In our production of HT300 grade inoculated cast iron for machine tool castings, we previously employed conventional 75SiFe inoculant with a high inoculation amount at the furnace front. This approach proved challenging to control due to the wide variability in raw material sources and compositional fluctuations, leading to difficulties in stabilizing the melting process. As a result, we frequently encountered issues such as inadequate strength and hardness, significant hardness variations, and poor machinability in the machine tool castings. Additionally, the castings exhibited high deformation tendencies, susceptibility to pinhole porosity, and subsurface blowhole defects revealed after machining, all of which compromised the overall product quality of the machine tool castings. To address these problems, we transitioned to using high Si/C ratio inoculated cast iron for producing HT300 grade CA series machine tool bed castings. This method has effectively improved the microstructure and physical-chemical properties of the HT300 inoculated cast iron, while reducing or preventing the occurrence of pinhole porosity and subsurface blowhole defects commonly associated with sand casting processes for machine tool castings.
The CA series machine tool bed castings demand specific material properties to ensure optimal performance in industrial applications. These machine tool castings require high mechanical properties, particularly a high elastic modulus, to withstand operational stresses. For thick sections, achieving consistent hardness is crucial, with uniform hardness distribution along the guideways being essential. The castings must exhibit low stress tendencies and high resistance to compressive deformation, ensuring dimensional stability over time. Since these machine tool castings operate under lubricated abrasive wear conditions, the material must possess excellent wear resistance to prolong the service life of the bed guideways. After precision machining, the guideways require high accuracy and surface finish, necessitating good machinability in the castings. Furthermore, the machine tool castings must demonstrate high density, sound casting properties, and suitability for batch production of approximately 3,500 units annually, relying on widely available raw materials without excessive alloying elements. These requirements highlight the critical role of material selection in producing high-quality machine tool castings.
Previously, our technical measures for producing HT300 inoculated cast iron involved increasing the scrap steel content in the charge to reduce carbon levels and applying high inoculation amounts of 75SiFe (over 1%) to refine the microstructure and enhance properties. However, this process often resulted in excessive hardness in thin sections of the machine tool castings, such as the 18 mm thick bed head, reaching over 270 HB. In contrast, thicker sections like the 55 mm guideways frequently failed to meet hardness requirements (around 170 HB) and showed uneven distribution, leading to inadequate hardness after quenching. Addressing this issue through conventional inoculation methods and composition control was challenging under our specific conditions. The inherent characteristics of gray iron solidification structures made it difficult to meet all requirements simultaneously. While higher-grade gray iron could improve elastic modulus, it increased stress tendencies and impaired casting properties, failing to satisfy the needs of our machine tool castings. After extensive research, we identified high Si/C ratio inoculated cast iron as a solution that aligns with our production requirements for machine tool castings, offering better control over melting processes and consistent physical-chemical properties.
The problems we faced were primarily attributed to the aluminum content in the inoculant. According to relevant literature, when the aluminum content in inoculated iron melt ranges from 0.01% to 0.10%, it readily reacts with mold gases in sand casting, leading to pinhole and subsurface blowhole defects. The 75SiFe inoculant we used contained 1.5% to 2.0% aluminum, and with a furnace front inoculation amount of 0.7% to 1.0%, the resulting aluminum content in the iron melt fell between 0.0105% and 0.0300%, which was a major factor in defect formation. This underscored the need for an alternative approach to improve the quality of our machine tool castings.
To systematically evaluate the benefits of high Si/C ratio cast iron, we conducted production trials comparing the conventional and new processes. The CA series bed castings, made of HT300, followed a standard production workflow: sand preparation for molding and coring, molding, core making, oven drying, core setting, mold closing, iron melting, pouring, shakeout, cleaning, stress relief heat treatment, fettling, inspection, painting, and storage. The melting equipment consisted of a 5 t/h hot blast cupola with a forehearth, using raw materials including pig iron, scrap steel, returns, scrap machine iron, 75SiFe, and 60MnFe, with 75SiFe as the inoculant.
The chemical compositions for the conventional and high Si/C ratio processes are detailed in Table 1. For the conventional process, the pre-inoculation composition was 2.9–3.1% C, 0.9–1.1% Si, 0.8–1.0% Mn, P < 0.15%, and S < 0.12%, while post-inoculation (with 0.7–1.0% 75SiFe added at the spout during tapping at temperatures above 1,430°C) it was 2.9–3.1% C, 1.3–1.5% Si, 0.8–1.0% Mn, P < 0.15%, and S < 0.12%. In contrast, the high Si/C ratio process had a pre-inoculation composition of 2.8–3.0% C, 1.7–1.9% Si, 0.9–1.1% Mn, P < 0.15%, and S < 0.12%, and post-inoculation (with 0.2% 75SiFe added at the spout during tapping above 1,420°C) it was 2.8–3.0% C, 1.8–2.0% Si, 0.9–1.1% Mn, P < 0.15%, and S < 0.12%. The Si/C ratio can be calculated using the formula: $$\text{Si/C} = \frac{\text{Si\%}}{\text{C\%}}$$ which highlights the increased ratio in the new process, contributing to improved properties in the machine tool castings.
