The pursuit of advanced wear-resistant materials with an optimal balance of hardness and toughness is a constant driver in material science and engineering. Low-alloy wear-resistant cast steels, typically alloyed with elements like Mn, Cr, Ni, and Mo, represent a significant class of materials for demanding industrial applications. These steel castings derive their properties from their specific microstructure, which is carefully tailored through composition design and heat treatment. Among various microstructures, bainite offers an attractive combination of strength and ductility. Recent research has focused on further enhancing the properties of bainitic cast steels through microalloying, with rare earth (RE) elements gaining considerable attention for their ability to refine microstructures and purify grain boundaries.
Heat treatment, particularly tempering, is a critical step in achieving the final desired set of mechanical properties in these steel castings. The tempering process of bainitic structures involves a series of complex and often overlapping transformations, including: the redistribution of carbon from supersaturated ferrite, precipitation of carbides from the bainitic ferrite, recovery and possible recrystallization of the ferritic laths, and the decomposition of metastable retained austenite or M/A (Martensite/Austenite) constituents. The specific trajectory of these transformations is highly sensitive to the steel’s chemical composition. Therefore, understanding the microstructural evolution during tempering of RE-modified bainitic cast steels is not only crucial for optimizing their performance but also enriches the fundamental theory of bainite tempering and clarifies the role of RE elements in solid solution and precipitation processes.
In this work, I investigated the tempering behavior of a low-alloy bainitic wear-resistant cast steel with a base composition of Fe-0.2C-1.0Mn-1.0Cr-1.9Ni-0.5Mo (in wt%), modified by the addition of a lanthanum-cerium (La-Ce) mischmetal. The as-cast state of this steel casting exhibits a granular bainite microstructure. A systematic study was conducted by tempering specimens at temperatures ranging from 200°C to 650°C for 1 hour. The primary objective is to characterize and analyze the sequence of microstructural changes, with a focus on carbide precipitation, the stability and decomposition of M/A islands, and the onset of recovery processes, providing a comprehensive view of the tempering response in this RE-containing system.
Materials and Experimental Procedures
The material under investigation was a laboratory-cast steel casting. Its chemical composition, determined by optical emission spectrometry, is provided in Table 1. The key feature is the intentional addition of approximately 0.01 wt% total rare earth (RE) in the form of a La-Ce mischmetal.
To determine the amount of rare earth elements in solid solution within the as-cast matrix, a two-step analytical procedure was employed. First, non-aqueous electrolysis was performed on as-cast samples under controlled conditions to selectively dissolve the metallic matrix while leaving non-metallic inclusions (including RE-containing inclusions) undissolved. These inclusions were subsequently filtered out. The electrolyte, now containing only the dissolved metallic elements from the matrix, was then analyzed using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) to quantify the concentrations of La and Ce. This measured value represents the fraction of RE that is atomically dissolved in the steel’s lattice rather than tied up in inclusions.
Samples for heat treatment were sectioned to a thickness of 4 mm from the as-cast material. They were subjected to tempering treatments at temperatures of 200, 250, 300, 350, 400, 450, 500, 550, 600, and 650°C. Each sample was held at the target temperature for 1 hour in a muffle furnace, followed by air cooling to room temperature.
The microstructural evolution was characterized using a combination of techniques. General microstructural features were observed using a QUANTA 400 environmental scanning electron microscope (SEM). For higher resolution analysis of fine precipitates, dislocation structures, and internal details of the bainitic ferrite, a JEM-2100F field-emission transmission electron microscope (TEM) equipped with an energy-dispersive X-ray spectroscopy (EDS) system was utilized.
To identify and quantify the precipitates formed during tempering, a physicochemical phase analysis was conducted. Carbides were extracted from a sample tempered at 650°C (where precipitation is most extensive) using an electrolytic extraction method with a non-aqueous electrolyte. The extracted powder was analyzed by X-ray diffraction (XRD) to determine its crystal structure. Subsequently, the powder was dissolved, and the solution was analyzed by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) to determine the precise chemical composition of the carbides, including any alloying and RE elements they contained.
The volume fraction of retained austenite in the microstructure after different tempering temperatures was measured quantitatively using XRD. A D8 ADVANCE diffractometer with Cu Kα radiation was used. The integrated intensities of the austenite {111} and {200} peaks and the ferrite {110} peak were measured. The volume fraction of retained austenite, $V_{\gamma}$, was calculated using the direct comparison method based on the following generalized formula:
$$ V_{\gamma} = \frac{1}{1 + G \frac{I_{\alpha}}{I_{\gamma}}} $$
where $I_{\alpha}$ and $I_{\gamma}$ are the integrated intensities of a selected ferrite and austenite peak, respectively, and $G$ is a material constant that depends on the specific crystal planes used and incorporates the structure factor, Lorentz-polarization factor, and multiplicity factor for those planes. The values were averaged from calculations using multiple peak pairs to improve accuracy.
