High-Strength Girth Gears Cast Steel Defect Welding and Repair Process
High-Strength Girth Gears Cast Steel Defect Welding and Repair Process
Feb 26, 2025
High-Strength Girth Gears Cast Steel Defect Welding and Repair Process
1. Technical Background and Current Status Low-alloy high-strength steel is widely used in mechanical engineering, power generation, pressure vessels, the automotive industry, mining machinery, and the oil and marine sectors. It is characterized by its ultra-high strength, good ductility and toughness, and favorable weldability. Based on the alloy element systems, high-strength steel can be broadly categorized into four types:
Mn-Si system: Based on Mn-Si, with the addition of small amounts of Cr, Ni, Mo, and V, the tensile strength is ≥600 MPa.
Mn-Si-Cr-Ni-Mo system: Based on the Mn-Si system, with the addition of small amounts of V, the tensile strength is ≥700 MPa.
Mn-Si-Cr-Ni-Mo-Cu-V system: Based on the Mn-Si-Cr-Ni-Mo system, with the addition of a certain amount of B, the tensile strength is ≥800 MPa.
Ultra-alloy system: Based on the Mn-Si-Cr-Ni-Mo-Cu-V system, with the addition of a higher amount of Ni, the tensile strength is ≥1000 MPa, and it exhibits good impact resistance.
Currently, in the industry, the welding repair of low-alloy high-strength steel generally faces issues such as cracking in the weld fusion zone, local softening in the heat-affected zone (HAZ), significant hardness differences between the weld repair area and the base material, and uneven hardness of the cast steel components. To ensure good crack resistance of the welded joints, a low-strength matching principle can be adopted, where welding materials with lower strength than the base material are used for repair. Although this method ensures good ductility and toughness of the welded area and reduces the risk of cracking in the welded joints, it results in a weld strength lower than that of the base material, which cannot guarantee the use of the cast steel components under more demanding operating conditions. The Girth Gears cast steel studied in this paper belongs to the second to third series of high-strength steel, which has the welding characteristics of ordinary low-alloy high-strength steel. As an important transmission component, the Girth Gears is required to have not only high strength but also high hardness, especially for the toothed areas, which must have uniform hardness. Non-uniform hardness can lead to different degrees of wear on the gear surfaces during later use, ultimately reducing gear transmission efficiency and even causing tooth breakage. Girth Gears cast steel components are core transmission parts in mining machinery. The difficulties in the defect welding and repair process mainly include weld cracking, especially significant cracking in the heat-affected zone, significant hardness differences between the weld repair area and the base material, and poor hardness uniformity. In particular, after precision machining and tooth cutting, the hardness of the tooth grooves is significantly lower than that of the tooth end faces, resulting in noticeable hardness attenuation, which occurs in both the weld and the base material. 2. Weldability Analysis of Girth Gears High-Strength Steel The Girth Gears high-strength steel studied in this paper has the material grade SAE8635, and its main composition and properties are shown in Tables 1 and 2. According to the carbon equivalent (CE) calculation formula recommended by the International Institute of Welding (IIW): CE (IIW) = ω(C) + ω(Mn)/6 + [ω(Cr) + ω(Mo) + ω(V)]/5 + [ω(Ni) + ω(Cu)]/15, the CE value of the base material in Table 1 is calculated to be between 0.67% and 0.69%. Based on the cold crack sensitivity index formula proposed by Japan, Pcm = ω(C) + ω(Si)/30 + [ω(Mn) + ω(Cu) + ω(Cr)]/20 + ω(Ni)/60 + ω(Mo)/15 + ω(V)/10, the Pcm value of the base material in Table 1 is calculated to be between 0.44 and 0.46. When the carbon equivalent value is greater than 0.4%–0.6%, the weldability is poor. Therefore, the low-alloy high-strength steel with the grade SAE8635 has poor weldability, a high tendency to harden, and a high likelihood of cold crack defects. Thus, the selection of welding materials, control of welding parameters, and the type of heat treatment after welding are key points in controlling welding quality. Table 1: Chemical Composition of SAE8635 (mass fraction, %) Table 2: Mechanical Properties of SAE8635 3. Pre-Welding Preparation 3.1 Welding Material Selection The selection of welding materials generally follows two principles: performance matching and composition matching. Performance matching is commonly used for structural steel welding, while composition matching requires more consideration of the