In our production of high-speed milling-boring machines, we faced significant challenges with the hardness consistency of machine tool castings, specifically the bed guideways. These machine tool castings are critical components that require precise mechanical properties to ensure long-term performance and accuracy. The bed, made of HT300 gray iron, had a mass of over 20 tons and dimensions of 6000 mm in length, with guideways extending the full length. The required hardness for the guideways was 180–230 HB, with no defects such as shrinkage porosity or gas holes permitted on the main sliding surfaces. Additionally, the castings underwent two stress relief treatments. However, initial productions revealed unsatisfactory hardness values, with large variations between thick and thin sections and before and after machining. This inconsistency threatened the functionality and durability of the machine tool castings. Through a systematic approach involving metallurgical and process improvements, we successfully addressed these issues, achieving stable hardness and meeting all design specifications for these essential machine tool castings.
Our initial production process utilized continuous water-cooled cupola melting and furan resin self-hardening sand molding. The type sand had a tensile strength of 0.8 MPa. An analysis of the chemical composition from previous productions indicated that carbon content was often at the upper limit or beyond, while silicon was at the lower limit or below. This composition resulted from the tendency of cupola melting to increase carbon content, especially when producing high-grade gray iron like HT300, which requires low carbon for proper inoculation. Although adding steel scrap to the charge reduced carbon and increased chill tendency, it adversely affected fluidity and shrinkage propensity, leading to hardness inconsistencies. The table below summarizes the typical chemical composition from initial productions, highlighting the deviations from standard ranges.
| Element | Average Composition (wt%) | Standard Range (wt%) |
|---|---|---|
| C | 3.09 | 2.8–3.2 |
| Si | 1.29 | 1.3–1.8 |
| Mn | 1.20 | 0.8–1.2 |
| P | 0.067 | <0.12 |
| S | 0.090 | ≤0.12 |
Hardness testing of the guideways from these initial machine tool castings revealed significant disparities. For instance, values varied widely between different sections of the same casting and showed considerable drops after machining. This indicated microstructural non-uniformity, which could lead to reduced wear resistance and precision in the final machine tool. The following table illustrates the hardness values measured before and after machining on several bed castings, underscoring the problem.
| Bed Identification | Hardness Before Machining (HB) | Hardness After Machining (HB) |
|---|---|---|
| #1 | 180, 182, 224 | 165, 178, 207 |
| #2 | 182, 195, 197 | 162, 176, 180 |
| #3 | 165, 192, 241 | 156, 177, 187 |
To resolve these issues, we implemented a multi-faceted strategy focused on enhancing the metallurgical quality and casting process of the machine tool castings. The key measures included increasing the tapping temperature, refining the grain structure, applying alloying treatments, and optimizing the casting design. Each of these interventions contributed to improving the hardness consistency and overall performance of the machine tool castings.
First, we focused on increasing the tapping temperature of the iron melt. Higher temperatures promote cleaner iron with reduced inclusions and oxidation, which directly enhances hardness. Research indicates that for every 100°C increase in temperature, hardness can rise by approximately 19 HB. This relationship can be expressed as: $$\Delta HB = k \cdot \Delta T$$ where $\Delta HB$ is the change in hardness, $k$ is a constant approximately equal to 0.19 HB/°C, and $\Delta T$ is the temperature change in °C. In our case, the initial tapping temperature ranged from 1380°C to 1400°C, which was insufficient for optimal hardness. We modified the cupola internal structure by shortening the well height and shaping the tuyere zone to a constricted form, which intensified combustion and heat transfer. Additionally, we upgraded the blower system to increase air supply and used high-quality coke in larger batches. These changes elevated the tapping temperature to 1420–1465°C, resulting in a measurable hardness improvement. The enhanced temperature also reduced carbon pickup, aiding in achieving the desired chemical composition for high-grade machine tool castings.
