Forging Knives

Heat & Temperature

Updated 12/27/08

Roofing Torch 50,000 BTU: RT 1 1/2-10 C

To forge steel it needs to be heated. Traditionally this was done on a coal or coke fire, we'll use propane. This torch was chosen for a few reasons: It's compact and puts out a good heat, comes with a pressure regulator and gauge, but the big reason is that it has a thumb valve much like that on a oxyacetylene cutting torch. Full heat is delivered only when the lever is depressed, which means the forge isn't roaring all the time--only when you need it. At 7000 feet elevation this torch has plenty of punch.

Build a muffle (heating chamber) out of soft kiln bricks or fireplace bricks to trap the heat. Ask about kiln bricks at the clay store where you picked up the clay. Loose stack the bricks, there's no need for mortar. Kiln bricks are better (more insulative) for our purposes, but more expensive. Kiln bricks can be cut with a handsaw, fireplace bricks need brick cutting equipment. Since the bricks are loose stacked they can be repositioned in new configurations as the need arises.

Be aware that a propane torch uses a substantial amount of oxygen
and your workspace should be vented accordingly.

Dempsey's Forge sells good safety/forging glasses. Get the #2 Green Filter Lenses.

Decarburization happens when hot steel oxidizes, creating a thin flaky oxide coating. HeatBath makes several water soluble stop-off protective coatings for steel, one of which is called No-Carb Green, which sounds like an eco-diet.

Temperature:

There are some special temperatures associated with steel called transformation temperatures. You don't have to learn their names or symbols, but it's nice to have some idea of what they mean.

When heating O1 steel (red line) at about 1350F Austenite begins to form. The rate of temperature rise slows as heat is being consumed to create this transformation. At about 1400F the transformation to Austenite is complete and the temperature rise resumes it's original rate. At the Peak Temperature, which is of our choosing, the steel is withdrawn from heat and begins cooling (blue line). At about 1295F Austenite begins transforming back into Ferrite and the rate of cooling slows as heat is being released during transformation. When the transformation is complete, 1240F, the original cooling rate resumes.

Some points about transformational temperatures:

All these changes, except the formation of martensite, occur at lower temperatures during cooling than during heating, and depend on the rate of change of temperature.

When a piece of steel is heated to Ac1, it continues to absorb heat without appreciably rising in temperature, although its immediate surroundings may be hotter than the steel. This is the decalescence point. Similarly, steel cooling slowly from a high heat will, at Ar3, actually increase in temperature, although its surroundings may be colder. This takes place at the recalescence point. The recalescence point is lower than the decalescence point by anywhere from 85 to 215 degrees F., and the lower of these points does not manifest itself unless the higher one has first been fully passed.

Now for Ac3. This is the important one, this is what everybody talks about. It's also called the critical temperature, the magnetic point, the Curie Temperature and so on. Call it Critical. When steel has transformed to Austenite it loses it's magnetic properties, it will no longer attract a magnet. And this is the test for Critical. It's not so important to know the temperature (the number) of Critical for the steel you're working with. You might be curious and want to know but otherwise it matters little. The magnet test identifies Critical and what it tells you that the steel has undergone a structural transformation, it doesn't actually say anything about temperature.

Each steel has it's own transformation temperatures, although for classes of steels these temperatures are similar.

Every material glows the same color at the same temperature. Whether it be steel, glass, ceramic or horse whiskers. This is just a fact of quantum physics. If you learn the color (see the chart below) of Critical for a particular steel and have a particularly acute eye and memory then you can identify Critical by sight alone--maybe. Everyone else should use a magnet to test for Critical.

There's one other temperature, but it's not in the graphic above. It's called Ms and it's the temperature at which Martensite starts to form, it's usually around 350-450F and Martensite formation progresses as the temperature falls.

We'll revisit this temperature stuff in the OzForging and heat treatment sections.

The temperature thing--how does one know steel temperatures with any degree of certainty?

The short answer is you don't. This is really the crux of hand forging, it's a craft rather than an industrial process. If you know the temperature within 25F you're doing very well, 50F is more like it. And things are always changing, the whole process of forging is dynamic rather than static.

You can know the Critical temperature by using a magnet. With careful heat control and a careful eye, you can know the decalescence and recalescence points. Borax melts at 1366F. Aluminum melts at 1220F. Use Tempilstiks, which are temperature markers that melt close to the specified temperature, get the 1300F, 1400F, and 1500F sticks. Beyond these, you're pretty much on your own.

If you want greater temperature control a kiln may be in order. Paragon sells knife making kilns. But then you're faced with the somewhat the same problem, how do you know is the dial is reading correctly? Try an Infrared Temperature gun if you want. The real test for temperatures and all the rest of hand forging is the results. If you create a knife which makes you happy, you must have done it right. To get all the things right, you'll have to go through about the same struggles steel forgers have all gone through since antiquity. You're not the first to wrestle with these problems.

Temperatures and Colors:

Using 1018 steel is a good way to become familiar with the colors of temperatures There are two color/temperature regions each arising from a different effect. While these color charts may not be perfect given different monitors, software, and the vagaries of the web, they'll give you an idea of the different color/temperatures.

In the forging-range, color is emitted by the steel. In the tempering-range, color is the result of oxides formed on the surface of the steel. To get a good reading in the tempering-range the oxides need to be filed or sanded off to allow new and current oxide colors to form. A quick swipe with a file or sand paper will do the trick. Practice seeing both sets of colors while forging 1018 steel.