When steel is raised in temperature and held at that constant temperature in which the austenite state is achieved in the steel, there is a strong influence on the grain size and, consequently, there will be the problem of distortion occurring.

A typical process that does require extended times at an austenitizing temperature is the process of carburizing, particularly deep case carburizing. We know that grain growth occurs as a function of time and temperature. We also know that with diffusion processes there is surface volume displacement as a result of the diffusion of nitride- or carbide-forming elements following nitriding or carburizing. Growth is caused by the carbon or nitrogen diffusing into the surface of the steel being treated.

Another source of potential growth will occur at the particular phase-change temperature when the material undergoes its natural volumetric phase change.

• Ferrite to austenite: Nine-atom structure to a 14-atom structure. Body Centered Cubic lattice to Face Centered Cubic lattice structure.
• Austenite phase: It is while in the austenite phase that the steel will be exposed to a high-temperature state for extended periods of time.
• Austenite transformation to martensite: When austenite is created followed by an appropriate cooling rate, there will be a further phase change from austenite to fresh untempered martensite. The untempered martensite has a geometric lattice structure of Body Centered Tetragonal (BCT) with a total of nine atoms. That is, of course, provided that there is sufficient carbon to create fresh martensite, and provided that both the austenitizing temperature and cooling rate is the appropriate temperature and cooling rate.
• Induced residual stress: Residual stress is induced into the steel from the manufacturing and machining process. It is a given that the phenomenon will occur.
• Induced phase-change stress: The induced phase-change stress will occur as a direct result of both temperature and cooling conditions. It is recommended that there be an intermediate stress-relieving procedure before the final heat-treatment procedures of carburizing and rapid cooling are conducted.
• Selection of the appropriate final austenitizing temperature: This is a most important selection procedure. It is a well-known fact that the higher the final austenitizing temperature, the greater the DELTA T from the selected temperature to the quench/cooling medium temperature. This can be a serious contributor for the created conditions of distortion to occur.

These are just a few thoughts for you to consider when trying to reduce the risk of distortion when carburizing. The long and short of it is that you will not eliminate distortion, but you can take steps to reduce it.