In the manufacturing of machine tools, the weight of machine tool castings typically accounts for 70-80% of the total weight of the complete machine. Due to the numerous influencing factors in the casting production process, it is difficult to completely avoid some localized defects. For these defective castings, provided the structural and performance requirements of the machine are not compromised, the practice of using welding repair to restore their service value is widely adopted both domestically and internationally. Practice has proven that this approach yields significant economic benefits. However, the weldability of cast iron is relatively poor. During the welding process, challenges such as the machinability of the heat-affected zone, color differences between the weld metal and the base metal, the mechanical strength of the joint, and thermal stresses must be addressed.
In our practice of repairing machine tool castings, we employ a self-developed strongly graphitizing cast iron electrode. By transferring graphitizing elements through the weld metal and applying preheating and stress-reduction measures in the process, the formation of hard, unmachinable cementite (white iron) structures in the weld seam is prevented, and welding thermal stresses are eliminated or minimized. Consequently, the repaired area exhibits good machinability and sufficient mechanical strength. Furthermore, since the weld metal also consists of a gray iron structure similar to the base metal, there is no significant color difference between them. The electrode and the welding procedure are two complementary and critical aspects ensuring repair quality, and they are elaborated upon separately below.
I. Development of the Welding Electrode
1. Core Wire
To prevent the formation of high-hardness cementite structures in the weld metal, it is essential to promote graphitization during the fusion welding process. Carbon and silicon are the most potent graphitizing elements. The graphitization coefficient K of cast iron is related to its chemical composition. For hypoeutectic cast iron with a ferrite-pearlite matrix, the graphitization coefficient is given by:
$$K = 4.28(\%\text{Si})\left[\frac{1}{5-0.31(\%\text{Si})}\right] + \%\text{C}$$
For ordinary non-alloyed cast iron, this coefficient can be approximated as:
$$K \approx \%\text{C} + 0.3 \times \%\text{Si}$$
The larger the graphitization coefficient, the shorter the time required for cementite to decompose into graphite. As can be inferred, when the graphitization coefficient is sufficiently high, the time needed for cementite to decompose into graphite is very short. Utilizing this principle, by adjusting the carbon and silicon content in the core wire to modify the graphitization coefficient of the weld metal, it is possible to ensure that cementite decomposes into graphite under the given welding conditions, thereby guaranteeing a gray iron structure in the weld metal. Practice has shown that maintaining the weld metal graphitization coefficient within the range of 7-9 is appropriate. Considering the potential loss of carbon and silicon due to arc burning and dilution by the base metal during fusion welding, the composition of the core wire was determined after repeated adjustments, as shown in Table 1.
| Carbon (C) | Silicon (Si) | Manganese (Mn) | Sulfur (S) | Phosphorus (P) |
|---|---|---|---|---|
| 3.0 – 3.5 | 3.5 – 4.0 | 0.5 – 0.8 | ≤ 0.05 | ≤ 0.10 |
2. Electrode Coating
Based on the metallurgical characteristics of cast iron, considerations for the coating formulation focus on arc stability, compensation capability for graphitizing elements, slag protection, and gaseous shielding of the molten iron pool, among others. After repeated trials, the coating formulation was determined as shown in Table 2.
| Component | Content (%) | Primary Function |
|---|---|---|
| Graphite Powder | 45 – 50 | Graphitization, Arc Stabilization |
| Fluorspar (CaF₂) | 15 – 20 | Slag Formation, Detoxification |
| Calcium Carbonate (Marble) | 10 – 15 | Gas Protection, Slag Formation |
| Aluminum Powder | 8 – 12 | Deoxidization, Strong Graphitization |
| Cryolite (Na₃AlF₆) | 5 – 8 | Improving Slag Fluidity |
| Ferrosilicon (75% Si) | 3 – 5 | Graphitization, Alloying |
3. Electrode Manufacturing Process
The coating ingredients are uniformly mixed according to the specified proportions. Sodium silicate (water glass), accounting for 25-30% of the total weight of the powdered raw materials, is added externally along with an appropriate amount of water to form a paste. This paste is then coated onto the core wire. The coating thickness is maintained at 1.2-1.5 mm. The coated electrodes are first air-dried in a well-ventilated area and subsequently baked in an electric furnace at 150-200°C for 2 hours. These electrodes have proven through long-term use that the hardness, strength, material density, and color matching of the weld zone all meet the requirements. Microstructural examination of test specimens generally reveals a gray iron structure of pearlite plus graphite in both the weld metal and the heat-affected transition zone.
