Grassy waves are a frequent defect in ultrasonic testing of large forgings. It is generally believed that grassy waves are produced by coarse crystals. Once the grass wave appears in the ultrasonic inspection, the most effective treatment method is to normalize the grain and eliminate the grass wave by normalizing.
However, in the production practice, it was found that in order to prevent the occurrence of grassy waves, one or two normalized forgings were carried out in the post-forging heat treatment, and grass-like waves may still appear during the flaw detection. It seems that the cause of the grass-like waves is still It is for further discussion.
According to current reports, there are two reasons for the formation of grassy waves: one is believed to be caused by coarse crystals, which is a generally accepted view; the other is caused by defects such as tiny cracks and holes. of. It can be seen that in order to prevent the grass from appearing in the ultrasonic inspection of the forging, it is necessary to create conditions to refine the grains and prevent micro cracks inside during the forging and post-forging heat treatment.
1. About refining grains
The reason why the grass wave is generated by the coarse crystal is that the normalizing can eliminate the grass wave, refine the grain by normalizing, and eliminate the grass wave, thereby inferring that the grass wave is generated by the coarse crystal.
The austenite grains in the large forgings are coarse and uneven. Therefore, refining the grains is one of the important tasks in the post-forging heat treatment of large forgings. For large forgings, there are not many methods for refining austenite grains. It is described in the literature that increasing the heating rate of the α→γ phase transition zone can refine the austenite grains, but it is necessary to increase α→γ for particularly large forgings. The heating rate of the phase change zone is very difficult, and the effect is also limited.
The most effective way to refine austenite grains is to normalize, and sometimes multiple normalizations are used to achieve the desired results. Whether the uniform austenite grains are obtained, or the relationship between the original coarse grains and the grain size after re-austenification is obtained, the first austenitizing heating temperature is required to be slightly higher in multiple normalizing. Some, when the austenite grains grow larger, the second austenitizing heating should use a temperature that does not cause the grains to grow significantly. For the Cr-Mo-V steel with relatively stable carbides, the second austenitization should also consider the dissolution of the carbides on the cleavage surface to a considerable extent, and the subsequent cooling process is expected to be A fine bainite structure is obtained by dissolving fine carbides as a core.
Air cooling or blast cooling is often used in multiple normalizing to reduce the transformation temperature to obtain finer grains. In order to lower the temperature and turn the supercooled austenite decomposition temperature in the center of the forging, the subcooling temperature should be selected to be lower. Whether the cooling rate in normalizing has an effect on the grain refining effect has not been reported, and for large forgings, rapid cooling is difficult to achieve.
2. About the healing of tiny cracks
In addition to coarse crystals, there will be other defects that will produce grassy waves. An in-depth understanding of the nature of the grass wave is important for the development of a reasonable heat treatment process to prevent the occurrence of grass waves. For the cold-rolled work roll blank, the sample was cut at the portion where the ultrasonic wave was detected by the ultrasonic wave, and it was found that many fine cracks were densely distributed in the central region f 50 to f 70 mm of the horizontal acid-immersed low-magnification test piece. And holes, no regularity, the cracks should not exceed 3mm. A crack formed in a strip shape or a hole in the longitudinal direction of the longitudinal fracture, and there is no inclusion in the crack.
The grass wave is also a defect wave, but it does not reach the specified defect equivalent under the specified sensitivity. It can be seen that the grass wave may be produced by a large number of small defects, and the loose and large number of tiny cracks formed by many tiny holes will also produce grassy waves.
It is widely believed that the reason for normalizing to eliminate grass waves is because the grains are refined. However, we should also see that in addition to refining grains, normalizing is also likely to cause small defects to heal. Cracks or hole defects in metal do not need plastic deformation at high temperatures, and can be repaired for a certain period of time, sometimes called Self-repairing. In the test, relatively large artificial defects can be repaired by heat treatment, that is, the crack can be repaired by keeping it at a certain temperature for a certain period of time. Hydrogen corrosion of methane bubbles and cracks in 304 stainless steel under high temperature and high pressure hydrogen environment was heat treated at 600 ° C for 6 h, and hydrogen bubbles and cracks completely healed. 20MnMo steel has a length of 6mm×0.3mm artificial pre-cracking. The repair speed is slower below 900°C, the repair speed is 900~1050°C, and it is completely repaired at 1200°C.
Some studies have carried out verification tests on self-repairing methods for five large tube sheet forgings that have been tested and scrapped. According to the material and size of different tube sheet forgings, the heating time, heating temperature and holding time were determined. After high temperature treatment, it was confirmed by re-testing that two defects were completely eliminated. As a qualified product, the other two cracks were reduced from f 900mm to f 300mm, and the single defect equivalent was reduced from f 14mm to f 9mm. The forgings are basically qualified. This method has also been used to treat six scrapped shaft forgings, which are also fully qualified. It can be seen that the crack repair speed has a strong dependence on the heating temperature, and prolonging the holding time has a certain promoting effect on the crack self-repair.
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