Plate materials are procured according to the materials specification and grade shown on the vessel drawing, or as proposed by the fabricator. These materials are selected to obtain the properties (strength, hardness, toughness, and corrosion resistance) required for the service conditions and process environment (see Section 500), and are designated by reference to a “SA Specification” in ASME Code, Section II.
The model specifications for pressure vessels (PVM-MS-4748, PVM-MS-4749, and PVM-MS-4750) impose a few requirements, such as CV-impact testing, to prevent brittle fracture that exceed ASME Code requirements. Furthermore, these specifications require the fabricator to verify that the required materials properties will be obtained by the materials in a condition that is representative of the completed vessel.
The fabricator orders the plate materials using a purchase specification that is a part of the Quality Control System that the fabricator must establish to obtain an ASME Code Certificate of Authorization for use of the “U” or “U2” symbols. These materials must comply with the requirements of the SA Specification in ASME Code, Section II, and the supplier must provide the fabricator with a “Materials Test Report” for each lot of material certifying that all of these requirements in the specification have been met. The authorized inspector (see Section 200) is required to examine these test reports to verify that they correctly represent the lots of material delivered to the fabricator and that all of the requirements of ASME Code, Section II, have been met.
The fabricator should incorporate any additional requirements in the pressure vessel specification into his materials purchase specification. The fabricator must also recognize that forming, welding, and postweld heat treatment can significantly alter the properties of the material after it is received from the supplier. Therefore, the fabricator must consider the changes in materials properties that will result from the fabrication procedures that will be used, and prepare his materials purchase specification to allow for these changes. The fabricator may perform supplemental tests required to confirm that the material properties specified in the pressure vessel specification will be obtained in the completed vessel, by simulating the anticipated thermal history (i.e., heat treatment, hot forming, reheat treatment, welding, and postweld heat treatment, etc.) with test coupons. Alternatively, the fabricator can have the materials supplier perform these tests, to be certain that the material supplied will meet the property requirements for the completed vessel. The Code, however, does not require this supplemental testing in all cases. The company inspector should review these test data to determine that the materials property requirements in the pressure vessel specification have been met.
The deformation of plate material associated with the forming of the shell and head components of a vessel can reduce the thickness of the plate. This must be estimated by the fabricator, based upon his experience; and plate materials that are somewhat thicker than the minimum required thickness (plus corrosion allowance) of the formed shell component must be procured from the supplier.
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