Improvement of creep strength of advanced ferritic steel welded joints

The microstructure and creep strength of simulated 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 crack and to improve the creep strength of welded joints at high temperature. Creep and creep rupture tests were carried out at 650°C (923 K) for up to about 104 h. Creep crack growth tests were also carried out using CT specimens. The creep rupture time of simulated HAZ specimens has its minimum after heating to AC3 temperature, which produces fine-grained microstructure. 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 even in the EB welded joints at low stress and long time conditions. The welded joint specimens were fractured in fine-grained HAZ and resulted in shorter creep life than those of base metal due to the formation of creep voids and cracks. Using a specially-designed FEM code for Type IV crack growth behaviour, the effect of creep ductility and void formation ahead of the crack tip on creep crack growth rate is successfully simulated. The Type IV fracture is suppressed by the addition of boron, which suppresses the formation of fine-grained region in HAZ.