In the manufacturing industry, machine tool castings constitute a significant portion of the total weight of equipment, often accounting for 60–80%. Due to various influencing factors in the production process, local defects in these machine tool castings are sometimes unavoidable. For such substandard castings, welding repair methods are widely adopted domestically and internationally to restore their usability, provided the structural performance of the machine tool is not compromised. Practical experience has demonstrated that this approach yields substantial economic benefits. However, the weldability of cast iron is relatively poor, presenting challenges such as machinability in the transition zone, color differences between the weld metal and the base material, mechanical strength of the joint, and thermal stress induced during welding. To address these issues, our team has developed a self-made strongly graphitizing cast iron electrode. By incorporating graphitizing elements into the weld and implementing preheating and stress-reduction measures, we prevent the formation of white iron structures in the weld, thereby minimizing or eliminating welding thermal stress. This ensures excellent machinability and sufficient mechanical strength in the repaired areas. Furthermore, since the weld metal consists of a gray iron structure similar to the base material, there is no significant color disparity. The electrode and welding process are two complementary aspects crucial to ensuring repair quality, and both will be elaborated upon in detail.
The development of the welding electrode is fundamental to achieving high-quality repairs for machine tool castings. To avoid the formation of high-hardness cementite structures in the weld metal, it is essential to promote graphitization during the 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 can be approximated by the formula: $$K = C + 0.3Si$$ where C and Si represent the percentages of carbon and silicon, respectively. A higher graphitization coefficient indicates a shorter time required for cementite to decompose into graphite. Experimental data shows that when the graphitization coefficient K is 5, the time needed for cementite decomposition is merely 0.5 seconds. By adjusting the carbon and silicon content in the electrode core, we can modify the graphitization coefficient of the weld metal, ensuring the formation of a gray iron structure. Practical experience indicates that maintaining the weld metal’s graphitization coefficient within the range of 4.5 to 5.5 is optimal. Considering the loss of carbon and silicon due to arc burning and dilution by the base metal during welding, the electrode core composition must be carefully calibrated. After repeated adjustments, the final composition of the electrode core was determined as shown in the table below.
| Element | Carbon (C) | Silicon (Si) | Manganese (Mn) | Sulfur (S) | Phosphorus (P) |
|---|---|---|---|---|---|
| Content (%) | 3.5–4.0 | 3.0–3.5 | 0.5–0.8 | ≤0.03 | ≤0.05 |
The electrode coating formulation is designed based on the metallurgical characteristics of cast iron, focusing on arc stability, compensation of graphitizing elements, slag protection, and gas shielding for the molten iron. After extensive experimentation, the coating composition was finalized as presented in the following table. The primary functions of each component include arc stabilization, slag formation, gas protection, and enhanced graphitization.
| Component | Electrode Powder | Fluorspar | Marble Powder | Aluminum Powder | Cryolite | Graphite |
|---|---|---|---|---|---|---|
| Content (%) | 30 | 20 | 15 | 10 | 10 | 15 |
| Main Function | Arc Stabilization | Slag Formation | Gas Protection | Graphitization | Slag Fluidity | Graphitization |
The electrode manufacturing process involves uniformly mixing the coating components according to the specified proportions. Water glass, accounting for 25–30% of the total powder weight, is added along with an appropriate amount of water to form a paste. This paste is then coated onto the electrode core, with a coating thickness maintained at 1.0–1.5 mm. The coated electrodes are air-dried in a well-ventilated area and subsequently baked in an electric furnace at 250°C for 2 hours. Long-term usage confirms that these electrodes yield weld metal with hardness, strength, density, and color matching that meet the required standards. Metallographic examination reveals that the weld metal and transition zone typically exhibit a gray iron structure composed of pearlite and graphite.
The welding repair process for machine tool castings involves several critical steps: defect cleaning, clay mold formation, preheating, welding, heat preservation, and post-weld cleaning. For small to medium-sized castings or areas with low stress in large castings, preheating may be omitted. However, for thick or highly stressed sections, local preheating at 400–500°C is generally necessary. The key steps are detailed below.
Defect cleaning entails removing all impurities from the damaged area. Subsequently, a clay mold is formed around the defect using a mixture of fireclay and refractory brick powder (70:30 ratio). This mold serves multiple purposes: it prevents the loss of molten iron, allows for controlled accumulation of iron to fill the defect, acts as a crucible to prolong solidification time for refining, and provides insulation to slow cooling. The mold isolates the casting from cold air and stores heat during preheating, contributing to gradual cooling.
