【Abstract】 Abstract: Through hardness measurement and metallographic inspection, it is confirmed that the cause of fatigue fracture of the rear axle axle of the automobile is improper heat treatment, which makes more ferrite appear in the structure of the axle, resulting in insufficient hardness and strength caused by.Chemical composition analysis and hardness determination 1. Chemical composition analysis (GB3077-881). 2. The hardness is measured according to the 40CrZB/T21004-89 "Technical Conditions for Automobile Axle" standard. After pre-conditioning and tempering, the core hardness of the shaft is 24^30HRC. After intermediate frequency quenching, the surface hardness of the rod is 52HRC3 hardness units.The actual test results, the...
Summary:
Through hardness measurement and metallographic inspection, it was confirmed that the cause of fatigue fracture of the rear axle half shaft of the automobile was caused by improper heat treatment, which caused more ferrite in the structure of the half shaft, resulting in insufficient hardness and strength.
Chemical composition analysis and hardness determination
1. Chemical composition analysis (GB3077-881).
2.硬度测定 40CrZB/T21004-89《汽车半轴技术条件》标准,该轴经预调质后心部硬度为24^30HRC,中频淬火处理后,杆部表面硬度52HRC3个硬度单位。实际检验结果,该半轴的硬度偏低。
Macro and micro inspection
1. The macroscopic inspection fracture occurs at the spline, the fracture is in the shape of a chrysanthemum, the entire section is an inverted cone, and the crack first occurs on the outside of the spline shaft.The central area, which is gray-black in color and lacks metallic luster, is the final instantaneous fracture area.
2 Crack Analysis
(1) There are few inclusions in the fracture semi-axis matrix, and there are no inclusions on both sides near the crack, but there is oxide scale in the crack, so it does not have the crack characteristics caused by non-metallic inclusions;
(2) There is no decarburization on both sides of the crack, the line is smooth, the tail is slender, and not round and bald, eliminating non-quenching cracks caused by defects in the raw material itself (white spots, looseness, skin turning, subcutaneous bubbles);
(3) The crack depth exceeds the hardened layer, and the sorbite and troostite in the hardened layer are fine and uniform, and quenching cracks caused by improper quenching such as excessive quenching temperature are excluded.
3 Microstructure
According to relevant literature, the unhardened layer of 40Cr steel after quenching and tempering treatment is troostite and sorbite, and ferrite is allowed in the core.
Samples were taken at the fracture of the spline shaft and observed under an optical microscope. The microstructure of the spline teeth was tempered sorbite and tempered troostite.The matrix structure of the semi-axial center is sorbite, on which there are ferrite distributed along the grain boundary in the form of reticulate and needle-like distribution, and the black agglomerates are troostite.It can be seen that the ferrite content gradually increases with the distance from the bottom of the spline to the center.
4 Discussion
According to the composition analysis, the chemical composition of the material used for the half shaft meets the composition requirements of 3077Cr in the standard GB88-40, and the steel is pure, so the factor of fracture caused by misuse of materials and poor steel can be excluded.
The hardness test results of the half shaft splines show that the hardness values from the core to the teeth are obviously low.Because the working environment of the half shaft is harsh, and it is subjected to bidirectional alternating torsional stress, and the spline shaft is the force fulcrum, insufficient hardness may lead to the formation of fatigue cores at the included angles of each groove, and at the same time along the radial direction At 45º, the two sides expand in an oblique direction, meet at the center of the shaft, and finally form a star-shaped fracture.
From the microstructure point of view, the semi-axes contain more ferrite, which are precipitated in the form of nets and needles along the grain boundaries.Generally, the half shaft of the automobile needs to be quenched and tempered, that is, the half shaft is heated to Ac3+ (30 ~ 50 ℃), kept for a period of time, and then cooled at a rate greater than the critical cooling rate, and the austenite is supercooled without touching the nose tip of the C curve. All transformed into martensite.If the cooling rate is less than the critical cooling rate, a part of austenite will transform into troostite and bainite, and ferrite will preferentially precipitate along the grain boundary, resulting in a reduction in the hardness and strength of the steel.There are generally two reasons for this situation: one is the improper selection of cooling medium; the other is due to mass production, too much furnace loading, and poor cooling and heating cycle due to the accumulation of parts, which makes the C curve to the left, and after quenching is bound to be. Troostite and undissolved ferrite structures appear.The undissolved ferrite in this quenched structure cannot be removed by high temperature tempering.The purpose of tempering is mainly to eliminate the internal stress caused by quenching and lattice distortion, reduce hardness, improve plasticity and toughness, and will not change the existing ferrite.The fine network ferrite that appears in this semi-axial microstructure is also the structure that exists after quenching.The difference is that the bulk ferrite is caused by low quenching temperature and insufficient holding time, and insufficient austenitization, while the reticulated ferrite is caused by the slow cooling rate during the cooling process, and the ferrite is preferential along the grain boundary. The reason for precipitation.
In the requirements of quenched and tempered steel, more free ferrite is not allowed in the tempered structure, especially the fine network free ferrite distributed along the grain boundary.It not only reduces the strength, but also directly affects the fatigue fracture.Because the damage of steel parts always starts from free ferrite with lower strength, especially for steel parts working under complex alternating stress, once there is free ferrite in the core, the ferrite is in cold working during operation. In the hardened state, with the prolongation of working time, it will develop from hardening to embrittlement to a certain limit, and then brittle fracture.In addition, due to the difference in the strength and plastic deformation of ferrite and sorbite, different plastic deformations are produced when the steel is subjected to the same stress, and in the two adjacent components, the grain boundary part produces greater stress Once the residual stress and deformation exceed the crack strength of the steel, it will cause grain boundary cracking, and when it continues to expand, it will become the main source of fatigue fracture.Therefore, more ferrite in the semi-axial structure is the root cause of fracture.