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After my presentation at the NADCA Die Casting Congress & Tabletop September 18, 2013 in Louisville, KY I talked to Prolong engineers about the big increase in die life possible when using Thermal Diffusion Surface Technology. What is Die Life? Dies are made of hot work tool steel which was melted, remelted under vacuum, forged, cut into size, machined to the required shape and finally heat treated. Even with the best efforts put in to increase the steel hardness, strength, toughness, ductility and thermal conductivity die tools can start wearing off after 10,000 - 20,000 castings made. Imaging now how much money to be saved by finding a way of increasing the die life to hundred thousand or more castings. Why Does Steel Wear Off? The best steel production process can't avoid internal microscopic cracks, impurities or distortions in the grain structure or vacancies in the crystalline. During casting production the steel surface gets heated up to 600C (1200F) over some seconds and cooled down back to room temperature to get heated up again right after. With the highest temperature changes right at the die surface and much less some millimeters into the steel stress creates due to thermal expanding and contraction. Over time the stress builds up, the steel starts cracking at the impurities and microscopic cracks open. With cracks increasing the poured liquid metal will be squeezed into the cracks and worsens the already high stress level. Cracks grow further. Depending on the casting surface requirements the piece of die steel has to be replaced sooner than later. How Does Thermal Diffusion Works? Compared to Nitriding which creates a surface layer onto the die steel, the Thermal Diffusion Surface Transformation process instills a highly complex chemistry to the surface of the steel which will resist many types of stresses. Diffusion takes place in the first 0.35mm (0.015 inches) below the die surface. By not changing the die component dimensions or the surface finish micro cracks close and toughness adds by increased plasticity and tensile strength. A positive side effect is the increase in wear resistance or in other words minimized erosion to the liquid melt. Soldering in High Pressure Die Casting As much as soldering is needed in multiple industries it is not wanted in a die casting die. Due to high affinity of aluminum to iron there will be a chemical reaction, the aluminum will stick on the die steel surface and has to be removed mechanical which decreases die life. Thermal Diffusion Surface Transformation is not only improving the die steel properties as toughness, strength and wear resistance the used chemistry prevents free iron reaction with aluminum and protects against soldering. Die Wearing Prevention Dies showing wearing can be repaired using Thermal Diffusion Surface Transformation the better way would be to prevent cracking and soldering at the first time. Fluid and heat flow simulation software are very handy to help. Simulation results will point to locations with high die wearing using melt velocity, temperature, time and residual stress results. Areas prone to these defects should be treated before starting casting production and be maintained frequently during production breaks to get the most out of the die.
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ralf kind, die casting, fluid flow simulation, engineering, die life, consulting, process design, high pressure die casting, foundry,
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