Fracture Analysis of Automobile Rear Axle

Abstract: Through hardness measurement and metallographic examination, it is confirmed that the cause of fatigue fracture of the rear axle half-axis of the automobile is caused by improper heat treatment, which causes more ferrite to appear in the structure of the half-axis, resulting in insufficient hardness and strength.
Chemical composition analysis and hardness determination
1. Chemical composition analysis (GB3077-881).
2. Hardness determination 40CrZB / T21004-89 "Technical conditions of automobile axles", the shaft hardness of the shaft is 24 ^ 30HRC after pre-quenching and quenching, and the surface hardness of the shaft is 52HRC. As a result of actual inspection, the hardness of the semi-axis is relatively low.
Macro and micro inspection
1. Macro inspection The fracture is generated at the spline, the fracture is chrysanthemum-shaped, the entire cross section is inverted cone, and the crack is first generated outside the spline shaft. The heart region, which is grayish black and has no metallic luster, is the final transient fracture region.
2 crack analysis
(1) There are few inclusions in the fractured semi-axle matrix, and there are no inclusions on both sides of the near crack, but there are oxide scales inside the crack, so it does not have the characteristics of cracks caused by non-metallic inclusions;
(2) There is no decarburization on both sides of the crack, its lines are smooth, its tail is slender, and it is not round and bald, excluding non-quenching cracks caused by defects in the raw material itself (white spots, looseness, peeling, and subcutaneous bubbles);
(3) The crack depth exceeds the hardened layer, and the microstructure of the sorbite and bainite in the hardened layer is small and uniform, excluding quenching cracks caused by improper quenching such as high quenching temperature.
3 microstructure
According to relevant literature, the unhardened layer of the 40Cr steel after quenching and tempering is bainite and sorbite, and ferrite is allowed in the heart.
Samples were taken at the spline shaft rupture and observed under an optical microscope. The spline tooth structure was tempered sorbite and tempered bainite. The matrix structure of the semi-axial part is sorbite, and there are reticular and needle-shaped ferrites distributed along the grain boundaries, and the black agglomerates are bainite. It can be seen that the ferrite content gradually increases with the distance from the bottom of the spline to the heart.
4 discussions
According to the composition analysis, it can be known that the chemical composition of the material used in the semi-axis meets the composition requirements of 40Cr in the standard GB3077-88, and the steel is pure, so the factor of fracture due to misuse of materials and poor steel can be ruled out.
The hardness test results at the half-axis splines show that the hardness values ??from the core to the teeth are significantly lower. Because the working environment of the half shaft is harsh, and it is subjected to two-way alternating torsional stress, and the spline shaft is a force fulcrum, insufficient hardness may cause a fatigue core to form at the angle of each groove, and at the same time The two 45 ° lateral extensions meet at the center of the shaft and form a star-shaped fracture.
From the microstructure point of view, the semi-axis contains more ferrite and precipitates in a network and needle shape along the grain boundaries. Generally, the automobile axle shaft needs to be quenched and tempered, that is, the axle shaft is heated to Ac3 + (30 ～ 50 ℃), held 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 of the C curve. All transformed into martensite. If the cooling rate is lower than the critical cooling rate, a part of austenite will transform to bainite and bainite, and ferrite will preferentially precipitate along the grain boundary, causing the hardness and strength of the steel to decrease. There are generally two reasons for this: one is the improper selection of cooling medium; the other is due to mass production, too many furnaces installed, poor thermal and thermal cycles caused by the accumulation of parts, making the C curve to the left, and bound to be hardened after quenching The appearance of bainite and undissolved ferrite. 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 there will be no change to the existing ferrite. The fine reticular 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 the low quenching temperature and insufficient holding time, and is not fully austenitized, while the network ferrite is due to the slower cooling rate during the cooling process, and the ferrite is preferential along the grain boundaries The reason for precipitation.
In the requirements of quenched and tempered steel, more free ferrite is not allowed to exist in the tempering structure, especially the fine mesh free ferrite distributed along the grain boundary. Not only does it reduce strength, it also directly affects fatigue fracture. Because the damage of steel parts always starts with low strength free ferrite, especially steel parts working under complex alternating stress. Once free ferrite is in the heart, ferrite is under cold working during work. In the hardened state, with the extension of working time, it will develop from hardening to brittleness to a certain limit, and then brittle fracture. In addition, due to the difference in strength and plastic deformation between ferrite and sorbite, steel parts undergo different plastic deformation when they are subjected to the same stress, which causes greater stress in the grain boundary part in two adjacent components. And deformation, this kind of residual stress and deformation will cause grain boundary fracture once it exceeds the crack strength of steel, and when it continues to expand, it will become the main source of fatigue fracture. Therefore, the presence of more ferrite in the semi-axial structure is the root cause of fracture.