INFLUENCE OF THE COOLING RATE DURING QUENCHING ON THE TOUGHNESS AT TYPICAL WORKING TEMPERATURES OF DIE-CASTING DIES

The influence of the cooling rate during quenching on the toughness of two hot-work tool steel gradesat ambient temperature and at working temperatures typical for die-casting dies has been compared.The heat treatments were performed in a vacuum furnace. Different cooling rates were obtained byvarying the nitrogen gas pressure and cooling fan velocity. The impact toughness was researchedby conventional and instrumented Charpy V-notch impact testing at various temperatures. Thefracture toughness was studied by KIc fracture toughness testing at room temperature and JIc fracturetoughness testing at elevated temperatures. The researched grades were Uddeholm Vidar Superior(X37CrMoV5-1 with reduced silicon content) and Uddeholm QRO 90 Supreme.The influence of the cooling rate on the toughness differed considerably between ambient and elevatedtemperatures. The decrease in toughness with decreasing cooling rate was more gradual at elevatedtemperatures. The reason for the difference was that the fracture type at elevated temperature wasfibrous while at room temperature it was more or less completely quasi cleavage