Creep rupture behaviour and recrystallisation in HR6W and Haynes alloy 230 cold bent boiler tubing for ultrasupercritical steam boiler applications

Abstract

Creep rupture experiments were conducted on tubes of HR6W and Haynes 230, candidate ultrasupercritical alloys, to evaluate the effects of cold work and recrystallisation during high temperature service. These creep tests were performed by internally pressurising cold bent boiler tubes at 775°C for times up to 8000 h. The bends were fabricated with cold work levels beyond the current ASME Boiler and Pressure Vessel Code section I limits for austenitic stainless steels. Destructive metallographic evaluation of the tube bends after testing was used to determine the effects of cold work and the degree of recrystallisation. The metallographic analyses, combined with an evaluation of the creep and rupture data, suggest that solid solution strengthened nickel based alloys can be fabricated for high temperature service at ultrasupercritical conditions utilising levels of cold work higher than the current allowed levels for austenitic stainless steels.

Keywords:

• NICKEL BASED ALLOYS
• COLD WORK
• PRESTRAIN
• CREEP
• RECRYSTALLISATION