Mechanical properties of weld metal are the key factors affecting the quality of heavy-wall X80 hot induction bends. The effects of bending parameters on the mechanical properties of weld metal for hot bends were investigated by simulation conducted on a Gleeble 3500 thermal simulator. Continuous cooling transformation （CCT） dia- grams of the weld metal were also constructed. The influences of hot bending parameters （such as reheating temperature, cooling rate, and tempering temperature） on the microstructure and mechanical properties of weld metal were also analyzed. Results show that the strength of all weld metal specimens is higher than the value indicated in the technical specification and increases with the increase of reheating temperature, cooling rate, and tempering tempera ture. The impact toughness is apparently related to the variation of reheating temperature, cooling rate, and tempering temperature.
Nb is often considered to be a powerful alloying element for controlling the recrystaUization process in mi- croalloyed high strength steels. However, Nb can be presented either as solute in solution, where it is thought to ex- hibit a strong solute drag effect, or as NbC precipitates, which are thought to be effective at pinning grain bounda- ries. Therefore, it is very important to quantitatively measure Nb in solution or in NbC precipitates. A quantitative analysis method of Nb in solution and in precipitates was proposed. The test procedure involved chemical dissolution, filtration and inductively coupled plasma atomic emission spectroscopic （ICP-AES） analysis. The amount of Nb in solution in Nb-microallyed steels under different treatment conditions was evaluated. The results show that the niobium and carbon contents in steels have a great effect on niobium dissolution kinetics. The solute Nb is more effective to retard dynamic recrystallization, while the NbC precipitates are more effective to inhibit static recrystaltization. The results may help to comprehend effect of Nb in steels, and provide some guides in the design of new high strength Nb-bearing steels.
XIAO Fu-renCAO Ya-binQIAO Gui-yingZHANG Xiao-bingLIAO Bo
To research the effect of large precipitates（size 〉 0.2 μm） on strain-induced dynamic transformation, the variation of V contents in large precipitates has been investigated quantitatively in two V–Ti micro-alloyed steels. The results showed that high N content promoted V precipitation on the surface of Ti large precipitates rapidly. Subsequently,large precipitates containing V induced the formation of intragranular ferrite, which accelerated the dynamic transformation process remarkably, promoted the occurrence of continuous dynamic recrystallization of ferrite and improved the refinement effect.
