Effect of Coarse Carbide Particle on SR embrittlement in the HAZ of 21/4Cr-1Mo steel

The authors pointed out that growth of the carbide particle is related to SR embrittlement in the HAZ of 2 1/4Cr-1Mo steel in a previous study. Suitable image processing was employed to evaluate the shape of a large number of carbide particles in a SEM image. An image-dividing technique was needed in for the difference of tone in each area of a SEM image. Image processing included image dividing, brightness and contrast, as well as a contraction and smoothing operation. The shape of the particle changed from the sphere type to the needle or polygon type because of the progress of SR treatment. The change of aspect ratio of the particle in the cross-section has a good correlation with the increment of SR treatment time. However, the aspect ratio of the particles on the fracture surface did not correspond with that of the cross-section in the long time range. This was confirmed by SEM observation on a pair of fracture surfaces, where coarse carbide particles were broken into pieces. Coarse carbide particle may accelerate SR embrittlement by acting as a path for crack propagation and a crack initiation site for brittle fracture. The type of carbide particles was investigated by X-ray diffraction. M2C (Mo2C type), M7C3 (Cr7C3 type) and M23C6 (Cr23C6 type) carbides were identified in the time range of SR embrittlement. The coarse carbide observed when SR embrittlement occurred significantly was confirmed as M23C6 type by TEM observation. The calculation results based on the Eshelby theory show that the change of shape of the carbide particle can affect crack propagation of brittle fracture.