Thread: Heat treating very thin blades

I am making a bunch of dovetail marking knives, and again the steel is from jigsaw blades. These end up being about 1 - 1.5mm thick, 6mm wide and 46mm long after I have surface ground them (on the disk sander).

I think the steel is just HCS (your basic Makita, etc) as it will burn if treated roughly. In the past I have gound carefully, quenched frequently, and the steel has held an edge reasonably well.

It occured to me that I could just heat half the blade red hot and quench in water (after it has been shaped and honed). This will make the blade very hard and lock in the edge. I tried it this way and it certainly works. Of course the downside it that the steel could become quite brittle and vulnerable to damage. And, of course, this is the issue.

My feeling is that these little blades are not chisels or planes blades. They are not treated as roughly and not subjected to the same stresses. The blades do need to have some flex, and hardening only half still permits this to remain present.

The alternative is to temper the blades so that they are still hard but not brittle. But how do I determine the duration and the temperature for tempering?

What are your thoughts?

Regards from Perth

Derek

Sharpen an old gents saw like a knife blade. Hold it againsed the pin and tap the back with a small hammer. Drawing a knife back agaised the pin or in the socket it can wonder, tapping straight down gives a "perfect" mark. Best way to mark them

Lignum

This is one time I want an answer, not a sidetrack. Not the time or place to debate how to mark a dovetail. Sorry if I am abrupt. I need an response to my query fairly quickly.

Regards from Perth

Derek

If it were me, I'd heat them hot enough to make them hard, quench in oil or water. I'd suggest oil, but even swinging them through the air will prolly work since they are thin. Clean them up so you have clean metal to look at.

Next step is up for debate.

One way is to temper the whole blade. I aim for a light straw yellow (not light emitting colour) most of the time, but anything from straw to red/brown will be ok. Blue is spring temper, and will be plenty flexible, but not that hard.

Another way would be to rest the working edge in some water, and blue temper what isn't a working edge by waving the flame over what sticks out of the water. Maybe a block of plasticine in a small bowl might work, since if things get too hot, the blade falls in cooling instantly. This way you get a glass hard edge backed up a springy body.

Hardening just the working edge will ruin any temper in the rest of the blade, inviting bends where the blade meets the handle.

Time for tempering need only be a second or two. The blades are thin, and will get up to temp all the way through very quickly. Simply put, I don't think it's a concern.

Before you go to all this trouble however, take a piece of blade, clean it up with a file (not a good one) and take not of how well the file bites into the steel, or doesn't bite as the case may be. then wave it in front of a flame until the clean steel goes blue. Apply the file again. If the file bites into the steel much harder, then it should be easily heat treated. If the file still does not bite, then it's HSS and don't bother changing the stuff, grind and shape to your hearts content. An awful lot of jigsaw blades here are imported from Europe, and the bulk of them are HSS.

Originally Posted by derekcohen

Lignum

This is one time I want an answer, not a sidetrack. Not the time or place to debate how to mark a dovetail. Sorry if I am abrupt. I need an response to my query fairly quickly.

Regards from Perth

Derek

What side trak? You will get an answer from someone. Im just offering an opinion. Whats whe big deal.

Originally Posted by derekcohen

I am making a bunch of dovetail marking knives, and again the steel is from jigsaw blades. These end up being about 1 - 1.5mm thick, 6mm wide and 46mm long after I have surface ground them (on the disk sander).

I think the steel is just HCS (your basic Makita, etc) as it will burn if treated roughly. In the past I have gound carefully, quenched frequently, and the steel has held an edge reasonably well.

It occured to me that I could just heat half the blade red hot and quench in water (after it has been shaped and honed). This will make the blade very hard and lock in the edge. I tried it this way and it certainly works. Of course the downside it that the steel could become quite brittle and vulnerable to damage. And, of course, this is the issue.

My feeling is that these little blades are not chisels or planes blades. They are not treated as roughly and not subjected to the same stresses. The blades do need to have some flex, and hardening only half still permits this to remain present.

The alternative is to temper the blades so that they are still hard but not brittle. But how do I determine the duration and the temperature for tempering?

What are your thoughts?

Regards from Perth

Derek

To temper punches for steel dies we heated a piece of 3/4 plate steel supported by a couple of refractory bricks so you can heat the steel from underneath with an oxy torch ,get it nice and hot like almost red.Then we would place the punch on the heated steel on end and watch the colour rise up the punch to the cutting edge ,when it got to straw colour we flicked the punch off the plate into a container of oil .Its a more controled method than trying to heat the punch directly with a flame.Of course the punch needed to be polished after the initial hardening.
I would be more inclined to make small knives from old hss power hacksaw blades.
But they can be brittle too ,but they are much thicker.

hope this helps
Cheers
Kev.

Thanks Schtoo and Kev

Kev, the blades are deliberately very thin - they need to run inside a saw kerf. I have made a large number of these before, but not messed with the heat treating before.

I was wondering if I could heat treat in the kitchen oven. What I am unsure of is how roughly long I should expect to get to a straw colour at 220/250 C? I suspect that this would take about 5 minutes with the metal this thin (the chisels and plane blades I have done in the past took about 45 minutes). I guess, if I am wrong, I could just start again.

