The extensive utilization of white cast iron in industrial applications stems from its exceptional hardness and superior wear resistance, coupled with its cost-effectiveness. However, the inherent brittleness and poor toughness of white cast iron pose significant challenges, particularly when defects necessitate repair via welding. The heat-affected zone (HAZ) formed during welding undergoes complex thermal cycles, drastically altering the microstructure and, consequently, the mechanical properties of the base white cast iron. This study systematically investigates the thermal sensitivity of a low-chromium molybdenum white cast iron by employing welding thermal simulation techniques to replicate the HAZ microstructures. We meticulously analyze the influence of varying thermal cycle parameters, specifically peak temperature, on the resultant microstructure, hardness, and impact toughness. Furthermore, fracture surface morphology is examined to elucidate the underlying failure mechanisms. The findings provide critical insights for developing viable welding and repair strategies for white cast iron components.
The fundamental issue with welding white cast iron lies in its extreme sensitivity to thermal cycles. The as-cast microstructure, typically comprising ledeburite (a eutectic mixture of cementite and austenite that transforms to pearlite), secondary cementite, and pearlite, is metastable. Heating during welding can dissolve carbides, alter carbon distribution, and upon cooling, lead to the formation of new, often detrimental, phases. The thermal simulation approach allows for precise control and reproduction of these thermal cycles, enabling a detailed study of microstructure-property relationships without the complexities of actual welding.
1. Material and Experimental Methodology
The subject material for this investigation is a low-chromium molybdenum alloyed white cast iron. Its chemical composition, determined via spectral analysis, is presented in Table 1. This specific alloying is chosen to provide a balance between hardness and some degree of stability against extreme carbide growth.
| Element | C | Si | Mn | Cr | Mo | Fe |
|---|---|---|---|---|---|---|
| Content | 3.2 – 3.6 | 0.4 – 0.8 | 0.5 – 0.9 | 1.5 – 2.2 | 0.8 – 1.5 | Bal. |
Welding thermal simulation was conducted on a Gleeble-1500 thermomechanical simulator. Specimens with dimensions of 10mm x 10mm x 55mm were prepared using wire-electrical discharge machining (EDM) and surface grinding. The thermal cycle parameters were derived from experimentally measured thermal profiles during manual metal arc welding of large white cast iron castings and from the phase transformation temperatures of the specific white cast iron.
The phase transformation behavior was characterized using a high-speed dilatometer. A specimen (Ø3mm x 10mm) was heated from room temperature to 1100°C, held for 300s, and then cooled. Critical transformation temperatures (Ac1, Ac3) were identified from the dilation-temperature curve. Based on this and actual weld thermal cycles, two categories of simulation were designed: single thermal cycles (simulating a single weld bead) and double thermal cycles (simulating a subsequent weld bead pass). The key parameters—peak temperature (Tp), heating time to Tp (th), hold time at Tp (thold), and cooling time between 800°C and 500°C (t8/5)—are summarized in Table 2.
| Cycle Type | Cycle ID | Tp (°C) | th (s) | thold (s) | t8/5 (s) | Remarks |
|---|---|---|---|---|---|---|
| Single | S1 | 780 | 12 | 1.5 | 35 | Simulated HAZ |
| S2 | 980 | 15 | 1.5 | 38 | Simulated HAZ | |
| S3 | 1100 | 18 | 1.5 | 42 | Simulated HAZ | |
| S4 | 1250 | 22 | 1.5 | 48 | Simulated HAZ near fusion line | |
| Double | D1 | 780 → 1100 | 12 → 18 | 1.5 → 1.5 | 35 → 42 | Simulated inter-critical + super-critical reheating |
| D2 | 780 → 1250 | 12 → 22 | 1.5 → 1.5 | 35 → 48 | Simulated inter-critical + super-critical reheating |
The thermal cycle can be conceptually described by the heating and cooling rate. For the heating stage, the average rate can be approximated as:
$$
\dot{T}_{heating} \approx \frac{T_p – T_{room}}{t_h}
$$
where $\dot{T}_{heating}$ is the average heating rate, $T_p$ is the peak temperature, $T_{room}$ is room temperature (~25°C), and $t_h$ is the heating time. This parameter significantly influences the dissolution kinetics of carbides in the white cast iron matrix.
Post-simulation, a 2mm-deep V-notch was machined at the center of the thermally affected zone of each specimen for Charpy V-notch impact testing. Microstructural analysis was performed using optical microscopy (OM) and scanning electron microscopy (SEM). The volume fraction of cementite was quantified using the linear intercept method on OM images. Microhardness profiles were measured across the simulated HAZ. Fracture surfaces of the impact specimens were examined in detail using SEM to characterize the fracture mode.
