The olive spring is the key part of the locomotive braking device, and it mainly uses the spring force to realize the parking brake and manual release functions when the locomotive is parked. When assembling, install the olive spring inside the cylinder and compress it to a certain height, and clamp it with an elastic snap ring: in actual use, the olive spring is also in a natural compressed state or a further compressed state after being filled with compressed air . Recently, individual fractures have occurred during the assembly and testing of the olive spring. This paper analyzes and discusses the fracture failure of the olive spring in order to grasp the failure mechanism, implement improvement measures, and ensure product quality.
1 Main technical parameters of olive spring
The shape of the olive spring is shown in Figure 1. It is a spiral with uniform diameter reduction. The diameter of the spring wire in the middle is the largest, about 16mm, and the diameter of the spring wire at both ends is slightly smaller, about 12mm. The material of the olive spring is 55siCrV, and its tensile strength can reach more than 1650MPa. It belongs to high-strength spring steel. After installation, the spring force in the initial compression state is 9925N, and the spring force in the working stroke can reach up to 12680N.
Figure 1 olive spring structure
2 Fracture phenomenon of olive spring
The fracture of the olive spring occurred after installation and during the test, the failure rate was 2.23%, and the fracture delay time was about 48h. After the fracture occurred, the parking brake was completely ineffective.
The fracture appearance and fracture condition are shown in Figure 2.
3 Brief description of the manufacturing process of olive spring
Olive spring is made of high-strength spring steel. The main process is coil spring, annealing, trimming, quenching, tempering, flaw detection, shot peening, compression, and surface spraying.
4 Fracture mechanism analysis of olive spring
4.1 Macro analysis of fracture
The color of the fracture section is gray, the section is relatively flat, and there is no obvious macroscopic plastic deformation near the fracture. Small radial ridges diverging inwards can be faintly seen on the edge of the fracture, and the crack source area is located at the convergent position of the radial ridges on the edge of the fracture, and the fracture macroscopically presents the characteristics of brittle fracture.
4.2 Fracture microscopic analysis
After ultrasonically cleaning the fracture with alcohol, the microscopic morphology of different regions of the sample surface was observed by scanning electron microscopy. The test results show that the microscopic morphology of the fracture source area is dominated by rock sugar-like intergranular brittle cracking, with flat crystal planes and clear grain boundaries, as shown in Figure 3(a): secondary cracks can be seen locally after zooming in and the typical chicken-claw morphology, the chicken-claw pattern is a typical morphology feature of hydrogen embrittlement fracture, as shown in Fig. It presents a mixed morphology composed of cleavage + dimples, and the farther away from the crack source area, the more dimples, as shown in Figure 3(c).
5 Analysis of the reason for the fracture of the olive spring
Through the macroscopic and microscopic analysis of the fracture, it can be determined that the failure of the olive spring is intergranular brittle delayed fracture caused by hydrogen embrittlement. The influencing factors of hydrogen embrittlement delayed fracture of high-strength spring steel generally include material itself and heat treatment factors. In the following, combined with the manufacturing process of the olive spring, these two factors are analyzed separately to clarify the external causes of hydrogen embrittlement fracture.
5.1 Material factor analysis
The chemical composition of the failed olive reed material was analyzed, and its chemical composition meets the product design requirements, that is, it meets the requirements for grade 55siCrV in cG/BT111-12T0, as shown in Table 1. Further sampling the hydrogen content of raw materials, fractured parts, and qualified parts, the detected hydrogen content is below T2-0 (Tppm), and there is no obvious correlation with fracture failure, so it can be speculated that the hydrogen embrittlement failure is not an absolute hydrogen failure. Due to the high content, it should be due to the presence of hydrogen accumulation or other abnormal properties that increase the susceptibility to hydrogen embrittlement.
5.2 Analysis of heat treatment factors
The hydrogen embrittlement of steel is closely related to its heat treatment structure, and the quenching + tempering process is the key link to determine the microstructure and strength level of spring steel. It can be seen from the literature search that the high-strength material 55siCrV can obtain a certain proportion of tempered troostite, tempered martensite and retained austenite after quenching + tempering heat treatment at an appropriate temperature and time. Among them, the tempered troostite ensures the high strength and high elastic limit of the material, and the structure should be small and stable.
5.1. T heat treatment process investigation
Through the retrospective investigation of the heat treatment process of this batch of springs, it was confirmed that the quenching and tempering temperatures used were normal production temperatures, and the conditions had not changed from the previous ones. The difference lies in the waiting time for tempering after quenching. In normal production, tempering is controlled within 1 hour after quenching. This batch of springs is tempered only 8 hours after quenching. There is a large change in the waiting time for heat treatment. Tempering heat treatment and waiting for aging generally affect material properties through microstructure, hardness, residual stress, etc.
