Nb Microalloying Enhances the Grain Stability of SAE8620H Gear Steel During High-Temperature Carburizing

Abstract

In modern industries, gears function as pivotal transmission elements whose operational performance is directly dependent on the microstructural characteristics of gear steels. While high-temperature carburizing (950–1050 °C) substantially improves process efficiency through accelerated carbon diffusion, it inevitably promotes austenite grain coarsening. This study investigates the effect of Nb microalloying on grain stability in SAE8620H gear steel during high-temperature carburizing. Experimental steels with varying Nb contents were prepared via vacuum induction suspension melting, followed by hot rolling, solution treatment, and pseudo-carburizing. Thermodynamic calculations, optical microscopy, transmission electron microscopy, and energy-dispersive spectroscopy were employed to analyze the mechanisms. Thermodynamic results revealed that higher Nb content retains more Nb(C, N) phases at elevated temperatures, effectively suppressing grain coarsening. Without preheating, increased Nb content refined grains but exhibited limited inhibition at high temperatures. Preheating (1330 °C × 10 min + water quenching) promoted uniform and fine Nb(C, N) precipitates, significantly enhancing grain refinement. When Nb content exceeded 0.053 wt.%, grain coarsening was fully inhibited under 1050 °C × 2 h carburizing. This study establishes the optimal Nb content range, elucidates the micro-mechanisms, and proposes a preheating process to improve high-temperature carburizing performance in gear steels.

1. Introduction

Gears serve as critical transmission components across strategic industries including aerospace, advanced manufacturing, and renewable energy systems [1,2]. Their performance and quality are critical determinants of the operational efficiency, reliability, and service life of mechanical systems. Consequently, optimizing the properties of gear steels has always been a focal point in materials research, with the carburizing treatment process being a core area of investigation [3,4,5].

Traditional carburizing temperatures typically span from 870 °C to 980 °C [6]. Within this temperature bracket, the diffusion rate of carbon in austenite remains relatively stable. However, this temperature range can no longer meet the escalating demands of modern industry for enhanced production efficiency. Recent advances demonstrate that elevated carburizing temperatures (≥980 °C) can dramatically enhance process kinetics. For example, Asi et al.’s research on SAE8620 gear steel indicated that, when the carburizing temperature increased from 940 °C to 980 °C, the carburizing time was reduced by over 50% [7]. Theoretical calculations further confirmed that, at 1000 °C, the time required to achieve the same carburized layer depth could be shortened to approximately one-third of the original duration [8]. As such, high-temperature carburizing has become a prominent approach to boost efficiency and cut costs, garnering significant attention from both academia and industry.

Nonetheless, detrimental austenite grain coarsening associated with high-temperature carburizing has severely limited its widespread adoption. The austenite grain size is a key microstructural parameter that governs the properties of gear steels. Research by K.A. Alogab et al. clearly demonstrated that SAE8620 gear steel exhibited severe grain coarsening in the carburized layer after being carburized at 950–1050 °C [9]. Coarse grains have a detrimental impact on the mechanical properties of gear steels. They reduce the material’s strength and toughness, making gears more susceptible to deformation and fracture under load [10]. In terms of fatigue performance, coarse grains act as initiation sites for fatigue cracks and facilitate their propagation [11,12], thereby substantially reducing the fatigue life of gears. This poses a significant safety risk in industries with stringent reliability requirements, such as aerospace and wind power.

To address this challenge, the topic of microalloying strategies has emerged as a prominent research area. Microalloying elements like Nb, V, Ti, and Al possess unique capabilities in inhibiting grain growth [13,14,15,16,17,18]. Among them, the Nb element has drawn particular attention due to its ability to form highly stable compounds with carbon and nitrogen. Multiple studies have provided strong evidence for its underlying mechanism. Morais et al. found that AISI 5115 gear steel with 0.038 wt.% Nb addition maintained fine grains after specific treatments, with the NbC precipitated phase being the primary contributing factor [19]. Gong et al.’s research indicated that adding an appropriate amount of Nb to SCr420 gear steel could effectively impede grain growth [20]. Zhang et al. from the Central Iron and Steel Research Institute also confirmed that adding slight amounts of Nb to 18CrNiMo7-6 gear steel could mitigate the grain growth tendency, with 0.03 wt.% Nb showing a pronounced effect [21]. These findings collectively highlight the significant potential of the Nb element in refining the carburized layer microstructure and optimizing mechanical properties, especially fatigue resistance.

