The pursuit of higher power density, improved fuel efficiency, and reduced manufacturing costs in modern diesel engines places continually escalating demands on critical components, particularly the crankshaft. As the core element responsible for converting reciprocating piston motion into rotational power, the crankshaft endures complex loading conditions including high-cycle bending and torsional fatigue, wear, and impact. Traditionally, forged steels such as 42CrMo and 45 steel have been the materials of choice for high-power, multi-cylinder engines due to their superior strength and toughness. However, their significant material and processing costs directly impact the overall competitiveness of the engine.
In this context, the development of high-performance nodular cast iron (ductile iron) presents a compelling alternative. Nodular cast iron offers a unique combination of properties: excellent castability for near-net-shape manufacturing leading to lower cost, good machinability, inherent damping capacity that reduces NVH (Noise, Vibration, and Harshness), and satisfactory wear resistance. Historically, grades like QT700-2 or QT800-2 have been successfully deployed in lower-load applications. The challenge for applying nodular cast iron to heavy-duty six-cylinder engines lies in reliably achieving and exceeding a stringent set of mechanical properties—specifically, a tensile strength ($R_m$) greater than 860 MPa, elongation ($A$) over 5%, and high bending fatigue strength—while ensuring consistent internal soundness in complex castings.
This work details a systematic development program aimed at engineering a high-strength nodular cast iron crankshaft capable of replacing forged steel counterparts in a six-cylinder diesel engine. The strategy integrates three core pillars: advanced material composition design, optimized foundry technology utilizing sand-coated iron mold (SCIM) casting, and a tailored heat treatment process. The goal is to achieve a fine, uniform microstructure free from defects, thereby unlocking the full potential of nodular cast iron for this demanding application.
Holistic Development Strategy and Material Optimization
The development philosophy was rooted in the fundamental principle that the final properties of nodular cast iron are a direct consequence of its microstructure, which in turn is governed by chemical composition, solidification conditions, and heat treatment. A synergistic approach was therefore adopted, where each stage of the process was designed to complement the others.
The primary objective of the material design was to achieve a fully pearlitic (or sorbitic) matrix with a high nodule count and perfect nodularity, ensuring high strength and good ductility. The base composition was carefully calibrated. Carbon (C) and Silicon (Si) contents were optimized to ensure good castability and graphitization potential while avoiding the formation of excessive primary carbides or graphite flotation. The Carbon Equivalent (CE) was a critical parameter controlled within a tight window:
$$CE = \%C + \frac{1}{3}(\%Si + \%P)$$
Alloying with Copper (Cu) and Antimony (Sb) was the key to enhancing performance without compromising processability. Copper, a mild graphitizer, dissolves in the ferrite/pearlite matrix, providing solid solution strengthening and significantly enhancing the hardenability and uniformity of microstructure, especially in heavier sections. Antimony, a powerful pearlite promoter, refines the pearlitic lamellae and improves the roundness of graphite nodules. However, its addition must be precisely controlled, as excess antimony can lead to carbide formation and embrittlement. Trace amounts of Molybdenum (Mo) were incorporated in subsequent trial runs to further improve hardenability and high-temperature stability. The target chemical composition is summarized in Table 1.
| Element | Target Range | Primary Function |
|---|---|---|
| C | 3.4 – 3.8 | Graphite formation, fluidity |
| Si | 1.7 – 2.2 | Graphitizer, strengthens ferrite |
| Mn | 0.4 – 0.5 | Pearlite promoter, solid solution strengthener |
| P | ≤ 0.03 | Minimized to avoid phosphide eutectic |
| S | ≤ 0.015 | Minimized for effective nodulization |
| Cu | 0.4 – 0.5 | Matrix strengthening, improves uniformity |
| Sb | 0.006 – 0.010 | Refines pearlite, improves nodularity |
| Mo | 0.15 – 0.30 (Pilot) | Enhances hardenability and thermal stability |
Advanced Casting Process: Simulation and Sand-Coated Iron Mold Technology
To translate the optimized chemistry into a sound casting, the foundry process was critically examined. Traditional resin sand molding, while flexible, often results in slower cooling rates, which can lead to coarser microstructures and a higher risk of shrinkage porosity in hot spots like the crankpin-to-web junctions.
Computer-aided simulation of solidification and feeding was employed upfront to design the gating and risering system. The simulation predicted thermal gradients and identified potential shrinkage defects, allowing for the optimization of riser size and placement to ensure directional solidification towards the feeders. The final design featured a side-gated system with carefully calculated choke ratios.
