Effect of powder oxidation on the impact toughness of electron beam melt Ti-6Al-4V.

Powder quality in additive manufacturing (AM) electron beam melt (EBM) of Ti-6Al-4V components is crucial in determining the critical material properties of the end item. In this study, we report on the effect of powder oxidation on the Charpy impact energy of Ti-6Al-4V parts manufactured using EBM. In addition to oxidation, the effects on impact energy due to hot isostatic pressing (HIP), specimen orientation, and EBM process defects were also investigated. This research has shown that excessive powder oxidation (oxygen mass fraction above 0.25 % and up to 0.46 %) dramatically decreases the impact energy. It was determined that the room temperature impact energy of the parts after excessive oxidation was reduced by about seven times. We also report that HIP post-processing significantly increases the impact toughness, especially for specimens with lower or normal oxygen content. The specimen orientation effect was found to be more significant for low oxidation levels.

Impact Toughness Modification of NIST Low-Energy Charpy Verification Specimens for Testing at Room Temperature.

The possibility for the National Institute for Standards and Technology (NIST) to certify Charpy reference specimens for testing at room temperature (21 °C ± 1 °C) instead of -40 °C was investigated in a previous study, in which a slightly increased likelihood of specimen jamming was observed at the low-energy level (13 J to 20 J). Moreover, there is a concern that the higher impact toughness of low-energy verification specimens at room temperature would not allow the same Charpy machine features to be verified as in the case of low-temperature (-40 °C) tests, namely, the linear elastic behavior of the sample and the very high maximum forces (typically larger than 33 kN). In this paper, we report on the change in the mechanical properties (hardness and absorbed energy) of the American Iron and Steel Institute (AISI) 4340 steel low-energy specimens that ensues from the modification of the temperature of the final tempering heat treatment. We established that, if low-energy verification specimens are tempered at 300 °C for 2 h and then air cooled, they exhibit equivalent impact toughness (13 J to 20 J) and postimpact behavior (specimen halves projected backward at high speed) at room temperature as compared to specimens currently on sale for testing at -40 °C. Their hardness is however increased to above 49 HRC on the Rockwell scale. The minimum hardness requirement for low-energy verification specimens, currently set at 44 HRC in NIST specifications, will have to be increased to 49 HRC.

Microstructure Refinement via Nucleation of Intragranular Acicular Ferrite Stimulated by Ti-Containing Core-Shell Structured Particles in Low-Carbon Steel.

This study investigated the microstructure, mechanical properties, and nucleation mechanism of acicular ferrite (AF) present in hot-rolled Ti deoxidized steel. In our experiments, the impact toughness of Ti deoxidized steel is significantly increased to 144 J at -20 °C, while those Mn and Al deoxidized steels are only 9 J and 18 J, respectively. Interlocked AF is the primary microstructure of Ti deoxidized steel. The second-phase particles of the core-shell-type structure, in which Ti2O3 is the nucleus and TiO is the outermost shell, act as effective nucleating agents to stimulate AF nucleation. The low lattice disregistry between TiO and AF is the main factor contributing to the production of AF. It is also revealed that Ti2O3 and MnS fulfill the particular orientation relationship, contributing to the formation of an Mn-depleted zone (MDZ) adjacent to MnS, proposed to be one of the possible mechanisms for promoting AF nucleation.

Influence of Nickel on Microstructure and Mechanical Properties in Medium-Carbon Spring Steel.

The effects of adding nickel on the phase transition temperature, microstructure, and mechanical properties of medium-carbon spring steel have been investigated. The results show that adding nickel reduces the martensite start (Ms) temperature, improves hardenability, and refines the sub-microstructure of the martensite, thereby improving yield stress. The yield strength of martensitic steel increases by approximately 100 MPa due to a synergistic combination of grain refinement strengthening and dislocation strengthening, with an increase in the nickel content from 0 wt.% to 1 wt.%. The cryogenic impact toughness of martensitic steel also improved with a higher nickel content due to packet and block refinement and an increase in the proportion of high-angle grain boundaries (HAGBs).

Strengthening and Embrittling Mechanism of Super 304H Steel during Long-Term Aging at 650 °C.

