Sharpening impulse hardened saw

[Timberwolf]

What temp?

Let it cool slowly or put in water when it comes out?...right away or natural cooling then plunge?  Should I put it on an old cookie sheet (and turn it) so the rack doesn't cause uneven heating???

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To "detemper the saw, just heat it until deep blue......then to re-harden it, heat it until a magnet wont be attracted to it, THEN plunge it quickly into any kind of oil...If it doesn't get hardened afterwards, repeat the process and quickly plunge it into water...This is better done over a bed of charcoal with a hair dryer for a blower to get it evenly hot enough..You could help get the blade hot over the charcoal using a propane torch, instead of a dryer.
I would just detemper a small spot at the tip of the blade first to see if it can be filed..it if cannot, I would not waste any more time on it.
My experience using a feather file on an asian saw has not been good...If you could find a diamond file to fit the shape of the teeth, you could do it pretty easily.

[JosephP]

Thank you Jack!  I may put it on the rainy day list.

I have had very good luck with the feather file on the Silky Sugoi saw.  After I touch up those, it is one good pull to cut clean through 1"-1.5" diameter green branches...keep your fingers back!

[Pedder]

Just raise the temperature until the blade gets blue. That is the hardness od ususal saw steel. Then take it out of the oven, let it cool and you can file it.

Cheers
Pedder

[AHill]

Good luck with your experiment.  I'm a metallurgical engineer by education.  Your biggest challenge will be keeping the blade flat when tempering it.  Your blade is very thin, and more susceptible to warping when heat treating, because any uneven stresses in the blade will react adversely during the heat treating process.  I seriously doubt you'll be able to temper the saw with a kitchen oven.  To temper to a Rockwell C Hardness of 52 (suitable for sharpening with standard saw files), you need to temper at 600°F.  Rather beyond what a kitchen oven can do.  If you can find an oven that gets hot enough, definitely DO NOT plunge the blade in water after you take it out of the oven.  Just let it cool down on its own.  If you quench the blade in oil, you'll get Rockwell C 66 hardness, and that's too hard to sharpen with saw files.  IMO, just bite the bullet and buy a new blade.

[Jim Waldron]

Okay, wait.  There's a lot of good info here, but for a newbie, it's going to get muddled because different steps keep getting discussed in isolation.  

To file impulse hardened teeth, it's necessary as a first step to anneal the steel of those teeth.  The annealing process is what makes the steel soft enough to work with easily.  That's too soft for sharpening, tho.  There are two ways to anneal; first is to "re-temper" the blade by heating to around 600 degrees F, as Pedder suggests.  (Tough to do in a home oven in the US.  Maybe German ovens go hotter.)  Then let it cool, preferably slowly.  In the oven with the heat turned off is good.  The second is to heat the steel with a torch (propane, MAPP gas, etc.) to the Curie point (that's where it loses its magnetic properties, easily tested with a small magnet) and then let it cool slowly in air.  The advantages of the second technique are that it is very quick, may be focused on the tooth line and not the whole blade, and it doesn't lead to an angry and upset wife.  On the other hand, there's always the chance when using a torch of burning down the house, so be careful.  

The annealed steel of the teeth is relatively soft and easy to work.  This is the time to do any re-configuring or reshaping of the teeth, but not the point for sharpening.  Any sharpening at the annealed stage is a waste and may even be harmful.  

The second step in the process is to harden the steel.  Hardening is just as easy: heat the steel to the Curie point (use that magnet again, when the steel is cherry red) and immediately quench it.  The heating part is just like the annealing part.  The quench is what is different.  Some steel hardens best quenched in oil, some does best in water and some like slow air cooling (preferably with no oxygen around).  Your saw is not going to be the air cooling variety, but you won't know whether it's an oil quenching steel or a water quenching steel.  So here's the way to figure it out:

Oil quenching is slower than water quenching.  So the first time around quench in oil and test for hardness.  If a file "skates" over the steel, it's very hard.  If the file cuts the steel, it's not hard.  In the "not hard" case, reheat and quench in water.  

Quenching is where things can go wrong in the whole process, so there are some things to watch out for.  

• Make sure you have enough volume and mass of the liquid to accept the entire blade in one quick plunge and quickly dissipate the heat of the blade.  
  
• When you introduce the blade into the liquid, insert it edge-wise and move the blade gently up and down a bit.  Don't stir at all as that's likely to cause the blade to warp.  
  
