Make a Plane Blade (DIY Heat-Treating)

Once you start making your own tools, you are going to need to learn how to heat treat tool-steel.  You can avoid this for a while by grinding and reshaping pre-hardened steel, or taking blades from other tools.  For instance, this chisel makes a pretty nice plane iron once you cut off the handle and do some reshaping. But if you make more than a handful of tools, it’s cheaper and more effective to harden your own blades.  It’s also not very difficult or expensive.

The process is simple:

1.)    Find a piece of tool-steal.

2.)    Grind and shape it close to its finished dimensions.

3.)    Heat it to the non-magnetic point.

4.)    Quench the hot steel in oil.

5.)    Temper the blade in a common oven.

6.)    Finish-shape and sharpen

Choosing Your Steel

There are dozens of different alloys that are good for knife and tool making, but most of these need specialized heat treating and they aren’t worth the trouble if you’re making a plane or a chisel.  You can make an excellent blade out of 1095 high-carbon steel.  This alloy is pretty much what Stanley and other manufacturers used in their tools for well over a century.  It’s nothing fancy, but it is cheap and dependable.

Another good choice is O1, which is a high-speed steel common in the metalworking industry. O1 is a complex alloy with several different ingredients.  It’s also a common choice among modern tool manufacturers who want to produce extra-hard blades.  Companies like Lie Nielson advertise their O1 blades, but you can work with this same steel in your own shop and produce blades of similar quality.

Both 1095 and O1 are easy to harden.  Heat treating other steels is like playing darts; off by just a little, and you don’t hit the bull’s eye.  Working with 1095 and O1 is more like throwing a grenade.  Get it close and you’re good.

Both of these metals are easy to find online from suppliers like Admiral Steel, Amazon and Ebay.  They’re not expensive.  I recently bought a 2”x 12” bar of 1095 and I paid $20.00, shipping included.  Always order more steel than you need.  You might mess up and need to start over.  Even if you don’t, you’re guaranteed to need more in the future, so you might as well just order it now.

Preparing the Blank

Most of the tool-steel you find online comes annealed, which is just a fancy way of saying “soft.” When it arrives, it will be similar to any low-carbon steel you’ve ever worked with.  You’ll be able to cut it with a hacksaw, grind it on a wheel, and shape it with a file.

Once you have your steal, cut it to length and grind the bevel close to the finished thickness while the steel is still soft and easy to work.  Don’t finish the bevel or make any part of the steel very thin.  Thin steal and fine edges will just burn up during heat treatment. 

Heating

For both of these steels, you need to get them to around 1400°, or little bit more.  The exact temperature doesn’t really matter since you’re not going to have the equipment to measure it anyway.  The handy thing about tool-steel is that its hardening temperature is right about where it loses magnetism.  So if you heat your steal to a nice cherry-red color and a magnet will no longer stick to it, then you are very close to your hardening temperature.

There are all kinds of forges and furnaces you can build for backyard heat treating.  You could use a charcoal grill and a blow-drier. Build a nice hot fire, heap up the coals, stick your blade in a blast it with air until it gets hot enough.

I like using a torch for heat-treating because it’s easy and the heat is concentrated.  I like the Bernzomatic MAP Gas Torch. MAP gas burns much hotter than plain propane, and gas cylinders only cost about $10. They also last a long time.  I’ve owned my MAP gas rig for over a year and I just emptied my first cylinder.  With a torch, there’s no fire to build or put out after you’re done.  For a small item like a chisel or a marking knife, you can just hold the steel in the flame until it’s hot enough and then quench.  

For a bigger blade like a plane iron, you’re going to need something to contain the heat.  This is where many people build or buy a small forge like the “paint can forge” you see all over the Internet.  You could also just pile up some soft fire breaks into a little igloo, stick a torch in the side, and heat up the interior.

I do my heat-treating inside a small length of heavy-gauge steel pipe.  The stuff I have is about ¼ inch thick and, crucially, is not galvanized or coated with any other mystery chemical that might cook off and kill me during the heat-treating process.  (If you don’t know what galvanized metal is, stop right now and read up on it.  Using galvanized metals in a forge will produce a gas that can literally kill you.)

 My forge is ugly as hell, but it works and it didn't cost a dime. 

My forge is ugly as hell, but it works and it didn't cost a dime. 

To make my forge, I just cut a little hole in the side of the pipe and then wired a piece of flat steel over one end.  This way, I stick the torch and the little hole, the blade in the big hole, and get heating.  This setup isn’t insulated in any way, so it doesn’t have the efficiency of a real forge, but it only takes a few minutes to heat up a plane iron, so the wasted fuel is pretty minor.

I stick the iron in and make sure the flame is pointed right at it.  It’s also helpful to have the torch pointed towards the back of the chamber and down at bit of an angle.  This way, the heat is aiming towards the closed off part of the chamber, where there will be less loss of energy.  I let the iron soak in the heat for a minute or so, and then turn it around.  As the steel begins to heat up, I turn it more often and move it around in the flame.  There is some danger of over-heating.  Tool-steal can be burned or melted if it gets too hot.  Over-heating can also cause some complicated metallurgical changes that I don’t understand, but I know they ain’t good.  So don’t just leave your steel in the direct flame for 10 minutes.  Move it around and keep the heat even.

