Quenched And Tempered Research Articles

Mechanistic understanding of banded microstructure and its effect on anisotropy of toughness in low carbon-low alloy steel

In this study, the relationship between the anisotropy of toughness and microstructure in low carbon low alloy steel treated by quenching and tempering (QT) heat treatment was investigated with the aid of scanning electron microscope, electron back-scattered diffraction and transmission electron microscope combined with energy disperse spectroscopy techniques. Results show that the impact toughness of the quenched steel plate along the longitudinal and transverse directions are little different. However, after tempering, the longitudinal impact toughness of QT steel plate is improved by 88%, while the transverse impact toughness is slightly decreased, leading to a significant anisotropy of toughness. Microstructural characterizations reveal that banded microstructure with coarse grain size exists along longitudinal direction (i.e., rolling direction) of steel plate, which is attributed to co-segregation of Mn and C and the resulting uneven recrystallization during hot rolling. After tempering, fine and dispersed carbides are precipitated in matrix microstructure, but high-density coarse carbides are formed within banded microstructure. It is suggested that the coarse grains and high-density coarse carbides significantly deteriorate the resistance against crack propagation along banded microstructure, leading to the anisotropy of toughness of QT steel plate. The findings of this study will aid to design metallurgical processes including chemical composition design, hot rolling, and heat treatments to eliminate the anisotropy of toughness of low alloy steels with high strength and toughness.

On the Origin of Enhanced Tempering Resistance of the Laser Additively Manufactured Hot Work Tool Steel in the As-Built Condition

Abstract In laser additive manufacturing (AM) of hot work tool steels, direct tempering (DT) of the tool from as-built (AB) condition without prior conventional austenitization and quenching results in enhanced tempering resistance. To date, intercellular retained austenite (RA) decomposition, leading to a shift in secondary hardening peak temperature, and finer martensite substructure are reported to be responsible for such a behavior. In this work, authors aimed at studying the strengthening contributions by performing isothermal tempering tests for long times (up to 40 hours) at elevated temperatures (up to 650 °C) on DT and quenched and tempered (QT) specimens. The thermal softening kinetics and the microstructural evolution were evaluated with the support of computational thermodynamics. The results suggest that the main contributor to enhanced temper resistance in DT condition is the larger fraction of thermally stable and extremely fine (~ 20 nm) secondary (tempering) V(C,N) compared with QT. This could be explained by the reduction of available V and C in austenitized and quenched martensite for a later secondary V(C,N) precipitation during tempering, because of equilibrium precipitation of relatively large (up to 500 nm) vanadium-rich carbonitrides during the austenitization process. A complementary effect of the substructure refinement (i.e., martensite block width) in rapidly solidified highly supersaturated martensite was also quantified in terms of Hall–Petch strengthening mechanism. The significant effect of secondary V(C,N) was successfully validated by assessing a laser AM processed vanadium-free hot work tool steel in QT and DT condition, where no significant differences in strength and temper resistance between the two conditions were evident. Graphical Abstract

Effect of tempering temperature on mechanical properties and hydrogen embrittlement resistance of a dual-precipitation-hardened martensitic steel

ABSTRACT The effects of dual-precipitation on the mechanical properties and hydrogen embrittlement (HE) resistance of a Ti-Mo-alloyed martensitic steel were investigated using a quenching and tempering (QT) procedure. The precipitate-free QT200 sample exhibited high tensile strength and brittle fracture after hydrogen charging. In contrast, the QT300 sample demonstrated a dual-precipitation constitution of (Ti, Mo)C and ϵ-carbide, achieving a high yield strength (∼1400 MPa) without sacrificing tensile ductility (∼10%). More importantly, it showed higher resistance to HE (elongation loss: ∼4.2%) compared to the low-strength QT400 sample. The unusual HE susceptibility of the QT300 samples can be attributed to a higher hydrogen trapping ability of ϵ-carbide compared to (Ti, Mo)C precipitates. This results in high densities of irreversible hydrogen trapping sites and a completely ductile fracture mode characterised by numerous dimples.

