Suppression of Type IV fracture and improvement of creep strength of 9Cr steel welded joints by boron addition

The microstructure and creep strength of simulated heat-affected zone (HAZ) specimens and welded joints have been investigated for advanced 9–12% Cr steels in order to make clear the mechanisms responsible for Type IV fracture and to improve the creep strength of welded joints at elevated temperature. Creep and creep rupture tests were carried out at 650 °C (923 K) for up to about 2×104 h. The creep rupture time of simulated HAZ specimens has its minimum after heating to AC3 temperature, which produces fine-grained martensitic microstructure with high density of dislocations and large M23C6 carbides but no lath structure. The welded joints were fractured in fine-grained HAZ at low stresses, indicating Type IV fracture, which resulted in shorter creep life than those of base metal. Reducing the width of HAZ by means of EB welding is effective for the extension of creep life but the brittle Type IV fracture appears at low-stress and long-time conditions. The addition of about 100 ppm boron combined with minimized nitrogen of 10–20 ppm suppresses the formation of fine-grained region in HAZ and hence suppresses the Type IV fracture.