Thread: Tempering homemade knife blades

Just because there's no bluing left does not necessarily mean the remaining material is unaffected.
The blueing is the response of the steel surface in contact with air to the temperature reached.
Did you repeated cool the blank while shaping it?
This could reduce the extent of the detempering.

Before you put a handle on why not just try it out.
<ake up a temporary handle and see how well it sharpens and then try whittling some hard wood and see how the edge holds

Hi Bob,

No, I was using a 1mm cutting disc in the angle grinder to rough out. It caused no apparent issue during the cut other than at the very tip of the blade blank. The rest of the shaping was with hand files.

The whole project is an experiment, so no harm in trying anything really. I've not done it before and I've lost track of any links to tempering, although I'm sure there was another thread on here somewhere that I raised the issue in regard to blued HSS turning tools.

What do I do? Heat it to cherry red and slow cool?

Thanks,

Originally Posted by Fuzzie

Hi Bob,

No, I was using a 1mm cutting disc in the angle grinder to rough out. It caused no apparent issue during the cut other than at the very tip of the blade blank. The rest of the shaping was with hand files.

The whole project is an experiment, so no harm in trying anything really. I've not done it before and I've lost track of any links to tempering, although I'm sure there was another thread on here somewhere that I raised the issue in regard to blued HSS turning tools.

What do I do? Heat it to cherry red and slow cool?

Thanks,

Thin kerf cutting wheels are relatively cool cutting so you may indeed not have lost much temper. If you can shape them with a file I suspect they may be too soft already?

However, I would definitely put some time into trying it out before you mess sour with hardening a tempering.

The reason for this is you are flying blind as you don't really know what you have - there are plenty of different carbon steels that have different heat treatment regimes

Hardening is heat to cherry read and then quench in oil or water - depends on the steel - I'd use oil in the first instance

Then tempering is a slow longish heating at the temperature that is required for that particular steel. A colour will develop that will give you an indication of how this is going.
Look up tempering colour charts on the web.

Unless its a really really old blade - it could well be HSS...

One caution.... Asymmetrical shapes like to warp. A lot. Especially if you made a single bevel carving knife... These almost always go banana when heat treating. If you want a single bevel - grind the bevel after heat treating rather than before.

Anyway - i would give it a go.
Theres plenty of stuff out there on quenching and tempering steel. I would start by heating to cherry red/nonmagnetic and quenching in warm canola oil. Check for hard with a good file - the file will skate. If it doesnt take - try again except quench in brine (enough salt in the water to float a raw egg). Plain water causes a lot of problems with warping and cracking.

Then temper to your desired hardness.

Interestingly - most older carbon steel knives were tempered fairly soft (by todays standards) to make them much easier to sharpen as well as resistant to chipping when you make a twisting or levering cut. They ended up around Rc55 rather than up in the 60's.

Thnks Guys,
Yes it was a very old saw blade, no carbide teeth. It seems softer than an old hand saw plate when filing it. I guess I'll experiment with a heat treatment.

Cheers,

Originally Posted by Fuzzie

I guess I'll experiment with a heat treatment.

Let us know how it goes

Cheers

Doug

There are a couple of good articles on Ron Hock's site that cover the basics and variables of DIY heat treatment. Looks like I need to find some suitable long tongs, a can of peanut oil and a safe place to work first.

It wasn't meant to be a WIP, but no doubt this will take longer than I originally envisioned. Here's a pic of the current state of the blanks.
blanks.jpg

I just made my first attempt at heat treatment. In addition to the blade above I cut another strip to make a small blade to go into something more the shape Krenov shows in one of his books. I thought I may as well have a couple of pieces to test run.

My heat source was a Primus gas torch using a pinpoint jet. I do not think this really got the metal hot enough for the first stage hardening. The smaller blade got visibly red and held it there for a while, but the slightly larger blade above didn't seem to want to go truly red in daylight. This may simply be a problem doing things outside in the glare of daylight, but I suspect the heat just wasn't there. I wasn't sure how to do a nonmagnetic test without burning myself. I used Rice Bran oil to quench, that seemed to be the cheapest option in high flash point oils at Woolies.

I have now tempered the blades in the oven at 165C so we will see what happens next.

I guess I'm thinking I don't really want to fix the blades in handles if they wont hold an edge. I probably should sharpen them first and try them out although I'm not sure what to do as a proof test.

Franklin

I think you will battle to heat your blank sufficiently with that set up. You need at least the largest nozzle you can get for your gas torch. I normally use oxy acetelane with a no.15 nozzle for small jobs and a heating head for larger jobs. A forge of course is the preferred option.

Assuming your old saw blade is just high carbon steel, you need to heat to cherry red and quench rapidly in oil. As your knife is thin, use a large quantity of oil if possible and plunge the knife rapidly and vertically up and down in the oil using a different part of the oil. Do not stir with the blank: It will warp.

Clean the blade and reheat to a light straw colour ( this is why you have to clean the blade) and quench again. The straw colour comes up very quickly. Don't wait until you can be certain it is straw or it will be too late and start to turn blue. This is particularly so with a thin project like a knife.

Take precautions with the oil and have a large blanket or similar ready to to smother flames if the oil should catch fire. With a small job like yours it may not generate sufficient heat in the oil, but with large chisels or similar it often seems to happen. Not a big issue provided you are prepared for it.

If it goes pear shaped (not literally) just start again at the begining heating to cherry red.

Regards
Paul

When hardening - check with a fine file before you temper it. A file will skate on full hard steel. If it files - you didn't get it hard. Theres no sense going through all the additional gyrations of tempering if you didn't get it hard.

This will also allow you to validate your choice of a quench medium. It it didnt get hard in oil - try brine next.

165c is pretty cold for tempering assuming you got it full hard. Likely to be very chippy. Very likely to crack if you twist it or it gets any impact while carving (hitting a knot). Your tempering temperature for a useful knife is probably going to be somewhere in the range of 225-275C

if I was doing this.... Exploratory work...
Have a cup of warm oil, a cup of brine, a fine file, your torch, some vise grips, and a magnet all handy.

Torch to cherry red, test nonmagnetic with magnet and dunk straight in oil.
Fish it out and check with file. If it files - heat it back up, check nonmagnetic, then quench in brine. Check again with file. If its not hard - throw it away and find new steel.

If it is hard - proceed to tempering.

Originally Posted by truckjohn

If it is hard - proceed to quenching.

Proceed to TEMPERING?

Yes, good catch.

It appears I've opened a larger can of worms than I originally intended. If the current blades don't seem to work well I don't think I'll be in a position to rework the heat treatment at high temps.

Since the intent was a quickie bench knife, if I start again with an old hacksaw blade will I need to heat treat? Can I just cut and shape on a grinder?