While previous studies have established fundamental principles of Nb microalloying in gear steels, critical knowledge gaps persist regarding its specific effects on SAE8620H steel during high-temperature carburizing (>980 °C). The distinct chemical compositions and microstructures of different steel grades lead to variations in the mechanism and efficacy of Nb. Currently, the optimal Nb content range for high-temperature carburizing of the SAE8620H gear steels remains undefined, and the micro-mechanism by which it inhibits grain growth is not clear. In addition, existing research lacks a comprehensive and in-depth analysis of the impact of heat treatment processes on Nb precipitated phases in this steel grade, failing to fully exploit the potential of heat treatment optimization to enhance the effects of Nb microalloying.

This study on the SAE8620H gear steel has significant practical importance and innovation. By integrating material calculations and experimental investigations, this research precisely mapped the grain growth curves under different Nb contents, determined the optimal Nb content range for high-temperature carburizing, and filled a gap in existing research. Moreover, an innovative pre-heat treatment process was developed to precisely control the Nb precipitated phases, further enhancing the ability of Nb to refine grains. The developed process–structure–property relationships provide actionable guidelines for industrial practice, driving the innovation and development of gear steel manufacturing technology and meeting the escalating demands of modern industry for high-performance gear steels. Additionally, advanced micro-characterization techniques, such as high-resolution transmission electron microscopy, were utilized to comprehensively analyze the micro-characteristics of Nb precipitated phases and their interaction mechanisms with austenite grains. The fundamental insights into precipitation mechanisms advance microalloying theory for high-temperature steel applications.

2. Experimental Methods

2.1. Material Preparation

The chemical compositions of the materials used in this study are presented in

Table 1. The chemical composition of the experimental steel.

No.	C (wt.%)	Si (wt.%)	Mn (wt.%)	Cr (wt.%)	Mo (wt.%)	Ni (wt.%)	Cu (wt.%)	Als (wt.%)	N (wt.%)	Nb (wt.%)	Fe (wt.%)
1#	0.21	0.27	0.79	0.60	0.18	0.59	0.014	0.021	0.011	0.02	Bal.
2#	0.20	0.26	0.67	0.59	0.19	0.58	0.013	0.023	0.012	0.04	Bal.
3#	0.18	0.26	0.68	0.62	0.20	0.61	0.012	0.020	0.013	0.053	Bal.
4#	0.20	0.26	0.75	0.61	0.19	0.59	0.013	0.021	0.012	0.08	Bal.
5#	0.19	0.27	0.71	0.60	0.19	0.59	0.013	0.022	0.010	0.1	Bal.

. The experimental steels were microalloyed with different Nb contents based on SAE8620H steel. The melting process of the experimental steels was carried out in a vacuum induction levitation melting furnace, with each batch weighing 1 kg. During the melting process, alloying elements such as Si, Mo, Ni, Cu, and Fe remained stable without significant burn-out, while Mn and Cr experienced only minor losses. The oxygen content in the raw materials reduced the acid-soluble Al content, and the final acid-soluble Al content was expected to range between 0.02% and 0.025%.

Chemical characterization was performed using inductively coupled plasma atomic emission spectrometry (ICP-AES) for metallic elements, combustion-based carbon/sulfur analysis for C content, and inert gas fusion analysis for O/N levels. All analytical measurements were performed in ISO-accredited laboratories to ensure traceability and precision.

