[China Aluminum Network] White spots are often produced after hours or hours, or even longer, after forgings have cooled to room temperature. For example, for a martensitic alloy steel billet of 160 MM, white spots were not found at 12, 24, and 48H after cooling, and white spots were not observed until 72H. In addition, after the white point starts to appear, it continuously expands and produces a new white point during the subsequent continuous cooling and placement. Therefore, check that the white point should be cooled after some time.
There are many theories about the formation of white spots. However, it is more convincing and can be proved by practice: the white point is the result of a combination of hydrogen and tissue stress in the steel. The structural stress here mainly refers to the internal stress formed when austenite is transformed into martensite and pearlite. Without a certain amount of hydrogen and more significant tissue stress, white spots cannot be formed. However, if only the hydrogen content is high and the tissue stress is not large, white spots do not generally occur. For example, in single-phase austenite and ferritic steels, because there is no tissue stress in the phase transition, white spots rarely occur.
How does hydrogen and tissue stress contribute to the formation of white spots? The current understanding of these problems is roughly as follows: 1) When steel contains hydrogen, the plasticity of the steel is reduced. When the hydrogen content reaches a certain value, the plasticity decreases sharply, causing hydrogen embrittlement. Especially when there is stress in the steel for a long time, hydrogen can diffuse into the stress concentration zone (dissolved hydrogen atoms in the gap have a tendency to concentrate in the lattice that sustains tensile stress), and the plasticity is reduced to almost zero. Brittle fracture occurs when the stress is large enough. For example, when 25CR2NI2MO steel contains 14.5CM3/100G of hydrogen, the elongation at 900°C is normalized, the elongation after tempering at 600°C falls to 0.6%, the reduction of area is reduced to 0, and the hydrogen content of 7.84CM3/100G is quenched. When the elongation and reduction of area are both reduced to 0.20 steel containing 170CM3/100G of hydrogen, the elongation in the annealed state is reduced to 0.2%, the reduction in area is 0; the hydrogen contained in 12.76CM3/100G is quenched. Elongation rate and reduction in area are both reduced to 0; 2) Hydrogen absorbed in steel during steelmaking is precipitated as the solubility of the steel ingot decreases. Figure 3-38 shows the solubility of hydrogen in iron. It is too late to escape the surface of the steel ingot and exists in the internal void of the steel ingot. Hydrogen is dissolved in steel before heating, and the dissolution of hydrogen reduces the solubility of hydrogen in the steel due to the decomposition of austenite and the temperature during the cooling process. The hydrogen atoms precipitate from the solid solution to some microscopic voids inside the billet. Department. Hydrogen atoms will combine into sub-states here, and will generate considerable pressure (when the amount of hydrogen in the steel is 0.001% and the temperature is 400°C, this pressure can be as high as 1200 Mpa or more). In addition, hydrogen reacts with carbon in the steel to form methane (CH4), which also causes significant molecular pressure. This is confirmed by decarbonization on the surface of some white spots; 3) The stress caused by the phase change in the cooling process of the billet can reach a considerable value under certain conditions (the more serious the dendritic segregation, the faster the cooling rate The faster the hardenability is, the better the microstructure stress is. Therefore, the hydrogen embrittlement of the steel loses its plasticity. Under the combined action of the tissue stress and the internal stress caused by the hydrogen evolution, the steel undergoes brittle fracture, which forms a white spot. The additional stress caused by the inhomogeneous deformation during the pressure processing and the thermal stress during cooling also have a certain influence on the formation of white spots.
Due to the large number of internal voids in the cast steel, hydrogen does not cause significant internal stress and therefore is insensitive to white spots. Ferritic and austenitic steels do not undergo phase transformation due to cooling and do not have a structural stress, so no white spots generally occur. Although the cooling of the beryllite steel has a large amount of structural stress, it may be due to hydrogen forming a stable hydride in these steels and due to complex carbides hindering the precipitation of hydrogen, etc., and does not produce white spots.
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