The white point is one of the shortcomings that the forging produced during the post-forging cooling process. Its shape is a fine hairline acute angle crack on the lateral low-magnification test piece, and the fracture is silvery white freckles. Photo 6-13 is the white point on the longitudinal fracture of the Cr-Ni-Mo steel forging. Its shape is not standard, the size is very different, very small long axis size is only 2mm, very large is 24mm. The white point is a brittle sharp edge crack with great risk and is a very dangerous shortcoming in martensite and pearlitic steel.
The cause of white spots is that hydrogen in steel is concentrated in the tensile stress region under stress, which causes so-called hydrogen embrittlement and brittle fracture, so that the joint use of hydrogen and additional stress is caused by white spots.
The countermeasures are:
1) The hydrogen content in the low steel, such as paying attention to the baking charge, fully boiling during smelting, vacuum degassing, refining outside the furnace and so on.
2) Accepting the heat treatment to eliminate white spots, the important task is to disperse hydrogen in the steel and eliminate stress, such as hydrogen expansion annealing heat treatment. See the shortcomings of the post-forging clearing process in the most common shortcomings in the casting process.