TIN BRONZE, LEADED TIN AND BRONZE

Because of their good mechanical properties and corrosion resist­ance, these alloys are often used in castings subject to hydrostatic or air pressures, as in valves, pumps, etc, The quality of such castings must be unusually high; they must be free of internal porosity, shrinkage, or other defects.
Good foundry practice is of critical importance in the casting of tin bronzes and leaded tin bronzes. All of these alloys have a wide freezing range of from 200" to 300" F. To produce sound castings in any of the tin bronzes, gates and risers should be designed so that the closest possible approximation is made to true directional solidification.
Gating must be non-turbulent and the metal flow distributed through as many gates as possible to avoid hot spots. The mold should be filled quickly. Risers should be placed so that they do not unnecessarily delay the freezing of heavy sections. Such delay results in a coarse. open structure that will leak under pressure. Risers should preferably feed hot metal directly into the casting without forcing it to cross through any thin section of the casting.
The use of chills with these alloys is highly desirable, and it is often necessary to use them liberally if complex castings are to be made sound.
It is very difficult to feed tin bronze into a parallel section. one inch or more thick. by a riser alone.. Tin bronzes begin freezing at the sand-metal interface and freeze progressively toward the center. leaving a pasty mixture of liquid and solid in the center. The flow of molten metal from the riser into a high, thick section will usually be obstructed by the already solidified portion of the wall. The liberal use' of chills in the drag of such a casting, or riser hot-­topping compounds and riser insulators, may be helpful in produc­ing sound sections.
Because of the solidification pattern of tin bronzes, ga5CS may aggravate the shrinkage condition by enlarging cavities until they are interconnected and can cause leakage. Following the melt­ing practices listed in Part I of this Section should help minimize this condition.
The tin bronzes improve in cast ability and soundness when phosphor copper is used as a deoxidizer.