Why should carbon steel undergo the process of hardening and tempering prior to use? What are the benefits that can be derived from subjecting it to such treatments? If you find yourself asking these questions, here are some things you definitely need to know.
Carbon Steel Hardening and Tempering 101
In a nutshell, carbon steel is hardened and tempered so that it can stand up to its intended level of usage. By subjecting carbon steel to the appropriate hardening temperature or heating it between 800oC to 900oC, quenching it in water, oil or gas, and then tempering or soaking it at a lower temperature, the material will develop the optimum hardness, toughness and strength required for its intended application.
Hardening and tempering can be carried out in an open furnace or it can take place in a protective environment if neutral or clean hardening is required. Materials such as bars and forgings that are designed to be fully machined into components are best hardened in open furnaces while those that will be used in the production of precision components are best hardened under tightly controlled conditions. As such, nuts and bolts, bearings, springs, and most automotive component parts are usually hardened using the neutral hardening technique.
However, you need to be aware that not all carbon steel can be hardened. Only medium and high carbon steel or those containing more than 0.30% carbon can be subjected to such treatments. Mild carbon steel, on the other hand, can be hardened by coating it with other materials in a process known as case hardening.
Exploring the Limitations
There are several limitations to the process. Here are some of them.
Limiting section size. All steel can only be hardened up to a certain point.
Excessive aluminum content. Aluminum has a negative effect on the hardening process. As such, low alloy and plain carbon non-alloy steels that contain excessive amounts of aluminum may not harden as expected. This can be a serious problem so you need to make sure that you know how much aluminum is present in your materials.
Steel condition. Sometimes, steels that are procured after open treatments lose some carbon from their surface layers while those that are processed in cold worked conditions may contain residual stresses. These imperfections need to be removed before hardening to reduce excessive distortion and cracking.
Temper embrittlement. Alloy steels that contain nickel, chromium and manganese are prone to temper embrittlement or reduced impact toughness when it is heated or cooled at a temperature range of 400oC to 600oC.
Hardening and tempering carbon steel prior to use can provide it with the optimum mechanical properties required for its intended application but you need to take note of their limitations to prevent any problems along the way.
