The pursuit of advanced materials capable of withstanding severe abrasive wear under demanding conditions has been a persistent focus in industrial metallurgy. Among the leading candidates, high chromium white cast iron stands out due to its exceptional combination of high hardness and respectable toughness, derived from its unique microstructure featuring hard M7C3 carbides embedded in a metallic matrix. This material represents the third generation in the evolution of white cast irons, succeeding ordinary and nickel-hard variants. While considerable research has been dedicated to standard compositions (typically 2.5–3.5 wt% C), the behavior of low-carbon, high-chromium white cast iron intended for elevated temperature service remains less explored. When the chemical composition and casting process are fixed, the heat treatment protocol becomes the critical lever for achieving an optimal balance between hardness and impact toughness, which is paramount for applications in sectors like mining, cement production, and power generation.
In this comprehensive study, we investigate the effects of key heat treatment parameters on the microstructure and mechanical properties of a low-carbon, high-chromium white cast iron. The primary objective is to determine an optimized thermal processing route that maximizes the synergy between hardness and impact resistance. We employ a systematic orthogonal experimental design to efficiently analyze the influence and interaction of four crucial factors: austenitizing temperature, austenitizing time, tempering temperature, and tempering time. The findings provide a detailed roadmap for tailoring the performance of this important class of wear-resistant white cast iron.
1. Experimental Methodology and Material System
The investigated white cast iron was prepared using a medium-frequency induction furnace with a capacity of 15 kg. The charge consisted of foundry pig iron, steel scrap, low-carbon ferrochromium, nickel-manganese iron, and ferrosilicon. Melting was conducted at temperatures ranging from 1450 to 1500°C, with a pouring temperature of approximately 1480°C. Prior to casting, aluminum flakes were added for deoxidation. The molten metal was poured into green sand molds to produce cast bars with dimensions of 22 mm × 22 mm × 110 mm. The final chemical composition of the low carbon high chromium white cast iron is detailed in Table 1.
| C | Cr | Si | Ni | Mn | S | P | Co | V | Re | Fe |
|---|---|---|---|---|---|---|---|---|---|---|
| 1.90 | 18.10 | 0.69 | 2.15 | 0.30 | 0.07 | 0.05 | 0.06 | 0.05 | Trace | Bal. |
The design of the heat treatment parameters was based on the Fe-Cr-C phase diagram and established principles for this class of white cast iron. Austenitizing above a critical temperature allows for the precipitation of secondary carbides from the supersaturated as-cast austenite. This precipitation lowers the carbon and alloy content in the matrix, raising the martensite start (Ms) temperature and promoting martensite formation upon cooling. Consequently, three austenitizing temperatures were selected: 970°C, 1010°C, and 1050°C. For each temperature, a suitable holding time is required for diffusion-controlled processes; thus, austenitizing times of 3, 4, and 5 hours were chosen. Given the high alloy content and associated tempering resistance, tempering in the range of 300–500°C was selected, with holding times of 3, 4, and 5 hours. Both quenching and tempering cooling were performed in sand to mitigate thermal stress and the risk of microcracking.
A four-factor, three-level orthogonal array L9(34) was adopted to structure the experiments efficiently. The factors and their levels are summarized in Table 2.
| Level | A: Austenitizing Temp. (°C) | B: Austenitizing Time (h) | C: Tempering Temp. (°C) | D: Tempering Time (h) |
|---|---|---|---|---|
| 1 | 970 | 3 | 300 | 3 |
| 2 | 1010 | 4 | 400 | 4 |
| 3 | 1050 | 5 | 500 | 5 |
Following heat treatment, specimens were machined into standard unnotched impact test bars (10 mm × 10 mm × 55 mm). Impact toughness was measured using a JB-30A pendulum impact tester, with the average of three tests reported. Macro-hardness was determined on an HR-150A Rockwell hardness tester (HRC scale), averaging five indentations per specimen. Microstructural analysis was conducted using scanning electron microscopy (SEM, FEI QUANTA200). Phase identification was performed via X-ray diffraction (XRD, D8 ADVANCE). Specimens for metallography were etched using a solution of 1 g picric acid, 5 mL hydrochloric acid, and 100 mL alcohol.
