China Manufacturer of edm machining process

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 helpful to analyze the cause of the crack. China Manufacturer of edm machining process

First of all, it is necessary to define the concepts of "raw material cracks" and "forging cracks". Cracks that appear after forging should be understood as "forging cracks", but the main factors that cause forging cracks can be subdivided into:

Forging cracks caused by raw material defects;
Forging cracks caused by improper forging process. The macroscopic shape of the crack is roughly distinguished first. The transverse direction is generally independent of the base metal, and the longitudinal crack needs to be analyzed in combination with the crack shape and the forging process. There is decarburization on both sides of the crack, which must be produced 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 the same position, the extension is shallow under the microscope, and there is decarburization on both sides. The material cracks do not necessarily appear repeatedly in the same position, and the depths under the microscope are different. There are certain rules to see and analyze more. Most of the material cracks are consistent with the longitudinal direction of the material. There are two types of forging cracks, one is caused by overheating and overburning, and there is oxidation and decarburization near the crack. Another is that hitting cold iron will also cause cracking, which is a phenomenon of lattice damage and tearing. It can be distinguished from the metallographic appearance. China Manufacturer of edm machining process

Purpose of forging:

Forming requirements;
Improve the internal structure of the material, refine the grain, and uniform the element composition and structure;
Make the material denser (shrinkage holes or looseness in the original unexposed air inside the forged material, etc.), and the streamline distribution is more reasonable;
Through reasonable post-forging heat treatment, serve the next process. Therefore, certain defects inside the forging and forging raw materials are the responsibility. Large-scale castings and forgings often start directly from the forging of steel ingots, and there must be a large number of smelting defects inside the steel ingots. Obviously, reasonable forging can forge the so-called “defects”. Therefore, the rationality of the forging process is the main reason for determining whether the forging will crack. Of course, relative to a stable forging process, if there is a clear requirement for raw material defect level control for the raw materials before forging, when the raw material defect level exceeds the requirements and the cracking phenomenon occurs in the original forging process, we can consider it as “raw material”. Forging cracks caused by defects”. The specific analysis of the crack problem should be considered in combination with the analysis of the process, including whether there is a protective atmosphere during the heating process. Forging should be forging and sealing the cracks of the raw materials. The oxide skin is usually dense and gray, and the dirt caused by the sample preparation process is very loose and dark in color. It can be seen at a high magnification. China Manufacturer of edm machining process

forging crack

Forging cracks are generally formed at high temperatures. During forging deformation, due to crack expansion and contact with air, under a microscope of 100X or 500X, it can be seen that the cracks are filled with oxide scale, 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 detailed tips, relatively round and pure, and without detailed directionality. In addition to the above typical forms, sometimes some forging cracks are relatively thin. Not fully decarburized but half decarburized around the crack

heat treatment cracks

The cracks generated during quenching heating and those formed during forging heating are significantly different in nature and morphology. 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, long holding time or rapid heating, early cracking will occur during the heating process. Cracks distributed along the coarser grain boundaries are generated; there is a slight decarburization structure on both sides of the crack, and the parts are heated 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 filled with Has oxide skin. Sometimes due to the failure of high-temperature instruments and the temperature is very high, the structure of the parts is extremely coarse, and the cracks are distributed along the coarse grain boundaries. China Manufacturer of edm machining process

Typical examples of quench cracks:

At 500X, it is in a zigzag shape, with wide cracks at the beginning, and small to no cracks at the end. No abnormal metallurgical inclusions are found at the cracks, and there is no decarburization phenomenon. The cracks extend in a zigzag shape, which has the typical characteristics of quenching cracks.

