A Reaffirmation of Fracture Toughness Requirements for ASME Section VIII Vessels for Service Temperatures Colder Than 77 K

Abstract

To assure adequate fracture resistance of cryogenic pressure vessels designed to operate at a minimum design metal temperature (MDMT) colder than 77 K (−196 °C or −320 °F), current American Society of Mechanical Engineers (ASME) Code, Section VIII, Division 1, UHA-51 Impact Test rule requires that the weld metal (WM) meets or exceeds 0.53 mm (21 mils) lateral expansion at 77 K, i.e., LE77K ≥ 0.53 mm (21 mils), as determined using Charpy V-notch (CVN) impact testing. To the credit of this rule, cryogenic pressure vessels fabricated to date meeting the above requirement had continued to serve well—without any adverse incident—in numerous applications across the world, at cryogenic temperatures colder than 77 K. However, a critical examination of the underlying research which relied on a regression equation relating ratio of fracture toughness to yield strength obtained at 4 K, i.e., [KIc/YS]4K with LE77K, revealed that the technical basis for establishing the above requirement is metallurgically unsustainable. To successfully overcome this, the present research employed dimensional analysis and balancing of the previously published regression equations and proposed [KIc/YS]277K as a valid fracture resistance parameter applicable for MDMT 77 K and warmer, as well as MDMT colder than 77 K. Related efforts offered equivalent fracture resistance as an insightful concept, wherein the minimum fracture resistance parameter for a MDMT colder than 77 K is equated as a simple multiple of the minimum fracture resistance parameter at 77 K MDMT. Concluding efforts applied numerical analysis to the equivalent fracture resistance equation to reaffirm the current minimum 0.53 mm (21 mils) CVN LE77K requirement for WM when MDMT is colder than 77 K and to identify minimum required [KIc/YS]277K values for cryogenic service at MDMT 77 K and warmer, and MDMT colder than 77 K. Inherently, the use of [KIc/YS]277K as a fracture resistance parameter offers a tremendous benefit to cryogenic equipment manufacturers, particularly in schedule and cost savings, as LE, KIc, and YS measured at 77 K can be used to successfully assess the fracture resistance at MDMT 77 K and warmer, as well as MDMT colder than 77 K.