This recommended practice (RP) includes materials and fabrication requirements for new 11/4Cr-1/2Mo steel and 1Cr-1/2Mo pressure vessels and heat exchangers for high temperature service. It applies to vessels that are designed, fabricated, certified, and documented in accordance with ASME Code Section VIII, Division 1. This document may also be used as a resource when planning to modify existing pressure vessels. The interior surfaces of these pressure vessels may have an austenitic stainless steel, ferritic stainless steel, or nickel alloy weld overlay or cladding to provide additional corrosion resistance. This recommended practice is applicable to wall (shell) thicknesses from 1 in. (25 mm) to 4 in. (100 mm). Integrally reinforced nozzles, flanges, tubesheets, bolted channel covers, etc. can be greater than 4 in. (100 mm). At shell or head thicknesses greater than 4 in. (100 mm), 11/4Cr-1/2Mo and 1Cr-1/2Mo have been shown to have difficulty meeting the toughness requirements given in this document, but this does not preclude the use of this alloy if these properties can be met or if the equipment is designed with stresses below the threshold for brittle fracture. Although outside of the scope, this document can be used as a resource for vessels down to 0.5 in. (12.7 mm) shell thickness with changes defined by the purchaser. This recommended practice is not intended for use for equipment operating below 825 °F (440 °C). Refer to API 934-C for information on design for equipment operating at lower temperature ranges. Since hydrotreaters typically are designed to temperatures lower than 825 °F (440 °C), the guidelines in this recommended practice do not apply to most hydrotreaters. Also, since coke drums typically fail due to fatigue and not due to reheat cracking; these recommended practices may not be appropriate for all aspects of coke drums.