Alloy Design of MARBN for Boiler and Turbine Applications at 650 ^oC

ABSTRACT

Soluble boron stabilises fine distributions of M₂₃C₆ carbides near grain boundaries (GBs) in 9Cr steel during creep, enhancing GB precipitation hardening, and suppresses the Type IV fracture in welded joints at 650^oC. Excess addition of boron and nitrogen produces BN inclusions at high temperature, degrading creep strength and rupture ductility. A 9Cr-3W-3Co-0.2V-0.05Nb steel with 130 ppm boron and 80 ppm nitrogen, designated martensitic 9Cr steel strengthened by boron and nitrides (MARBN), exhibits not only the much higher creep rupture strength of base metal than Gr.92 but also no Type IV fracture in welded joints at 650^oC. The boron and nitrogen concentrations in MARBN are located just inside the phase boundary for BN at normalising heat treatment temperature in the phase diagram, where we can obtain enough soluble boron and soluble nitrogen but no BN inclusion. The oxidation resistance is improved by the pre-oxidation treatment in argon gas.

KEYWORDS:

• 9Cr steel
• alloy design
• creep strength
• rupture ductility
• welded joint
• grain boundary
• boron
• M₂₃C₆ carbide
• oxidation

Disclosure statement

No potential conflict of interest was reported by the author(s).