The cylinder of a hydraulic support column cracked after only 7 months of use. The reasons for the cracking of the cylinder were analyzed by chemical composition analysis, optical metallographic structure observation, tensile test, impact test, SEM fracture observation, etc. The results show that the strength of the material does not meet the national standard, which will have an adverse effect on the cracking of the cylinder. The presence of microcracks in the inner hole cladding layer and its welding heat affected zone, as well as the penetration of welding metal, are the main reasons for the cracking of the cylinder during use.

As a country rich in coal and less in oil, my country’s oil and natural gas are heavily dependent on imports, and coal will continue to be the main energy strategy for a long time. More than 90% of my country’s coal mining work needs to be completed underground, and the mining process is inseparable from the normal operation of various mining equipment. Among them, hydraulic supports, as an important component of the coal mining equipment system, not only ensure the safety of underground personnel and equipment, but also improve production efficiency and expand the working space. As the core component of hydraulic support, hydraulic cylinder will rust, rust pits and even crack during use due to its complex working conditions and huge loads all year round. Therefore, in order to gain a foothold in the coal market, it must rely on excellent quality, which has become the goal pursued by all enterprises.

Figure 1 shows a hydraulic cylinder of a hydraulic support column produced by a certain enterprise, which cracked during use. The product has been used for only 7 months since it was shipped. The cylinder of this product is made of 30CrMnSi, and the raw material is hot-rolled. After quenching and tempering, the outer circle is laser clad and the inner hole is MIG welded bronze for surface treatment. This paper analyzes and explores the reasons for the cracking of the cylinder body of the hydraulic cylinder by testing the raw materials of the cracked samples, observing the optical metallographic structure, tensile testing, impact testing, SEM fracture observation, etc.

1 Crack macromorphology

The macromorphology of the crack is shown in Figure 2. The main crack extends along the axial direction, and bifurcates when approaching the two ends, but does not penetrate the entire axial direction. At the main crack, it can be found that the crack has penetrated along the wall thickness direction. Observation and cylinder size measurement show that the outer diameter of the cracked part of the cylinder increases, indicating that the cylinder deformed before cracking, that is, there is a cylinder expansion phenomenon. In Figure 2 (a), the outer surface of the cylinder body is bright, which is the stainless steel layer clad by laser, and no peeling is found. The inner surface is yellow, which is the copper plating layer of MIG welding, as shown in Figure 2 (b).

2 Experimental analysis

The sample was extracted from the cracked cylinder body, and the chemical composition test, optical metallographic structure observation, tensile test, impact test, SEM fracture observation were performed on it, and the possible causes of its cracking were analyzed.

2.1 Chemical composition test

The raw material of the cylinder body is 30CrMnSi. The sample was cut near the fracture and its chemical composition was tested. Table 1 shows the chemical composition test results of the sample taken from the raw material.

By comparing with the composition specified in GB/T 3077-2015 “Alloy Structural Steel”, the analysis shows that the raw material composition meets the national standard.

2.2 Optical metallographic structure observation

The samples were taken from the uncracked cylinder barrel. After grinding, polishing and corrosion treatment, the inner wall of the cylinder barrel and the laser cladding side of the outer wall were observed using an optical microscope, as shown in Figure 3. Figure 3 (a) shows the morphology of the joint of the inner wall of the cylinder barrel, with the substrate on the left and the copper plating on the right. It was found that the copper plating had an obviously uneven thickness, with a thickness of 0.5 to 1 mm. From Figure 3 (b), it can be clearly seen that there are cracks on the copper plating on the cladding side, and copper can be seen penetrating along the substrate grain boundary (or crack) on the substrate side, which may be the main cause of the cracking. Figure 3 (c) and (d) are the local magnified morphology of the substrate on the cladding side. The surface structure of the substrate is troostite with martensite orientation. As the distance from the surface increases, ferrite appears in the structure (white structure in the figure) and the amount of ferrite increases with the distance, as shown in Figure 3 (e). Figure 3 (f) is the structure morphology of the laser cladding layer and the substrate. The lower white bright layer in Figure 3 (f) is the laser cladding layer, and the upper part is the substrate. The thickness of the laser cladding layer is uniform, and its structure is a cast dendritic structure. No abnormality is found in the laser cladding layer and its bonding interface.

2.3 Tensile test analysis

Tensile test is a basic way to measure the mechanical properties of materials. The tensile strength and yield strength of materials can truly reflect the performance of materials. The sample was extracted from the center of the cylinder, with a diameter of 10 mm and a gauge length of 50 mm. The tensile test was carried out in accordance with GB/T 228.1-2021 “Tensile Test of Metallic Materials Part 1: Room Temperature Test Method”, and the results are shown in Table 2.

As can be seen from the table, the elongation of the sample meets the national standard, but the tensile strength and yield strength do not meet the national standard requirements. The sample was sampled from the middle part (core) of the cylinder along the wall thickness. Due to the presence of ferrite structure in the core, the strength index of the material is low. However, the heat treatment of the product only clearly requires quenching and tempering, but there is no clear requirement for the structure and performance of the core after quenching and tempering, so it is difficult to judge whether its tensile performance meets the product performance requirements. The tensile fracture morphology is shown in Figure 4. It can be seen that the tensile test bars all show obvious necking, indicating that the material has good plasticity. No obvious metallurgical defects were found at the fracture.

