A Realistic Evaluation
An honest presentation of the advantages and limits of both steel component forms allows users to make informed choices when considering a forged or cast design.
By Malcolm Blair and Raymond W. Monroe
Steel Founders’ Society of America, Crystal Lake, Illinois
The principal mechanical properties of interest to designers are strength, ductility and hardness. But how does the user know the mechanical characteristics of a part?
For cast steel, it is relatively easy. If the component is made from a standard alloy, the characteristics are given in a standard specification. If it is made from any other alloy, standard foundry tests will provide the answers. The values will apply to that component regardless of the axis along which measurements were made.
Many metal parts are made from rolled products like bars or plates. The rolling process changes the properties of the metal. The major advantage is that the strength is increased in the rolling direction or the longitudinal axis. Both forgings and fabrications have directional properties as a result of the rolling process.
However, tensile strength, elongation and impact properties decrease in the transverse and axial directions. Thus, wrought steel and forgings in particular, are anisotropic (exhibiting different values of a property in different directions). For equivalent alloys, the ductility and impact strength of steel castings generally lie between the longitudinal and transverse values of forgings. In castings, the metal is isotropic, with similar properties in all directions (see Fig. 1–2).
Fig. 1. These graphs demonstrate the relation between the mechanical properties of rolled steel and the angle of inclination of the test specimen
Fig. 2. This figure illustrates the influence of forging reduction on anisotropy for a 0.35% carbon wrought steel. Properties for a 0.35% carbon cast steel are shown in the graph by a star (*) for purposes of comparison.
With respect to the mechanical characteristics of a forging, most forging references provide only longitudinal characteristics. To obtain the transverse or axial characteristics, the user will probably have to request them specifically.
Additionally, the service conditions of the components must be carefully evaluated. If the loading is uni-axial along the longitudinal axis, then the directionality of the forging is an advantage. As the stresses increase in any other direction, directionality becomes a problem. Pressure vessels are good examples of applications where stresses are triaxial.
The design code (ANSI B16.34) used by most flow control manufacturers doesn’t indicate that forged products offer any mechanical property advantage over cast products (Table 2).
In terms of temperature extremes, corrosion resistance and wear resistance, “equivalent” castings and forgings generally perform equally well throughout the temperature range and are generally equally resistant to corrosion and wear.
With regard to corrosion, however, cast stainless steels with controlled amounts of ferrite will probably be superior to their forged counterparts, since ferrite generally increases corrosion resistance. With regard to wear, work-hardenable steels can, for practical purposes, only be obtained as castings.
Check back next week to read part 3.
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