anealing and tempering metal

**Bill Hoffman** · Feb 11, 2015, 02:43 PM

Generally, CP is in the 1400's F, and CT is in the 1500's F. If I'm forge heat treating, I go to CP,
note the color and let the steel sit until the color is a LITTLE more to the orange side.

You can also see the recalescence/decalescence shadows pass across the steel as the carbon
enters/leaves solution. The shop should be pretty dark to see this well, but after you've seen
it once you won't miss it again.

I generally quench in Parks 50 oil for hardening, and temper oven is a gas grill with multiple thermometers.
Temper about an hour, let cool to room temp, 3 times. Most of my heat treating is done on
knife blades.

**CedarSlayer** · Feb 11, 2015, 04:18 PM

The Critical temperature is often below the Curie point. The Curie point is used by blacksmiths when they don't know the ideal temperature, cannot precisely control the temperature or cannot measure the temperature precisely. If you purchased the metal knowing what it was, you may have the first, but unless you have a good steel working furnace, the second two are unlikely. Since the loss of magnetism is related to the loss of structure, in general a blacksmith can count on the magnet test. In cases where the loss of structure that allows for magnetism to happen is reached and the carbides are not dissolved, the blacksmith might or might not have better luck going with a longer wait at temperature or a bit higher temperature.

In the end a blacksmith's final judgment is based on results. If it does not work out, the metal is not considered useable. In some cases a blacksmith will go for a longer hold temperature. In an old fashioned or more primative environment, where the metal is not wrapped in stainless foil and put in a furnace capable of multiple hold times at a range of temperatures, the odds are that the curie point will be exceeded by a bit anyway.

There are a lot of odd metals used in knives, but apart from top end rather expensive steel knives and pretty much all stainless steel knives, knife steel can be handled by a blacksmith using the magnet test.

**Hotz** · Feb 11, 2015, 04:23 PM

Really good read ... Congratulations to the teachers.

Hotz

**CedarSlayer** · Feb 11, 2015, 08:56 PM

Here is an odd tip that some metallurgists and machinists know but few blacksmiths. Hot oil quenches faster than cold oil does. The reason is that low viscosity gives better circulation than high viscosity. Since the oil immediately vaporizes and forms a gas envelop around the steel, the gas is replaced faster when the oil flows more easily. A rapid stir and moving oil will help collapse/replace the envelope, but the advantage of fluid oil outweighs the advantage of cooler oil.

On the downside, if you don't have enough oil, hot oil can quickly be brought to a temperature that will allow an oil fire. I used to use a small long container to quench and not waste oil. Now I want a large enough volume so the oil temperature will not be raised too much as the metal transfers heat.

Bob

**Bill Hoffman** · Feb 11, 2015, 09:05 PM

I very respectfully disagree. I've never heard of a common high carbon steel which has a CT
below the Curie Point. Now that I've stuck my neck out, I'm only familiar with blade steels.
Some hi-tec stainless steels may be different, but I've never heard of one.

What I've learned from 50+ years of making knives is that without a carefully temp controlled
heat treat oven, mine's a Paragon, you need to refer to the calescent shade change to insure
that the carbon has entered into the iron matrix to form Martensite from the lower temperature
Austenite. The magnet test will get you into the ballpark, but the transition is somewhat
higher than the Curie Point. The Japanese bladesmiths have for centuries used the "shade" test
to determine when the steel is ready for quench.

You may get some hardening if you quench from Curie Point or below, but it will not be the max
the steel is capable of.

This is great. Civil discourse of an issue from which we all can learn. If I'm wrong, then
apologies all around.

Bill
ABS Apprentice

**CedarSlayer** · Feb 12, 2015, 12:34 AM

I love a good discussion and I tend to learn more when I am wrong than when I am right.

As a confession, my actual education is in the area of physics, but electronics and light are more of my area of mastery. Furthermore, I have used a magnet to test the concept, but I go by eye and use a cherry red glow in dim light to judge temperature. So my experience with using magnets to test has been limited to 01 steel.

That said there are a lot of rather amazing blacksmiths that will look at you with a blank stare if you start this discussion with them. A lot the old timers consider pounding hot steel to be important in the formation of a good blade and steel temperature charts rarely take that into effect.

The critical temperature, is a term that moves depending on what you are referring to. For magnetism, the Curie Point is the critical temperature. For carbon to be made available for producing carbides the critical temperature is where austenite is forming. Since the question of where austenite forms is so important, those who know that there is a difference in Curie Point and carbon being made available tend to label this the Critical point. At this critical point in the discussion, we lose most of our audience. For those who are persistent, austenite is one of the many crystaline forms that carbon steel can take. This form is particularly good for forming carbide crystals during a quench.

You are correct that in simple carbon steel, the temperature for austenite formation is above the critical point after the carbon content goes over 0.77%. This is however not the entire story.

Alloys of carbon steel can lower the temperature where austenite forms. O1 steel is typically held at 1400 in order to anneal. The Curie point for o1 is 1420 F. So for a steel with 0.95% carbon, the Curie point is a pretty good temperature for annealing.

Since I personally use a cherry red glow, this is hardly an issue. Cherry red is just bit below 1500 so I am typically am way over. I try to not get much further into the red orange zone, since a lot of steels such as o1 can develop retained austenite with over heating.

In any case, better blacksmiths that I will ever be regularly use magnets to test temperature and seem to manage quite well.