The pursuit of advanced engineering materials that combine excellent mechanical properties with economic viability is a constant driver in manufacturing. Among these, ductile iron casting stands out for its unique combination of castability, good strength, and appreciable toughness derived from its spherical graphite morphology. However, a traditional limitation of conventional grades has been the inverse relationship between strength and ductility; high-strength pearlitic grades often exhibit limited elongation, while highly ductile ferritic grades lack sufficient strength. This trade-off restricts their application in demanding sectors. Therefore, developing a new generation of as-cast ductile iron with harmonized high strength and toughness without resorting to energy-intensive heat treatments represents a significant technological and economic advancement. This work focuses on characterizing the dry sliding wear behavior of such a newly developed high-performance as-cast QT800-5 ductile iron, investigating the influence of critical operational parameters and elucidating the underlying wear mechanisms.
The development of this material centers on a strategic alloy design and processing methodology within a standard green sand casting process. The chemical composition is pivotal, employing a composite alloying approach with Si, Cu, Ni, Mo, and Sn to tailor the matrix structure and enhance solid solution strengthening. The base materials include high-quality pig iron, steel scrap, and returns. After melting and superheating, a multi-stage inoculation process is critical for achieving a high nodule count and a fine, uniform microstructure. This involves a base inoculation, followed by a wire-feeding treatment for both nodulization (using FeSiMg wire) and post-inoculation, a transfer ladle inoculation, and a final flow inoculation during pouring. This rigorous processing protocol ensures the desired as-cast microstructure and properties.
The resulting microstructure from this optimized ductile iron casting process is characterized by a high nodularity exceeding 90%, with an average graphite nodule diameter of approximately 15.3 µm and a nodule count of about 420 nodules/mm². The matrix consists of a mixture of pearlite and ferrite, with the pearlite fraction being dominant at around 88%. A key feature is the refined interlamellar spacing within the pearlite colonies, measured to be about 0.28 µm. This fine microstructure, combined with the solid solution strengthening from alloying elements, yields a compelling set of mechanical properties in the as-cast state: a tensile strength of 843 MPa, a yield strength of 577 MPa, an elongation of 6.3%, and a hardness of 263 HB. This successfully demonstrates the achievement of high strength coupled with good toughness directly after casting.
To evaluate the tribological performance of this high-strength ductile iron casting, a series of dry sliding wear tests were conducted using a ball-on-disc configuration on a high-temperature tribometer. An Al₂O₃ ball served as the counterbody. The wear rate was calculated from the measured dimensions of the wear track. The investigation systematically varied three key parameters: normal load (5, 10, 15, 20 N), sliding speed (0.1, 0.2, 0.3, 0.4 m/s), and ambient temperature (25, 50, 100 °C), while keeping other parameters constant as per standard test procedures.
The wear rate as a function of these parameters is summarized in the table below. The data clearly shows that the material offers good wear resistance under mild conditions but is significantly influenced by increasing severity of the test parameters.
| Test Parameter | Values | Wear Rate (10⁻⁶ mm³/m) | Trend |
|---|---|---|---|
| Normal Load (at 25°C, 0.2 m/s) | 5 N | ~4.50 | Gradual increase, sharp rise above 15 N |
| 10 N | ~5.80 | ||
| 15 N | ~7.27 | ||
| 20 N | 16.25 | ||
| Sliding Speed (at 25°C, 15 N) | 0.1 m/s | ~6.50 | Steady increase, highest at max speed |
| 0.2 m/s | ~7.27 | ||
| 0.3 m/s | ~13.50 | ||
| 0.4 m/s | 19.23 | ||
| Ambient Temperature (at 15 N, 0.2 m/s) | 25 °C | 7.27 | Very sharp exponential increase |
| 50 °C | 32.77 | ||
| 100 °C | 55.54 |
The contact mechanics for the ball-on-disc test can be described by Hertzian theory. The maximum contact stress (σ_max) and the relative displacement (S) are given by:
$$
\sigma_{\text{max}} = 0.388 \sqrt[3]{\frac{P E^2}{R^2}} \quad \text{and} \quad S = 1.231 \sqrt[3]{\frac{P^2}{E^2 R}}
$$
where $P$ is the normal load, $E$ is the composite elastic modulus, and $R$ is the ball radius. Although the calculated contact stresses are high (e.g., ~1.43 GPa at 15 N), the wear rates at lower loads remain within a mild wear regime, underscoring the inherent wear resistance of this ductile iron casting.
