Effect of Heat Treatment Process on Microstructure and Mechanical Properties of CrNiMoBNb16-16 Heat-Resistant Bolt Steel
However, with the increase of temperature and pressure under some working conditions, the performance requirements of bolts are correspondingly increased. Therefore, the steel must have high strength, good plasticity and toughness, and heat and corrosion resistance. So far, there have been few reports on the influence of the heat treatment process system on the microstructure and properties of steel, which has brought many inconveniences to the practical application of the steel. For this reason, the influence of different heat treatment process parameters on the microstructure and mechanical properties of steel was studied. On this basis, the reasonable heat treatment process parameters of the steel were obtained.
Sample preparation and experimental methods The experimental materials are rolled and processed according to the heat treatment conditions specified in the standard, that is, first rolling, followed by solution treatment [1]. And as required, the material should be stress relieved prior to delivery. According to the standard, the chemical composition is shown in the table.
According to the normal temperature and high temperature standards, the samples subjected to different heat treatments were subjected to room temperature stretching and high temperature tensile tests using standard samples. The metallographic samples under different heat treatment systems were prepared, and the cross-section of the sample was polished and polished, and then the processing technology of the solution treatment of ferric chloride and hydrochloric acid was used; the telephone; the chemical composition mass fraction heat treatment process of the steel to CrNiMoBNb16 -16 Effect of heat-resistant bolt steel on microstructure and mechanical properties He Wei 1, Yu Jixing 1, Wang Weili 2, Hualin School of Materials Science and Engineering, Wuhan University of Technology, Hubei Wuhan Shanghai Steam Turbine Co., Ltd., Shanghai Abstract: From heat treatment process to room temperature, high temperature The effect of tensile properties and the effect of test temperature on tensile properties analyzed the relationship between steel microstructure and mechanical properties. The test results show that as the test temperature increases, the strength decreases more obviously, while the plasticity decreases. For this experimental steel, warm forging is better than the comprehensive performance at high temperature forging. Through metallographic analysis, the change in its properties is closely related to the enthalpy in its chemical composition.
The heat treatment fluid of the first phase of the "Hot Processing Technology" was subjected to metallographic corrosion, and its microstructure was observed under the scanning electron microscope in the Netherlands.
The ideal heat treatment process is to obtain a uniformly distributed fine-grained austenite structure of the steel, and the comprehensive performance of strength and plasticity is good at high temperature, and at the same time, the corrosion resistance can be enhanced. Therefore, the following two heat treatment process schemes have been proposed: scheme deformation, water cooling; scheme deformation, air cooling, solid solution experiment results and analysis of the effects of deformation on mechanical properties at different temperatures. After the above two schemes, each sample is obtained. At the same time, a raw material sample was taken for room temperature and high temperature tensile test. The results are shown in Fig. 1. It can be seen that with the increase of high temperature tensile test temperature, the decline trend of tensile strength is divided into three stages, the first stage is At 20 ~ 300 ° C, its tensile strength drops faster. The second stage is 300 to 550 ° C. At this time, the tensile strength drops slowly, but there is a significant decline in the third stage, but there is no first stage. As the temperature increases, the grain coarsens, the length of the dislocation clusters in the grain increases, and the adjacent grain dislocation source is more easily activated, and plastic deformation is easy to occur, and the grain coarsening causes the number of grain boundaries. The reduction, that is, the number of obstacles that hinder the movement of dislocations, is reduced, and the tensile strength is also lowered.
It is found that the tensile strength of the scheme has been significantly improved, and the strength of the scheme and raw materials is similar, and there is no major change. This shows that the effect of improving the strength of the steel is obvious.
In addition, although the overall elongation of the solution has decreased, it still meets the requirements of the steel under high temperature conditions.
