Automatic welding process for nickel material surfacing on sealing surface of gray cast iron valve body

Abstract

The present application relates to the field of welding, in particular to an automatic welding process for welding nickel material on the sealing surface of a gray cast iron valve body, which is achieved by preheating the valve body before welding, selecting reasonable welding parameters, and slowly cooling the valve body after welding to reduce the hardened structure at the welding site, thereby improving the sealing effect of the valve body welding. The present application standardizes each step of the welding process, and has a strict and reasonable control range for the process parameters at each stage, i.e. using correct and effective preheating before welding, selecting suitable welding material, protective gas, reasonable welding parameters, robot welding trajectory, coordination between the welding robot and the positioner, and slow cooling after welding, to ensure the feasibility and sealing detection of the valve body welding.

Description

Technical Field

[0001] This invention relates to the field of welding, specifically to an automatic welding process for overlaying nickel material onto the sealing surface of a gray cast iron valve body. Background Technology

[0002] Welding high-quality nickel (greater than 95%) welding materials onto gray cast iron valve bodies and seats as valve sealing surfaces can significantly improve valve lifespan, corrosion resistance, and wear resistance, while saving a significant amount of precious metals. However, gray cast iron has high levels of impurities such as sulfur and phosphorus, and a high carbon content, with carbon distributed in the form of flake graphite. This results in low tensile strength, high brittleness, and almost no plastic deformation capacity, leading to extremely poor weldability. This manifests as follows: after welding, even after observing the weld surface using dye penetrant testing, ultrasonic testing, and X-ray testing, no defects are visible, yet the sealing test fails, failing to meet the zero-leakage requirement. The reason for this is that extremely small, undetectable microcracks are generated during the welding process. Summary of the Invention

[0003] To ensure the feasibility of valve body welding and the pass rate of sealing tests, this application provides an automatic welding process for overlaying nickel material onto the sealing surface of a gray cast iron valve body.

[0004] The following technical solution is adopted:

[0005] An automated welding process for overlaying nickel material onto the sealing surface of a gray cast iron valve body reduces the hardened structure at the weld by preheating the valve body before welding and slowly cooling it after welding, thereby improving the welding sealing effect of the valve body.

[0006] The process is as follows:

[0007] The first step is to select welding materials;

[0008] The second step is pre-welding cleaning;

[0009] The third step is to verify the welding trajectory.

[0010] Step 4: Preheating before welding;

[0011] Step 5: Welding;

[0012] Step 6: Post-weld treatment;

[0013] Step 7: Post-weld inspection.

[0014] Furthermore, the welding material used is ERNi-CI welding wire conforming to AWS A5.15 standard, with a diameter of 1.2 mm and a nickel content greater than 95%.

[0015] The higher the nickel purity of the welding wire, the better its ductility, the less constraint it exerts on the gray cast iron substrate after welding, the less stress it generates, and the less likely it is to crack. In addition, nickel promotes the graphitization of carbon elements in gray cast iron, which has a positive effect on reducing the white cast iron structure in the fusion zone during welding.

[0016] Furthermore, both the preheating before welding and the heating after welding are achieved using a welding torch.

[0017] Preheating before welding reduces the cooling rate of the weld and heat-affected zone, decreases the tendency to harden, and prevents cold cracking. After welding, heating with a welding torch further heats the weld and the area around the valve seat, homogenizing the heating and reducing stress generation.

[0018] The reason for not using a heating furnace for preheating is for assembly line welding operations. If the temperature is too high after preheating, it will be difficult for personnel to position the valve body at the welding station. By the time the positioning is completed, the preheating temperature has already dropped significantly.

[0019] Furthermore, the preheating before welding includes the following steps:

[0020] 1) Preheat the area around the valve body, especially the outer ring of the valve body corresponding to the valve seat welding surface, for 2-4 minutes;

[0021] 2) After preheating the base material around the valve seat welding surface for 2-3 turns, start preheating the valve seat welding surface;

[0022] 3) Alternately preheat the valve seat welding surface and its surroundings. Initially, preheat the area around the sealing surface more, and later preheat the valve seat welding surface more. Stop preheating when the preheating temperature of the straight section of the valve seat welding surface reaches 220-250℃, the preheating temperature of the inner bend section reaches 240-300℃, and the preheating temperature of the outer bend section reaches 300-350℃.

[0023] Furthermore, after preheating before welding, ensure that the real-time temperature of the valve body and valve seat welding surfaces is greater than 175℃ during welding.

[0024] Furthermore, the valve body is welded using welding wire with a nickel content greater than 95%, and is protected by a mixture of argon (Ar) and carbon dioxide (CO2) gas. The welding current is 90-150A, the arc voltage is 16-24V, the gas flow rate is 10-15L / min, and the welding speed is 10-15cm / min.

[0025] Furthermore, the post-weld heating and slow cooling process includes:

[0026] 1) Heating: After welding is completed, continue to heat the weld to 300±10℃, then stop heating;

[0027] 2) Cooling: Perform stress-relieving annealing on the valve body that has not cooled down, and slowly cool it to room temperature.

[0028] Furthermore, the valve body is slowly cooled by placing it in a 250°C heating furnace for 30 minutes of stress-relief annealing, followed by air cooling; or by placing the valve body in an asbestos-insulated box until it cools to room temperature.

[0029] Furthermore, the weld height of the valve body after welding is not less than 5mm, and the weld width is 14-16mm.

[0030] The above technical solution can guarantee the subsequent machining allowance of the blank.

[0031] Furthermore, after the valve body is welded and precision machined, the machined surface has no pores larger than 0.4mm and no cracks visible to the naked eye; and the sealing performance is tested by water pressure. Under the pressure of the valve body in the specified place of use, and after a specified time, if no air bubbles overflow from the sealing surface, the test is qualified.

