Temper cracks are a common problem that can arise during heat treatment and have multiple causes. To effectively prevent these defects, it is crucial to start the prevention process during the product design phase. This involves carefully selecting the right materials, carrying out a well-structured project and proposing suitable technical requirements for heat treatment.
Furthermore, it is essential to properly organize the process route, including making reasonable choices for heating temperature, holding time, heating medium, cooling medium, cooling method and operation mode.
In terms of materials
Carbon is a crucial factor in determining the tempering tendency of steel. As the carbon content increases, the melting point (MS) decreases, making the steel more susceptible to quench cracking. To minimize this risk, it is advisable to choose steel with the lowest possible carbon content, whilst maintaining the desired hardness and resistance properties.
The impact of alloying elements on quenching tendency is seen primarily in their effects on hardenability, MS point, grain size growth, and decarburization. The effect of alloying elements on hardenability can also affect the likelihood of quench cracking. However, increasing hardenability also tends to increase toughness. For parts with complex shapes, it is recommended to choose steel with good hardenability and use a tempering medium with a weaker cooling capacity to avoid deformation and cracking.
Alloying elements have the greatest impact on the MS point. Generally, the lower the MS point, the greater the tendency for hardening cracks. However, if the MS point is high, the martensite formed during the transformation process can self-temper, reducing the transformation stress and preventing quenching cracks. Therefore, when choosing steel, it is better to select a small amount of alloy elements or steel with elements that have less influence on the MS point.
Finally, it is important to consider sensitivity to overheating when selecting steel. Steel that is sensitive to overheating is more prone to cracking, so it is essential to pay close attention when making your selection.
Structural design of parts
The section size is uniform.
During heat treatment of parts with rapid changes in cross-sectional dimensions, internal stresses can cause cracking. To minimize this risk, it is recommended to avoid sudden changes in section size and maintain a uniform wall thickness.
If necessary, holes can be drilled in thick-walled parts that are not essential to the application, but it is advisable to drill these holes in through holes whenever possible.
For parts with varying thicknesses, a split design can be used and the parts can be assembled after heat treatment. This helps reduce internal stress and minimize the risk of cracking.
Rounded corner transition.
Parts with corners, sharp edges, grooves and transverse holes are susceptible to stress concentration, which can cause cracks. To mitigate this risk, it is advisable to design parts without stress concentration and round sharp corners and steps.
Variations in the rate of cooling during quenching can also result from shape factors. Cooling speed can vary depending on the shape of the part, and even different parts of the same object can have different cooling rates due to various factors. To avoid quenching cracks, it is important to minimize excessive differences in cooling rates.
Technical heat treatment conditions
It is advisable to use local or surface hardening techniques. The local hardness of quenched parts must be adjusted accordingly based on the service conditions.
When the local hardness requirement is low, there is no need to force uniform hardness throughout the part. It is also important to consider the mass effect of the steel.
When tempering, avoid the brittle zone of the first type of tempering to avoid cracking. By taking these precautions, the risk of cracking can be minimized and the performance of hardened parts can be optimized.
Reasonable arrangement of process routes and process parameters
Once the material, structure and technical conditions of the steel parts have been determined, heat treatment technicians will perform a process analysis to determine an optimal process route.
This involves properly arranging the positions of the heat pretreatment, cold working and hot working processes and determining the heating parameters. Process analysis helps ensure that the heat treatment process is efficient and effective in achieving the desired results.
Erase crack
Under 500X magnification, the surface appears to have a serrated appearance with a wide crack at the beginning and a small crack at the end.
Microscopic analysis revealed abnormal metallurgical inclusions and a zigzag pattern in the crack morphology. After corrosion with 4% nitric alcohol, there was no evidence of decarburization and the micromorphology is illustrated in the attached figure.
1# sample
No abnormal metallurgical inclusions were detected in the cracks of the product and there was no evidence of decarburization. The cracks had a zigzag pattern and exhibited the typical characteristics of temper cracks.
2# sample
Conclusions:
The sample composition complies with standard specifications and matches the original furnace composition.
Microscopic examination revealed no unusual metallurgical inclusions in the cracks of the sample and there were no signs of decarburization.
The cracks have a zigzag pattern and have characteristics typical of temper cracks.
Forging crack
Typical cracks caused by materials, the edges are oxides.
According to microscopic observation, the bright white layer on the surface is believed to be the secondary quenched layer, while the dark black layer below it is the high-temperature quenched layer.
Conclusions:
The presence of decarburized cracks helps to determine whether the cracks are due to defects in the raw material or not.
Typically, if the decarburization depth in the crack is equal to or greater than the surface decarburization depth, it is considered a raw material crack.
On the other hand, if the decarburization depth in the crack is less than the surface decarburization depth, it is considered a forging crack.