5.1.1 Metallographic structure analysis
In order to further study the effect of tempering aging on the spring structure after quenching, the metallographic structure analysis of the fractured and qualified parts was carried out, as shown in Figure 4. Both the matrix structure of the fractured piece and the qualified piece showed needle-shaped tempered troostite structure, and there was no obvious difference between the surface and the core, and there was no slag inclusion, grain coarsening, and a large amount of retained austenite, etc., based on which it was judged that Large changes in tempering waiting time did not have a significant impact on the metallographic structure of the spring itself.
Fracture Failure Analysis and Improvement of Olive Spring Used in Locomotive Braking Device
In addition, on the surface of the spring at the crack source, continuous strips of secondary quenched martensite at a depth of about 8.0 m were found, as shown in the white layer in Figure 5. Intermittent strips at a depth of about 3.0 m can also be observed on the surface of the spring near the crack source. secondary quenched martensite. The sprayed paint layer at the source of the crack did not fall off, indicating that the formation of the burnt tissue was after the heat treatment and before the painting process, that is, the shot peening process, and it is speculated that there is an abnormality in the shot peening process. Secondary quenching martensite is a typical wear and burn structure, which is hard and brittle, which greatly increases the hydrogen embrittlement susceptibility of the structure, and is prone to hydrogen embrittlement delayed fracture under continuous tensile stress loading. And because of the difference in hardness, it is easy to form stress concentration points at the interface between the white bright layer and the substrate during use, shortening the fatigue life.
Fig.5 Secondary quenching martensite structure (white strip) at the crack source of olive spring
5.2.3 Hardness analysis
Sampling and testing the hardness of olive spring fractured parts and qualified parts. The results are shown in Table 2. It can be seen that the hardness of the broken parts is generally higher than that of the unbroken parts. Based on this, it can be judged that the spring has not been tempered in time after quenching. In the state, the hardness is high and uneven. The higher the strength of the material, the greater the sensitivity to hydrogen, and the risk of hydrogen embrittlement fracture will increase significantly.
5.2.4 Stress analysis
After the olive spring is wound, there is residual stress in itself, the inner side of the traveler bears the tensile stress, and the outer side bears the compressive stress. The subsequent heat treatment process needs to achieve effective release and uniform distribution of residual stress. The residual stress test is carried out on the upper and lower positions of the same batch of olive spring fractured products (1# pieces) and other batches of qualified products (2# pieces). The results are shown in Table 3.
It can be seen that the residual stress value of the test point on the same batch of fractured products is significantly higher than that of other batches of qualified products. Based on this, it is judged that the olive spring has not been tempered in time after quenching, resulting in the inability to effectively release the deformation stress caused by quenching. In the presence of concentrated or large residual stress, the susceptibility to hydrogen embrittlement fracture is further enhanced.
6 Analysis conclusion
Through the macroscopic and microscopic analysis of the olive spring fracture, it has obvious hydrogen embrittlement fracture characteristics, the fracture is flat, radial ridge, intergranular brittle fracture, chicken claw pattern can be seen, and the expansion zone is a mixture of toughness and dimples. The nature of the olive spring fracture is hydrogen embrittlement.
At present, there are various hypotheses and theories about the mechanism of crack growth under hydrogen embrittlement, but the sensitivity of hydrogen embrittlement to material properties and heat treatment process is a basic consensus, and it is also the main factor affecting hydrogen embrittlement fracture of steel. Based on the above analysis, the following conclusions can be drawn:
(1) High-strength springs are highly sensitive to hydrogen embrittlement. Even if the absolute hydrogen content of the material is not high, hydrogen embrittlement fracture may still occur due to improper heat treatment.
(2) The olive spring was not tempered in time after quenching. Although it did not cause significant changes in the metallographic structure, it resulted in high hardness of the material, large residual stress, and a significant increase in sensitivity to hydrogen embrittlement.
(3) The secondary quenched martensite produced abnormally in the shot peening process is hard and brittle, which greatly increases the susceptibility to hydrogen embrittlement of the structure, and easily forms stress concentration points at the interface with the matrix, shortening the fatigue life. Further investigation confirmed that the shot peening process was abnormal, there were holes in the filter screen, and metal debris from the broken blades of the cooling fan were mixed in the sandblasting, resulting in the appearance of a white layer of secondary quenched martensite.
(4) The brittle fracture of the olive spring mainly occurs during the assembly and test process before the formal use of the product, and it bears continuous load maintenance and certain impact, and has delay.
7 suggestions for improvement
Through the above analysis, the basic mechanism and cause of olive spring fracture failure can be determined. In order to effectively improve product quality and reduce the risk of hydrogen embrittlement, the following improvement measures can be taken:
(1) Reasonably control the tempering aging of the olive reed after quenching to avoid abnormally high hardness and residual stress of the product and brittle performance.
(2) Strengthen the control and maintenance of key equipment status in the olive spring manufacturing process to avoid performance defects caused by abnormal processes.
(3) Before the olive spring leaves the factory, the load-holding time and load impact are added on the basis of the original load-holding test to help screen the early failure of the olive spring.