You also don’t need to get the whole thing hot.  In fact, it’s better if the rear end of the blade remains soft so that it won’t chip when tapped with the adjustment hammer.  Just getting an inch or two of the business end up to critical temperature is more than enough.  Even many commercial irons are not hardened through the whole length.

Safety

This whole process is moderately dangerous.  I see people on the Internet treating it pretty casually, and it freaks me out because you can easily get burned or start a fire doing this.  If you can’t do your heat-treating in a dedicated metal-shop, then just do it outside.  Just doing it outdoors hugely reduces the risks. 

Obviously, don’t heat-treat around anything flammable.  Have a fire extinguisher handy.

You should at least wear safety glasses, but a full-face shield is even better.  Have a pair of thick welding gloves, or at least sturdy leather gloves.  Wear long sleeves and long pants made of natural fiber and wear leather shoes.  Synthetic fibers can melt and stick to your skin if they get hot.  This can cause a severe burn, where a natural fiber would just scorch or burn off. Hold your torch steady with something. I like to use a screw-clamp

Finally, handle your hot steel with the longest pair of pliers you can find.

 My complete hardening setup. 

My complete hardening setup. 

Quenching

 My iron right after the quench. Note the flaky scale. 

My iron right after the quench. Note the flaky scale. 

For both 1095 and O1, you want to quench in oil.  Motor oil is fine, but I typically go with canola oil since it releases fewer toxic gases when it gets heated up.  Your quenching oil shouldn’t be cold, so use your forge to test-heat a chunk of plain steal and quench that to get your oil warm.  Then, heat up your blade until it loses magnetism.  Both of the steels we’re discussing need to be a bit hotter than non-magnetic in order to harden properly.  I typically heat until I reach non-magnetic, then just stick the blade back in the forge for another minute.  It’s not very precise.

Once your steel is hot enough, pull your blade out and plunge it quickly but carefully into the quench oil.  Move the blade around in small circles or figure-eight patterns until the oil stops bubbling.  Be prepared for the oil to smoke and possibly burst into flames.  Neither of these is very dangerous, but keep your face and arm off to the side while quenching.  Once the oil stops bubbling, leave the iron in until it’s still warm but no longer hot.

Testing

A properly hardened blade is usually covered with a flaky, black scale.  This flaking generally indicates successful hardening.  Once the iron is cool enough to touch, scuff off all the scale and degrease the blade with mineral spirits or acetone.

A fully-hardened blade should be harder than a file, so gently rub a file along the edge of your blade.  It should skate off and make a high, glass-like sound.  If the file refuses to bite in, then your blade is hard.

You should now sand your blade with fine sandpaper.  You want the surface to be shiny and silver, especially close to the edge.

Tempering

Your hardened blade is now too hard and brittle to be usable.  You’ll need to soften it a little bit by heating it up and then cooling it slowly.  A common kitchen oven or toaster oven is fine for this sort of thing.

You can’t trust the oven’s internal thermometer, so buy an inexpensive oven thermometer like this one and stick it inside before you heat the oven.  Your oven will probably think it’s up to temperature long before it actually is.  Give it at least a ½ hour to warm up before you change the temperature setting.

Once your oven is up to temperature, put the iron inside and leave it there for 2 hours.  If your blade has been properly cleaned and degreased, then it won’t smell up the kitchen.  After 2 hours, turn the oven off, but leave the blade inside to cool as slowly as possible.  Remove the blade when you can touch it comfortably.

Tempering temperatures can vary depending on which steel you are using, and I’ve had to go through a bit of trial and error with different projects.  The important thing is that it’s very easy to make a hardened steel softer, but if you make it too soft, then it needs to be re-hardened all over again.  I often do my tempering more than once.  I start with a temperature that is probably a little bit too cool, test the blade and then temper again at a higher temperature, if necessary.

For my most recent plane iron, I heat-treated a piece of 1095 and tempered it at 400°.  The resulting blade was nice and hard, but I couldn’t get it to sharpen on my oil stones.  Too hard.

I re-tempered the same blade at 500°and found that it was extremely hard but still sharpenable with my normal methods.  Perfect.

 A standard color chart for tempering.

A standard color chart for tempering.

You can also use the surface of the blade to tell you when you’ve tempered correctly.  Clean, shiny steel will form an oxide layer as it’s heated.  You can use the color of this oxide layer to tell you when your tempering is correct. 

Most steels are correctly tempered when they reach a gold or “straw” color.  I tempered my most recent blade to straw, but found that too hard.  When I tempered again, it was more of a light blue color and this ended up being correct.  Like I said, a little bit of trial and error.

Overall, hardening your own steel at home is neither expensive nor very difficult.  You need to invest in a little bit of equipment, be conscious of safety, and be ready to do things over.  Unless you severely overheat your blade in the forge, any botched treating job can just a redone.  It won’t hurt the steel at all.

Rex Krueger