In engineering applications, the failure of components due to fatigue under cyclic contact stress is a primary cause of equipment downtime and degraded processing quality, with rolling contact fatigue (RCF) being a key contributing factor. Mechanical parts such as rolling bearings, gears, and wheel-rail contacts suffer from various forms of surface damage leading to performance degradation or failure. Typical damage from rolling contact fatigue includes pitting, spalling, cracking, and “bruise-like” damage, which often evolve into cracks propagating through the entire rolling body. As noted in literature, the accumulation of cyclic contact stresses induces localized plastic strain, and the accumulation of this plastic flow is a primary cause of crack initiation in RCF.
Grey iron castings are valued for their low cost and favorable properties, including low melting point, good fluidity, castability, excellent machinability, and decent wear resistance. However, their fatigue wear resistance is often considered poor in contact fatigue environments due to the presence of low-hardness, pore-like flake graphite, which acts as a defect. Researchers have employed various surface modification techniques to enhance the fatigue wear performance of grey iron castings, such as physical vapor deposition (PVD), chemical vapor deposition (CVD), thermal spraying (TS), and plasma transferred arc (PTA) welding, with some success. Nevertheless, these methods have limitations: coating thickness is often limited to less than 100 μm, restricting use in high-stress applications; material mismatch at the coating-substrate interface risks crack formation; and compositional differences lead to chemical discontinuity. Consequently, failures in such coatings or at interfaces can be more severe than in untreated materials.
Recent work has explored laser remelting to modify surfaces locally without altering bulk properties, often inspired by biological surfaces to create patterned “bionic” structures with alternating hard and soft phases to improve properties like wear and fatigue resistance. For grey iron castings, laser remelting leverages the solid solution and precipitation of carbon in ferrite to form hardened phases and carbides. However, the inherent inhomogeneous distribution of graphite in grey iron castings leads to microstructural variability—regions away from graphite form low-carbon martensite with few carbides, while areas near graphite form high-carbon martensite with abundant carbides. To enhance microstructural homogeneity in the treated zone, this study introduces a laser carburizing (LC) process, where carbon is introduced into the substrate to promote a more uniform distribution. Combining finite element analysis (FEM) with experimental investigation, this work studies the rolling contact fatigue behavior and damage mechanisms of laser-carburized grey iron castings under different laser energy densities, elucidating the strengthening mechanisms.
Materials and Experimental Methods
The substrate material was a grey iron casting conforming to type HT 150. Its nominal chemical composition is provided in Table 1. The microstructure consisted of flake graphite (Type A), pearlite, and ferrite, with the graphite flakes embedded in a pearlite-ferrite matrix. The powder used for the coating was 98% pure carbon.
| C | Si | Mn | P | S | Fe |
|---|---|---|---|---|---|
| 3.0–3.2 | 1.5–1.8 | 0.7–0.9 | <0.15 | <0.12 | Bal. |
Samples were cut into blocks and their surfaces were polished prior to testing. Before laser treatment, a carbon powder pre-coating was applied using sodium silicate as a binder. A pulsed Nd:YAG laser was used for surface modification under an argon shield. By adjusting the focal length, different laser energy densities were achieved. The primary processing parameters are summarized in Table 2. The treated stripes are referred to as “units.” For comparison, laser-remelted (LM) samples and untreated parent material (PM) samples were also prepared.
| Sample ID | Spot Dia. (mm) | Coating Thickness (mm) | Energy Density (J/mm²) | Description |
|---|---|---|---|---|
| 1 (PM) | — | — | — | Base Material |
| 2 | 0.8 | 0 | 10.7 | Laser Remelting (LM) |
| 3 | 0.8 | 0.2 | 10.7 | Laser Carburizing (LC) |
| 4 | 1.0 | 0 | 6.84 | Laser Remelting (LM) |
| 5 | 1.0 | 0.2 | 6.84 | Laser Carburizing (LC) |
| 6 | 1.2 | 0 | 4.75 | Laser Remelting (LM) |
| 7 | 1.2 | 0.2 | 4.75 | Laser Carburizing (LC) |
Following laser treatment, cross-sections were examined using optical microscopy and scanning electron microscopy (SEM). Phase identification was performed using X-ray diffraction (XRD). Microhardness was measured using a Vickers hardness tester. Tensile and compressive tests were conducted on a material testing system.
Rolling contact fatigue (RCF) tests were performed on a custom-built tester under lubricated conditions using SAE 46 mechanical oil. The test involved a rolling bearing with rollers applying a cyclic load on the sample surface. Sample mass was measured at intervals to determine mass loss (ML) and mass loss rate (MLR) as indicators of fatigue wear resistance. Worn surfaces were examined using microscopy and 3D profilometry.