[0032] By assembling and pressurizing the water, all welded valve bodies are subjected to a sealing test to ensure that the finished products meet the sealing requirements.

[0033] Furthermore, the preheating, welding, and postheating processes of the valve body are all performed on the positioner, and three pads are evenly distributed in a ring around the center of the positioner platform between the valve body and the positioner platform surface.

[0034] The above technical solution can minimize the contact area between the flange surface of the valve body and the platform surface of the positioner, reduce heat conduction, reduce the rate at which heat is lost to the platform surface of the positioner, effectively ensure the initial temperature of the pre-welded surface, and slow down the cooling rate of the welded surface during welding.

[0035] Furthermore, the valve body is pre-cleaned before welding preheating to ensure that the valve seat welding surface is free of rust, moisture, oil and other impurities.

[0036] By adopting the above technical solutions, the adverse effects of rust, moisture, oil, and other impurities on the fusion of the weld can be avoided, thus ensuring welding quality.

[0037] This application has the following beneficial effects:

[0038] 1. This application reduces the formation of brittle and hard structures and the occurrence of microcracks by preheating before welding, thereby improving the welding sealing effect of the valve body;

[0039] 2. In this application, the weld and valve seat surface are heated and baked by a welding torch after welding, which further heats the weld and the surrounding area, making the heating more uniform. This can minimize the chance of forming a brittle and hard structure, thereby reducing the generation of hot cracks and reducing structural stress.

[0040] 3. By using slow cooling, the cooling rate after welding is reduced, thus decreasing the probability of white iron structure appearing.

[0041] In summary, the automated welding process for overlaying nickel material onto the sealing surface of gray cast iron valve bodies described in this application features strict and reasonable control of parameters at each stage of the process, and a standardized workflow. The use of correct and effective preheating, selection of suitable welding wire and shielding gas, reasonable welding parameters, robot welding trajectory, coordination between the welding robot and the positioner, and slow post-weld cooling are all key factors ensuring the feasibility of valve body welding and passing the sealing test. This reduces the structural stress in the weld joints and bends, prevents the formation of brittle cementite or martensite structures, reduces the generation of microcracks, and ensures sealing performance. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of the valve body installation and welding.

[0043] Figure 2 for Figure 1 BB section view;

[0044] Figure 3 A schematic diagram of the welding trajectory on the valve seat welding surface;

[0045] Figure 4 Metallographic image of the weld microstructure of a flange that has passed the post-weld sealing inspection;

[0046] Figure 5 Metallographic image of the weld microstructure of a flange that failed the post-weld sealing test (cracks appear in the fusion zone);

[0047] Figure 6 Metallographic image of the weld microstructure of a flange that failed the post-weld sealing test (cracks appeared in the gray cast iron base material);

[0048] Figure 7 Metallographic image of the weld microstructure of a flange that failed the post-weld sealing test (with obvious white cast iron structure);

[0049] In the figure: 1. Valve body; 11. Valve seat welding surface; 2. Positioner platform surface; 3. Pad; 4. Weld; 5. Fusion zone; 6. Gray cast iron substrate; 7. Crack; 8. White cast iron structure. Detailed Implementation

[0050] The specific embodiments of this application will be further described below with reference to the accompanying drawings.

[0051] like Figure 1 The valve body 1 shown, the valve seat welding surface 11 is the welding point, and its welding process is as follows:

[0052] The first step is to select welding materials;

[0053] The second step is pre-welding cleaning;

[0054] The third step is to verify the welding trajectory.

[0055] Step 4: Preheating before welding;

[0056] Step 5: Welding;

[0057] Step 6: Post-weld treatment;

[0058] Step 7: Post-weld inspection.

[0059] Specifically, when the weld matrix is ​​ferrite or pearlite, and the graphitization process is sufficient, the weld is less prone to cracking. This is because graphitization is accompanied by volume expansion, which can relax some of the shrinkage stress. The main cause of cracking at this point is the morphology and distribution of graphite; coarse and long flake graphite is more prone to cracking than fine and short flake graphite. In this case, the higher the nickel purity of the nickel portion of the weld, the stronger its shrinkage strain capacity, and the more it can co-expand and shrink with the gray cast iron base material. Therefore, the welding material used in this application must conform to AWS A5.15 standard, be ERNi-CI welding wire, 1.2 mm in diameter, and have a nickel content greater than 95%. Higher nickel purity in the welding wire results in better ductility, less constraint on the gray cast iron base material after welding, less stress, and a lower chance of cracking. Furthermore, nickel also promotes the graphitization of carbon elements in gray cast iron, which has a positive effect on reducing the precipitation of white cast iron in the fusion zone during welding.

[0060] The second step, pre-welding cleaning, mainly involves cleaning the valve seat surface of rust, moisture, oil, and other impurities to avoid adverse effects on the fusion of the weld.

[0061] The preheating, welding, and post-weld treatment of the valve body are all completed on the positioner using a welding robot. The valve body's installation state on the positioner is as follows: Figure 1 As shown, three small pads 3, each 15mm long, 15mm wide, and 10mm high, are evenly distributed around the center of the positioner platform 11. The size of the flange face varies depending on the valve body specification, and the distance from the center of the pads can be adjusted to ensure that the three pads can evenly support the flange. The pre-welded valve body bottom flange is brought into contact with the three pads, positioned using positioning holes and pins, and then fixed using tooling fixtures. This step uses three pads of equal height to ensure the parallelism between the valve body flange face and the positioner platform, while also separating the contact area between the valve body flange face and the positioner platform surface, reducing heat conduction and the rate of heat loss to the positioner platform surface. This effectively ensures the initial temperature of the pre-welded surface and alleviates the cooling rate of the welded surface during welding.

