Effect of composition and processing on the thermal fatigue and toughness of high performance die steels

Dies cost about 20% of the total cost of producing aluminum die castings. Improvements in die life could result in annual savings of over {dollar}200 million in US with only a 20% life extension. The objective of this study was to obtain this improvement in average die life by optimizing die steel composition and the die processing. Four different steels, KDA-1, QRO-90, Chromo-N and Premium Grade H-13 have been investigated for thermal fatigue resistance and toughness. Premium Grade H-13 is now the standard die steel for aluminum die castings used in US. KDA-1 and QRO-90 are lower in vanadium content and higher in molybdenum than conventional H-13 die steel. KDA-1 also has a lower silicon content that should presumably minimize interdendritic segregation. Optimum heat treatment processing has been determined for each steel with respect to austenitizing temperature and tempering conditions. The effect of the quenching rate on the thermal fatigue resistance and toughness of the die steels and the effect of Electro-Discharge Machining (EDM) on the thermal fatigue resistance were also determined. The immersion thermal fatigue specimen developed at CWRU was used to determine the thermal fatigue resistance as characterized by the two parameters of average maximum crack length and total crack area. The Charpy V-notch impact test was used over a {dollar}{lcub}-{rcub}100spcirc{dollar}F to 450{dollar}spcirc{dollar}F testing temperature range to evaluate the toughness and the brittle-ductile transition behavior.; KDA-1 has been identified as superior in performance compared to Premium Grade H-13. QRO-90 and Chromo-N provide lower toughness and thermal fatigue resistance than H-13. Faster cooling rates provide higher thermal fatigue resistance and toughness. Higher austenitizing temperatures such as 1925{dollar}spcirc{dollar}F compared to 1875{dollar}spcirc{dollar}F provide better thermal fatigue resistance, but lower austenitizing temperatures of 1875{dollar}spcirc{dollar}F provide better toughness. Higher hardness improves thermal fatigue resistance, but reduces toughness. A minimum of Rc 46 hardness is desired for aluminum die casting dies. EDM reduces the thermal fatigue resistance compared to conventional machining operations. When the EDM process of multiple small steps of decreasing energy and post-EDM treatments are employed, the effect can be reduced to a very slight amount.