Causes of Forging Cracks and Heat Treatment Cracks

Reasons for forging cracks: During the forging process of steel, due to the surface and internal defects of the steel, such as hair lines, blisters, cracks, inclusions, subcutaneous bubbles, shrinkage holes, white spots and interlayers, etc., may become forging cracks. s reason. In addition, due to poor forging process or improper operation, such as overheating, over-burning or the final forging temperature is too low, the cooling rate after forging is too fast, etc., it will also cause the forging to crack. China Manufacturer of edm machining process
Causes of heat treatment cracks: Quenching cracks are macroscopic cracks, mainly caused by macroscopic stress. In the actual production process, steel workpieces are often due to unreasonable structural design, improper steel selection, incorrect quenching temperature control, and inappropriate quenching cooling rate. On the one hand, increasing the internal stress of quenching will make the formed quenching microscopic The cracks expand to form macroscopic quenching cracks. On the other hand, due to the increased sensitivity of microscopic cracks, the number of microscopic cracks is increased, and the brittle fracture resistance Sk of the steel is reduced, thereby increasing the possibility of quenching cracks. .

Factors Affecting Quenching Cracking

There are many factors that affect quenching cracks, here are only a few situations that are often encountered in production. China Manufacturer of edm machining process

Quenching cracks caused by defects in the raw materials: If there are defects such as cracks or inclusions on the surface and inside of the raw materials, which are not found before quenching, quenching cracks may be formed;
Cracks caused by inclusions: If there are serious inclusions in the part, or cracks have been hidden due to serious inclusions, cracks may occur during quenching;
Quenching cracks caused by poor original structure;
Quenching cracking caused by improper quenching temperature: Improper quenching temperature causes quenching cracking of parts. Generally, there are two situations:

(1) The temperature indicated by the instrument is lower than the actual temperature of the furnace, which makes the quenching temperature on the high side, resulting in overheating of quenching and cracking of the workpiece. Coarse grains and coarse martensite exist in all metallographic structures cracked by superheat quenching. China Manufacturer of edm machining process

(2) The actual carbon content of the steel is higher than the content specified by the steel grade. If it is quenched according to the normal quenching process of the original grade, it means that the quenching temperature of the steel is increased, which is easy to cause the parts to overheat and grain growth. The increased stress causes quenching cracks.

Quenching cracks caused by improper quenching and cooling: Improper cooling during quenching will also cause quenching cracks in parts.
Quenching cracking caused by machining defects: due to poor machining, rough and deep tool marks are left on the surface of the part, even if it is a very simple part or a place where stress is not concentrated, it will also cause cracking during quenching, or Early damage occurred during service.
The influence of the shape of the parts on the quenching cracks: The geometry of the parts is unreasonable, or the thickness of the transition area of the cross-section varies greatly, and cracks are prone to occur due to stress concentration during quenching.
Cracking caused by untimely tempering: If not tempering in time after quenching, cracks may be caused due to excessive quenching residual stress. China Manufacturer of edm machining process

How to identify cracks

How to distinguish whether it is a quenching crack, a tempering crack, a forging crack or a grinding crack is very important, so that it is easy to accurately find which process the crack occurs in, and it is beneficial to analyze the cause of the crack.

First, note the difference in the morphology of quenched and ground cracks. 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 to pollutants. 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 form of a tortoise shell. The depth of grinding cracks is shallow, while the quenching cracks are generally deeper and larger, and have nothing to do with the grinding method. China Manufacturer of edm machining process

Second, pay attention to where the crack occurs. Cracks at sharp concave-convex corners, edges of holes, engravings, stencils, and surface defects caused by machining are mostly quenching cracks.

Third, by observing the fracture surface of the part to distinguish 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 determined as a quenching crack. If the crack section is dark brown, or even oxygen skin appears, it is not a quenching crack, it is before quenching. The existing cracks are the cracks formed when the part is forged or rolled, and these cracks will be enlarged by quenching. Since quenching cracks are basically formed below the MS point, the cross-section will not be oxidized. China Manufacturer of edm machining process

Fourth, in the microstructure, the quenching crack is a fracture along the grain boundary. If it is not fractured along the grain boundary, but is fractured along the grain, it is a fatigue crack.

Fifth, if there is a decarburized layer around the crack, it is not a quenching crack, but a crack that exists before quenching, because the quenching crack is generated during quenching and cooling, and decarburization will never occur. China Manufacturer of edm machining process