Being able to accurately judge the temperature of heated high carbon steel is especially necessary to determine the steel's transformation range. The transformation range is often referred to as the critical range. I prefer using the term transformation range because it seems more definitive to me. This is the range of temperature at which austenite forms during heating and transforms during cooling. When the cooling is performed at a rapid rate, as in quenching, a martensitic structure is formed which is hardened steel. If you quench the steel before reaching this temperature unreliable results will occur.

Accurately judging the temperature of the steel while forging and heat treating can be accomplished by a variety of methods. Most smiths gauge the temperature by the color of the hot steel. This method takes experience under the same repeated working conditions. This is unreliable for the beginner. Temperature color charts are popular in books on blacksmithing and knifemaking. Atmospheric conditions and ambient light can fool the eye, and opinions on what a specific color is called will differ. What you call sunrise red might be slightly different to another smith's eye.

The high tech way of determining the temperature of steel is with a pyrometer. If the pyrometer is located in a stationary position in your forge, it will only tell you the temperature of the atmosphere around the instrument not the steel's core temperature. Therefore, you must hold this temperature for quite some time in order to be sure the steel has leveled out to the same temperature. You could use a pyrometer directly against the surface of the steel, but you still must hold it at the temperature you want to be certain that the core of the steel is the same. Quality pyrometers are not cheap and can be awkward.

The most reliable way I have found to accurately determine the transformation range of high carbon steel is with a simple magnet. When high carbon steel reaches its transformation range it loses its magnetic attraction. This seemingly magical occurrence can be explained with solid-state metallurgy, but that isn't all that important here. What is important is that it works and it works every time. Colors can fool you, gauges can become damaged, but the magnet never lies.

Different high carbon steel alloys will have different transformation temperatures but the magnet doesn't care, it accurately judges them all. The magnet doesn't just tell you surface temperature either it examines the steel completely throughout. It's cheap, easy, and readily available. I use a broken speaker magnet clamped in a pair of vise grips. When I think the steel is coming close to the right temperature, I touch the magnet to the surface. If I feel any attraction I wait a little longer, and let the heat rise, until suddenly there's no pull. Then I know with absolute confidence that I have reached the beginning of the transformation range. If you still feel that color is accurate enough, then try this simple experiment. Heat up a bar of high carbon steel in the middle of the day until it loses its magnetic attraction. Make note of its color. Then heat up the same bar of steel at night to what you believe is the exact same color. Now touch the magnet to the surface. I'll gamble that the magnet will stick to the steel, and you'll be surprised at how much longer you will need to heat it up until it will no longer be attracted.

If you heat treat your steel in an oven equipped with a pyrometer you can check the accuracy of the gauge, if you know the temperature data of the steel you're using. Heat the steel in the oven until you have reached the prescribed transformation temperature then check to see if there is any pull with the magnet. If there is, you will need to adjust the temperature or recalibrate your gauge.

A lot of times the simplest way of doing something turns out to be the better way. The notion that a craftsman is only as good as his tools was probably started by a machinery salesman. If you insist on always buying the fanciest and most expensive tools, you will wind up being a slave to a high maintenance shop when your time could have been spent developing better hand/eye coordination.

In actual forging practices, I use colors to gauge temperature most of the time myself. But I use the magnet to get my bearings and I always use it to get a precise temperature before annealing or quenching.
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