1. Forging heat treatment of large forgings
After forging, the subsequent heat treatment is called post-forging heat treatment. The purpose of post-forging heat treatment of large forgings is mainly to destress, recrystallize and refine grains, and simultaneously dehydrogenate.
(1) Recrystallization treatment Large forgings undergo multiple recrystallization treatments to refine grains, improve microstructure, and improve performance.
(2) Dehydrogenation annealing reduces the hydrogen in the forging to below the limit hydrogen content and makes it evenly distributed to avoid the danger of white spots and hydrogen embrittlement.
Since the solubility and diffusion coefficient of hydrogen in the γ phase and the α phase are different, in particular, the solubility of hydrogen in the α phase is low, and the diffusion coefficient is large, so that hydrogen is continuously diffused outward during annealing.
Dehydrogenation annealing is often combined with recrystallization. The temperature for dehydrogenation annealing is usually 650 °C. After annealing, it should be cooled as slowly as possible to prevent new internal stresses. Generally, the cooling is divided into two stages: above 400 °C. Since the plasticity of the steel is good, the internal stress is not easy to form, so the cooling can be faster; below 400 °C, the cooling rate is slowed down.
For forgings with more alloying elements and higher performance requirements, one or even multiple recrystallization heat treatments are required after forging to improve the structure and properties of the forgings.
2. Precautions in post-forging heat treatment of large forgings
(1) Purchase and storage of steel materials should be selected in the spring, summer, autumn and winter seasons when purchasing steel. In order to ensure the quality of steel in terms of purity and avoid the impact of the annual rainy season on steel, there must be one in the quantity of steel. Quantitative reserves are urgently needed under the premise of ensuring quality.
(2) Sensitive forgings of white spots During the cooling process, the hydrogen on the surface easily diffuses and escapes, while the hydrogen in the core is not easily discharged. Therefore, the larger the cross section of the forging, the easier it is to produce white spots. As the cross section of the forging increases, the degree of segregation of the steel becomes more serious, and it is easier to promote the formation of white spots.
The larger the surface area of ​​the forging, the easier the precipitation of hydrogen. Therefore, hollow forgings (such as ring gears) have less white point sensitivity than solid forgings (such as gear shafts).
The white point sensitivity of low carbon steel with w C <0.3% is extremely low. As the carbon content increases, the white point sensitivity of the steel increases. The faster the forging is cooled after forging, the greater the white point sensitivity.
Steel that does not produce white spots: such as austenitic steel, ferritic steel, austenitic-ferritic steel, and lysitic steel. Under normal production conditions, carbon steels with w C <0.3% and w Mn <0.6% will not form white spots. Therefore, it is not necessary for such steels to perform dehydrogenation annealing after forging.
Steel sensitive to white spots: pearlite steel has less sensitivity to white spots, martensite-pearlite steel has greater white point sensitivity, and martensitic steel has the greatest white point sensitivity. Such as: 20Cr2Ni4, 18Cr2Ni4W, 20CrNi2Mo, 40CrNi2Mo, 40CrNiMo and so on. The white point limit hydrogen content of martensitic steel is ≤1.8×10-6.
(3) Forging of large forgings that have been whitened and scrapped. Forgings that have produced white spots can be processed into smaller-sized parts blanks by hot working such as forging and forging. Slow cooling and annealing can save some of the lost losses.
(4) Providing a quality inspection report while delivering the forgings While the forging factory is delivering the forgings, it must produce: 1 purity [H], [N], [O] content. 2 chemical composition, mechanical properties. 3 forging ratio. 4 Quality inspection report of related content such as ultrasonic nondestructive testing.
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