Lath Martensite Research Articles

Effect of boron content and quenching temperature on the microstructure and wear resistance of high boron steel

Abstract This article investigates the change rule of the microstructure, hardness and wear resistance of high boron steel containing 0.1 wt.% to 0.5 wt.% boron in the cast state and after quenching at different temperatures. The results show that the microstructure of cast high boron steel with different contents of boron is composed of pearlite, ferrite and eutectic boride, which has lower hardness and wear resistance, and the higher the content of boron, the higher the hardness and the better the wear resistance. After quenching at 900–1,000 °C, pearlite and ferrite change into a large number of lamellar martensite and a small amount of lath martensite. After high-temperature quenching at 1,050 °C, retained austenite appears in the microstructure in addition to martensite, and borides are partially dissolved. The hardness and wear resistance are significantly improved compared to the as-cast high boron steel. As the quenching temperature increases, the dissolution of boride is obvious, the hardness and wear resistance are firstly increased and then decreased. When the content of boron is 0.5 wt.% and the quenching temperature is 1,000 °C, the hardness reaches a maximum value of 59.0 HRC, and the abrasion resistance is the best.

Microstructure and mechanical-property evolution of the explosive welding joint from the same RAFM steels under explosive welding and post-weld heat treatment

This study investigated the morphological evolution behaviors and the mechanical property of the weld interface during explosive welding for the flyer plate and base plate sourcing from the same reduced activation ferrite/martensitic (RAFM) steel. The relationship between fine-grain formation and the occurrence of dynamic recrystallization was analyzed based on the strain and temperature field distributions at the explosive-weld interface. Although post-weld tempering (PWT) treatment eliminated bent martensite laths, but it did not eliminate fine equiaxed grains and δ ferrite grains near the explosive-weld interface; therefore, the PWT treatment improved the mechanical properties of the explosive-weld RAFM steel joint only minimally. However, implementation of a normalizing process after the explosive welding process (“PWN” treatment) significantly eliminated the gradient structure and the residual δ ferrite grains at the explosive-weld interface of the RAFM steel joint. Based on the PWN state, the inclusion of a tempering treatment (“PWNT” treatment) promoted the uniform precipitation of M23C6 carbides, which significantly improved the tensile strengths of the explosive-weld RAFM steel specimens under both room and high temperatures. To study the influence of different microstructures on the plastic deformation behavior at the explosive-weld interface, the representative volume element (RVE) models basing on crystal plasticity finite element (CPFEM) method was constructed for different post-weld heat treatments (PWHTs). At the explosive-weld interface of the PWT-state RAFM steel joint, the plastic deformation was mainly concentrated in the δ ferrite grains, rather than in the fine equiaxed grains. In addition, only minimal deformation concentration was observed for the PWNT-state RAFM steel joint. In order to reveal the independent influence of the fine equiaxed grains at the weld interface on the mechanical properties of explosive-weld RAFM steel joint, the microstructural and mechanical distinctions between the RAFM steel joints in the N-PWT and PWNT states were investigated detailly. Finally, through optimization of the PWHT method, an explosive-weld RAFM steel (PWNT state) joint with a uniform microstructure and ideal mechanical properties was obtained.

On the Strengthening Mechanism in Lath Martensite of As-Quenched Fe–Ni Alloys

On the Strengthening Mechanism in Lath Martensite of As-Quenched Fe–Ni Alloys

Microstructural characteristics of the internal oxidation zone of ferritic/martensitic steel exposed to LBE at 550°C for 1000 h

The oxidation products formed on ferritic/martensitic (F/M) steel exposed to oxygen-saturated lead-bismuth eutectic (LBE) at 550 °C for 1000 h were investigated using various characterizations. The results indicate that the corrosion products consist of three distinct layers from the inside to the outside: the inner oxidation zone (IOZ), a middle oxide layer, and the growth front facing LBE. In the IOZ, although O has penetrated the entire oxidation layer, the martensite laths and ferrite grains remain visible. However, significant segregation of Cr and O along the laths and grain boundaries has occurred, resulting in the formation of strip-shaped Cr2O3 particles. In the matrix adjacent to the IOZ, Cr atoms have segregated at the grain boundaries and martensite laths, forming Cr-rich regions, while O atoms have not yet infiltrated. During the growth of Fe3O4 grains, the priority formed strip-shaped Cr2O3 particles are pushed toward the corrosion front until they detach from the F/M steel and disperse into the LBE, resulting in nearly pure Fe3O4 grains in the middle layer. The gradient three-layer structure of the oxidation product is closely associated with the segregation of Cr and the gradient distribution of oxygen partial pressure (PO2).

