Continuing our conversation on different types of castings and related treatments, let’s turn our attention to bronze treatments. The heat treatment of a gear bronze is rare, because such treatment dissolves the hard delta phase. This dissolving occurs at 1100ºF; if lower temperatures are used it is possible to relieve the segregation of tin in the matrix without affecting the delta phase. Of major importance are the chilling rate and pouring temperature of the molten metal.
Cast aluminum alloys: These alloys are grouped according to their response to heat treatment. They are all suitable for sand casting, and the majority of them are produced by gravity die-cast. Only a few can be pressure die-cast as cast aluminum alloys—including alloys of magnesium-manganese, copper-silicon, and silicon—receive no benefit from heat treatment. The heat treatable aluminum alloys include, copper-magnesium-silicon-nickel, and copper-magnesium-silicon.
Steel castings: More than 40 ASTM specifications refer to steel castings, and they are not easy to comprehend. Standard specifications do exist, and they are the most popular. Many areas can be specified, but they should always include dimensions, composition, mechanical properties, casting integrity, and inspection requirements. Extra time is required in checking a new specification, and a standard specification that is written around the application’s needs should always be used. Tolerance grade tables for castings of different weights and sizes are available in the Steel Casting Handbook. The ASTM specifications allow the user to obtain the required quality without over specifying. A gear casting should provide the required ultimate tensile strength and fatigue properties, impact toughness, ductility, and on occasion, corrosion resistance. When the composition and mechanical properties are to an ASTM specification, they must be verified by test.
When it is a standard alloy steel casting, the mechanical properties are stipulated in the specification. If it is a nonstandard alloy, conventional foundry tests supply the data. The property values will be non-directional (isotropic), i.e. they will be the same regardless of the axis along which they are measured. Mechanical properties of steel castings can be:
• Tensile properties—elongation, reduction of area, tensile, and yield strength;
• Impact properties—The term toughness in steel is defined as the property determining the amount of energy absorbed before fracture, involving both ductility and strength;
• Fatigue properties—Data for S/N curves are obtained by testing on rotating or bending beam machines;
• Hardenability—This will determine the depth and distribution of hardness that can be induced by quenching. It is not the ability of the material to harden, but to be through-hardened. A choice of production methods is available such as open hearth, electric arc, and induction furnace. Carbon and alloy steel castings are mostly used for gears that are through-hardened and, to a lesser extent, for case-hardened gears.
The hardenability of steel is achieved by increasing the alloying content with such elements as manganese, chromium, and molybdenum. Castings are supplied in a wide range of designs such as being cast in segments, or as a solid or split ring. Considering casting defects, the most common defects that are to be avoided are:
• Blowholes—These cavities are not always observable. They can appear during machining and when the teeth are being cut. They have several causes, primarily, excessive moisture in the mold, foreign material in the poured metal, and trapped gases;
• Porosity—A defect that is due to dirt or faulty composition of the metal, it will prevent the casting from being pressure proof;
• Sponginess—This usually appears in heavy sections that were not properly fed during the pouring process;
• Scabbiness—This occurs when portions of the sand break away from the mold cavity and stick to the casting surface;
• Hard spots—When the material has hard spots it becomes a problem for the cutting tool during machining. Too-rapid cooling of the thin sections and poor metal composition are the prime causes;
• Cold shuts—This is a problem created by two streams of molten metal failing to combine in the mold by pouring too slowly or at too low a temperature;
• Core displacement—Any movement of the core results in a poor casting;
• Cracks—They will occur if there is uneven cooling or the metal is poured at too high a temperature.