Carbon Steel Quenching Secrets

by Pete Stanaitis

Why did that chisel break after I heat treated it? What happens when you quench carbon steel? Why do some people quench the steel in water while others quench in oil? How do I know if my parts got fully hard? Why should I care?

Well, if you think I am going to fully answer all these questions, you are wrong, because the full answers would constitute a complete course in heat treating. But what I do want to do is to make you a little more aware of the process that takes place when you DO quench a piece of carbon steel.

The experts describe the hardening characteristics of a given steel on a graph called a "Time Temperature Transformation" chart or "TTT curve" for short.

Figure 1 is one such chart for a typical medium carbon steel. The shape of the curves varies somewhat for each different kind of steel, but I will use this curve to illustrate my point for this article.

First, water provides a faster quench than oil does. Carbon steels and low alloy steels are quenched in water. High alloy steels are often quenched in oil or in air, depending upon their ingredients.

Generally, whenever you make a project from steel with enough carbon to render it hardenable, you will want to first harden it and then "draw" or "temper" it to some particular balance between brittleness and toughness. Okay.

The whole point of this article is that if you do not fully harden the part when quenching it from its critical temperature, then you won't know exactly how hard it got. And if you don't know how hard it got, then you can't depend upon the oxidation colors you get during the tempering process! The tool you are making could end up harder or softer than you planned and you won't know what happened. This means that you don't have control of the process you are using!!!!

Now, why is this true? Look at Fig. 1. Notice that the horizontal axis is labeled "Time" and that 1.0 second is pretty close to the left side of the graph and 100 seconds is a little past the middle. Now look at the vertical axis. This one is calibrated in degrees C on the left side and in degrees Fahrenheit on the right side.. This particular steel must be heated all the way through to at least 1350 degrees F. if we are to harden it.. If we had thermocouples connected to the part, and had a good stopwatch, we could plot the Time/temperature coordinates on the graph at any given time as the part cooled off.

Next, look at the "Critical Quenching" curve. If we stay to the LEFT of that curve as we cool the part to about 400 degrees F, we will get FULL hardness. See the curve labeled "Successful Quenching". The number 697 is the hardness attained on the Brinell hardness scale. This implies that we MUST cool it pretty darn fast, because the "Critical Quenching" curve hits 400 degrees F in only a second or so! If we can't or don't cool the part fast enough to stay to the left of the "Critical Quenching" curve, then we get less than full hardness, and that is why we lose control of the process! You can see examples of this in the other curves. For instance, the curve labeled "Slow Quench" gives only a hardness of 429 even though the part will probably get down to 400 degrees F in 8 or 9 seconds.

The moral of the story is that you have to quench carbon steels and low alloy steels in water, and not in oil. You don't CHOOSE an oil quench for a carbon steel if you don't want to quench fast; instead, you choose a steel that is designed for an oil quench.

There are exceptions to this rule but you need a lot of knowledge and much better temperature measurement equipment than is found in most blacksmith shops to take advantage of them. If this little lesson makes you curious, go to the library and get a book on Metallurgy.