The figure shows the metallographic structure of the surface of the sample under different processes, and the microstructure is single-phase austenite. The grain size of the matrix is ​​5 ~ 10μm, and the distribution is uniform. The grain size of the matrix is ​​5 ~ 20μm, and the distribution is uneven. This shows that warm forging is one of the factors that make the final grain refinement. At this temperature, the crystallization of the metal is a dynamic recrystallization process. Due to the large nucleation rate during dynamic recrystallization, the rapid growth of the crystal grains is impossible, and the microstructure of the steel is alternated with the deformation. Refined, so the resulting organization is very small. In both figures, many carbides are precipitated. In the figure, the carbides are mainly distributed in the grain boundaries, while the carbides in the figure are mostly distributed on the grain boundaries. After energy spectrum analysis, the granular form is a ruthenium carbon compound and an elemental ruthenium. Since the amount of niobium in the steel is around, it far exceeds the carbon content. However, niobium is a strong carbide forming element, and it is easy to form intermetallic compounds when heated for a long time, which can eliminate or reduce the intergranular corrosion tendency, stabilize the formation of carbides, and avoid precipitation at the grain boundaries, thereby causing grain boundaries. It is depleted in chromium and inhibits intergranular corrosion. Insoluble NbC, due to the difference in thermal expansion between the creep process and the matrix, causes dislocations around the material. The temperature of the raw material solution / ° C tensile strength raw material solution elongation temperature / ° C chart mechanical properties test results a program 1 map test The metallographic structure of the surface is turned down. The effect of heat treatment on the surface of the second stage of the "Hot Processing Technology" is not negligible. The alloying elements mainly affect the diffusion coefficient by affecting the carbon activity in the steel [6]. When the silicon content is high, the cementite decomposes and the carbon exists in a free state, so that the carbon activity can be increased to promote the decarburization of the spring steel. Both vanadium and chromium are carbide-forming elements. When the chromium content is low, they are easily soluble in cementite. As the content of the element increases, the concentration of carbon in the steel increases, so the chromium element can reduce the steel off. Carbon tendencies. Vanadium has a strong affinity with carbon and is relatively easy to form very stable carbides with carbon. The vanadium and chromium carbides are segregated on the grain boundaries, which reduces the voids at the grain boundaries and reduces the atomic mobility at the grain boundaries, thereby reducing the role of the grain boundaries as diffusion channels. Observing the decarburization layer changes of the two test steels, it was found that the decarburization process developed along the original austenite grain boundaries, so the presence of carbides reduced the carbon diffusion rate and reduced the decarburization sensitivity of the steel. From the composition of the two steels, the test steel and the carbon content are the same, the former content is significantly higher than the latter, and the test steel has the highest content. Because the effect on the carbon activity is opposite, the depth of the decarburization layer of the test steel under the same heating process is smaller than that of the test steel. At the same heating temperature, the influence of the holding time on the decarburization layer of the test steel is mainly manifested as The depth of the full decarburization layer changes, and the depth of the full decarburization layer increases with the increase of the holding time. The influence of the holding time on the decarburization layer of the test steel is mainly manifested by the change of the depth of the total decarburization layer. Prolonged, the depth of the total decarburization layer increases sharply.
At the same holding time, the depth of the full decarburization layer of the test steel will be the maximum, and there is a sensitive temperature at the depth of the total decarburization layer. The heating temperature of the test steel exceeds 950 ° C, and the full decarburization layer The depth change is small, the total decarburization depth will decrease, but the excess will gradually increase with the increase of heating temperature.
And can form carbides, reduce carbon activity, thereby reducing the decarburization sensitivity, and can improve the carbon activity and the ability of carbon to diffuse in austenite, thereby improving the decarburization sensitivity. In order to reduce the decarburization sensitivity, the carbon and silicon content should be reduced in the design of new spring steel alloy elements, and microalloying elements such as vanadium and chromium should be appropriately used. When the existing spring steel is processed, as far as the process requirements are met, Reduce heating temperature and shorten holding time.
[2] Xu Dexiang, Yin Zhongda. Development status and trend of high strength spring steel [J]. Steel [3] Xiaohong Red. Control of surface decarburization during hot rolling of silicon-containing medium carbon steel [J]. Physical and chemical [4] Li Yesheng, Rao Ke.
Research on new low carbon martensitic spring steel 35Si2CrVB [5] Cao Jie, Xiang Changxiang, Chen Dong, et al. Oxidative decarburization behavior of several high speed steels [J]. North [6] Nie Yihong, Fu Shuhong, Hui Weijun, and so on. The decarburization sensitivity of boron to high-strength spring steel is linked to the dislocations that are to be precipitated to pin new dislocations, and these new dislocations enhance the strength of the steel. Some bismuths exist in the form of simple substances. These solute atoms act as dragging on the grain boundaries, preventing recrystallization and refining the grains.
Conclusion After high temperature deformation heat treatment, the strength of the steel has been improved. Especially in the case of warm forging, the strength has been obviously improved to 700 MPa, the steel is deformed more than the strength of the raw material, the tensile strength after air-cooling and solution treatment is deformed, and the tensile strength after water-cooling treatment is improved. The reason is mainly caused by the presence of bismuth in the chemical composition, and the ruthenium in the form of a compound suppresses the corrosion of the grain boundary and improves its strength. The ruthenium in the form of a simple substance refines the grains.
It is determined that the deformation heat treatment method of the steel is warm forging, so that the toughness has the best match.
[1] Hao Hongyuan, Cao Shirui, Hao Hao. Effect of solution/aging treatment on microstructure and properties of austenitic heat-resistant steel [D]. Taiyuan: Department of Materials Engineering, North University of China, 2006.
[2] Liu Dezhen. The effect of deformation on the microstructure and properties of austenite at moderate temperature isothermal transformation [D]. Beijing: Department of Materials Physics, University of Science and Technology Beijing, 2004.
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