Overheating

Dimensional changes after heat treatment are a result of crystalline structure changes and are usually less than an eighth of a percent in linear dimension. Warpage is usually the result of bad practices in process including rapid heating, overheating, non-uniform heating, non-uniform cooling,non-uniform agitation,and water contamination in the quenching oil.
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.