[0062] Based on the characteristics of the valve body and valve seat welding surfaces, the welding trajectory is divided into... Figure 3The segments shown are: straight segment I (point A to point B), inner curve segment (point B to point F), straight segment II (point F to point G), and outer curve segment (point G to point A). Linking the welding robot's motion trajectory points with the positioner's rotation ensures that the welding surface remains horizontal in real time.

[0063] To ensure welding accuracy, the welding robot's welding trajectory needs to be verified before welding begins; this is the third step. The welding robot is then used to verify that its welding trajectory on the valve seat welding surface is consistent with the specified path. Figure 3 The set trajectory shown should be consistent; if it is inconsistent, correction is required. If it is a batch of blanks and rough-machined parts, which are repeatable, this step can be ignored after positioning.

[0064] Depend on Figure 3 As shown in the schematic diagram of the valve seat welding surface, the direction of stress varies depending on the welding location. The two straight segments are only subjected to contraction stress in the straight direction, making it relatively easy to ensure their sealing performance. However, for the two curved segments, located in three-dimensional space, the microstructure is subjected not only to the contraction force from the cooling of the straight segments but also to the vertical contraction stress and tensile stress from the weld seam within the curved segments. Under these multiple and complex stresses, the probability of cracks appearing in the two curved segments is high. The outer curved segment, in particular, is significantly affected by the external ambient temperature at the valve body opening, resulting in a faster cooling rate. Therefore, different treatment methods are required for different parts of the valve seat welding surface to ensure the final sealing effect of the valve body.

[0065] Therefore, in order to ensure the sealing of the valve body after welding, this application adopts preheating measures before welding to prevent or reduce hardened structures; at the same time, it adopts heating and slow cooling measures after welding to reduce the probability of white cast iron structure.

[0066] The fourth step, the preheating process before welding, is as follows:

[0067] 1) Ignite and control the acetylene gas output to a moderate level, turn on the oxygen, and extinguish the flame;

[0068] 2) Preheat the area around the valve body, especially the outer ring of the valve body corresponding to the valve seat welding surface, for 2-4 minutes;

[0069] 3) After preheating the base material around the valve seat welding surface for 2-3 turns, start preheating the valve seat welding surface;

[0070] 4) Alternately preheat the valve seat welding surface and its surroundings. Initially, preheat the area around the sealing surface more, and later preheat the valve seat welding surface more. Stop preheating when the preheating temperature of the straight section of the valve seat welding surface reaches 220-250℃, the preheating temperature of the inner bend section reaches 240-300℃, and the preheating temperature of the outer bend section reaches 300-350℃.

[0071] To verify the effect of preheating before welding on the sealing performance of the valve body, this application implemented the following 5 embodiments and 5 comparative test examples. As shown below, Table 1 shows the sealing performance of different preheating temperatures applied to different welding sections:

[0072] Table 1: Sealing results of different preheating temperatures in various welding sections

[0073] Example 1

[0074] Taking a 6-inch cast iron valve body as an example, the welding process requirements for using the welding process described in this application are as follows:

[0075] The first step is to select welding materials: Welding wire conforming to AWS A5.15 standard, grade ERNi-CI, diameter 1.2mm, nickel content greater than 95%, and actually 98% nickel content welding wire is used.

[0076] The second step is pre-welding cleaning: remove rust, moisture, oil and other impurities from the valve seat surface to ensure that there are no impurities or stains on the valve seat welding surface that may affect the welding operation or welding effect.

[0077] Step 3, Welding trajectory verification:

[0078] 1) such as Figure 1 As shown, three pads 3, each 15mm long, 15mm wide, and 10mm high, are evenly distributed in a ring around the center of the positioner platform. Then, the pre-welded valve body is placed on the three pads, and the bottom flange of the valve body is made to contact and fix with the three pads to ensure that the flange face of the valve body is parallel to the positioner platform.

[0079] 2) According to Figure 3 The preset welding direction shown verifies the welding trajectory of the welding robot. If the actual welding trajectory of the welding robot is different from the preset welding trajectory, the robot is adjusted and corrected in real time until it matches the preset welding trajectory.

[0080] Step 4, Preheating before welding:

[0081] 1) Ignite and control the acetylene gas output to a moderate level, turn on the oxygen, and extinguish the flame;

[0082] 2) Preheat the area around the valve body, especially the outer ring of the valve body corresponding to the valve seat welding surface, for 3.5 minutes;

[0083] 3) After preheating the base material around the valve seat welding surface for 3 turns, start preheating the valve seat welding surface;

[0084] 4) Alternately preheat the valve seat welding surface and its surroundings. Initially, preheat the area around the sealing surface more, and later preheat the valve seat welding surface more.

[0085] 5) Stop preheating when the preheating temperature of the straight section of the valve seat welding surface reaches a minimum of 220℃, the preheating temperature of the inner bend section reaches a minimum of 240℃, and the preheating temperature of the outer bend section reaches a minimum of 300℃.

[0086] Step 5, Welding: Operate the robotic welding torch to the arc starting point ( Figure 3 At point A in the diagram, following the predetermined counterclockwise welding trajectory, a mixed gas of 98% argon (Ar) and 2% carbon dioxide (CO2) is used for protection. The welding current is 110A, the arc voltage is 19V, the gas flow rate is 12L / min, and the welding speed is 11cm / min. Through the collaboration of the welding robot and the positioner, the welding of the entire valve seat surface is completed.

[0087] Step 6, Post-weld treatment:

[0088] 1) Heating: After welding is completed, continue heating the weld (especially the inner and outer bends) to 300℃.

[0089] Stop heating;

[0090] 2) Cooling: The valve body that has not cooled down is then removed and placed in a heating furnace at 250°C for 30 minutes of stress-relieving annealing, followed by air cooling to room temperature;

[0091] 3) Machining: The weld overlay effect obtained at this time is a weld layer height of about 5mm and a weld seam width of 14-16mm. The valve body is turned to the machining center and the weld layer is precision machined to achieve the precision machining dimensions. At this time, the weld overlay layer is 2-3mm thick.

