1995 年 13 巻 4 号 p. 573-578
The creep rupture strength (CRS) of welded joints of 9Cr-1Mo-V-Nb-N steel was studied by means of welding heat cycle simulation. The reduction of CRS in the welded joints has been successfully reproduced, and the reasons for the reduction was discussed from the points of fine precipitates in the matrix.
The CRS of simulated HAZ (Heat-Affected Zone) changes with the peak temperature of welding heat cycle simulation (PT). It goes down sharply beyond Ac1 temperature, then gradually reaches to the bottom at Ac3 temperature of 925°C. At higher temperatures than Ac3, it goes up steadily and shows almost the same CRS as base metal at 1100°C.
The weakest CRS of simulated HAZ lies just at the lowest limit of CRS data band of various welded joints prepared by GTAW, SMAW and SAW with the same base material. The simulated HAZ is useful for evaluating and analysing the reduction of CRS in the welded joints.
Heating up to Ac3 temperature by welding changes the shape of VN from thin plate to sphere. This makes the size of VN smaller. Moreover, the Ac3 temperature being too low for the solution of Nb, VN that precipitaes further during PWHT (Post-Weld Heat Treatment) contains little amount of Nb ; It causes the contraction of lattice constant of VN, and the decrease of coherency strain between VN and matrix, which is beneficial for the suppression of local diffusion around VN. These changes could be the reasons for the reduction of CRS of HAZ.