Microstructural analysis of flame-sprayed and PTA-deposited nickel-based self-fluxing alloy coatings

In this paper, the results of microstructural analyses, including optical microscopy, scanning electron microscopy with energy dispersive spectroscopy and X-ray diffraction analysis, of the Ni-based self-fluxing alloys NiCrBSi, NiCrBSi–WC, and NiBSi–WC deposited on a previously quenched and tempered (QT) steel substrate 42CrMo4 by flame spraying with simultaneous fusing and plasma transferred arc (PTA) process are presented. The aforementioned microstructural analysis was carried out to determine the microstructural characteristics of the investigated coatings, especially at the coating/substrate interface, and the influences of the spraying and welding technology on the steel substrate. The analysis revealed a change in the microstructure of the coating/substrate interface. Specifically, the diffusion characteristics of certain chemical elements (carbon and iron) from the coating to the substrate and from the substrate to the coating were observed. Additionally, the analysis established the existence of new phases within the coating that arose as a result of the aforementioned diffusion and reaction with chemical elements from the coating. The diffusion of chemical elements was most pronounced in the area of the coating/substrate interface, while it decreased away from this area.

Strain rate-temperature effects and constitutive models for Q690D QT steel

This paper investigates the mechanical properties of the high-strength structural steel Q690D QT (Quenched and Tempered) over a wide temperature range and a wide strain rate range. Firstly, a series of quasi-static and dynamic tests adopting the universal testing machine and Split Hopkinson Pressure Bar were carried out to investigate the mechanical properties at strain rates from 0.00025 s−1 to 5000 s−1 and temperatures from room temperature (20 °C) to 700 °C. Stress-strain curves and dynamic yield strength increase factors were extracted and investigated. Test results show that Q690D QT exhibits different strain rate effects at different temperatures, more significant at the temperatures of 600 and 700 °C, as well as the high-temperature softening effect. Dynamic strain aging effect is identified at temperatures between 300 − 600 °C for plastic strain rates of 800 s−1 and 2300 s−1, nevertheless, this effect is observed at a higher temperature range of 500–700 °C for higher plastic strain rates of 4000 s−1 and 6400 s−1. From the analysis, the adiabatic thermosoftening effect cannot be ignored when the plastic strain rates reach 4000 s−1. Finally, an improved form of the Johnson-Cook model is proposed for capturing the adiabatic temperature effects at different temperatures and validated by the test data. A parameter dynamic temperature factor is proposed to describe the comprehensive effect of strain rate, test temperature, and adiabatic thermosoftening.

Effect of austenitisation and tempering treatments on the mechanical properties of advanced high strength spring steel SAE 9254

In response to increasingly stringent global regulations within the automotive industry, there is a growing demand for lightweight yet high-strength steel suspension coil springs. This study undertakes a comprehensive examination of the critical factors influencing the mechanical properties of SAE 9254, a well-established spring material, in its quenched and tempered (QT) state. The primary objective is to optimise its strength and ductility, especially in high-performance applications with an emphasis on lightweight design. The investigation encompasses a detailed exploration of the effects of varying austenitisation temperature (AT) and austenitisation time (At) on the microstructure and mechanical properties of the steel. Of particular interest is the study of tempering temperatures ranging between 300°C and 350°C to elucidate the mechanisms responsible for enhancing the steel's strength. Notably, the findings highlight dislocation density as the primary factor driving the strengthening of martensitic spring steel. Furthermore, the pivotal role of austenite grain size before quenching becomes evident, influencing toughness, ductility, martensite block size, and the percentage of retained austenite (γR). This study provides valuable insights into optimising the mechanical properties of SAE 9254 spring steel for high-performance applications, achieving ultimate tensile strengths (UTS) exceeding 2100 MPa and a reduction in area (RoA) exceeding 30%. Notably, the properties attained for this grade are comparable to those achieved with microalloying additions. Consequently, this research underscores the significance of controlling austenite grain size and dislocation density as critical factors for tailoring the properties of martensitic steels, positioning them as competitive materials, particularly in the pursuit of lightweight design.

High strength and high ductility of 60Si2CrVAT spring steel through a novel quenching and partitioning (Q-P) process

The balance between strength and ductility in spring steel is a perennial issue. However, production using traditional quenching and tempering (Q-T) process is challenging because tempered martensite provides high tensile strength but significantly reduces ductility. To solve this problem, we propose a novel quenching and partitioning (Q-P) process to enhance the comprehensive properties of 60Si2CrVAT spring steel. The novel Q-P process introduced 31.56% excellent mechanical stability retained austenite, which existed mostly as a film-like between the martensite and bainite laths, and ensured a continuous and long-lasting transformation-induced plasticity (TRIP) effect. Moreover, Q-P process reduced the effective grain size due to the diffusion of carbon, and changed the fracture behavior due to the coordinated deformation of multiphase microstructure. Consequently, the total elongation of the spring steel increased by approximately 180%, from 7.8% to 21.7%, while the tensile strength of the spring steel was not affected and remained at a high of 1809 MPa. In conclusion, the novel Q-P process adjusted the volume fraction of the martensite, bainite, and retained austenite multiphase microstructure, and fully leveraged the synergistic effect of the multiphase microstructure to optimize the properties of spring steel.