Next, we employed grain refinement to achieve a more uniform and finer microstructure, which is crucial for consistent hardness in machine tool castings. finer grains enhance strength and hardness by providing more grain boundaries that impede dislocation movement. The Hall-Petch equation describes this phenomenon: $$\sigma_y = \sigma_0 + \frac{k}{\sqrt{d}}$$ where $\sigma_y$ is the yield strength, $\sigma_0$ is the lattice friction stress, $k$ is the strengthening coefficient, and $d$ is the average grain diameter. A smaller $d$ results in higher $\sigma_y$ and, consequently, higher hardness. To accomplish this, we used 75% ferrosilicon (75SiFe) as an inoculant, added at 0.4–0.5% of the melt weight. Inoculation was performed both at the furnace spout and during pouring via instantaneous methods, ensuring effective grain refinement throughout the casting. This approach minimized hardness variations between different sections of the machine tool castings, contributing to more predictable machining behavior and longer service life.
Alloying was another critical step in enhancing the hardness of our machine tool castings. Initially, we increased manganese content to the upper limit of 1.2%, but this alone did not yield significant improvements. After evaluating various elements, we selected tin (Sn) due to its low melting point (231.5°C) and minimal oxidation losses when added to the ladle. Tin acts as a pearlite stabilizer in gray iron, enhancing hardness without promoting excessive chilling. We conducted trials with Sn additions, and the results demonstrated notable hardness increases. The table below shows hardness values from beds produced with Sn alloying, indicating improved consistency and values closer to the target range.
| Bed Identification | Hardness Before Machining (HB) | Hardness After Machining (HB) |
|---|---|---|
| #1 | 229, 210, 229 | 194, 182, 194 |
| #2 | 228, 207, 246 | 191, 185, 198 |
| #3 | 229, 207, 216 | 200, 181, 191 |
| #4 | 225, 210, 220 | 195, 185, 188 |
However, Sn is a costly alloy, so we optimized its usage by revising the casting process. Originally, the gating system involved simultaneous filling from both sides, requiring Sn addition to two ladles. We redesigned it to a sequential filling system, where one ladle specifically filled the guideway sections. This allowed us to add Sn only to that ladle, reducing overall consumption while maintaining hardness benefits. The illustration below depicts the optimized gating system, which contributed to cost-effective production of high-quality machine tool castings.
Furthermore, we considered the carbon equivalent (CE) as a key parameter in controlling the properties of gray iron machine tool castings. The carbon equivalent is calculated as: $$CE = C + \frac{Si + P}{3}$$ where C, Si, and P are the weight percentages of carbon, silicon, and phosphorus, respectively. A lower CE favors higher strength and hardness but can increase shrinkage tendencies. By balancing CE through controlled scrap addition and inoculation, we achieved a microstructure conducive to consistent hardness. The interplay between chemistry and process parameters is vital for producing reliable machine tool castings.
The combined implementation of these measures yielded significant improvements in the TKG6920 type bed castings. We produced multiple units under the refined process, and hardness tests confirmed that values met the design specifications. The following table presents hardness data from two beds after rough and semi-finish machining, demonstrating minimal variation and values within the 180–230 HB range.
| Bed Identification | Hardness After Rough Machining (HB) | Hardness After Semi-Finish Machining (HB) |
|---|---|---|
| #1 | 201, 201, 215 | 190, 183, 196 |
| #2 | 206, 200, 198 | 196, 185, 181 |
Additionally, we analyzed the hardness uniformity across different sections of the machine tool castings. The maximum hardness difference between thick and thin walls was reduced to 15–39 HB in the as-cast state and 12–19 HB after machining, compared to much larger variations initially. This consistency is critical for the dimensional stability and wear resistance of machine tool castings in service. The success of these measures underscores the importance of a holistic approach to producing high-performance machine tool castings.
In conclusion, by systematically addressing the factors affecting hardness—through elevated tapping temperatures, grain refinement, strategic alloying, and process optimization—we achieved our goal of producing machine tool castings with reliable and uniform hardness. The hardness values now consistently fall within 180–230 HB, with minimal differences before and after machining and across varying section thicknesses. This achievement not only meets the stringent requirements for high-speed milling-boring machines but also sets a benchmark for quality in machine tool castings. Our experience demonstrates that with careful analysis and targeted improvements, it is possible to overcome production challenges and deliver superior machine tool castings that enhance the performance and longevity of precision machinery. Future work may explore further alloy combinations or advanced inoculation techniques to push the boundaries of what is achievable with gray iron machine tool castings.