Quenching cracks are common quenching defects, and the causes are various.
Since the defects of heat treatment start from product design, the work of preventing cracks should start from product design.
It is necessary to properly select materials, conduct structural design reasonably, propose appropriate heat treatment technical requirements, properly arrange the process route as well as choose a reasonable heating temperature, holding time, heating medium, cooling medium, cooling method and operation mode.
In terms of materials
- Carbon is an important factor affecting quenching tendency. As the carbon content increases, the MS point decreases, and the tendency to quench cracking increases.
Therefore, under conditions that meet basic properties such as hardness and strength, a lower carbon content should be selected as much as possible to ensure that it is not easy to crack.
The influence of alloying elements on quenching tendency is mainly reflected in the effects on hardenability, MS point, grain size growth tendency, and decarburization.
The effect of alloying elements on hardenability affects the tendency to quench cracking. Generally speaking, hardenability and hardenability increases.
However, while the hardenability is increased, a quenching medium with a weak cooling capacity can be used to reduce the quenching deformation to prevent the deformation and cracking of complex parts.
Therefore, for parts with complex shapes, in order to avoid quenching cracks, it is a better solution to choose steel with good hardenability and use a quenching medium with weak cooling capacity.
Alloying elements have a greater effect on the MS point. Generally speaking, the lower the MS, the greater the tendency for quench cracking.
When the MS point is high, the martensite formed by the transformation may be self-tempered immediately, thereby eliminating part of the transformation stress and avoiding quench cracking.
Therefore, when the carbon content is determined, a small amount of alloying elements or steels with elements that have less influence on the MS point should be selected.
- When selecting steel, consider overheating sensitivity.
Overheat sensitive steels are prone to cracks, so attention should be paid when selecting materials.
Structural design of parts
- The section size is uniform.
During the heat treatment of a part whose cross-sectional dimensions change rapidly, cracks occur due to internal stress.
Therefore, try to avoid sudden changes in section size and uniform wall thickness.
When necessary, holes can be made in thick-walled parts that are not directly related to the application, the holes shall be made into through holes as far as possible.
For parts with different thicknesses, split designs can be made and assembled after heat treatment.
- Rounded corner transition.
When the part has corners, sharp corners, grooves and transverse holes, these parts are prone to stress concentration, which causes the part to crack.
For this reason, parts should be designed as far as possible without stress concentration, and rounded at sharp corners and steps.
- Difference in cooling rate caused by shape factors.
The speed of cooling during the quenching of parts varies with the shape of the part. Even in different parts of the same part, there will be different cooling rates due to various factors.
Therefore, try to avoid excessive cooling differences to prevent quench cracking.
Heat treatment technical conditions
- Try to use local hardening or surface hardening.
Adjust the local hardness of the quenched parts reasonably according to the service conditions of the parts. When the local quenching hardness requirement is low, try not to force the overall hardness to be consistent.
Pay attention to the mass effect of steel.
Avoid tempering in the brittle zone of the first type of tempering.
Reasonable arrangement of process routes and process parameters
Once the material, structure and technical conditions of the steel parts are determined, the heat treatment technicians will perform a process analysis to determine a reasonable process route.
That is, the positions of the pre-heat treatment, cold working and hot working processes are correctly arranged and the heating parameters are determined.
- At 500X, it is serrated, with a wide crack at the beginning and a small crack at the end.
- Microscopic analysis: abnormal metallurgical inclusions, crack morphology extending in a zigzag manner. Observed after corrosion with 4% nitric alcohol, there is no decarburization, and the micro morphology is shown in the following figure:
1 # sample
No abnormal metallurgical inclusions were found at the cracks of the product, and there was no decarburization. The cracks extended in a zigzag pattern and had the typical characteristics of quenching cracks.
2 # sample
- The composition of the sample meets the standard requirements and corresponds to the original furnace number composition.
From the microscopic analysis, no abnormal metallurgical inclusions were found at the cracks of the sample, and no decarburization was observed. The cracks extend in a zigzag pattern and have the typical characteristics of quench cracks.
- Typical cracks caused by materials, the edges are oxides.
- Microscopic observation, the surface bright white layer should be the secondary quenched layer, and the dark black under the secondary quenched layer is the high temperature tempered layer.
The existence of decarburized cracks is to distinguish whether the cracks are raw material cracks.
Generally, the crack decarburization depth greater than or equal to the surface decarburization depth is the raw material crack.
If the crack decarburization depth is less than the surface decarburization depth, it is a forging crack.