From Experience – December 2017: Casting Light on Cast & Wrought Stainless Steel Valves
Stainless steel valves and pipe fittings are manufactured from either “cast” or “wrought” metal (see definitions in Experience in Brief). The differences between cast and wrought provide advantages for the manufacturer and quality of the component for the application intended:
- A valve body formed by a cast alloy allows for a wide range of valve design configurations and minimizes the amount of metal machining. Some degree of metal finishing and polishing may still be required to achieve the required finish specification for internal food-contact surfaces and external finishes for cleaning and appearance.
- Valves and parts manufactured from wrought alloy begin as cast ingots that are worked into the approximate shape and dimensions, and then machined to create the required configuration, cavity spaces, etc. This process provides additional mechanical properties to the metal. High precision parts, and divert and mix-proof valves are manufactured in this manner.
Standards agencies have devised nomenclature to describe the composition of the metal and whether it has been cast or wrought. Most people are very familiar with the wrought alloy designations (304 SSL and 316 SSL) established by the American Iron and Steel Institute (AISI). Stainless steel castings are specified by the Alloy Casting Institute (ACI) designations. These cast components are sometimes identified with an alpha-numeric code embossed on the exterior to reflect the composition of the metal. The table below provides the correlation between the same metallurgical composition in the cast and wrought forms.
For example, the photo below taken by Hixson shows stamping on the exterior of a cast PBM sanitary valve with tri-clamp connections. The stamping indicates that the valve:
- Was manufactured by PBM (Pittsburgh Brass Manufacturing)
- Is two inches in size (2″)
- Was cast from metal suitable for corrosive service (“C”)
- Contains a particular ratio of chromium (17-21%) and nickel (8-12%) (“F”), and a maximum carbon content of less than 0.03% (“3”) and molybdenum (“M”)
The wrought metal equivalent for the metal in this valve is 316L stainless as defined by the AISI.
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