In modern mechanical systems, gear shafts play a pivotal role in transmitting motion, torque, and bending moments, often under severe conditions such as impact, vibration, and unbalanced loads. The performance of gear shaft steel is critical, requiring a balance of high strength, hardness, and ductility to ensure reliability and longevity. Traditional heat treatment methods, including direct quenching, have been widely applied but often fall short in achieving an optimal combination of strength and plasticity, as measured by the strength-elongation product (SEP). The quenching-partitioning (Q-P) process has emerged as a promising technique to enhance these properties by stabilizing residual austenite and tailoring microstructure. This study investigates the influence of partitioning temperature and time on the tensile properties, phase composition, microstructure, and fracture morphology of gear shaft steel, with a focus on maximizing the SEP for improved performance in demanding applications.
The gear shaft steel used in this work is a 20CrMnTi alloy structural steel, commonly employed in automotive and industrial machinery due to its excellent hardenability and toughness. The chemical composition of the steel is detailed in Table 1, which highlights key elements such as carbon, manganese, chromium, and titanium that contribute to its mechanical properties. The critical transformation temperatures, including the complete austenitization temperature (Ac₃), martensite start temperature (Mₛ), and martensite finish temperature (M_f), were determined using a dilatometer. These temperatures are essential for designing the Q-P process, as they define the range for quenching and partitioning steps. Specifically, Ac₃ was found to be 862°C, Mₛ at 398°C, and M_f at 203°C, guiding the selection of quenching end temperatures between 203°C and 398°C to facilitate partial martensite formation and subsequent carbon partitioning.
| Element | C | Si | Mn | P | S | Cr | Ni | Cu | Ti | Fe |
|---|---|---|---|---|---|---|---|---|---|---|
| Content | 0.21 | 0.23 | 0.98 | 0.009 | 0.002 | 1.17 | 0.02 | 0.01 | 0.07 | Bal. |
The heat treatment process involved austenitizing the gear shaft steel specimens at 910°C for 3 minutes, followed by quenching to specific end temperatures (T_Q) that coincide with the partitioning temperatures (T_P). This was done to maintain consistency in the thermal cycle. The partitioning times (t_P) varied from 10 s to 150 s, after which the samples were water-quenched to room temperature. A direct quenched sample, subjected only to austenitization and immediate water quenching, served as a reference for comparison. The detailed parameters for all 13 samples are summarized in Table 2, illustrating the systematic variation in partitioning conditions to analyze their effects. The tensile specimens were machined according to standard dimensions, and room temperature tensile tests were conducted at a strain rate of 2 mm/min, with results averaged over three tests for reliability.
| Sample No. | Sample Type | Quenching End Temp. T_Q (°C) | Partitioning Temp. T_P (°C) | Partitioning Time t_P (s) |
|---|---|---|---|---|
| 1 | Q-P | 225 | 225 | 10 |
| 2 | Q-P | 225 | 225 | 30 |
| 3 | Q-P | 225 | 225 | 60 |
| 4 | Q-P | 225 | 225 | 90 |
| 5 | Q-P | 225 | 225 | 120 |
| 6 | Q-P | 225 | 225 | 150 |
| 7 | Q-P | 275 | 275 | 10 |
| 8 | Q-P | 275 | 275 | 30 |
| 9 | Q-P | 275 | 275 | 60 |
| 10 | Q-P | 275 | 275 | 90 |
| 11 | Q-P | 275 | 275 | 120 |
| 12 | Q-P | 275 | 275 | 150 |
| 13 | Direct Quench | – | – | – |
Tensile properties, including ultimate tensile strength (UTS) and elongation to fracture, were evaluated to understand the mechanical behavior of the gear shaft steel under different Q-P conditions. The strength-elongation product (SEP), defined as the product of UTS and elongation, was calculated to assess the overall performance: $$ \text{SEP} = \sigma_b \times \delta $$ where $\sigma_b$ is the tensile strength in MPa and $\delta$ is the elongation in percentage. This metric is crucial for gear shaft applications, as it indicates the material’s ability to withstand deformation without failure. The results, presented in Table 3, show that for a fixed partitioning temperature of 225°C, increasing the partitioning time from 10 s to 150 s led to a gradual decrease in UTS from approximately 1580 MPa to 1500 MPa, while elongation initially increased, peaking at 15.9% for t_P = 120 s, before decreasing. Similarly, at T_P = 275°C, UTS decreased from 1560 MPa to 1440 MPa, with maximum elongation of 15.7% at t_P = 90 s. The SEP followed a similar trend, reaching maxima at t_P = 90 s for both temperatures, with values significantly higher than that of the direct quenched sample (SEP ≈ 13,786 MPa·%).
| Partitioning Temp. T_P (°C) | Partitioning Time t_P (s) | Tensile Strength σ_b (MPa) | Elongation δ (%) | SEP (MPa·%) |
|---|---|---|---|---|
| 225 | 10 | 1580 | 14.0 | 22120 |
| 30 | 1560 | 14.5 | 22620 | |
| 60 | 1540 | 15.2 | 23408 | |
| 90 | 1520 | 15.9 | 24168 | |
| 120 | 1500 | 16.0 | 24000 | |
| 150 | 1480 | 14.8 | 21904 | |
| 275 | 10 | 1560 | 13.5 | 21060 |
| 30 | 1520 | 14.0 | 21280 | |
| 60 | 1480 | 14.8 | 21904 | |
| 90 | 1440 | 15.7 | 22608 | |
| 120 | 1420 | 14.5 | 20590 | |
| 150 | 1400 | 13.2 | 18480 | |
| Direct Quench | – | 1603 | 8.6 | 13786 |
Fracture morphology analysis revealed ductile failure characteristics in all Q-P treated gear shaft steel samples, with dimple structures observed on the fracture surfaces. For instance, at T_P = 225°C and t_P = 10 s, the dimples were relatively shallow and small, indicating moderate ductility. In contrast, at t_P = 90 s, the dimples became deeper and larger, correlating with higher elongation values. Similarly, for T_P = 275°C, the optimal t_P = 90 s sample exhibited pronounced dimples, suggesting enhanced plastic deformation. The direct quenched sample, however, showed finer and shallower dimples, consistent with its lower elongation. This morphology aligns with the tensile data, underscoring the role of Q-P treatment in improving the toughness of gear shaft steel.
