Differentiation of forging cracks, heat treatment cracks and raw material cracks

19/01/2023
There are many different types of cracks: raw material cracks, heat treatment cracks, forging cracks, etc. It is dizzying to see how to identify them. How to identify them is a very important lesson, so that it is easy to find out exactly where the cracks occur and to analyze the causes of cracks.


    First of all, it is necessary to determine the concept of "raw material crack" and "forging crack" first, and the cracks that appear after forging should be understood as "forging crack", except that the main factors that lead to forging crack The main factors that lead to forging cracks can be further divided into.


1, forging cracks caused by defects in raw materials.


2, forging cracks caused by improper forging process.





From the crack macroscopic form first for general distinction, horizontal generally has nothing to do with the base material, longitudinal cracks need to be combined with the crack pattern and forging process and other combined analysis.


    Cracks on both sides of the decarburization, is certainly the forging process, as to whether it is the raw material or forging process caused by, it is necessary to analyze according to the metallographic and technological processes.


    For the same batch of the same type of workpiece, forging cracks are basically in one location, extending relatively shallow under the microscope, with decarburization on both sides. And material cracks do not necessarily recur in the same location, the depth under the microscope varies. Look more and analyze, there is still a certain rule.

 
Material cracks are mostly consistent with the longitudinal direction of the material. And there are two kinds of wrought cracks, one is caused by overheating and overburning, and there is oxidation decarburization near the crack. There is also a kind of hitting cold iron can also cause hairline cracks, this one has the phenomenon of lattice destruction tearing. It can be distinguished from metallographic.


Purpose of forging.


1, forming requirements.


2, to improve the internal organization of the material, refining the grain, uniform elemental composition and organization.


3, to make the material more dense (forging together the material inside the original unexposed air shrinkage or sparse, etc.), more reasonable distribution of flow lines.


4, through a reasonable post-forging heat treatment, for the next process.


Therefore, it is the duty of the forging to forge certain defects inside the raw material. Large castings and forgings are often directly from the ingot forging start, ingots must exist a large number of internal smelting and casting defects, obviously, reasonable forging, can be the so-called "defects" forging. Therefore, the reasonableness of the forging process is the main reason to decide whether the forgings will crack.


    Of course, relative to a stable forging process, if the forging before the raw material to put forward a clear level of raw material defects control requirements, when the raw material defect level exceeds the requirements and forging under the original forging process cracking phenomenon, we can be recognized as "raw material defects caused by forging cracks".


Cracking problem specific analysis, combined with the process analysis, including the heating process there is no protective atmosphere should be considered, forging should be the raw material crack forging dense only. Oxide is usually dense gray, the sample making process caused by the dirt is very loose color blackish, a look at the high magnification will know, it is impossible to distinguish the direct play energy spectrum must be able to distinguish.




Forging cracks are generally formed at high temperatures, forging deformation due to crack expansion and contact with air, so under the microscope of 100X or 500X, you can see cracks filled with oxide, and both sides are decarburized, the organization is ferrite, the morphological characteristics of the crack is relatively thick and generally through the form of multiple exist, no bright fine tip, more round and pure, no bright fine directional, in addition to the above typical form, sometimes In addition to the above typical patterns, some forging cracks are sometimes found to be relatively fine. The crack is not surrounded by full decarburization but by semi-decarburization.


Examples of typical forging cracks.


More oxide at the edges.



Quenching heating process cracks and forging heating process formed in the nature and form of cracks have obvious differences. 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 less than the forging temperature. Due to high heat treatment heating temperature, holding time is too long or rapid heating, will produce early cracking in the heating process. Produced along the coarser grain boundary distribution of the crack; crack on both sides of a slight decarburization organization, parts heated too fast, will also produce early cracking, this crack on both sides of no obvious decarburization, but the crack and its tail filled with oxide skin. Sometimes due to high-temperature instrumentation failure, the temperature is very high, resulting in the organization of the part is extremely coarse, the crack along the coarse grain boundary distribution.


Example of a typical quench crack.


At 500X, it is serrated, with a wide crack at the beginning end and a fine to no end fracture crack, no abnormal metallurgical inclusions found at the crack, no decarburization, and the crack extends in a serrated pattern, with the typical characteristics of a quenched crack.




Forging cracks and heat treatment cracks generated by the causes


1, forging crack causes: steel in the forging process, due to the presence of surface and internal defects in steel, such as hairline, sand eye, cracks, inclusions, subcutaneous bubbles, shrinkage, white spots and laminations, etc., may become the cause of forging cracking. In addition, due to poor forging process or improper operation, such as overheating, overburning or final forging temperature is too low, forging cooling speed is too fast, etc., will also cause forging cracking.


