Notes on Heat Treatment of Carbon Steel

[Phil White]

Notes on Heat Treatment of Carbon Steel tools

 

When I first started carving seriously, I was short of cash, and in need of tools. I began to make my own, and with the help of a local gunsmith, who I got to know, I began to learn a few things about tool making. This took me down the separate, but parallel path of metalworking.

 

Most of following notes on heat treatment of carbon tool steel are a collection of musings that have been passed along to me by three older gunsmiths and blacksmiths that I have known, befriended, or otherwise worked with in the museum world. The most significant of which was a Norwegian master that I informally apprenticed with, who worked exclusively with the bare basics of tools.

 

All of these master craftsmen were trained to work by hand, and by eye, in a very low-tech manner. Their milling machine was a hammer, a good file, and maybe a good cold chisel.

 

The rest I have picked up on my own, from putting these practices to use.

 

These are the traditional techniques that have been used by edged toolmakers and gunsmiths for hundreds of years, before any thought was given to crystal size, temperature gauges, or modern alloys. Although some readers may have other preferred techniques for making tools, and may prefer a more measured or technical approach, I know that these simple materials and methods work for me.

 

Carbon Steels:

 

Carbon steel is generally considered to be an alloy of iron and carbon, with other elements occasionally thrown in for various reasons, such as to improve tensile strength or wear resistance. The percentage of carbon can very between about 0.1 percent to about 1.2 percent The lower end of this range is considered to be mild steel, such as “hot rolled steel” or “cold rolled steel”, and can not be hardened by heating and quenching, and is therefore useless for making tools.

 

There are a large range of steels available, but the basic carbon steels are identified by a four-digit code that indicated the percentage of carbon. For example 1070 steel has 0.7% carbon, 1080 steel has 0.8% carbon, 1095 steel has 0.95% carbon, etc.

 

Generally, the more carbon in the steel, the higher the potential for hardness, wear resistance and edge-holding ability. However, with this comes the potential for brittleness. The lower the carbon, the tougher the steel becomes, with more ability to absorb shock and torque, but it is less likely to hold a fine edge over time. For example, High carbon steels can be found in files and engraving tools, where wear resistance is important. Medium carbon steels can be found on edge tools, where both wear resistance and flexibility are required. Low carbon steels can be found in tools where shock absorption is important, such as axes and hammers.

 

 

Tools that have been made from medium to low carbon steels

 

 

Tools that have been made from 1095 steel

 

Steel “drill rod” or “silver steel” is a tough high quality tool steel that can be hardened and tempered easily, and will hold a very superior edge. It is used where a hard and tough steel are required, such as making taps or drills, or for small chisels and gouges. It is available in round stock, polished in various diameters from 1/16” on up. It comes annealed and can be filed, ground, or machined with conventional tools. It can be a bit hard to work though, but with patience it will make an excellent graver or fine chisel.

 

 

Tools that have been made from drill rod

 

Gunsmith’s suppliers are one of the best places to pick up small quantities of carbon tool steel, as gunsmiths are frequently required to make small tools and parts that are not easily available. Brownells at: www.brownells.com is one of the best, and sells carbon spring steel in small quantities of both flat bar stock and round rod. Another source is Metals Supermarket. They are online at: www.metalsupermarkets.com is also very good. If you happen to have one nearby, they also sell metals such as bronze sheet. Drill rod is available from Metals Supermarket, as well as many hardware stores. I believe I have seen it in the metals rack at Home Depot.

 

Steel can also be obtained through recycling old files, scrapers, hay rake teeth, very old leaf springs, or any source where similar steel was once used. On of my favorite recycled steels comes from worn out files.

 

 

Tools that have been made from old files

 

Shaping

 

Shaping of the steel should be done before hardening and tempering. It is beyond the scope of this tutorial to get into the methods of working steel, however, it will suffice to say that carbon steels can be sawn, ground, filed, machined, forged, or any combination thereof. Most tool steels are purchased soft, or annealed, unless otherwise noted. If using recycled steel such as a file, the steel will have to be softened before it can be machined of filed to shape.

 

Annealing can be done by heating the steel to a red heat and allowing it to cool slowly in ashes. Putting the file into a fireplace with a good wood fire, and leaving it to cool with the ashes after the fire has gone out will usually do the trick. If hard spots are encountered while working the steel, heating to a very dull red and burying in ash for a couple of hours will usually soften them.

Heat source:

 

Any heat source that will bring the steel up to a bright red or “cherry red” will do. For small tools under ¼ in I use a propane brazing torch, just like the ones that you buy in a hardware store in the plumbing section. An oxy acetylene torch works well also, or a coal or propane forge. I have not specified a tempering oven, because if you have one, you know what you are doing anyway.

