A systematic analysis was conducted on the failure of the three-eccentric butterfly valve stem of the gas extraction system and the gate valve stem of the drainage system in the Sangshuping No. 2 Mine, and it was determined that the main cause of failure was the uniform corrosion of media such as H2S, CO2, and SO2. In order to improve the corrosion resistance of the valve stem, a repair plan of laser cladding Inconel 625 alloy (powder and wire) on its surface was proposed. Systematic microstructural observation, chemical composition test, microhardness test and electrochemical test were carried out on the laser cladding layer to evaluate its reliability. The results show that the microstructure distribution of Inconel 625 wire and powder cladding layers is similar. The microstructure of the cladding layers is dendrite, which is well bonded to the substrate without cracks and pores. The Cr content in the interdendritic region of the cladding layer is higher than that in the intradendritic region, and there are Nb-rich precipitates between the dendrites. The overall element distribution of the cladding layer is relatively uniform, and the main elements such as Ni, Cr, Fe, Mo, and Nb show a transitional distribution between the parent material at the interface. The hardness of the Inconel 625 cladding layer is slightly higher than that of the parent material, and the self-corrosion potential is higher than that of the parent material. The self-corrosion current density Icorr (8.62×10′-7A/cm2) of the cladding layer is lower than that of the parent material (8.23×10′-6A/cm2). After equivalent circuit fitting, the charge transfer resistance of the Inconel 625 cladding layer is 9.9×104 Ω·cm2, which is much higher than 2.6×103 Ω·cm2 of the parent material, indicating that the corrosion resistance of the cladding layer is better than that of the substrate. The repaired valve stem structure has been in service in the coal mine for 6 months without corrosion failure and is still operating normally, while the untreated valve stem structure has corrosion failure after 1 month of service, indicating that the Inconel 625 cladding layer has a good protective effect on the valve stem material.

A butterfly valve is a valve that uses a circular butterfly plate as an opening and closing member and rotates with the valve stem to open, close and adjust the fluid channel [1]. The triple eccentric butterfly valve is based on the double eccentric butterfly valve, with the valve seat sealing surface offset at a certain angle. Its valve seat sealing surface is a conical surface. The valve plate seal will separate from the valve seat sealing surface when it is opened, and there is almost no friction between the two. Compared with the double eccentric butterfly valve, the sealing performance and service life of the butterfly valve are greatly improved [2]. The structure of the triple eccentric butterfly valve is shown in Figure 1a. The wedge gate valve uses the gate as the opening and closing member. When working, the gate movement direction is perpendicular to the fluid direction. The wedge gate valve generally completes the opening and closing of the valve by moving the valve stem up and down [3-5]. The wedge gate valve has a good seal because its valve sealing surface is at a certain angle to the vertical center line, so it can withstand greater pressure. The structure of the wedge gate valve is shown in Figure 1b. In general, the valve stems of the three-eccentric butterfly valve and the wedge gate valve (as shown by the arrow in Figure 1) have good corrosion resistance and abrasion resistance after tempering and surface nitriding.

However, due to the deteriorating service environment underground, the above structures have suffered serious corrosion failure during their service life [6]. The gas extraction butterfly valve is the main equipment for controlling and throttling the gas extraction system underground in coal mines, and the water supply and drainage wedge gate valve is the main equipment for the water supply and drainage system underground in coal mines. The valve stem is the rotating core of the above equipment, and its reliability and stability are crucial to production safety.

This paper systematically analyzes the failure of the butterfly valve stem and gate valve stem in service at Sangshuping No. 2 Mine, and proposes a solution to improve the corrosion resistance of the valve stem by laser cladding Inconel 625 alloy based on the analysis results. Inconel 625 alloy belongs to the Ni-Cr-Mo alloy system and has excellent resistance to oxidation corrosion [7-10]. Laser cladding has the characteristics of small heat-affected zone and excellent mechanical properties of formed parts. It is the preferred method for surface repair of failed structural parts [11-14]. The reliability of the cladding layer was comprehensively evaluated by systematic microstructural observation, chemical composition, microhardness test and electrochemical test of the laser cladding layer.

1 Failure analysis

From the appearance of the disassembled butterfly valve stem and gate valve stem (Figure 2), it can be seen that the corrosion on the surface of the two valve stems is relatively serious, mainly pitting corrosion. The valve shaft stem was tested for chemical composition and confirmed that its material is 2Cr13 stainless steel. The main cause of failure is: the valve stem is in direct contact with the shaft hole without isolation, and the high-temperature medium (H2S, CO2, SO2, etc.) in the service environment penetrates into the gap between the valve stem and the shaft hole, forming gap corrosion, and the valve stem material has a weak ability to resist the above-mentioned corrosive medium.