The pivotal process change was the adoption of the Sand-Coated Iron Mold (SCIM) process. In this method, a thin shell (typically 4-6 mm) of resin-bonded sand is formed over a reusable metal mold. This hybrid approach combines the advantages of both methods: the high and consistent cooling rate provided by the metal mold, and the flexibility and surface finish afforded by the precision sand layer. The significantly faster heat extraction compared to full sand molds has several profound benefits for nodular cast iron:
- Increased Undercooling: Leads to a much higher nucleation rate for both graphite nodules and the austenite matrix.
- Refined Microstructure: Results in a finer pearlitic matrix and a larger population of smaller graphite nodules.
- Improved Structural Soundness: Promotes quicker passage through the pasty freezing range, reducing the time available for shrinkage pore formation.
The relationship between cooling rate ($\dot{T}$), nodule count ($N_v$), and secondary dendrite arm spacing (SDAS, $\lambda_2$) is well-established and can be approximated by:
$$N_v \propto (\dot{T})^n \quad \text{and} \quad \lambda_2 \propto (\dot{T})^{-m}$$
where *n* and *m* are positive exponents. The SCIM process effectively increases $\dot{T}$, thereby increasing $N_v$ and decreasing $\lambda_2$, both contributing to enhanced mechanical properties.
Melting was conducted in a medium-frequency induction furnace. Treatment involved a sandwich method in the ladle using a FeSiMgRE alloy for nodulization, followed by a primary inoculant. A stream inoculation was applied during pouring to combat fade and ensure a high nodule count. Pouring temperature was tightly controlled between 1380°C and 1400°C.
Microstructure Transformation through Isothermal Normalizing Heat Treatment
The as-cast microstructure of alloyed nodular cast iron, while strong, often contains a mixture of pearlite, ferrite, and possibly some free carbides from the alloying additions (Sb, Mn). To achieve a homogeneous, high-strength sorbitic matrix (very fine pearlite) and eliminate any detrimental carbides, a tailored two-stage austenitizing followed by isothermal transformation was developed.
The heat treatment cycle, as shown in the schematic below, was designed based on the principles of the Time-Temperature-Transformation (TTT) diagram for alloyed nodular cast iron:
- Stage 1 Austentitizing (920°C): The higher initial temperature ensures the complete dissolution of any free carbides (e.g., cementite) that may be present, particularly from antimony.
- Stage 2 Austenitizing (900°C): The temperature is then lowered to homogenize the austenite carbon content without causing excessive grain growth. This step is crucial for achieving a uniform transformation product later.
- Isothermal Transformation (600°C): The parts are rapidly transferred to a salt bath or furnace held at 600°C. At this temperature, which is just below the pearlite “nose” of the TTT curve, the austenite transforms isothermally to a very fine mixture of ferrite and cementite known as sorbite. This structure offers an optimal balance of strength and ductility, superior to coarse pearlite. Holding at this temperature until transformation is complete also minimizes residual stresses compared to continuous cooling.
The final microstructure target was a matrix consisting of >90% fine sorbite, with a nodule count >140 nodules/mm², nodularity >90%, and virtually no free carbides or phosphide eutectic.
Comprehensive Performance Evaluation of the Developed Nodular Cast Iron Crankshaft
A pilot batch of six-cylinder crankshafts was produced using the integrated SCIM and isothermal normalizing process. Extensive testing was conducted on samples machined from various critical locations along the crankshaft axis (e.g., web regions, journal fillets) to evaluate uniformity and ensure all specifications were met.
Metallographic and Microstructural Analysis
Microstructural examination confirmed the success of the process. Table 2 summarizes the key metallographic parameters at different sampling locations, demonstrating excellent consistency.
| Sample Location (Web #) | Nodularity Grade | Nodule Count (nodules/mm²) | Matrix Constituent | Free Carbide Content |
|---|---|---|---|---|
| 1 | 2 | 155 ± 5 | >90% Sorbite | < 1% |
| 5 | 2 | 171 ± 5 | >90% Sorbite | < 1% |
| 9 | 2 | 167 ± 5 | >90% Sorbite | < 1% |
| 12 | 2 | 145 ± 5 | >90% Sorbite | < 1% |
The microstructure was characterized by a uniform, fine sorbitic matrix with well-dispersed, spherical graphite nodules (Grade 6 size). No shrinkage porosity, slag inclusions, or chilling was detected via non-destructive testing and sectioning, validating the casting simulation and SCIM process. The refined structure is a direct result of the fast cooling from the SCIM process and the subsequent isothermal heat treatment.
