In our extensive research on engine cylinder head materials, we have focused on the development and production stability of high-grade vermicular iron, often compared to traditional gray iron casting due to its widespread use. Gray iron, particularly in forms like gray iron casting and grey iron, has been a staple in the industry for its excellent machinability and thermal conductivity. However, the increasing demands for higher performance and durability in heavy-duty diesel engines have driven the adoption of vermicular iron, which offers superior tensile strength and fatigue resistance. This paper elaborates on our findings regarding the production challenges and solutions for high-grade vermicular iron, drawing parallels with gray iron casting to highlight advancements. We will explore material development, common production issues, and optimization strategies through detailed tables and formulas to summarize key insights.
The journey of vermicular iron began internationally in the 1940s, with early studies identifying its unique graphite morphology介于片状和球状石墨之间. In contrast, gray iron casting has long been favored for its cost-effectiveness and ease of production. For instance, gray iron typically exhibits lower tensile strength but better thermal conductivity, making it suitable for applications where heat dissipation is critical. Our analysis of material properties, as summarized in Table 1, shows a clear comparison among ordinary gray iron, alloyed gray iron, and vermicular iron. This table underscores why vermicular iron is gaining traction despite the challenges, as it provides a balance of strength and performance that gray iron casting cannot match.
| Property | Ordinary Gray Iron (HT250) | Alloyed Gray Iron (HT300) | Vermicular Iron (RuT450) |
|---|---|---|---|
| Tensile Strength (MPa) | 250 | 300 | 450 |
| Elastic Modulus (GPa) | 105 | 115 | 145 |
| Thermal Conductivity (W/m·K) | 46 | 38 | 36 |
| Bending Fatigue Strength (MPa) | 150 | 170 | 300 |
In our production experience, high-grade vermicular iron for cylinder heads requires stringent technical specifications: hardness between 200-260 HB, pearlite content ≥85%, vermicularization rate ≥80%, and tensile strength ≥400 MPa. These demands often lead to stability issues not commonly seen in gray iron casting. For example, we observed significant fluctuations in material properties, such as tensile strength ranging from 400 to 530 MPa and vermicularization rates dropping as low as 75%. Additionally, shrinkage porosity at local hot spots, like bolt holes and guide holes, posed major defects. This variability highlights the narrow stable zone for vermicular iron compared to the more forgiving nature of grey iron. The effective magnesium content control range is approximately 0.008%, as described by the relationship: $$ \text{Mg}_{\text{effective}} = k \cdot \text{CE} $$ where CE is the carbon equivalent, and k is a constant dependent on processing conditions. This formula illustrates the delicate balance required, unlike in gray iron casting, where such precise control is less critical.
To address these challenges, we implemented an OCC-based production process, which involves precise control systems for wire feeding and real-time analysis. This method contrasts with traditional gray iron casting processes, where simplicity often leads to higher consistency. Our production flowchart, as outlined in the OCC system, includes melting in electric furnaces, component analysis via spectrometers and thermal analyzers, and controlled wire feeding. Key to this is maintaining raw material stability, as fluctuations in elements like sulfur and oxygen can derail the process. For instance, in gray iron casting, sulfur levels might be tolerated up to 0.04%, but for vermicular iron, we严格控制硫含量 below 0.02% to avoid multiple desulfurization steps that increase production time and variability.
In terms of raw material control, we focused on selecting high-quality pig iron, scrap steel, carburizers, and returns. Table 2 details the chemical composition requirements for pig iron, emphasizing low sulfur content to minimize interference with vermicularization. This is a stark contrast to gray iron casting, where higher sulfur levels are often acceptable. Similarly, for scrap steel, we prioritized单一来源 materials with stable titanium content (0.05-0.08%), as titanium helps expand the stable vermicularization zone by preventing excessive nodular graphite formation. The formula for carbon equivalent, $$ \text{CE} = C + \frac{1}{3}Si $$, is critical here; we aimed for a CE that places the molten iron near the eutectic point to reduce shrinkage tendencies, typically with a liquidus temperature of 1136-1146°C. This approach reduces the shrinkage propensity, which is less of a concern in grey iron due to its different solidification behavior.
| Grade | C (%) | Si (%) | Mn (%) | P (%) | S (%) |
|---|---|---|---|---|---|
| Z14 | ≥4.0 | 1.25-1.60 | ≤0.50 | ≤0.06 | ≤0.04 |
| Q10 | ≥4.0 | 0.8-1.0 | ≤0.20 | ≤0.06 | ≤0.03 |
| Q12 | ≥4.0 | 1.0-1.4 | ≤0.50 | ≤0.06 | ≤0.03 |
Optimizing the gating system was another critical area. In gray iron casting, simple designs often suffice, but for vermicular iron, we adjusted iron feeding positions and incorporated chills to address local hot spots. For example, in a 9L cylinder head, moving the feeding point away from shrinkage-prone areas reduced defects by over 30%. Similarly, using external chills and sand core chills effectively minimized porosity in bolt and guide holes. This level of optimization is rarely necessary in grey iron, where the material’s inherent properties handle thermal stresses better. The solidification behavior can be modeled using equations like $$ \frac{\partial T}{\partial t} = \alpha \nabla^2 T $$, where T is temperature, t is time, and α is thermal diffusivity, highlighting how vermicular iron’s narrower solidification range demands precise thermal management compared to gray iron casting.
Melting process improvements were pivotal for consistency. We optimized charge ratios, using 20% pig iron, 40% scrap steel, and 40% returns, balancing strength and shrinkage tendencies. This ratio contrasts with gray iron casting, where higher scrap proportions might be used without significant issues. Additionally, we developed contingency plans for off-spec iron, such as secondary wire feeding for under-vermicularized iron or adding raw iron for over-vermicularized batches. For instance, if the magnesium index (a key parameter in OCC systems) falls below target, we add rare earth elements or perform secondary wire feeding to correct it, as summarized in Table 3. This adaptability is less common in grey iron production, where reprocessing is simpler due to broader tolerance ranges.
| Initial Mg Index | Correction Method | Added Material | Resulting Vermicularization Rate (%) | Tensile Strength (MPa) |
|---|---|---|---|---|
| 11.2 | Rare Earth Addition | 1.0 kg | 90 | 436 |
| 10.3 | Secondary Wire Feeding | 4.0 m | 90 | 441 |
| 17.7 | Raw Iron Addition | 60 kg | 90 | 471 |
Detection and control strategies were enhanced through combined methods, including thermal analysis, spectrometry, and traditional wedge tests. For example, we use the carbon equivalent formula $$ \text{CE} = C + \frac{1}{3}Si + \frac{1}{4}P $$ in some cases to account for phosphorus influences, ensuring accurate control. In gray iron casting, such comprehensive checks are often unnecessary due to the material’s robustness. Moreover, we optimized wire composition and particle size distribution to improve absorption consistency, reducing issues like wire breakage or incomplete melting. This attention to detail underscores the higher complexity of vermicular iron production compared to standard grey iron processes.
In conclusion, our research demonstrates that high-grade vermicular iron offers significant advantages over gray iron casting in terms of strength and fatigue resistance, but its production requires meticulous control of raw materials, gating systems, and melting processes. By learning from the stability of gray iron casting, we have developed strategies to manage the narrow stable zone of vermicular iron, such as optimizing charge ratios, implementing secondary treatments, and using advanced detection methods. These approaches have improved production continuity and reduced defect rates, making vermicular iron a viable alternative for high-performance engine components. Future work will focus on further refining these methods to bridge the gap between the simplicity of grey iron and the advanced requirements of modern applications.