A Woodworker’s Guide to Tool Steel and Heat Treating

howarth cast steel logoEver wonder by what magic steel can be made either hard or soft? Me too. After hardening my first plane iron, I decided to find out. This page is the result of my research. While there are dozens of different tool steel formulations, the only one with which I have any experience is O-1 Oil Hardening Tool Steel. I use O-1 for the following reasons:

In the following discussions, the terms "steel", "tool steel", and "carbon steel" should be understood as referring to O-1. While the physical changes and phase relationships in heat treating are substantially the same for all tool steels, the temperatures required (and other physical properties) vary widely according to composition. Note also that other fluids (water, air) besides oil are used to quench different tool steels, and tempering procedures also differ.

Chemical Composition of O-1 Tool Steel Physical Properties of O-1 Tool Steel
Iron 97.1% Hardening (°F) 1450 - 1500
Carbon 0.90% (°C) 788 - 816
Chromium 0.50% Tempering (°F) 350 - 550
Manganese 1.00% (°C) 177 - 228
Tungsten 0.50% Hardness Range (Rc) 64 - 58

Definitions:

Annealing - Softening the tool steel for working, by heating to the hardening temperature and cooling slowly. Slow cooling can be accomplished by burying the steel in an insulating medium such as lime or vermiculite and allowing it to cool to room temperature.

Hardening - Heating the steel to the hardening temperature and cooling suddenly by quenching in an oil bath.

Tempering - Reheating the hardened steel to the tempering temperature in order to relieve stress induced in the hardening process, and remove some of the hardness in exchange for toughness. Untempered, hardened tool steel is nearly as brittle as glass.

What Happens During Heat Treating of Tool Steel?

Carbon and Iron exist together in several different phases, depending on Carbon percentage and temperature. A Fe-C phase diagram shows these relationships.
Fe-C Phase Diagram Simplified Fe-C Phase Diagram (Steel Portion)
Four phases are important for our discussion: Note that the diagram shown is only for the steel portion of the system. For carbon contents of 2 - 6.67%, the alloy is cast iron. Above 6.67% carbon, the alloy consists of cementite and graphite.

An alloy consisting of exactly 0.76% Carbon and 99.24% Fe has the lowest temperature at which the conversion from ferrite and cementite to austentite is complete. This is known as a eutectoid steel. Increasing Carbon above this amount, as well as the addition of other alloying elements, also increases the temperature of complete phase transformation (i.e., hardening).

Fully annealed carbon steel consists, in addition to impurities and other alloyed elements, of a mechanical mixture of iron and iron carbide. The iron takes the crystalline form ferrite, and the iron carbide takes the crystalline form cementite. The overall structure consists of bands of these two components and is known as pearlite. In this state the steel is soft and workable.

As the steel is heated above the critical temperature, about 1335°F (724°C), it undergoes a phase change, recrystallizing as austenite. Continued heating to the hardening temperature, 1450-1500°F (788-843°C) ensures complete conversion to austenite. At this point the steel is no longer magnetic, and its color is cherry-red.

If the austenitic steel is cooled slowly (the process known as annealing), it will return to the pearlite structure. If, however, it is cooled suddenly by quenching in a bath of oil, a new crystal structure, martensite, is formed. Martensite is characterized by an angular needle-like structure and very high hardness.

While martensitic steel is extremely hard, it is also extremely brittle and will break, chip, and crumble with the slightest shock. Furthermore, internal stresses remain in the tool from the sudden quenching; these will also facilitate breakage of the tool. Tempering relieves these stresses and causes partial decomposition of the martensite into ferrite and cementite. The amount of this partial phase change is controlled by the tempering temperature. The tempered steel is not as hard as pure martensite, but is much tougher.

Effects of alloying elements on tool steel properties:

Including these elements in varying combinations can act synergistically, increasing the effects of using them alone.

Types of tool steel other than O-1:

The A and D series (A-2, D-2, etc) contain more chromium and are thus more wear resistant. The S series contain more silicon and are thus more shock resistant. The M and T series contain either more Molybdenum or Tungsten, and so are high-speed steels, with much greater hot strength. All of these require substantially higher temperatures for hardening than O-1, and are not really suitable for home-shop hardening.

Why treat tool steel cryogenically?

In some alloy tool steels, significant amounts of austenite are not converted to martensite on quenching. This is particularly true of A-2 and D-2; these steels are chosen for edge tools for their higher wear resistance, which is lessened by the retained austenite. Because austenite is not stable at room temperatures, it gradually converts to martensite over a period of time - but we're talking years, a little too long to be practical. So supercooling the quenched, tempered, steel speeds the conversion to a matter of hours. The tool is then retempered after the cryogenic treatment, and studies show that this results in substantially increased wear resistance. Because the martensitic transformation is more complete in O-1, cryogenic treatment is not necessary for that tool steel.

Tool Steel Color vs Temperature

2000°F Bright yellow 1093°C
1900°F Dark yellow 1038°C
1800°F Orange yellow 982°C
1700°F Orange 927°C
1600°F Orange red 871°C
1500°F Bright red 816°C
1400°F Red 760°C
1300°F Medium red 704°C
1200°F Dull red 649°C
1100°F Slight red 593°C
1000°F Very slight red, mostly grey 538°C
0800°F Dark grey 427°C
0575°F Blue 302°C
0540°F Dark Purple 282°C
0520°F Purple 271°C
0500°F Brown/Purple 260°C
0480°F Brown 249°C
0465°F Dark Straw 241°C
0445°F Light Straw 229°C
0390°F Faint Straw 199°C

Steel exhibits different colors depending on temperature. Temperatures above 800°F (427°C) produce incandescent colors; the atoms in the steel are so energized by heat that they give off photons. Temperatures below 800°F (427°C) produce oxidation colors. As the steel is heated, an oxide layer forms on the surface; its thickness (and thus the interference color as light is reflected) is a function of temperature. These colors may be used in tempering tool steel.