To understand the stress distribution, a numerical simulation using the finite element method (FEM) was conducted, modeling a single rolling contact under a constant load. The parameters used are listed in Table 3.
| Region | Elastic Modulus (GPa) | Poisson’s Ratio | Density (g/cm³) |
|---|---|---|---|
| Parent Material (PM) | 130 | 0.23 | 7.4 |
| Laser-Treated Unit | 150 | 0.25 | 7.8 |
Characteristics of Laser-Carburized (LC) and Laser-Remelted (LM) Units
During laser processing, the substrate is rapidly heated above its melting point and then undergoes rapid cooling, resulting in phase transformations. The cross-sectional geometry of the treated zone typically approximates an inverted parabola. The dimensions of the units under different conditions are summarized in Table 4. The width increases with laser spot size, while the depth decreases significantly. The unit area is inversely proportional to the laser spot size due to the corresponding decrease in energy density. Comparing units made under the same conditions, Laser-Remelted Units (LMUs) are wider but shallower than Laser-Carburized Units (LCUs), resulting in a smaller treated volume fraction for LMUs.
| Sample ID | Width (mm) | Depth (mm) | Cross-sectional Area (mm²) |
|---|---|---|---|
| 2 (LMU) | 1.26 | 0.84 | 0.71 |
| 3 (LCU) | 1.03 | 1.10 | 0.76 |
| 4 (LMU) | 1.43 | 0.24 | 0.23 |
| 5 (LCU) | 1.25 | 0.50 | 0.42 |
| 6 (LMU) | 1.52 | 0.10 | 0.10 |
| 7 (LCU) | 1.37 | 0.15 | 0.14 |
The difference in size between LCUs and LMUs is attributed to the increased laser beam absorptivity due to the carbon coating. The effective heat input considering absorptivity ($\eta$) can be expressed as:
$$ Q_{eff}^{‘} = \eta \frac{P}{V} $$
where $P$ is laser power and $V$ is traverse speed. The absorptivity of a bare grey iron casting surface is around 20%, but it can be as high as 90% for a dark carbon-coated surface. The higher absorbed energy density leads to a higher melt pool temperature, promoting deeper heat penetration and thus a deeper unit for LCUs. The cross-sectional area increases with energy density, and LCUs consistently have a larger area than LMUs due to the enhanced absorptivity.
XRD analysis confirmed the phases present. The parent material showed graphite and ferrite peaks. After laser treatment, new phases including martensite, retained austenite, and cementite (Fe$_3$C) were identified. While the phase composition of LCUs and LMUs was similar, the diffraction peaks for LCUs were broader, indicating finer grains and/or higher lattice strain from the austenite-to-martensite transformation.
Microstructural analysis revealed that flake graphite was completely dissolved in both LMUs and LCUs. The microstructure of LMUs was less uniform, with significant retained ferrite, due to the heterogeneous carbon distribution in the original grey iron castings. In contrast, LCUs exhibited a more homogeneous microstructure. The introduced carbon facilitated the more complete transformation of ferrite, leading to a structure comprising martensite, retained austenite, lamellar pearlite, and a network of ledeburite (eutectic mixture of austenite and cementite). The percentage of ledeburite was significantly higher in LCUs (60-70%) compared to LMUs (40-45%), and increased with energy density for both types.
The microhardness of the treated units is presented in Figure 5. Two key trends are evident: (1) the hardness of both LCUs and LMUs increases with applied energy density, and (2) for a given energy density, LCUs are consistently harder than LMUs. This is directly attributed to the finer, denser microstructure and the higher volume fraction of hard carbides/ledeburite in the laser-carburized units of the grey iron castings.
Mechanisms of Fatigue Wear Improvement
Suppression of Crack Initiation and Propagation
In grey iron castings, flake graphite acts as a pre-existing defect or crack. Under cyclic loading, cracks readily initiate at the tips of these graphite flakes. Neighboring graphite flakes can become connected by propagating cracks, leading to macro-scale failure. Laser treatment eliminates these graphite flakes, thereby removing the primary sites for crack initiation. Furthermore, the refined grain structure within the treated unit increases the energy required for crack propagation, whether it occurs transgranularly or intergranularly. The treated region exhibits a higher resistance to fatigue crack growth and a larger threshold stress intensity factor range compared to the base material of grey iron castings.
