There are various kinds of cracks: raw material cracks, heat treatment cracks, forging cracks, etc., which make people dizzy. How to identify them is a very important course, so that it is convenient to find out exactly which process the cracks occur in and to analyze the causes of the cracks.

The solution is "forging crack", but the main factors leading to forging crack can be further divided into:

1. Forging cracks caused by defects in raw materials;

2. Forging cracks caused by improper forging process.

The macroscopic morphology of the crack should be roughly distinguished first. The transverse crack is generally independent of the base metal. The longitudinal crack should be analyzed in combination with the crack morphology and forging process.

There is decarburization on both sides of the crack, which must be caused by the forging process. As for the raw material or the forging process, it needs to be analyzed according to the metallographic and technological process.

For the same batch of workpieces of the same model, the forging cracks are basically in the same position, and the extension is shallow under the microscope, with decarburization on both sides. However, the material cracks do not necessarily recur at the same location, and the depth is different under the microscope. There is a certain rule to see more and analyze more.

Most material cracks are consistent with the longitudinal direction of the material. There are two kinds of forging cracks, one is caused by overheating and burning, and there is oxidation decarburization phenomenon near the crack. Another is that striking cold iron will also cause cracking, which has the phenomenon of lattice damage and tearing. It can be distinguished by metallography.

Purpose of forging:

1. Forming requirements;

2. Improve the internal structure of the material, refine the grain, and homogenize the element composition and structure;

3. Make the material more dense (shrinkage or porosity of the original unexposed air inside the forged material, etc.), and streamline distribution more reasonable;

4. Serve the next process through reasonable post-forging heat treatment.

Therefore, it is the responsibility of forging and forging raw materials to have certain internal defects. Large castings and forgings often start directly from the forging and pressing of steel ingots. There must be a large number of smelting and casting defects inside the steel ingots. Obviously, the so-called "defects" can be forged together by reasonable forging. Therefore, the rationality of forging process is the main reason to determine whether the forging will crack.

Of course, relative to a certain stable forging process, if a clear requirement for raw material defect level control is put forward in advance for raw materials before forging, when the raw material defect level exceeds the requirement and the cracking phenomenon occurs during forging under the original forging process, we can consider it as "forging crack caused by raw material defect".

The specific problem of crack should be analyzed in combination with the process analysis, including whether there is a protective atmosphere in the heating process. Forging should be done by forging the cracks of raw materials. The oxide scale is usually dense and gray, and the dirt caused by the sample preparation process is loose and the color is black. At a high magnification, it can be seen that it can't be resolved directly by energy spectrum.

Forging cracks

Forging cracks are generally formed at high temperatures. During forging deformation, due to the crack expansion and contact with air, it can be seen under a 100X or 500X microscope that the crack is filled with oxide skin, and both sides are decarburized, and the structure is ferrite. Its morphological characteristics are that the crack is relatively thick and generally exists in multiple forms, without detailed tip, relatively round and pure, without detailed directionality, except for the above typical morphology, Sometimes some forging cracks are relatively fine. The crack is not completely decarburized but semi-decarburized.

Heat treatment crack

There are obvious differences in nature and morphology between the cracks formed during quenching and heating and the cracks formed during forging and heating. For structural steel, 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 far less than the forging temperature. Due to the high heating temperature of heat treatment, long holding time or rapid heating, early cracking will occur during the heating process. Cracks distributed along the boundaries of coarser grains are produced; There is a slight decarburization structure on both sides of the crack. If the heating speed of the part is too fast, it will also cause early cracking. There is no obvious decarburization on both sides of the crack, but the crack and its tail are filled with oxide skin. Sometimes due to the failure of high temperature instruments, the temperature is very high, resulting in the extremely coarse structure of the parts, and its cracks are distributed along the boundaries of coarse grains.

Causes of forging crack and heat treatment crack

1. Cause of forging crack: During the forging process of steel, due to the surface and internal defects of steel, such as hairline, sand hole, crack, inclusion, subcutaneous bubble, shrinkage cavity, white spot and interlayer, etc., may be the cause of forging crack. In addition, due to poor forging process or improper operation, such as overheating, overburning or too low final forging temperature, and too fast cooling rate after forging, the forging will also crack.

