We continue David Pye's blog series on the causes and effects of distortion. Part 2 can be found here.

Heat Treatment

Stresses can also be caused by differential expansion due to thermal gradients in the steel. These stresses will increase along with the thermal gradient and will cause plastic deformation as the yield strength is exceeded. Stress relieving (discussed later) can help to reduce this type of distortion.

When parts are quenched during heat treatment, there is a relationship between phase changes and dimensional size. If not quenched uniformly or tempered correctly, distortion can be the result.

The probable factors that will contribute to heat-treat distortion include:

  • Non-uniform heating
  • Austenitizing temperature too low
  • Austenitizing temperature too high
  • Phase changes on heating and cooling
  • Non-uniform quenching
  • Quench-medium temperature (too high/low)
  • Incorrect tempering temperature selection


Beyond distortion, cracking can occur if heat-treat stresses are excessive. Material irregularities can enhance this problem. Some of the potential causes of cracking are:

  • Long parts with differing cross-sectional thickness
  • Non-symmetrical shapes
  • Holes too near corners
  • Keyways
  • Grooves


If a component such as a splined shaft is distorted, do not attempt to straighten the shaft in the cold condition. There is a serious risk of cracking the shaft if straightening is attempted at room temperature. It must first be heated.

What temperature should be selected for the straightening operation? This depends on what type of heat treatment has been given to the part.

  • Has the shaft been carburized and quenched?
  • Has the shaft been manufactured from a heat-treatable through-hardening steel?


The shaft should be heated to a temperature that is below the tempering temperature. This will both reduce the risk of crack initiation and prevent softening of the part.
 

Stress Relieving

The stress-relieving process is very valuable in reducing the risk of distortion at the final heat-treatment operation. One should not underestimate the value of intermediate stress relieving. It does not guarantee distortion elimination, but it will do a great deal to reduce the final heat-treatment distortion.

Figure 4 provides a suggested sequence of the manufacturing operations and when the stress-relief procedure could be introduced. If stress relieving is not considered and conditions within the metal component are not addressed, the metal will stress relieve itself during heating to hardening temperature.

We continue David Pye's blog series on the causes and effects of distortion. Part 2 can be found here.

Heat Treatment

Stresses can also be caused by differential expansion due to thermal gradients in the steel. These stresses will increase along with the thermal gradient and will cause plastic deformation as the yield strength is exceeded. Stress relieving (discussed later) can help to reduce this type of distortion.

When parts are quenched during heat treatment, there is a relationship between phase changes and dimensional size. If not quenched uniformly or tempered correctly, distortion can be the result.

The probable factors that will contribute to heat-treat distortion include:

  • Non-uniform heating
  • Austenitizing temperature too low
  • Austenitizing temperature too high
  • Phase changes on heating and cooling
  • Non-uniform quenching
  • Quench-medium temperature (too high/low)
  • Incorrect tempering temperature selection


Beyond distortion, cracking can occur if heat-treat stresses are excessive. Material irregularities can enhance this problem. Some of the potential causes of cracking are:

  • Long parts with differing cross-sectional thickness
  • Non-symmetrical shapes
  • Holes too near corners
  • Keyways
  • Grooves


If a component such as a splined shaft is distorted, do not attempt to straighten the shaft in the cold condition. There is a serious risk of cracking the shaft if straightening is attempted at room temperature. It must first be heated.

What temperature should be selected for the straightening operation? This depends on what type of heat treatment has been given to the part.

  • Has the shaft been carburized and quenched?
  • Has the shaft been manufactured from a heat-treatable through-hardening steel?


The shaft should be heated to a temperature that is below the tempering temperature. This will both reduce the risk of crack initiation and prevent softening of the part.
 

Stress Relieving

The stress-relieving process is very valuable in reducing the risk of distortion at the final heat-treatment operation. One should not underestimate the value of intermediate stress relieving. It does not guarantee distortion elimination, but it will do a great deal to reduce the final heat-treatment distortion.

Figure 4 provides a suggested sequence of the manufacturing operations and when the stress-relief procedure could be introduced. If stress relieving is not considered and conditions within the metal component are not addressed, the metal will stress relieve itself during heating to hardening temperature.