Preheating temperature depends on the casting volume and wall thickness, with higher temperatures for larger sections. Typical preheating temperatures for various machine tool castings are summarized in the table. Preheating is performed using coke or gas flames. Crucially, the preheating location must be selected strategically to minimize thermal stress. For instance, in frame structures with cracks, preheating only the cracked area can lead to tensile stresses during cooling due to constraint from unheated adjacent zones, potentially causing re-fracture. Therefore, the heating stress reduction method involves preheating both the defect area and surrounding regions to relieve constraints.
| Casting Type | Preheating Temperature (°C) |
|---|---|
| Various Beds | 450–500 |
| Headstocks, Saddles | 400–450 |
| Small to Medium Castings | 350–400 |
Welding is conducted using a DC welding machine, with the current determined empirically by the formula: $$I = K \cdot d$$ where I is the current in amperes, d is the electrode diameter in millimeters, and K is an empirical coefficient typically set at 40. Welding begins at the lowest point of the defect, and slag accumulating in the molten pool is regularly removed with an iron hook. If slag inclusions are observed, the arc is lengthened and directed towards them with slight electrode oscillation to facilitate flotation. To prevent excessive oxidation, welding is paused if the molten iron emits dazzling light, or small pieces of cast iron electrode remnants are added to the mold. As the mold nears fullness, the arc is gradually lengthened, and after the surface is leveled, the arc is slowly extinguished. Upon completion, the weld is covered with charcoal to insulate and slow the cooling process.
Over the years, we have successfully repaired various defects in machine tool castings, such as porosity, cracks, and mechanical damage, using this method. For example, on the guide rails of a bed casting, common repair areas include shrinkage cracks, sand holes, and impact damage. In one case, an entire corner was broken off and restored by building a contour clay mold for shape welding. Another instance involved repairing a crack in the foot of a bed casting. To prevent re-cracking, the crack was cut through to form a U-shaped groove with a gap approximately twice the electrode diameter, facilitating manipulation. Accounting for weld shrinkage δ of about 1–2 mm, a screw jack was used to pre-expand the gap by an equivalent amount before welding. After filling the mold, the jack was released during solidification to impose compressive stress, counteracting tensile stresses and ensuring joint integrity.
Compared to other methods, such as cold welding with nickel-based electrodes, this approach, though requiring better working conditions (e.g., via remote-controlled welding tongs), offers superior comprehensive performance in terms of hardness, strength, and color match. Thus, it remains a vital technique for repairing machine tool castings and warrants broader adoption. However, current industry standards often restrict welding on critical surfaces like guide rails, leading to unnecessary scrap. We recommend emphasizing defect prevention while leveraging welding technology, supported by stringent quality standards, to reduce the annual remelting of thousands of tons of machine tool castings.
The graphitization process in machine tool castings is critical for achieving desired microstructures. The time required for cementite decomposition as a function of the graphitization coefficient K can be modeled by the equation: $$t = A e^{-BK}$$ where t is time, A and B are constants, and K is the graphitization coefficient. This relationship highlights the importance of controlling chemical composition to facilitate rapid graphitization during welding. Optimizing the electrode composition and process parameters ensures that the weld metal in machine tool castings mirrors the base material’s properties, enhancing overall reliability.
In practice, the dilution effect of the base metal on the weld pool must be considered. The effective graphitization coefficient Keff after dilution can be estimated as: $$K_{\text{eff}} = \frac{K_{\text{electrode}} \cdot V_{\text{electrode}} + K_{\text{base}} \cdot V_{\text{base}}}{V_{\text{total}}}$$ where V represents the volume contributed by the electrode and base metal, respectively. This calculation aids in fine-tuning the electrode composition to maintain Keff within the optimal range of 4.5–5.5. Additionally, the cooling rate influences graphitization; slower cooling promoted by the clay mold allows sufficient time for carbon diffusion and graphite precipitation.
Mechanical properties of repaired machine tool castings are paramount. The ultimate tensile strength σ of gray iron can be correlated with its graphite structure and matrix using empirical formulas such as: $$\sigma = C_1 – C_2 \cdot G$$ where G represents graphite factor, and C1, C2 are material constants. By ensuring a fully graphitized structure, the weld achieves compatibility with the base machine tool castings in terms of strength and machinability. Hardness tests on repaired sections typically show values between 180–220 HB, matching the parent material.
Thermal stress management during welding of machine tool castings involves preheating to reduce temperature gradients. The stress σth induced can be approximated by: $$\sigma_{\text{th}} = E \alpha \Delta T$$ where E is Young’s modulus, α is the thermal expansion coefficient, and ΔT is the temperature difference. Preheating minimizes ΔT, thereby lowering residual stresses. Furthermore, the use of stress-relief techniques, such as the jack method described, introduces beneficial compressive stresses to counteract shrinkage effects.
Economic analyses demonstrate that repairing defective machine tool castings via welding reduces costs by 60–70% compared to producing new castings. This approach also aligns with sustainable manufacturing by minimizing material waste. Future work should focus on automating the process and developing advanced electrodes for specialized machine tool castings to further enhance efficiency and quality.
In summary, the integration of specially designed electrodes and meticulous welding procedures enables effective restoration of machine tool castings, preserving their functionality and extending service life. Continued refinement of these practices will contribute significantly to the economization and sustainability of machinery production.