Influence of Cell Structure on Yield Strength and Strain‐Hardening Behavior in Additively Manufactured H13 Hot Work Tool Steel

Mechanical properties including yield strength, hardness, and strain hardening are superior in additively manufactured H13 hot work tool steel (AMH) compared to conventional H13 (CMH), which are correlated with microstructural characteristics. Comprehensive microstructural analysis reveals that the strain is highly localized in the cell structure with retained austenite in AMH. Quenching and tempering (QT) heat treatment removes the cell structure and retains austenite from AMH, resulting in a microstructure and mechanical properties similar to CMH after QT. The constitutive equation for yield strength shows that retained austenite with sufficient amount of dislocation density in martensite contributes to higher yield strength in AMH than CMH and identical strengthening mechanism between AMH and CMH after QT. Furthermore, microstructural changes during plastic deformation are investigated to find the origin of high strain‐hardening rate in AMH. The relationship between hardness and indentation depth yields the higher statistically stored dislocation density in AMH than CMH. In addition to higher dislocation accumulation, transformation of retained austenite to α′ martensite results in transformation‐induced plasticity effect, which contributes to strain hardening.

Tailoring Low‐Alloyed Cryogenic Steel Strong and Tough Introduces Fine‐Grained Equiaxed Ferrite by Intercritical Subquenching

Herein, a novel approach is developed to enhance the strength, ductility, and low‐temperature toughness of high‐strength low‐alloy steels, overcoming the traditional trade‐off. By incorporating an intercritical subquenching step during the conventional quenching and tempering (QT) process, a microstructure featuring refined lath bainite and a small amount of fine equiaxed ferrite is achieved. The resulting steel exhibits a strength similar to that of QT steel with significantly improved ductility and low‐temperature impact toughness. Compared to traditional QT steel, the new process steel maintains a similar strength. In addition, the elongation increases from ~22.2% to ~24.9%, and the impact energy significantly increases at −50 °C from ~233 to ~347 J. The fine grain size and dense lath structure contribute to the minimal reduction in strength. The presence of equiaxed ferrite and bainite improves the ductility, while plastic deformation and special grain boundaries play a crucial role in enhancing the low‐temperature toughness. This innovative heat treatment method offers an economical solution for producing high‐strength low‐alloy cryogenic vessel steels without compromising strength while greatly improving ductility and low‐temperature toughness.

Grey-Taguchi analysis and experimental assessment of 1 GPa HSLA steel treated by quenching and tempering

The effect of quenching and tempering (QT) process on the mechanical properties of the experimental high-strength low-alloy (HSLA) steel was analyzed by Grey-Taguchi method, thus achieving the optimum combination of parameters based on the appropriate nine sets of experiments on an orthogonal array. The grey relational analysis (GRA) reveals that the quenching temperature T1 has the greatest effect on each response variable, followed by the tempering temperature T2, while the quenching time t1 and tempering time t2 have similar and the least effect. Experimental assessment of microstructure evolution was performed by multi-scale characterizations and in-situ investigation combined with modeling, focusing on martensite transfomation kinetics that controlled by the quenching process. Results indicate that the prior austenite grain (PAG) size and the substructure of martensite are significantly refined, as the quenching temperature decreased from 950 to 850 °C. The PAG refinement leads to an increase in driving force for martensite transformation initiation, thus shifting the martensite transformation temperature to lower levels for a higher degree of undercooling, consistent with the experimental and modeling results. The refined microstructure obtained at low quenching temperature contributes to strength improvement, and the various carbides precipitated during tempering process offset the tempering softening of the steels. In general, the value increase of process parameters (T1, T2, t1 and t2) leads to a decrease of strength property, but an increase of ductility and toughness. Based on the theoretical and experimental basis, a 1 GPa grade HSLA steel with improved comprehensive mechanical properties can be produced.