2.4 Impact test analysis

In accordance with GB/T229-2020 “Metallic Materials Charpy Pendulum Impact Test Method”, V-notch standard impact specimens were prepared. To compare the impact toughness at different positions, three specimens were taken for testing. Among them, specimen 1 was taken from the copper-plated side close to the inner wall of the cylinder, specimen 2 was taken from the center of the cylinder, and specimen 3 was taken from the laser cladding side close to the outer wall of the cylinder. The impact test results of the specimens are shown in Table 3.

According to the test results, the impact toughness values ​​of the three specimens are quite different. This is directly related to the sampling position. Since specimen 1 is close to the copper-plated side of the inner wall of the cylinder, the inner wall of the cylinder is severely affected by heat during cladding, and there are defects such as cracks, so its impact toughness is poor, which is much less than the standard value of 49J/cm2. Specimen 2 is located at the center of the cylinder, and its impact toughness is slightly lower than the standard value. The impact toughness of sample 3 is much higher than the standard value. Due to the lack of impact toughness of samples without cladding and laser cladding after quenching and tempering, it is impossible to determine the effect of laser cladding on impact toughness, but it can be determined that laser cladding does not have a significant negative impact on the impact toughness of the material. The above analysis shows that the impact toughness of the material after laser cladding can meet the standard requirements, but the cladding heat affected zone of the inner wall will have a significant negative impact on the impact toughness of the material. Therefore, it can be basically concluded that the cracking of the cylinder body is caused by the cladding of the inner wall and its heat affected zone.

2.5 SEM fracture observation

According to the test results of Sections 2.2 and 2.4, the metallographic observation and impact test of the laser cladding side showed good performance without any abnormality. Based on this, the fracture of the cladding copper coating-substrate side was observed using a scanning electron microscope in order to find the real cause of the cracking of the column cylinder. Take the fracture sample of the inner wall surfacing side, place it in an ultrasonic cleaning machine to clean it, and then observe it using a scanning electron microscope. The observation results are shown in Figure 5. Figure 5 (a) shows the fracture morphology of the copper-plated layer side joint. It can be seen that there are two morphologies of rough and uneven fracture and smooth fracture on the copper-plated layer side. The upper rough and uneven fracture is the matrix, which is a ductile fracture; the lower smooth fracture is the copper plating layer, which is a brittle fracture. This also reflects that the copper plating layer is not tough enough during the fracture process. Figure 5 (b) is a partial enlarged view of the matrix side fracture. There are obvious dimples and secondary cracks on the matrix side fracture. The secondary cracks are generally generated before the component breaks. Since the copper plating layer only accounts for a small part compared to the matrix, the fracture is mainly a ductile fracture.

3 Analysis and discussion

The main crack of the cylinder is parallel to the axial direction, and the cracking of the surface cylinder is mainly caused by the tensile stress in the circumferential direction. Further analysis of the reasons for the cracking mainly includes the excessive pressure in the inner cavity during the use of the cylinder, which eventually causes the cylinder to crack and the insufficient strength of the product’s own material. The cylinder has an increase in the outer diameter, there is a bifurcation at the end of the crack, and the fracture is a ductile fracture, indicating that the cracking of the cylinder is a ductile fracture. Ductile fracture is generally caused by overload, so it is not ruled out that the cracking is caused by excessive pressure inside the cylinder. Due to the lack of data on the pressure of the cylinder during use, the following analysis is only based on the product’s own problems.

Through chemical composition testing, optical metallographic structure observation, tensile test, impact test, SEM fracture observation and other analyses of the cracked middle cylinder, it was found that the composition of the material meets the requirements. Inner wall surfacing will have a greater impact on the impact toughness of the material. The thickness of the laser cladding layer is uniform, and no abnormal defects are found. The thickness of the inner hole copper plating layer is uneven, and there are cracks in the cladding layer and the welding heat affected zone. The surface structure of the substrate is tempered bainite. As the distance from the cladding layer/substrate interface increases, the ferrite content in the substrate structure increases. At the same time, there is also a phenomenon of cladding melt infiltration into the substrate. By observing the fracture of the cladding side through a scanning electron microscope, it was found that there were secondary cracks on the substrate surface. The microcracks in the inner hole cladding layer and its welding heat affected zone and the infiltration of welding metal are the source of the cylinder cracks and the main cause of cracking. In addition, the mechanical properties of the substrate of the sample did not meet the national standard requirements, and the strength index was low, which would affect the safe use margin of the cylinder. However, the overall size of the product, especially the thickness, is large, and the mechanical properties of itself are uneven, and the manufacturer currently has no clear index requirements for this.

4 Conclusion

(1) The microcracks in the inner hole cladding layer and its welding heat affected zone, as well as the infiltration of welding metal, are the main causes of cracking. The surfacing process should be improved to reduce its thermal impact on the cylinder body.

(2) The material has good plasticity, but the strength does not meet the national standard, which will have an adverse effect on the cracking of the cylinder body. The mechanical performance standards after quenching and tempering should be clarified to ensure that the strength of the product after quenching and tempering meets the national standard.