The crankshaft stands as the central and critically loaded component within an internal combustion engine, translating the reciprocating motion of pistons into rotational torque. Its performance, longevity, and reliability are paramount to the engine’s overall operation. For heavy-duty applications, particularly in large-bore, six-cylinder engines, the material demands are exceptionally high, requiring an outstanding combination of high tensile strength, good ductility, high fatigue resistance, and excellent wear characteristics. Traditionally, forged steel was the material of choice for such demanding components. However, with significant advancements in ductile iron casting technology and metallurgical control, high-grade ductile iron has successfully emerged as a superior alternative for heavy-duty crankshafts.
The shift from forged steel to ductile iron casting offers a compelling array of advantages. Ductile iron crankshafts are more cost-effective to manufacture, simplify production logistics, and exhibit inherent vibration damping properties. Crucially, they possess a high degree of rigidity, excellent fatigue strength, and a notable insensitivity to surface notches or micro-cracks compared to their steel counterparts. The primary challenge lies in achieving the required mechanical properties in the as-cast condition to avoid the costs, energy consumption, and potential distortion associated with subsequent heat treatment processes like normalizing. This article details the comprehensive development and production methodology for a heavy-duty, six-cylinder crankshaft made from as-cast ductile iron with a guaranteed minimum tensile strength of 850 MPa and a minimum elongation of 4.5%, designated as grade QT850-4.5.
Production Infrastructure and Foundry Methodology
The successful production of high-integrity ductile iron casting components for heavy-duty applications hinges on a synergistic combination of precise process control, suitable equipment, and high-quality raw materials.
Foundry Equipment and Process Selection
Melting was conducted using a 1-ton medium-frequency induction furnace, which provides excellent control over temperature and composition with minimal element loss. The defining manufacturing process employed was the Iron Mold with Sand Lining technique. This process involves creating a thin layer (4-8 mm) of resin-coated sand on the internal cavity of a reusable metal mold. This hybrid method offers distinct advantages critical for producing sound, high-performance ductile iron casting:
- Rapid Heat Extraction: The metal mold provides rapid cooling of the molten metal after pouring, promoting a fine, dense microstructure.
- High Mold Rigidity: The combination of the rigid iron mold and the cured sand lining creates a mold of high strength and dimensional stability.
- Exploitation of Graphite Expansion: The rigid mold constrains the casting during solidification, allowing the expansion associated with graphite nodule formation to counter the shrinkage of the iron matrix, effectively enabling natural feeding and minimizing shrinkage porosity.
- Enhanced Yield and Quality: This natural feeding mechanism significantly improves casting yield and reduces scrap rates related to shrinkage defects.
- Superior Surface Finish: Castings produced by this method exhibit excellent surface finish, typically achieving a roughness of Ra 12.5 μm or better after shot blasting.
For quality assurance, a suite of analytical equipment was utilized: chemical titration and CS analyzers for carbon and sulfur, a 19-channel Baird optical emission spectrometer for full elemental analysis, a 30-ton hydraulic universal testing machine for tensile properties, and a Brinell hardness tester.
Stringent Raw Material Selection
The foundation of a high-strength, high-ductility ductile iron casting is laid with the careful selection of raw materials. Impurities and trace elements must be meticulously controlled.
- Base Iron: High-purity pig iron or specially processed low-residual base iron is essential. This is critical to prevent the formation of degenerate graphite forms (like chunky or exploded graphite) in the thermal centers of heavy sections and to ensure superior mechanical properties. Key elements in the base iron must be minimized: Si < 1.0%, Mn < 0.2%, S < 0.01%, P < 0.04%. Anti-nodularizing (or “poisoning”) elements such as Ti, V, Sn, and W must be strictly controlled, with Titanium kept below 0.04%. The total content of trace elements (∑T) should ideally be less than 0.1%.
- Steel Scrap: Only high-quality, low-alloy carbon steel scrap should be used. Manganese content should be low (Mn < 0.4%), and the scrap must be free from contaminants and excessive levels of the harmful trace elements mentioned above.
