Analysis of Outer Fold Defect of HS140HC Casing

The hot-charging and hot-conveyance process is used to produce the Φ273.05 mm×13.84 mm HS140HC casing. After the hot-charging temperature is raised，the outer fold defect occurs. It is regarded via relevant analysis that the cause for the defect is that due to the too high temperature of the blank pipe as being conveyed，the pipe surface temperature falls to the third brittle zone and the dual-phase zone，which leads the original weak area of the pipe body to be jointly affected by the phase transformation，the V carbonitride precipitation，and the stress concentration at the abnormal grain boundary to eventually form cracks that after deformed via rolling，turn into outer folds. Accordingly，it is suggested to get the low alloy steel blank pipe that contains such chemical elements as Nb，V and Ti，etc. cooled down below 500 °C，and then hot-charged and hot-conveyed so as to reduce the occurrence of the said defect.