There are various types of cracks: such as cracks in raw materials, cracks caused by heat treatment, and forging cracks, which can be confusing.
Identifying them is an important course of action as it helps locate where the crack occurred accurately, which can aid in analyzing the reason why the crack occurred.
Firstly, we need to clarify the concepts of “cracks in raw materials” and “forging cracks”. The cracks that occur after forging should be considered as “forging cracks”.
However, the main factors which lead to the formation of forging cracks can be classified into the following:
- Forging cracks caused by defects in raw materials;
- Forging cracks caused by improper forging techniques.
To roughly differentiate, cracks can be classified by their macroscopic morphology. Transverse cracks are generally unrelated to the parent material, while longitudinal cracks require analysis that combines crack morphology and forging process.
Decarburization on both sides of a crack indicates that it occurred during forging. As for whether it was caused by the raw material or the forging process, analysis based on metallography and process is required.
For workpieces of the same model and the same batch, forging cracks are usually located in the same position and extend relatively shallowly under the microscope with decarburization on both sides. Material cracks may not repeatedly occur at the same location and may have varying depth under the microscope. There is still some regularity to observe and analyze.
Material cracks mostly align with the material’s longitudinal direction. There are two types of forging cracks: one caused by overheating and oxidation leading to decarburization near the crack, while the other caused by the tearing phenomenon of lattice damage during cold iron forging can be distinguished through metallography.
The purpose of forging is as follows:
- Meet the forming requirements;
- Improve the internal structure of the material, refine the grain size, and homogenize the element composition and structure;
- Make the material more dense (forging can close internal defects such as unexposed air holes or looseness), and distribute the streamline more reasonably;
- Serve the next process through appropriate post-forging heat treatment methods.
Therefore, there must be certain defects inside the raw material for forging. Large cast forgings are often directly forged from steel ingots, which inevitably contain a large number of casting defects. Reasonable forging can also forge these so-called defects.
Therefore, the rationality of the forging process is the main factor that determines whether the forging will crack.
Of course, based on a stable forging process, if explicit control requirements for raw material defect grades are made before forging, and the cracking phenomenon appears due to raw material defects exceeding the requirements during forging, this can be considered as “forging cracks caused by defects in raw materials”.
Crack problems require specific analysis, coupled with analysis of the forging process, and consideration of whether there is protective atmosphere during the heating process.
Forging should be the process of forging and compacting raw material cracks. Oxide scale is usually tight and grey, while dirty and loose debris from the sampling process is black.
Looking under high magnification can reveal the difference, however, energy spectrum analysis can always distinguish them when other methods fall short.
Forging cracks
Forging cracks generally form at high temperatures during forging deformation. When cracks expand and come into contact with air, under a microscope with a magnification of 100X or 500X, the cracks can be seen with oxidation scales and decarburization on both sides, with the structure being primarily ferritic.
The morphological characteristics of these cracks are that they are relatively thick and often exist in multiple forms, without a clear pointed end, relatively round and pure, and without clear directionality.
In addition to these typical forms, sometimes finer forging cracks may appear. The decarburization around the crack is not complete but partial.
Typical examples of forging cracks include:
More oxide on the edges of the crack.
Heat treatment crack
Cracks produced during the quenching and heating process have significant differences in nature and morphology compared to those formed during the forging and heating process.
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 insulation time is much shorter than that of the forging temperature. Early cracking can occur during the heating process as a result of excessively high heat treatment temperatures, producing cracks distributed along coarser grain boundaries.
When the heating speed of the part is too fast, early cracking can also occur, with slight decarburization on both sides of the crack, but oxidation scales are present inside and at the tail of the crack.
Sometimes, due to instrument malfunction, extremely high temperatures can cause the material’s coarse-grained structure, with the crack distributed along the tubby crystal boundary.
A typical example of quenching cracks is as follows:
Under a microscope with a 500X magnification, the crack appears serrated, with a wide starting end and a small ending fracture. There is no abnormal metallurgical inclusion or decarburization present at the crack, which extends in a serrated manner, having typical characteristics of quenching cracks.
Reasons for forging cracks and heat treatment cracks
Causes of forging cracks:
During the forging process, steel may crack due to defects on the surface or inside the material, such as hairline cracks, sand holes, inclusions, subsurface bubbles, shrinkage holes, white spots, or laminations.
