The investigation of the heat treatment records supplied by the mandrel supplier indicates that the mandrel hardened
from the high side of the austenitizing temperature range and only slightly tempered with a low temperature treatment
(400 8C) which is likely insufficient to fully reduce the brittleness of the mandrel. The heat treatment guide, for used S7 tool
steel calls this steel to be tempered at 480–540 8C. Double or triple tempering is usually necessary for tool steels. The first
temper relieves the stresses from the as-quenched martensite and at the same time conditions the retained austenite to
transform into martensite upon cooling from the tempering temperature. If the second tempering is omitted, the tool will
remain in the as-quenched condition and will eventually break. Correct hardening and double tempering is important. A
third tempering is advisable for heavy cross sections such as this mandrel.