Results: Microstructural Observations
As-Cast and Low-Temperature Tempering (200-300°C)
The microstructure of the as-cast steel casting is shown in the provided SEM image, which reveals a classic granular bainite morphology. The matrix consists of bainitic ferrite laths, upon which island-like constituents are distributed. These islands are the M/A constituents, comprising regions of high-carbon retained austenite and/or martensite.
After tempering at 200°C for 1 hour, the SEM morphology showed no significant change compared to the as-cast state. The M/A islands remained clearly visible and seemingly intact. TEM examination of the 200°C tempered sample confirmed a high density of dislocations within the bainitic ferrite laths, a characteristic feature of this phase. Notably, no discrete carbide precipitates were observed within the ferrite at this stage. This indicates that although the bainitic ferrite is supersaturated with carbon, the thermal activation at 200°C is insufficient to trigger significant carbide nucleation and growth. The tempering process at this temperature is likely dominated by very early-stage clustering of carbon atoms and slight relaxation of stresses.
Upon increasing the tempering temperature to 300°C, subtle changes became apparent in the SEM. The bainitic ferrite matrix began to appear less distinct, and the boundaries of some M/A islands showed signs of incipient decomposition, becoming slightly irregular.
Intermediate Temperature Tempering (400-500°C)
The tempering at 400°C marked a more pronounced change. SEM observation revealed that the M/A islands had undergone significant decomposition. Their originally smooth, island-like morphology became highly disrupted and “ragged,” indicating active transformation of the high-carbon austenite within them.
Tempering at 500°C provided critical insights into the precipitation behavior and dislocation recovery. TEM images from this condition, such as the one provided, clearly showed the presence of fine, plate-like or granular carbide precipitates within the bainitic ferrite laths. EDS analysis on one such particle confirmed the presence of Fe, C, and significant amounts of alloying elements like Cr and Mn, suggesting it was an alloy carbide. Interestingly, a minor peak for La was also detected in the EDS spectrum from the carbide. This intriguing finding suggests that the precipitated carbide may have incorporated RE atoms that were originally in solid solution in the matrix at that location.
Despite the occurrence of precipitation, the bainitic ferrite in the 500°C tempered sample still retained a high density of dislocations. Furthermore, no obvious merging or coarsening of adjacent bainitic ferrite laths was observed, indicating that the recovery process had not yet commenced in a significant, microstructurally visible way at this temperature.
High-Temperature Tempering (650°C)
The microstructure after tempering at 650°C exhibited features indicative of advanced tempering stages. TEM examination revealed several key phenomena. First, the carbide precipitates had coarsened noticeably compared to those at 500°C. They were often located at dislocation lines and at interfaces, such as prior austenite or bainite lath boundaries. EDS analysis on these larger carbides again showed them to be alloy carbides containing Cr, Mn, Mo, and traces of RE elements.
Second, and most significantly, clear evidence of recovery was observed. TEM images displayed the formation of dislocation walls and subgrain boundaries within the bainitic ferrite. These are low-energy configurations formed by the rearrangement and annihilation of dislocations, marking the onset of polygonization. An example is the clear network of dislocations forming a sub-boundary. This suggests that at 650°C, the thermal activation is sufficient to allow substantial dislocation mobility. The prior precipitation of carbides, which scavenges carbon from solid solution, reduces the pinning effect on dislocations (as carbon atoms strongly pin dislocations), thereby enabling the recovery process.
Results: Precipitate Analysis and Retained Austenite Quantification
Identification and Chemistry of Precipitates
The XRD pattern obtained from the electrolytically extracted powder of the 650°C tempered sample is shown in the provided figure. All diffraction peaks were successfully indexed to the orthorhombic cementite structure (M3C-type, where M represents metal atoms). The calculated lattice parameters were in the range of a = 0.4523-0.4530 nm, b = 0.5088-0.5180 nm, c = 0.6743-0.6772 nm. This confirms that the equilibrium carbide phase formed during high-temperature tempering in this steel casting is alloy cementite.
The chemical composition of this extracted cementite, as determined by ICP-AES, is summarized in Table 2. It reveals that the cementite is significantly alloyed, particularly with chromium and manganese. Crucially, it also contains 0.0011 wt% (11 ppm) of RE (La+Ce).
