Currently, the laser additive manufactured ultra-high strength and ductility steels (UHSDSs) exhibit undesirable corrosion resistance and necessitate complicated post-heat treatment for the strength-ductility balance. To develop corrosion-resistant UHSDSs suitable for laser direct energy deposition (LDED) without post-processing, a novel strategy combining thermodynamic computation with machine learning was proposed for composition design and optimization of Fe-C-Cr-Ni-Nb-Ti stainless UHSDS in this work. Based on the metallurgy domain knowledge, the microstructural characteristic parameters were selected as the key features for thermodynamic modelling and composition optimization. Subsequently, the differential evolution algorithm was adopted to identify the optimal solution via exploring the alloy composition space through a machine learning-assisted metaheuristic approach. The LDED-fabricated UHSDS achieved an ultimate tensile strength (UTS), yield strength (YS), and elongation of 1662 MPa, 1321 MPa and 9.01%, respectively. The excellent mechanical properties can be attributed to the refined martensite laths and the nanoscale precipitates induced by the intrinsic heat treatment during the LDED process. Meanwhile, the corrosion resistance of the as-deposited UHSDSs surpasses that of AISI 420 martensitic stainless steel, due to the refined grain and impeded precipitation of Cr-rich particles. This research can provide a guideline for developing cost-effective, short-process and high-performance advanced alloys fabricated by laser additive manufacturing.
Dissimilar metals welding presents significant challenges due to differences in thermal conductivity, coefficient of thermal expansion, and chemical composition, which can lead to residual stress formation, the development of undesirable phases, and degradation of mechanical properties. Using buffer layers can be an effective way to solve this problem. To this end, this study investigates the effects of buffer materials, including 309L stainless steel, 316L stainless steel, and 70S-6 carbon steel, on the mechanical properties and microstructural features of dissimilar arc stud-welded joints between St37 carbon steel base plates and 304 stainless steel studs. After the arc stud welding process, mechanical properties were evaluated through tensile and cantilever bend testing, while microstructural characterization was performed using FESEM and EDS. The results demonstrate that all buffer materials improved tensile properties relative to the non-buffered condition, with the 70S-6 buffer yielding the highest fracture strength (67% higher) and absorbed energy (451% higher). In bending testing, however, the 316L buffer produced the greatest improvements in bending strength (15% higher), flexural stiffness (51% higher), and absorbed energy (40% higher). Microstructural analysis revealed that the 316L buffer minimized alloying element dilution, suppressed undesirable phase formation, and produced the thinnest partially melted zone and lowest delta ferrite (δ) content. Considering both tensile and bending performance collectively, the 316L buffer demonstrated the most balanced and superior overall mechanical performance among all conditions examined.
This study develops a low-cycle fatigue (LCF) testing method under cathodic electrolytic hydrogen charging for the seismic resistance assessment of hydrogen pipelines. Currently available hydrogen pipeline standards lack specifications for seismic design. Thus, we investigated the effect of hydrogen on the LCF life of pipeline steel X65 and examined the processes from the incubation period to crack initiation, crack growth, and fracture in LCF under hydrogen charging by observing surface cracks and crystallographic analyses of subsidiary cracks after interrupted LCF testing. Strain-controlled LCF tests at a strain amplitude of up to ±3% under hydrogen charging revealed that hydrogen adversely affects the LCF life of X65 steel. In addition, hydrogen locally intensified serrations on hysteresis loops at a strain rate of 6×10–4 s–1. Close observation of the cracks using FE-SEM images revealed that hydrogen-related cracks initiated before persistent slip band formation and propagated along the {011} slip planes, {001} cleavage planes, grain boundaries, and nonspecific indices without subgrain formation. These findings indicate that in a hydrogen environment, the pinning and release of dislocation motion are emphasized, leading to localized plastic deformation, and cracks initiate and propagate without significant plastic deformation. This study contributes to the safety assessment of hydrogen pipelines subjected to seismic motions.
