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Fracture scanning electron microscopy (SEM) analysis and the low-magnification of the spring fracture and the electron micrograph of the source region, respectively, reveal clear spread fringes and show that the extended fringes originate from microcracks entering the matrix from the surface. Produced. As the electron microscopic characteristics of the fatigue fracture found near the source region, the transient fault zone is characterized by the transgranular fracture and the microscopic morphology of the secondary crack, and the final fracture zone has a small dimple. The surface of the spring has a large number of strains, widened cracks in the grain boundaries, and partial shedding.
After etching: the surface opening defects are mostly along the grain boundary, and the local defects are as deep as nearly ten grains. There is no obvious difference between the surface and the heart tissue, and the tissue is also single-phase austenite. The cross-section grains are equiaxed, and the longitudinal section grains are directional in the axial direction, and the deformation slip line is less. 3 spring polished state (500@) spring polished state (500@)0 spring section surface metallurgy (200@) 1 spring cross section surface metallography (200@) 2 spring cross section metallographic (200@) 3 spring longitudinal section Metallographic (200@) 316 comprehensive analysis and comprehensive analysis can be seen that the fracture spring material is 1Cr18Ni9, such spring can work in corrosive environment, is a spring with corrosion resistance.
In addition, the microscopic observation of the fracture surface of the injection pump of a certain oilfield shows that the spring exhibits fatigue fracture characteristics, the crack source comes from the defect of the outer surface of the spring, the fatigue fringing feature is visible near the source region, and the instantaneous fault zone is The cleavage fracture and the transgranular fracture are mainly accompanied by the presence of secondary cracks. It can thus be judged that the breakage of the spring is a fragile fatigue failure caused by surface defects.
With the continuous action of the alternating load, the slip line of the front edge of the fatigue source expands into a crack, and a broom-like slip beam is formed at the end thereof, so that one slip line in the slip becomes a crack again, which makes the crack The development has got a leap. When the crack develops so that the shear stress on the section exceeds the tensile strength, the spring breaks. The existence of surface defects can not only greatly accelerate the fatigue failure process of the spring, but also change the spring failure property to make it appear as instantaneous brittle fracture. In addition, the effects of environmental media on the oxidation and corrosion of the spring also contribute to the fatigue fracture of the spring.
Conclusions and recommendations (1) The chemical composition of the spring meets the standard value requirements of 1Cr18Ni9 stainless steel, and its hardness and metallographic structure are normal. (2) The spring is brittle fatigue fracture failure, and the surface defects (including surface strain, grain boundary widening crack, surface pit and surface burr) generated by the spring are the main causes of spring fracture failure. The oxidative corrosion effect of the environmental medium promotes the fatigue fracture of the spring. (3) Improve the drawing quality of the spring steel wire, improve the spring winding process, and spray-shoot the formed spring to improve the surface quality of the spring, which can effectively reduce the fatigue failure probability of the spring.
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