2. Causes of heat treatment cracks: quenching cracks are macroscopic cracks, mainly caused by macroscopic stress. In the actual production process, due to the unreasonable structure design, improper selection of steel, incorrect control of quenching temperature, improper quenching cooling rate and other factors, the steel workpiece will expand the formed quenching micro-cracks and form macroscopic quenching cracks by increasing the internal stress of quenching, on the other hand, the sensitivity of micro-cracks will be increased, the number of micro-cracks will be increased, and the brittle fracture resistance of steel will be reduced, Sk, Thus increasing the possibility of quenching crack formation.

Factors affecting quenching crack

There are many factors that affect the quenching crack. Here, we will only introduce some cases that are often encountered in production.

1. Quenching crack caused by existing defects of raw materials: if there are cracks or inclusions on the surface and inside of raw materials, which are not found before quenching, quenching cracks may be formed;

2. Cracks caused by inclusions: if the internal inclusions of the parts are serious, or cracks have been hidden due to the serious inclusions, cracks may occur during quenching;

3. Quenching crack caused by poor original structure;

4. Quenching crack caused by improper quenching temperature: quenching crack of parts caused by improper quenching temperature generally has two conditions:

(1) The indicated temperature of the instrument is lower than the actual temperature of the furnace, which causes the quenching temperature to be on the high side, resulting in overheating of the quenching and cracking of the workpiece. The metallographic structure of all overheated quenching cracks has coarse grain and coarse martensite.

(2) The actual carbon content of steel parts is higher than the content specified by the steel grade. If the steel is quenched according to the normal quenching process of the original grade, the quenching temperature of the steel is increased, which is easy to cause overheating and grain growth of the parts, and increase the stress during quenching, resulting in quenching crack.

5. Quenching crack caused by improper quenching and cooling: due to improper cooling during quenching, parts will also have quenching crack accidents.

6. Quench crack caused by machining defects: due to poor machining, rough and deep knife marks are left on the surface of parts. Although it is a very simple part or not a place where stress is concentrated, it will also cause cracking during quenching or early damage during service.

7. The influence of part shape on quenching crack: the part geometry is unreasonable, or the thickness of the section transition zone varies greatly, which is easy to cause cracks due to stress concentration during quenching.

8. Cracks caused by untimely tempering: if the tempering is not conducted in time after quenching, the cracks may be caused by excessive quenching residual stress.

Crack resolution method

It is very important to distinguish whether it is quenching crack, tempering crack, forging crack or grinding crack, so as to find out exactly which process the crack occurs in and analyze the cause of the crack.

First, pay attention to the difference between quenching crack and grinding crack. For cracks not found during quenching but found after grinding, it is necessary to distinguish between quenching cracks and grinding cracks. It is easier when the crack is not attached with pollutants. At this time, pay attention to the shape of the crack, especially the direction of the crack development. The grinding crack is perpendicular to the grinding direction, in the form of parallel lines, or in the form of tortoiseshell cracks. The depth of grinding crack is shallow, while the quenching crack is generally deeper and larger, which has nothing to do with the grinding side, and is mostly in the form of straight knife cutting.

Second, pay attention to the location of the crack. Sharp concave-convex corners, hole edges, engraving, steel stamping and surface defects caused by machining, etc. The cracks in these parts are mostly quenching cracks.

Thirdly, by observing the crack section of parts, we can distinguish whether it is caused by quenching crack, forging crack before quenching or other conditions. If the crack section is white, dark white or light red (water rust caused by water quenching), it can be determined as a quenching crack. If the crack section is dark brown, even with oxygen skin, it is not a quenching crack. It is a crack that existed before quenching, and it is a crack formed when the parts are forged or rolled, and these cracks will be expanded due to quenching. Because the quenching crack is basically formed below the MS point, its section will not be oxidized.

Fourth, in the microstructure, the quenching crack is broken along the grain boundary. If it is not broken along the grain boundary, but along the grain, it belongs to fatigue crack.

Fifthly, if there is decarburization layer around the crack, it is not a quenching crack, but a crack that existed before quenching, because the quenching crack is generated during quenching and cooling, and decarburization will never occur.

Source: Hot Working Forum

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