There are various types 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 easy to find out exactly which process the crack occurred in, and it is beneficial to analyze the cause of the crack.
First of all, it is necessary to define the concepts of “raw material cracks” and “forging cracks”. The cracks that appear after forging should be understood as “forging cracks”. However, the main factors that lead to forging cracks can be divided into:
1. Forging cracks caused by raw material defects;
2. Forging cracks caused by the improper forging process.
From the macroscopic form of the crack, we can roughly distinguish it. The transverse direction is generally not related to the base material, and the longitudinal crack needs to be analyzed in combination with the crack form and the forging process.
There is decarburization on both sides of the crack, which must have occurred during the forging process. As for whether it is caused by 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 type, the forging cracks are basically in one position, the extension is relatively shallow under the microscope, and there is decarburization on both sides. However, material cracks do not necessarily appear repeatedly at the same position, and the depths are different under the microscope. More watching and more analysis, there are still certain rules.
Most of the material cracks are consistent with the longitudinal direction of the material. There are two kinds of forging cracks, one is caused by overheating and overburning, and there is oxidation and decarburization near the cracks. Another is that hitting cold iron can also cause cracking, which is a phenomenon of lattice damage and tearing. Can be distinguished from the metallographic.
Purpose of forging:
1. Forming requirements;
2. Improve the internal structure of the material, refine the grain, and uniform element composition and structure;
3. Make the material denser (shrinkage cavity or looseness of the original unexposed air inside the forging material, etc.), and the streamline distribution is more reasonable;
4. Serve the next process through reasonable heat treatment after forging.
Therefore, certain defects inside the forging and forging raw materials are the responsibility. Large castings and forgings are often directly forged from steel ingots, and there must be a large number of casting defects inside the steel ingots. Obviously, reasonable forging can forge the so-called “defects” in them. Therefore, the rationality of the forging process is the main reason for determining whether the forging will crack.
Of course, relative to a certain stable forging process, if there is a clear control requirement on the raw material defect level before forging, when the defect level of the raw material exceeds the requirements and the cracking phenomenon occurs during forging under the original forging process, we can consider it as “raw material”. Forging cracks caused by defects”.
The crack problem should be analyzed in detail, combined with the analysis of the process, including whether there is a protective atmosphere during the heating process. Forging should be to forge and seal the cracks of the raw materials. Oxidized skin is usually dense and gray. The sample preparation process makes it very loose and dark in color. It can be seen at high magnifications. It is really impossible to distinguish. It must be able to distinguish directly by energy spectrum.
• Forging cracks
Forging cracks are generally formed at high temperatures. During forging deformation, due to the expansion of cracks and contact with air, when observed under a 100X or 500X microscope, it can be seen that the cracks are filled with oxide skin, and both sides are decarburized, and the structure is ferrite. , Its morphological characteristics are that the cracks are relatively thick and generally exist in multiple forms, without sharp tips, relatively round and pure, and without clear directionality. In addition to the above typical forms, sometimes some forging cracks appear thinner. Around the crack is not fully decarburized but half decarburized.
Example of a typical forging crack:
More oxides on the edges.
• Heat treatment cracks
There are obvious differences in nature and shape between the cracks generated during the quenching heating process and the cracks formed during the forging heating process. 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 much shorter than the forging temperature. Due to the high heating temperature of heat treatment, too long holding time or rapid heating, early cracking will occur during the heating process. Cracks distributed along the boundaries of coarser grains are generated; there is a slight decarburization structure on both sides of the crack, and the heating speed of the part is too fast, and early cracking will also occur. There is no obvious decarburization on both sides of the crack, but the crack and its tail are full There is scale. Sometimes due to the failure of high-temperature instruments, the temperature is very high, resulting in the structure of the part is extremely coarse, and its cracks are distributed along the boundaries of coarse grains.
Typical quench crack example:
Under 500X, it is jagged, the crack at the starting end is wide, and the crack at the end is small to none. No abnormal metallurgical inclusions are found in the crack, and there is no decarburization phenomenon. The crack extends in a zigzag shape, which has the typical characteristics of quenching cracks.
• Causes of forging cracks and heat treatment cracks
1. Reasons for forging cracks: During the forging process of steel, due to the surface and internal defects of steel, such as hairlines, trachoma, cracks, inclusions, subcutaneous air bubbles, shrinkage cavities, white spots and interlayers, etc., may become forged cracks s reason. In addition, due to poor forging process or improper operation, such as overheating, overburning or too low final forging temperature, too fast cooling rate after forging, etc., the forgings will also crack.
