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Vacuum Heat Treatment of 42CrMo Steel High Strength Bolts

1.Materials and Methods

(1) Materials

The raw material is φ40mm double vacuum smelting steel bar produced by a steel plant

(2) Method

The processing process of high-strength bolts is to roughen the φ40mm bar → stamping → necking → vacuum annealing → tapping → vacuum quenching → vacuum tempering → performance testing → high-strength bolts. The vacuum quenching and tempering process is carried out on the production line of the idler mesh belt furnace. The specific process is as follows: the mesh belt quenching furnace of the idler mesh belt furnace production line is heated to 400°C, kept for 1 hour, and then heated to 600°C for 1 hour. , when the temperature rises to 800 °C, methanol and acetone are introduced as a protective atmosphere to avoid decarburization of the surface of high-strength bolts and oxidation of the hearth of the mesh belt quenching furnace. When the carbon potential is stabilized at 0.4%, the high-strength bolts to be treated are put in and kept for 1.5h (the diameter of the high-strength bolts in this test is 36mm, so the holding time is 1.5h). Temper at the specified vacuum tempering temperature, keep for 2h (similarly, the diameter of the high-strength bolt is 36mm, and the vacuum tempering time is determined to be 2h), and the furnace is water-cooled. In this paper, the orthogonal test is used to design three influencing factors of heat treatment process parameters, and the orthogonal test is used to find the interaction between them and determine the best parameters of each factor.

2.Results and Analysis

Three parameter factors and three levels of carbon potential, water-soluble quenching cooling medium and vacuum tempering temperature during vacuum heat treatment of 42CrMo steel high-strength bolts were designed by orthogonal experiment.

According to 3 parameter factors and their 3 levels, the orthogonal experiment design was used to conduct 9 groups of experiments. It can be seen that when the concentration of water-soluble quenching cooling medium is 1% to 6%, 6% to 12% and 12% to 18%, and the vacuum tempering temperature is 520 ° C, 530 ° C and 540 ° C, respectively, 42CrMo steel high-strength bolts σb are 1158MPa, 1170MPa and 1160MPa respectively, σ0.2 are 1044MPa, 1052MPa and 1046MPa respectively, δ5 are 13.3%, 14% and 12.7% respectively, AKU are 84J, 88J and 87J respectively, Ψ are 48.3% and 51% respectively and 49.3%. It can be seen that the mechanical properties of 42CrMo steel increase first and then decrease with the increase of vacuum heat treatment parameters of water-soluble quenching cooling medium concentration and vacuum tempering temperature. Therefore, when the water-soluble quenching cooling medium concentration is 6% to 12 %, when the vacuum tempering temperature is 530 ℃, the performance of 42CrMo steel high-strength bolts is 1170MPa, σ0.2 is 1052MPa, δ5 is 14%, AKU is 88J, Ψ is 51%, reaching the maximum value, its comprehensive performance Also best. At the same time, studies have shown that the carbon potential of 42CrMo steel high-strength bolts in the process of vacuum quenching and heating mainly plays the role of protective gas, which inhibits the surface decarburization of high-strength bolts and prevents the oxidation of the idler mesh belt quenching furnace furnace. Generally, the carbon potential is controlled at About 0.4% is appropriate, which has little effect on the organization and performance of high-strength bolts.

(1) The effect of different vacuum heat treatment parameters on the microstructure of 42CrMo steel.

Microstructures of 42CrMo steel under the first, fifth and ninth group of vacuum heat treatment parameters, respectively. The bright matrix is martensite, and the irregular particles or strips are bainite and ferrite. The black matrix is lath martensite, the brighter is bainite, and the bright spot is undissolved. The end of carbide or bainite structure intersects the observation surface, and the bright spots on ferrite are carbides precipitated during the transformation process.

The bainite structure is all carbon-free bainite, but there are great differences in morphology. There is a common carbon-free bainite structure, but it is more needle-like, and the distribution is uneven, and there is segregation. Under the parameters of carbon potential (0.4%), water-soluble quenching cooling medium concentration (6%-12%) and vacuum tempering temperature of 530℃, the bainite structure of 42CrMo steel is mostly granular and has adjacent bainite The phenomenon of interconnected body structure indicates that the early formation of carbon-free bainite is not granular, but mostly needle-shaped and evenly distributed, because of the high energy and composition fluctuations at the grain boundaries, Facilitates the nucleation of bainitic ferrite. The growth of carbon-free bainite is not sufficient, and a bundle of bainitic ferrite that grows in parallel from the grain boundary to the grain is formed.

The formation of carbon-free bainite is mainly due to the strong diffusion ability of carbon in ferrite and austenite when 42CrMo steel is quenched and cooled in an appropriate concentration of water-soluble quenching cooling medium. After the nucleation of the ferrite, its supersaturated carbon can diffuse out quickly, and can diffuse into the austenite over a long distance, which ensures that there is no carbide in the bainite and ferrite during and after growth. Precipitate. At the same time, it also shows that the carbon content of bainite and austenite around ferrite is not too high, otherwise carbides will be precipitated and their growth will be hindered. Therefore, under the appropriate carbon potential, water-soluble quenching cooling medium concentration and vacuum tempering temperature and other vacuum heat treatment process parameters, the microstructure of 42CrMo steel is more refined and homogenized, and the overall performance is better.