The selected Nb concentration gradient (0.02–0.1 wt.%) was strategically designed to systematically investigate concentration-dependent effects on high-temperature austenite grain stability. While prior investigations [19,20,21] typically employed 0.03 wt.% Nb for grain refinement in gear steels, this study aimed to explore the enhanced precipitation strengthening potential at elevated temperatures through incremental Nb additions. The gradient approach enabled statistical analysis of content–response relationships, and although minor compositional discrepancies occurred during melting, the observed variations (±0.005 wt.%) were within acceptable experimental error margins relative to the 0.08 wt.% concentration span, thus preserving the integrity of the study’s conclusions.

The melted ingots were then subjected to hot rolling with a reduction ratio of 80%. Prior to rolling, a solution treatment at 1200 °C for 30 min was performed. The hot-rolling process was executed using a two-high rolling mill (Model: 200 × 300, Wuxi Guancheng Metal Technology Co., Ltd., Jiangyin, China), which incorporates precision hydraulic adjustment systems and temperature-monitoring sensors to ensure controlled deformation at 1250 ± 10 °C. The 80% reduction was achieved via a five-pass schedule with incremental thickness reductions: 30 mm → 25 mm → 20 mm → 15 mm → 10 mm → 6 mm. Ingots were reheated to 1250 °C in a programmable resistance furnace between passes to maintain austenitic stability and minimize work hardening, ensuring uniform deformation and preventing cracking.

This treatment was designed to precipitate fine and dispersed precipitated phases during the deformation process, thereby refining the sample grains. After hot rolling, the samples were air-cooled to room temperature, and multiple metallographic specimens with dimensions of 10 × 10 × 6 mm were cut for subsequent pseudo-carburizing tests under different conditions. The pseudo-carburizing conditions mainly involved holding at temperatures ranging from 980 °C to 1050 °C for 1–8 h, followed by water quenching. To further enhance the control of Nb microalloying over the structural stability, a pre-solution and water-quenching process was applied to the experimental steels before pseudo-carburizing. This pre-treatment consisted of holding at 1330 °C for 10 min, followed by water quenching, and was aimed at investigating the structural stability of the samples after pseudo-carburizing at temperatures above 1000 °C for 2–6 h.

2.2. Microstructural Characterization

The pseudo-carburized samples were processed using standard mechanical grinding and polishing techniques. Given that gear steels undergo a γ-α phase transformation around 700 °C, and the γ grains formed during carburizing cannot be retained at room temperature, specific methods were employed to reveal the original γ grain boundaries. The corrosion process was as follows: The quenched specimens were first tempered at 300 °C for 2 h and then corroded using the Labtech LBTK03 corrosive (Labtech Test Technology (Wuhan) Co., Ltd., Wuhan, China). The water bath temperature was maintained at 70 °C for 15 min, and the sample surface was cleaned every 5 min. The etched samples were then examined under an optical microscope (OM) to measure the average grain size and evaluate the grain fineness. For transmission electron microscopy (TEM) analysis, the samples were prepared by grinding thin slices to a thickness of 50 μm, followed by punching. Subsequently, a 10% perchloric acid–alcohol solution was used for electrolytic double-jetting at a voltage of 28 V and a temperature of −25 °C. A Thermo Fisher Scientific Talos F200i transmission electron microscope (Waltham, MA, USA) operated at 200 kV accelerating voltage with a point resolution of 0.19 nm was utilized to observe and analyze the Nb-containing precipitated phases in the samples. The system was equipped with an Oxford X-Max 80 mm² Silicon Drift Detector (SDD) (Oxford Instruments, Abingdon, Oxfordshire, UK) for energy-dispersive X-ray spectroscopy (EDS), operated at a working distance of 10–15 mm with ≤1 nm spatial resolution for STEM-EDS mapping. EDS spectra were acquired over 60 s per analysis spot and quantified using Cliff–Lorimer k-factor calibration.

