Tempering is a function of time and temperature and is greatly affected by the various alloys in the steel. A polished piece of steel will oxidize as it is reheated and begin to show colors, the higher the temperature the thicker the oxide layer and the darker the color. Careful control of the temperature and the time of the tempering cycle will give more reliable results. Each steel responds differently to tempering and you should make sample test pieces and test them thoroughly to find the best combination for the steel and job it is expected to perform. The purpose of tempering is to relieve the stresses built up during the transformation from austeninte to martensite and to control the hardness of the steel to suit the use of the tool.
Tempering can also trip retained austenite to make fresh untempered martensite.
A common and effective practice is to double temper steel. The first tempering heat is for at least a one hour cycle and then a second heat is used to temper any fresh martensite that has been transformed. A two to three hour drawing cycle is more effective than a shorter duration higher temperature draw. Care should be taken not to over heat the piece while tempering since it will soften the steel and diminish its potential. Tempering may be done in almost any heat source that can be accurately controlled.