Chain sharpening...

[agg221]

Can we please get away from tempering, you can't temper over temper or under temper it. Tempering is for treating high carbon alloy steel that has been hardened by heating. Cutter bodies are made of bog ordinary mild steel, it can be hardened but it dosent posess the exotic alloys that respond to tempering.

 

Actually, not quite correct.

 

Cutter bodies are carbon steel, not mild steel. The difference is the amount of carbon present. If there's less than 0.08% carbon it's soluble in the iron at room temperature (mild steel = unhardenable), forming the ferritic phase. When the carbon content is higher it is only soluble after the high temperature phase transition to the martensitic structure. If you cool quickly (quench) it locks in the martensitic phase. Cool slowly (anneal) and you get a mixture of ferrite and pearlite (alternating layers of ferrite and iron carbide within the grain). If the carbon content is even higher (8%) you get precipitated graphite, as found in cast iron.

 

The martensitic structure is stressed and hence the atoms don't move easily, i.e. it's hard. It's also brittle. Re-heating gently (tempering) causes some of the martensite to convert to pearlite/ferrite. Conveniently, hardness drops more slowly than toughness increases with increasing temperature. A characteristic colour of oxide film is also formed with temperature, hence tempering straw, blue, purple is progressively softer and tougher. You choose your tempering temperature depending on what you're doing, so scalpels are left harder and more brittle (straw), metal cutting tools are taken to light blue, springs to dark blue etc.

 

This matters on a chainsaw because although the chromium is the cutting edge, the softer the backing, the more easily the surface layer flakes off (think ice on mud). If you under-temper, the chain is too hard to file, over-temper and it won't support the chromium cutting edge.

 

If you overheat the chain when grinding it cools quickly (as Spud says) and hence becomes harder. Re-tempering is theoretically possible with a hot air gun, but difficult to control, so you're better off taking care when grinding and not doing it!

 

Alec

Edited September 11, 2012 by agg221

[slack ma girdle]

Get one of Rob's 12v chainsaw sharpeners, and then it does not matter if the teeth have been hardened or not. And you will always have a perfectly sharp saw where ever you are.

 

alaskan mill I home of the portable chainsaw mill

[treequip]

Actually, not quite correct.

 

Cutter bodies are carbon steel, not mild steel. The difference is the amount of carbon present. If there's less than 0.08% carbon it's soluble in the iron at room temperature (mild steel = unhardenable), forming the ferritic phase. When the carbon content is higher it is only soluble after the high temperature phase transition to the martensitic structure. If you cool quickly (quench) it locks in the martensitic phase. Cool slowly (anneal) and you get a mixture of ferrite and pearlite (alternating layers of ferrite and iron carbide within the grain). If the carbon content is even higher (8%) you get precipitated graphite, as found in cast iron.

 

The martensitic structure is stressed and hence the atoms don't move easily, i.e. it's hard. It's also brittle. Re-heating gently (tempering) causes some of the martensite to convert to pearlite/ferrite. Conveniently, hardness drops more slowly than toughness increases with increasing temperature. A characteristic colour of oxide film is also formed with temperature, hence tempering straw, blue, purple is progressively softer and tougher. You choose your tempering temperature depending on what you're doing, so scalpels are left harder and more brittle (straw), metal cutting tools are taken to light blue, springs to dark blue etc.

 

This matters on a chainsaw because although the chromium is the cutting edge, the softer the backing, the more easily the surface layer flakes off (think ice on mud). If you under-temper, the chain is too hard to file, over-temper and it won't support the chromium cutting edge.

 

If you overheat the chain when grinding it cools quickly (as Spud says) and hence becomes harder. Re-tempering is theoretically possible with a hot air gun, but difficult to control, so you're better off taking care when grinding and not doing it!

 

Alec

 

 

None of which changes the fact that of you grind hard the tooth hardens and it's the chrome that forms the durable cutting edge.

[Stubby]

From my "O" Level metal work classes (was there such a thing?) I remember "hardening & tempering" as being the process where the correct grade of carbon steel was first highly polished, then heated red-hot before being "quenched" in water or oil (or menstural blood for Zulu spears apparently), to rapidly cool, this steel would then have been so hard as to be brittle, therefore then repolished before being heated to blue heat or straw coloured heat (showing in a rainbow effect across the polished steel) dependant on the level of hardness to be retained in the steel, before being allowed to air cool, ie slowly.

I cannot therefore see how grinding "hardens" as the chain is not "quenched"

Rather one is overheating the steel and loosing the hardening effect by overtempering. It should then actually be softer and easier to file BUT will not retain the edge in use.

I liked Metal Work.

