Study of creep rupture strength in heat affected zone of 9Cr‐1Mo‐V‐Nb‐N steel by welding thermal cycle simulation

Summary

This paper describes an investigation of the creep rupture strength (CRS) of 9Cr‐1Mo‐V‐Nb‐N steel welded joints by welding thermal cycle simulation. The reduction in the CRS of the welded joints is successfully reproduced, and the reasons for this reduction are discussed from the perspective of fine precipitates in the matrix.

The CRS of the simulated HAZ (heat affected zone) varies with the peak temperature of welding thermal cycle simulation (P_T). It falls sharply beyond the A_c1 temperature, gradually reaching the minimum at the A_c3 temperature of 925 °C. At any higher temperature than A_c3, it rises steadily, showing almost the same CRS as the base metal at 1100°C.

The weakest CRS of the simulated HAZ lies at the lowest limit of the CRS data band of various welded joints prepared by GTA (TIG), MMA, and submerged‐arc (SA) welding with the same base metal. The simulated HAZ is useful for evaluation and analysis of the CRS reduction of welded joints.

Heating to the A_c3 temperature by welding changes the shape of VN from thin plates to spheres. This makes the VN smaller‐sized. Moreover, since the A_c3 temperature is too low for Nb to enter into solid solution in the VN precipitated by heating equivalent to post‐weld heat treatment (PWHT), this results in a reduction in the VN lattice constant and a decrease in the coherency strain between the VN and matrix, promoting suppression of local diffusion around the VN.

The above‐noted reduction in VN size and the reduction in misfit with the matrix may well be the reasons for the reduction in the CRS of welded joints.