Keywords:The characteristics and differences of quenching crack,forging crack and grinding crack are summarized by China Hardware
Quenching crack is a common quenching defect, and the causes are various. Because the defect of heat treatment starts from the product design, the work of preventing crack should start from the product design. The material should be chosen correctly, the structure design should be carried out reasonably, the technical requirements of heat treatment should be put forward, the process route should be arranged properly, the heating temperature, the holding time, the heating medium, the cooling medium, the cooling method and the operation mode should be selected reasonably.
- Carbon is an important factor affecting the tendency of quenching cracking. With the increase of carbon content, the MS point decreases and the tendency of quenching crack increases. Therefore, under the condition of satisfying basic properties such as hardness and strength, lower carbon content should be selected as far as possible to ensure that it is not easy to crack.
- The influence of alloying elements on quenching tendency is mainly reflected in the influence of hardenability, MS point, grain size growth tendency and decarbonization. Alloying elements affect the tendency of quenching through their influence on hardenability. Generally speaking, hardenability increases and hardenability increases, but at the same time, quenching medium with weak cooling ability can be used to reduce quenching deformation to prevent deformation and crack of complex parts. Therefore, for the parts with complex shapes, in order to avoid quenching cracks, it is a better plan to choose steel with good hardenability and use quenching medium with weak cooling capacity.
Generally speaking, the lower the MS point, the greater the quenching tendency of steel. When the MS point is high, the martensite generated by phase transformation may be self-tempered immediately, so as to eliminate part of the phase transition stress and avoid quenching. Therefore, when the carbon content is determined, steel with a small amount of alloying elements or elements that have little effect on the MS point should be selected.
When choosing steel, the sensitivity to overheating should be considered. Overheat sensitive steel, easy to crack, so in the selection of materials should be paid attention to.
Structural design of parts
Uniform section size. The parts with rapid change in section size will crack due to internal stress during heat treatment. Therefore, the design should try to avoid the abrupt change of section size. The wall thickness should be uniform. If necessary, holes can be opened in thick wall parts not directly related to the use. Holes should be made through holes as far as possible. For parts with different thicknesses, they can be designed separately and assembled after heat treatment.
Fillet transition. When the parts have edges, sharp corners, grooves and transverse holes, these parts are easy to produce stress concentration, resulting in parts quenching crack. To this end, the parts should be designed as far as possible not to occur stress concentration shape, in the sharp corners and steps processing into rounded corners.
Differences in cooling rate caused by shape factors. The cooling speed of parts quenching is different with the different shapes of parts. Even in different parts of the same part, there will be different cooling rates due to various factors. Therefore, excessive cooling differences should be avoided as far as possible to prevent quenching cracks.
Technical conditions for heat treatment
1.Use local quenching or surface hardening as far as possible.
2.Adjust the local hardness of the quenched parts according to the service conditions of the parts. When the local quenching hardness requirement is low, try not to force the whole hardness to be consistent.
3. Note the mass effect of steel.
4.Avoid tempering in the first type of tempering brittle zone.
Arrange process route and process parameters reasonably
When the steel material, structure and technical conditions are determined, the heat treatment process personnel will carry out process analysis, determine the reasonable process route, that is, the correct arrangement of preparation heat treatment, cold working and hot working process position and determine the heating parameters.
At 1.500x, the crack was serrated, the initial crack was wide, and the end crack was fine to none.
Microscopic analysis: abnormal metallurgical inclusion, crack morphology is serrated extension; After corrosion with 4% nitric acid alcohol, no decarbonization phenomenon was observed. The microscopic morphology is shown in the following figure:
No abnormal metallurgical inclusions were found at the crack of the product, and no decarbonization phenomenon was found. The crack extended in serrated shape and had the typical characteristics of quenching crack.
The composition of the sample meets the standard requirements and corresponds to the composition of the original furnace number.
According to the microscopic analysis, no abnormal metallurgical inclusions were found at the crack of the sample, and no decarbonization phenomenon was found. The crack extended in serrated shape and had the typical characteristics of quenching crack.
Crack caused by typical material, edge of oxide.
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