Forging cracks are generally formed at high temperatures. When forging deformation, the cracks expand and contact with air. Therefore, under the microscope of 100X or 500X, it can be seen that the crack is filled with scale, and both sides are decarburized, and the structure is ferrite. The morphological feature is that the crack is relatively thick and generally exists in multiple forms, no sharp tip, relatively round and pure, and no directionality. In addition to the above typical forms, some forging cracks sometimes appear thinner. The crack is not completely decarburized but semi-decarburized.
The cracks generated during the quenching heating process and the cracks formed during the forging heating process are significantly different in properties and morphology. For structural steels, the heat treatment temperature is generally much lower than the forging temperature, even for high-speed steel and high-alloy steel, the heating and holding time is much smaller than the forging temperature. Due to the high heat treatment temperature, too long holding time or rapid heating, early cracking will occur during the heating process. Cracks are generated along the coarser grain boundaries; there are slightly decarburized structures on both sides of the cracks, and the parts are heated too fast, which will also cause early cracking. There is no obvious decarburization on both sides of the cracks, but the cracks and tails are filled. There is oxide scale. Sometimes due to the failure of the high temperature instrument, the temperature is very high, resulting in a very large part of the structure, and its cracks are distributed along the coarse grain boundaries.
Common defects in structural steel:
1 Forging defects (1) Overheating, over-burning: The main feature is that the grains are coarse and have obvious Wei's structure. The occurrence of over-burning defects indicates that the heating temperature is high, the fracture grains are coarse, the unevenness is uneven, there is no metallic luster, and there is oxidative decarburization around the grain boundary.
(2) Forging crack: It is often caused by coarse grain, stress concentration or segregation of alloying elements, and the inside of the crack is often filled with scale. If the forging temperature is high, or the terminal temperature is low, cracks are likely to occur. Another type of crack is formed by cooling water after forging.
(3) Folding: surface defects are caused by punching, cutting, blade wear, rough forging, etc. In the subsequent forging, defects such as surface oxide scale are wound into the forging body to form a crease. When observed on a microscope, significant decarburization was observed around the fold.
2 Heat treatment defects (1) Quenching: It is characterized by being stiff and straight, showing a transgranular distribution, a wide starting point, and a slender tail. Such cracks are mostly generated after martensite transformation, so the microstructure around the crack is not significantly different from other regions, and there is no decarburization.
(2) Overheating: The microstructure is coarse. If it is mildly overheated, secondary quenching can be used to save.
(3) Over-burning: In addition to coarse grains, some grains tend to melt and the grain boundaries are extremely thick.
(4) Soft point: The microstructure has massive or reticular wrought body and undissolved ferrite. Insufficient heating, insufficient holding time, and uneven cooling will produce soft spots.
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