressure towards the edge and use the spine as the guide that it is rather than sharing the wear with the edge. If the wear ever did get ahead of the edge due to poor honing technique , tape can be used to get back to the desired bevel angle.

Hamon can be cool. If I were going for that, my decisions of where to put it would be purely aesthetic. As to different flex; contrary to what seems intuitive, the modulus of elasticity is identical regardless of the heat treat of a given alloy steel up to yield where the curves diverge. The yield point is what changes on the stress/strain curves with different degrees of heat treat. Since we are not flexing our razors beyond yield (permanent deformation) softer portions of the blade behind the edge are not a problem. The same would apply to thin grinds.

If you look at the edge profile of my razors you will see that I leave the shoulder square. This is mainly an aesthetic consideration, but it does result in the surface area of the hone wear increasing rapidly and then slowing because of that increase. I don't have a problem with hone wear on my razors, several of the razors on my page were honed sans tape

I've never worked with W1, but everyone I've talked to regard it as pretty much identical to 1095.

ok, so its not really a big deal.

I will pay more attention next time I go to set bevels. it may be that I am putting more pressure on the spine instead of using it as a guide like you are suggesting. or perhaps I left it really square and I was just seeing the initial wear before it starts to slow.

I was also wondering how to leave that shoulder. I would think that the more rounded I can leave it, the nicer it would be to the strop. or possibly leave it square, then set the bevels, flatten it out a bit while setting the bevels with the blade at a fairly coarse surface finish, and then smoothing it out as the blade finish gets finer.

this w-1 seems like it could be nice stuff. forged easily, grinds fast. but I do need to temper it a bit higher than my 1095 to get the edge at the right hardness it seems.

They have made frameback razors with brass spines that have lasted many years, I would not worry about the spine being softer on a differential hardened blade.

Originally Posted by caltoncutlery

ok, so its not really a big deal.

I will pay more attention next time I go to set bevels. it may be that I am putting more pressure on the spine instead of using it as a guide like you are suggesting.

I usually set my first bevel with tape and then re set it without if I'm going to hone without tape.

Originally Posted by bluesman7

Hamon can be cool. If I were going for that, my decisions of where to put it would be purely aesthetic. As to different flex; contrary to what seems intuitive, the modulus of elasticity is identical regardless of the heat treat of a given alloy steel up to yield where the curves diverge. The yield point is what changes on the stress/strain curves with different degrees of heat treat. Since we are not flexing our razors beyond yield (permanent deformation) softer portions of the blade behind the edge are not a problem. The same would apply to thin grinds.
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I have to take back part of what I said. On the razor that I'm currently finishing I seemed to be having more of the wrinkly problem despite grinding to my normal thickness. When I etched the blade, I saw a distinct temper line in about the center of the blade. I'm thinking that since martensite is less dense than pearlite , the edge is in higher compression than it would be if the temper line were higher in the blade. So, IMO, temper lines combined with very hollow grinds can be problematic.

I haven't had many blades with the wrinkly problem. like this last batch of w1 razors, I forged 4 of them from the same 1" square bar. in the same forge, with the same hammers, same arm, same anvil, and all were thermocycled at the same time, at as close to the same temp as my eye could tell, quenched in the same oil, tempered at the same time. and only one showed the wrinklies.

and about the only fix ive ever seen for the wrinklies is either the scrap pile, or grinding the edge back and then regrinding it thicker so there is more steel to support the straightness that you want. I remember a 8" chef I made that the edge started wrinkling when I got it to the thickness I was after, so I ground it back, then reground the blade, and as soon as I got it to the thickness I wanted again, it started wrinkling. I do remember that it seems like I get more blades with the wrinklies that are thinner before the quench than those that are left with more meat on them. now that being said, this w1 razor that had the wrinklies was left pretty thick prior to the quench.

I'm thinking that on blades where I want a hardening line, I will leave them a bit thicker, and put the hardening line as far up the blade as I can to be on the safe side.

Originally Posted by caltoncutlery

I'm thinking that on blades where I want a hardening line, I will leave them a bit thicker, and put the hardening line as far up the blade as I can to be on the safe side.

There's a couple of threads where a hamon was revealed on Iwasaki razors. It's right up on the spine.

On top of the previous answers: my default is to always hone with tape. Most of what I make is damascus or wootz anyway, and you don't want those shiny wear lines on the spine of an otherwise greyish and patterened blade.