Sudden warped edge while grinding

[Case_D]

I was rough grinding the bevels on this blade, and got it to the dimensions I liked. I switched to 120 grit and started polishing when suddenly this small section on the edge warped. It sprung out and now the edge is no longer straight, while before this happened it was perfect. I don't know what's causing this or if it can be remedied, and I've never had this happen before.

This blade is 26C3. Forged and fully heat treated. I was going for a brute de forge finish, so it was forged as close as can be in most dimensions. I did make sure to leave plenty around the edge to grind off because of decarb. The whole time I was forging and heat treating it I made sure to keep everything as straight as possible. I also did a stress relief cycle before hardening to mitigate any potential warping. I staved off warping almost entirely for the last handful of knives this way. Quenched in Parks 50. The hardness should be somewhere in the ballpark of 65HRC. I ground the bevels in to 60 grit on a radius platen, so it gets really thin at the edge and behind it.

The warped section is springy. If I push it straight, it springs back when I let go. It's like it's being buckled from the sides. Because it's so thin, the straight portion of the edge will flex with some pressure by my fingers but not to the degree of the warped section.

https://imgur.com/a/maucap4

 

[Robert Erickson]

You may have overheated it while grinding is my assessment based on what you've described. How thick was the edge before heat treating?

 

[FunkCoaster]

FWIW, I had the same problem. Also 26C3, but mine had a temper line (hamon). But a brut de forge finish may have the same factors at play due to unhardened outer regions.

My theory is that the steel expands when it converts to Martensite. The thicker and non-Martensite spine is preventing the hardened edge from expanding.
Once you grind it thin enough behind the edge, there is no longer enough material to contain the tensioned Martensite.

The details are in this post : https://www.bladeforums.com/threads...to-wavy-after-grinding.1890816/#post-21494922

In short, I used a carbide hammer to elongate and it seems to have helped. Hope this is useful for you as well.

 

[Case_D]

You may have overheated it while grinding is my assessment based on what you've described. How thick was the edge before heat treating?

I grind with a water mister and dunking frequently. The blade rarely even got warm to the touch since it was cold outside. The edge thickness before heat treating was somewhere between 1/16" and 1/8" and I ground the edge back about 1/8"

 

[Case_D]

FWIW, I had the same problem. Also 26C3, but mine had a temper line (hamon). But a brut de forge finish may have the same factors at play due to unhardened outer regions.

My theory is that the steel expands when it converts to Martensite. The thicker and non-Martensite spine is preventing the hardened edge from expanding.
Once you grind it thin enough behind the edge, there is no longer enough material to contain the tensioned Martensite.

The details are in this post : https://www.bladeforums.com/threads...to-wavy-after-grinding.1890816/#post-21494922

In short, I used a carbide hammer to elongate and it seems to have helped. Hope this is useful for you as well.

I'll probably give it a try. The post says you hammered around the hamon? I can usually see the decarb line and sometimes some auto hamon when doing blades like this. If I were to hammer on the edge, I'd crack or chip it for sure since it's so thin.

 

[Joshua Fisher]

Measure the spot the buckled and measure around it that stayed straight, my guess is you ground too thin in that spot, likely in the 3-4 thou range. When you see a flat/convex grind you’ll often see makers going down to just a few thou or almost sharp but you don’t have to go that thin with a hollow to get the same geometry and if you go too thin and aren’t even in thickness sometimes the edge can warp like that in the over ground spots.

 

[Case_D]

Measure the spot the buckled and measure around it that stayed straight, my guess is you ground too thin in that spot, likely in the 3-4 thou range. When you see a flat/convex grind you’ll often see makers going down to just a few thou or almost sharp but you don’t have to go that thin with a hollow to get the same geometry and if you go too thin and aren’t even in thickness sometimes the edge can warp like that in the over ground spots.

It's possible that's the case. I left the heel a little thicker on purpose and the rest as thin as I'd ever push it. Maybe that transition caused some stress. This was the 3rd kitchen knife I'd ground on a radius platen, and on the first one I took it too thin. It ended up with a decent recovery and given away as a Christmas present. On the radius platen, it gets deceptively thin compared to a flat grind.

 

[Joshua Fisher]

Yeah my guess is just too thin in a spot, maybe you were pushing harder in that area and caused it to buckle. For kitchen knives I shoot for around 8 thou edge thickness with my hollows on a radius platen, for flax/convex I go down to 5 and find they have about the same cutting geometry at the edge.

 

[Stacy E. Apelt - Bladesmith]

Nothing to lose by trying to straighten it with a brass hammer on a hardwood board.

 

[Thescrawnylumberjack]

It’s very common for blades to move/warp when grinding. Especially when they’re long thin kitchen knives. I would guess left over stress in the blade with optionally an uneven grind causing the edge to warp. I’ve had it where I grind in a distal taper on one side and the blade is warpped until I grind it in on the other side. Sometimes I need to use the carbide straightening hammer persuade the students.

 

[Stuart Davenport Knives]

I know exactly what you are talking about. It is not a normal warp. It is just a certain section of the edge. It pops out and you can push it straight with your fingers but it springs right back out. I grind under a flood of water, so heat was not the issue at all. Edge measured the same thickness from tip to heel. I have no idea why that happened. Mine was a tall thin santoku in 52100. That was before I had the carbide hammer, and that knife became a template! I hope you're able to get it straight. Luckily that has only happened to me once.