Removing too much jacket material on san mai chef knife?

[Leon Husock]

Whenever I make san mai chef's knives I end up with the majority of the jacket material ground away and the transition line between materials often occurs 2/3 of the way towards the spine, often with the jacket completely ground away closer to the tip and only the core steel showing. I assume this is because of the extremely gradual, high flat grinds chef knives require, but I also notice other people don't have this problem. How can I avoid it? Is it just a matter of using thinner core steel?

 

[HSC ///]

Stock removal or forged?
Purchased san mai or you made it?

if you didn’t forge it you can have this condition

 

[Leon Husock]

Stock removal or forged?
Purchased san mai or you made it?

if you didn’t forge it you can have this condition

Homemade san mai, forged as close to shape as I can before grinding. In the worst case I made a wrought iron/nickel/W2 billet and ended up grinding the wrought off completely except for a few spots.

 

[DevinT]

Make the core thinner to start with. The % thickness of the core in the initial stack = % rise on a flat ground blade.

Example: 3/8” clad plus 1/4” core plus 3/8” clad = 25% core thickness

Forged out flat, flat ground 2” tall blade would have 1/2” core showing or 25% showing.

Hoss

 

[Leon Husock]

Make the core thinner to start with. The % thickness of the core in the initial stack = % rise on a flat ground blade.

Example: 3/8” clad plus 1/4” core plus 3/8” clad = 25% core thickness

Forged out flat, flat ground 2” tall blade would have 1/2” core showing or 25% showing.

Hoss

Awesome, thank you! This is an extremely helpful guideline (and somehow it had never occurred to me that I could make my jacket thicker and keep the core the same. I was kicking myself for not ordering some 1/8" thick stock when I bought steel a few days ago!)

 

[HSC ///]

FWIW, the laminated steel I use (purchased) is about 1/3 core...if I make it, I also shoot for about 1/3 or error to the low side (like Devin advised)
Usually I get this result, the first 2 are purchased,the last one is made with a 26c3 core

 

[Leon Husock]

FWIW, the laminated steel I use (purchased) is about 1/3 core...if I make it, I also shoot for about 1/3 or error to the low side (like Devin advised)
Usually I get this result, the first 2 are purchased,the last one is made with a 26c3 core

That's what I usually shoot for too. I generally do each piece at 1/4". It's possible I need to forge it closer to shape; I've only had one san mai with a nice clean line close to the bottom, and that was also the only one I drew out on a press rather than by hand. So it could be I'm hammering it too unevenly when I'm drawing the billet out?

 

[HSC ///]

That's what I usually shoot for too. I generally do each piece at 1/4". It's possible I need to forge it closer to shape; I've only had one san mai with a nice clean line close to the bottom, and that was also the only one I drew out on a press rather than by hand. So it could be I'm hammering it too unevenly when I'm drawing the billet out?

Mine are all forged hammered by hand, hammering unevenly pushes the core to one side or the other.
I would suggest that you are not forging thin enough,

you can see on this one the raw forged finish, I believe that one was forged to about .1 at the spine and tapering,
Thus the grinding took me about 10 minutes to rough grind out.
If you zoom click on the heel area you can see the core height.

 

[Leon Husock]

Mine are all forged hammered by hand, hammering unevenly pushes the core to one side or the other.
I would suggest that you are not forging thin enough,

you can see on this one the raw forged finish, I believe that one was forged to about .1 at the spine and tapering,
Thus the grinding took me about 10 minutes to rough grind out.
If you zoom click on the heel area you can see the core height.

Gotcha. Yeah, I've probably just been being too cautious about how thin I forge. I like to grind my san mai at least a little before heat treat after the time I didn't grind at all and the core steel split right down the center (something about the mild jacket constricting the expanding martensite in the quench) so I worry about it getting too thin pre-quench.

 

[HSC ///]

Gotcha. Yeah, I've probably just been being too cautious about how thin I forge. I like to grind my san mai at least a little before heat treat after the time I didn't grind at all and the core steel split right down the center (something about the mild jacket constricting the expanding martensite in the quench) so I worry about it getting too thin pre-quench.