Super 304H has been a crucial material for ultra-supercritical boilers. However, the relationship between microstructure evolution, strengthening mechanism, and embrittling behavior during long-term aging was lacking investigation. This investigation aimed to reveal the strengthening and embrittling mechanism from precipitates in Super 304H. The results showed that the hardness increment came from the grain boundary's M23C6 (GB's M23C6) and intragranular nano Cu-rich particles. After being aged for 5000 h, the GB's M23C6 and nano Cu-rich particles provided a hardness increment of approximately 10 HV and 30 HV, respectively. The impact toughness gradually decreased from 213 J/cm2 to 161 J/cm2 with the extending aging time. For the aged Super 304H, the GB's M23C6 provided a higher cracking source. In addition, the nano Cu-rich particle restricted the twin-induced plastic deformation of austenitic grain and depressed the absorbed energy from austenitic grain deformation.

Comparison of the Mechanical Properties of Hardfacings Made by Standard Coated Stick Electrodes and a Newly Developed Rectangular Stick Electrode.

Cladding with a stick electrode is one of the oldest arc processes for adding a deposit on a base material. The process is suitable for outdoor working, but the disadvantages are low productivity and large dilution rates. In this work, a simple solution is proposed, which would enable cladding of a larger area with one pass and decrease the dilution rate at the same time-a new type of electrode was developed, exhibiting a rectangular cross-section instead of a round one. Hardfacings, welded with E Fe8 electrodes according to EN 14 700 Standard were welded on mild steel S355 J2 base material with three different coated stick electrodes. The first one was a commercially available, standard, round hardfacing electrode, the second was the same, but with a thinner coating, and the third one was a newly developed rectangular electrode. All three types had equal cross-sections of the metallic core and the same type of coating. Manufacturing of the rectangular electrodes in the laboratory is explained briefly. One- and multi-layer deposits were welded with all three types. Differences were observed in the arc behavior between the round and rectangular electrodes. With the rectangular electrode, the microstructure of the deposit was finer, penetration was shallower, and dilution rates were lower, while the hardness was higher, residual stresses predominantly compressive, and the results of instrumented Charpy impact tests and fracture mechanics tests were better.

Investigation of Microstructure and Mechanical Properties of High-Depth-to-Width-Ratio Horizontal NG-GMAW Joint for S500Q Steel.

A novel high depth-to-width ratio of 15:1 narrow-gap gas metal arc welding technique was developed for the welding of S500Q steel in a horizontal butt joint. The bead arrangement of the I groove was optimized to produce a high-quality connection with the upper sidewall of the joint. The microstructure and mechanical properties were observed and evaluated by optical microscopy, scanning electron microscopy, tensile testing, and micro-hardness and impact toughness testing at 1/5, 2/5, 3/5, and 4/5 thickness of the joint. The 3/5 T position exhibited the highest strength, which was attributed to the presence of finer carbide precipitates. The highest micro-hardness appeared at 4/5 T. The highest impact toughness appeared at 3/5 T. The formation of coarse granular bainite was the major reason for the decrease in impact toughness in other regions. A microscopic fracture at 1/5 T and 3/5 T was further analyzed. It was observed that the width of the fibrous zone at 3/5 T was significantly larger than that at 1/5 T. The radial zones at 1/5 T were observed to exhibit cleavage, with secondary cracks on the fracture surface.

Impact toughness and dynamic constitutive model of geopolymer concrete after water saturation.

The dynamic compression test of geopolymer concrete (GC) before and after water saturation was carried out by the split Hopkinson pressure bar (SHPB). And the effects of water saturation and strain rate on impact toughness of GC were studied. Based on Weibull statistical damage distribution theory, the dynamic constitutive model of GC after water saturation was constructed. The results show that the dynamic peak strain and specific energy absorption of GC have strain rate strengthening effect before or after water saturation. The impact toughness of GC decreases after water saturation. The size distribution of GC fragments has fractal characteristics, and the fractal dimension of GC fragments after water saturation is smaller than that before water saturation. The dynamic constitutive model based on Weibull statistical damage distribution theory can accurately describe the impact mechanical behavior of GC after water saturation, and the model fitting curves are in good agreement with the experimental stress-strain curves.

Microstructural Dependence of the Impact Toughness of TP316H Stainless Steel Exposed to Thermal Aging and Room-Temperature Electrolytic Hydrogenation.

