Metallurgical Analysis of Chip-off Observed in 0.3%C-Cr-Mo-V Flow-Formed Steel

Flow forming is the most demanding metal forming technique for fabricating thin-walled cylinders with excellent surface finish and strength without material wastage. A flow-formed cylinder made up of 0.3%C-Cr-Mo-V electroslag refined ESR steel as a part of fabrication of motor cases for launch vehicle. During the sectioning activity of the flow-formed cylinder, a chip-off occurred circumferentially near the inner diameter (ID) side. The chip-off location was subjected to a detailed metallurgical investigation to identify the reason for cracking. Fractography analysis confirms that the material fractured in a brittle manner, with transgranular cleavage features observed at the fracture surface. After detailed analysis, it was found that the chipping-off of the flow-formed cylinder was attributed to the presence of residual stress in the material resulting from radial compression and axial stretching of the hardware during flow forming, which got released/ redistributed and generated chip-off. Crack initiated at aligned carbide particles present in between the highly deformed inter-martensitic lath layers and propagated through the layer of fine martensitic grains.