2 Experimental materials and methods

2.1 Laser cladding repair

The valve stem substrate of the old butterfly valve is 2Cr13, and the laser cladding uses Inconel 625 wire with a wire specification of 1.2 mm. The chemical composition of the material is shown in Table 1. Each valve stem of the old butterfly valve has two sections, with specifications of 55 mm×200 mm and 55 mm×468 mm respectively. Before laser cladding, the surface of the old valve stem is turned to remove 0.5 mm thickness, and then the Inconel 625 wire is laser clad on the turned part, with a layer of cladding and a thickness of 1.2 mm. Finally, the valve stem is machined to restore to its original size. Wire laser cladding
The power used is 2500 W, the cladding speed is 14 mm/s, and the spot diameter is 4.5 mm. The morphology of the butterfly valve stem wire before and after laser cladding is shown in Figure 3.

The old gate valve was disassembled and its stem material was 17-4PH. Inconel 625 powder was used for laser cladding. The powder morphology is shown in Figure 4. The powder sphericity is good and the particle size is 150~300 mesh.

The specifications of the disassembled old valve stem are 38×360 mm. Before laser cladding, the part to be clad was turned off by 0.5 mm thickness, and then the turned part was laser clad with Inconel 625 powder. The thickness of the cladding layer was 1.2 mm. The power of powder laser cladding was 2500 W, the cladding speed was 40 mm/s, the powder feeding speed was 1.2 r/min, the feed distance was 1.6 mm, and the spot size was 4 mm×4 mm. The morphology of the gate valve stem powder before and after laser cladding is shown in Figure 5.

2.2 Experimental method

The microstructure of the cross section of the laser wire and the laser powder cladding layer was analyzed using an Olympus optical microscope. The Zeiss field emission scanning electron microscope was used to observe the microstructure of the cladding layer, and the EDS probe was used to determine its micro-region composition. The microhardness of the cross-section of the cladding layer was tested by a micro-Vickers hardness tester. The test load was 300 g, the holding time was 15 s, the measuring point spacing of the cladding layer was 100 μm, and the measuring point spacing of the parent material was 200 μm. The electrochemical test of the cladding layer was carried out on the Shanghai Chenhua CHI600E series electrochemical test workstation. The three-electrode system was selected to evaluate the electrochemical behavior of each cladding layer. The platinum mesh and Ag/AgCl were used as the auxiliary and reference electrodes in this test, respectively, and 3.5% NaCl was selected as the electrolyte used in the test.

3 Results and discussion

3.1 Organization analysis

3.1.1 Metallographic organization analysis

Figures 6 and 7 are the metallographic structures of Inconel 625 wire and powder laser cladding layers, respectively. The cladding layers are all dendrite structures, which are well bonded to the parent material, without cracks and pore defects. As can be seen from Figures 6b and 7b, the middle part of the cladding layer presents an equiaxed crystal morphology. This is because the feed distance is less than the spot diameter during the wire and powder laser cladding process, and the latter cladding layer fully remelts the previous cladding layer. There is no obvious difference in the metallographic structure between the Inconel 625 wire and powder laser cladding layers.

3.1.2 Scanning electron microscopy and EDS analysis

Field emission scanning electron microscopy was used to observe the Inconel 625 wire cladding layer/substrate bonding interface and the middle area of ​​the cladding layer, as shown in Figure 8.

The EDS spectrum test results of the typical area in Figure 8 are shown in Table 2. Combined with the spectrum test results, the structure of the wire cladding layer near the interface is mainly fine dendrite morphology, among which the Fe content of the area close to the parent material interface (spectrum 1) is relatively high (15.37 wt.%); the protruding interdendritic area (spectrum 2) is compared with the dendrite area (spectrum 3), and there is enrichment of Mo and Nb elements. In the middle area of ​​the cladding layer, the Cr content of the interdendritic area (spectrum 4) is slightly higher than that of the dendrite area (spectrum 5); there is a bright, granular precipitate phase in the interdendritic area, and the EDS spectrum test results show that it is a Nb-rich phase (spectrum 6). With the help of the above EDS data, the dilution rate of the wire laser cladding layer is calculated to be about 4%~5%.

Figure 9 shows the field emission scanning electron microscope observation results of the powder laser cladding layer, and Table 3 shows the EDS spectrum detection results of the typical area. The organization and element distribution of the powder laser cladding layer are similar to those of the wire laser cladding layer. Near the interface of the parent material, the Cr content in the interdendritic region (spectrum 1) is slightly higher than that in the dendrite region (spectrum 2), and there is a Nb-rich precipitate phase in the interdendritic region (spectrum 3). In the middle area of ​​the cladding layer, the bright Nb-rich precipitate phase (spectrum 6) is distributed in the dendrites in a granular manner; the Cr, Mo, and Nb contents in the intradendritic region (spectrum 5) are slightly lower than those in the interdendritic region (spectrum 4). With the help of the above EDS data, the dilution rate of the powder laser cladding layer is calculated to be about 5%~6%, which is close to the dilution rate of the wire cladding layer. Due to the concentrated energy density of the laser heat source, the dendrite structure of the above two cladding layers is relatively fine.

Figure 10 shows the EDS line scan results of the laser cladding layer. Since the microstructure distribution of the wire cladding layer and the powder cladding layer is similar, only the element distribution of the powder cladding layer is presented. It can be seen that the overall composition distribution of the cladding layer is uniform, with local fluctuations (Nb-rich precipitation phase between dendrites). At the interface, there is a transition distribution characteristic between the main elements Ni, Cr, Fe, Mo, and Nb of the cladding layer and the parent material.

