The selection of a particular material of construction for bodies and bonnets is often based on economics, material strength, erosion resistance, and the compatibility of the selected materials with the process fluid (corrosion resistance). A basic description of some common body and bonnet materials follows. Additional information on material designations, characteristics, and applications is included in Figure 12, page 18.
1) Historically, carbon steel has been the standard material for control valve bodies and bonnets. Cast carbon steel welds easily, which allows simple installation and weld repair of damaged bodies. ASME SA216 Grade WCB has been quite popular throughout the valve industry. Many valve manufacturers have switched from WCB to WCC because WCC offers higher strength and increased pressure-temperature ratings. WCC and WCB should not be applied at temperatures below -20 degrees F. LCC and LCB are the standard low-temperature carbon steel grades, and they can be applied at temperatures as low as -50 degrees F.
2) Alloy steels (also known as chrome-moly alloy steels) are often specified when the application pressures and/or temperatures exceed the limits for carbon steel. Alloy steels also provide greater erosion resistance than ordinary carbon steels; therefore, alloy steel bodies and bonnets are typically specified for erosion resistance; i.e., for applications where the fluid contains erosive particles or vapor droplets.
Historically, ASME SA217 Grade C5 has been very popular; however, C5 tends to form cracks during the casting process and whenever it is welded. WC6 is another alloy that has enjoyed some popularity because its pressure and temperature ratings are somewhat greater than the ratings for C5; however, it has the same tendency to crack as C5. WC9 is another chrome-moly steel that has greater strength than C5 or WC6 under most conditions. WC9 casts well, and it can be weld-repaired if necessary. For these reasons, some manufacturers have standardized on WC9 as a standard chrome-moly body and bonnet material.
3) Stainless steels provide excellent high and low temperature performance and corrosion resistance in a wide variety of environments. CF8 (type 304 stainless steel) is preferred for some corrosive fluid applications. CF8M (type 316 stainless steel) offers a good balance of strength, corrosion resistance, and economics. Generally speaking, CF8M bodies have higher pressure-temperature ratings than CF8.