2, heat treatment cracking causes: quenching cracks are macro cracks, mainly caused by macro stress. In the actual production process, the steel workpiece is often due to unreasonable structural design, improper selection of steel, incorrect quenching temperature control, quenching and cooling speed is not appropriate and other factors, on the one hand, increase the quenching stress, will make the formation of quenching microcrack expansion, the formation of macroscopic quenching cracks, on the other hand, due to increase the sensitivity of the microcrack, increase the number of microcracks, reducing the steel brittle fracture resistance Sk , thus increasing the possibility of quenching crack formation.



Factors affecting quenching and cracking


Many factors affect the quenching crack, here will be the production of only a few commonly encountered in the case of an introduction.  


1, raw materials have defects and quenching crack: If the raw material surface and internal cracks or inclusions and other defects, not found before quenching, may form quenching cracks.


2, cracking caused by inclusions: if the internal inclusions of the part is serious, or itself due to serious inclusions have hidden cracks, quenching will likely produce cracks.


3, quenching cracks caused by poor original organization.


4, quenching temperature improperly caused by quenching crack: quenching temperature improperly caused by parts quenching crack, there are generally two cases.


(1) the instrument indicates that the temperature is lower than the actual temperature of the furnace, so that the quenching temperature is high, resulting in quenching overheating, resulting in cracking of the workpiece. Where the superheated quenching cracking metallographic organization are coarse grains and coarse martensite.


(2) the actual carbon content of the steel is higher than the content of the steel grade, if the original grade of the normal quenching process quenching, which is equivalent to raising the quenching temperature of the steel, easy to cause parts overheating and grain growth, so that the quenching stress increases and causes quenching and cracking.


5, quenching and cooling caused by improper quenching crack: quenching due to improper cooling, will also make the parts quenching and cracking accidents.


6, machining defects caused by quenching crack: due to poor machining, leaving a coarse and deep tool marks on the surface of the part, although it is a very simple part or not a stress concentration, will also cause cracking during quenching, or early damage occurs during service.


7, the shape of the part on the impact of quenching cracks: part geometry is unreasonable, or cross-sectional excess area thickness difference is large, in quenching are prone to cracking due to stress concentration.


8, not timely tempering caused by cracking: after quenching without timely tempering, will likely be due to quenching residual stress is too large and lead to cracking.


How to distinguish cracks


It is important to distinguish between quenching cracks, tempering cracks, forging cracks or grinding cracks, etc. This makes it easier to find out exactly where the cracks occur and helps to analyze the causes of the cracks.


First, pay attention to the difference between quenching cracks and grinding cracks. For cracks that are not found during quenching but only after grinding, it is important to distinguish whether they are quenching cracks or grinding cracks. It is easier when the crack is not attached to the contaminant. At this time, pay attention to the morphology of the crack, especially the direction of the crack development, while grinding cracks are perpendicular to the grinding direction, in the form of parallel lines, or in the form of tortoise-like cracks. The depth of grinding cracks is shallow, while quenching cracks are generally deeper and larger, unrelated to the grinding direction, and are mostly in the form of straight cut cracks.


Second, pay attention to the site where the crack occurs. Sharp concave and convex corners, the edge of the hole, engraving, steel marks and surface defects caused by mechanical processing, etc., in these parts of the crack is mostly quenching crack.


Third, by observing the crack section of the part to distinguish whether it is quenched cracks or quenching before the forging cracks or cracks caused by other circumstances. If the crack section is white or dark white or light red (water quenching caused by water rust), can be determined as quenching crack, if the crack section is dark brown, or even the appearance of oxygen skin, it is not quenching crack, is quenched before the existence of the crack, is the crack formed when the part is forged or calendered, these cracks will be expanded due to quenching. Because quenching cracks are basically formed below the MS point, the section is not oxidized.


Fourth, in the microstructure, quenched cracks are fractured along grain boundaries. If they are not fractured along grain boundaries but along intracrystalline fractures, they are fatigue cracks.


Fifth, if a decarburization layer exists around the crack, it is not a quenching crack, but a crack that exists before quenching, because quenching cracks are generated during quenching and cooling, and decarburization will never occur.



Source:https://mp.weixin.qq.com/s/ZwLvBOl_VCHnNS-nUATIUA