008618266359149
There are various types of cracks: raw material cracks, heat treatment cracks, forging cracks, etc., which can make people dizzy and disoriented. How to identify them is an important course, which facilitates accurate identification of the process where cracks occur and is beneficial for analyzing the causes of crack formation.
Firstly, 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". However, the main factors that cause forging cracks can be further divided into:
1. Forging cracks caused by defects in raw materials
2. Forging cracks caused by improper forging processes.
Roughly distinguish from the macroscopic morphology of cracks, horizontal cracks are generally unrelated to the base metal, and longitudinal cracks need to be analyzed in combination with crack morphology and forging technology.
There is decarburization on both sides of the crack, which is definitely caused by 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 process.
For workpieces of the same batch and model, forging cracks are generally located in one position, with shallow extension under the microscope and decarburization on both sides. And material cracks may not necessarily appear repeatedly at the same location, with varying depths under the microscope. There is still a certain pattern after reading and analyzing more.
Material cracks are mostly 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. There is also a phenomenon of lattice damage and tearing that can cause cracking when using cold iron. It can be distinguished from metallography.
The purpose of forging:
1. Forming requirements;
2. Improve the internal structure of materials, refine grain size, and achieve uniform elemental composition and structure;
3. Make the material denser (such as shrinkage holes or looseness that are not exposed to air inside the forged material), and the streamline distribution is more reasonable;
4. Serve the next process through a reasonable post forging heat treatment method.
Therefore, the responsibility lies in the internal defects of the forging raw materials. Large castings and forgings often start directly from forging steel ingots, and there are inevitably a large number of casting defects inside the ingots. Obviously, reasonable forging can fit the so-called "defects". So, the rationality of forging process is the main reason for determining whether the forging will crack.
Of course, for a relatively stable forging process, if clear requirements are made in advance for controlling the defect level of raw materials before forging, when the defect level of raw materials exceeds the requirements and cracks occur during forging under the original forging process, we can consider it as "forging cracks caused by raw material defects".
The specific problem of cracking should be analyzed in detail, combined with the process analysis, including whether there is a protective atmosphere during the heating process. Forging should be done by forging and sealing the cracks in the raw material tightly. Oxidized skin is usually dense and gray in color, and the dirt caused by the sample preparation process is very loose and has a dark color. At high magnification, it can be seen at a glance, but it is difficult to distinguish. It can be resolved directly by energy spectrum analysis.
Forging cracks
Forging cracks are generally formed at high temperatures. When forging deformation occurs, due to the expansion of the crack and contact with air, it can be observed under a 100X or 500X microscope that the crack is filled with oxide scale, and both sides are decarburized. The structure is ferrite, and its morphological characteristics are that the crack is relatively thick and generally exists in multiple forms, without detailed tips, relatively round and pure, and without detailed directionality. In addition to the typical shapes mentioned above, sometimes some forging cracks may appear finer. The area around the crack is not completely decarburized but partially decarburized.
Typical forging crack example:
More oxides on the edges
Heat treatment cracks
The cracks generated during quenching heating process have significant differences in nature and morphology compared to the cracks formed during forging heating process. For structural steel, the heat treatment temperature is generally much lower than the forging temperature, and even for high-speed steel and high alloy steel, the heating and insulation time is much shorter than the forging temperature. Due to the high heating temperature during heat treatment, prolonged holding time, or rapid heating, early cracking may occur during the heating process. Generate cracks distributed along the boundaries of coarser grains; There is slight decarburization on both sides of the crack, and if the heating speed of the part is too fast, early cracking may occur. There is no obvious decarburization on both sides of the crack, but there is oxide scale inside and at the end of the crack. Sometimes, due to the malfunction of high-temperature instruments and extremely high temperatures, the structure of the parts becomes extremely coarse, and cracks are distributed along the boundaries of coarse grains.
Typical examples of quenching cracks:
At 500X, it shows a serrated shape with a wide crack at the beginning and fine to no fracture lines at the end. No abnormal metallurgical inclusions were found at the crack site, and no decarburization phenomenon was observed. The crack extends in a serrated shape and has typical characteristics of quenching cracks.
Causes of Forging Cracks and Heat Treatment Cracks
1. Causes of forging cracks: During the forging process of steel, surface and internal defects such as hairlines, sand holes, cracks, inclusions, subcutaneous bubbles, shrinkage cavities, white spots, and interlayers may all be the causes of forging cracks. In addition, due to poor forging technology or improper operation, such as overheating, overburning, or too low final forging temperature, or too fast cooling rate after forging, it can also cause cracking of forgings.
