Surface cladding repair of 40Cr steel shafts by CO2 gas shielded welding

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At a large sugar manufacturing plant in our city, two large shafts used for transporting sugar syrup and transmitting high torque have worn out over time; as a result, the seals at those shafts are no longer effective, causing sugar syrup to leak during production. This leads to serious waste of raw materials and disrupts production processes. Because the cost of repurchasing is high. To save costs, it was decided to perform surfacing repair on the two shafts to allow them to be used continued. I. Material of the shafts and condition of damage The material of both shafts is 40Cr steel. The composition is as follows (in %): C: 0.37–0.44; Si: 0.17–0.37; Mn: 0.5–0.8; Cr: 0.8–1.10. The 40Cr steel used for manufacturing these shafts has been treated through quenching and tempering to obtain a low-alloy high-strength steel with excellent overall properties. The dimensions of the shafts are ψ350X4000 mm, and there is a hole with a diameter of ψ60 MM running through the center of each shaft. The total weight of the shafts is 2 tons. The wear at the gasket area is approximately 300 MM in width and 3 MM in depth. II. Analysis of the weldability of the shaft material 40Cr steel is a medium-carbon alloy steel; it belongs to the category of steels with poor weldability, and it has relatively high levels of Cr and Mn. Therefore, it has strong hardenability and a tendency to harden; coupled with the large volume of the shaft, the faster cooling rate during welding exacerbates these two tendencies. 　Steel 40Cr has a low Mf point, which leads to the formation of large amounts of martensite in the area near the weld seam; as a result, it is prone to cold cracking. To prevent such cracking and to avoid the peeling off of the surfacing layer under high torsional forces during use, in addition to using low-hydrogen welding materials with good plasticity and toughness, preheating before welding and carrying out stress-relief treatments after welding are also necessary. 　Furthermore, especially when no recovery heat treatment can be carried out after welding, and welding is performed in the quenched and tempered state, an excessive line energy leads to an increase in the layer of high-temperature tempering softening in the heat-affected zone, reducing the strength of the shaft’s surfacing area and affecting its performance, thus resulting in failed repairs. 　Similarly, a high linear energy input still cannot prevent the formation of martensite; however, it increases the degree of superheating and stability of austenite. This leads to the formation of coarse martensite structures in the quenched area, increasing the tendency toward brittleness and resulting in brittle failure. 　So, how can we ensure that excessive wire energy is not generated, while at the same time slowing down the cooling rate of the heat-affected zone after welding? This can only be achieved by using proper pre-warming before welding, strictly controlling the interpass temperature; in other words, by employing a welding method that results in a narrow weld bead and thus lower wire energy. When selecting welding materials, it is also necessary to choose those with strong crack resistance and low levels of S and P. III. Selection of Welding Method To avoid the problems mentioned above and to ensure the desired performance of the shaft, the best welding method is CO2 gas shielded welding. In practice, short-circuit transfer, narrow weld beads, and rapid welding are employed. This helps to reduce the duration of high temperatures in the heat-affected zone, thereby minimizing the degree of austenite embrittlement in that area and enhancing the stability of its microstructure. 　At the same time, by combining appropriate pre-welding heating, strict control of interpass temperature, and post-weld slow cooling, the microstructural properties of the quenched area can be improved, thereby enhancing resistance to cold cracking. When these methods are used appropriately, welding with low wire energy also helps to reduce the softened zone and minimize the degree of local softening in the shaft, satisfying the requirements for practical use. IV. Welding repair process: 1. Construct a support that allows the shaft to roll freely on it. Clean the worn areas of the shaft of any dirt to expose the metallic surface. 2. It has a large volume, which facilitates rapid heat dissipation; heating with an acetylene flame is slow. Instead, steam is passed through the central hole in the shaft to raise its temperature overall to above 100 degrees. Then, a HO--20 type welding torch is used to heat the worn areas evenly to 200–250 degrees before starting welding. 3. CO2 gas shielded welding: the wire grade is H08Mn2SiA with a diameter of 0.8 MM; the welding current is 100 A and the arc voltage is 18–21 V. Short-circuit transfer is used. First, the main shaft is rotated evenly to the starting position to ignite the arc and begin welding, so that the weld bead moves forward in a spiral pattern, thereby ensuring uniform heating of the weld bead over one full circle and preventing deformation or bending of the shaft. 