Effect of Laser Powder Bed Fusion Parameters on the Evolution of Melt Pool, Densification, Microstructure, and Hardness in 420 Stainless Steel Parts

Laser powder bed fusion (LPBF) involves depositing, melting, and solidifying metal powder particles layer by layer to create 3D components. In this study, a deep fundamental understanding on how process parameters—laser power, scan speed, and hatch spacing—affect the melt pool, densification, microstructure, hardness, and thermal behavior of 420 stainless steel (420SS) parts produced by such technology is provided. The conducted investigation considers five levels of laser power and hatch spacing, and four scan speeds. Optimal single tracks, based on geometry and profile, are achieved with laser powers between 40 and 80 W and a scan speed of 10 mm s−1. In the multitrack analysis, it is indicated that a dense, smooth surface is obtained with a hatch spacing of 250 μm, corresponding to an overlapping rate of ≈30%. The 420SS samples show high densification (≈99%) and low surface roughness (≈3.62 μm). The microstructure consisted of martensite laths and retained austenite. The hardness and thermal conductivity of the samples are measured at 540 HV and 15.3 W m−1 K−1, respectively. In this study, the understanding of the process–structure–property relationships in LPBF of 420SS is expanded.

The evolution and toughening mechanism of austenite in high Co–Ni ultrahigh strength steel

The austenite, as a “soft” phase, has a significant impact on the mechanical properties in ultrahigh strength steel. The morphology, size, and distribution of austenite play a critical role in toughness of steel. This study concentrates on morphology, size, and distribution of austenite subjected to solid solution, cryogenic, and aging treatments and explores its evolution characterized by transmission electron microscopy, scanning electron microscopy, electron backscatter diffraction and x-ray diffraction. The toughening mechanism of austenite with different characterization was investigated through charpy impact testing and fracture morphology analysis. The results indicate that filmy and blocky austenite existed at the lath boundary of martensite. Blocky austenite and some filmy austenite were eliminated after cryogenic treatment, while approximately less than 15 nm filmy reverted austenite was formed after aging treatment. Filmy reverted austenite at the boundary of martensite laths can enhance toughness. Retained austenite without cryogenic treatment can induce the formation of larger blocky austenite, which decreased toughness.

Laser melting deposited a laminated H13/17–4PH heterostructure alloy to achieve strength-ductility synergy and outstanding high-temperature wear resistance

To inhibit the cracking behavior induced by the poor ductility of LMD-ed H13 steel, we have adopted a soft/hard laminated composite strategy to achieve a 17–4PH/H13 heterostructure alloy. Also, the effect of its hard-soft interfacial characteristics on tensile properties and high/room-temperature wear resistance has been studied. The research results show that this laminated alloy mainly comprises two alternately stacked martensite layers with a layer width of 350 µm, including lath martensite (H13) and lath martensite (17–4PH). The lath-like martensite layer possesses a high average microhardness value of over 500 HV, far higher than that of LMD-ed 17–4PH steel. Besides, the laminated alloy shows a high σb (∼1721.59 MPa) with a sufficient δ (∼5.21 %), far higher than that of LMD-ed H13 steel (i.e., σb ∼1701.41 MPa and δ ∼ 1.27 %), which the fracture mode is mixed by ductile and cleavage fracture. The H13/17–4PH laminated alloy has excellent wear resistance at RT comparable to the LMD-ed H13 steel. Interestingly, the H13/17–4PH laminated alloy maintains low friction coefficients and wear rates at 600 ℃. Specifically, its high-temperature wear rate (22.28×10−6 mm3 N−1·m−1) is one-eighth as low as that of LMD-ed H13 steel (157.17×10−6 mm3 N−1·m−1). The above results provide a reference for re-manufacturing die steels in industrial applications of high-temperature wear resistance.