[0092] 4) Grind the burrs around the weld seam to avoid scratching the rubber part of the valve core that seals with the valve body after assembly.

[0093] Step 7, Post-weld inspection:

[0094] 1) After the cylindrical surfaces of the valve body and valve seat are precision machined and polished, they are assembled, pressure tested with water, and their sealing performance is tested. Zero leakage is required, that is, under the specified pressure and for the specified time, no air bubbles are allowed to overflow from the sealing surface. Taking the 6-inch specification as an example, the sealing test pressure is 1.2MPa, the holding time is 45 seconds, and no air bubbles are allowed to overflow from the sealing surface.

[0095] 2) Check that the machined surface of the valve seat has no pores larger than 0.4mm, any cracks visible to the naked eye, etc.

[0096] In this embodiment, 20 valve bodies were tested for welding, and the one-time pass rate for sealing performance was 100%. Furthermore, the valve seat surface showed no cracks or pores.

[0097] Example 2

[0098] The difference between this embodiment and embodiment 1 is that, in the fourth step, step 5) of preheating before welding, preheating is stopped when the preheating temperature of the straight section of the valve seat welding surface reaches 230°C, the preheating temperature of the inner bend section reaches 260°C, and the preheating temperature of the outer bend section reaches 310°C.

[0099] In this embodiment, 20 valve bodies were tested for welding, and the one-time pass rate for sealing performance was 100%. Furthermore, the valve seat surface showed no cracks or pores.

[0100] Example 3

[0101] The difference between this embodiment and embodiment 1 is that, in the fourth step, step 5) of preheating before welding, preheating is stopped when the preheating temperature of the straight section of the valve seat welding surface reaches 240°C, the preheating temperature of the inner bend section reaches 280°C, and the preheating temperature of the outer bend section reaches 320°C.

[0102] In this embodiment, 20 valve bodies were tested for welding, and the one-time pass rate for sealing performance was 100%. Furthermore, the valve seat surface showed no cracks or pores.

[0103] Example 4

[0104] The difference between this embodiment and embodiment 1 is that, in the fourth step, step 5) of preheating before welding, preheating is stopped when the preheating temperature of the straight section of the valve seat welding surface reaches 250°C, the preheating temperature of the inner bend section reaches 290°C, and the preheating temperature of the outer bend section reaches 335°C.

[0105] In this embodiment, 20 valve bodies were tested for welding, and the one-time pass rate for sealing performance was 100%. Furthermore, the valve seat surface showed no cracks or pores.

[0106] Example 5

[0107] The difference between this embodiment and embodiment 1 is that, in the fourth step, step 5) of preheating before welding, preheating is stopped when the preheating temperature of the straight section of the valve seat welding surface reaches 250°C, the preheating temperature of the inner bend section reaches 300°C, and the preheating temperature of the outer bend section reaches 350°C.

[0108] In this embodiment, 20 valve bodies were tested for welding, and the one-time pass rate for sealing performance was 100%. Furthermore, the valve seat surface showed no cracks or pores.

[0109] Comparative Test Example 1

[0110] The difference between this experimental example and Example 1 is that the fourth step of the preheating process before welding is not performed.

[0111] In this test, 10 valve bodies were welded, and the first-time pass rate for sealing was 0%. Leakage was concentrated in the outer and inner bends.

[0112] The metallographic test results of the flanges that failed the sealing test in this test example are as follows: Figure 7 As shown, the microstructure contains obvious white cast iron. Because the shrinkage rate of white cast iron (cementite) is approximately 2.3%, while that of gray cast iron (lamellar graphite + ferrite or pearlite) is approximately 1.26%, the different cooling rates result in different microstructures. Since white cast iron cannot withstand large shrinkage stresses, significant shrinkage stresses will be generated between the microstructures after welding. The more cementite (white cast iron) in the weld, the more prone it is to cracking.

[0113] Comparative Test Example 2

[0114] The difference between this experimental example and Example 1 is that, in the fourth step, step 5) of preheating before welding, the preheating is stopped when the preheating temperature of the straight section of the valve seat welding surface reaches 100°C, the preheating temperature of the inner bend section reaches 100°C, and the preheating temperature of the outer bend section reaches 100°C.

[0115] In this test, 10 valve bodies were welded, and the pass rate for the sealing test on the first attempt was 0%.

[0116] The leakage is concentrated in the outer and inner bends.

[0117] Comparative Test Example 3

[0118] The difference between this experimental example and Example 1 is that in the fourth step, step 5) of preheating before welding, the preheating is stopped when the preheating temperature of the straight section of the valve seat welding surface reaches 200°C, the preheating temperature of the inner bend section reaches 200°C, and the preheating temperature of the outer bend section reaches 200°C.

[0119] In this test, 10 valve bodies were welded, and the first-time pass rate for sealing was 0%. Leakage was concentrated in the outer and inner bends.

[0120] Comparative Test Example 4

[0121] The difference between this experimental example and Example 1 is that in the fourth step, step 5) of preheating before welding, the preheating is stopped when the preheating temperature of the straight section of the valve seat welding surface reaches 280°C, the preheating temperature of the inner bend section reaches 350°C, and the preheating temperature of the outer bend section reaches 400°C.

[0122] In this test, 10 valve bodies were welded, and the first-time pass rate for sealing tests was 30%. Leakage was concentrated in the outer and inner bend sections.

[0123] Comparative Test Example 5

[0124] The difference between this experimental example and Example 1 is that in the fourth step, step 5) of preheating before welding, the preheating is stopped when the preheating temperature of the straight section of the valve seat welding surface reaches 300°C, the preheating temperature of the inner bend section reaches 400°C, and the preheating temperature of the outer bend section reaches 450°C.