2. Causes of heat treatment cracks: quenching cracks are macro cracks, mainly caused by macro stress. In the actual production process, steel workpieces are often due to factors such as unreasonable structural design, improper steel selection, incorrect quenching temperature control, and unsuitable quenching cooling rate. On the one hand, increasing the quenching internal stress will make the formed quenching microscopic Cracks expand to form macroscopic quenching cracks. On the other hand, due to the increase in the sensitivity of microcracks, the number of microcracks increases and the brittle fracture resistance Sk of steel is reduced, thereby increasing the possibility of quenching cracks. .
• Factors Affecting Quench Cracking
There are many factors that affect quenching cracking. Here we will only introduce several situations that are often encountered in production. the
1. Quenching cracks caused by existing defects in raw materials: If there are defects such as cracks or inclusions on the surface and interior of raw materials, which are not found before quenching, quenching cracks may form.
2. Cracking caused by inclusions: If the inclusions inside the part are serious, or there are already hidden cracks due to serious inclusions, cracks may occur during quenching.
3. Quenching cracks caused by poor original tissue.
4. Quenching cracks caused by improper quenching temperature: Quenching cracks caused by improper quenching temperature, there are generally two situations:
(1) The temperature indicated by the instrument is lower than the actual temperature of the furnace, so that the quenching temperature is too high, resulting in overheating of the quenching and cracking of the workpiece. Coarse grains and coarse martensite exist in all metallographic structures cracked by overheating quenching.
(2) The actual carbon content of the steel part is higher than the content specified in the steel grade. If it is quenched according to the normal quenching process of the original grade, it is equivalent to increasing the quenching temperature of the steel, which will easily cause overheating of the parts and grain growth, making the quenching time Stress increases to cause quenching cracking.
5. Quenching cracks caused by improper quenching and cooling: due to improper cooling during quenching, quenching cracks will also occur in parts.
6. Quenching cracks caused by machining defects: Due to poor machining, thick and deep tool marks are left on the surface of the parts. Even if it is a very simple part or not a place of stress concentration, it will also cause cracks during quenching, or Early damage occurs during service.
7. The influence of part shape on quenching cracks: the geometric shape of parts is unreasonable, or the thickness of the cross-sectional transition zone is quite different, and cracks are prone to occur due to stress concentration during quenching.
8. Cracking caused by untimely tempering: If it is not tempered in time after quenching, cracks may occur due to excessive quenching residual stress.
• How to identify cracks
How to distinguish whether it is quenching cracks, tempering cracks, forging cracks or grinding cracks is very important, so that it is easy to accurately find out which process the crack occurs in, and it is beneficial to analyze the cause of the crack.
First, pay attention to the difference between quenching cracks and grinding cracks. For the cracks that were not found during quenching but were discovered after grinding, it is necessary to distinguish whether they are quenching cracks or grinding cracks. It is easier when no pollutants are attached to the crack. At this time, pay attention to the shape of the crack, especially the direction of crack development. The grinding crack is perpendicular to the grinding direction, in the form of parallel lines, or in the shape of a tortoise shell. The depth of grinding cracks is shallow, while quenching cracks are generally deep and relatively large, which has nothing to do with the grinding method, and most of them are linear knife-like cracks.
Second, pay attention to where the crack occurs. The sharp concave-convex corners, the edge of the hole, the engraved place, the stamped place, and the surface defects caused by mechanical processing, etc., the cracks that occur in these parts are mostly quenching cracks.
Third, by observing the fracture surface of the part, it is distinguished whether it is a quenching crack or a forging crack before quenching or a crack caused by other conditions. If the crack section is white or dark white or light red (rust caused by water quenching), it can be concluded that it is a quenching crack. If the crack section is dark brown, or even has oxygen skin, it is not a quenching crack, it is before quenching. The cracks that exist are the cracks formed when the parts are forged or rolled, and these cracks will be enlarged due to quenching. Because the quenching crack is basically formed below the MS point, its cross section will not be oxidized.
Fourth, in the microstructure, the quenching crack is fractured along the grain boundary. If the fracture is not along the grain boundary but along the intragranular fracture, it is a fatigue crack.
Fifth, if there is a decarburization layer around the crack, it is not a quenching crack, but a crack that exists before quenching, because the quenching crack is produced during quenching and cooling, and decarburization will never occur.