This work deals with the effects of two individual isothermal aging experiments (450 °C/5000 h and 700 °C/2500 h) and the subsequent room-temperature electrolytic hydrogen charging of TP316H stainless steel on its Charpy V-notch (CVN) impact toughness and fracture behavior at room temperature. Microstructural analyses revealed that aging at 700 °C resulted in the abundant precipitation of intermediary phases, namely, the Cr23C6-based carbide phase and Fe2Mo-based Laves phase, whereas aging at 450 °C resulted in much less pronounced precipitation of mostly intergranular Cr23C6-based carbides. The matrix phase of 700 °C-aged material was completely formed of austenitic solid solution with a face-centered cubic (FCC) crystal structure, whereas an additional formation of ferritic phase with a base-centered cubic (BCC) structure was detected in 450 °C-aged material. The performed microstructure observations correlated well with the obtained values of CVN impact toughness, i.e., a sharp drop in the impact toughness was observed in the material aged at 700 °C, whereas negligible property changes were observed in the material aged at 450 °C. The initial, solution-annealed (precipitation-free) TP316H material exhibited a notable hydrogen toughening effect after hydrogen charging, which has been attributed to the hydrogen-enhanced twinning-induced plasticity (TWIP) deformation mechanism of the austenitic solid solution. In contrast, both aging expositions resulted in significantly lowered hydrogen embrittlement resistance, which was likely caused by hydrogen trapping effects at the precipitate/matrix interfaces in thermally aged materials, leading to a reduced TWIP effect in the austenitic phase.

Increasing the Strength and Impact Toughness of Carbon Steel Using a Nanosized Eutectoid Resulting from Time-Controlled Quenching.

High-carbon steels are normally used as tool materials. The use of such steels for construction is limited due to their increased brittleness and poor weldability. However, it appears that high-carbon steels possess certain hidden reserves for enhanced plasticity and strength if properly heat-treated. An unconventional heat treatment was applied to carbon eutectoid steel (0.8 wt.% C) in order to increase its strength and impact toughness simultaneously. Samples for tensile and impact testing were held at 800 °C for different time ranges from 3 min to 9 min with subsequent cooling in oil. It was established that for each type of sample, an optimal holding time exists that is responsible for increased strength and high impact toughness. The hardness and microhardness levels of the surface and under-surface regions of the samples reached 390 HV after optimal heat treatment. An X-ray revealed a shift of the (211)α-peak to the lower 2-theta angles after heat treatment with the optimal holding time; this indicates an increase in carbon content in alpha solid solutions of approximately 0.12 wt.%. Thus, a nanostructured mixture of low-carbon martensite and thin cementite plates is formed in the under-surface region of carbon eutectoid steel after heat treatment, with a controlled holding time at the austenitizing temperature.

Microstructure and Mechanical Properties of High-Strength AA6011 Aluminum Alloy Welding with Novel 4xxx Filler Metals.

Welding high-strength 6xxx aluminum alloys using a commercial ER4043 filler often results in inferior joint strength. This study investigated the effects of newly developed Al-Si-Mg filler metals with varying Mg (0.6-1.4 wt.%) and Mn (0.25-0.5 wt.%) contents on the microstructure evolution and mechanical performance of high-strength AA6011-T6 plates using gas metal arc welding. Two commercial fillers, ER4043 and ER4943, were used as references for comparison. The results revealed that increasing the Mg and Mn contents in the novel fillers resulted in sufficiently high alloying elements in the fusion zone (FZ), leading to higher microhardness. Under as-welded conditions, the weakest region of the joint was the heat-affected zone (HAZ). The joint strength was almost independent of the filler type and was controlled by the HAZ strength, measuring a UTS of 230 and 241 MPa for ER4043 and the other joints, respectively. The higher Mg contents in the novel fillers promoted the precipitation of a large volume fraction of fine ß″-MgSi in the FZ during post-weld heat treatment (PWHT), resulting in superior strength and higher welding efficiency relative to the reference fillers. The optimal Mg content of the novel fillers was 0.6 wt.%. Increasing the Mn content of the filler metal had an insignificant effect. The FMg0.6 filler with 0.6% Mg achieved the best combination of strength (UTS of 410 MPa) and elongation (6.7%) as well as the highest welding efficiency (94%) after PWHT, among all of the fillers studied. However, the newly developed fillers adversely affected the impact toughness of the joints.

Research on the Low-Temperature Impact Toughness of a New 100-mm Ultra-Thick Offshore Steel Fabricated Using the Narrow-Gap Laser Wire Filling Welding Process.

