Heat Treating Steel Forgings

When a forging with around 0.8% carbon comes off the press and is cooled to ambient temperature, it has a microstructure of ferrite plus pearlite; over 0.8% carbon, one of pearlite plus cementite. Ferrite is effectively pure iron, pearlite is ferrite plus cementite. Under a microscope, it resembles Mother of Pearl, hence its name. Cementite is iron carbide. The occurrence of these different phases is part of what makes steel such a versatile material. What makes the alloy additionally versatile is what happens to it when it is heated and cooled, to what temperature it is heated (and for how long), and how quickly it is cooled.

We haven’t yet mentioned the master in all this, namely Austenite. When steel of any carbon content up to 2% is heated, at 1333ºF (723ºC) its structure starts to change, from ferrite plus pearlite in steels up to 0.8% carbon, to one of ferrite plus austenite. The pearlite is dissolving in the austenite, and as the temperature increases above 1333ºF (723ºC), the amount of austenite increases until the steel becomes fully austenitic. For example, steel with 0.5% carbon would be totally austenitic at 1400ºF (760C.)

For steels with over 0.8% carbon, the starting structure is pearlite plus cementite, and over 1333ºF (723ºC) it changes to austenite plus cementite. From 1333ºF (723ºC) to 2065 ºF (1130ºC) the temperature at which a steel becomes 100% austenitic increases until at 2065ºF a 2% carbon steel is 100% austenite.

The idea is to heat to the so-called austenitizing temperature gradually; not too quickly. Rapid heating may not be good for complex-shaped parts, nor for highly alloyed grades. Time at temperature may be something like 30 minutes per inch of cross section, but in the long run is a matter of experience.

Those who have experience with carbon steel, and its forming and its heat treatment, know that heating, forming and cooling, then reheating and cooling again, are very important aspects of the treatment of steels. For example, the quicker we cool carbon steel from a hot forming temperature, or from a heat treatment temperature, the harder the steel will be. So, a water quench will give a harder steel than an oil quench, which will in turn give a harder steel than an air cool. When we water quench carbon steel, martensite is formed, an acicular-shaped phase that is very hard and brittle. Hardening is followed by tempering, both to relieve stress and to meet required mechanical properties.

Cooling in a furnace from a heat treatment temperature, (or annealing), may render a steel about as soft as it could get, except for something called an isothermal anneal, which produces a more homogeneous microstructure within the steel and is faster and less expensive than full annealing. It is typically performed on hypoeutectoid steels, and it is usually not performed on hypereutectoid steels. Steels with 0.84% carbon are called eutectoid steels, those with less than 0.84% carbon are called hypoeutectoid, and those with more than 0.84% carbon are called hypereutectoid steels.

At All Metals & Forge Group we understand the importance of controlling temperature and time in the forging and heat treating of steel. We also understand the effects of these variables, plus that of cooling rate, and why they lead to the production of different phases in a steel’s structure. Coincidentally, we understand the role of the heat treater, and their part in bringing back the art aspect of that sometimes idiosyncratic process. Heat treatment is a combination of art and science, and we will do well to remember that.