[0125] In this test, 10 valve bodies were welded, and the first-time pass rate for sealing tests was 20%. Leakage was concentrated in the outer and inner bend sections.

[0126] As can be seen from the above Examples 1-5 and Comparative Experiments 1-5:

[0127] The two straight sections of the valve body weld surface will pass the sealing test regardless of whether preheating is performed or the preheating temperature. This is because the straight sections are only subjected to contraction in the straight direction, making adjustment easier.

[0128] Because the inner and outer bends are located in three-dimensional space, their microstructure is subjected not only to the contraction force of the cooling straight section but also to the vertical contraction and tensile stresses of the weld within the bend. Under these multiple and complex stresses, the probability of cracks appearing in both bends is high. The outer bend, in particular, is located at the valve body opening and is significantly affected by the external ambient temperature, resulting in a relatively faster cooling rate and making it more prone to microstructural cracking. Therefore, the preheating temperature of the outer bend should be slightly higher than that of other sections, and extra care should be taken during post-weld baking.

[0129] When the preheating temperature of the straight section of the valve seat welding surface reaches 220-250℃, the preheating temperature of the inner bend section reaches 240-300℃, and the preheating temperature of the outer bend section reaches 300-350℃, the temperature measuring gun continuously measures the real-time temperature of the valve seat welding surface during this process. Ensuring that the real-time temperature of the valve seat surface during welding is greater than 175℃ will result in the optimal welding pass rate and cost. The metallographic image of the weld seam of a flange that has passed the post-weld sealing inspection is shown below. Figure 4 As shown.

[0130] Appropriate preheating temperature range has a positive effect on the sealing performance of the weld. However, when the preheating temperature is greater than 400℃, hot cracks are easily generated, which is also detrimental to the sealing performance.

[0131] Furthermore, in order to verify the influence of the welding process on the sealing effect of the valve body, this application implemented the following embodiments and test examples to verify the influence of welding parameters, including current, voltage, shielding gas ratio, welding speed, etc., on the welding results.

[0132] Table 2: Influence of welding parameters, shielding gas ratio, and welding speed on welding results Example 6

[0133] Similarly, taking a 6-inch cast iron valve body as an example, the welding process requirements for using the welding process described in this application are as follows:

[0134] The first step is to select welding materials: Welding wire conforming to AWS A5.15 standard, grade ERNi-CI, diameter 1.2mm, nickel content greater than 95%, and actually 98% nickel content welding wire is used.

[0135] The second step is pre-welding cleaning: remove rust, moisture, oil and other impurities from the valve seat surface to ensure that there are no impurities or stains on the valve seat welding surface that may affect the welding operation or welding effect.

[0136] Step 3, Welding trajectory verification:

[0137] 1) such as Figure 1 As shown, three pads 3, each 15mm long, 15mm wide, and 10mm high, are evenly distributed in a ring around the center of the positioner platform. Then, the pre-welded valve body is placed on the three pads, and the bottom flange of the valve body is made to contact and fix with the three pads to ensure that the flange face of the valve body is parallel to the positioner platform.

[0138] 2) According to Figure 3 The preset welding direction shown verifies the welding trajectory of the welding robot. If the actual welding trajectory of the welding robot is different from the preset welding trajectory, the robot is adjusted and corrected in real time until it matches the preset welding trajectory.

[0139] Step 4, Preheating before welding:

[0140] 1) Ignite and control the acetylene gas output to a moderate level, turn on the oxygen, and extinguish the flame;

[0141] 2) Preheat the area around the valve body, especially the outer ring of the valve body corresponding to the valve seat welding surface, for 3.5 minutes;

[0142] 3) After preheating the base material around the valve seat welding surface for 3 turns, start preheating the valve seat welding surface;

[0143] 4) Alternately preheat the valve seat welding surface and its surroundings. Initially, preheat the area around the sealing surface more, and later preheat the valve seat welding surface more.

[0144] 5) When the preheating temperature of the straight section of the valve seat welding surface reaches 220℃, the preheating temperature of the inner bend section reaches 240℃, and the preheating temperature of the outer bend section reaches 300℃, stop preheating.

[0145] Step 5, Welding: Operate the robotic welding torch to the arc starting point ( Figure 3 At point A in the diagram, following the predetermined counterclockwise welding trajectory, a mixed gas of 98% argon (Ar) and 2% carbon dioxide (CO2) is used for protection. The welding current is 90A, the arc voltage is 16V, the gas flow rate is 10L / min, and the welding speed is 10cm / min. Through the collaboration of the welding robot and the positioner, the welding of the entire valve seat surface is completed.

[0146] Step 6, Post-weld treatment:

[0147] 1) Heating: After welding is completed, continue to heat the weld (especially the inner and outer bends) to 300℃.

[0148] Stop heating;

[0149] 2) Cooling: The valve body that has not cooled down is then removed and placed in a heating furnace at 250°C for 30 minutes of stress-relieving annealing, followed by air cooling to room temperature;

[0150] 3) Machining: The weld overlay effect obtained at this time is a weld layer height of about 5mm and a weld seam width of 14-16mm. The valve body is turned to the machining center and the weld layer is precision machined to achieve the precision machining dimensions. At this time, the weld overlay layer is 2-3mm thick.

[0151] 4) Grind the burrs around the weld seam to avoid scratching the rubber part of the valve core that seals with the valve body after assembly.

[0152] Step 7, Post-weld inspection:

[0153] 1) After the cylindrical surfaces of the valve body and valve seat are precision machined and polished, they are assembled, pressure tested with water, and their sealing performance is required to be zero leakage. Taking the 6-inch specification as an example, the sealing test pressure is 1.2MPa and the holding time is 45 seconds. No air bubbles are allowed to overflow from the sealing surface.

[0154] 2) Check that the machined surface of the valve seat has no pores larger than 0.4mm, any cracks visible to the naked eye, etc.

