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 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).
Fig. 2. Three-dimensional illustrated view of the various crystal structures. Left: Face-centered-cubic lattice structure; Center: Body-centered; Right: Body-centered tetragonal
I have often addressed the effects of induced mechanical stresses in the metal and the ways that those stresses affect the dimensional movement when the metal is subjected to thermal conditions (hot or cold). The question is often asked: How long does it take to remove the induced mechanical stresses caused by physical mechanical conditions being applied to the metal (e.g., machining stresses)?
If the metal (steel) is mechanically forged or rolled, grain deformation results. If the steel is cooled rapidly from the austenite region of the iron-carbon equilibrium diagram, surface cracking is another potential problem as the martensite phase is initiated.
Fig. 3. Factors causing distortion
Fig. 4. Steps to reduce distortion
It is generally accepted that shape distortion is the result of either residual or externally applied stresses. These stresses result from some or all of the factors illustrated in figure 3.
I have often been told, “We can heat treat without distortion.” I do not fully agree with that comment. Distortion can be reduced but not eliminated. There are simply too many variables that the heat treater does not have any control over that will create a distorted heat-treated component. Distortion can be minimized, but it cannot be eliminated.
In order to reduce the risk of distortion, extremely careful steps need to be considered and taken prior to heat treating the component. Some of these distortion-reducing steps are shown in figure 4.
This discussion will be continued in our next quarterly Technical Talk column.