Most heat treaters have been told by in-house customers, as well as external customers, that we need to have the component “heat treated without distortion.” That is essentially an impossibility. The component will distort, but the skill of the heat treater must now create the impression that distortion has not occurred even though heat treatment created a metallurgical phase change.

If we consider the rise of a steel part temperature, molecular changes occur in the structure of the material.

  • Once heat is applied to the steel, there will be movement from the relieving of machining stresses (i.e., the part will undergo possible movement, which we know as distortion).
  • As the temperature of the steel escalates upward to the transition area (approximate temperature 1300°F; dull-red heat), the metallurgical structure now begins to transform from what is known as ferrite.
  • The steel is now continuously increasing in temperature. Once it increases above the A1 line, the physical molecular change continues into a new phase.
  • As a result, there are three volumetric size changes (including the room-temperature molecular structure).


Please be aware that when the process temperature of the steel reaches a transformation line, it does not mean that the complete phase area is immediately transitioning from one phase to the next. It means that the volumetric size of the crystal structure is changing as the steel process temperature is changing. The completeness of each transformation will depend on:

  • Maximum cross-section
  • Residence time at the selected temperature (dependent on process temperature selection)
  • Total analysis
  • Areas of grain growth within the body of the material
  • Change in temperature rise and the relief of residual machining stresses
  • Changes in cooling rate (and/or quench-medium type)


Distortion can be caused by changes in shape and/or size, which are related to heating or cooling time and temperature. An additional influencing factor that is not often considered is the effect of induced mechanical stresses caused by the working of the metal (this also applies to nonferrous materials).

Most heat treaters have been told by in-house customers, as well as external customers, that we need to have the component “heat treated without distortion.” That is essentially an impossibility. The component will distort, but the skill of the heat treater must now create the impression that distortion has not occurred even though heat treatment created a metallurgical phase change.

If we consider the rise of a steel part temperature, molecular changes occur in the structure of the material.

  • Once heat is applied to the steel, there will be movement from the relieving of machining stresses (i.e., the part will undergo possible movement, which we know as distortion).
  • As the temperature of the steel escalates upward to the transition area (approximate temperature 1300°F; dull-red heat), the metallurgical structure now begins to transform from what is known as ferrite.
  • The steel is now continuously increasing in temperature. Once it increases above the A1 line, the physical molecular change continues into a new phase.
  • As a result, there are three volumetric size changes (including the room-temperature molecular structure).


Please be aware that when the process temperature of the steel reaches a transformation line, it does not mean that the complete phase area is immediately transitioning from one phase to the next. It means that the volumetric size of the crystal structure is changing as the steel process temperature is changing. The completeness of each transformation will depend on:

  • Maximum cross-section
  • Residence time at the selected temperature (dependent on process temperature selection)
  • Total analysis
  • Areas of grain growth within the body of the material
  • Change in temperature rise and the relief of residual machining stresses
  • Changes in cooling rate (and/or quench-medium type)


Distortion can be caused by changes in shape and/or size, which are related to heating or cooling time and temperature. An additional influencing factor that is not often considered is the effect of induced mechanical stresses caused by the working of the metal (this also applies to nonferrous materials).