The devil is in the details...
I’m going to suggest you’ve got to provide more specifics, if you want the proper assistance. So how thin are you forging to at the edge profile?

in four years of quenching laminated steels in straight water I’ve never had one split on me. This is maybe 300 quenches.

I forge thin and have never ground before heat treat quench.

so when you say - “(something about the mild jacket constricting the expanding martensite in the quench) ”

I don’t believe others experience this. I can quench a .060 thick blade and grind afterwards and have no concerns about the material separating

you are going to do what works for you of course but just as a comparison , I forge thin and Grind after heat treat. Always

 

[Leon Husock]

The devil is in the details...
I’m going to suggest you’ve got to provide more specifics, if you want the proper assistance. So how thin are you forging to at the edge profile?

in four years of quenching laminated steels in straight water I’ve never had one split on me. This is maybe 300 quenches.

I forge thin and have never ground before heat treat quench.

so when you say - “(something about the mild jacket constricting the expanding martensite in the quench) ”

I don’t believe others experience this. I can quench a .060 thick blade and grind afterwards and have no concerns about the material separating

you are going to do what works for you of course but just as a comparison , I forge thin and Grind after heat treat. Always

That particular one was a kukri made from mild jacket and 1095 core. I forged it and left it relatively thick (can't recall the exact thickness at the edge, probably 3/16" at the spine) and made the mistake of waiting a few hours from quench to temper. When I checked back on it the spine was split right down the core, not along the welds. After some googling it seemed like the core wanted to expand during the martensite conversion and the mild steel had constricted it since, with no grinding, the high carbon core was completely encased in the mild steel.

In general I should probably take a few more measurements of thickness during the forging process, since I can't really tell you how thick I usually forge to. My guess would be that the edge usually clocks in just shy of 1/8", with the spine closer to 5/32" or 3/16". Next knife I forge I'll check with some calipers.

 

[HSC ///]

Well I think your thickness is fine

why your lamination is splitting is a whole another topic... and Frankly beyond me

split down there core meaning the 1095 ripped itself in half...? I have really never heard of this

you say the core is completely encased, but that’s not really accurate is it? Isn’t the core exposed at the spine and the edge profile? It should be...

 

[Leon Husock]

Well I think your thickness is fine

why your lamination is splitting is a whole another topic... and Frankly beyond me

split down there core meaning the 1095 ripped itself in half...? I have really never heard of this

you say the core is completely encased, but that’s not really accurate is it? Isn’t the core exposed at the spine and the edge profile? It should be...

Ah yeah true, and it was since it was a stack not a taco. And yeah, the 1095 ripped itself in half while the welds held together. It was really bizarre, it's never happened again but I'm always wary of it now.

 

[jake pogg]

In the words of Kevin Cashen



"It is the expansion of the hardenable steel as it is setting up that creates the issues when it is held by another steel that is not moving at all. While the austenite to martensite transformation is occurring the lattice is more susceptible to coming apart and so if the outer surfaces are held fast it own expansion tears it in half. Others have had luck with not burying as much of the insert in the iron. Leave the iron thin, the steel thick and leave more of the steel hanging out the end and see how it works. "

 

[Willie71]

That particular one was a kukri made from mild jacket and 1095 core. I forged it and left it relatively thick (can't recall the exact thickness at the edge, probably 3/16" at the spine) and made the mistake of waiting a few hours from quench to temper. When I checked back on it the spine was split right down the core, not along the welds. After some googling it seemed like the core wanted to expand during the martensite conversion and the mild steel had constricted it since, with no grinding, the high carbon core was completely encased in the mild steel.

In general I should probably take a few more measurements of thickness during the forging process, since I can't really tell you how thick I usually forge to. My guess would be that the edge usually clocks in just shy of 1/8", with the spine closer to 5/32" or 3/16". Next knife I forge I'll check with some calipers.

the split was likely from waiting too long from quench to temper. The steel had no where to move with the mild jacket. This suggests your welds are pretty solid.

https://knifesteelnerds.com/2018/04/23/what-happens-during-tempering-of-steel/

for the thickness, it’s helpful to have a piece of steel with slots milled into the edge at different thicknesses lime 3/8”, 1/4”, 3/32”, and 1/8”. Keep it by your anvil. You can see easily how close you are, and not worry about damaging your calipers. I am making one for myself this weekend.