Fretting Behavior of WC-Co-Cr Coatings Against QT Steel in Bolted Joint

Fretting damage on contacting surfaces introduces major challenges in mechanical assemblies. Thermal sprayed hardmetal coatings are extensively used for surface modification in tribological applications under demanding conditions and may also be subjected to fretting-inducing loading. In the present work, the fretting behavior of High Velocity Oxy-Fuel (HVOF) and High Velocity Air–Fuel (HVAF) sprayed WC-10Co-4Cr coatings against quenched and tempered (QT) steel was studied by using a single bolt joint-type fretting experiment. This experimental approach was selected to obtain realistic data on the fretting fatigue performance of the contact pairs (both coating-to-steel and coating-to-coating). Experimental results were completed with continuum-scale linear elastic finite element method (FEM) calculations.

Microstructural evolution of secondary carbides during spheroidized annealing and quenching and tempering in 60Cr16MoMA martensitic stainless steel

The microstructural changes in secondary carbides during spheroidized annealing (SA) and quenching and tempering (QT) processes in 60Cr16MoMA martensitic stainless steel has been comprehensively studied. Using scanning electron microscope, X-ray diffraction, inductively coupled plasma-atomic emission spectrometry, and transmission electron microscopy, a substantial dissolution of submicron-sized M23C6 carbides after QT treatment was observed, leading to an increase in the mean carbides diameter. The highest solubility occurs at a 0.5 μm diameter, and carbide roundness improves during QT. Notably, the transformation of M7C3 carbides into M23C6 carbides was observed in QT samples through a multi-site in-situ nucleation mechanism. Additionally, it has been observed that Nb-rich MC carbides effectively restrict the growth of alloy carbides during heat treatment.

Effects of Pre-Strain and Tempering on Mechanical Properties in High-Strength Martensitic Steels

Effects of pre-strain and tempering on mechanical properties in high-strength martensitic steels were investigated. In this study, strain tempering (ST) and quenching and tempering (QT) martensitic steels were prepared, and their mechanical properties were studied. In the tensile tests at the deformation temperatures between 296 and 573 K, the ST sample increased both of tensile strength (TS) and uniform elongation (U.El) from 473 to 523 K whereas the QT sample increased U.El with little change of TS. From the in situ neutron diffraction experiments, stress partitioning to the bcc phase increased with an increase in the deformation temperature from 296 to 523 K. The difference of phase stress between the bcc and cementite phases decreased with increasing the temperature because of a decrease in the cementite strength. In the ST sample, Pre-straining of 0.5% increased YS at 296 K with slight work hardening. The initial value of dislocation density (ρ) decreased at 523 K but ρ increased significantly after yielding, leading to better combination of TS and U.El. The combinations of pre-strain, tempering, and deformation temperatures have changed ρ before deformation and the increase of ρ after yielding of the martensitic steels.

Toughening mechanism and delayed fracture analysis of 4145H steel under intercritical quenching heat treatment process

Tuning thermal treatment schedule has been confirmed as an effective technical route for achieving optimum properties of steels. In the present work, the intercritical quenching tempering(IQT) process was applied to steel 4145H for oil drilling tools, and the performance enhancement and delayed fracture mechanism of the IQT process were comprehensively elucidated using multiscale characterization methods and various mechanical tests. Compared with the quenching and tempering(QT), the IQT process results in a separate island ferrite plus finely tempered sostenite organization, which reduces the dislocation density in the tempered organization and increases the proportion of recrystallized grains. Due to the grain refinement effect of multiple quenching, the pristine austenite grains were refined by 58 %. At the same time, the proportion of large-angle grain boundaries increased from 68.6 % to 80.3 %. The results of the mechanical property tests show that the intercritical 790 °C quenching treatment and tempering at 600 °C exhibits a tensile strength of 1087 MPa, a high elongation of 21.72 %, and a high impact work of 104 J compared to QT. The improved impact toughness is mainly attributed to the grain refinement effect of multiple quenching and the large-angle grain boundaries provided by the composite ferrite organization and M23C6 carbides. The impact of different ratios of large-angle grain boundaries on crack extension was also evolved by means of the crystal phase field, and the delayed fracture effect of large-angle grain boundaries was verified.

