Heavy armor must be designed to take a lot of punishment. It will receive direct blows from all sorts of weapons while protecting the wearer. Such armor tends to be made from a few large pieces rather than lots of small pieces like light armor.
Iron and steel are easy to work. Just heat them up and pound them back into shape. You can even use a camp fire for field repairs. Avoid filing off any of the metal. Always try to conserve the metal and work it back into shape.
If a piece needs a lot of hammering, it may become brittle. Reheating the armor every now and then can reduce the brittleness after severe repairs. Once the hammering is done, be sure to oil it well. The freshly hammered surfaces will rust more quickly and need to be protected.
Dwarven and Orcish armor require small and large hammers. Heat should be used sparingly, particularly with Orcish. Both types respond better to many small hammer strokes rather than fewer heavy strokes.
Ebony can only be hammered when heated. It will develop small cracks that eventually shatter the material if hammered cold. Daedric should always be worked on at night... ideally under a new or full moon, and never during and eclipse. A red harvest moon is best.