Metal casting—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, but today these factors are closely controlled, with new techniques, processes, and computers providing precise control. Modern-day foundries are completely mechanized, and a crude art has now become a controlled science.
Cast iron can also be produced through continuous casting. 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, and lengths up to 20 feet. They can be austempered, through-hardened, and 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 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 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:
Sand Casting: This 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. The sand must be cohesive and refractory (withstand heat without fusing), permeable (porous enough to let gases escape), and strong enough to support the weight and cores. Such a cast gear, even with extra care, can only achieve a quality level of A14.
Die Casting: Split molds are known as a die. Die-casting is a large production method for castings that weigh less than 100 pounds. The two most popular die methods are gravity-die and pressure-die. The molds and machinery are expensive items, since gravity-die production requires a minimum of 500 pieces while the pressure-die process requires 2,000 or more. Gravity-die castings are superior to sand and pressure-die castings both in structure and strength. When every effort is made, the quality can be as high as A9. Precision-casting, investment-casting, and lost-wax process are various names for the process by which small castings can be produced to a close dimensional tolerance and good surface finish. It is an expensive process, but maybe the only way to produce an intricate casting. Continuous casting is the method used to produce ingots that can be reheated and rolled to produce more convenient sizes, such as billets and slabs.
Vacuum Casting: Vacuum castings provide the capabilities associated with investment castings at costs equivalent to green sand castings. The process has only been viable since the 1970s, requiring microprocessor technology. It is a cost effective alternative to stamping and forging on parts less than 150 pounds. The advantages claimed are that the undesirable impurities evaporate, there are consistent properties, and the formation of oxides and nitrides is minimized. New alloys with high-temperature properties have been developed as a result.
Iron Casting Heat Treatments: Iron castings can be given a full annealing to improve their machinability 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 microstructure, 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.