II. Welding Repair Procedure for Machine Tool Castings
When using this electrode to repair small-to-medium-sized machine tool castings or areas with low stress concentration on large castings, preheating may be omitted. However, for thick, heavy sections or areas subject to high stress, local preheating of the casting to 400-550°C is generally required. The repair process sequence is as follows:
Defect Preparation → Mud Mold Fabrication → Preheating → Welding → Post-weld Heat Retention → Final Cleaning.
The key steps are detailed below.
1. Defect Preparation and Mud Mold Fabrication
Taking a typical defect as an example, after the defect area is thoroughly cleaned, a mud mold is sculpted around it using a dry mud mixture containing fireclay and refractory brick powder. The functions of this mud mold are threefold:
- It prevents the loss of molten iron, allowing the weld pool to accumulate and gradually fill the defect cavity according to a defined shape.
- Acting like a crucible, it prolongs the solidification time of the molten iron, enabling refining. As welding continues and the molten pool grows, impurities and gases have sufficient time to escape, and the diffusion of graphitizing elements becomes more complete, promoting a uniform, dense gray iron structure upon solidification.
- The mud mold insulates the casting from cold air. During preheating, the mold itself stores significant heat, contributing to slow cooling and stress reduction after welding.
2. Preheating
The preheating temperature depends on the casting volume and wall thickness. Thicker and larger machine tool castings require slightly higher preheating temperatures, and vice versa. Reference preheating temperatures for various types of castings in our practice are listed in Table 3. Preheating is performed using coke or gas flame.
| Type of Casting | Preheating Temperature (°C) |
|---|---|
| Various Bed Bodies, Headstocks, Saddles | 500 – 550 |
| Medium-sized Castings | 450 – 500 |
| Small Castings | 400 – 450 |
In addition to controlling the temperature, selecting the correct preheating area is crucial. Consider a frame structure with a crack at location A. If only area A is preheated, it will expand upon heating. During subsequent cooling and contraction after welding, areas B and C, which were not preheated, will restrain the shrinkage of area A, potentially inducing tensile stresses high enough to cause re-cracking. A more rational approach is to preheat areas B and C appropriately along with area A to eliminate their constraining effect. This method of reducing thermal stress by selectively heating specific areas is known as the “heating for stress reduction” method. In practice, various other stress-reduction techniques are employed based on the specific part geometry.
3. Welding Operation
When using a DC welding machine, the welding current can be determined by an empirical formula:
$$I = k \cdot d \quad \text{(Amperes)}$$
where \( I \) is the current intensity, \( d \) is the electrode diameter (mm), and \( k \) is an empirical coefficient chosen between 35 and 40.
Welding begins by striking the arc at the lowest point of the defect. Slag accumulating in the pool should be removed with an iron hook. If slag inclusions are observed in the molten iron, the arc should be slightly lengthened and directed towards the inclusion with a gentle weaving motion to help it float to the surface. The temperature of the molten iron in the mud mold should not be allowed to rise excessively to avoid oxidation. Therefore, if the molten iron emits a dazzling white light, pause welding briefly or add small pieces of leftover cast iron electrode to the pool.
When the mud mold is nearly filled, the arc should be slightly lengthened. After the entire pool surface is leveled, gradually break the arc to finish. Upon completion, as the iron begins to solidify, cover the area with some charcoal to ensure slow cooling.