[0155] In this embodiment, 20 valve bodies were tested for welding, and the one-time pass rate for sealing performance testing was 100%. No pores were found on the precision-machined valve seat surface, and the requirement of pore size on the valve seat surface being less than 0.4mm was met.

[0156] Example 7

[0157] The difference between this embodiment and embodiment 6 is that in the fifth step, welding: operating the robot welding torch to the arc starting point ( Figure 3 At point A in the diagram, following the predetermined counterclockwise welding trajectory, a mixed gas of 98% argon and 2% carbon dioxide is used for protection. The welding parameters are 120A welding current, 20V arc voltage, 12L / min gas flow rate, and 12cm / min welding speed. Through the collaboration of the welding robot and the positioner, the welding of the entire valve seat surface is completed.

[0158] In this embodiment, 20 valve bodies were tested for welding, and the one-time pass rate for sealing performance testing was 100%. No pores were found on the precision-machined valve seat surface, and the requirement of pore size on the valve seat surface being less than 0.4mm was met.

[0159] Example 8

[0160] The difference between this embodiment and embodiment 6 is that in the fifth step, welding: operating the robot welding torch to the arc starting point ( Figure 3 At point A in the diagram, following the predetermined counterclockwise welding trajectory, a mixed gas of 98% argon (Ar) and 2% carbon dioxide (CO2) is used for protection. The welding parameters are 150A welding current, 24V arc voltage, 15L / min gas flow rate, and 15cm / min welding speed. Through the collaboration of the welding robot and the positioner, the welding of the entire valve seat surface is completed.

[0161] In this embodiment, 20 valve bodies were tested for welding, and the one-time pass rate for sealing performance testing was 100%. No pores were found on the precision-machined valve seat surface, and the requirement of pore size on the valve seat surface being less than 0.4mm was met.

[0162] Comparative Test Example 6

[0163] The difference between this comparative test example and Example 6 lies in the fifth step, welding: a mixed gas of 80% argon (Ar) and 20% carbon dioxide (CO2) is used for protection, the welding current is 110A, the arc voltage is 19V, the gas flow rate is 12L / min, and the welding speed is increased from 11cm / min to 13cm / min; the welding of the entire valve seat surface is completed through the collaboration of the welding robot and the positioner.

[0164] In this embodiment, welding tests were conducted on 10 valve bodies. The first-time pass rate for sealing tests was 90%. However, due to the increased welding speed, the amount of welding wire melting per unit time was insufficient to cover the valve seat surface. The resulting weld bead was approximately 4mm high and 9-10mm wide, failing to achieve the expected weld height of approximately 5mm and width of 14-16mm. This incomplete coverage of the valve seat surface is unacceptable. Furthermore, the shielding gas contained a high concentration of carbon dioxide, resulting in numerous pores on the valve seat surface, some dispersed and some clustered, with some larger than 0.4mm. This is also unacceptable.

[0165] Comparative Test Example 7

[0166] The difference between this comparative test example and Example 6 lies in the fifth step of welding: following the predetermined counterclockwise welding trajectory, a mixed gas of 90% argon (Ar) and 10% carbon dioxide (CO2) is used for protection, with welding current of 130A, arc voltage of 21V, gas flow rate of 12L / min, and welding speed of 13cm / min. The entire valve seat surface is welded through the collaboration of a welding robot and a positioner.

[0167] In this embodiment, 20 valve bodies were tested for welding, and the pass rate for the sealing test was 90% on the first attempt.

[0168] The valve seat surface is fully covered, but after finishing, there are many small pores, some scattered and some clustered, some larger than 0.4mm. Overall, there are fewer pores than in comparative test 6, but it is still unacceptable.

[0169] Comparative Test Example 8

[0170] The difference between this comparative test example and Example 6 lies in the fifth step of welding: following the predetermined counterclockwise welding trajectory, a mixed gas of 75% argon (Ar) and 25% carbon dioxide (CO2) is used for protection, with welding current of 150A, arc voltage of 24V, gas flow rate of 15L / min, and welding speed of 12cm / min. Through the coordination of the welding robot and the positioner, the welding of the entire valve seat surface is completed.

[0171] In this embodiment, welding tests were conducted on five valve bodies. Multiple weld collapses occurred on the straight and curved sections of the valve seat surface. The cause was analyzed to be the use of a high welding current, which increased the amount of welding wire melting per unit time. The welding speed decreased from 15 cm / min to 12 cm / min, making the valve seat surface width insufficient to support the accumulation of molten wire, resulting in localized weld pool collapse. This may have left areas without machining allowance; therefore, subsequent machining and sealing tests were not performed. This weld collapse is unacceptable.

[0172] As demonstrated in Examples 6-8 and Comparative Experiments 6-8 above, shielding gas plays a crucial role in protecting molten metal droplets during welding and significantly impacts weld quality. Shielding gas prevents oxidation of the solidifying molten weld and also blocks impurities and moisture from the air, protecting the high-temperature metal from external gases. A commonly used mixed shielding gas is 80% argon (Ar) + 20% carbon dioxide (CO2). As the argon content increases, its protective effect as an inert gas on the weld strengthens, reducing the amount of gas entering the molten pool. Experimental results confirm that 2% CO2, as an active gas, plays a positive role in expelling existing gases from the molten pool.

[0173] In addition, the matching of welding current and welding speed is also very important. If the current is low, the welding speed should be slow to obtain the ideal weld width, otherwise the surface to be welded will not be fully covered. If the current is high, the welding speed should be increased accordingly, otherwise the weld pool will accumulate too much and the weld will collapse.

[0174] Based on the principle of obtaining a low dilution rate, a low current should be selected if it meets the welding requirements.

[0175] Furthermore, in order to verify the effect of the cooling process after welding on the sealing effect of the valve body, the present application implemented the following embodiments and test examples.