This measured RE content in the cementite is remarkably consistent with the amount of RE found in solid solution in the as-cast state via the ICP-MS method (3.5 ppm La + 5.5 ppm Ce ≈ 9 ppm total RE). This correlation strongly supports a mechanism where the RE atoms, initially in solid solution in the austenite and subsequently trapped in the bainitic ferrite upon transformation, are not stable in the α-Fe lattice during high-temperature exposure. Upon tempering at 650°C, they redistribute and partition into the growing cementite precipitates. This finding aligns with literature suggesting that RE atoms like La can substitute for Fe atoms in the cementite lattice, forming an (Fe, RE)3C type alloy cementite. The precipitation sequence can thus be described as: Supersaturated α (Bainitic Ferrite) → Transition Carbides (not prominently observed here) → Alloy Cementite (Fe, Mn, Cr, Mo, RE)3C.
The total weight fraction of the extracted cementite phase, $W_{cem}$, can be estimated from the balance of a key alloying element. Using chromium as an example, a mass balance equation can be set up:
$$ [Cr]_{steel} = W_{cem} \cdot [Cr]_{cem} + (1 – W_{cem}) \cdot [Cr]_{\alpha} $$
where $[Cr]_{steel}$ is the bulk Cr content (1.04%), $[Cr]_{cem}$ is the Cr content in cementite from Table 2 (0.314%), and $[Cr]_{\alpha}$ is the Cr content remaining in the ferrite matrix, assumed to be near negligible after equilibrium partitioning at 650°C. Solving gives $W_{cem} \approx \frac{1.04}{0.314} \approx 3.3$ wt%, which is a reasonable estimate for the precipitate fraction.
| C | Si | Mn | P | S | Cr | Ni | Mo | RE (La+Ce) |
|---|---|---|---|---|---|---|---|---|
| 0.20 | 0.52 | 1.04 | <0.01 | 0.002 | 1.04 | 1.89 | 0.51 | 0.010 |
| Mn | Cr | Ni | Mo | RE (La+Ce) |
|---|---|---|---|---|
| 0.131 | 0.314 | 0.011 | 0.108 | 0.0011 |
Decomposition of Retained Austenite / M/A Constituents
The XRD patterns for samples tempered between 200°C and 500°C are shown in the provided figure. Distinct diffraction peaks for austenite (γ) are visible for tempering temperatures up to 400°C. As the tempering temperature increases, the intensity of these austenite peaks progressively diminishes. By 450°C, the austenite peaks are no longer detectable above the background noise of the diffractometer.
The volume fraction of retained austenite, $V_{\gamma}$, calculated from these XRD patterns, is plotted as a function of tempering temperature in the accompanying graph. The data clearly demonstrates a continuous decrease in $V_{\gamma}$ with increasing temperature. The as-cast material and the 200°C tempered sample contain the highest amount of retained austenite (approx. 10-12 vol%). This amount decreases gradually up to 350°C, then drops sharply between 350°C and 400°C. By 450°C, $V_{\gamma}$ has effectively reached zero. This quantitative result perfectly corroborates the SEM observations: the M/A islands remain largely stable at 200°C, begin decomposing around 300°C, and undergo massive decomposition between 400-450°C.
The thermal stability of the M/A islands in the as-cast granular bainite of this steel casting is relatively high due to two factors. First, the austenite within these islands is heavily enriched with carbon and alloying elements (Mn, Ni, Cr, Mo) during the bainitic transformation, which were rejected from the forming ferrite. This enrichment significantly lowers its martensite start (Ms) temperature, stabilizing it to room temperature. Second, the overall alloy design, including the RE addition, impacts transformation kinetics. The decomposition during tempering likely involves the diffusion of carbon out of the austenite, leading to its destabilization and subsequent transformation to ferrite and cementite. The tempering temperature provides the necessary activation energy for this diffusion-controlled process.
| Tempering Temperature (°C) | Volume Fraction of Retained Austenite, $V_{\gamma}$ (%) |
|---|---|
| As-Cast / 200 | ~11.5 |
| 250 | ~9.0 |
| 300 | ~7.5 |
| 350 | ~5.5 |
| 400 | ~2.0 |
| 450 | 0 (Not Detectable) |
Discussion: Mechanisms of Microstructural Evolution
The comprehensive experimental data allows for the construction of a detailed picture of the tempering-induced microstructural evolution in this RE-modified bainitic steel casting. The processes are temperature-activated and occur in overlapping stages.
Stage I (≤ 300°C): Stability and Early Changes. At the lowest tempering temperatures, the primary event is the relaxation of internal stresses and very limited carbon redistribution within the bainitic ferrite. The M/A islands, due to their high chemical stability, remain largely unchanged. The high dislocation density inherited from the bainitic transformation is preserved, providing potent nucleation sites for future precipitation.
