The aim of this research is improving the performance of H11 hot-work tool steel by optimizing the heat treatment parameters. response surface method(RSM) experimental design technique was performed to optimize the heat treatment parameters. After the initial investigations, three parameters including austenizing time and tempreture and tempering temperature were selected from most effective parameters on toughness, hardness and thermal fatigue in order to design the heat treatment experiments. A two step tempering with optimum time duration of 2 hours was applied. The lowest and highest levels for austenizing temperatures were 995°C and 1025°C, for austenizing time, were 60 and 100, and for tempering temperature, were 600°C and 670°C. Responses including impact energy (as a measure of toughness), hardness, and crack density were selected. The purpose was to achieve the highest impact energy, the optimum hardness (320 Vickers), and the lowest crack density. Hardness measurement, impact test and thermal fatigue test were used. Heat treatment cycles were evaluated by scanning electron microscopy, EDS analysis, X-ray diffraction analysis, micro-hardness and crack density measurement by image processing software. The frequent thermal and mechanical stresses induced on the surface of the substrates caused as a consequent of the stress concentration in high energy areas such as grain boundaries, fatigue-crack growth originated from these areas and propagated along the grain boundaries of initial austenite. After statistical investigations, the austenizing temperature of 995°C, the austinizing time of 100 minutes, and tempering temperature of 646°C were identified as optimal conditions. The average hardness of 320 Vickers, average impact energy of 54 J, and average crack density of 6.8 were obtained after applying the optimum heat treatment cycle. Microstructural investigations showed that the finer austenite grain size resulted in greater toughness and more heat checking resistance. Keywords : H11 tool steel, heat treatment, design of experiment, heat checking