4. Weld 45 turns per layer in a spiral pattern, for a total of two layers. Maintain the surface temperature of the shaft at 200 degrees Celsius per cycle and between layers, and allow it to cool naturally for 2 hours after welding. Using this method, the two axes were welded and repaired one by one. V. Conclusion After welding repairs were carried out on the two shafts, their operation remained normal, and no quality issues resulting from the repairs were observed. It is feasible to use large 40Cr steel shafts, along with a low wire energy and appropriate auxiliary processing techniques; this approach not only prevents the formation of large amounts of martensite in the heat-affected zone, which could make the shaft brittle, but also reduces the size of the softened zone, ensuring that the strength of the shaft remains unaffected. At a large sugar manufacturing plant in our city, two large shafts used for transporting sugar syrup and transmitting high torque have worn out over time; as a result, the seals at those shafts are no longer effective, causing sugar syrup to leak during production. This leads to serious waste of raw materials and disrupts production processes. Because the cost of repurchasing is high. To save costs, it was decided to perform surfacing repair on the two shafts to allow them to be used continued. I. Material of the shafts and condition of damage The material of both shafts is 40Cr steel. The composition is as follows (in %): C: 0.37–0.44; Si: 0.17–0.37; Mn: 0.5–0.8; Cr: 0.8–1.10. The 40Cr steel used for manufacturing these shafts has been treated through quenching and tempering to obtain a low-alloy high-strength steel with excellent overall properties. The dimensions of the shafts are ψ350X4000 mm, and there is a hole with a diameter of ψ60 MM running through the center of each shaft. The total weight of the shafts is 2 tons. The wear at the gasket area is approximately 300 MM in width and 3 MM in depth. II. Analysis of the weldability of the shaft material 40Cr steel is a medium-carbon alloy steel; it belongs to the category of steels with poor weldability, and it has relatively high levels of Cr and Mn. Therefore, it has strong hardenability and a tendency to harden; coupled with the large volume of the shaft, the faster cooling rate during welding exacerbates these two tendencies. 　Steel 40Cr has a low Mf point, which leads to the formation of large amounts of martensite in the area near the weld seam; as a result, it is prone to cold cracking. To prevent such cracking and to avoid the peeling off of the surfacing layer under high torsional forces during use, in addition to using low-hydrogen welding materials with good plasticity and toughness, preheating before welding and carrying out stress-relief treatments after welding are also necessary. 　Furthermore, especially when no recovery heat treatment can be carried out after welding, and welding is performed in the quenched and tempered state, an excessive line energy leads to an increase in the layer of high-temperature tempering softening in the heat-affected zone, reducing the strength of the shaft’s surfacing area and affecting its performance, thus resulting in failed repairs. 　Similarly, a high linear energy input still cannot prevent the formation of martensite; however, it increases the degree of superheating and stability of austenite. This leads to the formation of coarse martensite structures in the quenched area, increasing the tendency toward brittleness and resulting in brittle failure. 　So, how can we ensure that excessive wire energy is not generated, while at the same time slowing down the cooling rate of the heat-affected zone after welding? This can only be achieved by using proper pre-warming before welding, strictly controlling the interpass temperature; in other words, by employing a welding method that results in a narrow weld bead and thus lower wire energy. When selecting welding materials, it is also necessary to choose those with strong crack resistance and low levels of S and P. III. Selection of Welding Method To avoid the problems mentioned above and to ensure the desired performance of the shaft, the best welding method is CO2 gas shielded welding. In practice, short-circuit transfer, narrow weld beads, and rapid welding are employed. This helps to reduce the duration of high temperatures in the heat-affected zone, thereby minimizing the degree of austenite embrittlement in that area and enhancing the stability of its microstructure. 　At the same time, by combining appropriate pre-welding heating, strict control of interpass temperature, and post-weld slow cooling, the microstructural properties of the quenched area can be improved, thereby enhancing resistance to cold cracking. When these methods are used appropriately, welding with low wire energy also helps to reduce the softened zone and minimize the degree of local softening in the shaft, satisfying the requirements for practical use. IV. Welding repair process: 1. Construct a support that allows the shaft to roll freely on it. Clean the worn areas of the shaft of any dirt to expose the metallic surface. 2. It has a large volume, which facilitates rapid heat dissipation; heating with an acetylene flame is slow. Instead, steam is passed through the central hole in the shaft to raise its temperature overall to above 100 degrees. Then, a HO--20 type welding torch is used to heat the worn areas evenly to 200–250 degrees before starting welding. 3. CO2 gas shielded welding: the wire grade is H08Mn2SiA with a diameter of 0.8 MM; the welding current is 100 A and the arc voltage is 18–21 V. Short-circuit transfer is used. First, the main shaft is rotated evenly to the starting position to ignite the arc and begin welding, so that the weld bead moves forward in a spiral pattern, thereby ensuring uniform heating of the weld bead over one full circle and preventing deformation or bending of the shaft. 