M2 high-speed steel (HSS) is a critical alloy for high-performance precision cutting tools. This study systematically investigated the effects of austempering at 230-290°C on laser powder bed fusion (L-PBF) fabricated M2 HSS to exploit the beneficial characteristics of lower bainite. Increasing the austempering temperature reduced retained austenite from 14.6% to ≤1.3%, increased the lower-bainite fraction from 24.8% to 33.1%, and coarsened the bainite laths, with their mean length increasing from 1.2 to 2.8 μm and their thickness from 132 to 359 nm. Austempering also promoted partial recovery of the high dislocation density inherited from rapid L-PBF solidification and modestly increased the average BCC grain size from 0.59 μm to 0.67-0.93 μm. The treatment decreased the yield strength from 1097.8 MPa to 772.1-856.4 MPa while increasing the ultimate tensile strength from 1369.2 MPa to > 1638.1 MPa, the elongation from 1.3% to 3%-4.6%, and the impact toughness from 3.1 J cm-2 to 6.9-8.5 J cm-2. Hardness decreased only slightly, from 62.3 HRC to 58.3-59.8 HRC. The favorable strength-toughness balance is attributed to the fine lower-bainite laths, dislocation recovery, crack deflection by bainite packets, and strain accommodation by thin-film retained austenite. These findings demonstrate that controlled austempering provides a tunable balance of hardness, strength, and toughness, making L-PBF fabricated M2 HSS a promising material for demanding high-speed tooling applications.
Additively manufactured nickel-based superalloys contain directionally connected columnar grain-boundary networks; however, their role in solute-mediated embrittlement during post-build joining remains insufficiently quantified. In this study, transient liquid phase (TLP) bonding of additively manufactured ZGH451 superalloy was performed with the bonding interface either perpendicular to the build direction (hereafter denoted the vertical joint) or parallel to it (the horizontal joint), and the resulting diffusion-affected zone (DAZ) precipitation and fracture behavior were systematically compared. Although both joints achieved complete isothermal solidification, the bonding-interface orientation altered the effective length and spatial connectivity of columnar grain boundaries embedded in the DAZ. When the bonding interface was parallel to the build direction, long grain-boundary segments acted as fast boron-diffusion and segregation paths, promoting continuous M3B2-type boride chains and a connected brittle network. In contrast, when the interface was perpendicular to the build direction, the DAZ intercepted only short boundary segments, resulting in discrete blocky borides rather than a continuous network. The connected boride network in the horizontal joint caused local strain concentration, intergranular cracking, and premature failure, whereas discrete borides in the vertical joint promoted microvoid-coalescence-dominated fracture. Consequently, the vertical joint reached 1075 ± 75 MPa and 24% ± 6% elongation, corresponding to 91% of the strength of the base metal loaded in the same direction, whereas the horizontal joint showed only 720 ± 15 MPa and 3% ± 0.6% elongation, i.e. 64% of the corresponding base metal strength. These results demonstrate that the connectivity of DAZ grain-boundary borides, rather than the ISZ grain size, governs the mechanical integrity of TLP-bonded anisotropic AM superalloy joints.
B4C–TiB2 composites with enhanced mechanical properties were fabricated by a hydrolysis-based coating process, followed by ball milling with stainless-steel media, and consolidated by reactive hot pressing. Milling with 9Cr18 stainless-steel balls introduced Fe–Cr debris, which generated a liquid phase during sintering, promoting the formation of coarse, plate-like (Ti,Cr)B2 solid-solution grains and a newly formed FeB phase. The optimal flexural strength of 745.7 ± 37 MPa is achieved, attributed to solid-solution strengthening and improved microstructural homogeneity. Meanwhile, the highest fracture toughness and Vickers hardness of 5.40 ± 0.28 MPa·m1/2 and 37.4 ± 1.3 GPa are achieved when the milling time is prolonged to 3 h, representing a 27% and 9.67% improvement, respectively, compared to the composites without ball-milling treatment. These results demonstrate that controlled introduction of Fe–Cr debris by stainless-steel ball milling enables simultaneous enhancement of flexural strength, fracture toughness, and hardness in B4C–TiB2 composites through liquid-phase sintering.
Hot corrosion is one of the critical factors decreasing the creep life of single-crystal turbine blades with film cooling holes (FCHs) in marine service environments. Creep tests were conducted on a nickel-based single crystal superalloy with close-packed FCHs at 980 °C and 270-330 MPa in air and hot corrosion environments to investigate the mechanism of hot corrosion-induced creep failure. The creep curves showed that hot corrosion reduced the creep life by 55%-63%, decreased the creep strain by approximately 50%, and increased the steady-state creep rate. During creep, hot corrosion changed crack propagation from plastic deformation along the inter-hole ligaments to tearing around the holes, deviating from the ligament direction. Meanwhile, hot corrosion led to the formation of a thick and loose corrosion product layer and a γ′-depleted layer exhibiting obvious recrystallization around the FCHs, accompanied by multiple damage features, including interfacial cracks, grain boundary cracks, and microcracks in the matrix alloy. Microstructural and stress analyses around the FCHs showed that hot corrosion altered the local stress distribution, weakened the inter-hole interaction, and induced a combined Mode I cracking-corrosion layer tearing mode between the FCHs, thereby accelerating inter-hole fracture. This study provides a new basis for creep life design of turbine blades under hot corrosion conditions.
