Crack is one of the most serious defects in heat treatment. This kind of defect is usually irreparable, and the parts can only be scrapped, so it has attracted special attention of heat treatment workers.

1. Causes of heat treatment cracks of parts

In the process of heat treatment, parts will produce great internal stress (microstructure stress and thermal stress). When these stresses exceed the yield strength of steel, they will cause deformation of parts; when the stress is greater than the tensile strength of steel, they will cause cracking of parts. There are two kinds of stress acting on the parts: compressive stress and tensile stress. The tensile stress formed during quenching is the main cause of quenching crack. However, when the plasticity of the steel is high, even if there is a large tensile stress, it will not cause cracking of the parts, such as stress relief annealing without microstructure transformation, austempering with more retained austenite, etc. Only when the stress is large and the structure is high hardness and brittleness, it is easy to cause the cracking of the parts. Therefore, there must be two conditions for the formation of quenching cracks: one is brittle structure; the other is that the tensile stress exceeds the tensile strength of the steel (of course, other conditions can also promote the occurrence of parts cracks, such as raw material defects, defects caused by improper design and machining, etc.).

2. About the type of crack

There are various classification methods of cracks. According to the direction of cracks, there are longitudinal cracks, transverse cracks, arc cracks and network cracks (also known as cracks); according to the location of cracks, there are surface cracks (or surface cracks) and internal cracks; according to the different processes, there are forging cracks, welding cracks, quenching cracks, tempering cracks, cold treatment cracks, pickling cracks and grinding cracks. In the process of heat treatment, quenching cracks are the most common.

3. Crack resolution method

It is very important to distinguish whether it is quenching crack, tempering crack, forging crack or grinding crack, so as to find out exactly which process the crack occurred in and analyze the causes of the crack.

First, pay attention to the different shapes of quenching cracks and grinding cracks. For the cracks not found during quenching but only found after grinding, it is necessary to distinguish between quenching cracks and grinding cracks. It is easy to crack when there is no contaminant attached. At this time, pay attention to the crack morphology, especially the crack development direction. The grinding crack is perpendicular to the grinding direction, and is in parallel line shape (as shown in Figure 1), or in tortoise shell shape. The depth of grinding crack is very shallow, while that of quenching crack. Generally, it is deep and large, and has nothing to do with the grinding direction. Most of the cracks are linear knife cutting.

Second, pay attention to the crack location. Sharp concave convex corner, hole edge, engraving, steel seal and surface defects caused by machining, the cracks in these parts are mostly quenching cracks.

Thirdly, by observing the crack section of the parts, we can distinguish whether it is caused by quenching crack, forging crack before quenching or other conditions. If the crack section is white or dark white or light red (water rust caused by water quenching), it can be determined as quenching crack. If the crack section is dark brown, even with oxygen skin, it is not quenching crack. It is a crack existing before quenching, it is a crack formed during forging or calendering of parts, and these cracks will be expanded due to quenching. Because the quenching crack is basically formed below the m point, its cross section will not be oxidized.

Fourthly, in the microstructure, the quenching crack is along the grain boundary, if it is not along the grain boundary, but along the intergranular fracture, it belongs to fatigue crack.

Fifthly, if there is decarburization layer around the crack, it is not the quenching crack, but the crack before quenching, because the quenching crack is produced during quenching and cooling, and decarburization will never occur. After sandblasting, the parts can be observed directly with naked eyes, and the surface of the parts can be observed with a magnifying glass. When the crack cannot be seen with eyes or magnifying glass. You can also use the oil immersion flaw detection method to check, that is, immerse the parts in kerosene, gasoline and other oil, take out the parts later, wipe them clean with cotton yarn, and then coat them with lime powder or other white powder. If there are cracks, there will be oil seepage in the white part. Experienced inspectors can also use the knocking method to check whether there are cracks, that is, gently tap the parts with a small hammer. If there is a clear metal sound and the tail sound is long, it can be considered that there is no crack; on the contrary, if there is a heavy sound, there is a crack. Magnetic flaw detection and fluorescent flaw detection can also be used to check whether the parts have cracks.

5. Quenching overheating and quenching crack

If the parts are heated to a certain quenching temperature higher than the specified process temperature and within a certain heating time, the workpiece will be overheated. When using medium frequency electric furnace or high frequency equipment for surface quenching, it is easy to cause overheating because of high electrical efficiency and fast temperature rise, and the overheating temperature will vary with different steel grades. Once the workpiece is overheated, the austenite grain coarsens and the martensite needle is thick, which is easy to produce martensite microcracks. This kind of martensite microcrack is the excitation source of quenching crack and develops into quenching crack. Overheating makes the properties of the steel worse, and the strength and plasticity are greatly reduced. In the production site, quenching cracks caused by overheating are the most common in tool steels, especially in high-speed tool steels. The reason is that the quenching temperature of high-speed steel is close to its melting point, so a little carelessness can cause overheating, even overburning. The modulus of a batch of W18Cr4V Steel after heat treatment in a factory is m= 12. When the outer diameter of 170 mm disc gear milling cutter is used, the quenching heating temperature specified in the process is 1270 ~, but due to the failure of the temperature control instrument, the temperature of the indicator is 35 ℃ lower than the actual furnace temperature. Fortunately, the operator found that the furnace temperature changed, and immediately took measures to stop production and re measure the temperature, avoiding the occurrence of a large number of overheated products, but there are still a small number of gear milling cutter cracks caused by overheating . Metallographic examination showed that the quenched grains were coarse, and the cracks occurred at the network carbide. It can be seen that the cracks were caused by too high heating temperature. The eutectic carbides formed by overheating are distributed in a network along the grain boundary, forming a hard shell on the grain boundary, causing great brittleness of the steel, hindering the plastic deformation of the steel, and causing quenching cracks under the action of the great stress produced during quenching and cooling. The main measures to prevent parts from cracking due to overheating are as follows:

① Technologists should make correct and reasonable heating temperature and heating method. For example, for some high-speed steel tools with large carbide segregation, the lower limit of quenching temperature of high-speed steel should be adopted. In order to ensure sufficient heating, the heating time can be appropriately extended. For some parts with sharp corners, the distance from the workpiece to the electrode in the salt bath furnace should not be less than 50 mm, so as to avoid overheating of sharp corners.

② The temperature measuring instrument and temperature control instrument should be checked and maintained regularly to ensure that the instrument is always in normal operation. The instrument staff shall ensure the accuracy of temperature measurement, temperature setting and temperature regulation.

③ The quenching worker should observe the temperature at any time. If abnormal temperature change is found, he should find the technologist or instrument man and other relevant personnel to solve the problem. If necessary, he should stop production immediately and measure the temperature again. When the furnace temperature returns to normal, he can start production again.

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