And how to minimize them.
Upon heating, steel parts change volume as they change crystal structure (I’ll spare you the technical terms and details). When these heated parts are quenched, their internal crystal structure changes again, and that volume change is not necessarily sufficient to offset the change upon heating. This change of volume can cause  dimensional distortion.  The rule of thumb that I have used for medium carbon alloy steels is  to expect a change in linear dimensions of about 0.125% maximum. That is, one eighth of a percent of the linear dimensions could be the change encountered  from heat treatment and quench. It generally is less, but 0.125% gives me a rule of thumb to evaluate capability to hold dimensions after heat treat. What rule of thumb do you use to estimate part growth as a result of heat treat?
Warpage or shape distortion  as a result of heat treat is different because it is usually a result of process and design issues rather than the expected phase changes of the material.

Salvador Dali understood!
Salvador Dali understood!

Here are 8 reasons steel parts can warp upon quench and tempering:

  1. Rapid heating.
  2. Overheating.
  3. Non-uniform heating.
  4. Non-uniform cooling.
  5. Non-uniform agitation.
  6. Water contamination in oil. 
  7. Large changes of mass and section.
  8. Asymmetric features.

Rapid heating can cause stresses to develop in parts due to excessive temperature gradients. Overheating similarly lowers mechanical properties,  potentially leading to parts sagging or creeping depending on orientation in the furnace. Non-uniform heating also creates differences in properties within the parts as well as leading to incomplete transformation products or hybrid structures upon quenching. Non-uniform cooling allows unbalanced stresses to develop during the quench, as does non-uniform agitation of quench medium.  Often non-uniform heating or cooling result from the way parts are stacked or piled in the basket or on the belt such that gradients of temperture are created. Water contamination in oil. This is difficult to figure out, but in addition to warped parts, inconsistent hardness readings between parts or on the same part are a sign of this. Parts with large section changes or that have asymmetric features are also more likely to warp than parts with balanced and uniformly distributed mass, regardless of process control.
Choosing steels with higher hardenability (alloys rather than plain carbon steels), finer grain size, and paying attention to the details of loading, time at temperature, and quenchant delivery are all steps that can minimize warpage distortion, even when part design is less than optimum.