Metallurgical Design: Composition and Rationale
The chemical composition of ductile iron is the primary lever for controlling its microstructure and, consequently, its mechanical and casting properties. The target composition was developed through extensive trials and is summarized in Table 1.
| State / Element | C | Si | Mn | P | S | Mg | RE | Mo | Ni | Cu |
|---|---|---|---|---|---|---|---|---|---|---|
| Base Iron | 3.6-3.8 | 1.3-1.4 | 0.3-0.4 | <0.03 | 0.015-0.020 | – | – | 0.15-0.20 | 0.2-0.4 | – |
| Final (Treated) | – | 2.1-2.5 | – | – | <0.01 | 0.035-0.045 | 0.01-0.02 | – | – | 0.7-0.8 |
The rationale behind this composition is detailed below:
- Carbon Equivalent (CE): The CE, calculated as $$CE = \%C + \frac{\%Si}{3}$$, profoundly influences fluidity, shrinkage behavior, and graphitization potential. A high CE improves fluidity and feeding but risks graphite flotation. For this heavy-section casting, the target was carefully balanced between 4.4% and 4.5% to ensure soundness without defects.
- Carbon (C): High carbon content (3.6-3.8%) promotes a high nodule count, smaller nodule size, and improved nodularity. It enhances the natural feeding effect during solidification, leading to denser castings. The upper limit is constrained by the risk of graphite flotation.
- Silicon (Si): Silicon is a potent graphitizer. It is primarily added via inoculation. A final Si content of 2.1-2.5% ensures sufficient graphitization force to achieve a fully ferritic-pearlitic matrix in the as-cast state, avoiding undesirable carbides, and contributes to strength solid-solution hardening.
- Manganese (Mn): Manganese is a mild pearlite promoter but has a strong tendency to segregate at cell boundaries, where it can form embrittling carbides. Its content is kept as low as possible (0.3-0.4%) solely from the raw materials, with no intentional addition.
- Phosphorus (P) and Sulfur (S): P forms low-melting, brittle phosphide eutectics at grain boundaries, severely impacting toughness. It is kept below 0.04%. S is a powerful anti-nodularizer. A small but controlled base S level (0.015-0.02%) is maintained before treatment because MgS/MgCeS particles can act as nucleation sites for graphite. After treatment, S must be below 0.01% to ensure good nodularization.
- Magnesium (Mg) and Rare Earths (RE): Mg is the essential nodularizing element. Residual Mg is tightly controlled between 0.035% and 0.045%. RE (typically Cerium-Lanthanum mixtures) are added with the Mg alloy to counteract the harmful effects of trace elements like Pb, Sb, and Ti. Residual RE is kept low (0.01-0.02%) to avoid causing graphite degeneration.
- Alloying Elements (Cu, Mo, Ni): To achieve the high strength (850 MPa) in the as-cast condition without quenching, pearlite content must be high and refined. A multi-alloying approach is used:
- Copper (Cu, 0.7-0.8%): A very effective pearlite promoter with minimal segregation tendency. It refines the pearlite lamellae and enhances strength and hardness with a moderate impact on ductility.
- Molybdenum (Mo, 0.15-0.20%): A strong hardenability agent and carbide former. It increases strength and refines the matrix but must be carefully controlled as excess Mo severely reduces ductility and promotes intercellular carbides.
- Nickel (Ni, 0.2-0.4%): A graphitizing element that promotes pearlite formation while reducing section sensitivity (the tendency for properties to vary from surface to center). It helps to achieve a uniform, high-strength microstructure throughout the heavy crankshaft section.