3. Results and Discussion

3.1. Thermodynamic Calculations

The variation in the Nb(C, N) phase content with temperature (from 500 °C to 1500 °C) for different Nb contents (0.02%, 0.04%, 0.06%, 0.08%, and 0.1%) is depicted in Figure 1. The elaborated calculation method was as follows: First, the chemical compositions from Table 1 were input into the Thermo-Calc software (Thermo-Calc 2020b). Next, we selected the appropriate thermodynamic database (TCFE10) to ensure reliable thermodynamic parameter support. Then, we configured calculation parameters, including setting the temperature range (500–1500 °C) and defining equilibrium calculation conditions according to the software’s standard protocols. Finally, the software executed computations based on thermodynamic equilibrium theory to obtain the Nb(C, N) phase content.

As shown in the figure, with an increase in temperature, the Nb(C, N) phase content corresponding to each Nb content exhibits a decreasing trend. In the low-temperature range (approximately 500–1000 °C), the decrease is relatively gradual, and the differences between the curves for different Nb contents are minimal. However, when the temperature exceeds 1000 °C, the rate of decrease accelerates. Moreover, the higher the Nb content, the relatively higher the Nb(C, N) phase content at the same temperature, resulting in an increasing divergence between the curves.

During the high-temperature carburizing process of gear steels, the traditional carburizing temperature range fails to meet the efficiency requirements of modern industry. High-temperature carburizing, while enhancing carbon diffusion, leads to austenite grain coarsening, which compromises the performance of gear steels. In microalloying technology, the Nb element is of particular interest because of its ability to form highly stable Nb(C, N) compounds with carbon and nitrogen. The Nb(C, N) precipitated phase has excellent high-temperature stability and can pin the austenite grain boundaries during high-temperature carburizing, effectively inhibiting grain growth and maintaining a fine-grained structure [13,21].

From Figure 1, it can be observed that, at higher Nb contents, the Nb(C, N) phase can maintain a relatively high content at elevated temperatures. This implies a more effective pinning of the grain boundaries, thereby suppressing grain coarsening. For example, when the temperature reaches above 980 °C, the curves corresponding to higher Nb contents still exhibit a certain level of Nb(C, N) phase content. This is of great significance for maintaining the microstructure integrity of the SAE8620H gear steel during high-temperature carburizing. However, the optimal Nb content range for high-temperature carburizing of this steel grade and the underlying micro-mechanism remain to be further investigated.

3.2. Grain Growth Behavior Without Pre-Heating

In the absence of pre-heat treatment, the Nb-microalloyed experimental steels after hot rolling were directly subjected to pseudo-carburizing tests. Figure 2 presents the original austenite grain morphology of the experimental steels after pseudo-carburizing at 1000 °C for 4 h. It is evident that, after Nb microalloying, as long as the Nb content is not less than 0.04%, the SAE8620H gear steel can maintain a fine-grained structure under the 1000 °C × 4 h pseudo-carburizing regime. The average grain size decreased from 11.7 μm to approximately 10.2 μm. Nb, as a strong carbide-forming element, combines with carbon during the austenitization process of steel to form fine and uniformly dispersed Nb(C, N) particles. These particles preferentially precipitate at the austenite grain boundaries and effectively impede the migration of the grain boundaries through a pinning mechanism [13,21]. When the Nb content is low (e.g., 0.02%), the grains are relatively coarse, and abnormal grain growth may occur. The lack of Nb(C, N) particles due to the low Nb content could be the reason. As the Nb content increases, the number of Nb(C, N) particles increases, leading to a denser distribution of pinning sites and a stronger inhibition of grain boundary movement. This results in a more pronounced grain refinement effect, enabling the maintenance of a stable fine-grained structure even after a relatively long holding time of 4 h.

When the temperature is increased to 1050 °C and the holding time is shortened to 2 h, although the holding time is reduced, the diffusion rate of atoms at high temperatures increases sharply, and the migration rate of grain boundaries accelerates, resulting in a more rapid tendency of grain growth. In this case, at low Nb contents, due to the limited number of Nb(C, N) particles, their pinning effect on the grain boundaries is insufficient to resist the strong driving force for grain boundary migration at high temperatures, causing the grains to grow rapidly. Even at high Nb contents, where there are more Nb(C, N) particles, the inhibitory effect on grain growth is weakened.

