In our investigation, we focused on improving the service life of excavator sleeves by employing centrifugal casting combined with austempering to produce ductile iron castings with superior mechanical and tribological properties. Ductile iron castings are renowned for their excellent combination of strength, ductility, and wear resistance, making them ideal for heavy-duty applications like construction machinery. The centrifugal casting process ensures a dense and uniform microstructure, which is critical for achieving consistent performance in demanding environments. This study delves into the microstructural characteristics, mechanical properties, and wear behavior of these ductile iron castings under both dry and lubricated conditions, providing insights into their potential for replacing traditional materials like carbon steel.
The centrifugal casting technique was utilized to fabricate sleeves with an outer diameter of 95 mm, inner diameter of 80 mm, and height of 70 mm. We prepared a total of four castings from a melt processed in a 500 kg medium-frequency induction furnace, with the pouring temperature maintained between 1540°C and 1560°C. The chemical composition of the ductile iron castings was carefully controlled, as summarized in Table 1, to include elements like copper, which enhances density and graphite distribution. After casting, the sleeves underwent austempering heat treatment: they were heated to 900°C for 1.5 hours, quenched in oil at 100°C, and then isothermally treated at 260°C for 100 minutes before air cooling. This process aims to develop a fine microstructure comprising lower bainite, spheroidal graphite, and retained austenite, which contributes to the high performance of ductile iron castings.
| Element | C | Si | Mn | Cu | S | P | Fe |
|---|---|---|---|---|---|---|---|
| Content | 3.5 | 1.9 | 0.9 | 0.5 | <0.04 | <0.04 | Balance |
To evaluate the mechanical properties, we conducted tensile tests and hardness measurements on six samples extracted from the ductile iron castings. The results, presented in Table 2, demonstrate an average tensile strength of 1099 MPa, a reduction in area of 0.5%, and an average hardness of 52.8 HRC. These values indicate that the austempering treatment significantly enhances the strength and hardness of ductile iron castings, making them suitable for high-stress applications. The microstructure examination revealed a dense arrangement of acicular lower bainite, uniformly distributed spheroidal graphite, and retained austenite, which collectively improve toughness and wear resistance. The presence of copper in the composition promotes finer bainite formation and accelerates the austempering kinetics, reducing processing time and costs for ductile iron castings.
| Sample | Tensile Strength (MPa) | Reduction in Area (%) | Hardness (HRC) |
|---|---|---|---|
| 1 | 1150 | 0.61 | 52 |
| 2 | 1068 | 0.44 | 54 |
| 3 | 1123 | 0.58 | 57 |
| 4 | 1053 | 0.38 | 49 |
| 5 | 1169 | 0.55 | 55 |
| 6 | 1034 | 0.47 | 50 |
Wear resistance tests were performed using a ball-on-disk tribometer under both dry and oil-lubricated conditions, with a GCr15 steel ball as the counterface. The normal load was set to 30 N, sliding speed to 10 mm/s, and test duration to 1800 s. The friction coefficient and mass loss were measured to assess the performance of the ductile iron castings compared to 45 steel. Under dry friction, the average friction coefficient for the ductile iron castings was approximately 0.38, which is about two-thirds that of 45 steel (0.58). In oil-lubricated conditions, the friction coefficient dropped to 0.12 for ductile iron castings,仅为 one-quarter of the 45 steel value. The wear mass loss, calculated as the percentage reduction in sample mass, further highlighted the superiority of ductile iron castings, with values of 3.59% under dry friction and 0.82% under lubrication, compared to 21.8% and 8.95% for 45 steel, respectively. These results underscore the exceptional wear resistance and self-lubricating properties of ductile iron castings, attributable to their unique microstructure.
The enhanced wear behavior of ductile iron castings can be explained through microstructural mechanisms. The fine acicular bainite and retained austenite in the matrix provide high hardness and toughness, while the spheroidal graphite acts as a solid lubricant. During wear, graphite particles detach and form a protective layer between sliding surfaces, reducing friction and wear. This self-lubricating effect is particularly pronounced in oil-lubricated conditions, where the graphite-filled pores trap lubricant, enhancing the fluid film formation. The transformation of retained austenite to martensite under stress further increases surface hardness, creating a wear-resistant “shell.” To quantify this, we can model the wear rate using the Archard equation: $$ W = \frac{K \cdot L \cdot s}{H} $$ where \( W \) is the wear volume, \( K \) is the wear coefficient, \( L \) is the load, \( s \) is the sliding distance, and \( H \) is the hardness. For ductile iron castings, the high hardness and low wear coefficient result in significantly reduced wear compared to 45 steel.
In addition to wear resistance, the mechanical properties of ductile iron castings contribute to their durability in excavator sleeves. The tensile strength and hardness data follow a normal distribution, which we can express as: $$ \sigma = \frac{\sum_{i=1}^{n} \sigma_i}{n} $$ where \( \sigma \) is the average tensile strength and \( n \) is the number of samples. The variability in properties is minimal, indicating consistent quality in ductile iron castings. Furthermore, the role of copper in refining the microstructure can be described by the equation for growth kinetics: $$ G = k \cdot t^{1/2} $$ where \( G \) is the grain size, \( k \) is a constant, and \( t \) is time. Copper addition reduces \( k \), leading to finer bainite and improved mechanical properties in ductile iron castings.
Under lubricated conditions, the friction behavior of ductile iron castings can be analyzed using the Stribeck curve, which relates friction coefficient to the Hersey number \( \eta \cdot v / P \), where \( \eta \) is viscosity, \( v \) is speed, and \( P \) is pressure. For ductile iron castings, the curve shifts downward due to the self-lubricating effect, resulting in lower friction across regimes. The wear mass loss data from our tests are summarized in Table 3, highlighting the dramatic improvement offered by ductile iron castings. This makes them ideal for applications where reliability and longevity are critical, such as in construction machinery sleeves.
| Condition | Material | Initial Mass (mg) | Final Mass (mg) | Wear Loss (%) |
|---|---|---|---|---|
| Dry Friction | Ductile Iron Castings | 3235 | 3119 | 3.59 |
| Dry Friction | 45 Steel | 3218 | 2516 | 21.8 |
| Oil Lubrication | Ductile Iron Castings | 3223 | 3141 | 0.82 |
| Oil Lubrication | 45 Steel | 3241 | 2951 | 8.95 |
The economic and environmental benefits of using ductile iron castings in excavator sleeves are substantial. By extending service life and reducing maintenance frequency, ductile iron castings lower operational costs and downtime. The centrifugal casting process ensures high yield and minimal waste, aligning with sustainable manufacturing practices. Moreover, the ability of ductile iron castings to perform under harsh conditions without rapid degradation reduces the need for frequent replacements, contributing to resource conservation. In our future work, we plan to explore the fatigue resistance and impact toughness of ductile iron castings to further validate their suitability for dynamic loading applications.
In conclusion, our study demonstrates that centrifugally cast and austempered ductile iron castings exhibit exceptional wear resistance, low friction, and high mechanical strength. The microstructure, characterized by fine bainite and graphite, provides self-lubricating properties that significantly outperform 45 steel. These attributes make ductile iron castings a promising material for excavator sleeves and similar components in construction machinery. As we continue to optimize the processing parameters, ductile iron castings are poised to revolutionize the industry by offering longer lifespan and improved reliability.