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Deformation correction of 20CrMo steel plate by vacuum carburizing and quenching
Steel plate quenching has been a difficult problem in the heat treatment industry for many years. When the surface requires high hardness, leveling is the biggest problem. It is an innovative method to use a cast iron plate to press and quench the parts with small deformation to ensure surface hardness and no cracking. The surface hardness of the 20CrMo steel plate after vacuum carburizing and quenching was measured to be 53 HRC-65 HRC, and the 15mm thick steel plate was completely quenched. The method is simple and easy to implement, especially suitable for handling small batches or single-piece production. This process can also be popularized and used in heat treatment deformation correction of other plate-shaped parts.
The liner on the aluminum foil cold rolling mill has an area of 330 mm × 330 mm, and has three types of thickness: 5 mm, 8 mm, and 15 mm. The process requires that the surface hardness is greater than 55HRC. Heart hardness 25-30 HRC. In order to meet the technical requirements, 20CrMo steel plate is selected, and the conventional carburizing process and one-time heating and quenching process are adopted. After the quenching oil treatment, the workpiece is greatly deformed. The maximum deformation exceeds 5mm-10mm. Due to the high hardness required for the surface, leveling is the biggest challenge. Pressure quenching is generally used for processing plate-shaped parts, and usually non-professional factories do not have special pressure quenching equipment. On the basis of the experiment, the simple method was adopted, the problem was successfully solved, and good results were obtained.
2 Materials and experimental methods
(1) Material selection In order to meet the technical requirements, the test material selected 20CrMo steel plate with good hardenability as the test material, which was processed into a test piece with an area of 150 × 150 mm and a thickness of 5 mm, 8 mm and 15 mm.
(2) Vacuum heat treatment process
Vacuum carburizing: The specimen is heated to 900°C, kept for 7 hours, and then cooled down for cooling.
Vacuum quenching: The test piece after carburizing is reheated to 850 ℃ and kept for 15 minutes. After being released from the furnace, it is quickly placed in the middle of two cast iron plates for cooling, and the heat conduction effect of the iron plate is used to cool down and quench.
Inspection: Use HR150 hardness tester to test the surface hardness of the quenched specimen to be 53 HRC-64HRC. Core hardness 25 HRC-30 HRC.
The heat treatment process curve is as follows:
3 Test results
(1) Microhardness measurement results
20CrMo steel has good hardenability, no temper brittleness, and high thermal strength. It is a better carburizing steel. The steel plate adopts the direct oil quenching process. Due to the influence of the thermal stress and the structural transformation stress of the parts, the deformation is inevitable. Considering the method of tempering adjustment, the tempering temperature is low, which can ensure that the surface hardness is not reduced, but the deformation is difficult to adjust, and the increased deformation will cause the surface to crack. When the tempering temperature is increased, even if the deformation amount can be adjusted back, the hardness and wear resistance decrease, which cannot meet the technical requirements. The pressure quenching process is usually applied, but special pressure quenching equipment must be available.
The test adopts cast iron plate cooling, and the parts have almost no deformation after quenching. The surface hardness of the 20CrMo steel carburized and quenched parts is measured to be 53 HRC-64HRC, and the core hardness is 25-30 HRC. Due to the concern of quenching and cracking of the parts, the low magnification and high magnification metallography were used to observe the metallographic structure of the surface and the core. The observation of metallographic structure shows that the surface of the quenched part is flaky martensite structure and retained austenite, and the core is lath martensite. No cracks were found in the surface layer of the quenched parts by low and high magnification metallographic observations. More than ten pieces of products are quenched by cast iron plate, all of which meet the technical requirements.
Steel plate quenching has been a difficult problem in the heat treatment industry for many years, mainly because the deformation is difficult to control. It is a simple and effective method to use the cast iron plate pressure quenching process. To ensure the surface hardness of the parts, no cracking, no deformation. is an innovative approach. It is especially suitable for small batch or single-piece production. Experiments show that the surface hardness of the carburized layer reaches 53HRC-64HRC, and the 15 mm thick steel plate is fully quenched when the 20CrMo steel is treated by pressure quenching of the cast iron plate. This process can also be used for reference to other plate-shaped parts heat treatment deformation correction.
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