HEAVY SECTION 9Cr-1Mo STEEL PLATE WITH IMPROVED WELDABILITY AND CREEP RUPTURE STRENGTH OF WELDED JOINTS

Effect of alloying elements on weld cracking susceptibility and toughness of the heat affected zone (HAZ) were studied on 9Cr-1Mo steels for tube sheet and shell plate of Fast Breeder Reactor (FBR). A mechanism of ferrite formation during welding and creep rupture strength of welded joints was also discussed. The results are shown as follows: 1) The results observed in the microstructure of the HAZ of 9Cr steels welded by GTAW or SAW is regarded as delta ferrite, and the delta ferrite content decreases with the increase in heat input up to a critical cooling rate(γ → α transformation). 2) Reducing the ferrite content is effective for improving HAZ toughness. Higher contents of ferrite are found in the coarse grain region of HAZ than in the base metal. Controlling the ferrite content in welds is important in alloy design of 9Cr-steel in order to obtain high toughness both in base metal and in welds. 3) A large difference in creep strength between the softened zone of welds and the base metal brings about strain concentration to the weak softened zone, resulting deterioration in creep strength of welded joints. 4) 20, 50, 250, 300 and 550 mm thickness 9Cr-1Mo-V-Nb steel plates were manufactured by commercial scale, They exhibited excellent toughness, a high weld cracking resistance and a high creep rupture strength of welded joints.