2. Results and Analysis: Deciphering Parameter Influence
The as-cast white cast iron exhibited a hardness of 42.5 HRC and an impact toughness of 4.93 J/cm². The results for the nine treatment conditions per the L9(34) orthogonal array are presented in Table 3. A significant observation is that all heat-treated conditions showed a marked increase in hardness (above 51 HRC) compared to the as-cast state, albeit with a slight, variable reduction in impact toughness.
| Run No. | A | B | C | D | Hardness (HRC) | Impact Toughness (J/cm²) |
|---|---|---|---|---|---|---|
| 1 | 1 | 1 | 1 | 1 | 54.6 | 4.37 |
| 2 | 1 | 2 | 2 | 2 | 53.0 | 4.26 |
| 3 | 1 | 3 | 3 | 3 | 51.3 | 4.82 |
| 4 | 2 | 1 | 2 | 3 | 56.0 | 4.81 |
| 5 | 2 | 2 | 3 | 1 | 55.7 | 4.62 |
| 6 | 2 | 3 | 1 | 2 | 53.2 | 4.74 |
| 7 | 3 | 1 | 3 | 2 | 56.2 | 4.60 |
| 8 | 3 | 2 | 1 | 3 | 53.6 | 4.60 |
| 9 | 3 | 3 | 2 | 1 | 54.8 | 4.41 |
To quantify the influence of each factor, range analysis (R-value) was performed. The R-value represents the difference between the maximum and minimum average response for a given factor across its levels; a larger R-value indicates a stronger influence on the target property. The results of the range analysis are compiled in Table 4.
| Response | Factor | |||
|---|---|---|---|---|
| A | B | C | D | |
| Hardness (HRC) | 6.0 | 7.5 | 2.4 | 4.2 |
| Impact Toughness (J/cm²) | 1.32 | 0.95 | 0.43 | 1.39 |
Based on the R-values, the order of significance for the factors is determined:
- For Hardness: B (Austenitizing Time) > A (Austenitizing Temperature) > D (Tempering Time) > C (Tempering Temperature).
- For Impact Toughness: D (Tempering Time) > A (Austenitizing Temperature) > B (Austenitizing Time) > C (Tempering Temperature).
This analysis reveals a crucial insight: while austenitizing parameters predominantly control the final hardness of this white cast iron, the tempering duration is the most critical factor for governing its impact toughness. The tempering temperature itself shows the least influence on both properties within the studied range, highlighting the high thermal stability of the alloyed matrix in this white cast iron.
Plotting the mean response for each level of the four factors (Figure 1) provides further clarity on trends:
- Hardness initially increases and then decreases with rising austenitizing temperature, peaks at the shortest austenitizing time, and generally decreases with prolonged tempering time and higher tempering temperature.
- Impact Toughness shows a maximum at an intermediate austenitizing temperature, improves with longer austenitizing time, is slightly degraded by higher tempering temperature, and exhibits a complex, non-monotonic relationship with tempering time (decreasing then increasing).
From this orthogonal analysis, preliminary optimal level combinations can be theorized: A2B1C2D1 for maximum hardness and A2B3C1D3 for maximum toughness. However, these are single-objective optima. To achieve a balanced property profile suitable for high-impact service, a compromise must be engineered. Considering that all conditions already yield high hardness, priority was given to improving toughness. Following a comprehensive trade-off analysis, the verification heat treatment was selected as A2B3C2D3: Austenitizing at 1010°C for 5 hours (sand cool), followed by Tempering at 400°C for 5 hours (sand cool).
3. Verification and Microstructural Evolution
Specimens subjected to the verification heat treatment protocol achieved a hardness of 55.2 HRC and an impact toughness of 4.9 J/cm². This represents an excellent compromise: a substantial 12.7 HRC increase over the as-cast state while retaining the original high level of impact toughness. This confirms the effectiveness of the optimized parameters for this low carbon high chromium white cast iron.
The underlying microstructural changes explain this property enhancement. XRD analysis (Figure 2) confirms that the as-cast structure consists primarily of austenite and M7C3 carbides. The high alloy content suppresses the martensitic transformation during casting. After the optimized heat treatment, distinct martensitic peaks appear in the XRD pattern, accounting for the drastic hardness increase. Carbide peaks remain, and a measurable amount of retained austenite is still present, contributing to the material’s toughness.
SEM micrographs provide vivid detail. The as-cast white cast iron (Figure 3a) shows a typical microstructure of primary austenite dendrites surrounded by a network of discontinuous eutectic M7C3 carbides. The optimized heat treatment (Figure 3b) transforms this structure profoundly. The eutectic carbide morphology remains largely unchanged, but the former primary austenite regions are now filled with a very fine, dense dispersion of secondary carbides. These precipitated carbides, which can be approximated as (Cr,Fe)7C3 or (Cr,Fe)23C6, play a dual role: they provide potent secondary hardening through Orowan strengthening, and, critically, their formation depletes the matrix of carbon and chromium. This depletion raises the Ms temperature, enabling a high fraction of martensite to form upon cooling. The final, high-performance microstructure of the heat-treated white cast iron is therefore a composite of: martensitic matrix + discontinuous eutectic M7C3 carbides + fine, dispersed secondary carbides + a minor fraction of retained austenite.
