Creep Rupture Properties of Weded Joints of W Containing 9Cr Ferritic Heat Resistant Steel

Abstract

The creep rupture properties of 9Cr-Mo-W welded joints were studied. The creep rupture property of HAZ was also studied by means of simulated HAZ subjected PWHT (Post Weld Heat Treatment). The effect of W on creep rupture strength of welded joint was discussed.
The creep rupture tests of GTA welded joint were conducted. The longest stress rupture time is about 20000 hours. In the creep rupture test, the welded joints ruptured in the base metal at higher applied stresses, while they ruptured in the fine grained HAZ adjacent to the base metal with low ductility at lower applied stresses.
When the welded joints ruptured in the base metal, the creep rupture strength was as high as that of the base metal. However, when the welded joints ruptured in HAZ, the creep rupture strength of welded joints was lower than that of the base metal. The crack which occurred in HAZ was Type IV cracking which tends to occur in the welded joints of ferritic heat resistant steel.
In the creep rupture test, the simulated HAZ heated up to a temperature around Acl and Ac3 temperatures gave lower creep rupture strength than that of the base metal. Especially the simulated HAZ heated up to Ac3 temperature gave the lowest creep rupture strength. The Type IV cracking owes to the fine grained HAZ.
The comparison of creep rupture strength of welded joints between 9Cr-Mo-W and 9Cr-Mo-Nb-V (Mod. 9Cr-Mo) indicated that W improved the creep rupture strength of welded joint as well as base metals.