4. Weld 45 turns per layer in a spiral pattern, for a total of two layers. Maintain the surface temperature of the shaft at 200 degrees Celsius per cycle and between layers, and allow it to cool naturally for 2 hours after welding. Using this method, the two axes were welded and repaired one by one. V. Conclusion After welding repairs were carried out on the two shafts, their operation remained normal, and no quality issues resulting from the repairs were observed. It is feasible to use large 40Cr steel shafts, along with a low wire energy and appropriate auxiliary processing techniques; this approach not only prevents the formation of large amounts of martensite in the heat-affected zone, which could make the shaft brittle, but also reduces the size of the softened zone, ensuring that the strength of the shaft remains unaffected. I. Material of the shafts and condition of damage The material of both shafts is 40Cr steel. The composition is as follows (in %): C: 0.37–0.44; Si: 0.17–0.37; Mn: 0.5–0.8; Cr: 0.8–1.10. The 40Cr steel used for manufacturing these shafts has been treated through quenching and tempering to obtain a low-alloy high-strength steel with excellent overall properties. The dimensions of the shafts are ψ350X4000 mm, and there is a hole with a diameter of ψ60 MM running through the center of each shaft. The total weight of the shafts is 2 tons. The wear at the gasket area is approximately 300 MM in width and 3 MM in depth. II. Analysis of the weldability of the shaft material 40Cr steel is a medium-carbon alloy steel; it belongs to the category of steels with poor weldability, and it has relatively high levels of Cr and Mn. Therefore, it has strong hardenability and a tendency to harden; coupled with the large volume of the shaft, the faster cooling rate during welding exacerbates these two tendencies. 　Steel 40Cr has a low Mf point, which leads to the formation of large amounts of martensite in the area near the weld seam; as a result, it is prone to cold cracking. To prevent such cracking and to avoid the peeling off of the surfacing layer under high torsional forces during use, in addition to using low-hydrogen welding materials with good plasticity and toughness, preheating before welding and carrying out stress-relief treatments after welding are also necessary. 　Furthermore, especially when no recovery heat treatment can be carried out after welding, and welding is performed in the quenched and tempered state, an excessive line energy leads to an increase in the layer of high-temperature tempering softening in the heat-affected zone, reducing the strength of the shaft’s surfacing area and affecting its performance, thus resulting in failed repairs. 　Similarly, a high linear energy input still cannot prevent the formation of martensite; however, it increases the degree of superheating and stability of austenite. This leads to the formation of coarse martensite structures in the quenched area, increasing the tendency toward brittleness and resulting in brittle failure. 　So, how can we ensure that excessive wire energy is not generated, while at the same time slowing down the cooling rate of the heat-affected zone after welding? This can only be achieved by using proper pre-warming before welding, strictly controlling the interpass temperature; in other words, by employing a welding method that results in a narrow weld bead and thus lower wire energy. When selecting welding materials, it is also necessary to choose those with strong crack resistance and low levels of S and P. III. Selection of Welding Method To avoid the problems mentioned above and to ensure the desired performance of the shaft, the best welding method is CO2 gas shielded welding. In practice, short-circuit transfer, narrow weld beads, and rapid welding are employed. This helps to reduce the duration of high temperatures in the heat-affected zone, thereby minimizing the degree of austenite embrittlement in that area and enhancing the stability of its microstructure. 　At the same time, by combining appropriate pre-welding heating, strict control of interpass temperature, and post-weld slow cooling, the microstructural properties of the quenched area can be improved, thereby enhancing resistance to cold cracking. When these methods are used appropriately, welding with low wire energy also helps to reduce the softened zone and minimize the degree of local softening in the shaft, satisfying the requirements for practical use. IV. Welding repair process: 1. Construct a support that allows the shaft to roll freely on it. Clean the worn areas of the shaft of any dirt to expose the metallic surface. 2. It has a large volume, which facilitates rapid heat dissipation; heating with an acetylene flame is slow. Instead, steam is passed through the central hole in the shaft to raise its temperature overall to above 100 degrees. Then, a HO--20 type welding torch is used to heat the worn areas evenly to 200–250 degrees before starting welding. 3. CO2 gas shielded welding: the wire grade is H08Mn2SiA with a diameter of 0.8 MM; the welding current is 100 A and the arc voltage is 18–21 V. Short-circuit transfer is used. First, the main shaft is rotated evenly to the starting position to ignite the arc and begin welding, so that the weld bead moves forward in a spiral pattern, thereby ensuring uniform heating of the weld bead over one full circle and preventing deformation or bending of the shaft. 4. Weld 45 turns per layer in a spiral pattern, for a total of two layers. Maintain the surface temperature of the shaft at 200 degrees Celsius per cycle and between layers, and allow it to cool naturally for 2 hours after welding. Using this method, the two axes were welded and repaired one by one. V. Conclusion After welding repairs were carried out on the two shafts, their operation remained normal, and no quality issues resulting from the repairs were observed. It is feasible to use large 40Cr steel shafts, along with a low wire energy and appropriate auxiliary processing techniques; this approach not only prevents the formation of large amounts of martensite in the heat-affected zone, which could make the shaft brittle, but also reduces the size of the softened zone, ensuring that the strength of the shaft remains unaffected.