Validity of Hardness Criteria to Demonstrate Acceptable Temper Bead HAZ Impact Properties for Nuclear Power Applications

Abstract

Temper bead welding is used routinely for weld repair on low alloy pressure vessel steels in the nuclear power industry. Temper bead procedure qualification is contingent on demonstration that the welding process does not degrade the mechanical properties, including fracture toughness, in the base metal weld heat affected zone (HAZ). Historically for temper bead qualification acceptance, adequate HAZ properties have been verified by tensile, impact, and bend testing, while hardness criteria has not been specified. In fact, temper bead welding has been successfully applied for welding on low alloy steels without any hardness criteria for many years. In 2004, ASME Section IX added hardness testing for temper bead procedure qualification when impact testing is not required. The Eurocode, ISO standards, and numerous other European specifications include maximum hardness criteria for general welding procedure qualification and have invoked these same criteria for temper bead procedures. Test results indicate that imposing maximum hardness criteria can actually lead to acceptance of less than optimum fracture toughness in the temper bead weld HAZ due to rapidly changing microstructures in low alloy steels. Impact properties for such microstructures can vary widely even though similar levels of hardness are exhibited. This paper investigates the legitimacy of using maximum hardness criteria to demonstrate acceptable HAZ fracture toughness in low alloy pressure vessel steels.