The synergistic effect of these alloying elements can be conceptually modeled for tensile strength (TS in MPa) as a function of matrix constituents. While precise empirical formulas are complex, a simplified representation highlighting key contributors is:
$$TS \approx f_{pearlite} \cdot (A \cdot \%Cu + B \cdot \%Mo + C) + f_{ferrite} \cdot D + \sigma_{nodule}$$
where $f_{pearlite}$ and $f_{ferrite}$ are the volume fractions of pearlite and ferrite, A, B, C, D are constants, and $\sigma_{nodule}$ represents the strengthening contribution from the graphite nodule dispersion.
Melting, Pretreatment, and Treatment Protocol
The melting and treatment sequence is a critical phase in ductile iron casting production, determining the final nodule count, morphology, and matrix structure.
- Charge and Melting: The furnace is charged with the selected high-purity pig iron and steel scrap. Graphitizing recarburizer is added to adjust the carbon content to the target range. Molybdenum and Nickel master alloys are added to the molten bath once the charge is fully liquid, approximately 3-5 minutes before tapping, to ensure complete dissolution.
- Silicon Carbide (SiC) Pretreatment: A key step involved the addition of silicon carbide (SiC) to the superheated molten metal. SiC acts as a potent preconditioner. It dissolves endothermically, providing heterogeneous nuclei for future graphite formation and helping to reduce the oxygen potential of the melt. This step is crucial for achieving a fine, uniform graphite structure in heavy sections and is represented by the dissolution reaction: $$SiC_{(s)} \rightarrow Si_{(l)} + C_{(dissolved\ in\ Fe)}$$
- Tapping and Treatment: The treated iron is tapped at approximately 1,540°C into a preheated 1-ton ladle. The treatment station uses the sandwich (or冲入法) method:
- The bottom of the ladle’s well is filled with a precisely weighed amount of nodularizing alloy (containing 5-7% Mg, 2-3% RE, balance Si-Fe).
- Copper shot is placed over the nodularizer to aid in submergence and reaction control.
- A primary inoculant (0.6-0.7% of the Fe weight, 3-8 mm granular, Ba/Ca-bearing FeSi) is placed in the stream during tapping.
The vigorous reaction ensures efficient Mg recovery (typically 30-40%) and desulfurization. The treated iron is then quickly slagged.
- Inoculation Strategy: A multi-stage inoculation approach is vital to combat fading (the loss of nucleation potency over time) and ensure a high nodule count throughout the pouring period.
- Primary Inoculation: As described, during tapping (0.6-0.7%).
- Secondary (Stream) Inoculation: During pouring of each mold, a fine-grained (0.2-0.7 mm) inoculant is added to the metal stream at a rate of 0.1-0.13%. This “instantaneous” inoculation creates a fresh population of nuclei just before solidification, maximizing graphite nucleation efficiency. The efficiency of inoculation fading can be described by a time-dependent function for nodule count, $N(t)$: $$N(t) = N_0 \cdot e^{-kt}$$ where $N_0$ is the initial nodule count and $k$ is the fading rate constant, which is minimized by using late stream inoculation.
Process Control and Solidification
Strict procedural control is maintained from treatment to shakeout:
- Temperature and Holding: The high superheat (1540°C+) followed by a brief holding period aids in slag aggregation and removal, purifying the ductile iron casting.
- Treatment-to-Pour Time: The entire process from the end of the nodularizing treatment to the completion of pouring is strictly limited to 8-10 minutes. This minimizes Mg fading, inoculation fading, and potential sulfur reversion from slag.
- Solidification in Rigid Mold: The Iron Mold with Sand Lining process is crucial. The rigid mold wall prevents expansion-driven mold wall movement, effectively “squeezing” the casting during the graphite expansion phase. This pressure compensates for the shrinkage of the austenite, dramatically reducing internal porosity. The pressure $P$ generated within the casting can be related to the expansion strain $\epsilon_g$ and the system’s rigidity: $$P \propto E_{mold} \cdot \epsilon_g$$ where $E_{mold}$ is the effective modulus of the mold system.
- Shakeout: To maximize the pearlitic transformation and achieve the desired hardness and strength, castings are shaken out from the mold as soon as they are sufficiently solid to avoid distortion, allowing for accelerated cooling in air.