 

In the following photos, a forge has been used as a heat source.

 

 

The forge

 

 

The forge fire

 

Heat Treatment (Hardening and Tempering)

 

There are two basic procedures for creating a lasting edge or surface on a steel tool: hardening and tempering.

 

During the hardening process, a piece of steel is heated to temperature that allows the carbon (iron carbide) in the steel to come out and blend with the iron in a grainy crystalline structure. While at this heat (known as the eutectic) the steel is cooled very quickly by quenching, which freezes the crystalline structure in place, resulting in a very hard material.

 

In the following photos, two pieces of steel, a round bar of mild steel and a hexagonal bar of medium carbon steel (1070) are heated to a “cherry red” heat and quenched in water. The two pieces are then tested for hardness with a file to make sure that they are hard. The file skates across the hexagonal piece without biting in, while the round piece is easily cut. The hexagonal bar has also turned grey, indicating physical changes have taken place.

 

With the range of steels previously discussed, that heat can be judged by it’s color, which is a bright glowing red, like a maraschino cherry. This color was known to the old toolmakers as “cherry red” This is putting it very simply, of course, but for the home toolmaker, that is all we really need to know.

 

 

Mild steel and carbon steel heated to cherry red

 

 

Quenching the two steels for hardness comparison

 

 

Testing the two steels with file

 

 

Results of test

 

Ideally, the steel should just be heated enough that the carbon goes into solution, and no more. This will produce a fine in the steel, which is very hard.

 

To illustrate this, the steel is hit with a hammer, breaking off a small piece, and revealing a fine grain structure.

 

 

Brittleness test with hammer

 

 

Close up of break

 

It should be noted that the higher the steel is heated, the larger the grains in the crystal structure get, and the more brittle the steel will become when quenched. The following photos show the same piece of hexagonal steel heated beyond the ideal temperature to a bright yellow heat, nearly melting the surface. The steel is quenched in water, and scored every ½ inch, then broken off in sections to show the grain structure. At the tip, the steel is so weak that only a light tap is necessary to break off a piece. Towards the left, where it was relatively cool when quenched, it will not break at all, indicating that it has not been heated enough to harden.

 

 

Returning rod to fire to heat higher

 

 

Results on grain at yellow heat quench

 

 

Close-up of grain where toughest

 

Understanding how grain size affects the process is important, in that if a finished tool breaks, and the grain is coarse, as in the previous photos, you can be relatively sure that the steel was quenched too hot and adjust your quenching temperature accordingly.

 

After quenching, the steel will be very hard, often referred to as “glass hard” but very brittle. Some of the brittleness must be taken out before the tool before it can be used for its intended purpose. This is done by re-heating, or “tempering” the piece, just a little, to break down the crystals enough to restore some strength.

 

The steel is usually cleaned of any scale or soot, to get down to the bare metal, and slowly heated until a color appears on the surface. The colors will appear in the order of: pale yellow (pale straw), yellow (straw), brown, purple, dark purple, bright blue, pale blue. The metal is then quenched again to stop the heating process.

 

The more the tool is heated, the more flexible it will become. The less it is heated, the more hardness is retained. Edged tools that are not tempered enough may chip at the edges too easily or even break. If heated too much, they mat bend or not hold an edge. Balance of hardness and flexibility are a personal preference. I prefer the tool to be on the hard side, so it will retain an edge, but I have to be careful not to pry.

 

The following photo shows a piece of polished drill rod that has been heated from the left to allow the temper colors to creep up the rod. The hotter end shows a pale blue, while the cooler end next to the polish shows a pale yellow.

 

 

Temper colors on polished steel

 

The following chart was taken from Appleton’s Cyclopedia of Applied Mechanics, and accurately shows the temper colors as they apply to various types of tools.

 

 

Quenching medium:

 

The role of a quenching medium is to cool the metal quickly, freezing the crystal structure that is formed at high heats. There are two basic quenching mediums: water and oil. Both have different effects. Water cools steel very fast, and is ideal for maximum hardness, such as small hammer faces, engraving chisels. Oil cools more slowly, and is ideal for toughness, such as knives, carving tools, and springs. Some tool steels are sold as water hardening or oil hardening, which implies the medium that they should be quenched in. For example, an oil-hardening steel may become too brittle, or even crack from the shock of being quenched in water. Conversely, a water-hardening steel will not become completely hard if quenched in oil.

