The question of composition has two parts: what is obtainable from foundries and forging shops, and what is or isn't castable or forgeable?

Forgings are produced from billets obtained from a steel mill and in compositions produced by the mill. Mills tend to produce limited grades of steel and special orders can be prohibitively expensive. Because steel foundries are more flexible, the number of chemical compositions obtainable from steel foundries is virtually unlimited.

Although a single foundry cannot supply every conceivable alloy, it is always possible to obtain a unique composition to meet a unique requirement from a variety of foundries at lower cost than competitive product forms.

The presence of controlled amounts of ferrite in certain stainless steels leads to increased corrosion resistance, higher crack resistance and better weldability. Ferrite occurs normally in most cast stainless steels, with the ferrite level controllable to produce the desired combination of characteristics. However, ferrite impairs hot working properties and is normally not present in forged components.

The important class of work-hardenable steels also are not forgeable. Work-hardenable steels are generally high-manganese (approximately 13% Mn) alloys that become harder the more they are worked. Thus, they are ideal for dipper teeth, compactor feet and other earth-moving and excavation applications.

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).

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 tri-axial. 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.

Table 2. Tables 2-1.1 and 9 (ANSI B16.34)