If colors are a problem:

It's not always practical to use color to determine temperature. Five to ten percent of the male population are color-blind; further, colors of hot steel are much harder to judge in the sun if you do your heat treating outdoors, which you should unless you have a ventilation hood and chimney in your shop. Tempering can be done in an oven with an accurate thermometer. For hardening temperature, there are several solutions:

Magnetism: Remember that, at the critical temperature, when the phase change to austenite begins, the steel will become non-magnetic.

Pyrometers: While good pyrometers are expensive, a type-K thermocouple can be purchased for a few bucks at a glass-making or ceramics supply shop. Plug it into a digital multimeter, download a millivolt to temperature chart for the thermocouple, and you're all set. I've done this, passing the thermocouple through a small hole in the side of my gas forge, and it seems pretty accurate. Some online resources:

Tempilstiks: Tempilstiks are color-coded crayons which are guaranteed to melt within 1% of their rated temperature. Available in a wide range of temperatures up to 2500°F, they can be purchased at blacksmith/forge supply dealers (online at Centaur Forge).

Guide to Hardening and Tempering O-1 Tool Steel

Start with annealed steel. At this stage the steel is soft enough to work with a file. Do all of your shaping now. If you're making an edge tool, however, don't grind a sharp edge yet - stop just short of sharp, leaving it blunt.

small gas forge fired with self-aspirated burner Small gas forge with self-aspirated burner
(A sharp edge during heat treating will introduce unwanted stresses into the tool.)

Using an appropriate heat source (or Building the Reil Burner), heat the steel to the critical temperature. How do you know when you reach the critical temperature? Austenite, the iron/carbon crystal structure that forms above the critical temperature, is non-magnetic. I keep an old magnet held in a pair of vice-grips handy when hardening. When the steel is hot enough, the magnet won't stick. At this point, the steel is cherry red.

red hot steel The image actually shows dark yellow approaching bright yellow; the cherry red can be seen further down the blade.
Now, remove the steel from the heat and immediately quench in oil. Any kind of oil will suffice; I've quenched with used motor oil, but now prefer cheap vegetable oil in a metal 5 gallon pail. (I'd rather be thinking of french fries than an oil-burning motor.) When you plunge the red-hot steel into the oil, do it vertically - if you plunge it in at an angle, it will warp. Agitate it carefully in the oil, in an up-and-down motion; a stirring motion may also cause warping. It's
quenching the steel Although it looks like I'm going in at an angle in the picture, the tongs are gripping the tool at the same angle, and the actually motion of the tool, and my arm, is vertical.
important to keep it moving to replenish the oil on the surface of the steel; otherwise a vapor layer will form resulting in a slower than desirable quench. If the quench is too slow, the tool won't be hard enough. Keep the steel in the oil until the oil stops bubbling.

As soon as the steel is cool enough to handle, wipe it off and test its hardness. If you've done right so far, a file won't bite - it'll just skate off the edge of the tool. If it's plenty hard enough, it's time to temper; it's important to temper as soon as possible after the quench. You can just put the tool in the oven if you trust its temperature setting (maybe a decent thermometer would be a good investment), or you can temper the way smiths do - by heating the tool until it reaches the right color. In order to see oxidation colors, you'll have to shine up the tool on some coarse emory paper. We're not talking mirror finish here - just enough to expose the bare metal (maybe up to about 220 grit). Now, using an appropriate heat source, carefully heat the tool from the non-business edge. The idea is to soften toward the cutting edge, so the cutting edge will be harder than the other end of the tool. For example, a knife would be harder at its edge than along its back - the back would be tempered more to give it flexibility. As the tool heats up, the first color you should see is a faint straw color. Keep heating and allow this color to spread toward the cutting edge. Just as it reaches the cutting edge, plunge the tool into some water to prevent it from tempering too much. You're done if the tool is a plane iron or chisel - all you have to do now is flatten the back and sharpen it. For a tool that needs more toughness (less brittleness), like maybe a cold chisel, you should temper a little higher. For real flexibility, like a spring, go all the way to blue.

Hardness vs Tempering Temperature

Tempering Temperature
°F
Tempering Temperature
°C
Approximate Hardness
Rc

300

149

63/65

350

177

62/64

400

204

61/63

450

232

60/62

500

260

58/60

600

316

55/57

700

371

51/53

800

427

48/40

900

482

44/47

1000

538

40/44

Disclaimer

Fire and red-hot metal are inherently dangerous. The author and the hosts of this site accept no responsibility for and exclude all liability in connection with browsing this site, use of information on this site and downloading any material from it including but not limited to any liability for errors, inaccuracies, and physical injury resulting from application of this information.

More Resources

The Metallargy of Carbon Steel
A Brief History of Steel
Steel from a Materials Science and Engineering point of view
Steel and Tool Making before 1870
Art of the Edge Tool
Brent Beach's 'Testing Plane Irons'
Blade steels, steel analysis, and heat treating methods
Heat Treating
Heatreat
University of Manchester Internet Microscope

at the Sign of the Three Planes