 

One note of caution when quenching is that thinly shaped edges may deform or even crack from the shock of being quenched. Therefore, it is ideal to leave a bit of thickness at the edge when shaping an edged tool, to avoid problems when hardening it. Keep in mind Joseph Moxon’s saying from the 17th century “If you will a good edge win, temper thick and then grind thin”

 

There are other options that can be used as well, such as wax or salt water, but I will stick to water for now.

 

 

Case Hardening

 

I have included this section for the purposes of explaining the term “case hardening”, rather than for any real practical purposes, although I have used the technique from time to time. The technique is not used on carbon steel.

 

Case hardening is a method of hardening a mild steel or iron surface to improve wear resistance. During this process, the iron is heated to the same temperatures as you would with a carbon steel, but in close contact with a carbon source. During the heating, the iron absorbs some of the carbon into the surface, effectively turning it into steel.

 

Traditionally, the iron parts that were to be hardened were placed in a crucible, and then covered with a carbon-rich material. The composition of this material varied, but usually consisted of ground charcoal and ground bone.

 

 

 

Burying iron parts in a crucible

 

The top of the crucible was then covered and sealed to keep oxygen out. This is an essential step. Without oxygen, the carbon will absorb into the surface of the iron, forming a layer of iron carbide on the surface.

 

 

Covering the lid of thee crucible and heating

 

The crucible was then heated for a period of time to allow the iron to migrate into the iron surface. The amount of time depended on the size of the piece, but ½ to 1 hour was common. The pieces were then dumped into water, straight out of the crucible. This created a hardened steel surface on the piece that is very resistant to wear. This technique was commonly used where moving parts were bearing against each other, such as in flintlock or percussion lock mechanisms on firearms. Similar results can be produced by heating the iron part red hao, sprinkling with 'Casenite" powder, re-heating to red, then quenching in water.

 

This flintlock was made by me using mild steel, which was case hardened, 1095 steel for the springs, and drill rod for the internal working parts. The following photos show the mottled colors of case hardening on the larger pieces, the pale blue on the springs where flexibility is necessary, and the purple or brown on the internal parts, where wear resistance is important.

 

 

Flintlock showing case hardening colors and temper colors of various carbon steels

 

With a little experimentation on a couple of pieces of steel, a feeling for these techniques will be achieved very quickly. By following these methods tools can be custom-made for special purposes, and they will hold an edge that is superior to any tools purchased off the shelf.

 

 

 

Phil

[wunderlich]

Hi Phil,

 

this is a really impressive tutorial. These old methods are rarely seen on the web. Thanks for sharing.

 

Some days ago I found in our library in Berlin in an old book (1844) on jewellers methods.

 

To harden small tools like drills and related stuff a white onion was used. The steel parts are heated to a cherry red glow and pushed into the onion. This should give a very good hardness.

 

I did not checked this recipe out, yet. But it´s well worth to try.

 

regards,

Karl

[toscano]

Super!

Thanks Phil for this wealth of information.

Very much appreciated.

 

-t

[magnus homestead]

Hello Phil,

 

This is a very nicely done tutorial on a subject important to all carvers really. I have been making my own tools for quite a while but have never been that fluent on the hardening aspect - I have gotten by but I think I might improve my technique considerably by following your information!

Thanks so much,

Magnus

[Guest ford hallam]

Hello Phil,

 

what can I say?, that is a beautiful account of heat treatment and the requirements of our tools. I can't speak for non-metal bashers but I loved seeing these images, very nicely done. As you said, there are more technical approaches, but when it comes down to it, I think we need to keep it simple and concentrate on actually putting our tools to sensitive use.

 

thanks and cheers,

 

Ford

[Phil White]

Thanks for your comments guys.

 

Karl, I have tried a few alternate materials for quenching, such as wax, and linseed oil, with interesting results, but have never tried vegetables before. I have seen other references to onions before, so someone must have found that it worked for them.

 

Phil

[Karl Carvalho]

Aloha Phil,

 

Many thanks for your lucid and to the point explanation of heat treatment. I liked the part about progressive crystal formation with pictures of sections illustrating the point. I am probably guilty of overheating, but it is easy to do with small tools.

Question: I have been told that overheating to anneal can "burnout" carbon content. True? Will casenite replace it; only at the surface?

To add a bit, I have been buying Kasenit from Blue Ridge Machinery. They also have a nice little trade text (#1 - part of a series) with color temperature charts and easy to understand info.

 

 

Karl,

We have a lot of Filipino ex-pats here who do landscape work. They make their own wicked looking tools out of leaf springs and the like. They swear that the only way to quench is to plunge red hot steel into a large squash.

 

(another ) Karl

[Dick Bonham]

Phil,

What a great tutorial on tool making. I learned a great deal. Did you make the flintlock? That is a beautiful lock.! Having hand made a few I know how much work they are to create. It even has a water proof pan. Was it forged or cast?