working environment of the component, such as the need for alloy elements like Cr, Mo, W, V, and Nb in heat-resistant steel welding materials to be comparable to those in the base material; and the need for alloy elements like Cr, Ni, and Mo in corrosion-resistant steel welding materials to be comparable to those in the base material. For the Girth Gears casting with the grade SAE8635, the main consideration is whether the performance of the welding material can meet the requirements of the base material. In addition, the selection of welding materials should also consider the type of heat treatment after welding, that is, whether stress-relief heat treatment or quenching and tempering will be performed after welding, which is particularly important for Girth Gears cast steel components. From Table 2, it can be seen that the Girth Gears casting with the material grade SAE8635 has high strength and hardness. In particular, the issue of hardness uniformity is prominent, with significant differences in hardness between the welding area and the base material, and attenuation of hardness in the repair area compared to the base material. Table 3 shows the composition of several target welding materials. Table 3: Composition of Welding Materials (mass fraction, %) The DI value is calculated according to the ASTM A255 end-quench test standard, with the formula as follows: DI (in) = 0.54ω(C) × (0.7ω(Si) + 1) × (3.3333ω(Mn) + 1) × (2.16ω(Cr) + 1) × (3ω(Mo) + 1) × (0.363ω(Ni) + 1) × (0.365ω(Cu) + 1) × (1.73ω(V) + 1), which is applicable to materials with ω(C)% < 0.4%. The final selection of welding materials also needs to consider the type of heat treatment after welding. Since the Girth Gears product has high requirements for hardness uniformity, quenching and tempering is usually performed after welding. In this case, the DI value is mainly considered in the selection of welding materials, while the CE value is also taken into account. Compared with the values in Table 3 and Table 1, the DI and CE values of welding material 1 are closest to those of the base material, making it the most suitable for welding followed by quenching and tempering. If stress-relief heat treatment is to be performed after welding, the CE value of the welding material should be mainly considered. However, a high CE value of the welding material will increase the risk of weld cracking and lead to significant hardness differences between the repair area and the base material. Therefore, if stress-relief heat treatment is to be performed after welding, welding material 4 is more suitable, with a lower Pcm value being preferred. 3.2 Defect Removal and Bevel Cleaning For the Girth Gears high-strength steel casting, defects are mainly removed by carbon arc gouging. Based on Table 1, the base material has high CE and Pcm values. To prevent base material cracking during the gouging process, preheating and insulation treatment should be carried out before and during the gouging process. The contour of the Girth Gears casting is mainly circular or semi-circular. A preheating pipe with a shape matching the contour of the Girth Gears is selected, and the casting is placed horizontally, which facilitates preheating, gouging, and welding operations. The specific preheating operation is shown in Figure 1. A preheating pipe with the same shape as the Girth Gears casting contour is selected. The distance between the preheating pipe and the casting is adjusted according to the length of the preheating flame, with a distance of 50 mm being appropriate. The preheating pipe has evenly distributed gas outlet holes. When ignited, the flame length from the holes should be 150 mm–200 mm to ensure uniform heating. When the Girth Gears has a large wall thickness (wide cross-section), an additional preheating pipe is added to ensure uniform preheating temperature that meets the required temperature. The preheating temperature should be controlled at 250℃–300℃, and the temperature difference between all defect areas should not exceed 50℃. When the gouging stops, continuous preheating and insulation should be maintained. After the gouging is completed, the bevel needs to be ground. During grinding, the temperature must be kept above 200℃. After the bevel is ground to a bright finish, dry powder testing is carried out to ensure that there are no crack defects in the bevel area. Preheating and insulation before welding are also carried out according to Figure 1. Figure 1: Preheating Schematic 4. Welding Operation Process 4.1 Welding Parameters and Techniques To effectively control heat input and prevent softening in the heat-affected zone, welding rods with a diameter of φ4.0 mm are preferably used for multi-layer and multi-pass welding. The