As shown in Figure 3, even with the high addition of 0.1% Nb, the Nb(C, N) particles cannot maintain a fine-grained state at this high temperature. The grains grow completely, and the coarsening trend is difficult to suppress. This indicates that, under extremely high-temperature conditions, the grain refinement effect of Nb alone is limited, and it may be necessary to combine other process methods to further control the grain size.

3.3. Grain Growth Behavior After Pre-Heating

The coarsening kinetics of precipitate particles during carburizing represents the predominant factor governing grain growth behavior [18]. Therefore, if the coarsening rate of precipitated phase particles during the carburizing process can be retarded, the fine-grain control ability for a specific composition can be significantly enhanced. From the perspective of particle coarsening kinetics, compared with a wide size distribution state of particles, an initial particle state with a narrow size distribution is less conducive to the progress of coarsening. Thus, this paper attempts to obtain an initial precipitated phase particle state with a narrow size distribution through heat treatment process optimization, aiming to improve the fine-grain control ability during the carburizing process.

The specific idea of this process is as follows: First, the Nb(C, N) precipitated phases in the as-rolled structure are completely dissolved back into the matrix by a high-temperature solution treatment. Then, water quenching is used to suppress precipitation in the high-temperature region, obtaining a supersaturated matrix with a very small number of fine precipitated phases. Subsequently, during carburizing, the Nb(C, N) precipitated phases precipitate isothermally at the grain boundaries, resulting in uniformly fine precipitated phase particles and achieving an initial precipitated phase state with a narrow size distribution. According to the thermodynamic calculation results in Figure 1, the highest theoretical dissolution temperature of Nb(C, N) does not exceed 1320 °C. Therefore, the solution temperature is set at 1330 °C.

A new pre-heat treatment process of solution + water quenching was adopted, and the microstructures of the Nb-microalloyed specimens after pseudo-carburizing at 980–1050 °C for 1–8 h were re-studied. Some of the microstructural images after pseudo-carburizing are presented below.

Figure 4 presents the austenitic grain morphology of pre-treated test steels with varying Nb contents following pseudo-carburizing at 1050 °C for 2 h. It can be seen that, after maintaining the solution at 1330 °C for 10 min + the water quenching process, when the Nb content is not less than 0.053%, the SAE8620H test steel can maintain non-coarsened grains under the 1050 °C × 2 h carburizing regime, which is a carburizing regime that cannot be achieved by conventional processes (Figure 3). This demonstrates the enhanced grain refinement capability achieved through the optimized pre-heat treatment combined with critical Nb microalloying.

Figure 5 illustrates the temporal evolution of grain structure in 0.1 wt.% Nb-modified steel subjected to extended carburizing durations at 1050 °C. It can be seen that, when the Nb content is increased to 0.1% and the pre-solution + water quenching process is applied, the SAE8620H test steel can maintain a fine-grained state under the carburizing regime of up to 1050 °C × 4 h, greatly improving the high-temperature carburizing structure stability of this gear steel. However, when the carburizing time is increased to 6 h, abnormal growth of individual grains occurs. The pinning effect of Nb(C, N) precipitated phase particles on the grain boundaries provides important support for grain refinement. However, with the extension of the carburizing time, the coarsening and dissolution of Nb(C, N) particles will reduce the pinning effect. When the driving force for grain boundary migration exceeds the pinning force of the particles, grain coarsening occurs.

3.4. Analysis and Discussion

During the high-temperature pseudo-carburizing process of gear steels, the presence of Nb(C, N) precipitated phase particles plays a critical role in microstructural control, especially in suppressing grain coarsening. As evidenced in Figure 1, the actual precipitation temperature range of the precipitated phases is wide. Under conventional slow-cooling conditions after hot rolling, prolonged exposure to elevated temperatures facilitates complete Nb(C, N) precipitation, generating a wide particle size distribution prior to carburizing. These heterogeneous particles undergo substantial coarsening during subsequent high-temperature carburizing, resulting in increased size disparity and larger average particle dimensions. A modified approach incorporating post-rolling solution treatment and water quenching significantly alters this precipitation sequence. The solution treatment first dissolves existing precipitates, while rapid quenching minimizes dwell time in the precipitation temperature zone, creating an under-precipitated state with a constrained particle size distribution. During subsequent carburizing, isothermal precipitation conditions promote simultaneous nucleation and growth of Nb(C, N) particles. This kinetic control yields a homogeneous dispersion of fine precipitates with enhanced size uniformity, effectively maintaining grain boundary pinning throughout high-temperature exposure. The comparison demonstrates that thermal history manipulation directly governs precipitate evolution pathways. Strategic process modification enables the optimization of precipitation characteristics for superior microstructural stability under extreme thermal conditions.