[0176] Table 3: Verification of the impact of post-weld cooling method on valve body sealing performance

[0177]

[0178] Note: The welding parameters in this table are as follows: 98% argon (Ar) + 2% carbon dioxide (CO2) mixed gas protection, welding current 110A, arc voltage 19V, gas flow rate 12L / min, welding speed 11cm / min.

[0179] Example 9

[0180] Similarly, taking a 6-inch cast iron valve body as an example, the welding process requirements for using the welding process described in this application are as follows:

[0181] The first step is to select welding materials: Welding wire conforming to AWS A5.15 standard, grade ERNi-CI, diameter 1.2mm, nickel content greater than 95%, and actually 98% nickel content welding wire is used.

[0182] The second step is pre-welding cleaning: remove rust, moisture, oil and other impurities from the valve seat surface to ensure that there are no impurities or stains on the valve seat welding surface that may affect the welding operation or welding effect.

[0183] Step 3, Welding trajectory verification:

[0184] 1) such as Figure 1 As shown, three pads 3, each 15mm long, 15mm wide, and 10mm high, are evenly distributed in a ring around the center of the positioner platform. Then, the pre-welded valve body is placed on the three pads, and the bottom flange of the valve body is made to contact and fix with the three pads to ensure that the flange face of the valve body is parallel to the positioner platform.

[0185] 2) According to Figure 3 The preset welding direction shown verifies the welding trajectory of the welding robot. If the actual welding trajectory of the welding robot is different from the preset welding trajectory, the robot is adjusted and corrected in real time until it matches the preset welding trajectory.

[0186] Step 4, Preheating before welding:

[0187] 1) Ignite and control the acetylene gas output to a moderate level, turn on the oxygen, and extinguish the flame;

[0188] 2) Preheat the area around the valve body, especially the outer ring of the valve body corresponding to the valve seat welding surface, for 3.5 minutes;

[0189] 3) After preheating the base material around the valve seat welding surface for 3 turns, start preheating the valve seat welding surface;

[0190] 4) Alternately preheat the valve seat welding surface and its surroundings. Initially, preheat the area around the sealing surface more, and later preheat the valve seat welding surface more.

[0191] 5) When the preheating temperature of the straight section of the valve seat welding surface reaches 220℃, the preheating temperature of the inner bend section reaches 240℃, and the preheating temperature of the outer bend section reaches 300℃, stop preheating.

[0192] Step 5, Welding: Operate the robotic welding torch to the arc starting point ( Figure 3 At point A in the diagram, following the predetermined counterclockwise welding trajectory, a mixed gas of 98% argon (Ar) and 2% carbon dioxide (CO2) is used for protection. The welding current is 110A, the arc voltage is 19V, the gas flow rate is 12L / min, and the welding speed is 11cm / min. Through the collaboration of the welding robot and the positioner, the welding of the entire valve seat surface is completed.

[0193] Step 6, Post-weld treatment:

[0194] 1) Heating: After welding is completed, continue to heat the weld (especially the inner and outer bends) to 300℃, then stop heating;

[0195] 2) Cooling: The valve body that has not cooled down is then removed and placed in a heating furnace at 250°C for 30 minutes of stress-relieving annealing, followed by air cooling to room temperature;

[0196] 3) Machining: The weld overlay effect obtained at this time is a weld layer height of about 5mm and a weld seam width of 14-16mm. The valve body is turned to the machining center and the weld layer is precision machined to achieve the precision machining dimensions. At this time, the weld overlay layer is 2-3mm thick.

[0197] 4) Grind the burrs around the weld seam to avoid scratching the rubber part of the valve core that seals with the valve body after assembly.

[0198] Step 7, Post-weld inspection:

[0199] 1) After the cylindrical surfaces of the valve body and valve seat are precision machined and polished, they are assembled, pressure tested with water, and their sealing performance is required to be zero leakage. Taking the 6-inch specification as an example, the sealing test pressure is 1.2MPa and the holding time is 45 seconds. No air bubbles are allowed to overflow from the sealing surface.

[0200] 2) Check that the machined surface of the valve seat has no pores larger than 0.4mm, any cracks visible to the naked eye, etc.

[0201] In this embodiment, 20 valve bodies were tested for welding, and the one-time pass rate for sealing performance testing was 100%. No pores were found on the precision-machined valve seat surface, and the requirement of pore size on the valve seat surface being less than 0.4mm was met.

[0202] Example 10

[0203] The difference between this embodiment and Embodiment 9 lies in step six, post-weld processing:

[0204] 1) Heating: After welding is completed, continue to heat the weld (especially the inner and outer bends) to 300℃, then stop heating;

[0205] 2) Cooling: Then remove the valve body that has not cooled down and place it in an asbestos-insulated box until it cools down to room temperature;

[0206] 3) Machining: The weld overlay effect obtained at this time is a weld layer height of about 5mm and a weld seam width of 14-16mm. The valve body is turned to the machining center and the weld layer is precision machined to achieve the precision machining dimensions. At this time, the weld overlay layer is 2-3mm thick.

[0207] 4) Grind the burrs around the weld seam to avoid scratching the rubber part of the valve core that seals with the valve body after assembly.

[0208] In this embodiment, 20 valve bodies were tested for welding, and the one-time pass rate for sealing performance was 100%. No pores were found on the precision-machined valve seat surface, and the requirement of less than 0.4mm of pores on the valve seat surface was met.

[0209] Comparative Experiment Example 9:

[0210] The difference between this comparative experiment and Example 9 lies in step six, post-weld treatment:

[0211] 1) Heating: No baking is required after welding is completed;

[0212] 2) Cooling: Allow to cool naturally to room temperature;

[0213] 3) Machining: The weld overlay effect obtained at this time is a weld layer height of about 5mm and a weld seam width of 14-16mm. The valve body is turned to the machining center and the weld layer is precision machined to achieve the precision machining dimensions. At this time, the weld overlay layer is 2-3mm thick.