The kinetic process during austenitizing can be conceptually described. The dissolution of pre-existing carbides and the homogenization of the austenite are counterbalanced by the precipitation of secondary carbides. An oversimplified representation of the driving force for secondary carbide nucleation from supersaturated austenite can be related to the undercooling below the solubility limit:
$$
\Delta G \propto -RT \ln \left( \frac{C}{C_e(T)} \right)
$$
where $\Delta G$ is the driving force, $C$ is the actual carbon/chromium concentration in austenite, and $C_e(T)$ is the equilibrium solubility at temperature $T$. Prolonged holding (Factor B) allows this precipitation process to approach a more stable equilibrium, optimally conditioning the austenite for subsequent transformation.
4. Discussion: Synthesizing Mechanism and Property Control
The experimental results underscore a fundamental principle in heat-treating high-alloy white cast iron: the separation of transformation strengthening (martensite formation) from precipitation strengthening (secondary carbides). Both are controlled by the austenitization step (Factors A and B).
Austenitizing Temperature (A): At 970°C, the driving force for secondary carbide precipitation is high, but diffusion rates are lower. The matrix may retain excessive carbon, leading to a high retained austenite content and lower than optimal hardness. At 1050°C, solubility increases, potentially re-dissolving some carbides and leading to a highly alloyed austenite that is too stable, again resulting in excessive retained austenite upon cooling and a potential decrease in hardness if auto-tempering or carbide coarsening occurs. The intermediate temperature of 1010°C appears to offer the best kinetic balance for achieving a matrix composition that transforms predominantly to martensite with a beneficial dispersion of secondary carbides.
Austenitizing Time (B): This is the most critical factor for hardness in this white cast iron. Shorter times (3h) may lead to an incomplete precipitation reaction, leaving the austenite too stable. The increasing toughness with longer times suggests that more complete precipitation and potentially some tempering of the eventually formed martensite (due to slower sand cooling from a higher temperature) contribute to a more compliant matrix. The optimal 5-hour time maximizes toughness while maintaining high hardness through a fine, stable dispersion.
Tempering Parameters (C & D): The relative insensitivity to tempering temperature (C) is characteristic of high-chromium white cast iron due to strong secondary hardening effects and resistance to overtempering. The pronounced effect of tempering time (D) on toughness is significant. It suggests that extended tempering facilitates beneficial stress relaxation, possible further precipitation, and stabilization of the microstructure, thereby improving ductility and impact resistance without catastrophic loss of hardness. This makes the control of tempering time a powerful tool for tailoring the toughness of this white cast iron.
The synergistic effect leading to the final properties can be summarized by a conceptual additive model for hardness:
$$
HRC_{total} \approx HRC_{M} \cdot f_M + HRC_{SC} \cdot V_{SC} + HRC_{EC} \cdot V_{EC} + HRC_{RA} \cdot f_{RA}
$$
where $f_M$, $V_{SC}$, $V_{EC}$, and $f_{RA}$ are the volume fractions of martensite, secondary carbides, eutectic carbides, and retained austenite, respectively, and $HRC_{i}$ are their intrinsic hardness contributions. The heat treatment optimizes the first three terms while controlling the fourth.
5. Conclusions
Through a systematic orthogonal experimental approach, this study has successfully identified and validated an optimized heat treatment protocol for a low-carbon, high-chromium white cast iron. The key conclusions are:
- The significance of heat treatment parameters on the mechanical properties of this white cast iron follows distinct orders:
- Hardness: Austenitizing Time > Austenitizing Temperature > Tempering Time > Tempering Temperature.
- Impact Toughness: Tempering Time > Austenitizing Temperature > Austenitizing Time > Tempering Temperature.
This highlights that while austenitizing conditions are primary for hardening, tempering duration is the principal lever for toughening this alloy.
- The optimized heat treatment cycle—Austenitizing at 1010°C for 5 hours (sand cool) followed by Tempering at 400°C for 5 hours (sand cool)—produces an outstanding balance of properties: 55.2 HRC hardness and 4.9 J/cm² impact toughness.
- The enhanced properties are directly linked to a refined microstructure consisting of a martensitic matrix strengthened by a fine dispersion of secondary carbides, coupled with the original network of discontinuous eutectic M7C3 carbides and a controlled amount of retained austenite. This microstructure exemplifies the successful engineering of a wear-resistant white cast iron for demanding applications.
This work provides a clear, data-driven framework for the heat treatment of low-carbon, high-chromium white cast irons, demonstrating that precise control over process parameters is essential to unlock their full potential for high-performance service where both hardness and impact resistance are required.