Analysis of the wear scar morphology via SEM and 3D profilometry provides insights into the active wear mechanisms. Under varying normal loads at room temperature, the wear surfaces showed evidence of adhesive wear as the dominant mechanism. At low loads (5 N), the surface was relatively smooth with minor grooves and some small delamination pits. Graphite nodules within the track remained largely intact. As the load increased to 20 N, the damage became more severe: graphite nodules were heavily deformed or smeared, and the number and size of delamination pits increased significantly, indicating pronounced material transfer and adhesive tearing.
The effect of sliding speed at a constant load (15 N) and temperature (25°C) revealed a transition in surface features. At lower speeds (0.1-0.2 m/s), grooves and delamination pits were observed. As the speed increased to 0.4 m/s, the wear surface exhibited large, continuous patches of adhered material and oxide layers. The increased sliding speed generates more frictional heat, leading to thermal softening of the near-surface material and enhanced oxidation. The wear mechanism thus evolves from primarily adhesive wear to a combination of adhesive and oxidative wear at higher speeds.
The most dramatic effect was observed with increasing ambient temperature. At 25°C, the wear track showed fine grooves and some evidence of graphite smearing, acting as a solid lubricant. At elevated temperatures (50°C and 100°C), the wear surfaces were characterized by deep, coarse grooves, extensive oxidation, and large, continuous adhered layers. The increase in ambient temperature, compounded by frictional heating, significantly reduces the material’s surface hardness and accelerates oxide formation. These oxides can fracture into hard abrasive particles. Consequently, the wear mechanism under elevated temperature is a severe mix of oxidative wear, adhesive wear, and abrasive wear from the oxide debris.
The 3D profilometry data quantitatively supports the SEM observations. The width and depth of the wear tracks increased with all three parameters. For normal load, depth increased sharply from 0.54 µm at 15 N to 0.94 µm at 20 N. For sliding speed, both width and depth grew substantially, with depth reaching 1.68 µm at 0.4 m/s. The most severe dimensional increase was under elevated temperature, where the depth surged from 1.54 µm at 50°C to 2.26 µm at 100°C, directly correlating with the exponential rise in wear rate.
The dominance of different wear mechanisms can be conceptually summarized. Under the primary influence of normal load, the cyclic stress and strong adhesive forces between the asperities of the ductile iron casting and the Al₂O₃ ball lead to plastic deformation, subsurface crack nucleation, and eventual material removal as delaminated sheets. This is classic adhesive wear. When sliding speed is the key variable, frictional heating becomes significant. This heat promotes surface oxidation and thermal softening. The process involves adhesive material transfer, but the formed oxides continually fracture and may act as a mild abrasive, leading to a mixed adhesive-oxidative mechanism. Under high ambient temperature, the bulk material temperature rise is substantial. This causes severe softening and prolific oxidation. The contact surface is covered by a thick, unstable oxide layer that is constantly removed and reformed. The removed oxide debris participates in three-body abrasion, creating deep grooves. Simultaneously, the softened metal readily adheres to the counterface. Thus, the regime is characterized by simultaneous and severe oxidative, abrasive, and adhesive wear.
In conclusion, this study demonstrates the successful development of a high-strength, high-toughness QT800-5 grade via advanced ductile iron casting techniques employing composite alloying and intensive inoculation. This as-cast material exhibits a favorable combination of tensile strength over 840 MPa and elongation above 6%. Its wear resistance is excellent under mild operating conditions of room temperature, low load, and low sliding speed, with wear mechanisms dominated by adhesive wear. However, its performance degrades under more severe conditions, particularly with increasing temperature, where oxidative and abrasive mechanisms become prominent and dramatically increase the wear rate. The findings provide crucial design guidelines for applying this advanced as-cast ductile iron casting in components where weight-saving, high strength, good toughness, and controlled wear resistance are simultaneously required, potentially replacing more expensive or heavily processed materials.