[0214] 4) Grind the burrs around the weld seam to avoid scratching the rubber part of the valve core that seals with the valve body after assembly.

[0215] In this embodiment, 10 valve bodies were tested for welding, and the pass rate for the sealing test on the first attempt was 20%.

[0216] Comparative Test Example 10

[0217] The difference between this comparative experiment and Example 9 lies in step six, post-weld treatment:

[0218] 1) Heating: After welding, bake at 200℃;

[0219] 2) Cooling: Then remove the valve body that has not cooled down and place it in an asbestos-insulated box until it cools down to room temperature;

[0220] In this embodiment, 10 valve bodies were tested for welding, and the pass rate for the sealing test was 50%.

[0221] As can be seen from Examples 9-10 and Comparative Test Examples 9-10 above, the valve body sealing qualification rate is significantly higher after post-weld baking and slow cooling than that of rapid cooling. Rapid cooling may cause cracks in the fusion zone (as shown in the metallographic diagram at this time). Figure 5 As shown in the figure, cracks may also appear in the heat-affected zone of the gray cast iron substrate (as shown in the metallographic diagram). Figure 6 As shown in the image, all of these will cause leakage.

[0222] During welding, the temperature in the fusion zone near the weld reaches 1150–1250℃, causing all graphite to dissolve into austenite. During weld cooling, if the cooling rate is rapid, carbon atoms in the austenite often do not have enough time to precipitate graphite, existing as Fe3C and forming white cast iron. The faster the cooling, the easier it is for white cast iron to form in the fusion zone. White cast iron is both hard and brittle, with a hardness between 500 and 800 HBS. If white cast iron appears in the weld or fusion zone, it not only easily leads to cracking but also makes subsequent machining difficult. This application uses post-weld heating with a welding torch to further heat and bake the weld and the area around the valve seat surface, ensuring uniform heating. The aim is to minimize the probability of white cast iron formation, thereby reducing crack initiation and structural stress.

[0223] This application achieves the required quality testing standards through preheating before welding, slow cooling after welding, the use of suitable welding materials, and a reasonable welding process. It boasts advantages such as safety, reliability, good stability, absence of porosity, weld beads, undercut, and weld collapse. It exhibits excellent weld formation without defects like incomplete fusion or cracks, ensuring proper fusion of nickel welding materials and gray cast iron while guaranteeing its sealing performance, thus fulfilling the most crucial sealing requirement for valve products.

[0224] It should be noted that the above specific embodiments are exemplary. Under the teachings of the present invention, those skilled in the art can make various improvements and modifications based on the above embodiments, and these improvements or modifications fall within the protection scope of the present invention.

Claims

1. An automatic welding process for overlaying nickel material onto the sealing surface of a gray cast iron valve body, characterized in that, By using a preheating process before welding and a slow cooling process after welding on the valve body, the hardened structure at the weld joint is reduced, thereby improving the welding sealing effect of the valve body. The preheating before welding includes the following steps: 1) Preheat the outer ring of the valve body around the valve body, corresponding to the welding surface of the valve seat, for 2-4 minutes; 2) After preheating the base material around the valve seat welding surface for 2-3 turns, start preheating the valve seat welding surface; 3) Alternately preheat the valve seat welding surface and its surroundings. Initially, preheat the area around the sealing surface more, and later preheat the valve seat welding surface more. Stop preheating when the preheating temperature of the straight section of the valve seat welding surface reaches 220-250℃, the preheating temperature of the inner bend section reaches 240-300℃, and the preheating temperature of the outer bend section reaches 300-350℃. The valve body is welded using welding wire with a nickel content greater than 95%, and is protected by a mixed gas of 98% argon (Ar) and 2% carbon dioxide (CO2). The welding current is 90-150A, the arc voltage is 16-24V, the gas flow rate is 10-15L / min, and the welding speed is 10-15cm / min. The post-weld heating and slow cooling process includes: Heating: After welding is completed, continue heating the weld to 300±10℃, then stop heating; Cooling: The valve body that has not cooled down is subjected to stress-relieving annealing and slowly cooled to room temperature.

2. The automatic welding process for overlaying nickel material onto the sealing surface of a gray cast iron valve body according to claim 1, characterized in that, Both the preheating before welding and the heating after welding are carried out using a welding torch.

3. The automatic welding process for overlaying nickel material onto the sealing surface of a gray cast iron valve body according to claim 1, characterized in that, After preheating before welding, the real-time temperature of the valve body and valve seat welding surfaces is greater than 175℃ during welding.

4. The automatic welding process for overlaying nickel material onto the sealing surface of a gray cast iron valve body according to claim 1, characterized in that, The valve body is slowly cooled by placing it in a 250°C heating furnace for 30 minutes of stress-relief annealing, followed by air cooling; or by placing it in an asbestos-insulated box until it cools to room temperature.

5. The automatic welding process for overlaying nickel material onto the sealing surface of a gray cast iron valve body according to claim 4, characterized in that, The weld height of the valve body after welding is not less than 5mm, and the weld width is 14-16mm.

6. The automatic welding process for overlaying nickel material onto the sealing surface of a gray cast iron valve body according to claim 5, characterized in that, After the valve body is welded and precision machined, the machined sealing surface has no pores larger than 0.4mm and no cracks visible to the naked eye; the sealing performance is tested by water pressure. Under the pressure specified for the valve body's use, the pressure is maintained for 45-90 seconds. If no air bubbles overflow from the sealing surface, the test is qualified.

7. The automatic welding process for overlaying nickel material onto the sealing surface of a gray cast iron valve body according to claim 1, characterized in that, The preheating, welding, and postheating processes of the valve body are all performed on the positioner. Three pads are evenly distributed in a ring around the center of the positioner platform between the valve body and the positioner platform surface.

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