Wednesday, May 16, 2007 8:30 AM Room: 343

AB DRASTICALLY IMPROVING DIE CASTING DIES PERFORMANCE: TACKLING THE PROBLEM FROM THE THERMO-ELASTIC SIDE I. Valls, ROVALMA S.A., Terrassa , Spain

Taking advantage of some recently developed quantum mechanics models for phonon and electron scattering in steels as a function of alloying elements presence and distribution, a hot work tool steel with a thermal conductivity of more than 60W/mK (450 Btu in/ ft2hºF) has been developed. This steel has more than double thermal diffusivity than H11, it has also a 15% lower Young’s modulus and 7% lower coefficient of thermal expansion than H11. Mechanical properties at high temperature like yield strength, creep resistance, fracture toughness, elongation, and resilience as well as hardness retention and tempering resistance are usually higher but rather similar to those of H11. For applications where the Biot number is small, Merit index is more than doubled, leading to a severe reduction of the strain amplitude (the same die, for the same application, made with the new tool steel has to withstand half the surface thermal stress amplitude than if it were made with H11). In the case of thermal fatigue checking this can translate into an order of magnitude increase in expected life time. Thermal shock resistance is improved in the same way and thus resistance to gross cracking. The high thermal diffusivity has a strong effect on the die surface temperature, this is again piece-geometry dependant, but for most cases over a 100ºC (180 ºF) surface temperature drop is to be expected. This has a direct impact on the mechanical properties of the steel at the working zone. Nucleation of heat checking cracks, soldering, wear resistance and washout resistances are also enhanced this way. The much faster cooling potential can lead to increased productivity and improved surface finish and mechanical properties (smaller interdendritic spacing) of the produced cast piece.