III. Theoretical Foundation: Graphitization Kinetics
The core principle enabling successful welding of machine tool castings without forming brittle white iron lies in controlling graphitization. The graphitization coefficient \( K \) is a direct indicator of this tendency. The relationship between the time \( t \) required for cementite decomposition and the graphitization coefficient can be conceptually described by an exponential decay function:
$$t = A \cdot e^{-B \cdot K}$$
where \( A \) and \( B \) are material constants. This implies that for a given set of welding thermal conditions, as \( K \) increases, the time needed for the metastable cementite to transform into stable graphite decreases dramatically. When \( K \) reaches a critical threshold (empirically found to be in the 7-9 range for our process), this decomposition occurs rapidly within the thermal cycle of the weld, preventing the retention of cementite. This fundamental understanding guides the design of both the filler metal (electrode core and coating) and the thermal process (preheat, mud mold). The carbon equivalent \( CE \), often used for cast irons, is related but distinct; our focus is specifically on the graphitizing power for the welding context: \( K \approx \%C + 0.3\%Si \). Adjusting the weld pool composition to achieve a high \( K \) is paramount.
IV. Application Examples and Advanced Techniques
Over an extended period, we have successfully used the aforementioned method to repair various defects in machine tool castings, such as gas holes, shrinkage cracks, and mechanical damage. The results have been consistently satisfactory. Some representative examples are provided for reference.
Example 1: Repair of Bedway Surfaces. Common repair areas on the guide rail surfaces of a lathe bed include shrinkage cavities, sand holes, and impact damage. In some cases, the repair area for a shrinkage crack can be as large as 300 cm². For a completely broken-off corner, the method involves sculpting a mud mold to the original shape and then building up the weld to replicate the form.
Example 2: Repair of Fractured Bed Feet with Pre-deformation. This involves a complex structure where a crack is located in a high-stress area. If only the crack area is preheated, the weld shrinkage during cooling often leads to re-fracture. An advanced technique involves cutting open the crack to create a U-shaped groove with a root gap approximately twice the electrode diameter to facilitate manipulation. Empirical measurements indicate that the weld metal contraction \( \Delta \) is about 1-1.5 mm. Therefore, before welding, a screw jack is placed in an adjacent opening to widen the gap by an amount equivalent to \( \Delta \), making the actual gap \( G + \Delta \). Welding then proceeds to fill the mud mold. Shortly after arc extinction, when the weld metal is still hot but beginning to solidify, the jack is quickly released. The elastic recovery of the casting applies a compressive stress to the weld, effectively counteracting the tensile shrinkage stresses and preventing re-cracking. This application of pre-deformation principle yields excellent results.
For over a decade, our factory has repaired thousands of tons of scrapped machine tool castings using the method described herein. This method is applicable to any casting that has not undergone finish machining. Compared to cold welding methods using other electrodes (including nickel-based electrodes), although our method has slightly more demanding working conditions, these can be greatly improved by using a form of remote-controlled welding torch. In terms of repair quality, our hot welding method more readily achieves superior comprehensive properties (machinability, strength, color match, density). Therefore, it has not been entirely superseded by cold welding and remains a vital technique for repairing machine tool castings, possessing significant value for wider promotion.
V. Summary and Industrial Recommendations
The welding repair of machine tool castings is a technically demanding yet economically vital practice. The success hinges on a deep understanding of cast iron graphitization, meticulously designed filler materials, and a carefully controlled thermal process. Our approach integrates a strongly graphitizing electrode with a tailored procedure involving defect preparation, mud molding, selective preheating for stress relief, and controlled welding and cooling. The electrode ensures the weld metal transforms into a machinable gray iron, while the process minimizes detrimental thermal stresses.
Historically, some technical standards have prohibited weld repairs on critical surfaces like sliding or rolling guideways and major locating surfaces to promote casting quality. While aiming to prevent issues, this has led to the unnecessary scrapping of many salvageable machine tool castings. Across the industry, this results in thousands of tons of castings being remelted annually, which is a considerable waste. Facing this reality, we recommend that while efforts to prevent defects (“prevention”) should continue, the valuable technology of welding repair (“cure”) should be fully utilized and not unduly restricted. The need is not for blanket prohibitions but for the establishment of detailed, stringent quality standards specifically governing the welding repair of machine tool castings. Such standards would ensure repairs are performed to a high, reliable level, maximizing resource efficiency and economic benefit without compromising the final product’s integrity.