2. Results: Influence of Thermal Cycles on White Cast Iron
2.1 Mechanical Properties: Hardness and Impact Toughness
The response of white cast iron to thermal cycling is profoundly evident in its mechanical properties. Table 3 summarizes the macrohardness (HV) and impact energy of the white cast iron in the as-cast condition and after various simulated thermal cycles.
| Condition | Cycle ID / Tp (°C) | Avg. Impact Energy (J) | Avg. Macrohardness, HV30 |
|---|---|---|---|
| As-cast | – | 4.8 | 520 |
| Single Cycle | S1 (780) | 2.1 | 380 |
| S2 (980) | 3.5 | 650 | |
| S3 (1100) | 3.8 | 720 | |
| S4 (1250) | 3.2 | 680 | |
| Double Cycle | D1 (780→1100) | 2.8 | 580 |
| D2 (780→1250) | 3.1 | 610 |
A critical observation is that all thermal cycles degrade the impact toughness of the white cast iron compared to its as-cast state. However, the degree of degradation and the trend in hardness are non-monotonic with peak temperature. The white cast iron subjected to a Tp of 780°C exhibits the lowest hardness and the poorest impact toughness. In contrast, white cast iron experiencing higher peak temperatures (980°C, 1100°C, 1250°C) shows significantly recovered hardness and moderately better, though still low, impact energy. The double thermal cycles demonstrate that a subsequent high-temperature cycle can ameliorate the properties of the softened zone created by a prior lower-temperature cycle, with improvement scaling with the second peak temperature.
2.2 Microstructural Evolution
The drastic changes in properties are directly linked to microstructural transformations induced by the thermal cycles in the white cast iron.
As-cast microstructure: The baseline structure consists of a continuous network of ledeburite (transformed to pearlite), blocky secondary cementite, and fine lamellar pearlite in the inter-eutectic regions.
Single Thermal Cycles:
– Tp = 780°C: This intercritical heating temperature partially austenitizes the matrix. Upon cooling, the resulting microstructure is predominantly cementite (undissolved and possibly slightly spheroidized) and coarse lamellar pearlite. The significant softening is attributed to this coarse pearlitic matrix.
– Tp = 980°C & 1100°C: These super-critical temperatures lead to substantial dissolution of carbides into the austenite, significantly increasing its carbon content. The rapid cooling (simulated t8/5) results in a hard matrix of twinned martensite with retained austenite, surrounding the remaining primary cementite network. The high carbon content promotes martensite formation and high hardness.
– Tp = 1250°C: Near the fusion line, carbide dissolution is extensive. The microstructure is similar to the 1100°C condition but may exhibit a slightly coarser prior-austenite grain size and a different cementite morphology, leading to a slight reduction in maximum hardness.
The degree of austenitization and carbon enrichment can be conceptually related to the peak temperature. The carbon concentration in austenite, $C_{\gamma}$, at a given Tp influences the martensite start temperature ($M_s$) and final hardness. A simplified relation is:
$$
M_s \approx M_s^0 – K \cdot C_{\gamma}
$$
where $M_s^0$ is the $M_s$ for pure iron and $K$ is a constant. Lower $M_s$ leads to more retained austenite and higher lattice strain in martensite.
Double Thermal Cycles: The first cycle to 780°C creates a softened zone (cementite + coarse pearlite). A subsequent high-temperature cycle (1100°C or 1250°C) re-austenitizes this region. However, the starting microstructure is not homogeneous. The final structure is a mixture of cementite, martensite, and some residual regions of refined pearlite. The properties are intermediate between the fully softened and the fully hardened single-cycle states.
The microhardness of individual phases provides deeper insight. Table 4 shows the microhardness of the cementite and the matrix for key conditions.
| Condition (Tp °C) | Cementite Avg. (HV) | Matrix Avg. (HV) | Matrix Constituents |
|---|---|---|---|
| As-cast | 1250-1400 | 280-320 | Fine Pearlite |
| 780 (S1) | 1250-1350 | 180-220 | Coarse Pearlite |
| 1100 (S3) | 1300-1450 | 750-850 | Martensite + Retained Austenite |
| 780→1250 (D2) | 1280-1400 | 400-550 | Martensite + Fine Pearlite |
The data confirms that the dramatic shifts in macrohardness are primarily due to changes in the matrix hardness, not the cementite. The hardness of the white cast iron is therefore a composite function:
$$
HV_{composite} \approx f_{cem} \cdot HV_{cem} + (1 – f_{cem}) \cdot HV_{matrix}
$$
where $f_{cem}$ is the cementite volume fraction. While $f_{cem}$ decreases with higher Tp due to dissolution (see Table 5), the exponential rise in $HV_{matrix}$ from pearlite to martensite dominates the overall hardness trend.
| Condition (Tp °C) | Cementite Volume Fraction, fcem (%) |
|---|---|
| As-cast | 32 ± 2 |
| 780 | 30 ± 2 |
| 980 | 25 ± 2 |
| 1100 | 18 ± 2 |
| 1250 | 15 ± 2 |
2.3 Fracture Surface Analysis
Fractography reveals the micromechanisms responsible for the observed toughness values in the thermally treated white cast iron.