2. Reason for heat treatment cracks: Quenching cracks are macroscopic cracks mainly caused by macroscopic stress. In the actual production process, steel workpieces are often subjected to factors such as 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 can cause the already formed quenching microcracks to propagate and form macroscopic quenching cracks. On the other hand, increasing the sensitivity of microcracks and the number of microcracks reduces the brittle fracture resistance Sk of steel, thereby increasing the possibility of quenching crack formation.
Factors affecting quenching cracking
There are many factors that affect quenching cracking, and here we will only introduce a few common situations encountered in production.
1. Quenching cracking caused by defects in raw materials: If there are cracks or inclusions on the surface and inside of the raw material that are not detected before quenching, quenching cracks may form;
2. Cracking caused by inclusions: If the internal inclusions of the part are severe, or if there are hidden cracks due to severe inclusions, cracks may occur during quenching;
3. Quenching cracking caused by poor original organization;
4. Improper quenching temperature causing quenching cracking: Improper quenching temperature causing quenching cracking of parts generally has two situations:
(1) The instrument indicates a temperature lower than the actual temperature of the furnace, causing the quenching temperature to be too high, resulting in overheating during quenching and causing cracking of the workpiece. Any metallographic structure that undergoes quenching and cracking due to overheating contains coarse grains and coarse martensite.
(2) The actual carbon content of steel parts is higher than the content specified by the steel grade. If quenched according to the normal quenching process of the original grade, it is equivalent to increasing the quenching temperature of the steel, which can easily cause overheating and grain growth of the parts, increase the stress during quenching, and cause quenching cracking.
5. Quenching cracking caused by improper cooling during quenching: Improper cooling during quenching can also cause quenching cracking accidents in parts.
6. Quenching cracking caused by machining defects: Due to poor machining, coarse and deep tool marks are left on the surface of the part, which can cause cracking during quenching or early damage during service, even if it is a simple part or not a stress concentrated area.
7. The influence of part shape on quenching cracks: If the geometric shape of the part is unreasonable or there is a significant difference in thickness in the transition zone of the section, cracks are easily generated due to stress concentration during quenching.
8. Cracking caused by untimely tempering: Failure to temper in a timely manner after quenching may result in cracking due to excessive residual stress after quenching.
Methods for distinguishing cracks
It is important to distinguish whether it is a quenching crack, tempering crack, forging crack, or grinding crack, so as to accurately identify which process the crack occurred in and analyze the cause of the crack.
Firstly, pay attention to the differences in the morphology of quenching cracks and grinding cracks. For cracks that were not discovered during quenching but were discovered after grinding, it is necessary to distinguish whether they are quenching cracks or grinding cracks. It is relatively easy when there is no pollutant attached to the crack. At this time, pay attention to the shape of the crack, especially the direction of crack development. Grinding cracks are perpendicular to the grinding direction, in the form of parallel lines or in the shape of a turtle shell. The depth of grinding cracks is shallow, while quenching cracks are generally deeper and larger, regardless of the grinding direction, and often crack in a straight cutting shape.
Secondly, pay attention to the location where the crack occurs. Cracks occurring at sharp concave convex corners, edges of holes, engraved areas, stamped areas, and surface defects caused by mechanical processing are mostly quenching cracks.
Thirdly, by observing the fracture surface of the component, it is possible to distinguish whether it is a quenching crack, a forging crack before quenching, or a crack caused by other conditions. If the crack section is white, 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 has oxygen skin, it is not a quenching crack, but a crack that existed before quenching. It is a crack formed during forging or rolling of the part, and these cracks will be enlarged due to quenching. Since quenching cracks are mostly formed below the MS point, their cross-section will not be oxidized.
Fourthly, in the microstructure, quenching cracks fracture along grain boundaries. If they do not fracture along grain boundaries but along intragranular boundaries, they belong to fatigue cracks.
Fifth, if there is a decarburization layer around the crack, it is not a quenching crack, but a crack that existed before quenching, because quenching cracks are generated during quenching cooling and decarburization will never occur.
2024 July 4th Week KYOCM Product Recommendation:
Rotary bearings consist of an inner ring and an outer ring, one of which usually contains a gear. Together with the connecting holes in the two rings, they enable optimized power transmission through simple and fast connections between adjacent machine parts. Bearing raceways are designed with rolling elements, cages or gaskets to accommodate loads acting individually or in combination in any direction.
Features and advantages:
High carrying capacity
High stiffness for rigid bearing applications
Low friction
Long service life
Surface protection and corrosion resistance
Integrate other features including:
Driving mechanism
Control device
Lubrication system
Monitoring system
Sealed cassette tape
https://www.kyocm.com/products/Slewing-Bearing/745.html
2024-07-30