specific repair process is shown in Figure 2. During welding, the position of the casting is adjusted to ensure that the defective welding area is in a flat welding position. Each weld layer thickness should be less than or equal to 3 mm. The welding process involves lateral oscillation with an amplitude less than or equal to 10 mm, and each weld pass width is controlled between 10 mm and 15 mm. The welding current is set at 140 A–160 A, and the voltage at 17 V–22 V. For large-volume defects in a vertical welding position, a two-layer base welding is first performed, followed by filling welding. The specific requirements for base welding are as follows: Using a φ4.0 mm welding rod for the first layer of base welding (see position 7 in Figure 2), the current is set at 150 A, and the voltage at 25 V. This ensures good penetration in the base layer and improves the composition of the base material near the weld through dilution by the base material, without significantly increasing heat input and avoiding excessive grain coarsening in the heat-affected zone. After the first layer of base welding is completed, a φ5.0 mm welding rod is used for the second layer of base welding at the position indicated by number 8 in Figure 2, with the current set at 190 A and the voltage at 25 V. After the two layers of base welding are completed, a centralized heating device is used to perform a high-temperature post-weld heat treatment on the base layer, maintaining the temperature between 400℃ and 450℃ for at least 2 hours. The temperature is then slowly reduced to around 200℃. The base layer is then ground to a bright finish, and dry powder testing is conducted to ensure no cracks are present. Filling welding is then carried out according to Figure 2(b) until the defect is fully welded. Figure 2: Welding Repair Schematic 4.2 Post-Weld Heat Treatment and Hydrogen Removal After welding is completed, the casting is subjected to post-weld heat treatment and hydrogen removal using the method shown in Figure 1. The overall temperature of the casting is maintained at 300℃–350℃. During the entire post-weld heat treatment process, insulation materials such as insulating cotton or heat-insulating fabric can be used to cover and insulate the repair area, ensuring that the temperature difference between the repair area and the base material area is ≤50℃. The temperature should not be allowed to drop during the entire process until the casting enters the furnace for heat treatment. 5. Conclusions
For Girth Gears high-strength steel, the selection of welding materials can be made more scientifically and reasonably by comparing the CE, DI, and Pcm values of the welding materials with those of the base material. The type of heat treatment after welding should also be considered. If stress-relief heat treatment is to be performed after welding, the CE value of the welding material should be consistent with that of the base material, while also considering the Pcm value of the welding material. If quenching and tempering is to be performed after welding, the DI value of the welding material should be consistent with that of the base material, while also considering the CE and Pcm values of the welding material.
To ensure uniform hardness of the Girth Gears, quenching and tempering is preferably chosen after welding. However, the end-quench test according to ASTM A255 should be conducted to verify the hardness attenuation of the welding and base materials under specific heat treatment processes. Combined with the final tooth cutting dimensions of the Girth Gears, it is necessary to ensure that the hardness attenuation after tooth cutting meets the base material hardness requirements. If not, the welding material should be reselected, or the quenching process should be adjusted.
For Girth Gears high-strength steel welding materials, manual welding rod welding is preferred, and small-diameter welding rods should be used whenever possible. This helps control heat input, reduces welding heat effects, and eliminates softening and cracking caused by overheating.
During defect removal, preheating is also necessary. The preheating temperature depends on the carbon content of the casting. The higher the carbon content, the higher the preheating temperature. For products with w(C)% < 0.4%, a minimum preheating temperature of 250℃ is generally sufficient. During the pause in welding, continuous preheating and insulation should be maintained above 250℃, and the temperature difference in the welding area should be controlled to prevent cracking due to uneven preheating.
It is crucial to maintain post-weld heat treatment and insulation before heat treatment of the casting. Otherwise, severe cracking may occur in the welding area.
