Simple Solutions That Work! Issue 11

I ntroduction: Nothing is more frustrating in foundry operations than finding an outbreak of subsurface porosity after costly machining operations. (see Figure 1) and nitrogen absorption in steel casting operations can result from alloy additions and reactions at the mold metal interface. The ability of molten steel to absorb large amounts of hydrogen and nitrogen is shown in Figure 2. Molten steels can readily absorb hydrogen in excess of the solubility limit. When a casting that contains high hydrogen levels solidifies, subsurface porosity will inevitability result. Hydrogen pickup can occur from almost any source of moisture, such as refractories, green sand molds, chemical mold and core binder decomposition, DR. R.L. (ROD) NARO AND D.C. WILLIAMS ASI INTERNATIONAL, Inc. ARTICLE TAKEAWAYS: • Understanding humidity and resulting porosity • Elimination of Sub-Surface Porosity in High Alloy Steels • Case study shows effective solution for preventing subsurface porosity ELIMINATION OF SUB-SURFACE PINHOLE POROSITY DEFECTS IN ALLOY STEELS BY FERROSELENIUM ADDITIONS 4 Outbreaks of subsurface porosity seemingly always occur during high humidity seasons, and especially during the spring. The prime culprit almost universally is blamed on hydrogen absorption into molten steels. The hydrogen atom is the smallest and simplest of all the elements. It must be noted here that hydrogen can only be absorbed in its mono-atomic state. Along with hydrogen, nitrogen can also play a significant role in the development of porosity, especially in the production of alloy and high alloy steel castings. Both mono-atomic hydrogen Figure 1: Cartoon from Foundry Management and Technology circa 1973 Figure 2: Solubility of Hydrogen and Nitrogen in molten iron (1)