Ultra-thick offshore steel, known for its high strength, high toughness, and corrosion resistance, is commonly used in marine platforms and ship components. However, when offshore steel is in service for an extended period under conditions of high pressure, extreme cold, and high-frequency impact loads, the weld joints are prone to fatigue failure or even fractures. Addressing these issues, this study designed a narrow-gap laser wire filling welding process and successfully welded a 100-mm new type of ultra-thick offshore steel. Using finite element simulation, EBSD testing, SEM analysis, and impact experiments, this study investigates the weld's microstructure, impact toughness, and fracture mechanisms. The research found that at -80 °C, the welded joint exhibited good impact toughness (>80 J), with the impact absorption energy on the surface of the weld being 217.7 J, similar to that of the base material (225.3 J), and the fracture mechanism was primarily a ductile fracture. The impact absorption energy in the core of the weld was 103.7 J, with the fracture mechanism mainly being a brittle fracture. The EBSD results indicated that due to the influence of the welding thermal cycle and the cooling effect of the narrow-gap process, the grains gradually coarsened from the surface of the welded plate to the core of the weld, which was the main reason for the decreased impact toughness at the joint core. This study demonstrates the feasibility of using narrow-gap laser wire filling welding for 100-mm new type ultra-thick offshore steel and provides a new approach for the joining of ultra-thick steel plates.

Mechanism Analysis for the Enhancement of Low-Temperature Impact Toughness of Nodular Cast Iron by Heat Treatment.

The low-temperature impact toughness of nodular cast iron can be significantly enhanced by heat treatment, and thus meet the severe service requirements in the fields of high-speed rail and power generation, etc. In order to explore the enhancement mechanism, microstructure, hardness, composition and other characteristics of as-cast and heat-treated nodular cast iron is systematically tested and compared by optical microscopy, microhardness tester, EBSD, SEM, electron probe, and impact toughness testing machine in this study. The results show that heat treatment has little effect on the morphology and size of graphite in nodular cast iron, ignores the effect on the grain size, morphology, and distribution of ferritic matrix, and has little effect on the hardness and exchange of elements, while it is meaningful to find that heat treatment brings about significant decrease in high-angle grain boundaries (HAGB) between 59° and 60°, decreasing from 10% to 3%. Therefore, the significant enhancement of low-temperature impact toughness of nodular cast iron by heat treatment may result from the obvious decrease in HAGB between 59° and 60°, instead of other reasons. From this perspective, the study can provide novel ideas for optimizing the heat treatment process of nodular cast iron.

Microstructure and Mechanical Properties of IN690 Ni-Based Alloy/316LN Stainless-Steel Dissimilar Ring Joint Welded by Inertia Friction Welding.

Inertia friction welding (IFW) was used to join large-diameter hollow bars made of Inconel 690 and 316LN successfully. The interfacial characteristics, microstructure, mechanical properties and fracture mechanism of welded joints under different process parameters were investigated. The results indicated that a joining mechanism with mechanical interlocking and metallurgical bonding was found in IFW joints. There was a significant mechanical mixing zone at the welding interface. The elemental diffusion layer was found in the "wrinkles" of the mechanical mixing zone. A tiny quantity of C elements accumulated on the friction and secondary friction surfaces. The tensile strength and impact toughness of the joints increased with the total welding energy input. Increasing the friction pressure could make the grain in all parts of the joint uniformly refined, thus enhancing the mechanical properties of welded joints. The maximum tensile strength and impact toughness of the welded joint were 639 MPa and 146 J/cm2, reaching 94% and 68% of that for Inconel 690, respectively, when the flywheel was initially set at 760 rpm, 200 MPa for friction pressure, and 388 kg/m2 for rotary inertia. Due to the Kirkendall effect in the welded joint, superior metallurgical bonding was at the welding interface close to the Inconel 690 side compared to the 316LN side.

Effects of Normalizing Temperature on Microstructure and Impact Toughness of V-N Micro-Alloyed P460NL1 Steel.