– As-cast and Tp=780°C (Low Toughness): The fracture surface is characterized by quasi-cleavage of the cementite phase and intergranular fracture along the cementite/matrix interface. The smooth facets of cleaved cementite and the brittle separation at interfaces require low energy, leading to very low impact values.
– Tp=980°C, 1100°C, 1250°C (Higher Toughness): The fracture mode shifts to predominantly transgranular cleavage. The crack path now cuts through the hardened martensitic matrix as well as the cementite. While still brittle, fracturing the strong martensite consumes more energy than decohesion at weak interfaces, resulting in a measurable, though modest, improvement in impact energy. The fracture surface shows larger cleavage facets and river patterns.
– Double Cycles: Fracture surfaces exhibit mixed-mode characteristics, with regions of transgranular cleavage and residual intergranular facets, consistent with their intermediate microstructures and properties.
3. Discussion: Mechanisms Governing Thermal Sensitivity in White Cast Iron
The extreme thermal sensitivity of white cast iron is a direct consequence of its composite nature—a hard, brittle carbide network embedded in a metallic matrix. The welding thermal cycle acts as a high-temperature treatment that alters both phases and their interfaces.
The most detrimental condition occurs at a peak temperature around 780°C. At this intercritical range, the matrix transforms to austenite with a relatively low carbon content, as only a minimal amount of cementite dissolves. Upon cooling, this leads to the formation of coarse, high-temperature transformation products like pearlite or upper bainite, which are soft. The thermal cycle also anneals and smoothens the carbide/matrix interface, reducing mechanical interlocking. Furthermore, potential segregation of impurity elements to these boundaries during reheating can further embrittle them. Consequently, under stress, cracks initiate and propagate easily along these weakened interfaces (intergranular fracture) or through the brittle cementite (cleavage), requiring minimal energy. This explains the paradox where a softer matrix leads to lower overall toughness in this white cast iron.
In contrast, high peak temperatures (e.g., 1100°C) cause significant carbide dissolution, creating a high-carbon, highly hardenable austenite. Subsequent cooling forms a strong, twinned martensitic matrix. Although martensite is brittle, its high strength forces cracks to propagate through it in a transgranular manner. The energy required for cleaving through the martensitic lattice and the remaining cementite is higher than for interfacial decohesion. Thus, the white cast iron exhibits higher hardness and slightly improved impact resistance. The relationship between matrix strength ($\sigma_m$), interface strength ($\sigma_i$), and effective fracture stress ($\sigma_f$) can be conceptualized. For the softened condition, $\sigma_i < \sigma_m$, promoting interface failure. For the hardened condition, $\sigma_m$ is elevated and may approach or fall below $\sigma_i$ due to martensite brittleness, but the overall crack propagation energy increases as $\sigma_f$ becomes governed by $\sigma_m$ and the fracture toughness of the cementite.
The beneficial effect of a second high-temperature cycle on a previously softened zone is technologically significant for multi-pass welding. It demonstrates that the poor properties of an intercritical HAZ sub-region in white cast iron can be partially remedied by the heat from a subsequent weld pass, as it re-transforms the coarse pearlite into a harder microstructure.
4. Conclusions
This comprehensive investigation into the thermal sensitivity of low-chromium molybdenum white cast iron using welding thermal simulation leads to the following key conclusions:
- White cast iron is profoundly sensitive to welding thermal cycles. All simulated thermal conditions reduced the impact toughness compared to the as-cast state, highlighting the inherent challenge in welding this material.
- The peak temperature of the thermal cycle is the dominant factor controlling the final microstructure and properties of the white cast iron HAZ.
- A peak temperature of ~780°C produces the most detrimental condition: a soft matrix of coarse pearlite leading to the lowest hardness and impact toughness due to a weak carbide/matrix interface promoting intergranular fracture.
- High peak temperatures (980–1250°C) result in a hardened matrix of martensite and retained austenite, yielding high hardness and relatively better (though still poor) impact toughness associated with a transgranular cleavage fracture mode.
- The volume fraction of cementite decreases with increasing peak temperature due to dissolution. However, the properties of the white cast iron are primarily governed by the matrix microstructure, not the carbide fraction.
- Double thermal cycles demonstrate that the properties of a softened HAZ region in white cast iron can be improved by the heat from a subsequent weld pass. The degree of improvement increases with the peak temperature of the second cycle.
- The fracture mode transitions from intergranular/cleavage (low energy) in softened white cast iron to transgranular cleavage (higher energy) in hardened white cast iron, explaining the correlation between matrix hardness and impact energy.
These findings underscore the critical importance of thermal management when welding or repairing white cast iron components. To minimize the extent of the softened, embrittled zone (associated with intercritical peak temperatures), welding processes with low heat input and techniques that allow rapid cooling may be advantageous. Alternatively, for multi-pass repairs, strategic sequencing of weld beads can be designed to temper or re-harden susceptible zones, leveraging the beneficial effect of double thermal cycles observed in this study on white cast iron.