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Effect of Vacuum Heat Treatment Process on Properties of Spring Steel

The effects of vacuum and air heat treatment processes on the fatigue properties and microstructure of 50CrVA spring steel were studied using optical metallography, scanning electron microscopy and fatigue tests. The results show that the fatigue properties of the materials under the two heat treatment processes are significantly different. The fatigue properties of the vacuum heat treatment process are significantly better than those of the air heat treatment process, with the median fatigue limit increased by 30.8%. The macro and micro morphology of the fracture surfaces of the two heat treatment processes are indistinguishable. The obvious difference is that the cracks originate from a single source on the surface. The surface state of the material has a great influence on the fatigue life. The sand blowing process of the air heat treatment process causes microscopic surface stress concentration on the sample surface, reducing the fatigue performance.

Due to its high strength, fatigue resistance and anti-elasticity, spring steel is often used to manufacture parts such as shock absorption, energy storage, transmission, and support, and plays an important role in aircraft manufacturing. Among them, 50CrVA spring steel has excellent properties such as good hardenability, high fatigue strength, and high yield ratio. The addition of vanadium refines the grains of the steel, reduces overheating sensitivity, and improves strength. Therefore, 50CrVA spring steel has good mechanical properties. And process performance, it is a higher-grade spring steel, widely used in various machinery.

As a means to improve the quality of heat treatment, vacuum heat treatment has the characteristics of no oxidation, no decarburization, small deformation, and low pollution compared with air heat treatment. The air heat treatment process requires a sand blowing process after the heat treatment of the parts to remove oxide scale, while the vacuum heat treatment process does not require a sand blowing process. Therefore, the two heat treatment processes have different effects on the surface state of the parts, which may lead to differences in fatigue performance.

The spring raw material selected for this test is 50CrVA. The quenching temperature and tempering temperature of the heat treatment are 850 and 385 ℃ respectively. After the heat treatment is completed, a fatigue test is performed.

Fatigue test: high cycle fatigue at room temperature, stress ratio R=0.06, sample stress concentration coefficient Kt=1, test frequency is 120±15 Hz. The test includes 2 parts: group method and lifting method. The fracture surface of the sample is scanned, and after ultrasonic cleaning, it is observed under a scanning electron microscope, the macro-fracture and micro-fracture are photographed, and the failure mode, crack source location, etc. are analyzed.

The influence of vacuum heat treatment process

The axial tensile high-cycle fatigue stress-life (S-N) curve of the 50CrVA smooth specimen is shown in the figure. The effective test results of the group method and the lifting method are listed in the figure (the numbers on the right side of the figure indicate the use of lifting The number of specimens under corresponding strength during the method).

Under the same load level, the vacuum heat treatment data points are basically all on the right side of the air heat treatment data points, indicating that the fatigue life of vacuum heat treatment is longer. The vacuum heat treatment curve is generally above the air heat treatment, indicating a higher fatigue limit. Therefore, it can be seen that the high-cycle fatigue test results of vacuum furnace heat treatment specimens are significantly better than those of air furnace heat treatment. The median fatigue limit of air heat treatment of 50CrVA spring steel is 816.7 MPa, which is 50.4% of the material’s yield strength; the median fatigue limit of vacuum heat treatment is 1067.9 MPa, which is 64.0% of the material’s yield strength. Comparing the two, it can be seen that the median fatigue limit of vacuum heat treatment has increased by 251.2 MPa, an increase of 30.8%, and the improvement effect is significant.

Vacuum heat treatment has significant improvement effect

(1) The room temperature high cycle fatigue performance of 50CrVA using vacuum heat treatment process is significantly better than that of air heat treatment process. The median fatigue limit of vacuum heat treatment is 30.8% higher than that of air furnace heat treatment;

(2) There is no obvious difference in the fracture morphology of the two heat treatment processes. There is only one crack source on the fracture surface, and the crack originates from the surface of the sample, that is, the crack originates from a single source on the surface;

(3) The surface state of the material has a great influence on the fatigue crack life. The sand blowing process of the air heat treatment process removes the oxide scale and forms scratches, abrasions, sharp grooves or pits on the surface of the sample, causing the surface roughness to increase. , and the discontinuous plastic deformation formed on the surface causes uneven distribution of surface material properties, causing microscopic surface stress concentration and reducing fatigue performance. The vacuum heat treatment process does not require sand blowing, the surface condition is good, and the fatigue performance is better.