Stage II (300-500°C): Precipitation and M/A Decomposition. This is the most active temperature range for phase transformations. Two major processes occur concurrently:
1. Carbide Precipitation: With sufficient thermal activation, carbon atoms in supersaturated bainitic ferrite become mobile. They combine with iron and alloying atoms (primarily Cr and Mn at these temperatures) to nucleate fine cementite precipitates. These precipitates form preferentially on dislocations and lath boundaries, as observed. The initial precipitates are likely transition carbides that quickly evolve towards the stable alloy cementite structure as temperature and time allow for greater atomic mobility of metallic atoms.
2. M/A Island Decomposition: The high-carbon austenite within the M/A islands becomes thermodynamically unstable at these temperatures. Carbon diffuses out, lowering the carbon content of the austenite and raising its Ms temperature. This leads to its transformation, typically into a mixture of ferrite and carbides (often cementite), which disrupts the original island morphology. The sharp drop in retained austenite content between 350°C and 400°C indicates the temperature range where the diffusion rate of carbon becomes high enough to trigger rapid, bulk decomposition of these constituents. The presence of strong carbide-forming elements (Cr, Mo) and RE atoms, which tend to segregate to defects like boundaries, can retard carbon diffusion, slightly shifting this decomposition to higher temperatures compared to a simpler steel system.
Stage III (≥ 550-650°C): Coarsening and Recovery. At the highest tempering temperatures, the focus shifts from phase transformations to microstructural coarsening and defect annihilation.
1. Carbide Coarsening: Following Oswald ripening, smaller cementite particles dissolve, and larger ones grow to reduce the total interfacial energy. The precipitates observed at 650°C are noticeably larger than those at 500°C. The partitioning of alloying elements (Mn, Cr, Mo) and RE into the cementite reaches its equilibrium state, as confirmed by the phase analysis.
2. Recovery of Bainitic Ferrite: This is a key finding. The persistence of a high dislocation density up to 500°C is attributed to the pinning effect of both interstitial carbon atoms and the fine, closely spaced carbide precipitates. As temperature increases to 650°C, several factors promote recovery: a) further depletion of carbon from solid solution into coarser carbides reduces interstitial pinning, b) the coarsening of carbides increases the mean free path between obstacles, and c) the thermal energy provides the necessary activation for dislocation climb and cross-slip. This leads to the rearrangement of dislocations into lower-energy configurations, forming dislocation walls and subgrains, as clearly captured in the TEM images. The onset of recrystallization (the formation of new, strain-free grains) was not observed within the 1-hour timeframe at 650°C, suggesting it would require either longer times or higher temperatures in this alloyed steel casting.
The role of rare earth in this evolution appears to be multifaceted. In solid solution, RE atoms likely segregate to dislocations and boundaries, influencing early-stage carbon clustering and potentially delaying the onset of recovery by providing additional pinning points. The most definitive evidence is their incorporation into the alloy cementite during precipitation at high temperature. This incorporation suggests that RE atoms are not merely passive spectators but actively participate in the phase transformation kinetics and may modify the interfacial energy and coarsening behavior of the carbides, which could have implications for the long-term tempering stability of the steel casting.
Conclusions
Based on a systematic investigation of the tempering behavior of a La-Ce modified low-alloy bainitic wear-resistant steel casting, the following key conclusions can be drawn regarding its microstructural evolution:
- The as-cast microstructure consists of granular bainite with M/A constituents. Tempering between 200°C and 650°C induces a sequence of transformations including carbide precipitation, decomposition of M/A islands, and recovery of the bainitic ferrite.
- The equilibrium carbide phase that forms during high-temperature tempering is alloy cementite (M3C). Chemical analysis confirms that this cementite contains significant amounts of alloying elements (Cr, Mn, Mo) and, importantly, incorporates trace amounts of rare earth (La, Ce) atoms that were initially in solid solution. This represents a clear mechanism for the redistribution of solute RE during heat treatment.
- Recovery of the dislocation-rich bainitic ferrite is strongly suppressed at temperatures up to 500°C. Distinct evidence of recovery, in the form of subgrain formation via dislocation rearrangement, is only observed after tempering at 650°C for 1 hour. This is attributed to the strong pinning effect of carbon and fine precipitates at lower temperatures.
- The retained austenite within the M/A islands is stable up to approximately 300°C. Its decomposition becomes significant between 300°C and 400°C, and is essentially complete after tempering at 450°C for 1 hour, as confirmed by quantitative XRD analysis. This decomposition is a major microstructural change that significantly alters the phase balance and mechanical properties.
This study provides a fundamental understanding of the tempering process in RE-enhanced bainitic steel castings. The findings on RE partitioning into cementite and its effect on delaying recovery are particularly noteworthy. This knowledge is essential for designing optimized tempering schedules to achieve specific combinations of strength, toughness, and wear resistance in this class of advanced wear-resistant materials.