There are many theories about the formation of white spots. But it is more convincing and can be proved by practice: white spots are the result of the interaction of hydrogen and tissue stress in steel. The structural stress here mainly refers to the internal stress formed when austenite transforms into martensite and pearlite. Without a certain amount of hydrogen and more significant tissue stress, white spots cannot be formed. However, if the hydrogen content is high and the tissue stress is not large, white spots will generally not appear. For example, single-phase austenitic and ferritic steels have few white spots due to the absence of phase-change structural stress.
How does hydrogen and tissue stress contribute to the formation of white spots? At present, the understanding of these problems is as follows: 1) When the steel contains hydrogen, the plasticity of the steel is lowered. When the hydrogen content reaches a certain value, the plasticity drops sharply, causing hydrogen embrittlement. Especially in the case where there is a long-term stress in the steel, hydrogen can diffuse into the stress concentration region (the tendency of the dissolved hydrogen atoms in the gap to concentrate in the lattice subjected to tensile stress), and the plasticity is reduced to almost equal to zero. Brittle fracture occurs when the stress is sufficiently large. For example, when 25Cr2Ni2Mo steel contains 14.5cm3/100g of hydrogen, it is normalized at 900°C, the elongation after tempering at 600°C is reduced to 0.6%, the area shrinkage rate is reduced to 0; when it contains 7.84cm3/100g of hydrogen, the quenching state When the elongation and the area shrinkage rate are both reduced to 0.20 steel containing 170cm3/100g of hydrogen, the elongation in the annealed state is reduced to 0.2%, the area shrinkage is 0; when it contains 12.76 cm3/100g of hydrogen, the quenched state Elongation and reduction of area are reduced to 0; 2) Hydrogen absorbed in the molten steel during steelmaking precipitates due to reduced solubility when the ingot solidifies. Figure 3-38 shows the solubility curve of hydrogen in iron. It does not have time to escape the surface of the ingot and exists in the inner space of the ingot. When heated before pressure processing, hydrogen is dissolved in steel. During the cooling process after pressure processing, due to austenite decomposition and temperature decrease, the solubility of hydrogen in steel decreases, and hydrogen atoms are precipitated from the solid solution to some microvoids inside the slab. Where. Here, the hydrogen atoms will combine with the state of the component and generate a considerable pressure (when the amount of hydrogen in the steel is 0.001%, the pressure can be as high as 1200 MPa or more at a temperature of 400 ° C). In addition, hydrogen reacts with carbon in steel to form methane (CH4), which also causes a large molecular pressure. This point is confirmed by the decarburization phenomenon on the surface of some white spots; 3) The tissue stress caused by the phase change during the cooling process can reach a considerable value under certain conditions (the more severe the dendritic segregation, the faster the cooling rate The faster the steel, the better the hardenability, the greater the tissue stress. Therefore, the steel hydrogen embrittlement loses its plasticity, and under the joint action of the structural stress and the internal stress caused by the hydrogen deposition, the steel is brittle fracture, which forms a white point. The additional stress caused by uneven deformation during pressure processing and the thermal stress during cooling also have an effect on the formation of white spots.
Because of the large internal voids in the cast steel, hydrogen does not cause a large internal stress when it is precipitated, so it is not sensitive to white spots. Ferritic and austenitic steels do not undergo phase transformation due to cooling, and there is no structural stress, so white spots generally do not occur. Although the tensile stress of the Leysite steel is large, it may be due to hydrogen forming a stable hydride in these steels and preventing the precipitation of hydrogen due to complicated carbides, and no white spots are generated.
White spots are often produced after hours or tens of hours, or even longer, of the forgings after cooling to room temperature. For example, a 160 mm martensitic alloy structural steel billet has no white spots found at 12, 24, and 48 h after cooling, and white spots are not found until 72 h. In addition, after the white point begins to be generated, new white spots are continuously expanded and generated during the subsequent cooling and placement. Therefore, the inspection of the white point should be carried out after a period of time after cooling.
How does hydrogen and tissue stress contribute to the formation of white spots? At present, the understanding of these problems is as follows: 1) When the steel contains hydrogen, the plasticity of the steel is lowered. When the hydrogen content reaches a certain value, the plasticity drops sharply, causing hydrogen embrittlement. Especially in the case where there is a long-term stress in the steel, hydrogen can diffuse into the stress concentration region (the tendency of the dissolved hydrogen atoms in the gap to concentrate in the lattice subjected to tensile stress), and the plasticity is reduced to almost equal to zero. Brittle fracture occurs when the stress is sufficiently large. For example, when 25Cr2Ni2Mo steel contains 14.5cm3/100g of hydrogen, it is normalized at 900°C, the elongation after tempering at 600°C is reduced to 0.6%, the area shrinkage rate is reduced to 0; when it contains 7.84cm3/100g of hydrogen, the quenching state When the elongation and the area shrinkage rate are both reduced to 0.20 steel containing 170cm3/100g of hydrogen, the elongation in the annealed state is reduced to 0.2%, the area shrinkage is 0; when it contains 12.76 cm3/100g of hydrogen, the quenched state Elongation and reduction of area are reduced to 0; 2) Hydrogen absorbed in the molten steel during steelmaking precipitates due to reduced solubility when the ingot solidifies. Figure 3-38 shows the solubility curve of hydrogen in iron. It does not have time to escape the surface of the ingot and exists in the inner space of the ingot. When heated before pressure processing, hydrogen is dissolved in steel. During the cooling process after pressure processing, due to austenite decomposition and temperature decrease, the solubility of hydrogen in steel decreases, and hydrogen atoms are precipitated from the solid solution to some microvoids inside the slab. Where. Here, the hydrogen atoms will combine with the state of the component and generate a considerable pressure (when the amount of hydrogen in the steel is 0.001%, the pressure can be as high as 1200 MPa or more at a temperature of 400 ° C). In addition, hydrogen reacts with carbon in steel to form methane (CH4), which also causes a large molecular pressure. This point is confirmed by the decarburization phenomenon on the surface of some white spots; 3) The tissue stress caused by the phase change during the cooling process can reach a considerable value under certain conditions (the more severe the dendritic segregation, the faster the cooling rate The faster the steel, the better the hardenability, the greater the tissue stress. Therefore, the steel hydrogen embrittlement loses its plasticity, and under the joint action of the structural stress and the internal stress caused by the hydrogen deposition, the steel is brittle fracture, which forms a white point. The additional stress caused by uneven deformation during pressure processing and the thermal stress during cooling also have an effect on the formation of white spots.
Because of the large internal voids in the cast steel, hydrogen does not cause a large internal stress when it is precipitated, so it is not sensitive to white spots. Ferritic and austenitic steels do not undergo phase transformation due to cooling, and there is no structural stress, so white spots generally do not occur. Although the tensile stress of the Leysite steel is large, it may be due to hydrogen forming a stable hydride in these steels and preventing the precipitation of hydrogen due to complicated carbides, and no white spots are generated.
White spots are often produced after hours or tens of hours, or even longer, of the forgings after cooling to room temperature. For example, a 160 mm martensitic alloy structural steel billet has no white spots found at 12, 24, and 48 h after cooling, and white spots are not found until 72 h. In addition, after the white point begins to be generated, new white spots are continuously expanded and generated during the subsequent cooling and placement. Therefore, the inspection of the white point should be carried out after a period of time after cooling.
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