Abstract
A brief report of studies is made hereunder of the structure of first weld layer of AN 31 alloy steel mixed with about 5% ferrite by using the NC-32 welding rod, and of its bending creep strength at 650°C in air. The results obtained are as follows.
(1) The boundaries of austenite dendritic grains in the first weld layer are inclosed among the mixed ferrite grains, and no weld crack is observed in this region.
(2) The mixed ferrite phase dissolves into austenite +Cr23 C6 carbite by being heated at 650°C for a long period, and these Cr carbides dispersed in the grain boundary regions accelerate preferred oxidation at the grain boundaries during the creep test.
(3) During the creep test at 650°C the weld specimens mixed with ferrite in the first weld layer show higher creep rate in a steady state for a certain period than the specimens with no mixture
It is considered that the increase of creep rate is due to the fact that the continuity existing among the crystal grains under the maximum bending stress is broken by the preferred oxidation on the boundary.