Hydrogen-resistant steel is an important engineering material for core components in advanced weapon systems and nuclear physics experiments because of its high strength, high toughness, and excellent resistance to hydrogen embrittlement under high-temperature and high-pressure conditions. In this study, a high-temperature Split Hopkinson pressure bar (SHPB) system was used to characterize the dynamic mechanical response of hydrogen-resistant steel. True stress–strain curves were obtained under different strains, strain rates, and deformation temperatures. The results show that the true stress exhibits nonlinear strain-hardening behavior, while thermal softening causes it to decrease with increasing deformation temperature. The adiabatic temperature rise during plastic deformation was calculated by considering the temperature dependence of the specific heat capacity. The actual deformation temperatures were then determined, and the true stress–strain curves obtained under non-isothermal conditions were corrected to isothermal true stress–strain curves using a thermal-softening coefficient. Based on the corrected isothermal true stress–strain curves, Power-Law and Johnson–Cook constitutive models were established for hydrogen-resistant steel. The established models were fitted to the SHPB-derived isothermal true stress–strain data and assessed through two-dimensional cutting simulations. They captured the main cutting-force trends under the tested conditions, although deviations increased at high cutting speeds. Thus, the models may serve as phenomenological inputs for preliminary cutting simulations within the investigated range.
High-Ti microalloyed high-strength steels exhibit complex dynamic recrystallization (DRX) behavior owing to the strong interaction between strain-induced precipitation and austenite recrystallization. To clarify the hot deformation behavior and DRX mechanisms of thick-gauge high-Ti microalloyed high-strength steel, hot compression tests were conducted at 900–1150 °C and 0.01–10 s-1, and a hyperbolic-sine Arrhenius constitutive model was established based on the Zener–Hollomon parameter. The results show that the flow stress decreases with increasing deformation temperature and increases with increasing strain rate, with a hot deformation activation energy of 386.185 kJ/mol. The softening mechanism gradually evolves from incomplete dynamic recrystallization (DRX) characterized by necklace structures to a more extensive and homogeneous DRX process with increasing temperature. Meanwhile, at low strain rates, abundant strain-induced precipitation of nanoscale (Ti, Nb)(C, N) generates strong Zener pinning, suppressing DRX and promoting necklace-like recrystallization. In contrast, the kinetically suppressed precipitation at 10 s-1, together with intense adiabatic heating, eliminates the pinning effect and leads to anomalous equiaxed grain coarsening. Finally, processing maps were established to firmly identify the optimum hot-working window at 1050–1150 °C and 0.01–10 s-1. Furthermore, predictive models for the critical strain () and recrystallized grain size () were developed, providing rigorous theoretical guidance for the industrial thermomechanical processing of high-Ti microalloyed steels.
Owing to their high specific strength, excellent fatigue resistance, and superior corrosion resistance, the Ti-3Al-2.5V titanium alloy tubes are widely utilized in aerospace applications. In this study, the cold-rolled Ti-4.5Al-3V-1.5Zr-0.45Fe titanium alloy tubes which designed based on the composition and application of Ti-3Al-2.5V were employed as the raw materails, the high temperature deformation was conducted by the thermal compression, and the effect of thermal compression processes on hot deformation behavior and microstructure evolution of the titanium alloy was investigated. Results show that the titanium alloy presents equiaxed microstructure of the α phase under 900°C, while it transformed into coarse β phase with the improvement of deformation temperature. The grains are compressed perpendicular to the compression direction gradually with the increase of deformation rate. The true stress improved firstly with the increase of strain and reached the peak stress, then it kept stable followed by a slow decline, which shows the character of strain soften behavior. The peak stress value decreased with the improvement of deformation temperature and it increased with the accelerate of deformation rate. Reducing the strain rate and increasing the processing temperature are more conducive to dynamic recrystallization of titanium alloy. The microstructure transformed from equiaxed grain to bimodal followed by lamellar when the deformation temperature improved from 800 to 1050°C.
