Distortions Flashcards
What are distortions, and what can they be an evidence of?
A reversible or irreversible geometrical modification in shape or size. They are one of the first macroscopic evidence of residual stresses.
What are reversible distortions?
- Reversible distortions are related to heating and cooling cycles that can be
considered as homogeneous in the volume treated. - If no phase modifications is involved when the metal cools back to room
temperature, then the part assumes its original shape and size (no permanent distortion)
What causes irreversible distortions?
Phase transformations or inhomogeneous thermal / mechanical cycles in parts.
How can irreversible distortions happen during cooling of steels?
The austenite-to-martensite transformation can lead to:
* slight expantion of the crystal lattice (permanent size distortion)
* the inhomogeneous kinetics of the transformation (surface vs. centre) may lead to uncontrolled
distortion (shape distortion)
Which is the most dangerous of size distortion and shape distortion, and why?
Irreversible shape distortions can be of greater concern since they are not always predictable and therefore cannot be compensated.
How are shape distortions often generated?
During heat treatment due to uncontrolled heating or cooling conditions
What are some possible causes of shape distortions during heating?
- relief of residual stresses coming from previous manufacturing operations
- high temperature plastic deformation due to weight or stacking
of part - Constrained thermal expansion of parts that are in contact with other bodies
- Thermal gradients between different regions of the furnace chamber
- Large thickness reductions resulting in differential heating rates
How can distortions be avoided during heat treatments?
Each stage of heat exchange should in principle occur in the same period of time throughout the volume of the part. The size realationships (ex Area vs Thickness) also needs to be within a specified value.