Thank you again,

Dick

[Janel]

Thank you very much Phil! Thoughtfully written, and very nicely presented!

 

Janel

[Guest ford hallam]

those comments about quenching tools by pushing them into onions or squash reminds me of an account I once read of how Roman sword-smiths allegedly quenched theirs by plunging their red hot blades into unfortunate slaves. I'm certain that this is comes from the realm of myth though, as it seems a terrible waste of a slave!

[Phil White]

Thank you all for your comments!

 

Karl,

 

I suppose that overheating for annealing could burn some carbon out, but since it is mostly locked into the crystal structure, this would take a lot of overheating. When I first started making tools, I was trying to make a small chisel for carving bone from a file. It worked at first,but I wasn't satisfied with the temper. I heated and reheated it several times, and eventually ruined it. I was likely removing carbon to the point where it would not harden properly any more. Kasenit (or Casenite) will only add carbon into the surface for a few thousandths of an inch.

 

Dick,

 

I did make the flintlock. The parts were made from rough bar stock by forging sawing and filing. No machinery was used, and no casting. All hand work. It took 3 1/2 weeks to make, 8 to 10 hours a day.

 

Ford,

 

Have you ever read the 11th century treatise on metal working by Theophilus "De Diversis Artibus", or the Various Arts? It is a wonderful account of medieval metalworking technology. Mostly good practical stuff, with a few wildly fantastic accounts for turning copper into gold, etc. He advocates, in several situations using the urine of a small red haired boy, and the blood of a red haired man, as a quenching medium.

 

These quenching mediums are actually sound advice for cooling the steel quickly, but not as quickly as water(although, I don't see the importance of the red hair, and don't advocate the use of bodily fluids)... Water will often cool tools too quickly, and it is sometimes preferable to slow it down a bit, particularly for delicate tools. I usually use oil for this purpose, particularly for small thin knives. This is something that I should add to the tutorial.

 

Any and all feedback is greatly appreciated.

 

Phil

[Dick Bonham]

Phil,

What an amazing task to forge a flint lock. The pieces I have made have all been miniature (quarter to one third scale) A fraction of the work required to make a full size piece. I have watched "The Gunsmith of Williamsburg" about 20 times and every time I marvel at the skill required. And your piece has the addition of a water proof pan and a safety. Your work is the quality of the late 18th early 19th century English gunmakers.

Dick

[Phil White]

Thanks Dick,

 

Much appreciated. Although I have forged all of the parts that can be found on a flintlock, I must admit that the forging on this piece was limited to the springs, the frizzen, and the internal parts. The rest is filing and sawing.

 

I have had the opportunity to meet and/or work with a number of very talented people, including one of the gunsmiths from Williamsburg. I assisted him in a barrel-forging demonstration in Kempton Pennsylvania for a couple of days.

 

Phil

[Guest ford hallam]

Hi Phil,

 

yes, I read Theophilus many years ago, I seem to remember the use of those odd quenchants.

If I remember correctly he misses out the need for quickening the surface of the metal prior too fire gilding, something I cursed the mad monk for at the time.

From what I can gather you don't believe his prescription for turning copper into gold will work then , damn!

 

cheers, Ford

 

p.s. the work in creating that flint lock is impressive. Very tidy, I bet it's operation is a joy too.

[wunderlich]

Hi Ford,

 

this old monk wrote for a small elite. He could expect some basic knowlegde from his readers. Even in later centurys authours left some important things out to keep things a little arcane or enhance the readers imagination.

 

The trickiest part in gold-making is to catch a Basilisk. These little reptiles are rather quick...

 

Karl

[Phil White]

That, and watching the basilisk while it sits on the nest.

 

Most of his method are sound though. Interestingly, there are no references to tempering, only hardening. THere are aa lot of variations on the case hardening theme as well. It seems that the concept of tempering came about a little later, hench the need to have various quenching solutions to create different effects.

 

I'm afraid I've never tried fire gilding. Always been afraid of going mad from the fumes.

 

Phil

[Guest ford hallam]

Hello Karl,

 

I'd always fancied that I was one of those elite!

 

As for basilisks, I keep an ever- ready eye open for the little buggers.

 

Hi Phil,

 

interesting that he had nothing to say about tempering, I generally don't bother either. I quench only the tip of the chisel in water and withdraw as soon as the red colour is gone. I rely on the residual heat in the tool to temper the tip, it seems to work for me.

 

You may have a point about the mercury fumes though, but for me it would be hard to establish a "before and after" moment.

