Microstructure and Creep Rupture Strength of Ni-Cr-W and Ni-Cr-W-Mo Alloys

Abstract

To obtain the basic knowledge needed for the development of alloys used for heat exchanger which is operated at near 1000°C in the nuclear steel making process, microstructure and creep rupture strength of Ni-Cr-W and Ni-Cr-W-Mo alloys which contain 12-20%Cr, 0-35%W and 0-25%Mo at 1000°C, have been investigated.
The results are summerized as follows:
(1) Solubility limit of Cr and W in Ni-Cr-W system is shown as Cr+ W≅39 (wt%). Above this limit of content of chromium plus tungsten, α-W is observed as second phase. When Mo is added to Ni-Cr-W alloy, Win the second phase, α-W, is gradually substituted by Ni, Mo and finally the second phase changes to μ-phase.
(2) Creep strength of Ni-Cr-W solid solution alloy increases with increasing W content. The high creep strength is presumarably attributed to decrease of diffusion rate with increasing W content. The substitution of W by Mo decreases the creep strength.
(3) Metallic phase α-W increases the creep and creep rupture strength. Intermetallic compound μ, however, increases creep strength, but does not increase creep rupture strength. This is caused by the preferential nucleation of cracks on the interface between matrix and μ-phase.