Composite materials composed of titanium and steel are reasonably priced and exhibit excellent corrosion resistance. However, their widespread engineering applications are severely limited by insufficient interfacial bonding strength. To fill the technical gap in the fabrication of titanium/steel composite shafts, this study proposes an assembly method for heterogeneous metal composite shaft blanks and adopts cross-wedge rolling (CWR) technology to successfully fabricate high-bonding-strength titanium–steel composite shafts. Based on the CWR process, the effects of forming temperature on the interfacial microstructure and bonding strength of titanium/steel composite shafts were systematically investigated. The results show that the shear strength of the composite shafts rolled at 850°C–1000°C increases first and then decrease with rising temperature, reaching a maximum value of 287 MPa at 900 °C. The superior interfacial bonding performance of the composite shafts is attributed to three main mechanisms. (1) Tight surface contact reduces air entrapment at the interface and suppresses the formation of brittle intermetallic compounds. (2) Cyclic stress loading and strain fluctuation at the interface during forming promote the fragmentation of interfacial compound layers. (3) Multilayer embedding of the Fe matrix and the pinning effect of dispersed nano-TiC particles at the interface further improve the interfacial shear strength. The findings verify that precise temperature control in the CWR process enables the acquisition of titanium/steel composite shafts with high interfacial bonding strength.
The present investigation aims to evaluate the strain rate dependent nano-mechanical and elasto–plastic deformation behaviour of graphene reinforced similar and dissimilar AZ31B–AZ91D friction stir welded (FSW) joints using nanoindentation and Dao-based inverse analysis. Similar AZ31B+AZ31B, AZ91D+AZ91D and dissimilar AZ31B+AZ91D joints reinforced with 0.3 wt.% graphene were fabricated at a tool rotational speed of 900 rpm and traverse speed of 40 mm/min. Nanoindentation tests were conducted up to a maximum penetration depth of 2000 nm at strain rates ranging from 0.05 to 0.20 s-1. All joints exhibited increased deformation resistance with increasing strain rate. The AZ91D+AZ91D+0.3 wt.% graphene joint showed the highest indentation resistance, approximately 18–32% higher than the AZ31B+AZ31B joint, while the AZ31B+AZ91D joint exhibited nearly 10–22% improvement. Nanoindentation hardness increased from 1.72 to 2.24 GPa for AZ31B+AZ31B, 2.42 to 3.24 GPa for AZ91D+AZ91D, and 2.08 to 2.82 GPa for AZ31B+AZ91D with increasing strain rate. The corresponding strain rate sensitivity values were 0.08497, 0.08982 and 0.09549, respectively. Nix–Gao analysis confirmed a pronounced indentation size effect, with the AZ91D-based joint exhibiting nearly 35–45% higher H2 values than the AZ31B-based joint. Dao-based inverse analysis revealed that the yield stress increased from 146–181 MPa for AZ31B+AZ31B and 227–278 MPa for AZ91D+AZ91D with increasing strain rate. The enhanced mechanical response is attributed to graphene reinforcement, grain refinement through dynamic recrystallization, and the presence of β-Mg17Al12 intermetallic phases within the stir zone.
The strain-induced melt activation (SIMA) process based on uniaxial compression pre-deformation has great potential for fabricating semi-solid materials with fine and spheroidal grains. However, deformation heterogeneity during uniaxial compression often leads to non-uniform semi-solid microstructures. In this work, 6061 aluminum alloy was investigated by combining finite element simulation and experimental validation. The coefficient of variation of average grain size and average shape factor was introduced to quantitatively evaluate microstructural uniformity. The stress–strain distribution, deformation-induced stored energy evolution, and their effects on microstructural homogenization were systematically analyzed. A critical stress–strain criterion for achieving microstructural homogenization was established and validated. The results show that the effective stress and strain in the easily deformed zone (EDZ), free deformation zone (FDZ), and difficult-to-deform zone (DDZ) increase with increasing deformation amount. The effective stress in the EDZ stabilizes at approximately 439 MPa after 50% deformation. When the deformation amount reaches 55%, followed by semi-solid treatment at 590 °C for 10 min, the grain size and spheroidization degree among different regions become highly consistent, with the CV values of grain size and shape factor reduced to 7.59% and 2.86%, respectively. EBSD analysis indicates that increasing deformation from 45% to 55% mainly enhances deformation in the DDZ, increasing its GND density and stored energy while reducing regional differences. A critical criterion of effective stress above 400 MPa and effective strain exceeding 0.51 was determined. Axial and radial compression experiments further verified its applicability under different loading paths, providing guidance for designing highly uniform semi-solid billets.
