TECHNICAL BASIS FOR THE EXEMPTIONS TO MANDATORY POST WELD HEAT TREATMENT (PWHT) OF SA-738 GRADE B FOR SEC.Ill DIV.1 SUBSECTION NE APPLICATION

摘要

With the current practice in steel making and welding process control, thick and large structural components fabricated from the widely used steel materials such as SA-738 Grade B have good material properties sufficient enough to meet the increasing demand of avoiding a large amount of Post Weld Heat Treatment (PWHT) applications at both shop and site. The original rule in the ASME Boiler and Pressure Vessel Code (the Code), however, allows the exemption to mandatory PWHT for SA-738 Grade B by limiting the applicable material thickness equal to or less than 1 3/4 in. (44 mm) for Section III, Division 1, Subsection NE application. A Code Case was proposed with the purpose to increase the maximum thickness for the exemption of mandatory PWHT from 1 3/4 in. (44 mm) in the original rule to 2 3/8 in. (60 mm). In order to demonstrate the applicability of the PWHT exemption for material with increased thickness, material tests have been performed mainly for the test data required in the ASME Sec. III Div. 1 Subsection NE for three different heats of the plates. The tests performed also include additional data to those required by the Code, such as fracture toughness (K_(JC)) test, microstructural observation, hardness test and oblique Y-groove test for SMAW process. All the tests results for the base materials and weld joints have shown sufficient rationale to increase the exemption thickness to mandatory PWHT to 2 3/8 in. (60 mm).