 

Ford

[Phil White]

Hi Ford,

 

This technique of hardening and tempering in one heat works well also. I thought about mentioning it, but wanted to keep things simple. Perhaps I should add it though. This is an excellent way to maximize the hardness of the tip, while retaining strength behind the cutting edge. It is often used for stone carving tools and cold chisels. The only drawback is that as you use up the cutting edge and grind back, the metal gets softer, and must be retempered. Not really a problem for stone carving tools, or chisels for cutting metal, but difficult if part of your tool is buried in a wooden handle. I have also noticed a tendancy for long thin carving tools to bend if tempered this way.

 

I usually do a slight variation on wood carving tools and engraving tools, in that I harden the majority of the tool, then temper by applying a torch a couple of inches or so behind the cutting edge and allow the color to creap up to the edge. The edge is usually a brownish yellow, while purple behind, fading to blue towards the tang. The same technique can be used with small knives. Really small tools can be heated on top of a steel plate, with a torch applied from behind, allowing the heat to creep through the plate into the tool.

 

As for the fumes, I have no "before" to compare to.

 

Phil

[wunderlich]

Hi Ford,

the mad monk didn`t forgot us . He had given a recipe for a solution for quickening the surface of the metal. He used a watery solution of common salt and cream of tartar. He gave this into the vessel with some mercury and heated it on a charcoal fire. (chapter 38.)

 

Hi Phil,

how do you harden very small cold chisels (tip size smaller than 1mm)? I always burned the tip of the tool while heating- using a jewellers blow-pipe-burner.

Or the tip becomes hard but the tool bends after a few hammer hits. Now I am using old files and grind them into form. But this has drawbacks.

 

Karl

[Guest ford hallam]

Hello Karl,

 

that cleans the surface of the metal but doesn't actually help the amalgam to "wet" the surface. For that you must use a mercury/acid solution. It's preparation is described in Oppi Untracht's book on Jewellery. I'd suggest starting a new thread so we don't hijack Phils excellent tutorial but fire gilding is probably best not encouraged too much.

 

Cheers, Ford

[wunderlich]

I'd suggest starting a new thread so we don't hijack Phils excellent tutorial but fire gilding is probably best not encouraged too much.

 

Cheers, Ford

 

Hi Ford,

 

you`re quite right with this. I for myself never tried fire gilding (was also afraid of the toxic potential of mercury)- is was just a kind of historical concern. But other gilding methods are always of interest.

 

Karl

[Phil White]

Hi Karl,

 

It sounds as though you are either getting the metal too hot when hardening, or not tempering enough after.

 

I usually use a propane torch on a low flame, running quickly up and down the length. The propane torch doesn't heat the metal too quickly, and gives you more control. I have also used an oxy-acetylene torch, but it takes a little more care and flame control. When I get an even bright red color, I quench. I also keep the container for the quenching liquid close. To temper, I clean all of the area that has been heated, and run the torch quickly over the surface again, taking care not to heat the tip too much. I try to get a purple to bright blue color, with a yellow near the tip, then quench again. This makes the chisel tough along the length, but herd at the tip. If the tool is still too brittle at the tip, it can be heated again to soften it a bit.

 

I have played around quite a bit with tools for engraving steel, and I find that the tempering fine tools for metal is the most difficult. I have become familiar with drill rod, which is why I use it for all chisels for metal, though I have used old files with success.

 

In the photo of the tools that are made from drill rod, there are a few engraving chisels, and one chisel for wood that is less than 1mm. These were all done this way. I have made a couple of small chisels of this size that will cut mild steel without any problems, and retain an edge that doesn't chip or dull quickly.

 

Phil

[Karl Carvalho]

Aloha Karl,

 

Phil's advice all pertains, but I found that I had to keep the water really close. My teacher showed me a simple technique recently, during a tool making session for 20 gauge sen zogan. I held the chisel using vicegrips about six inches directly over the water (onion, squash), keeping the flame away from the tip. Hardening was relatively easy. Tempering was definitely not. I am quick, but it took 3 or 4 tries to travel that distance in a second or so to capture the straw color. I started to plunge almost before seeing it. For me, polishing to clean metal before tempering really helped. That and a lot of practice.

 

Karl

 

[Phil White]

Karl,

 

This is an excellent photo. It shows the temper colors exactly as I try to achieve with this type of tool. You are quite right about the water being very close, particularly with very small tools. Regarding the removal of the scale, I usually polish the metal after shaping, and before hardening, while it is still soft. This makes the removal of the scale after hardening much faster.

 

Phil

[wunderlich]

Thanks guys,

 

this helps a lot.

This is exactly the type of tool I meant. Hope I will get it now.

 

Karl