How to distinguish between forging cracks, heat treatment cracks and raw material cracks

  There are many different types of cracks: raw material cracks, heat treatment cracks, forging cracks, etc. It can be dizzying. How to identify them is a very important lesson, so that it is easier to find out exactly where the cracks occur and to analyse the causes of the cracks. 
  First of all, it is necessary to define the concept of "raw material cracks" and "forging cracks" first, and the cracks that appear after forging should be understood as "forging cracks", except that the main factors leading to forging cracks The main factors that lead to forging cracks can be further divided into:
1, forging cracks caused by raw material defects;
2. forging cracks caused by improper forging processes.
     The macroscopic form of the crack is first roughly distinguished from the horizontal, which is generally not related to the base material, and the longitudinal crack, which needs to be analysed in combination with the crack form and the forging process.
    Decarburisation on both sides of the crack is definitely a result of the forging process. As to whether it is caused by the raw material or the forging process, this needs to be analysed 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 shallowly under the microscope, with decarburisation on both sides. Whereas material cracks do not necessarily recur in the same location, with varying depths under the microscope. There are still certain patterns when you look and analyse more.
 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 over-burning, with oxidative decarburisation near the crack. There is also a type of cracking caused by hitting cold iron, which has lattice damage tearing. It can be distinguished from the metallographic aspects.
The purpose of forging:
1, forming requirements;
2, to improve the internal organisation of the material, refining the grain, uniform elemental composition and organisation;
3, so that the material is more dense (forging materials within the original not exposed to air shrinkage or sparse, etc.), more reasonable distribution of flow lines;
4, through a reasonable post-forging heat treatment, for the next process services.
    Therefore, the forging of certain defects within the forging material is the responsibility of the forging. Large castings and forgings are often directly from the forging of ingots, ingots are bound to have a large number of internal smelting and casting defects, obviously, reasonable forging, which can be the so-called "defects" forging together. Therefore, the reasonableness of the forging process is the main reason for deciding 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 defects level exceed the requirements and forging in the original forging process cracking phenomenon, we can be recognized as "raw material defects caused by forging cracks".
 Cracking is a 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 to close. The oxide skin is usually dense and grey, the sample making process causes it to be very loose and blackish in colour, which can be seen under high magnification, but it is impossible to distinguish, so if you play the spectrum directly, you will be able to distinguish.
- Cracked forging
Forging cracks are generally formed at high temperatures, forging deformation due to crack expansion and contact with air, so under the microscope at 100X or 500X, you can see cracks filled with oxide, and both sides are decarburised, the organisation is ferrite, the morphological characteristics of the cracks are relatively thick and generally exist in the form of multiple, no bright tip, more round and pure, no bright 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 decarburisation but by semi-decarburisation.
- Heat treatment crack
There is a clear difference in the nature and form of cracks produced during quenching and heating and those formed during forging. For structural steels, the heat treatment temperature is generally much lower than the forging temperature, even for high speed steels and high alloy steels where the heating and holding time is much less than the forging temperature. Due to the 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 is filled with oxide skin. Sometimes due to the failure of high temperature instruments, the temperature is very high, resulting in the organization of the part is extremely coarse, the crack along the coarse grain boundary distribution.
Typical examples of quench cracks:
500X, serrated, wide crack at the start, small to no end crack, no abnormal metallurgical inclusions found at the crack, no decarburisation, crack extending in a serrated pattern, typical of quench cracking.
- Causes of forging cracks and heat treatment cracks
1, forging crack generation causes: steel in the forging process, due to the presence of steel surface and internal defects, such as hairline, sand eyes, cracks, inclusions, subcutaneous bubbles, shrinkage, white spots and interlayer, etc., may become the cause of forging cracking. In addition, due to poor forging process or improper operation, such as overheating, over-burning or final forging temperature is too low, forging after 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, quenching temperature control is not correct, quenching cold speed is not appropriate and other factors, on the one hand, increase the quenching stress, will make the formation of quenching microscopic crack expansion, the formation of macroscopic quenching cracks, on the other hand, due to the increased sensitivity of microscopic cracks, increasing the number of microscopic cracks, reducing the brittle fracture resistance of steel Sk and thus increases the likelihood of quench crack formation.
- Factors influencing quench cracking
There are many factors affecting quenching and cracking, here will be only a few commonly encountered in the production of the situation is introduced one by one.　　
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, inclusions caused by cracking: if the internal inclusions of the parts are serious, or itself due to serious inclusions have hidden cracks, quenching will likely produce cracks.
3, quenching crack 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 situations: (1) the instrument indicates that the temperature is lower than the furnace:
(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 overheating quenching cracking metallographic organization, there 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 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 in 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 tempered in time to cause cracking: after quenching and tempering is not timely, will likely be due to quenching residual stress is too large and lead to cracking.
- Crack distinguishing method
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 analyse the causes of the cracks.
Firstly, note the difference between quenching and grinding cracks. For cracks that are not found during quenching but only after grinding, it is important to distinguish between quenching and grinding cracks. It is easier to distinguish between a quenching crack and a grinding crack when the crack is not attached to a contaminant, and at this point pay attention to the shape of the crack, especially the direction of the crack development. 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-line knife-cutting cracks. 
Secondly, pay attention to where the cracks occur. Sharp concave and convex corners, the edge of the hole, engraving, hitting steel marks and surface defects caused by mechanical processing, etc., in these parts of the crack is more quenching cracking.
Third, by looking at the cracked section of the part to distinguish whether it is a quenching crack or a forging crack before quenching or a crack 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 for quenching cracks, if the crack section is dark brown, even with oxygen skin, it is not quenching cracks, quenching before the existence of cracks, is the part after forging or calendering cracks formed, these cracks will be expanded due to quenching. Because quenching cracks are basically formed below the MS point, the section is not oxidised. 
Fourthly, in the microstructure, quenched cracks fracture along grain boundaries, if they do not fracture along grain boundaries but along intracrystalline fractures, they are fatigue cracks.
Fifth, if there is a decarburisation layer around the crack, it is not a quenching crack, but a crack that existed before quenching, because quenching cracks are generated during quenching and cooling, and decarburisation will never occur.
Source: Reproduced 
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