Poor forging processes or improper operations such as overheating, over-burning, or too low final forging temperatures, as well as too fast cooling after forging, can also cause cracking of forgings.
Causes of heat treatment cracks:
Quenching cracks are macroscopic cracks primarily caused by macroscopic stresses. In actual production, steel workpieces are often due to the structural design being unreasonable, improper selection of materials, insufficient temperature control during quenching, or inappropriate cooling rates, which on the one hand increases the internal stress during quenching, leading to the expansion of the formed microcracks to form macroscopic quenching cracks.
On the other hand, increasing the number of microscopic cracks decreases the material’s resistance to brittle fracture SK, increasing the likelihood of quenching crack formation.
Factors affecting quenching
There are many factors that affect quench cracking, and here we only introduce a few common cases encountered in production.
- Quench cracking caused by pre-existing defects in raw materials: If there are cracks or inclusions on the surface or inside of the raw materials, and they are not found before quenching, quenching cracks may be formed.
- Cracking caused by inclusions: If there are serious inclusions inside the parts, or there are hidden cracks due to serious inclusions, cracks may occur during quenching.
- Quench cracking caused by poor original structure.
- Quench cracking caused by improper quenching temperature: There are generally two cases of quench cracking caused by improper quenching temperature:
(1) The indicated temperature of the instrument is lower than the actual temperature of the furnace, resulting in a higher quenching temperature, causing the workpiece to overheat and crack during quenching. The metallographic structure of overheated quenched cracking always contains coarse grains and coarse martensite.
(2) The actual carbon content of the steel is higher than the content specified by the steel grade. When quenched according to the normal quenching process of the original grade, it is equivalent to increasing the quenching temperature of the steel, which is easy to cause overheating and grain growth of the parts, and increase the stress during quenching, causing cracking.
- Quenching cracking caused by improper cooling during quenching: Improper cooling during quenching can also cause quenching cracking of the parts.
- Quench cracking caused by machining defects: Due to poor machining, rough and deep tool marks are left on the surface of the parts. Even for simple parts or non-stress-concentrated areas, cracking may occur during quenching or early failure may occur during service.
- The influence of part geometry on quenching cracks: The unreasonable geometry of the parts or the large difference in thickness in the excessive section are both easy to cause cracking due to stress concentration during quenching.
- Cracking caused by failure to temper in a timely manner after quenching: Failure to temper in a timely manner after quenching may cause cracks due to residual stress from quenching.
Methods for distinguishing cracks
It is important to distinguish whether it is quenching cracks, tempering cracks, forging cracks, or grinding cracks, in order to accurately identify which process the cracks occurred in and analyze the reasons for their formation.
Firstly, pay attention to the differences in the morphology of quenching cracks and grinding cracks. To distinguish between quenching cracks and grinding cracks, which may not be detected during quenching but are found after grinding, pay attention to the shape of the cracks, especially the direction of crack development.
Grinding cracks are perpendicular to the grinding direction, appearing in a parallel line shape or a tortoise shell pattern. Grinding cracks are shallower while quenching cracks are generally deeper and larger.
Quenching cracks are not related to the grinding direction and often appear as straight knife-cut-like cracks.
Secondly, pay attention to where the cracks occur. Sharp corners, the edges of holes, inscriptions, stamping or mechanical surface defects, and other areas where cracks occur are mostly quenching cracks.
Thirdly, distinguish quenching cracks from forging cracks or cracks caused by other conditions by observing the fracture surface of the part.
If the crack surface is white, dark white, or light red (caused by water rust during water quenching), it can be determined that it is a quenching crack. If the crack surface is dark brown, with even oxide scale, it is not a quenching crack; it is a pre-existing crack that was formed during forging or rolling and has been expanded during quenching.
Since quenching cracks are formed below the MS point, their surfaces cannot be oxidized.
Fourthly, in the microstructure, quenching cracks fracture along grain boundaries. If they are not along grain boundaries but within the grains, they are fatigue cracks.
Fifthly, if there is decarburization around the cracks, it is not a quenching crack, but a pre-existing crack before quenching because quenching cracks are produced during quenching and decarburization does not occur.
In summary
Conducting metallographic analysis for defect analysis is a complicated process as the failure of a part can often be attributed to multiple causes. To ensure accurate defect analysis, a comprehensive investigation should be carried out to fully understand the facts and analyze them from various angles.