Regards from Perth

Derek

Originally Posted by Schtoo

Before you go to all this trouble however, take a piece of blade, clean it up with a file (not a good one) and take not of how well the file bites into the steel, or doesn't bite as the case may be. then wave it in front of a flame until the clean steel goes blue. Apply the file again. If the file bites into the steel much harder, then it should be easily heat treated. If the file still does not bite, then it's HSS and don't bother changing the stuff, grind and shape to your hearts content. An awful lot of jigsaw blades here are imported from Europe, and the bulk of them are HSS.

I've just made a marking knife from a used jigsaw blade I pulled out of the bin at work. It had been used (abused) until it had turned blue but still seemed to be quite hard (M2 HSS doesn't seem very interrested in scratching it). So as Schtoo suggests, there may not be any need to heat treat at all.

Other than the fact that heat (cherry red?) will not affect the temper of HSS, how does one determine if it is HSS or HCS? Can HSS go brown or blue?

Regards from Perth

Derek

ps Monday is a public holiday in Perth and I want to treat about 20 or so of this little buggers.

Look at the last part of Schtoo's post again.
What were the blades designed to cut? Steel cutting blades will probably be HSS or Bimetal. Wood/plastic cutters might only be HC steel.

Uhh, maybe 5-10 minutes in the oven, I'd set it at 230* since most ovens are pretty substandard for precision temp control. If it doesn't change colour right, raise the temp.

HSS can get even harder if raised to very high temperatures, turning all kinds of colours in the process. The tempering colours are formed when metal is heated with lots of oxygen around, and are often called 'oxide colours'. Naturally, if you heat up metal in air, it's going to change colour. What lies underneath the pretty colours determines what the metal is like, not the colours.

Handsaws are typically tempered to blue, or thereabouts. Simply take the blade to blue temper and try and file it. If it files, then chances are you should be able to heat treat it. If the file skips, don't even try to change it.

While you are fooling around with this stuff, I'd strongly suggest making up a tempering colour chart. Take a piece of steel, any kind will do. Clean off all crud/rust till you get shiny metal. Heat one end gently until it turns a deep blue/purple, then dunk in water. Dry it well and oil it, and you have a colour chart. What the colours mean is google-able, but you have the colours right there in front of you to compare to.

What Kev said about heating up your workpiece by sticking it on another bit of hot metal is good. The low rent version would be to put a piece of steel on the stove, heat it gently and place the items to be tempered on it, then rinse the blades under the nearby tap when done to hold that temper.

I'd think that if the blades were HSS, they would be marked as such. If no marking, then probably regular HCS and easily heat trreated.


Finally, don't let it worry you. If they can be heat treated, it's fairly easy to do. If not, then you won't be hurting the blades by trying it.

I'm pretty certain that the jigsaw blades are HCS. To begin with, I surface ground them on a 120 grit disk sander, then ground a bevel on a 240 grit belt sander. Finally, I was able to hone the bevel on a extra fine diamond stone. I don't think that HSS would work this easily.

I did experiment with a blade or two (each jigsaw blade produces two marking knife blades). I heated the edge cherry red (butane torch) and quenched in water. The colour did not spread more than a third way down the blade. I was able now to pare hard end grain cherry without folding over an edge. The "stalk" of the blade did not appear to lose its spring.

Here is an example of the marking knife (and an awl, for which I am also making blades). This is an example ffrom several months ago, but I am making others identical to this.



Regards from Perth

Derek

Why would you want to treat the blades just for little marking knives that will only lightly mark some timber and not be used in anger.

Hi Lignum

First off, my apologies for being testy earlier - I have tomorrow free and impatient to get these marking knives and awls done.

The reason for heat treating the cutting edges of the blades is that this will make life easier for all as the knives will need less sharpening. Slicing hard end grain is tough on blades. The knives are actually great for marking detail work as well, so I expect that most will use them as general purpose marking knives, and not just for dovetails.

Incidentally, your earlier suggestion of a saw blade to mark dovetails is a good one. It is one I learned from Tage Frid some years ago. I still use it. In the picture below, on the far left, is a chisel-like tool. I build this to hammer into saw kerfs for this purpose, and for half-blind dovetails.

Regards from Perth

Derek

Derek nice work there mate looks good, dont bother using your oven to heat steel up its a waste of time, to get a straw yellow colour you need at least 250 deg c, dull red 500 deg c, cherry red 900 deg c. Tempering a steel is usually below 500 deg c hardening of steel is 500 dg and over. I used to be a butcher so Ive sharpened a ???? load of knives and pieces of steel. The trick to keeping a neat sharp edge has nothing to with the type and quality of the case hardening but more to do with the material used. Jigsaw blades are an excellent idea. If you want to remove the flex from the blade then yes I would heat treat it and quench in oil, but as far as the cutting edge you dont need to do a thing except put a decent angle on it and in your case Id use 30 degrees for lasting capability 15 deg if you want it sharp sharp but it wont last as long
Steve