There are many kinds of cracks: raw material cracks, heat treatment cracks, forging cracks, etc., which make people dizzy. How to identify cracks is a very important process, which helps to analyze the causes of cracks.
First, it is necessary to define the concepts of raw material crack and forging crack. Cracks after forging shall be understood as forging cracks. However, the main factors causing forging cracks can be divided into:
1. Forging cracks caused by defects in raw materials;
2. Forging cracks caused by improper forging process. Starting from the macroscopic form of cracks, a rough distinction should be made first. Horizontal cracks are usually independent of the base metal. It is necessary to analyze longitudinal cracks in combination with crack form and forging process. During forging, both sides of the crack must be decarburized. For raw materials or forging process, it is necessary to analyze according to metallography and process. For the working parts of the same batch and model, the forging cracks are basically in the same position, relatively shallow under the microscope, and decarburization occurs on both sides. Material cracks may not occur repeatedly at the same location, and the depth under the microscope will also be different. There are some rules that can be seen and analyzed more. The material crack is mainly consistent with the material crack in the longitudinal direction. There are two kinds of cracks, one is caused by overheating and overheating, the other is cold crack and iron crack, which will lead to crystal crack.
Purpose of forging:
1. Form requirements;
2. Improve the internal structure, fine grain, uniform element and structure of the material;
3. Make the material more dense (original air shrinkage hole or loose internal forging material, etc.), and streamline distribution more reasonable;
4. Provide services for the next process through reasonable post-forging heat treatment methods. Therefore, it is responsible for some internal defects of forging raw materials. Large castings usually start directly from ingot forging, and ingots must have a large number of smelting and casting defects. Obviously, reasonable forging can be called defect forging. Therefore, the rationality of the forging process is the main reason to determine whether the forging crack will lead to the raw material crack. Of course, if a clear level of raw material defects is proposed in the forging process of raw materials, then when the raw material defects exceed the requirements of raw material defects, we can identify the main causes of raw material defects. The concrete analysis of the crack problem should be considered and combined with the process to ensure the existence of raw material defects. Forging shall be closely combined with crack forging of raw materials. Oxidized skin is usually dense gray. The dirt caused by the sample preparation process is very loose. The color is black. You can see it at a glance. This is really indivisible. It must be able to distinguish the direct spectrum.
Forging cracks 
Forging cracks usually form at high temperatures. When the forging deformation is observed under a 100X or 500X microscope, due to crack expansion and contact with air, the crack is full of oxidized skin, decarburized on both sides, and the structure is ferrite. Its morphological feature is that the crack is relatively thick, usually in many forms, without details, relatively pure, and no detailed direction. In addition to the above typical forms, some relatively fine forging cracks sometimes occur. The crack is not completely decarburized, but semi-decarburized. Typical example of forging crack: more oxide on the edge.
Heat treatment of cracks
There are significant differences in the nature and shape 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 far lower than the forging temperature, even for high-speed steel. The heating and holding time of high alloy steel is far lower than the forging temperature. As the heating temperature of heat treatment is too high, the holding time is too long or the heating is too fast, early cracking will occur during the heating process. Cracks distributed along the thicker grain boundary; There are slight decarburization structures on both sides of the crack. If the heating speed of parts is too fast, early cracks will also occur. There is no obvious decarburization on both sides of the crack, but the inside and tail of the crack are full of oxidized skin. Sometimes, due to high temperature instrument failure, the temperature is very high, the structure of the part is very thick, and the cracks are distributed along the coarse grain boundary.
Typical examples of quenching cracks:
At 500X, it is serrated. The crack at the starting end is very wide, and the crack at the starting end is very small until there is no crack. There is no abnormal metallurgical mixing and decarburization at the crack. The crack extends in a zigzag shape, with typical quenching crack characteristics.
The cause is forging crack and heat treatment crack
1. Causes of forging cracks: During the forging process, the causes of forging cracks may be the surface and internal defects of steel, such as hairlines, sand holes, cracks, sundries, subcutaneous bubbles, shrinkage holes, white spots, interlayers, etc. In addition, due to improper forging process or operation, such as overheating, overheating or low final forging temperature, too fast cooling rate after forging, etc., This can also lead to forging cracks.
2. Cause of heat treatment crack: quenching crack is a kind of macroscopic crack mainly caused by macroscopic stress. In the actual production process, steel parts often have problems due to unreasonable structural design and improper selection of steel. Quenching temperature control is incorrect. On the one hand, with the increase of quenching internal stress and the expansion of quenching internal stress, macro quenching cracks are formed. On the other hand, due to the increased sensitivity of microcracks, the number of microcracks increases, and the brittle fracture resistance SK of steel decreases, thus increasing the possibility of quenching cracks.
Factors affecting quenching crack.
How to distinguish quenching crack, tempering crack, forging crack or grinding crack is very important. It is easy to find the crack process accurately and is conducive to analyzing the cause of the crack. First, please note the different forms of quenching crack and grinding crack. Quenching crack or grinding crack shall be divided into quenching crack or grinding crack. When cracks and dyes appear, it is easier to pay attention to the form of cracks, especially in the direction of crack development. The grinding crack is perpendicular to the grinding direction, in parallel linear shape, or tortoiseshell crack. The depth of grinding crack is shallow, and the quenching crack is generally deep and large, independent of the grinding side. They are mostly straight cut cracks. Secondly, please pay attention to the forged part of the crack. Sharp concave and convex corners. The edge of the hole. Engraving part. Surface defects caused by steel printing and machining. The cracks in these parts are mainly quenching and quenching cracks.
If cracks caused by part cracks or other conditions (cracks caused by white or dark red rust), they can be determined as dark brown, even oxygen cracks, but not oxygen cracks. Cracks before quenching are caused by forgings or delayed parts caused by quenching and expansion. Since the quenching crack is basically formed below the MS point, the part will not be oxidized. Fourth, in the microstructure, the quenching crack breaks along the crystal boundary. If they do not break along the crystal boundary, but along the crystal interior, they are fatigue cracks. If there is decarburization layer around the crack, because the quenching crack is generated during quenching and cooling, and decarburization will never occur, it is not the quenching crack, but the crack before quenching.