EBSD investigations of carbides

Powell, G. and V. Randle (1997). "Effect of Si on the relationship between

orientation and carbide morphology in high chromium white irons." Journal of

Materials Science 32(3): 561-565.

Electron back-scatter diffraction (EBSD) has been shown to be the most appropriate

technique to study the orientation and carbide morphology of small ( less than 0.5 mu m)

regions of microstructure of high chromium white irons. The carbides in a slightly hypo-

eutectic Fe-Cr-C alloy show a distinct texture close to left bracket 101 over bar 1 right

bracket whereas those in a 1.3 wt% Si commercial white iron have a diffuse texture, with

regions near to major crystal directions, i.e., left bracket 0011 right bracket , left bracket 1

over bar 21 over bar 0 right bracket , left bracket 011 over bar 0 right bracket ,

unpopulated. Using EBSD, it has been shown that the interconnectivity of the eutectic (Cr,

Fe)7C3 carbide is less in a 1.3 wt% Si alloy compared with a low (0.1 wt%) Si alloy. This

reduced interconnectivity is consistent with the increased fracture toughness in the as-

cast condition. (Author abstract) [References: 10]

Randle, V. and G. Laird (1993). "A Microtexture Study of Eutectic Carbides in White

Cast Irons Using Electron Backscatter Diffraction." Journal of Materials Science

28(15): 4245-4249.

Wachter, O. and G. Bruemmer (1997). "Experiences with austenitic steels in boiling

water reactors." Nuclear Engineering & Design 168(1-3): 35-52.

Stabilized austenitic steels are susceptible to intergranular stress corrosion cracking

(IGSCC) under boiling water reactor (BWR) conditions. This important finding for the

German nuclear power station industry arises from the detection of cracks during the last

3 years in reactor hot water pipes made from titanium-stabilized steel AISI 321 in six

BWRs and in reactor core components made from the niobium-stabilized steel AISI 347 in

one BWR. All the observed cracks had a common feature: they had their origin in the

chromium carbide precipitates at the grain boundaries and in the associated chromium-

depleted region near the grain boundary. These microstructural features in the heat-

affected zones of the hot water pipe weldments were caused by the heat input during

deposition of the root bead. The TiC partially dissolved in the region near the fusion line

and the released carbon reacted to form chromium-rich M/sub 23/C/sub 6/. Regarding the

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cracks found in the core shroud and the core grid plates, it was shown that a sensitizing

heat treatment of rings taken from the same heat of steel could give rise to a

microstructure susceptible to IGSCC in the region of a weldment. High carbon contents

coupled with low stabilization ratios led to sensitization. Residual stresses developed

during welding provided the significant contributions to the tensile stress necessary for

IGSCC. With regard to the service medium, the influence of the electrochemical corrosion

potential (ECP) was recognized as a dominant factor, together with the conductivity. The

corrosion potential was mainly determined by the radiolytic formation of H/sub 2/O/sub 2/;

with increasing distance from the core, the H/sub 2/O/sub 2/ content decreased owing to

catalytic decomposition. For the pipes the problem of IGSCC could be resolved by the use

of optimized steel (lower carbon content with maximum allowable stabilization ratio). (29

References).

Zhou, Y., K. T. Aust, et al. (2001). "Effects of grain boundary structure on carbide

precipitation in 304L stainless steel." Scripta Materialia 45(No. 1): 49-54.

Lassen, N. C. K. and D. J. Jensen (1999). Automatic recognition of recrystallized

grains in partly recrystallized samples from crystal orientation maps. Twelfth

International Conference on Textures of Materials, McGill University, Montreal,

Canada, NRC Research Press.

Lassen, N. C. K., D. J. Jensen, et al. (1992). "Image-processing procedures for

analysis of electron back scattering patterns." Scanning Microscopy 6(1): 115-121.

The authors report the use of electron backscatter diffraction (EBSD) to elucidate more

precisely the orientation characteristics of carbides in both the hypereutectic and

hypoeutectic alloy specimens. The result confirmed previously obtained have X-ray data

which indicated a monocrystalline orientation of carbide rods within a grain in the

hypereutectic alloy specimen and random orientation of carbide rods in the hypoeutectic

specimen. A detailed study was also made of the mechanism of carbide growth. (7

References).

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