Re: Metallurgy manifesto from a metallurgist *LINK*
Andrew F in Australia
>Hi Adam.
I use an old Marples 1” firmer from around the turn of the century – it’s a good chisel I support your comments on this.
Now to the metallurgical part of the discussion.
There is a difference between RC62/63 and 64. Basically, Rockwell is a scale that determines resistance to penetration of a (from memory) a **hard** steel ball, otherwise it's a diamond pyramid, into a freshly polished surface of the steel.
It's exponential, so there's (guessing once again at numbers as I don't have time or calculator close by) about 3-5% difference between the two in hardness.
The 'toughness' we refer to here with chisels is the energy able to be absorbed by the chisel without deformation (= resilience.)
The dictionary definition of toughness (see linked on-line materials dictionary below) is not a resistance to bending, as much as area under the stress strain curve as you rip the chisel in half. As you point out, once an edge is bent, it's useless. But not as useless as if you maximise the dictionary definition of toughness and destructively test your chisel.
Going to my Materials Engineering/Metallurgy trade background, every time you use any edge tool and lever off it, it deforms (like a leaf spring.) Once again, like a leaf spring, you want the chisel to return to its original shape. This it termed elasticity. The point at which, after being stretched/bent, it ceases to return to its original shape is termed the elastic limit or yield strain.
As the steel is stretched/bent, it reacts and tries to return to its original shape. The amount of force required to do this (standardised to a per-area unit basis) is called stress.
At the elastic limit, the stress is called the yield stress.
The area under the yield stress/strain curve for steel is pretty close to a triangle, so resilience is basically the area under the stress/strain curve where no damage occurs, and is roughly proportional to yield stress times yield strain.
As well, the ratio between yield stress and yield strain is constant in steel, regardless of whether you've got a fully hardened steel or a fully annealed (soft) one (termed Young's modulus). So, a steel with a yield stress of 500MPa will stretch twice as far to get to its elastic limit as a steel with a yield stress of 250MPa.
In a practical sense, as you get the steel harder, the amount of work it takes before deformation increases, as well as the distance you can bend/stretch it before breaking also increases. This is how you control the resilience of the steel.
The alloys used in modern tool steel create precipitates that are ceramic (such as Chromium, Tungsten and Vanadium Carbides, and very hard (approx 78 - 85HRC from memory.) As well as standard heat treatment of steel (more later) these precipitates create a hard edge when they are successfully embedded in the edge or near it. They do this by resisting deformation as well as pinning the softer steel edge, preventing it somewhat from moving.
The steel structure surrounding these carbide grains consists of fine ‘grains’ of pearlite (= interspersed layers of iron carbide and iron) as well as fine grains of surplus iron that weren’t needed to make the pearlite.
Tool steels are designed for applications where the material they're working is sliding across the surface, wearing it down this way. In these situations, the precipitates embedded in the surface stop the tool steel from wearing away.
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Having said this, I use M2 plane blades - they take a beating and last fairly well between sharpening (and on our silica rich timbers, the edge used to blunt after about 2 plane strokes)
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The downside of using modern steel is that the precipitates get too large when poorly heat treated or at high alloy content.
Brittle ceramic at large sizes = fracture = edge failure.
At small precipitate sizes, when they do fracture, it's at a level where they don't impair the appearance of the finished surface.
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Conclusion 1: You're probably looking for a resilient chisel, not a tough one.
To get this, you need steel that has a fair amount of elastic limit and a proportionally fairly high yield stress.
The way you get this is small grain size, low impurity and control over the size of precipitates/carbides in the steel to maximise resilience of the steel.
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PART 2: Japanese steels and old tool steel
These steels are essentially alloys of carbon and iron. The iron are stacked at an atomic level like billiard balls – At temperatures above 800C the carbon atoms fit neatly in the holes between the iron atoms and stop the layers of iron atoms from sliding easily over each other. At room temperature, some of this carbon remains in solution. This is what gives low carbon steel its resistance to deformation (= hardness.)
The carbon atoms also stop the steel from changing form easily in heat treatment, which results in the hardenability of steel. When quenched from 1500F (820C), the iron can’t form is stable phases as easily. If it’s prevented from forming the stable phases (iron and iron carbide) then it forms a hard phase (martensite). When heat treated (tempered), the martensite turns into very fine layers of iron and iron carbide (= pearlite grains), as well as fine grains of iron.
These fine layers/grain structure give the steel resistance to deformation, which means that it can bend a fair bit and return to original shape, which also gives the steel its hardness. This is the classic hardened structure you get with W1/W2 steel.
Alloys of about 0.2% Carbon are fairly hard to heat treat (ie: they don’t form the hard phase), alloys of about 0.4-0.6% C are fairly easy to heat treat, alloys of about 0.8%C produce all pearlite. W1 is a 0.8%C alloy, W2 is a 0.8%C alloy with a little bit of vanadium added to produce some harder Vanadium Carbide grains as well.
The Japanese steels are a variation on this theme – they contain layers of pure iron as well as layers of carbides, all very fine grain size. The pure iron gives a cushioning layer between each layer of hard carbide, something similar to the modern tool steels, where the iron/carbon matrix cushions the carbide grains.
Japanese steels get the flexibility and high hardness by having the hard layers cushioned by ‘springs’ of softer layers. The hard layers are very hard, which allows the steel to reach an average 62-65HRC or so. The more times the steel is folded on itself, the finer the grain structure and the better the resilience.
Use of an old anchor that survived without rusting in a sea environment for 200 years is part fact as well as fiction – it means that the iron was fairly pure to start with. Impure iron would have rusted away.
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Conclusion 2:
There is some science behind the manufacture of Japanese steel, but there’s also a fair bit of marketing hype.
The different internal structure of Japanese chisels is what gives the high hardness and elastic limit.
Japanese steel and old tool steel both are basic iron/carbon steels. When they’re very fine grained, they should both hold an edge equally and work the same. Just that the Japanese chisels will take a bit more of a beating before failure.
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This is more of a manifesto than I planned to write. And it's still fairly basic.
Any questions, please feel free to ask
Andrew
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