The pursuit of durable materials for harsh engineering environments has consistently driven research in metallurgy and tribology. Among the various candidates, high-chromium white cast iron stands out due to its exceptional combination of hardness, wear resistance, and moderate corrosion resistance. This class of ferrous alloy derives its properties from a microstructure comprising hard carbide phases embedded within a metallic matrix. The nature of both the carbides and the matrix is paramount in determining the final performance of the white cast iron, especially under severe sliding conditions characterized by low speed and high load. While substantial research exists on the abrasion resistance of these materials, their behavior under dry sliding friction—a condition prevalent in applications like guide plates, rolls, and extrusion tooling—requires deeper exploration. The interaction between the hard carbides and the supporting matrix under such concentrated stress and frictional heating is complex, involving processes of plastic flow, fracture, and microstructural evolution. This article presents a comprehensive investigation into the dry sliding friction and wear behavior of high-chromium white cast iron with varied microstructures. We systematically examine the influence of carbide type, matrix constitution, applied pressure, and friction power density on key tribological parameters: the coefficient of friction and the wear rate. Furthermore, we delve into the mechanistic underpinnings of wear, elucidating the layered structural transformations that occur beneath the sliding surface and their implications for material removal.
1. Introduction and Background
White cast irons are defined by their microstructure, where carbon is present primarily in the form of cementite (Fe3C) or alloy carbides, rather than as graphite. This results in a very hard and wear-resistant material, albeit with limited ductility. The introduction of chromium in significant amounts (typically >12%) transforms the basic white cast iron into a high-performance alloy. Chromium serves multiple critical functions: it promotes the formation of harder and more stable chromium-rich carbides like (Fe,Cr)7C3 and (Fe,Cr)23C6, enhances corrosion and oxidation resistance, and allows for the adjustment of the matrix microstructure through heat treatment. The resultant high-chromium white cast iron has become a cornerstone material in mining, cement production, power generation, and steelmaking industries for components subjected to severe abrasive wear.
However, many service conditions involve not just abrasion but also sliding contact between metallic surfaces, often under high pressure with minimal lubrication (dry or boundary lubricated conditions). Examples include hot rolling mill guides, certain types of pump sleeves, and tools for metal forming. In these scenarios, the dominant wear mechanisms shift from pure micro-cutting and fracture induced by abrasive particles to adhesion, plastic deformation, surface fatigue, and tribo-chemical reactions. The performance of high-chromium white cast iron in such contexts is less documented. The hard carbides, while resisting penetration, can fracture or detach if the supporting matrix is too weak. Conversely, a tough matrix may flow plastically, exposing and eventually undermining the carbides. The interplay is governed by the mechanical properties of each phase and the integrity of their interface.
The matrix of high-chromium white cast iron can be austenitic, martensitic, or a pearlitic/bainitic mixture (often termed “eutectoid” structure), depending on the alloy composition and heat treatment. Austenitic matrices, often found in as-cast high-chromium alloys, offer high toughness and work-hardening capacity. Martensitic matrices, achieved through quenching, provide high hardness and strength. Eutectoid structures, resulting from sub-critical annealing or slow cooling, offer a balance but generally lower hardness and toughness. The primary carbide type, dictated mainly by the chromium-to-carbon (Cr/C) ratio, also plays a crucial role. A lower Cr/C ratio favors M3C-type carbides, intermediate ratios favor M7C3, and high ratios favor M23C6 carbides, where M represents Fe and Cr. These carbides differ in morphology, hardness, and fracture toughness.
This study aims to establish quantitative and qualitative relationships between these microstructural variables and the tribological outputs under controlled dry sliding conditions. We employ a systematic approach, preparing white cast iron samples with distinct carbide types and matrix structures. Through rigorous pin-on-ring testing under varied loads and speeds, we measure the friction coefficient and wear rate. A key parameter we utilize is the friction power density, which combines the effects of load and speed to represent the energy input per unit area at the interface. We also employ directional solidification to create well-aligned microstructures, enabling clearer observation of subsurface deformation zones. The ultimate goal is to construct a more predictive understanding of how high-chromium white cast iron fails under sliding contact, thereby informing better material selection and design for specific tribological applications.
2. Materials and Experimental Methodology
The foundation of this investigation lies in the preparation and characterization of high-chromium white cast iron samples with controlled variations. Three distinct alloy compositions were designed to yield different primary carbide phases, as detailed in Table 1. The alloys were melted in a medium-frequency induction furnace using raw materials including pig iron, scrap steel, and ferrochromium. The melt was cast into sand molds to produce cast bars.
| Alloy Designation | C | Cr | Mn | Si | Estimated Primary Carbide* |
|---|---|---|---|---|---|
| Alloy A | 3.83 | 8.0 | 0.42 | 1.08 | (Fe,Cr)3C |
| Alloy B | 3.47 | 17.3 | 0.75 | 1.22 | (Fe,Cr)7C3 |
| Alloy C | 3.20 | 30.7 | 0.90 | 1.15 | (Fe,Cr)23C6 |
*Based on Cr/C ratio and equilibrium phase diagrams.
