ELIMINATING POROSITY
Phase two of the experiment involved investigating remedies to defects in castings poured under adverse conditions.
Ti and Zr Additions
Titanium (Ti) has long been recognized as helpful in reducing subsurface N-related porosity. Varying levels of 70% FeTi (20 mesh and down) were added to the ladle before pouring. In addition, two commercial gray iron inoculants containing Ti were examined. The effect of zirconium (Zr) on porosity reduction was evaluated by adding 0.05% Zr as FeSiZr, as well as incorporating Zr into a high-potency inoculant.
In almost all cases, adding small amounts of Ti to the ladle was effective in eliminating subsurface porosity in castings made with cores bonded with excessive Part II. In the case of 70% FeTi additions, Ti additions of 0.05% were effective in removing subsurface porosity defects; however, a considerable amount of surface porosity or small pores remained.
Since 70% FeTi may be difficult to dissolve below 2700F (1482C) resulting in erratic recoveries, two Ti-containing gray iron inoculants also were investigated. Inoculant A, based on 50% FeSi, was effective in eliminating porosity when the Ti addition level was 0.03%.
Inoculant B is based on 75% FeSi, and since these inoculants dissolve more rapidly than those based on 50% FeSi, Inoculant B was more effective at lower Ti addition rates. No porosity was found when Ti addition levels of 0.025% were employed with Inoculant B. FeSiZr was almost as effective in eliminating porosity, but somewhat higher levels of 0.05% Zr were necessary.
Inoculant C is a potent gray and ductile iron inoculant containing 30-33% oxy-sulfide-forming elements that was modified by the addition of 9% Zr (in the form of FeSiZr). With Zr additions of 0.025%, trace-to-no subsurface porosity was found. Since Zr forms more stable nitrides than Ti, more Zr must be added due to its higher atomic weight. Therefore, higher levels of Zr must be added to Inoculant C for complete porosity elimination. Although Inoculant C did not entirely eliminate porosity, it was the most effective of the three inoculants, reducing chill, and produced the most uniform microstructure consisting of 100% Type A graphite flakes.
Metallographic inspection of the castings made with 70% FeTi showed that higher addition rates of Ti (0.05% and greater) were effective in tying up N as Ti compounds (TiCN or TiN) and preventing re-precipitation as gas holes during solidification. The inoculants produced similar results. The FeTi additions were not, however, effective in preventing surface reactions associated with lustrous carbon pockmarking reactions from high pouring temperatures.
|
[Binder Ratio] [Binder Level] [Casting Temperature] [Section Size]