Obvious different formability and mechanical properties of warm caliber rolling Q345 steel with normalized and tempered states

The formability and strengthening-toughening behaviors of the warm rolled steels are highly dependent on the raw heat treatment state. Herein, the normalized and tempered Q345 steels were both subjected to caliber rolling at 500 °C with different rolling reduction ratios (18%, 44%, and 68%) to obtain elongation grains and carbide particles with different distribution and size. With the increase of caliber rolling reduction ratio, the strength and fracture elongation of the rolled normalized Q345 steels are gradually decreased, whereas the rolled tempered Q345 steels exhibit obvious improvement in these properties. The warm rolled normalized steel contains severe uneven grain distribution and coarse carbides, which leads to high stress concentration and low mechanical properties, as well as poor formability. On the other hand, warm rolled tempered steel contains nanoscale carbides and homogeneous elongation grains that help to release high stress and strain concentration, resulting in improved formability and mechanical properties to a certain extent. Moreover, the warm rolled tempered steel (Tempformed/TF) exhibited a significant improvement in the impact absorb energy, about 3.1 and 47 times higher than those of the quenched and tempered (QT) state at 20 °C and −80 °C, respectively. This remarkable improvement in low-temperature impact toughness can be attributed to the delamination behavior and subsequent induced super-high plastic deformation energy. These findings have important implications for the design and development of high-performance materials with superior strength-toughness combinations.

Effect of Deep Cryogenic Time on the Microstructure and Mechanical Property of Cr-Mn-Si High-Strength Alloy Steel

Cr-Mn-Si alloyed high-strength steel was subjected to deep cryogenic treatment after quenching and tempering (Q-T), and the microstructure and property evolution of the alloy steel after deep cryogenic treatment were studied. The tensile strength increased by about 30 MPa, the yield strength decreased by about 10 MPa, and the grains of alloy steel were refined, indicating that the strength and toughness of the alloy steel can be relatively improved via the deep cryogenic treatment (−120 °C × 1 h); the secondary carbides precipitated inside the martensitic matrix were uniformly distributed; and the average size was also significantly reduced, presenting a more uniform microstructure than that of the Q-T samples. Furthermore, the dislocation density of alloy steel also evolved during the deep cryogenic treatment, with the highest dislocation density after the 2 h treatment, thus providing a dislocation-strengthening effect. Therefore, the overall properties of the alloyed steel could be comprehensively improved by the deep cryogenic treatment after Q-T.

Effect of Nb interlayer on microstructure and property of Ti-6Al-4V/690 MPa grade steel clad plate by vacuum rolling

Ti alloy clad plate is of great interest for marine and petrochemical industry applications. However, high interfacial bonding is hardly obtained in the Ti alloy clad plate due to the brittle intermetallic compounds (IMCs) forming at the interface. In this study, a Ti-6Al-4V alloy/EH690 steel clad plate was fabricated by vacuum rolling and subsequently tempered to control the mechanical property of the steel substrate. The tempered sorbite appeared in the EH690 steel after quenching and tempering (QT). A decarburized zone, Fe2Ti + TiC mixed phases layer, and β-Ti layer were formed at the interface between steel and Ti. The brittle IMCs with high lattice mismatch resulted in a low interfacial strength of 108 MPa. To improve bonding strength, Nb was inserted between Ti and steel as an interlayer, which effectively inhibited the inter-diffusion of Ti, Fe, and C without brittle TiC or Fe-Ti IMCs. A (Ti, Nb) solid solution layer with high plasticity was present at the Nb/Ti interface. Fe2Nb and Fe7Nb6 phases were detected at the steel/Nb interface, exhibiting less brittleness and hardness compared to Fe-Ti IMCs. The interfacial shear strength was up to 265 MPa after adding the Nb interlayer, showing a mixed fracture mode of brittleness and ductility.

Effect of Heat Treatment on Crevice Corrosion Behavior of 304 Stainless Steel Clad Plate in Seawater Environment.

With the application of stainless steel clad plate (SSCP)-enlarging in the marine engineering field, awareness of the consequences of heat treatment on ameliorating microstructure and mechanical properties in stainless steel (SS)/carbon steel (CS) joints is being raised. However, carbide diffusion from a CS substrate to SS cladding may damage the corrosion resistance during inappropriate heating. In this paper, the corrosion behavior of a hot rolling-produced stainless steel clad plate (SSCP) after quenching and tempering (Q-T) treatment, especially crevice corrosion, was studied by electrochemical and morphological methods, such as cyclic potentiodynamic polarization (CPP), confocal laser scanning microscope (CLSM) and scanning electron microscopy (SEM). Q-T treatment led to more significance in carbon atoms diffusion and carbide precipitation, which made the passive film of the SS cladding surface on the SSCP unstable. Subsequently, a device for measuring the crevice corrosion performance of SS cladding was designed; the Q-T-treated cladding showed lower re-passivation potential (-585 mV) during CPP when compared to as-rolled (-522 mV), with the maximum corrosion depth ranging from 70.1 μm to 150.2 μm. In addition, the processing of crevice corrosion on SS cladding could be divided into three parts, including the initiation, propagation and development stages, which were driven by the interactions between corrosive media and carbides. The generation and growth mechanism of corrosive pits in crevices were revealed.