X-ray diffraction (XRD) analysis was employed to quantify the phase composition, particularly the volume fraction of residual austenite, which is key to the enhanced ductility in Q-P processed gear shaft steel. The direct quenched sample exhibited almost no residual austenite (below 0.5%), whereas Q-P samples showed significant amounts, ranging from 3.0% to 6.4%. The volume fraction of residual austenite (V_γ) was calculated using the direct comparison method: $$ V_\gamma = \frac{I_\gamma}{I_\gamma + K I_\alpha} $$ where $I_\gamma$ and $I_\alpha$ are the integrated intensities of the austenite and ferrite/martensite diffraction peaks, respectively, and K is a constant dependent on the crystallographic planes. For example, the sample with T_P = 225°C and t_P = 120 s had V_γ = 6.4%, while T_P = 275°C and t_P = 90 s yielded V_γ = 5.4%. These values are summarized in Table 4, highlighting that higher residual austenite content generally corresponds to improved elongation and SEP, as austenite undergoes strain-induced transformation to martensite during deformation, absorbing energy and increasing ductility.
| Partitioning Temp. T_P (°C) | Partitioning Time t_P (s) | Residual Austenite V_γ (%) |
|---|---|---|
| 225 | 10 | 3.2 |
| 30 | 3.8 | |
| 60 | 4.5 | |
| 90 | 5.1 | |
| 120 | 6.4 | |
| 150 | 4.0 | |
| 275 | 10 | 3.0 |
| 30 | 3.5 | |
| 60 | 4.2 | |
| 90 | 5.4 | |
| 120 | 4.8 | |
| 150 | 3.6 | |
| Direct Quench | – | <0.5 |
Microstructural examination using scanning electron microscopy (SEM) revealed significant changes due to the Q-P process. The direct quenched gear shaft steel consisted of fine lath martensite, typical of as-quenched structures. In Q-P samples, extending partitioning time at a constant T_P promoted martensite tempering, carbide precipitation, and growth. For T_P = 225°C and t_P = 10 s, the microstructure was dominated by lath martensite with minor carbide precipitation. At t_P = 90 s, increased carbide formation and slight coarsening were observed, indicating tempering effects. Further extension to t_P = 150 s resulted in blurred martensite lath boundaries and coarse carbides, reducing strength. Similarly, at T_P = 275°C, the same trend occurred but with more pronounced carbide precipitation and growth due to higher temperature, leading to faster kinetics. The carbon diffusion during partitioning can be described by Fick’s law: $$ J = -D \frac{\partial C}{\partial x} $$ where J is the flux, D is the diffusion coefficient, and ∂C/∂x is the concentration gradient. This diffusion facilitates carbon enrichment of austenite, stabilizing it against transformation, but excessive time causes carbide coarsening and austenite decomposition, degrading properties.
The martensite transformation kinetics also play a role in the behavior of gear shaft steel. The M_s temperature can be estimated using empirical formulas such as: $$ M_s = 539 – 423C – 30.4Mn – 12.1Cr – 17.7Ni – 7.5Mo $$ where element concentrations are in weight percent. For the 20CrMnTi steel, this aligns with the measured M_s of 398°C. During Q-P processing, the initial quenching to T_Q between M_s and M_f results in partial martensite formation, and the partitioning step allows carbon to migrate from martensite to austenite, increasing its stability. The volume fraction of martensite (V_m) after quenching can be approximated by: $$ V_m = 1 – \exp[-k(M_s – T_Q)] $$ where k is a material constant. This relationship highlights how T_Q influences the initial microstructure, affecting subsequent carbon partitioning and residual austenite retention.
In terms of applications, the improved SEP from Q-P treatment is highly beneficial for gear shaft performance, as it enhances resistance to impact and fatigue. The optimal conditions identified—T_P = 225°C or 275°C with t_P = 90 s—yield a superior balance, making the gear shaft steel more suitable for high-stress environments. compared to direct quenching, Q-P processing not only increases ductility but also maintains adequate strength, reducing the risk of brittle fracture. Furthermore, the presence of residual austenite provides a transformation-induced plasticity (TRIP) effect, where austenite transforms to martensite under strain, dissipating energy and improving toughness. This is particularly important for gear shaft components subjected to dynamic loading.
In conclusion, the quenching-partitioning treatment significantly enhances the microstructure and mechanical properties of gear shaft steel. By carefully controlling partitioning temperature and time, it is possible to optimize the strength-elongation product, with maxima observed at t_P = 90 s for both 225°C and 275°C. The increase in residual austenite volume fraction, up to 6.4%, contributes to improved ductility, while microstructural changes such as martensite tempering and carbide evolution influence strength. These findings provide valuable insights for advancing heat treatment strategies in gear shaft manufacturing, ultimately leading to more durable and efficient mechanical systems.