To vary the matrix microstructure, specific heat treatments were applied to samples from each alloy, particularly focusing on Alloy C. The heat treatment schedules and the resulting microstructural features are summarized in Table 2. Standard metallographic preparation was followed, and etching with 4% nital was used to reveal the microstructure. Macro-hardness was measured using the Rockwell C scale (HRC).
| Sample ID (Base Alloy) | Heat Treatment | Primary Carbide | Matrix Structure | Avg. Hardness (HRC) |
|---|---|---|---|---|
| A-E | 950°C/1.5h → 690°C/1.5h → Furnace Cool | (Fe,Cr)3C | Eutectoid (Pearlitic/Bainitic) | 37.7 |
| B-E | 950°C/1.5h → 690°C/1.5h → Furnace Cool | (Fe,Cr)7C3 | Eutectoid (Pearlitic/Bainitic) | 39.3 |
| C-E | 950°C/1.5h → 690°C/1.5h → Furnace Cool | (Fe,Cr)23C6 | Eutectoid (Pearlitic/Bainitic) | 36.5 |
| C-M | 950°C/1.5h → Air Cool (Quench) | (Fe,Cr)23C6 | Martensite | 65.2 |
| C-A (As-Cast) | None | (Fe,Cr)23C6 | Austenite | 55.5 |
To gain profound insights into the subsurface deformation mechanisms, directionally solidified (DS) samples of a high-chromium white cast iron were also prepared using the Bridgman technique at a solidification rate of 80 mm/h. This process aligns the carbides parallel to the growth direction, creating a model microstructure that simplifies the analysis of flow and fracture patterns beneath the wear surface.
The dry sliding wear tests were conducted on a ring-on-block (M-200 type) configuration. The white cast iron samples, machined to precise blocks, were pressed against a rotating hardened 40Cr steel ring (HRC 51-53). The test parameters were carefully chosen to simulate low-speed, high-load conditions:
- Sliding Speeds (v): 0.4187, 0.5233, 0.8373, 1.0467 m/s.
- Applied Loads (G): Varied from 98 N to 1225 N.
- Test Duration: Adjusted for each speed to maintain a constant total sliding distance.
The frictional torque (M) was measured in-situ. From the measured data, two critical engineering parameters were calculated: the nominal contact pressure (P) and the friction power density (q). The contact pressure is given by:
$$ P = \frac{G}{A_c} $$
where the apparent contact area $A_c = B \cdot b$, with B being the sample width and b the measured wear scar width. The friction power density, representing the frictional energy dissipation rate per unit nominal contact area, is calculated as:
$$ q = \frac{F_f \cdot v}{A_c} = \frac{\mu \cdot G \cdot v}{A_c} = \frac{(M / r) \cdot v}{A_c} $$
where $F_f$ is the friction force, $\mu$ is the coefficient of friction, and $r$ is the radius of the steel ring. This parameter $q$ is crucial as it combines the effects of load, speed, and friction into a single intensive variable governing the thermal and mechanical state of the interface.
The wear rate (W) was determined gravimetrically using a precision balance:
$$ W = \frac{\Delta m}{t} $$
where $\Delta m$ is the mass loss in milligrams and $t$ is the sliding time in minutes.
Post-test analysis involved the examination of wear scars and, most importantly, the cross-sectional subsurface microstructure. Samples were sectioned perpendicular to the wear surface along the sliding direction, mounted, polished, and etched. Optical microscopy (OM) and scanning electron microscopy (SEM) were employed to characterize the deformation zones, crack initiation, and material removal features.
3. Results: Friction and Wear Behavior
The experimental data reveals clear and significant trends linking the microstructure of the white cast iron to its tribological response under dry sliding.
3.1 Influence on Wear Rate
The wear rate was found to be strongly dependent on the matrix structure, the type of carbide phase, and the imposed friction power density. Figure 1 (conceptual data plot) illustrates the relationship between wear rate and friction power density for Alloy C white cast iron with three different matrices: austenite (C-A), martensite (C-M), and eutectoid (C-E). A defining feature is the existence of a critical friction power density ($q_{crit}$). Below $q_{crit}$, wear proceeds at a relatively moderate rate. Upon exceeding this threshold, the wear rate increases dramatically, indicating a transition to a severe wear regime.