Third-generation Al-Li alloys are aerospace structural materials strengthened primarily by T1 (Al2CuLi) precipitates. Although the interaction of T1 precipitates with dislocations has been extensively studied, the effect of T1 thickness on the mechanical response of T1/Al interfaces remains unclear. In this work, AA2099 Al-Li alloy containing T1 phases with different layer thicknesses was produced by hot deformation. The dynamic precipitation behavior of T1 phases was characterized by TEM, CBED, and STEM-HAADF, while the effects of T1 thickness on the interfacial stability and tensile response of T1/Al interfaces were investigated using DFT calculations and deep potential molecular dynamics (DP-MD) simulations. The results show that higher temperatures and lower strain rates promote the coarsening and thickening of T1 phases, leading to coexisting single-layer and multilayer phases, where multilayers preferentially form under high-temperature, low-strain-rate conditions. The maximum T1 volume fraction and highest hardness were achieved at 280 °C and 0.025 s-1, where T1 precipitates were predominantly single-layer. DFT calculations reveal that T1/Al interfacial stability depends on layer number and Al stacking sequence, with increasing thickness enhancing stability only in the 3N Al matrix model. DP-MD simulations show limited differences in tensile response among the models, with the single-layer interface exhibiting slightly higher peak stress and more pronounced dislocation activity. These results identify 280 °C and 0.025 s-1 as the most favorable condition investigated and indicate that thermomechanical processing should favor predominantly single-layer T1 precipitates and limit excessive thickening, because multilayer formation provides no proportional tensile benefit despite its configuration-dependent enhancement of interfacial stability.
In this study, the effects of different pulsed-laser conditions on the microstructure, carbide precipitation, microhardness, and friction and wear properties of Fe-based cladding layers were investigated by jointly regulating the temporal duty cycle and laser-on power while maintaining approximately constant nominal average energy input. The results show that, as the duty cycle decreases from 95% to 50%, the morphology of the VC reinforcing phase in the cladding layer gradually evolves from non-oriented dendrites to oriented dendrites, coarsened dendrites, and fine equiaxed crystals. The volume fraction of the VC reinforcing phase first increases and then decreases. The VC volume fractions at duty cycles of 95%, 80%, 65%, and 50% are 23.51%, 28.00%, 32.64%, and 13.50%, respectively. Among them, the cladding layer prepared at an 80% duty cycle exhibits the best comprehensive performance, with the highest microhardness at the top layer, reaching approximately 1250 HV0.2, and the lowest wear volumes at both room temperature and elevated temperature, which are 0.45×107 μm3 and 0.89×107 μm3, respectively. Cutting experiments were conducted using the cladding layer tool prepared at an 80% duty cycle. The results indicate that, compared with the high-speed steel (HSS) tool, when the cutting distance reaches 10000 m, the flank wear of the cladding layer tool decreases from 189 μm to 151 μm, a reduction of approximately 20%, while the machined surface roughness decreases from 1.65 μm to 1.33 μm. In addition, the three-dimensional cutting forces are all reduced, demonstrating superior wear resistance, anti-adhesion performance, and machined surface quality.
Molybdenum (Mo) alloys are promising structural materials for advanced nuclear energy systems, yet conventional thermomechanical processing is limited in fabricating complex Mo-based components with high material utilization. Vacuum-based electron beam wire deposition (EBWD) exhibits unique advantages for the fabrication of Mo alloys compared with other non-vacuum additive manufacturing techniques. In this study, Mo-14Re circular tubes were fabricated via EBWD, and their crystallographic texture, metallurgical defects, room-temperature tensile deformation mechanism were investigated. The EBWD-deposited tubes exhibit a columnar structure with mixed <111> and <110> textures along the building direction (BD), and each columnar domain contains abundant fine grains. Due to bidirectional heat dissipation along the tube wall, a distinct radial microstructure heterogeneity emerges across the tube wall: finer grains, higher porosity and higher geometrically necessary dislocation (GND) density are observed near the inner and outer walls. The main metallurgical defects include random intralayer pores and interfacial cracks. Pores arise from vapor phases generated by impurity element evaporation and local keyhole collapse, while interfacial cracks form due to grain boundary (GB) impurity segregation-induced weakening combined with high thermal stress during deposition. The alloy delivers a prominently improved mechanical performance, with an average ultimate tensile strength of ∼376 MPa and an average fracture elongation of ∼13%, which is a significant enhancement over WAAM-fabricated pure Mo. Room-temperature plastic deformation is dominated by the activation of {110}<111> slip systems, assisted by secondary {112}<111> slip. This work provides insights into microstructure tailoring and performance optimization for additively manufactured Mo-Re alloys.