(2) Influence of water-soluble quenching cooling medium on mechanical properties of 42CrMo steel

The quenching cooling rate is determined by the concentration of the water-soluble quenching cooling medium. The lower the concentration, the faster the quenching cooling rate. When the concentration of the water-soluble quenching cooling medium is 1% to 6%, the quenching and cooling rate is too fast. The transformation of the body structure is insufficient, and defects such as segregation and distortion are easily generated in the structure of 42CrMo steel; and when the concentration is 6% to 12%, the quenching cooling rate is moderate, which provides enough time for the martensitic transformation, so it also effectively reduces It reduces the chance of distortion, cracking and other defects when the workpiece is quenched, the structure is more uniform, and the overall performance is better; when the concentration of the water-soluble quenching cooling medium continues to increase, the quenching cooling rate is too slow, causing the martensite to become coarse, reduce its mechanical properties.

(3) Effect of vacuum tempering temperature on mechanical properties of 42CrMo steel

Under different vacuum heat treatment parameters, the mechanical properties of 42CrMo steel high-strength bolts after vacuum quenching are consistent with the variation law of vacuum tempering temperature. Its strength, hardness, plasticity and toughness depend on the nature and content of the constituent phases, that is, on the strength, hardness, plasticity, toughness and volume percentage of martensite and ferrite. After vacuum tempering at different temperatures, its properties still depend on the martensite vacuum tempered structure and the properties of ferrite. Therefore, after the microstructure of 42CrMo steel is heated and vacuum quenched at the same critical temperature, the mechanical properties are consistent with the change of vacuum tempering temperature.

Hardness is the ability of a material to resist the pressing of hard objects into its surface, and it is one of the important performance indicators of metal materials. Hardness test is the most simple and easy test method in mechanical property test. Practice has proved that there is an approximate corresponding relationship between the hardness value and the strength value of metal materials. The hardness value is determined by the initial plastic deformation resistance and the continued plastic deformation resistance. The higher the hardness value, the higher the strength of the material and the higher the plastic deformation resistance. However, when the hardness reaches a certain value, the brittleness of 42CrMo steel increases, and its area shrinkage rate Ψ, impact energy AKU and elongation rate δ5 begin to decrease, which fails to meet the requirements for use.

With the increase of vacuum heat treatment parameters of water-soluble quenching cooling medium concentration and vacuum tempering temperature, there is a trend of first increase and then decrease. When the vacuum tempering temperature is 530 °C, its microstructure and properties are the best. This is because the low temperature vacuum During tempering, the strength of the martensite and ferrite phases is very different, which is easy to cause stress concentration and segregation, resulting in the inability to homogenize the structure and the performance does not meet the requirements. However, when 42CrMo steel high-strength bolts are vacuum tempered at 530 °C, the difference of the two-phase strength is reduced, which is beneficial to improve the stress distribution and the uniformity of the structure. When the vacuum tempering temperature is too high, the brittleness of the 42CrMo steel high-strength bolt is too high, resulting in some mechanical properties not meeting the requirements. Therefore, when the vacuum tempering temperature is 530 °C, its comprehensive mechanical properties are the best.

(4) Effect of vacuum tempering temperature on the microstructure of 42CrMo steel

42CrMo steel high-strength bolts are vacuum quenched and then vacuum tempered. The effect of vacuum tempering temperature on their microstructures is consistent, and the microstructures have experienced changes from coarse to fine. The different vacuum tempering processes after vacuum quenching are mainly the elimination of the quenched structure stress, the stabilization of the structure and the precipitation of carbides in the matrix structure. When vacuum tempered at 520℃, the microstructure is vacuum tempered martensite, undissolved ferrite and a small amount of retained austenite. The carbides precipitated during low temperature vacuum tempering are relatively fine. With the increase of vacuum tempering temperature, carbon atoms are continuously precipitated to form carbides, and the carbides precipitated first continue to grow to form vacuum tempered trotenite. When the temperature reaches 530 °C, the carbides continue to grow and become granular. The tensite is transformed into ferrite, forming vacuum tempered sorbite and undissolved ferrite. At this time, the strength and hardness of the steel are reduced, and the plasticity and impact toughness are greatly improved.

3.Conclusion

(1) The vacuum heat treatment parameters such as appropriate water-soluble quenching cooling medium concentration and vacuum tempering temperature make the structure of 42CrMo steel high-strength bolts refined and homogenized, and their comprehensive properties have also been greatly improved.

(2) 42CrMo steel high-strength bolts undergo the fifth group of vacuum heat treatment (heating at 860°C for 1.5h, carbon potential 0.4%) + cooling with water-soluble quenching cooling medium (6%~12%) + 530°C vacuum tempering for 2h + water cooling ) after the organization is homogenized and refined, and the overall performance is also the best.

Selection of vacuum heat treatment equipment: The RVT series vacuum tempering furnace produced by SIMUWU is a high-quality product for such processes. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum tempering process. With more than ten years of sales and manufacturing experience, it is exported to developed regions in Europe, America and Asia. It is a widely acclaimed vacuum furnace product.