iron casting

It is sometimes necessary to machine tension test specimens with a reduced section of 0.357 in. diameter from a casting, since the casting shape does not allow making a larger size specimen. Some casting users have raised the question of the reliability of the smaller specimens. Over a period of years, several size specimens have been taken from the same casting, and it has been found that, if machining is carefully done, the results are reliable. The data in Fig. 6 are representative and have been obtained from a small automotive clutch plate. The hardness and tensile strength data for this casting show that a 5/8-in. plate has a similar cooling rate to a 1.2-in. test bar.

Ductile Iron Casting In trying to predict tensile strength in a casting from Brinell hardness, there are more factors involved than mere sec kness. For sections from which heat flow during cooling is unimpeded, a very good hardness-tensile strength relationship can be established. For more complex castings, such as diesel engine cylinder heads having many cored passages, the cooling pattern may be complicated. The section within the head may freeze fairly rapidly, but, after the eutectic temperature interval is passed, there is a heat build up, and the section may cool more like a simple section two to four times as thick. For such cases, a correlation needs to be worked out for each type of casting.

Green Sand Casting Steel shows a rather minor influence of tensile strength and hardness on the modulus of elasticity, since it is mostly in the range of 29,000,000 to 30,000,000 psi. For gray iron, the modulus of elasticity not only varies with tensile strength but also with the stress level. As a result of these factors, the modulus of elasticity will vary from around 12,000,000 psi for a very soft iron to over 20,000,000 psi for a high strength iron. The stress-strain curve for gray iron in tension is almost a curved line from the origin. This has been reported by many investigators, and Morrogh[14], in reporting some work by Gilbert, suggests that the curve is a result of some volume changes in the spaces occupied by the graphite. They have also shown that some microcracking takes place between flakes. Some investigators have used resonant frequency measurements and also sound velocity measurements which are dependent on modulus of elasticity to predict tensile strength.

Resin Sand Casting In machine tool and other applications where maximum stiffness of a structure is desired, a high modulus of elasticity is desirable. There are other applications, notably those involving thermal fatigue for which a low modulus of elasticity is wanted to minimize the increase in stress levels associated with expansion resulting from temperature increases under operating conditions. High-duty brake drums are an example of this type of situation. It has been found that a rather high-carbon iron (3.60 to 3.92 percent) will give better service than a lower carbon iron. The higher-carbon irons nearly always have a lower modulus of elasticity. Unfortunately. the tensile strength tends to be low with such high-carbon irons. and it becomes necessary to add an alloy to strengthen the matrix.

Alloy Steel Casting engineers often look on percent elongation as obtained from tension test specimens as a measure of the ductility of the material. With this concept. gray iron would not be considered ductile.

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