Cold working as a blade hardening process- What are the possibilities/limits?

[Bert2368]

After participating recently in a thread about machetes?

https://www.kitchenknifeforums.com/threads/machete-recommendations.40688/

I did quite some googling and reading on the many technologies humans have used to slash back encroaching vegetaion they didn't care for.

I came upon much (new to me, hey, I usually just rent a brush cutter when I need to do that kind of thing?) information related to how, historically, manual brush, grass or grain cutting scythes might be maintained and sharpened.

Most particularly, I had NEVER heard about "peening" a scythe blade until last month. Let alone the various related fixtures an specialty hammers presently available for performing these kinds of blade tooling operations.

How applicable is such a technique to a culinary blade? It may be spanking the baby with an axe... Or have the scythe wielders never hung out with the blade smiths???

Can any speak to the possibilities and limits of such mechanical steel hardening processes? I am slightly familiar with work hardening in relation to sheet metal stampings for modern rifle receivers and their magazines, cold rolling of structural steel, auto body repair work, metal spinning and similar cold steel deformation operations. But had never considered the possibilities for hardening CUTTING TOOLS.

 

[milkbaby]

Most of our kitchen knives are relatively high hardness and need to be consistently hard along the edge and behind it to support being able to sharpen it many times. If you are thinking about peening soft annealed knife steel (mostly spheroidized pearlite I believe), I would be surprised if you could get it hard enough to support a good kitchen knife edge.

If you are talking about peening already heat treated kitchen knives, that can be done gently to straighten warped blades, but you are unlikely to get any useful increase in hardness. In fact, almost all kitchen knives are tempered after hardening to increase their toughness with a corresponding drop in hardness. Remember high hardness also means brittle, hit it hard enough and you can crack it.

Another edit: http://scytheconnection.com/consultants-report-on-blade-hardness/
Converting the Vickers hardness numbers to Rockwell C we are familiar with for knives, the possible work hardening was maybe from Rc 45 to 52, still too soft for our kitchen knives which tend to be Rc 60 and up. That scythes are produced at Rc 45 shows they require much more toughness than what is needed in a kitchen knife. For the kitchen we want much more hardness to support very acute edge angles which cut easier.

Edited to add: Heat treating and tempering are well developed techniques to make hard steel suitable for knives, basically to change the form of steel to hardened martensite phase. What is the goal you are trying to accomplish with cold working?

 

[HRC_64]

This all sounds about right to me... a german knife (54-56) is already harder than a work-hardened peened edge.

I've done it with cheap chineese garden tools...you get better edge geometry
thinnger, more acute angles wont fold, but the abrasion resistance etc is still poor
and the life of the edge is accordingly still relativley low.

just be aware that you will be re-working this edge quite a bit,
...the work hardened edge material is lost each cycle.

 

[Bert2368]

I was primarily interested in understanding WHY this was the edge hardening method of choice for European scythes. And wondered where else the technique might make sense?

I did some more reading after making the original post, along with thinking about what scythe use required of a cutting edge.

As noted, a scythe blade calls for great toughness/shock resistance and this dictates the accompanying softness in the (rather long) blade.

Along with shock resistance, a somewhat harder cutting edge for a bit of wear resistance was wanted- An edge harder than the spine, yet still soft enough that the user could re sharpen the edge with a scythe stone in the field very quickly.

Differential hardening of blade spine and edge as was done in Japanese traditonal blades would probably have resulted in too deep of a hard zone behind the edge, prone to cracking when the inevitable rock or log was encountered. The hard zone was wanted only for the edge itself, plus enough depth behind the edge to get through a day or two of work and the accompanying field sharpening.

After the work hardened zone was worn away from use and sharpening, the peening operation restored a desirable thin edge geometry plus imparting some edge hardness QUICKLY and without removing metal abrasively, thus prolonging tool life vs. a purely abrasive edge thinning technique.

I get it. The only food cutting related edge maintenance besides a scythe I can think of right off the bat where this approach might make sense could be tools for butchering whales? And that's not a big market these days...

 

[HRC_64]

That the peening is also a "thinning" type exercise if you think about it...one can plastically deform the edge to thin it and this work hardens it. Perfect for crazy sharp differential hardness, without being so brittle the tool is too delicate for peasants to use in the field.

For kitchen knives, its not super applicable iMHO because peened edge is too delicate (schyth cut only grass, nothing hard).

Kind of a neat thing to learn about, the stones they carried also were held in water on the waistbelt for quick touch ups.

 

[inferno]

I was primarily interested in understanding WHY this was the edge hardening method of choice for European scythes. And wondered where else the technique might make sense?

I did some more reading after making the original post, along with thinking about what scythe use required of a cutting edge.

As noted, a scythe blade calls for great toughness/shock resistance and this dictates the accompanying softness in the (rather long) blade.

Along with shock resistance, a somewhat harder cutting edge for a bit of wear resistance was wanted- An edge harder than the spine, yet still soft enough that the user could re sharpen the edge with a scythe stone in the field very quickly.

Differential hardening of blade spine and edge as was done in Japanese traditonal blades would probably have resulted in too deep of a hard zone behind the edge, prone to cracking when the inevitable rock or log was encountered. The hard zone was wanted only for the edge itself, plus enough depth behind the edge to get through a day or two of work and the accompanying field sharpening.

After the work hardened zone was worn away from use and sharpening, the peening operation restored a desirable thin edge geometry plus imparting some edge hardness QUICKLY and without removing metal abrasively, thus prolonging tool life vs. a purely abrasive edge thinning technique.

I get it. The only food cutting related edge maintenance besides a scythe I can think of right off the bat where this approach might make sense could be tools for butchering whales? And that's not a big market these days...

They probably did it because it was cheap and you could reproduce the results. it makes sense it austenitic stainless grades since these cannot be HTed. It also makes sense for low price medium strength bars that you machine stuff out of. since its a bit harder and stronger than the hot rolled stuff.

There is one steel that is famous for this: hadfield steel https://en.wikipedia.org/wiki/Mangalloy. its a manganese steel with i think 12% or so manganese, its used as wear plates in demolition equipment, excavators, dumpers, crushers and stuff, think heavy duty machines. I think they used it more many years back though. The thing that made it good was this: as the surface got worn by rocks and steel and scrap and whatever you have, you exposed fresh new material, and that workhardened as it wore. Magic steel!

These days i think they mostly use hardox steels for this though, since you can get it in any thickness you want, its easily welded, and very available, and not too expensive. I think the most used wear grade is the hardox 500. 500 brinell hardness or 52-53 or hrc i guess, well its somewhere around there at least.

 

[Bert2368]

Not really your kitchen knife, but:

Looking at Spyderco "Salt" knives with H1 steel blades, I find that WORK HARDENING BY ROLLING the stock to desired thickness before these blades are cut out and ground from the rolled stock IS the only hardening done to them. H1 steel only has .1% Carbon but cold rolling the stock from 7mm to 2mm gets it up to around 57 RHC.

I found an explanation of what H1 alloy is and why work hardening is used here:

https://knifesteelnerds.com/2019/06/24/h1-steel-how-it-works/