Simples

think of the point of a chisel for an example.

 

Kindfa what I said .

[Stubby]

I can see how you would come to that conclusion but here are a few facts to allow you to re considder

 

Hardening occurs when you heat the material, the rate of cooling will have an effect but even if you let the material air cool it will still be hard and brittle, its the heat that hardens not the cooling.

 

There is no such condition as over tempering, the material is either tempered or not but grinding cant temper the material because it wasn't hard in the first place.

 

Retaining the edge in use has little to do with the condition of the cutter, rather the cutting edge is the few microns of chrome on the surface of the tooth. The tooth itself is made of a softer steel that can be easily filed out from under the chrome, you can't file chrome so they make the cutter body suitable to be filed out from under the chrome leaving a good cutting edge.

 

No . Dissagree . Its not the heat alone its the rate of cooling and the carbon content that affect hardness . If you heat it and let it cool slowly it will aneal ( soften ) it . If you cool it rapidly it weill harden it , Too rapidly and it will becom brittal . You then temper it back to the desired harness ( Measured on the rockwell scale . Case hardening ( as metioned in the first post ) is done with a non carbone steel which is heated uop to a cherry red and then dipped in corbon granuels and cooled rapidly . This only hardens the skin and not right through as with carbon steels . Both can be tempered back . If you grind a chain and get it too hot and it cools slowly it will soften .

[treequip]

No . Dissagree . Its not the heat alone its the rate of cooling and the carbon content that affect hardness . If you heat it and let it cool slowly it will aneal ( soften ) it . If you cool it rapidly it weill harden it , Too rapidly and it will becom brittal . You then temper it back to the desired harness ( Measured on the rockwell scale . Case hardening ( as metioned in the first post ) is done with a non carbone steel which is heated uop to a cherry red and then dipped in corbon granuels and cooled rapidly . This only hardens the skin and not right through as with carbon steels . Both can be tempered back . If you grind a chain and get it too hot and it cools slowly it will soften .

 

But it dosent does it? Have you ever had a file on an overground chain?

 

It is as hard as hell, why is that? Because heat hardens.

 

The slow cooling that annealing steel requires takes a lot longer than the air cooling. From memory annealing steel takes a temperature drop of 15 to 20 degrees per hour, it is generally done in a conveyor oven. An overground tooth cools in a second or two so the cooling does not anneal the material.

 

I know this because in another life I served my time in the tool room of a copany that made and heat treated fasteners (nuts and bolts to you)

[g&l tree]

the higher the carbon content of the steel the harder and more brittle it becomes when hardened. to stop it breaking the steel is tempered (heat up to straw colour or blue and quench in oil or water), this softens the steel slightly retaining most of the hardness. If you overheat the steel and let it air cool it is normalised (softer again). I have used this principle at work many times to both harden and soften steel. Alloy steels will react differently

[farmer rod]

does anyone from their experience and observation think that it a grinding wheel working too hot can not only pick up steel in its pores, but could also push bits of stone into the steel workpiece.

 

Personally I have never used a grinder on a chain, but I can see by the animated discussion here that results are variable.

[Stubby]

But it dosent does it? Have you ever had a file on an overground chain?

 

It is as hard as hell, why is that? Because heat hardens.

 

The slow cooling that annealing steel requires takes a lot longer than the air cooling. From memory annealing steel takes a temperature drop of 15 to 20 degrees per hour, it is generally done in a conveyor oven. An overground tooth cools in a second or two so the cooling does not anneal the material.

 

I know this because in another life I served my time in the tool room of a copany that made and heat treated fasteners (nuts and bolts to you)

 

Yes you must be right I suppose because the cutter is of so little relative mass it does not retain the heat long enough so air cooling is rapid enough to harden it . Accepted !. So cyanide hardening followed by slow tempering in a convector oven is not what is happening to a small cutter ! Yep thinking about it you are right . Small point -- I don't think cutters are chrome plated , rather they arte made from steel with a chrome content . Also time served in a toolroom that made plastic injection mould tools . Hate the smell of slurry coolent !!!

[morten]

Yes you must be right I suppose because the cutter is of so little relative mass it does not retain the heat long enough so air cooling is rapid enough to harden it . Accepted !. So cyanide hardening followed by slow tempering in a convector oven is not what is happening to a small cutter ! Yep thinking about it you are right . Small point -- I don't think cutters are chrome plated , rather they arte made from steel with a chrome content . Also time served in a toolroom that made plastic injection mould tools . Hate the smell of slurry coolent !!!

 

Not so. Cutters ARE chrome plated (except the carbide tipped cutters that are not used in the arb industry).