Effect of Boron Addition on Creep Strength in 9Cr-3W-3Co-V-Nb Steels

Recently, boron is being added to high-strength steels for boilers and turbines in order to raise creep strength. However, there are various points that are not yet fully understood with regard to the creep-strengthening mechanism of boron addition. In order to investigate the influence of boron addition on creep strength, varying the amounts of boron and nitrogen were added in 9Cr-3W-3Co-V-Nb steels tempered two different conditions and investigated. Although creep strengths of all materials converged to become similar in the low-stress region, the materials tempering at 780°C with boron addition which exhibited the lowest strength on the high-stress region is seen to have slightly greater strength on the low-stress region. In analysis of the precipitates and grain boundary of creepruptured material conducted using TEM-EDS and Auger diffraction analysis equipment, the presence of boron could not be detected.