Simple Solutions That Work! Issue 7

requirements necessitate the use of a weaker than desired material, care must be taken in the design of the part and load on the part to reduce possible failure. Corrosion Resistance/Wettability: Ceramic materials that are used for direct contact with molten metal must be able to withstand the constant effects of metal corrosion and penetration. While some materials can take short term exposure, if extended exposure times (weeks, months) are required, the material must be tested and proven for use in the requested metal alloy, and at the required temperature. Remember that the addition of fluxes into your alloys can greatly change the chemical composition and reactivity of the metal. Also, metal can increase reactivity with increased temperature and this must be taken into consideration when the proper ceramics are selected. Flexural Strength/Modulus of Rupture (MOR): Flexural strength is a material property defined as the maximum stress in a material just before it yields (breaks) in a flexure test. As ceramics really do not bend like a metal would, this property test is the determining factor as to how strong a ceramic material is. Ceramic strength is something that must always be considered in use. While many ceramics are quite strong under compression, compared to metals, they are very brittle and cannot survive being twisted or bent in application. When mechanical strength is needed (ladles or stopper rods, for example), choosing a ceramic that has the highest flexural strength will help extend life in the application. Occasionally, when other property Figure 4. Corrosion test displaying negative results when using material not suited for application Contact: PHILIP GEERS [email protected] 21 Figure 2. Ceramic materials being MOR strength tested Figure 3. Corrosion test displaying non-wetting property of Blasch Nitron™ Below is a table with several common ceramic materials used in aluminum casting applications and their general properties. Blasch Description Part Eng/ Tolerances Shape Porosity Thermal Chemical Product Cost Setup Capability Shock /Corrosion Cost Resistance Resistance Oxytron Oxide Bonded SiC Low Medium +/- 1/2% High 15% High High Nitron Nitride Bonded SiC Medium Medium +/- 1/2% High 15% Medium High Altech Aluminum Titanate High Low Machined High Dense High High Fused Silica Low Low +/- 1% High 5% High Med As you look to improve your casting process and want to improve the life of your ceramic products, leverage your relationship with your refractory provider and share as much information you can regarding your process. Process changes in temperature, thermal cycles, and alloy types will all affect your choice of ceramic. Utilize the knowledge of your refractory supplier to provide you with the best options for improvement. With new ceramic materials being formulated all the time, it is good practice to check in often to see what new products might benefit your casting facility.