First blade from an old sawblade what did I do wrong?

[jcdoherty]

Oops just realized I got this posted in the wrong place, can a mod put it where it should be? Thanks,
Hi, I cut a knife out of an old 8" circular saw blade, about 8" long 1 1/2" wide and about a 1/16" thick.

I finished it with an oak handle epoxied and pinned and sharpened it with a Lansky sharpener and got it shaving sharp but I found it dulled very quickly.

I started reading and watching some youtube videos and figured out the steel was too soft and I should have hardened it.

So I took it all apart and heated the blade with a propane torch but had no magnet to test it so I went by eye, it never really got orangey but I was in full daylight.

I quenched in used motor oil but when I tested it with a file it dug in so I figured I didn't get it hot enough.

Tried again with acetylene and got it orange for sure, quenched again but the blade came out looking kind of funny.

The blade had a black coating from the tip back about an 1 1/2 then a 2" section where the blade was dark but not coated (could kind of see silver under or through the darker color) then a black coating from there to the handle just like the tip.

I checked it with a file and it was definitely harder but the file would still leave a mark with some effort.

I cleaned the blade with a metal scouring pad, the center section cleaned up but the coated areas didn't clean completely and I had to sand the coating off.

Since the file did cut some I thought I wouldn't need to temper it and proceeded to sharpen the blade with the Lansky sharpening kit.

After sharpening it at 25 degrees through 600 grit, it will cut printer paper and phone book paper fine but won't shave the hair off my arm when it would prior to hardening.

Did I screw up? I really wanted it to be shaving sharp and keep the edge a long time.

Any advice or criticisms gratefully accepted.

Thanks, Jim

 

[samuraistuart]

Jim...welcome to the world of heat treating! It is fun, but can be a challenge, especially since we don't know exactly what steel you were using. However, if it is a steel suitable to knives, you should be able to get it hard and serviceable.

Let's start from the ......... start. And let's just assume the steel from the sawblade is good knife steel.

Heat treat in a dark(er) room. And get that magnet. Since you aren't used to heat treating, judging by eye is just going to be a crap shoot. The magnet is much better. You can re-heat treat no problem. Heat the blade up with your torch or forge or whatever, and get the blade as evenly colored as you can. You'll notice that it will go from a dull red to bright red, somewhere in there at 1414F the magnet will no longer stick. But that is NOT hot enough. You need to be at 1500F to quench......so we usually say "One shade of red or two past the color when the magnet stops sticking." Once the entire blade has reached that shade of red at 1500F, quench it. Please Please do NOT use motor oil. There is no reason to use motor oil when canola and peanut oil are easily available. Warm the canola to 130F and quench the blade in the canola.

Once the blade is quenched, leave it in the oil until you can hold it bare hand without burning, let it cool to room temp. If you want to check it with the file...now is the time. After it as it room temp, run the file on the edge. It should bite in a little bit....because the heat treat caused the carbon to come out of the blade (very small layer) and form the black scale that you noticed. If the blade hardened right, and there was a small decarb layer (usually this is the case), the file will bite into that decarb layer, but will stop biting when it reaches the hard steel. It is VERY noticeable. The file will act as if you are trying to file glass....it will just run right over the edge and not even scratch it.

If it hardened right....go to temper it. Usually I do a one hour at 375F and then an hour at 420F and another at 420F for hunters. Less temp for kitchen knives.

 

[jcdoherty]

Thanks for the rundown Stuart, I see I messed up a few of things.

Why shouldn't I use motor oil? I had a container ready to be turned in to the auto shop and figured it wouldn't hurt.

I only held the blade in the oil until it stopped bubbling, spitting and smoking then set it on top of the can to cool, could it still have been hot enough to temper itself?

I guess I got lucky as my blade is definitely harder yet didn't break when I accidentally dropped it point first on a bare concrete floor.

I went through the sharpening process again and now it will shave the hair off my arm but I have to work it a bit unlike when it was softer but I think I'll leave it as is and try to get the next one done correctly and compare the two.

I have your procedure copied for future reference, Thank you for taking the time to let me know how it should be done.

I really appreciate it, have a good one. Jim

 

[Stacy E. Apelt - Bladesmith]

Welcome to Shop Talk, JC

It is unlikely that a circular saw blade will make a good knife. The steel is designed to be tough and shatter resistant, not specifically hard. If it was a carbide tipped b;lade, the steel won't harden much at all. Even if it is a knife grade steel ( again, unlikely) circular saw blade steel requires a much more complex HT than "git 'er hot and dunk 'er in oil".

Your best bet is to get a bar of 1084 from Aldo ( New jersey Steel baron) and make a knife from that.

Used motor oil is not a very efficient quench media, and it can have some rather nasty smoke when used. Canola oil is far better.

The Count will be along soon with his standard reply to Newbies. Until then, the stickys are full of the info you need.