| Process | Stage | C (%) | Si (%) | Mn (%) | P (%) | S (%) | Si/C Ratio |
|---|---|---|---|---|---|---|---|
| Conventional | Pre-inoculation | 2.9–3.1 | 0.9–1.1 | 0.8–1.0 | <0.15 | <0.12 | 0.29–0.38 |
| Conventional | Post-inoculation | 2.9–3.1 | 1.3–1.5 | 0.8–1.0 | <0.15 | <0.12 | 0.42–0.52 |
| High Si/C | Pre-inoculation | 2.8–3.0 | 1.7–1.9 | 0.9–1.1 | <0.15 | <0.12 | 0.57–0.68 |
| High Si/C | Post-inoculation | 2.8–3.0 | 1.8–2.0 | 0.9–1.1 | <0.15 | <0.12 | 0.60–0.71 |
The performance of the castings was evaluated using dry sand molds with 30 mm diameter test bars. For tensile strength, the conventional process yielded 280–310 MPa, while the high Si/C ratio process achieved 310–360 MPa, indicating a significant improvement. Hardness measurements were taken at critical locations: the 18 mm thick bed head and three points on the 55 mm guideway. In the conventional process, the bed head hardness exceeded 270 HB, and the guideway points showed values of 203, 195, and 185 HB, demonstrating high variability. In contrast, the high Si/C ratio process resulted in a bed head hardness of 210 HB and consistent guideway hardness of 203 HB at all three points, highlighting the uniformity achieved. The relationship between hardness and Si/C ratio can be expressed as: $$\text{Hardness} = k \times \left(\frac{\text{Si}}{\text{C}}\right) + b$$ where \(k\) and \(b\) are constants derived from empirical data, showing how higher Si/C ratios promote more stable hardness in machine tool castings.
| Process | Tensile Strength (MPa) | Hardness (HB) – Bed Head | Hardness (HB) – Guideway Points | Pinhole Defect Rate (%) | Subsurface Blowhole Rate (%) | Cracking Rate (%) |
|---|---|---|---|---|---|---|
| Conventional | 280–310 | >270 | 203, 195, 185 | 20 | 37 | 5 |
| High Si/C | 310–360 | 210 | 203, 203, 203 | 0 | 2 | 0 |
The defect rates further illustrate the advantages of the high Si/C ratio approach. In the conventional process, pinhole defects, subsurface blowholes, and cracking occurred at rates of 20%, 37%, and 5%, respectively. After switching to high Si/C ratio cast iron, these rates dropped dramatically to 0% for pinholes, 2% for subsurface blowholes, and 0% for cracking. This reduction is largely due to the lower aluminum intake from the reduced inoculant amount, minimizing reactions with mold gases. The improved performance can be modeled using defect probability equations, such as: $$P_d = \alpha \cdot [\text{Al}] + \beta$$ where \(P_d\) is the defect probability, \([\text{Al}]\) is the aluminum content, and \(\alpha\) and \(\beta\) are process-dependent constants. For machine tool castings, maintaining low aluminum levels is crucial to avoid such defects.
From a microstructural perspective, the high Si/C ratio promotes the formation of a finer graphite structure and more uniform pearlite matrix, which enhances the mechanical properties of the machine tool castings. The graphite morphology can be described by parameters like aspect ratio and distribution density, which influence strength and machinability. The effect of Si/C ratio on graphite formation can be represented as: $$G_f = f\left(\frac{\text{Si}}{\text{C}}\right)$$ where \(G_f\) is a graphite formation factor. Higher Si/C ratios reduce the undercooling tendency during solidification, leading to more consistent structures across varying section thicknesses in machine tool castings. This is particularly important for complex geometries like bed castings, where thermal gradients can cause inhomogeneities.
In terms of production economics, the high Si/C ratio process offers cost benefits by reducing the reliance on large inoculation amounts and minimizing scrap losses. The annual production of 3,500 machine tool castings now experiences lower rejection rates, improving overall efficiency. The cost savings can be estimated using: $$C_s = (R_c – R_n) \cdot U_c \cdot N$$ where \(C_s\) is the cost saving, \(R_c\) and \(R_n\) are the defect rates for conventional and new processes, \(U_c\) is the unit cost per casting, and \(N\) is the annual production volume. For our machine tool castings, this translates to significant financial advantages without substantial increases in raw material costs.
Furthermore, the high Si/C ratio cast iron exhibits better casting properties, such as improved fluidity and reduced shrinkage, which are vital for producing sound machine tool castings. The fluidity index can be correlated with the Si/C ratio through empirical relations: $$F_i = \gamma \cdot \left(\frac{\text{Si}}{\text{C}}\right) + \delta$$ where \(F_i\) is the fluidity index, and \(\gamma\) and \(\delta\) are constants. This ensures that the molten metal fills molds completely, reducing the likelihood of misruns or cold shuts in intricate machine tool casting designs.
In conclusion, the adoption of high Si/C ratio inoculated cast iron has resolved the issues of high deformation, cracking tendency, inconsistent physical-chemical properties, and defect formation in our HT300 grade machine tool castings. This material provides stable performance, with reduced deformation, low thermal cracking susceptibility, minimal stress distortion, and an increase in strength grade by half to a full level. The hardness differential is minimized to around 5 HB, ensuring excellent machinability, dense microstructure, and enhanced wear resistance. Importantly, it has virtually eliminated pinhole and subsurface blowhole defects, drastically reducing scrap rates and generating tangible benefits in cost reduction and product quality for our machine tool castings. This approach underscores the importance of optimizing composition and inoculation strategies in the production of high-performance machine tool castings, ensuring they meet the rigorous demands of industrial applications.