Marking knife - anneal the blade?

[rafezetter]

I'm going to make a marking knife from and old jigsaw blade as many have before, but as it was a metal cutting blade and I know it's seen some use should I anneal then harden / temper again or just use as is.

It's much wider than normal though about 20mm, and the section I'm using for the sharp bit is opposite where the original teeth were (however the tip of the blade will meet where the teeth were, and I'm doing a curved blade instead like ashley isles so won't be relying on just the point tip like most marking knives), do you think this may mean that this part of the blade has not been affected by friction heat, or should I do it anyway because of the tip to be sure?

 

[RogerP]

do you think this may mean that this part of the blade has not been affected by friction heat

If it is high speed steel it will be okay.

 

[ayuce]

If its a metal cutting jigsaw blade its either hss or bimetal. What i understand cutting edge will be untoothed side of jigsaw blade. If its bimetal, this side will be low carbon, soft metal and and wont fit your knife design. If its hss you can use it as it is. If marking is available on it, check with the manufacturer.

Anneal - harden - temper cycle is needed either its too hard to shape or you want a different hardness than the original hardness of jigsaw. Tempering temperatures usually above 200C and two cycle of 1 hour tempering is common. So if friction heated the jigsaw blade above 200C for some time, might lost some hardness while in use.

 

[rafezetter]

Thanks for the replies - I have no idea if it's HSS, or bi-metal as it came covered in black corrosion paint, a lot of which has worn off from use and I didn't pay that much attention to it when new, included with the jigsaw.

This is only my first marking knife making attempt, so it's fairly crude by most peoples standards I'd imagine, and with your advice and thinking about it a bit more, it's only to strike a line in wood so I guess it'll be fine.

 

[seaco]

To anneal just heat to cherry red until it's non magnetic and then let it cool slowly if the wife's been cooking in the oven coincide it with your blade bring it to temp then as you wife turns the oven off pop the knife in and let it cool it is now annealed.

shape the blade then when your ready to harden reheat to cherry red again then when it's non magnetic dunk in oil or water while it's still warm pop back in the pre heated oven at 230 for one hour to temper if the blade wasn't to dirty it should have a straw colour to it...

Job done!

 

[d c]

post some pics when done.
i've started making a couple of marking knives this very morning. one is cut from an old irwin jack saw blade and the other is a ground down 13mm spade bit.

 

[AES]

Roger,

to check if your blade is bi-metal or not look for a faint "join" line just "up" the blade, above/behind the teeth. That's a very fine weld BTW.

If the blade is grotty give it a stiff brush off with a wire brush or some old abrasive paper, and you may need a magnifying glass and a bright light to see (it's a very fine join line). But if it's bi-metal that line will be there, if none is visible it's almost certain to be HSS as it is/was a metal cutting blade.

HTH
AES

 

[rafezetter]

I gave it a good clean and I can't see any line, so I've gone ahead without anealing,, it's not finished though.. had a little hiccup, and need to make a part again, should be finished tomorrow with pics.

 

[sdjp]

I gave it a good clean and I can't see any line, so I've gone ahead without annealing ...

Which is good - if it's HSS, then annealing, hardening and tempering operations are significantly more complicated than for a simple carbon steel.

I mean, the basic principles are the same (Anneal: Heat above critical temperature, and cool slowly. Harden: Heat above critical temperature, and quench) - the problem is that the quench rate for some high speed steels is to air cool; and there's one or two where that's _too quick_, and need to be 'quenched' in a cooling oven. (Indeed, this property of 'air hardening' was the initial driver for this class of steel, before it was realised that the hardness would preserve to higher heats [0]). It's also the case that the critical temperature is higher, in general, and needs a long soak (to let the various alloy carbides dissolve properly), balanced against the grain enlargement problems.

In my opinion, Air hardening and HSS steels aren't worth the faffing to heat treat, unless one has a controlled ramp oven. Which, oddly, means that the wood cutting bits are in many ways easier to repurpose, as the various heat treatment steps are much simpler.

If one wanted to be rather silly, cheap 'HSS' blanks from China are cheap on eBay (a few pounds for a 2mm x 20 mm x 200 mm blank), and you can make a couple of rather nice knives out of this. I'm not sure that there's any real advantage to this route, unless you feel you need a _lot_ of practice at grinding - it's no-name HSS, so I wouldn't presume to put it to the limits; but it's plenty hard and tough enough for a carving knife. (I did that because I _did_ want lots of practice at grinding; having just acquired a few grinding tools)


[0] Mushet steel - the core idea is that the slower the quench, the lower the deformation, and risk of cracking on quenching. Air hardening allowed for more complex, and more precise, shapes to be made, without the expense of a lot of post hardening grinding.