In this work, the influence of normalizing temperature on vanadium micro-alloyed P460NL1 steel is studied in terms of microstructures and impact toughness. With the normalizing temperature increased from 850 °C to 950 °C, the V(C,N) particles are dissolved. The dissolution of V(C,N) particles leads to a reduction in their ability to pin the primitive austenite grain boundaries, resulting in the coarsening of the primitive austenite grain. Simultaneously, the number of precipitated particles promoting ferrite nucleation decreased. The combination of these two effects led to the coarsening of ferrite grains in the steel samples. Of note, in the sample normalized at a temperature of 850 °C, the ferrite and pearlite crystals clearly exhibited banded structures. As the normalizing temperature increased, the ferrite-pearlite belt phase weakened. The highly distributed belt phase resulted in poor impact toughness of the steel sample normalized at 850 °C. The belt phase was improved at a normalizing temperature of 900 °C. In addition to that, the microstructure did not undergo significant coarsening at this normalizing temperature, thereby allowing it to achieve the highest toughness among all samples that were prepared for this study. The belt phase almost vanished at the normalizing temperature of 950 °C. However, microstructure coarsening occurred at this temperature, resulting in the deterioration of impact toughness.

Impact of Cryogenic Treatment Process on the Performance of 51CrV4 Steel.

The working load on automotive components is continuously rising, and the mechanical performance requirements for component materials are rising along with the growth trend toward light weight and high dependability in automobiles. In this study, the response characteristics of 51CrV4 spring steel were taken to be its hardness, wear resistance, tensile strength, and impact toughness. Prior to tempering, cryogenic treatment was introduced. Through the Taguchi method and gray relational analysis, the ideal process parameters were discovered. The ideal process variables were the following: a cooling rate of 1 °C/min, a cryogenic temperature of -196 °C, a holding time of 24 h, and a cycle number of three. An analysis of variance revealed that the holding time had the greatest effect on the material properties, with an effect of 49.01%. The yield limit of 51CrV4 was increased by 14.95% and the tensile strength was increased by 15.39% with this group of processes, and the wear mass loss was reduced by 43.32%. The mechanical qualities had a thorough upgrade. Microscopic analysis revealed that cryogenic treatment resulted in refinement of the martensite structure and significant differences in orientation. Additionally, bainite precipitation occurred, exhibiting a fine needle-like distribution, which positively influenced impact toughness. Analysis of the impact fracture surface showed that cryogenic treatment led to an increase in dimple diameter and depth. Further analysis of the elements revealed that calcium (Ca) weakened the negative effect of sulfur (S) on 51CrV4 spring steel. The overall improvement in material properties provides guidance for practical production applications.

Dissimilar autogenous TIG joint of Alloy 617 and AISI 304H steel for AUSC application.

To reduce costs and improve high-temperature performance in Advanced Ultra Super Critical (AUSC) boilers, it is necessary to weld austenitic steel to Inconel alloy. In this study, the autogenous tungsten inert gas (TIG) welding process was used to join Alloy 617 and an austenitic AISI 304H steel plate of thickness 5 mm. Microstructural analysis showed that the microstructure formation was uneven along the weldments, with columnar and cellular dendrites near the interface while the central area of the weld exhibited a combination of columnar, cellular, and equiaxed dendrites. The use of energy dispersive spectroscopy and electron probe micro-analysis unveiled the presence of an unmixed layer at the interface between the weld and AISI 304H steel. Furthermore, a notable variation in the concentration of alloying elements such as Fe, Cr, Ni, Co, and Mo was observed. Within the weld metal, inter-dendritic areas showed the presence of precipitates rich in Cr, Ti, and Mo. Meanwhile, the heat-affected zone (HAZ) of Alloy 617 exhibited the presence of phases like Cr and Mo-rich M23C6 as well as Mo-rich M6C. Hardness tests showed non-uniform hardness along the weldments, with a hardness of 199 ± 6 HV in the weld metal and 225 ± 4 HV in Alloy 617 HAZ, and 207 ± 7 HV in AISI 304H HAZ. The Mo and Cr segregation in the inter-dendritic spaces led to a decline in the tensile properties of the welded parts and resulted in failure from the region of the weld metal.

Effect of Heat Treatment Parameters on the Modification of Nano Residual Austenite of Low-Carbon Medium-Chromium Steel.