- Matrix Effect (Constant Carbide): For white cast iron with (Fe,Cr)23C6 carbides, the austenitic matrix (C-A) demonstrated the highest resistance to wear, exhibiting the lowest wear rates across the power density range and the highest $q_{crit}$ value. The martensitic matrix (C-M) showed intermediate performance, while the eutectoid matrix (C-E) had the highest wear rate and the lowest $q_{crit}$.
- Carbide Effect (Constant Matrix): When comparing white cast irons with a eutectoid matrix but different carbides, the wear resistance ranking was: (Fe,Cr)7C3 (B-E) > (Fe,Cr)3C (A-E) > (Fe,Cr)23C6 (C-E). The critical power density $q_{crit}$, however, appeared to be more closely linked to the matrix than the carbide type for these eutectoid structures.
This can be summarized by a phenomenological model for wear rate:
$$ W = k \cdot q^n \quad \text{for} \quad q < q_{crit} $$
$$ W = K \cdot \exp(\beta q) \quad \text{for} \quad q \geq q_{crit} $$
where $k$, $n$, $K$, and $\beta$ are material-dependent constants influenced by the carbide and matrix properties.
| Microstructural Condition | Relative Wear Resistance (Low q) | Critical Power Density, $q_{crit}$ (MW/m²) | Dominant Wear Mechanism below $q_{crit}$ |
|---|---|---|---|
| (Fe,Cr)23C6 + Austenite | Best | ~5.5 | Oxidative/Mild Delamination |
| (Fe,Cr)23C6 + Martensite | Intermediate | ~5.0 | Micro-cracking/Delamination |
| (Fe,Cr)23C6 + Eutectoid | Poorest | ~4.5 | Ploughing/Delamination |
| (Fe,Cr)7C3 + Eutectoid | Best among eutectoid | ~4.5 | Micro-fracture/Ploughing |
| (Fe,Cr)3C + Eutectoid | Intermediate among eutectoid | ~4.5 | Ploughing/Carbide Fracture |
3.2 Influence on Coefficient of Friction
The steady-state coefficient of friction ($\mu$) exhibited a different set of dependencies. It was primarily influenced by the type of primary carbide phase and the applied contact pressure ($P$), but showed little systematic variation with the matrix structure for a given carbide type. Figure 2 (conceptual data plot) shows the friction coefficient as a function of contact pressure for the three eutectoid white cast irons with different carbides.
- Carbide Effect: The white cast iron containing (Fe,Cr)23C6 carbides (C-E) consistently exhibited the highest friction coefficient. The material with (Fe,Cr)7C3 (B-E) showed an intermediate value, and the white cast iron with (Fe,Cr)3C (A-E) displayed the lowest friction coefficient.
- Pressure Effect: For all materials, the coefficient of friction decreased with increasing nominal contact pressure. This trend can often be described by a power-law relationship:
$$ \mu = \alpha \cdot P^{m} $$
where $\alpha$ is a constant and the exponent $m$ is negative. - Matrix Independence: When comparing Alloy C white cast iron with austenitic, martensitic, and eutectoid matrices, the friction coefficient values clustered together at a given pressure, indicating that the nature of the matrix played a secondary role in determining the interfacial shear stress compared to the carbide phase present at the surface.
| Primary Carbide Phase | Relative Coefficient of Friction (High) | Approx. $\mu$ at P=10 MPa | Pressure Sensitivity Exponent (m) |
|---|---|---|---|
| (Fe,Cr)23C6 | Highest | 0.60 – 0.65 | -0.15 to -0.20 |
| (Fe,Cr)7C3 | Intermediate | 0.50 – 0.55 | -0.12 to -0.18 |
| (Fe,Cr)3C | Lowest | 0.45 – 0.50 | -0.10 to -0.15 |
4. Discussion: Microstructural Evolution and Wear Mechanisms
The disparate influences on wear rate and friction coefficient can be rationalized by examining the microstructural evolution beneath the wear surface. Analysis of the directionally solidified white cast iron samples provided a clear, layered view of this evolution, which is schematically and physically represented in Figure 3. From the wear surface inwards, four distinct zones are identified:
- Friction Layer (Tribolayer): This is the uppermost zone (40-75 μm thick) in direct contact with the counterface. The original microstructure is completely obliterated. Intense shear, compaction, and frictional heating cause severe plastic deformation, fragmentation of carbides, and mixing with the heavily deformed matrix material. This layer often undergoes oxidation and forms a mechanically mixed layer (MML). The properties of this friction layer, dictated by the initial carbide hardness and brittleness, govern the interfacial shear strength and thus the coefficient of friction. The higher friction associated with (Fe,Cr)23C6 may relate to its different fracture behavior and interaction with the steel counterface within this layer.