Figure 6 shows the TEM bright-field images of the sample with 0.08% Nb content after pseudo-carburizing at 1050 °C for 2 h before and after pre-heat treatment. It can be clearly seen that a large number of spherical particles (indicated by red arrows) are distributed in the matrix. According to Figure 6c, these particles are Nb(C, N) precipitated phases containing Nb, which can pin the austenite grain boundaries and hinder grain growth. Through statistics, the maximum size of the precipitated phase particles in the carburized sample before pre-heat treatment is 102.7 nm, the minimum size is 23.8 nm, and the average particle size is 50.7 nm. After pre-heat treatment, the maximum size of the precipitated phase particles in the carburized sample is 68.5 nm, the minimum size is 13.9 nm, and the average particle size is 38.0 nm. The comparison shows that, after the pre-solution + water quenching heat treatment process, the precipitated phase particles in the high-temperature carburizing state are more uniform and fine. The Zener pinning force of particles on the grain boundaries can be calculated by the formula [22]:

$$P_z=\sigma \sum _i{\xi }_i{\displaystyle \frac{f_i}{r_i}}$$

where f is the volume fraction of the precipitated particles, r is the particle radius, i represents the type of precipitated phase, and ξ is a dimensionless constant determined by the particle characteristics. According to this formula, when the volume fraction of the precipitated phase remains approximately unchanged, the smaller the particle size, the greater the pinning force of the particles on the grain boundaries, and the more favorable it is for inhibiting grain growth.

Figure 7 displays the TEM bright-field images of the experimental steel with 0.04% Nb content under different pseudo-carburizing regimes after pre-heat treatment, and Figure 8 shows those of the experimental steel with 0.08% Nb content under different pseudo-carburizing regimes after pre-heat treatment. It can be clearly observed that spherical particles are uniformly distributed in the matrix after pre-heat treatment and subsequent carburizing. According to the EDS results, these are Nb(C, N) precipitated phases containing Nb. With the change in the carburizing regime, the shape of the particles remains basically spherical, but the size changes significantly. The sizes of the precipitated phase particles are statistically presented in

Table 2. Average size and number density of precipitated phase particles under different carburizing regimes after preheating treatment.

		Solution + Quenching	Carburizing 1050 °C × 2 h	Carburizing 1050 °C × 6 h	Carburizing 1000 °C × 4 h	Carburizing 1000 °C × 8 h
Nb 0.04%	Size	22.1 nm	27.3 nm	60.3 nm	27.7 nm	46.6 nm
	Number density	13.1 × 1012 m−2	7.5 × 1012 m−2	4.9 × 1012 m−2	7.1 × 1012 m−2	11.7 × 1012 m−2
Nb 0.08%	Size	18.4 nm	38.0 nm	67.8 nm	29.6 nm	34.4 nm
	Number density	13.8 × 1012 m−2	11.3 × 1012 m−2	3.0 × 1012 m−2	16.1 × 1012 m−2	8.4 × 1012 m−2

.