Quality Validation and Results
To verify the properties of the final ductile iron casting, test coupons were taken from the crankshaft body itself (from non-critical, representative thick sections). This provides the most accurate assessment of the actual component’s properties.
Chemical Analysis
Spectrographic analysis of five separate body samples confirmed the precise adherence to the target composition ranges, as shown in Table 2.
| Sample | Si | Mn | P | S | Mg | RE | Mo | Ni | Cu |
|---|---|---|---|---|---|---|---|---|---|
| Body 1 | 2.20 | 0.30 | 0.037 | 0.010 | 0.038 | 0.015 | 0.163 | 0.222 | 0.75 |
| Body 2 | 2.15 | 0.28 | 0.036 | 0.009 | 0.037 | 0.018 | 0.181 | 0.225 | 0.72 |
| Body 3 | 2.10 | 0.34 | 0.036 | 0.008 | 0.038 | 0.017 | 0.165 | 0.254 | 0.78 |
| Body 4 | 2.35 | 0.33 | 0.037 | 0.010 | 0.036 | 0.015 | 0.168 | 0.291 | 0.77 |
| Body 5 | 2.27 | 0.31 | 0.036 | 0.010 | 0.035 | 0.017 | 0.187 | 0.291 | 0.75 |
Mechanical Properties
Tensile tests performed on specimens machined from these body samples yielded exceptional results, consistently exceeding the QT850-4.5 specification, as detailed in Table 3.
| Sample | Tensile Strength (MPa) | Elongation (%) | Hardness (HB) |
|---|---|---|---|
| Body 1 | 873 | 5.4 | 270 |
| Body 2 | 899 | 5.6 | 272 |
| Body 3 | 887 | 5.6 | 272 |
| Body 4 | 880 | 5.6 | 278 |
| Body 5 | 899 | 5.0 | 285 |
Metallographic Analysis
Microstructural examination revealed the high-quality internal structure of the ductile iron casting. The graphite was predominantly spheroidal, with a nodularity rating of Grade 2 (excellent), and a nodule size of Grade 6 (fine). The matrix consisted of approximately 95% fine, well-dispersed pearlite with negligible amounts of free carbides and phosphide eutectic (both < 0.05%). This microstructure—fine spherical graphite in a strong, predominantly pearlitic matrix—directly explains the achieved combination of high strength and good ductility.
Conclusion
The successful production of a heavy-duty, as-cast QT850-4.5 ductile iron crankshaft is a testament to the capabilities of modern ductile iron casting technology. The achievement was predicated on several interlinked factors:
- Precise Metallurgical Design: The careful balancing of carbon equivalent, silicon content, and the synergistic use of alloying elements (Cu, Mo, Ni) was fundamental to developing a composition capable of yielding high strength with sufficient ductility directly after casting.
- Foundation of High-Purity Materials: The use of low-residual base materials was non-negotiable to control harmful trace elements and ensure consistent, high-quality graphite formation throughout the heavy section.
- Optimized Treatment and Processing: The implementation of SiC pretreatment, a controlled nodularizing reaction, and a robust multi-stage inoculation strategy ensured a high, uniform nodule count and excellent graphite morphology. The selection of the rigid Iron Mold with Sand Lining process was critical to exploit the natural feeding properties of ductile iron, resulting in sound, dense castings free from shrinkage defects.
- Rigorous Process Control: Strict adherence to parameters—from superheating and slag removal to limiting the treatment-to-pour window—was essential for reproducibility and quality.
The validation tests confirm that the developed process reliably produces ductile iron casting components meeting and exceeding the stringent requirements for heavy-duty powertrain applications. The crankshafts exhibit a tensile strength consistently above 850 MPa paired with elongation values over 5.0%, all achieved in the as-cast condition, thereby eliminating the need for energy-intensive and distortion-prone heat treatments. This project underscores the position of advanced ductile iron as a superior, cost-effective engineering material for the most demanding automotive and industrial components.