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DESIGN FOR OPTIMUM ECONOMY
The price of a casting quoted by the foundry is
relatively unimportant both to the ultimate user and to the designer. Instead, the contribution of the casting cost to the total cost should be considered. This incorporates items such as machining, shipping costs, ease of repair, service life, and others.
Every step in the manufacturing process affects total production economy and each of these is influenced by the designer. A complete review here is neither necessary nor possible. Instead, four major factors influencing economy will be discussed. One additional factor, dimensional tolerance, will be discussed later.
FACTOR #1 STRENGTH REQUIRED
The grade of Ductile Iron selected will have a considerable effect on economy. With reference to the classification on the insert, grade 65-45-12 is the least expensive especially if machining costs are of some consideration. Grade 80-55-06 costs approximately the same to cast but is somewhat less easy to machine. Grade 100-70-03 is still relatively inexpensive and should be selected where its high strength and good wear resistance are utilized. Grades 60-40-18 and 120-90-02 are the most expensive to cast, but either may be the most economical choice if the particular material characteristics are of value in service. The evaluation of the very expensive austenitic grades of Ductile Iron must be considered individually on the
basis of their excellent corrosion, erosion and oxidation resistance, performance at elevated temperatures, magnetic properties, low thermal expansivity and other unique features.
FACTOR #2 MACHINING COST
Machining costs are frequently of major importance from the point of view of economy. Strength, weight, service life, and other considerations may be overruled on occasion to minimize machining cost. The grades easiest to machine are 3 and 4, followed by 5, then 2. Grade 1 is the least machinable although its machinability is superior to steels with the same hardness. The machinability of austenitic Ductile Irons is generally superior to that of stainless steels.
FACTOR #3 COOLING RATE
It will be shown how cooling rate (section thickness) affects the properties of Ductile Iron. Rapid cooling promotes a hard and brittle structure which is difficult-to-machine. In terms of wall thickness, 6 mm or heavier sections are relatively easy to produce without any embrittlement or unduly high hardness. Thinner walls are increasingly more difficult to produce without such deterioration in the as-cast condition. The brittleness and high hardness can be eliminated through heat treatment, but such treatment is expensive and also results in distortion to various degrees. Whenever practical, cast wall thickness should be at least 6 mm to facilitate as-cast delivery. Heat treatment increases casting cost by 10 to 30%.
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