Based on the Ostwald ripening theory, the DICTRA software (The simulation is performed by employing the thermodynamic database TCFE•8 and the kinetic database MOFE•3, with the Thermo-Calc 2020b software) was used to simulate the coarsening process of Nb(C, N) precipitated phases in SAE8620H test steels. Although the actual precipitated phase is Nb(C, N), nitrogen has a fast diffusion rate and a negligible impact on coarsening, as an interstitial atom. It is mainly the Nb element that dominates the coarsening process of the precipitated phase. Thus, the precipitated phase NbC can be used as a substitute for simulation. The parameters used in the simulation are as follows: the system is Fe-C-Nb, the interface energies are 0.54 J/m² for 1050 °C and 0.57 J/m² for 1000 °C [23], the molar volume is 1.345 × 10⁻⁵ m³/mol [24], and the initial particle size is the experimental value after pre-heat treatment before carburizing.

The curves of the average diameter of Nb(C, N) precipitated phases in the test steels changing with the holding time, as simulated by the DICTRA software, are shown in Figure 9. It can be seen that, with the increase in time, the diameters of Nb(C, N) in all four states gradually increase, and the experimental data points are basically consistent with the simulated curves. By comparing Figure 9a and Figure 9b, as well as Figure 9c and Figure 9d, it can be found that, at the same Nb content, when the temperature increases, the growth rate of the Nb(C, N) diameter accelerates. This is because high temperatures provide a more atomic diffusion rate, making it easier for Nb and C atoms to aggregate and form NbC particles for growth and coarsening. By comparing Figure 9a and Figure 9c, as well as Figure 9b and Figure 9d, it can be seen that, at the same temperature, an increase in the Nb content leads to an increase in the Nb(C, N) diameter. This is because the diffusion process of Nb is generally considered to be the rate-limiting step in the coarsening kinetics of Nb(C, N). An increase in the Nb content raises the concentration of Nb in the matrix, which enhances the diffusion rate of Nb, and consequently increases the size of Nb(C, N).

It is worth noting that the experimental data are in good agreement with the simulated curves when the holding time is short. When the holding time is long, the actual particle size of the precipitated phase is significantly larger than the simulated data. This may be attributed to the fact that the initial precipitated phase size distribution in the DICTRA coarsening simulation is a standard normal distribution, while the actual precipitated phase size distribution is wider, resulting in a faster coarsening rate of the actual Nb(C, N) than the simulation results. Charles et al. [25] have revealed this phenomenon in carburized steel as well that particles with a bimodal size distribution had a faster coarsening rate than those with a unimodal size distribution. In addition, the assumption in this model that the volume fraction of the precipitated phase does not change with time is slightly different from the actual holding process, which also leads to a difference between the actual particle size and the simulation results [26].

4. Conclusions

In this study, the influence of Nb microalloying on the high-temperature carburizing performance of SAE8620H gear steel was investigated. The main conclusions are as follows:

(1)

Thermodynamic calculations show that, with the increase in temperature, the Nb(C, N) phase content in SAE8620H test steels with different Nb contents decreases. The higher the Nb content, the relatively higher the Nb(C, N) phase content at the same temperature. A higher Nb content can maintain a certain amount of Nb(C, N) phase at high temperatures, which is of great significance for suppressing austenite grain coarsening during high-temperature carburizing.

(2)

Without pre-heat treatment, during pseudo-carburizing at 980–1050 °C, Nb microalloying can refine the grains of SAE8620H gear steel. The higher the Nb content, the more obvious the grain refinement effect. However, as the temperature increases, the grain growth trend intensifies, and relying solely on Nb for grain refinement is limited. At 1050 °C, even with 0.1% Nb addition, it is difficult to maintain a fine-grained state.

(3)

By using the pre-heat treatment process of maintaining the solution at 1330 °C for 10 min + water quenching, the Nb(C, N) precipitated phase particles can be made uniform and fine. When the Nb content is not less than 0.053%, the grains can be ensured not to coarsen under the 1050 °C × 2 h carburizing regime. When the Nb content is 0.1%, a fine-grained state can be maintained under the 1050 °C × 4 h carburizing regime.

(4)

After pre-heat treatment, the precipitated phase particles in the high-temperature carburizing state are more uniform and fine, with a greater pinning force, which is more conducive to suppressing grain coarsening. The coarsening trend of the precipitated phase particles is in good agreement with the DICTRA simulation results, indicating that the precipitated phases coarsen in accordance with the Ostwald theory.