Why are there white spots on forgings?

Release time:

2024-01-19 09:37

Usually the heat treatment of large forgings is combined with the cooling of the forgings with each other, due to the large cross-section size of large forgings and the complexity of the production process. In the heat treatment process, the organisation and properties of forgings due to non-uniformity, some forgings are prone to white point defects. Therefore, large forgings heat treatment in addition to eliminating stress, reduce the hardness, the main purpose is to prevent the forging of white point, improve the uniformity of the chemical composition of the forgings, adjust and refine the organisation of the forgings.

Large forgings in the white point is a very fine brittle cracks within the forgings, round or oval silver-white spots, diameter size from a few millimetres to tens of millimetres. According to the microstructure observation, in the neighbouring area of the white spot no traces of plastic deformation were found, so the white spot is a brittle rupture.

The presence of white spots in forgings not only leads to a sharp decline in mechanical properties, due to the white spots bring a high degree of stress concentration, heat treatment quenching will make the parts crack, or parts used in the sudden fracture, resulting in accidents caused by the destruction of the machine.

Therefore, the white point is a defect of the forging, large forgings technical conditions clearly stipulate that once found white point must be scrapped. On the theory of the formation of many white spots, the current more consistent view that: white spots are hydrogen and internal stress in steel (mainly organisational stress) the common effect of the results. Without a certain amount of hydrogen and greater internal stress, white point can not be formed.

In the forging after forging cooling overload, with the temperature reduction due to austenite transformation, in addition to the forging will produce internal stress (mainly tissue stress), while hydrogen in the steel solubility decreases, at this time, the internal stress so that dislocations in the sub-granular boundary convergence and constitute a sub-microcosmotic fissure, when the hydrogen atoms dissolved from solid solution dialysis to the sub-microcosmotic fissure, the hydrogen atoms in the fissure combined with a hydrogen molecule and produces a lot of pressure. Therefore, in the high hydrogen content of steel in the local brittle place, in the organisational stress and hydrogen precipitation stress, so that the sub-microscopic cracks continue to expand to rupture, resulting in very fine internal cracks, so that the formation of white point.

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