William Crosher: Tooth Tips

This is the first in a three-part series in which the author shares his thoughts on castings, discussing how the process has progressed from a crude art to a controlled science.

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Consider iron castings. This form of metal casting, which involves pouring molten metal into a mold until it solidifies, is an ancient process. Early civilizations learned that by using this method the mold shape would be duplicated. In early times little was known about solving casting problems, which can include shrinkage, cavities, inclusions, sulfur content, cooling rates, quenching, and mold material selection, together with other factors that determine the casting quality. Today, not only are these factors closely controlled, but new techniques, processes, and computers provide precise control. Modern day foundries are completely mechanized, and a crude art has now become a controlled science. The casting design knowledge, mold materials, and new quenchants result in high-quality, sound castings in shapes and sizes that can be within the capabilities of forgings.

Cast iron can also be produced through continuous casting. The continuous bar stock is a material very suitable for large production runs, and for gears above four inches in diameter there are usually material cost savings. Continuous-cast gray and ductile iron can perform similarly to free-machining steels, with lower sound levels. The bars range in diameter up to approximately 20 inches with lengths up to 20 feet. They can be austempered, through hardened, flame, or induction hardened.

High-test cast iron is the preferred material for gears that are to mate with non-metallic gears. Steel or bronze gears at a higher cost provide no advantage if they are to run with non-metallic gears. Metal alternatives frequently have a lower durability rating than the cast iron. The cast iron used should always be of high quality, close grained, and free of hard spots. When a cast iron gear is designed to run with a steel pinion, better durability is achieved than is with an untreated steel gear, but at the cost of lower strength.

When cast irons are used at high temperatures they have a tendency to scale due to oxidation. The temperature can also cause growth leading to cracks. The reasons for crack development are different from those seen in hardening carbon or alloy steels. Age strengthening can occur in gray iron castings, but not in steels. Castings fall into several categories including sand, shell molding, investment, permanent, centrifugal, and die. Each method provides different levels of dimensional accuracy.

Now let’s turn to iron casting heat treatments. Iron castings can be given a full annealing to improve their machineability by heating them to approximately 800º C and then slowly cooling to break down the excess carbides. Maintaining a heat of about 450 º C and then allowing slow cooling can relieve stresses. Cast iron gears, especially automotive gears, are frequently induction hardened. Usually gray and ductile (nodular) irons are used. More infrequently, malleable and compacted graphite irons are also induction hardened. There are some important differences to that of carbon steels due to the difference in the critical temperature. Gray irons are difficult to harden as they have a tendency to crack during the rapid heat-up or sudden cooling. It is important to have a good micro-structure, gear design, and proper processing for the best results.

Case hardening of cast iron requires a rapid heating of the surface, usually by flame or induction so the heat remains on the surface. The rapid heating provides a minimum of time for the carbon to diffuse and form a homogeneous austenite prior to quenching. To be effective, therefore, the structure should be fully ferritized, consisting only of ferrite grains and spheroidal graphite. Their case depth is well-defined, with a relatively shallow transition zone.

Sand casting is the oldest-known method of producing an intricate casting. Molten metal is poured into a non-permanent sand mold that has been prepared with a pattern. Green sand molds use synthetic sand or sand in its natural or green state, i.e, damp sand still containing moisture. Green sand was in use as early as the sixteenth century. The method is commonly used for casting small- to medium-size ferrous and non-ferrous castings. A dry sand mold contains sand artificially dried before the molten metal enters. The molds are much stronger than for green sand, as is usually required for the more-intricate castings. Less steam is produced, and this reduces the possibility of blow holes. The sand must be cohesive and refractory (so as to withstand heat without fusing), permeable (porous enough to let gases escape), and strong enough to support the weight and cores. A cast gear even with extra care can only achieve a quality level of A14. 

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is former director of the National Conference on Power Transmission, as well as former chairman of the AGMA's Marketing Council and Enclosed Drive Committee. He was resident engineer-North America for Thyssen Gear Works, and later at Flender Graffenstaden. He is author of the book Design and Application of the Worm Gear.