インコネル600粗大柱状晶を用いた水素割れの方位差依存性の研究

Abstract

The effect of misorientation on the intergranular cracking of cathodically hydrogen-charged specimens under the external tensile stress was investigated on Inconel alloy 600 with coarse columnar crystals. As the deviation of the growing direction from 〈100〉 orientation was within 10°, the grain boundaries were treated as simple tilt boundaries with 〈100〉 rotation axis. Misorientation of a grain boundary is described in terms of the three angles, relative tilt angle (ω), tilt angle (α) and symmetrical tilt angle (θ), determined with {100} planes nearest to the grain boundary plane. The depth of intergranular cracking (D) of the specimens water-quenched and sensitized at 973 K for 7.2 ks changed over a wide range up to 200 μm. There is no clear relation between D and ω. It was found that D increased with decreasing α in classified cases of ω=2°∼10° and ω=40°∼45°, and also with decreasing θ. This result shows that grain boundaries of which the planes are close to {100} of adjacent crystals have higher susceptibility to cracking. Markedly lower susceptibilities of symmetrical coincidence boundaries such as ∑5 and ∑13 are recoganized against the normal relation of D with θ. Shallower transgranular cracks compared with intergranular cracks were always observed along {100} plane. The relationship between misorientation of grain boundaries and carbide precipitation is not clear, expect the fact that symmetrical coincidence boundary and tilt boundary with ω=2°∼10° are carbide-free.