3.2 Microhardness test

In order to determine the performance changes of Inconel 625 wire and powder laser cladding layers along the thickness direction, micro-Vickers hardness tests were performed on them, and the test results are shown in Figure 11. The microhardness test starts from the substrate, perpendicular to the thickness direction, and ends at the outer surface of the cladding layer. It can be seen that the hardness value at the substrate is low, ranging from 140 to 180 HV0.3. The hardness of the laser cladding layer is higher than that of the substrate, among which the hardness of the powder laser cladding layer (average value of 254 HV0.3) is slightly higher than that of the wire laser cladding layer (average value of 248 HV0.3). This is mainly because the molten pool is small during the powder laser cladding process, and the temperature gradient is relatively large when the liquid molten pool solidifies, resulting in a finer cladding layer structure. In addition, the thermal effect of laser cladding (whether powder or wire) on the substrate is small, and the hardness change of the substrate heat affected zone is not obvious.

3.3 Electrochemical performance test

The electrochemical test results of Inconel 625 powder laser cladding layer and substrate material are shown in Figure 12. From the polarization curve in Figure 12a, it can be seen that the self-corrosion potential of the cladding layer is Ecorr=-0.399 V, which is higher than the self-corrosion potential of the parent material Ecorr=-0.872 V; at the same time, the self-corrosion current density of the Inconel 625 cladding layer Icorr=8.62×10′-7A·cm’-2 is lower than that of the parent material Icorr=8.23×10′-6A·cm’-2, indicating that the corrosion resistance of the cladding layer is significantly better than that of the parent material. From the impedance curve in Figure 12b, it can be seen that the AC impedance spectra of the parent material and the Inconel 625 cladding layer both show a single capacitive arc, indicating that the electrode reaction process is controlled by charge transfer. The larger the radius of the capacitive arc, the better the corrosion resistance. After equivalent circuit fitting, the charge transfer resistance of the Inconel 625 cladding layer is Rct=9.9×104 Ω·cm2, which is much higher than the Rct=2.6×103 Ω·cm2 of the parent material. The above test results show that cladding Inconel 625 alloy on the surface of the parent material can greatly improve the corrosion resistance of the parts, thereby increasing their service life.

The butterfly valve stem and gate valve stem repaired by the above two laser cladding processes have been in service underground for 6 months without corrosion failure, while the untreated valve stem has corrosion failure after 1 month of service, indicating that the Inconel 625 laser cladding layer has a good protective effect on the valve stem material.

4 Conclusions and Suggestions

This paper systematically analyzes the failure of the valve stem structure of the key rotating parts of butterfly valves and gate valves used in a mine. The main cause of the failure is uniform corrosion under corrosive media such as H2S, CO2, and SO2. In view of this, a repair scheme of laser cladding Inconel 625 alloy on the surface of the above-mentioned failed valve stem is proposed. Inconel 625 wire cladding and powder cladding were carried out respectively, and the reliability of the prepared cladding layer was evaluated by comprehensively analyzing the microstructure, chemical composition, microhardness and electrochemical test results. The following main conclusions were obtained:

(1) The metallographic structure observation shows that the structure of the wire laser cladding layer and the powder laser cladding layer are both dendrite structures, and the cladding layer is well bonded to the parent material matrix, and no cracks and pores are observed. The feed distance used in the laser cladding process is smaller than the spot size, and the obtained cladding layers (wire and powder) show equiaxed crystal structure.

(2) Field emission scanning electron microscopy and EDS analysis show that the segregation behavior of elements in the wire laser cladding layer and the powder laser cladding layer is similar. The Cr content in the interdendritic region is higher than that in the intradendritic region, and bright Nb-rich precipitates exist in the interdendritic region. The overall chemical composition of the cladding layer is evenly distributed, and there is a transition distribution between the main elements Ni, Cr, Fe, Mo, Nb and the parent material at the interface.

(3) The microhardness test results show that the hardness of the powder laser cladding layer (average value of 254 HV0.3) is slightly higher than that of the wire laser cladding layer (average value of 248 HV0.3), and the hardness value at the substrate is the lowest, ranging from 140 to 180 HV0.3.

(4) The electrochemical test results of the powder laser cladding layer show that the self-corrosion potential of the Inconel 625 cladding layer is higher than that of the parent material, and the self-corrosion current density of the cladding layer Icorr (8.62×10′-7A·cm’-2) is lower than that of the parent material Icorr (8.23×10′-6A·cm’-2). The charge transfer resistance Rct of the Inconel 625 cladding layer obtained after equivalent circuit fitting is 9.9×10’4Ω·cm’2, which is much higher than the parent material Rct of 2.6×10’3Ω·cm2, indicating that the corrosion resistance of the cladding layer is better than that of the substrate.

The repaired butterfly valve stem and gate valve stem served in the mine for 6 months without failure, while the untreated valve stem failed after 1 month of service, indicating that the Inconel 625 laser cladding layer has a good protective effect on the valve stem material.