General Properties Of Materials
Following is a list of materials that are commonly specified for control valves:
• Cast iron is a low cost, non-ductile (brittle) material that is typically specified only for low pressure steam, water, gas, and non-corrosive fluids. Cast iron is brittle, is easily fractured, and cannot be weld repaired. These are some of the reasons that Section 4.1.3 of SAES-J-700 disallows the use of cast or ductile iron for control valve bodies.
• Cast carbon steels (WCC, WCB) are popular body and bonnet materials because of their low cost and good strength over a wide range of temperature conditions.
• Cast alloy steels or chrome moly steels (C5, WC9) are also standard body and bonnet materials. Cast alloy steels are typically specified for applications that require more erosion resistance and/or higher pressure and temperature ratings than can be obtained with cast carbon steel.
• Various grades of cast stainless steel (CF8, CF8M) are typically specified for bodies and bonnets that are applied in high and low temperature applications and in corrosive applications. Stainless steel in several different grades (410, 416, 316) is also the industry standard trim material for mild to moderately corrosive and erosive applications. Increased durability and erosion resistance is achieved through hardening of the material (either by heat treating or cold work), through the application of coatings (such as electroless nickel coating, or ENC), or through the application of hardfacings such as Stellite (CoCr-A) or other erosion resistant materials.
• High nickel content alloys (including Monel, Inconel, Hastelloy, and others) are often selected for valve bodies and trim components that are applied in extremely corrosive or caustic applications.
• Cobalt alloys such as Alloy 6 (Stellite, or CoCr-A) are hard and tough materials that are commonly specified because of their superior erosion resistance.
Critical surfaces of trim and bodies may be hardfaced with cobalt materials (by weld depositing the material), or trim components may be machined from wrought and cast forms of the material.
• Other hard and tough materials such as tungsten carbide and ceramics (also referred to as cermets, or ceramics/metals), are sometimes specified to provide erosion resistance in exceptionally erosive applications.
Figure 12 includes a chart of popular materials, their primary characteristics, and their
applications.
