Heat treatment and its shop applications

February 2012 - In general, the terms heat treatment and hardening mostly are used to describe steel applications. For a deeper understanding as well as more defined and accurate terminology in heat treating steel, workers in a fabrication or machine shop should be aware of the differences among tempering steel, annealing steel and hardening and tempering processes.

Tempering
In general, tempering is the process of reheating a hardened steel workpiece to a temperature that is not high followed by cooling. Going back to the iron-carbon diagram, it can be identified in the uniform structure of a martensite when it is forced into formation by quenching.

During quenching, hard martensite crystals form in rapid cooling. This occurs only at the critical quenching speed. Cold water, for example, cools the steel very rapidly. This effect does not make the steel very stable. When heated slightly, it decays into ferrite and pearlite or into pearlite and cementite if the carbon content is high.

Generally, tempering toughens steel at the cost of hardness. Although steel becomes stronger with increasing hardness, it also becomes more brittle. When it is reheated to between 392 degrees Fahrenheit (200 degrees Celsius) and 680 degrees Fahrenheit (360 degrees Celsius), some of the hardness is lost but the brittleness disappears and the malleability increases. There is a simple applied rule to follow: The higher the tempering temperature, the lower the hardness and the higher the malleability.

For the shop application, it is useful for workers to observe and note the tempering colors (oxide colors). They can help estimate the tempering temperature. When the desired color is reached, the steel is quenched immediately.

With external tempering, the steel workpiece is heated in an oven or by the flame of a torch. With internal tempering, it uses the heat of a hardened steel workpiece. Only a part of the workpiece is quenched after hardening. An example is the cutting edge of a chisel. The shank of the chisel still contains enough heat to bring the cutting edge to the tempering temperature.

Annealing steel
The annealing process involves heating a steel workpiece to a particular temperature, maintaining it at the temperature and then cooling it gradually. Undesired stresses and structural hardness are removed with this method.

There are three common annealing processes:

Stress relief annealing or process annealing at 932 degrees Fahrenheit (500 degrees Celsius) to 1,112 degrees Fahrenheit (600 degrees Celsius) removes the internal stresses that developed during hot or cold working processes.

Soft annealing takes place at 1,256 degrees Fahrenheit (680 degrees Celsius) to 1,508 degrees Fahrenheit (820 degrees Celsius) for higher alloyed steels and reduces the hardness of steels so they can be machined more easily.

Normalizing takes place above the GSK line in the iron carbon phase diagram at 1,562 degrees Fahrenheit (850 degrees Celsius) to 1,742 degrees Fahrenheit (950 degrees Celsius), depending on the carbon content. This type of soft annealing transforms the grain structures that are too coarse and results in a new and uniform fine-grain structure.

Hardening and tempering steel
The general aim of hardening and tempering is to increase the toughness of steel at a given tensile strength. The workpiece is hardened and subsequently tempered at temperatures of 932 degrees Fahrenheit (500 degrees Celsius) to 1,202 degrees Fahrenheit (650 degrees Celsius).

This is a form of combined heat treatment to increase toughness. The steel contains 0.25 percent to 0.6 percent carbon. Those are very fine-grained alloyed steels and are treated in this way more frequently than carbon steels.

The steel workpiece is first normalized at 1,382 degrees Fahrenheit (750 degrees Celsius) to 1,832 degrees Fahrenheit (1,000 degrees Celsius) then hardened and tempered. Although this process reduces the hardness, the tensile strength rises enormously. This process is used for highly stressed machine parts. FFJ

Udo O.J. Huff is an independent consultant with project experience in machine building, welding engineering, training and development. He holds Master of Education and Bachelor of Science in Technology degrees from Bowling Green State University. Questions or comments? E-mail uhuff@sbcglobal.net.

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