Static Mechanical Properties
Tensile tests and hardness measurements were performed on specimens extracted from the crank webs. The results, presented in Table 3, not only meet but substantially exceed the baseline requirement of $R_m$ ≥ 860 MPa and $A$ ≥ 5%. The high yield strength ($R_{p0.2}$) is particularly noteworthy for fatigue resistance.
| Property | Average Value | Range (Across Locations) | Comparison: 45 Steel Forging | Comparison: 42CrMo Steel Forging |
|---|---|---|---|---|
| Tensile Strength, $R_m$ | 946 MPa | 925 – 964 MPa | ~640 MPa | ~1080 MPa |
| Yield Strength, $R_{p0.2}$ | 607 MPa | 595 – 620 MPa | ~360 MPa | ~930 MPa |
| Elongation, $A$ | 5.6 % | 5.2 – 6.3 % | >16 % | ~12 % |
| Brinell Hardness (Core) | 285 HB | 278 – 294 HB | ~240 HB | ~288 HB |
The hardness on the main and connecting rod journals after induction hardening and grinding was consistently between 59-62 HRC, with minimal variation (<2 HRC), meeting the requirement for wear resistance.
Bending Fatigue Performance
Fatigue life is the ultimate functional criterion for a crankshaft. Finished crankshafts, processed through induction hardening of the journals and fillet rolling for residual compressive stresses, were subjected to fully-reversed bending fatigue tests. Under a constant bending moment of 2900 N·m, all tested crankshafts achieved the run-out limit of 10×107 cycles without failure. This demonstrates that the high-strength nodular cast iron, when processed correctly, possesses exceptional fatigue strength, rivaling that of forged steel components in this application. The fatigue limit ($\sigma_f$) of nodular cast iron is strongly influenced by its tensile strength and microstructure homogeneity, often following a relationship like:
$$\sigma_f \approx 0.4 \times R_m \quad \text{(for high-quality, defect-free material)}$$
For our material with $R_m$ = 946 MPa, this suggests a fatigue limit near 378 MPa, which aligns with the successful test results under the applied stress.
The refined sorbitic matrix provides a high intrinsic strength, while the spherical graphite nodules act as less severe stress raisers compared to sharp inclusions or micro-shrinkage. The fillet rolling process introduces deep compressive stresses at the most critical stress-concentration points, further shielding the material from crack initiation.
Industrial Application and Cost-Benefit Analysis
The successful pilot batch led to a full-scale trial production. The process demonstrated high stability and a yield rate of 100%. The performance data conclusively proved that this grade of high-strength nodular cast iron is fully capable of replacing both 45 steel and 42CrMo forged steel crankshafts in this six-cylinder diesel engine model, meeting all technical specifications for static strength, hardness, and—most importantly—bending fatigue life.
The economic advantage is transformative. A detailed cost analysis reveals that the unit cost for the high-strength nodular cast iron crankshaft rough casting is approximately 44% lower than a 45 steel forging and a staggering 66% lower than a 42CrMo steel forging. This cost reduction stems from several factors: the lower raw material cost of iron versus alloy steel, the near-net-shape casting process which dramatically reduces machining scrap and energy consumption compared to forging and extensive machining from a billet, and the higher material utilization efficiency.
Conclusion
This development project successfully engineered a high-performance, cost-effective crankshaft solution for heavy-duty diesel engines through the intelligent integration of material science and advanced manufacturing processes. The key outcomes are:
- A novel alloy design based on nodular cast iron, optimized with Cu and Sb, and supplemented with Mo, was formulated to target a high-strength, fully sorbitic matrix.
- The Sand-Coated Iron Mold (SCIM) casting process was decisively selected and implemented. Its rapid cooling capability fundamentally refined the as-cast microstructure, increasing nodule count and ensuring casting soundness, providing a superior foundation for final properties.
- A two-stage austenitization followed by isothermal transformation heat treatment was developed. This process guarantees the complete dissolution of carbides and the formation of a homogeneous, fine sorbitic matrix, optimizing the balance between tensile strength ($R_m$ ~946 MPa) and ductility ($A$ ~5.6%).
- The comprehensive performance evaluation confirmed that the developed high-strength nodular cast iron crankshaft exceeds all technical requirements, including achieving a bending fatigue life beyond 10 million cycles at 2900 N·m. Its core properties surpass those of a standard 45 steel forging and approach those of a 42CrMo forging, making it a technically viable substitute.
- The transition to this high-strength nodular cast iron results in drastic manufacturing cost reductions of 44-66%, offering a significant competitive advantage without compromising performance or durability.
This work validates that through systematic optimization of composition, solidification control, and heat treatment, modern high-strength nodular cast iron can be elevated to meet the challenges of the most demanding powertrain components, delivering an unparalleled combination of performance, quality, and cost efficiency.