The effects of austenitizing and tempering temperatures on the microstructure, as-quenched and tempered hardness capability, and Charpy V-notch impact resistance of D2 and Hl 3 tool steels were investigated. Decarborization behavior of D2 and Hi3 tool steels was observed by heat treating the samples in vacuum and normal furnaces. Heat treatment in an open atmosphere furnace gave up to .018″(.45rnm) thick layer of decarborization and also results in the loss of precious alloying elements, which should be in controlled amounts for 02 and Hl 3 tool steels. The main results can be summarized as: (1) An increase in austenitizing temperature resulted in coarsening of the grain structure, increased dissolution of carbides, increased asquenched and tempered hardness capability, and decreased impact toughness; (2) Tempering three times in comparison with two times after hardening in a controlled atmosphere furnace gives an increase in Charpy impact toughness of up to 25%; (3) 02 and Hl3 steels hardened at 1038’~ followed by three temperings show relatively higher Charpy impact values versus those treated with one or two temperings. The failure mechanism of the impact tested 02 and Hl3 steels after heat treatment at 1 OZS*C, 1038’~, and 4 065’~ followed by the tempering up to three times at temperatures 205’~, 538’~, 593%, and 620’~ was studied through using Scanning Electron Microscopy. The resultant microstructure of D2 and Hi 3 steels after the three tempering process gives better plasticity than after two tem perings
