Suppression of Type IV fracture in welded joints of advanced ferritic power plant steels - effect of boron and nitrogen

The effect of boron and nitrogen on the grain refinement in HAZ during heating and on the Type IV fracture in welded joints has been investigated for P92 and 9Cr-3W-3Co-VNb steel containing boron and or nitrogen. Creep tests were carried out at 650 degrees C (923 K) for up to about 3 x 10(4) h for base metals, simulated HAZ specimens and welded joints. The addition of boron of 47 to 139 ppm to 9Cr-3W-3Co-VNb steel containing minimized nitrogen as low as 10-20ppm suppresses the grain refinement in HAZ and hence suppresses the Type IV fracture in welded joints at low stresses. The addition of excess nitrogen of 0.06% causes the grain refinement during heating at around Ac3 temperature, in both conditions of no boron and high boron of 130 ppm. A combination of high boron of 130ppm and low nitrogen of 80ppm causes no grain refinement in HAZ during heating at around Ac3 temperature as well as significant improvement of creep rupture strength of base metal by fine MX nitrides without any formation of boron nitrides.