The driving force for crack initiation and propagation stems from the accumulation of micro-strain due to cyclic stress. The fatigue life ($N_f$) related to ratcheting failure can be expressed as:
$$ N_f = \frac{\varepsilon_c}{\Delta \varepsilon_r} $$
where $\varepsilon_c$ is the critical strain for failure and $\Delta \varepsilon_r$ is the equivalent ratcheting plastic strain per cycle, given by:
$$ \Delta \varepsilon_r = \sqrt{(\Delta \varepsilon)^2 + \left(\frac{\Delta \gamma}{\sqrt{3}}\right)^2 } $$
Here, $\Delta \varepsilon$ and $\Delta \gamma$ are the average ratcheting axial and shear strains per cycle, respectively. Fatigue life is inversely proportional to $\Delta \varepsilon_r$. Mechanical testing showed that laser-carburized samples of grey iron castings exhibited significantly improved resistance to deformation under both tensile and compressive loading compared to both the base material and laser-remelted samples. This enhanced resistance to plastic strain accumulation directly contributes to a longer fatigue life, as per the equation above.
Fatigue Wear Performance and Stress Distribution
The fatigue wear resistance was evaluated by mass loss (ML) and mass loss rate (MLR). The MLR for all samples followed a similar trend: an initial period leading to a peak rate, followed by a steady-state period. The treated samples entered this steady-state period approximately 10 hours earlier than the untreated grey iron casting, indicating the delayed onset of severe wear due to the treated units. The average steady-state MLR and total ML after 70 hours for all samples are summarized in Table 5. Laser carburizing (Sample 3) yielded the best performance, with the lowest MLR and total ML.
| Sample ID & Treatment | Avg. Steady-State MLR (mg/10 h) | Total Mass Loss after 70h (mg) | Performance Ranking |
|---|---|---|---|
| 1 (PM) | 2.65 | 18.6 | Lowest (7) |
| 2 (LM, 10.7 J/mm²) | 2.27 | 15.9 | 5 |
| 3 (LC, 10.7 J/mm²) | 2.00 | 14.0 | Highest (1) |
| 4 (LM, 6.84 J/mm²) | 2.40 | 16.8 | 6 |
| 5 (LC, 6.84 J/mm²) | 2.27 | 15.9 | 5 |
| 6 (LM, 4.75 J/mm²) | 2.58 | 18.1 | 3 |
| 7 (LC, 4.75 J/mm²) | 2.45 | 17.2 | 4 |
Finite element analysis of the contact stress distribution provided critical insight. For the untreated grey iron casting surface, the maximum contact stress occurred at the edges of the roller contact. For the surface with laser-treated units, the maximum stress was concentrated within the harder units. Consequently, the contact stress on the untreated matrix regions between the units was lower than that on a fully untreated surface. According to the characteristic S-N curve, reducing the stress level on the bulk material significantly extends the fatigue life of the component. Although the units themselves experience a higher peak stress, their refined, graphite-free, and carbide-rich microstructure is more resistant to crack initiation and propagation. Therefore, while the untreated matrix areas of treated samples and the parent material showed severe spalling and delamination, the laser-carburized units exhibited only minor pitting. This combination of stress redistribution and localized strengthening is the core mechanism behind the improved performance of laser-treated grey iron castings.
Conclusions
This study demonstrates that localized laser carburizing is a highly effective surface engineering technique for significantly improving the rolling contact fatigue wear resistance of grey iron castings. Compared to simple laser remelting, the laser carburizing process, which involves the intentional introduction of carbon, produces a more homogeneous and refined microstructure within the treated units, characterized by a higher volume fraction of carbides and ledeburite. This results in superior hardness and enhanced resistance to plastic deformation under both shear and compressive stresses.
The presence of these hard, graphite-free units fundamentally alters the failure mechanisms in grey iron castings under cyclic contact loading. They eliminate the primary crack initiation sites (graphite flakes), impede crack propagation through grain refinement, and beneficially redistribute the surface contact stresses, lowering the stress burden on the surrounding untreated matrix material. Consequently, the fatigue life is extended. The degree of improvement is positively correlated with the laser energy density used during the carburizing process.
The primary failure modes observed were distinct: the laser-carburized units exhibited mild pitting, while the untreated regions of the grey iron castings suffered from severe spalling and delamination. This work validates the feasibility of laser carburizing as a practical and efficient method for enhancing the durability of grey iron castings in demanding applications involving rolling/sliding contact, offering advantages in terms of process flexibility, controlled treatment depth, and energy efficiency due to increased beam absorptivity.