- Rheological Layer (Plastic Flow Zone): Beneath the friction layer, extending to about 220 μm, lies a zone of intense plastic flow. The matrix undergoes severe plastic deformation, dragging and fragmenting the carbide phases. The aligned carbides in the DS sample lose their orientation and are broken into fragments. The viscous resistance to this solid-phase plastic flow is immense. This leads to poor material cohesion, resulting in the formation of micro-voids and cracks, often aligned parallel to the surface. This zone is critical for the wear rate. The toughness of the original matrix determines the stability of this layer. The ductile austenite matrix accommodates strain better, delaying crack nucleation and coalescence, leading to a higher $q_{crit}$. The brittle eutectoid and martensitic matrices facilitate earlier crack formation, leading to lower $q_{crit}$. The interconnected cracks eventually lead to macroscopic delamination and spallation, responsible for the severe wear regime.
- Strain Band (Elasto-Plastic Transition Zone): Below the rheological layer, plastic deformation diminishes. The carbides, still largely intact, are visibly bent or kinked due to the plastic strain imposed by the deforming matrix above. Some micro-cracking of carbides may occur here, especially in matrices with low ductility (eutectoid, martensite). This zone marks the transition from bulk plastic flow to elastic deformation.
- Unaltered Base Material: Beyond the strain band, the microstructure remains essentially unchanged from its original state.
The wear process is not uniform across the surface but occurs via a fluctuation mechanism. Local adhesion, fracture of the friction layer, or subsurface delamination in the rheological layer creates pits. The process then repeats in adjacent areas. Over time, this leads to an average steady-state wear rate. The superior performance of the austenitic white cast iron is attributed to its high strain-hardening capacity and fracture toughness, which stabilizes the subsurface zones against crack propagation. The (Fe,Cr)7C3 carbide, often possessing a hexagonal rod-like morphology, may offer a better combination of hardness and fracture resistance compared to the script-like (Fe,Cr)23C6 or the less hard (Fe,Cr)3C, explaining its better performance within the eutectoid matrix group.
The decoupling of friction and wear control factors is now clear. The coefficient of friction is a surface phenomenon governed by the shear strength of the friction layer/tribolayer, which is predominantly composed of fragmented carbides and work-hardened matrix material. The intrinsic properties and interaction of the dominant carbide phase with the counterface thus play the leading role. The wear rate, particularly the onset of severe wear, is a bulk subsurface phenomenon controlled by the ability of the material to withstand plastic deformation without crack initiation and propagation. This is primarily a function of the matrix toughness and the carbide-matrix interfacial strength, determining the stability of the rheological layer.
5. Conclusions and Implications
This comprehensive study on the dry sliding behavior of high-chromium white cast iron under low-speed, high-load conditions leads to several fundamental conclusions with practical significance:
- The tribological performance of white cast iron is governed by two semi-independent sets of material properties: surface properties affecting friction and bulk/sub-surface properties affecting wear resistance.
- The steady-state coefficient of friction in dry sliding is primarily determined by the type of primary carbide phase present in the white cast iron (e.g., (Fe,Cr)23C6 > (Fe,Cr)7C3 > (Fe,Cr)3C) and decreases with increasing contact pressure. The matrix microstructure (austenite, martensite, eutectoid) has a negligible direct influence on friction.
- The wear rate and the transition to severe wear are critically dependent on the matrix microstructure, the carbide type, and the friction power density ($q$). Austenitic matrices provide the highest resistance to severe wear, followed by martensitic, and then eutectoid matrices for a given carbide type. The critical friction power density ($q_{crit}$) for severe wear onset follows the same order.
- The wear mechanism involves the formation of a layered subsurface structure: a friction layer, a rheological layer of plastic flow and void formation, a strain band, and the unaltered base material. Catastrophic wear occurs via delamination fatigue within the rheological layer. The toughness of the matrix is key to inhibiting crack initiation and propagation in this layer.
- For applications involving severe dry sliding, selecting a white cast iron with an austenitic or martensitic matrix is preferable over a eutectoid structure. Furthermore, optimizing the carbide type and morphology, such as favoring (Fe,Cr)7C3, can yield additional wear resistance benefits. Design should aim to keep the operational friction power density below the material-specific $q_{crit}$ to avoid catastrophic wear rates.
This work underscores the complexity of wear in multi-phase materials like high-chromium white cast iron. It moves beyond a simple hardness-based selection criterion, highlighting the critical roles of fracture toughness, microstructural stability, and the specific energy conditions at the sliding interface. Future work could focus on quantitatively modeling the stress state in the rheological layer and linking it to the mechanical properties of the constituent phases to develop predictive life models for components made from this important class of wear-resistant white cast iron.