The ball milling lining board operates in a harsh environment, and the current materials fail to meet the requirements for large-sized boards due to the lack of synergistic properties between impact toughness and wear resistance. To address this issue, a low-carbon medium-chromium steel with martensite and nano residual austenite phases have been designed for future use. However, the residual austenite network could decrease the properties. Heat treatment, which includes processes like quenching and tempering, has the potential to alter the morphology and quantity of nano-scale residual austenite in the steel. In this study, the influence of heat treatment parameters on the morphologies and properties of steel has been investigated to address the wide-ranging fluctuations in impact toughness affected by nano residual austenite. Furthermore, the effect of cooling transformation on the microstructure has also been examined. The research findings indicate that modifying the quenching temperature of the steel within the range of 950-1100 °C results in a microstructure comprising martensite and nano residual austenite. At all quenching temperatures, the hardness exceeds 45 HRC, and the impact toughness shows a consistent improvement with increasing quenching temperature, indicating a modification of the nano residual austenite phase. The failure mode is primarily dimple fracture, with quasi-dissociation fracture as a secondary mode. The optimal heat treatment parameters are annealing at 930 °C, oil quenching at 1050 °C, and tempering at 250 °C. Under this condition, the steel exhibits a hardness of 51 HRC and impact toughness of 40 J/cm2 and an approximate fourfold increase compared to the untreated sample.

Super-Tough PLA-Based Blends with Excellent Stiffness and Greatly Improved Thermal Resistance via Interphase Engineering.

Super-tough poly(lactic acid)/polycarbonate (PLA/PC) (50/50) blends with an excellent balance of stiffness, toughness, and thermal stability were systematically designed and characterized. Poly(methyl methacrylate) (PMMA) was utilized as a novel, highly effective nonreactive interphase to promote PLA-PC phase compatibility. Partial miscibility of PMMA with both PLA and PC produced strong molecular entanglements across the PLA-PC phase boundary followed by an excellent phase adhesion. This was predicted from interfacial energy measurements and supported by dynamic mechanical thermal analysis, morphological observations, and mechanical tests. Ternary PLA/PC/PMMA blends exhibited an exceptional set of stiffness, tensile and flexural strength, tensile and flexural ductility, and thermal stability together with improved impact strength compared with neat PLA and uncompatibilized PLA/PC blends. Addition of nonreactive polybutadiene-g-styrene-co-acrylonitrile (PB-g-SAN) impact modifier to the compatibilized blend resulted in further dramatic improvements in the dispersion state of PC and PMMA phase domains followed by the development of an interconnected structure of PC, PMMA, and PB-g-SAN domains in the PLA matrix. Such a network-like morphology, with rubbery particles percolated at the interface between the dispersed structures and surrounding PLA matrix, produced a tremendous increase in impact resistance (≈700 J/m) and tensile ductility (≈200% strain) while maintaining excellent stiffness (≥2.1 GPa). The combined effects of interfacial localization of impact modifier particles, network-like morphology (extended over the entire volume of the blend), and strong phase interactions between the components (due to mutual miscibility) are described to be responsible for super-tough behavior. The role of PMMA as an efficient interphase adhesion promoter in the toughened quaternary blends is also clarified. Impact fractography revealed multiple void formations, plastic growth of microvoids, and the formation of void-fibrillar structures around as well as inside the dispersed structures as the main micromechanical deformation processes responsible for massive shear yielding and plastic deformation of blends. Blends designed in this work offer remarkable improvements in tensile and flexural ductility, impact resistance, and heat deflection temperature compared with neat PLA resin. The overall characteristics of these blend systems are comparable and/or superior to those of several commercial thermoplastic resins.

Strength-Toughness of a Low-Alloy 0.25C Steel Treated by Q&P Processing.

Quenching and partitioning (Q&P) treatments were applied to 0.25C steel to produce the microstructures that exhibit an improved balance of mechanical properties. The simultaneous bainitic transformation and carbon enrichment of retained austenite (RA) during the partitioning stage at 350 °C result in the coexistence of RA islands with irregular shapes embedded in bainitic ferrite and film-like RA in the martensitic matrix. The decomposition of coarse RA islands and the tempering of primary martensite during partitioning is accompanied by a decrease in the dislocation density and the precipitation/growth of η-carbide in the lath interiors of primary martensite. The best combinations of a yield strength above 1200 MPa and an impact toughness of about 100 J were obtained in the steel samples quenched to 210-230 °C and subjected to partitioning at 350 °C for 100-600 s. A detailed analysis of the microstructures and the mechanical properties of the steel subjected to Q&P, water quenching, and isothermal treatment revealed that the ideal strength-toughness combinations could be attributed to the mixture of the tempered lath martensite with finely dispersed and stabilized RA and the particles of η-carbide located in the lath interiors.