Abstract: As the material most widely used in manufacturing pressure vessels in China, Q345R steel has been permitted in ASME Code Case 2642 to be used for fabricating pressure vessels since 2010. It is listed in the material group corresponding to the exemption curve A for Charpy V-notched (CVN) impact test requirements. However, recent studies indicate that the mechanical property of Q345R has been underestimated and the curve A classification is over-conservative. In this paper, the K_1d–T relationship for two batches of Q345R produced in 2009 and 2014 is empirically obtained by curve–fitting and regression analysis from a large amount of CVN data based on K_1c–CVN correlations and the temperature-shift method. Based on the theory of derivation for the ASME exemption curves, the specific exemption curves for the two batches are generated by combining the K_1d–T relationship and the K_1(min)–t relationship developed from the failure assessment diagram (FAD). Such exemption curve is not in parallel to the ASME curves, and lies over curve C and between curves B and D, but better reflects the actual toughness and expands the impact test exemption area, especially for small components with a thickness less than 20 mm. Furthermore, the method presented in this paper (the Materials Properties Council (MPC) method) is compared with the master curve (MC) method, concluding that the two methods are reliable for determining the exemption curve, and the MC method expands a further area for the impact test exemption and results in a lower minimum design metal temperature (MDMT) than the MPC method.

Key words: Q345R steel; Exemption curve; Master curve (MC); Charpy V-notched impact test; Fracture toughness

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