• Keep the blade immersed for at  least two minutes.  Taking it out before it's cooled enough can prevent full hardening.  Leaving it in too long is not a problem.  
  
• Make sure of safety precautions as sticking a red hot piece of steel into a flammable liquid has been known to cause fire.  
  

Once the quench is done and hardness is confirmed with the file test, it's time for the next step: Tempering!

The hardened blade right out of the quench is so hard it will be brittle and if you drop it on your bench, you may fracture it.  Treat it like a valuable piece of antique glass.  Then you temper, the sooner the better.  Here you do need an oven.  If you ask politely (and take care to use an oil that's not too stinky for your quench, say peanut oil) you may get permission to do this: pre-heat the oven to 450 degrees F, put the blade in supported on the wire oven rack, and let it soak in the heat for at least half an hour.  This will reduce the hardness and increase the toughness.  Try with a file and you should be able to cut steel.

Now you can polish the blade and sharpen those teeth.  For most steel, you should end up with a hardness of Rc 58-60, which is very slightly harder than average saw blades, but softer than most chisels and plane blades.  

Of course working with such thin blades it's quite possible, as already noted above, it won't work at all.  But it's a disposable blade, so what's to lose for such a valuable life lesson?

[Pedder]

Boys, I tried that . It works.

You can just temper the hardened teeth to the hardness of any saw blade.  

No need to go back to red hot. Just temper to blue. Gerd says straw ist enough.

Edit: Certainly you can't get the "body of the saws blade" to a higher hardness grade than it is.

But that might be ok.

Cheers
Pedder

[Timberwolf]

I think this may be the best place to get a good answer for my question ...

I have several Silky brand hand saws for pruning trees. Some say they are sharpable others are impulse hardened to hold theur edge longer and are not sharpenable.  Replacement blades are not cheap. Can I use heat to soften an impulse harden blade and successfully sharpen it? I know it will not hold the edge as long. But it is at the end of its useful life as it is... so if it will work I might as well.

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After thinking a bit about the shape of the teeth and examining one of my japanese saws, instead of going to all the trouble to anneal, file, then rehardening a saw plate that is going to warp, and probably wont hold an edge very long, I would learn to use a Dremel tool with a thin abrasive disc to "touch-up" the teeth..All that you need to grind is the very tip of each tooth..The thin abrasive disc should allow you to get between each tooth so that you can restore the cutting tip fairly easily..I would "attack" the back side of each tooth.Just touch it lightly to remove a tiny amount..You wont grind enough to build up enough heat to draw the temper...
Definitely worth a try and if successful, you should be able to restore the cutting edge several times before the saw is used up.

[Admiral]

Jack gives good advice.

[AHill]

Jack gives good advice.

+1.

The OP cited these are pruning saws.  They are much thicker than your average dozuki or Western hand saw, but you still have very little saw plate to deal with when trying to take the hardness out of the impulse-hardened teeth.  A typical heat treat sequence in making steel plate is:  Anneal - Quench - Temper.  A Western Hand saw plate is already annealed, quenched, and tempered to achieve a Rockwell C hardness of Rc 50-52.  The plates are shipped to saw makers who then punch the teeth out, file, and set the teeth without any further heat treatment.  If the teeth are impulse hardened, it's done in a very controlled environment and it all happens in a matter of seconds.  If you only have a kitchen oven as your heat source, you will never, never ever be able to anneal the plate.  Annealing only happens at 1400-1500°F.  That's physics.  Annealing causes the individual metal crystals to recrystallize and it softens the steel.  You no longer have the crystals that made the steel hard to begin with. 

Tempering does not change the crystal structure of spring steel, but it does redistribute the carbides in the harder crystals to soften the steel.  You can temper the plate, which starts around 600°F, but while softening the teeth, you're also softening the plate.   The thickness AND the height of the blade conspire against you in order to get any kind of consistent properties out of the saw, and this is what might cause the blade to warp.  Even if you are successful tempering the teeth without warping the saw plate, you may very well get the teeth soft enough to sharpen, but then you've weakened the rest of the saw plate so much that you risk bending the teeth and/or breaking them off in use if you don't heat treat the blade after sharpening to get consistent metallurgical properties.

This is my humble, scientifically informed opinion.  You can try this at home, and if you're successful, please let us know.  I'm just very skeptical that you can sufficiently control the process at home and achieve the desired result.

[JosephP]

Already then...so Dremmel or new saw! Thanks all, great info here!