Post-Weld Heat Treatment of S690QL1 Steel Welded Joints: Influence on Microstructure, Mechanical Properties and Residual Stress

During the manufacturing of welded structures, some degree of residual stresses occurs. The classic approach to residual stress reduction is Post-Weld Heat Treatment (PWHT). In the case of structural grade mild steels, the thermal process is well established. In case of S690QL1 High Strength Steel (HSS), which is manufactured using the Quenching and Tempering (QT) process considered in this paper, only limited PWHT treatment is possible without deterioration of mechanical properties. Since this steel grade is susceptible to subsequent heat input, the challenge is to establish adequate PWHT parameters, achieving residual stress reduction while retaining sufficiently high mechanical properties. The paper considers X joint welded HSS steel plates with slightly overmatching filler metal. The welded coupon is prepared and subjected to PWHT treatment. The research on the influence of heat treatment was performed using the four different PWHT cycles and initial As-Welded (AW) material condition. The authors proposed those PWHT cycles based on available resources and the literature. Process holding temperature is considered the variable parameter directly related to the behaviors of material properties. The methodology of welded joint analysis includes experimental testing of mechanical properties, metallographic examination, and residual stress quantification. Testing of mechanical properties includes tensile testing, Charpy V-notch impact testing, and hardness testing in scope of complete welded joint (BM + HAZ + WM). Metallographic examination is performed in order to characterize the welded joint material in relation to applied PWHT cycles. In order to quantify residual stresses, all heat-treated samples were examined via the X-ray diffraction method. Mechanical properties testing determined that an increase in PWHT cycle holding temperature leads to degradation of tested mechanical properties. For specific zones of the welded joint, the decreasing trend from AW condition to Cycle D (max. 600 °C) can be quantified. Based on representative specimens comparison, strength values (BM ≤ 5.7%, WM ≤ 12.1%, HAZ ≤ 20%), impact testing absorbed energy (BM = 17.1%, WM = 25.8%, FL = 12.5%, HAZ = 0.6%), and hardness values (BM = 4.1%, WM = 3.2%, CGHAZ = 16.6%, HAZ = 24.2%) are all exhibiting decrease. Metallographic examination, using the light microscopy, after the exposure to PWHT thermal cycles, did not reveal significant changes in the material throughout all specific welded joint segments. Average relative reduction in residual stress in correlation with PWHT temperature can be observed (AW = 0%, Cycle A (max. 400 °C) = 72%, Cycle B (max. 530 °C) = 81%, Cycle C (max. 550 °C) = 93% and Cycle D (max. 600 °C) = 100% stress reduction). It can be concluded that S690QL1 HSS welded joints can generally be subjected to PWHT, while adhering to the limits of the material and process. In the authors’ shared opinion, it is advisable to use the PWHT Cycle C (max. 550 °C) with 93% RS reduction, while mechanical properties retain high values.

Study of an Economical and Effective Heat Treatment Method to Improve the Performance of Gear Steels

During heat treatment, gear steels undergo grain coarsening, so an appropriate heat treatment process that can efficiently control the austenite grain growth and ensure a good performance of gear steels is particularly important. Herein, the influence of a requenching treatment on the comprehensive behavior of low‐carbon alloy steel 20Cr2Ni4 is explored. The pristine steel samples with and without Nb microalloying subjected to conventional quenching and tempering (QT) treatment are compared with the requenched steel sample. The results show that different requenching temperatures considerably impact grain refinement and homogenization. The steel requenched at 830 °C exhibits the greatest tensile strength of 1444 MPa and excellent toughness, comparable to the mechanical properties of the steel upon Nb microalloying after the conventional QT treatment. The formation of twin martensite and uniform fine grains with a high dislocation density together with numerous low‐angle and high‐angle grain boundaries is responsible for the simultaneously improved strength and ductility of the requenched specimen; thus, the proposed requenching heat treatment is a promising, efficient, and reliable method to double‐refine prior austenite grains and their packet and block structures and improve the comprehensive properties with a combination of high strength and toughness, yielding high‐strength gear steel.