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.
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.
We will cover more of this topic in a later blog.
Part 1 can be found here.