This study investigates the cryogenic strength-ductility synergy of Co-free FCC multi-principal element alloy fabricated by laser powder bed fusion (LPBF), with emphasis on the effects of scanning speed, elemental segregation, and deformation mechanisms. Equiatomic CrFeNiMn alloys were fabricated at scanning speeds of 800-1200 mm/s and characterized by tensile testing at 298 K and 77 K. All specimens exhibited simultaneous increases in strength and ductility at 77 K. The specimen fabricated at 1200 mm/s showed the representative parameter set, with yield strength, ultimate tensile strength, and total elongation increasing from 609 MPa, 706 MPa, and 20.0% at 298 K to 949 MPa, 1170 MPa, and 26.4% at 77 K, respectively. The enhanced uniform elongation from 11.0% to 23.0% indicates sustained work hardening and delayed necking at cryogenic temperature. The as-fabricated alloy consisted of a single FCC phase with fine/columnar grains, low-angle grain boundary networks, and cellular dislocation structures. Mn-Ni co-segregation at cell walls generated local chemical and stress heterogeneity. At 77 K, reduced stacking-fault energy and increased flow stress promoted dense stacking faults, early deformation twinning, and extensive Lomer-Cottrell locks. These features, together with dislocation-cell strengthening, oxide-particle strengthening, and dynamic Hall-Petch strengthening, synergistically contributed to the superior cryogenic strength-ductility combination.
Active metal brazing (AMB) is pivotal in the manufacturing of complex industrial structures owing to its operability and efficiency. Ag-based fillers, an important feedstock for AMB, have garnered significant scientific and industrial interest due to their exceptional wettability, superior plasticity, and versatile applicability. However, a systematic review focusing on Ag-based fillers for AMB is currently deficient. This review systematically summarizes recent advances in Ag-based fillers for AMB, shedding light on key challenges and innovative control strategies. A comprehensive classification of currently employed Ag-based fillers is presented, with particular emphasis on novel fillers developed in recent years. The relationship between interface structure and performance characteristics, and the formation mechanism of joints using Ag-based fillers is thoroughly elucidated. The inherent issues and control strategies for the Ag-based fillers are critically discussed, including the recent breakthroughs in the application of advanced materials. Finally, the future outlook of the Ag-based fillers for AMB is put forward based on the current research status. This review presents a state-of-the-art perspective in Ag-based filler design, interface regulation, and optimization of the joint performance, aiming to provide a reference for further investigation and application of AMB technology.
The effects of austenitization, controlled cooling and subsequent tempering on the microstructure, precipitation behavior and hardness of J21 wheel steel containing 0.13 wt.% V were investigated using optical microscopy, transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energy-dispersive spectroscopy (EDS) and Vickers hardness testing. Increasing the austenitization temperature from 850 to 910 °C promoted dissolution of V-rich precipitates and reduced their number. After the 910 °C/60 min treatment, the mean size of the remaining particles increased because the finer particles were preferentially dissolved; limited ripening of the surviving population may occur concurrently. The calculated equilibrium complete-dissolution temperature of idealized VC is approximately 938 °C. Under the prescribed cooling schedules, decreasing the cooling rate above 500 °C increased the ferrite fraction and the population of V-rich precipitates. During subsequent tempering, the readily observable precipitates mainly grew and coarsened at 440-520 °C, whereas numerous fine V-rich particles below 20 nm were observed at 600 °C. The fine-particle population at 600 °C did not produce the highest hardness because precipitation strengthening was counteracted by matrix softening. The maximum hardness was 336.5 HV after tempering at 440 °C for 90 min. Under the present rolling-wear conditions, J21 exhibited lower wear loss and higher surface hardness than J12.