Effect of austenitizing temperature on variant selection of martensite transformation in a 2 GPa grade steel

The correlation of prior austenite grain (PAG) size, mechanical properties, and the variant selection in martensite transformation of a 2 GPa grade automobile steel has been systematically investigated by using a combination of scaning electron microscopy (SEM), electron backscatter diffraction (EBSD), and uniaxial tesile tests. As the austenitizing temperature decreases from 970 °C to 870 °C, the as-quenched microstructure of the studied steel is fully lath martensite. Meanwhile, the average size of PAG is refined from 7.2 ± 3.9 μm to 2.1 ± 1.2 μm, and the volume fraction of high angle grain boundaries (HAGB) is increased from 59.8% to 72.3%, accompanied with an increased number fraction of grains belonging to closely spaced planes (CP) group. With the refinement of the PAGs, the martensitic variant selectivity becomes strong, and the Bain group variant selection discretely transites to the CP group, resulting in a dominate CP group. Consequently, a tensile strength of 2014 ± 33 MPa, a tensile elongtation of 12.9 ± 0.5% and a maximum strength × ductility production of 26.0 ± 1.4 GPa·%, are obtained when the austenitization temperature is at 920 °C.

Microstructural and mechanical properties evolution for 9Cr[sbnd]Si heat resistant steel deposited metals under different heat treatment states

To develop the matched filler metal for the structural materials in Generation IV lead-cooled fast reactor (LFR), the microstructural evolution and mechanical property variation of 9CrSi heat resistant steels deposited metals were investigated both for the as-welded and as-aged (at 550 °C) weld metals. The experimental results indicate two precipitates in the as-welded deposited metals: the bigger M23C6 and the smaller MX. Si facilitates the nucleation of M23C6. The chain-like precipitates (consisting of M23C6) form at the boundaries of prior austenite and martensitic laths in high-Si specimens. They deteriorate not only the strength but also the impact energy. Meanwhile, a few δ ferrites form in high-Si deposited metal deteriorating the impact energy. Precipitates tend to nucleate at the interface between the δ ferrite and the martensite after aging. New types of precipitates (Mo-rich M6X and Laves) with rapid coarsening rate form in as-aged high-Si deposited metals as time increases. Coarse precipitates deteriorate the pinning effect, which causes the recovery of laths and dislocations. Subgrain pinned by M23C6 forms in deposited metals with post-weld heat treatment (PWHT) during aging. The structure hinders the formation of M6X and facilitates the formation of nano-sized MX. Therefore, the mechanical properties of deposited metals with PWHT are stable during long-term (up to 10,000 h) aging treatments.

Effects of heat input on microstructure evolution and corrosion resistance of underwater laser cladding high-strength low-alloy steel coating

The effects of heat input on the microstructure evolution and corrosion resistance of the high-strength low-alloy (HSLA) steel coating obtained by wire-feed underwater laser cladding were studied. Optical digital microscope, scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), transmission electron microscope (TEM), electron back-scattered diffraction (EBSD), electrochemical tests, laser microscope and other characterization methods were used. As the heat input increased, the smoothness of the underwater cladding coating improved, with little changes in elemental distribution and phase composition. However, the content of acicular ferrite increased, while the amounts of lath bainite, lath martensite, and granular bainite decreased. As the heat input increased from 0.4 to 0.55 kJ/mm, the grain size increased from 25.3 to 55.9 μm, the proportion of low-angle grain boundaries rose from 61.3 % to 69.1 %, and the texture intensity of the (110) crystal plane increased from 16.22 to 23.83. Meanwhile, the dislocation density decreased from 4.9 × 1014 to 4.1 × 1014 m−2. These changes enhanced the corrosion resistance of underwater coating. As the heat input increased from 0.4 to 0.55 kJ/mm, the corrosion current density decreased from 2.13 × 10−5 to 3.35 × 10−6 A/cm2, and the corrosion potential increased from −0.823 to −0.529 V. Additionally, the depth-to-width ratio of the corrosion pits decreased from 0.255 to 0.053. This study laid the foundation for high-quality in-situ underwater repairs of ships and submarines made of high-strength steel.

Designing a new ultra-high strength steel with multicomponent precipitates under material genetic design

Ultra-high strength steels with excellent strength, ductility and toughness have become one of indispensable high-performance structural materials. The development of ultra-high strength steels with higher performance has been widely concerned to break the strength-ductility trade-off. This study proposes a material inverse design strategy that combines material genetic design with thermodynamic calculation to search for new alloys with desirable microstructure and optimal strengthening contributions. Based on the advancement of the 'Pareto Front', a new steel with superior properties was successfully screened from 390,625 candidate individuals, which was verified by experiments. The experimental results show that a new steel Fe-0.25C-14.8Ni-17.0Co-2.54Mo-1.56Cr-0.53Al-1.06Cu-0.38V (wt. %) has been successfully developed. After quenching-cryogenic-aging process, the new steel achieved a superior combination of strength, ductility and toughness, with a yield strength of 2.2 GPa, an elongation-to-failure of 10% and a V-notch impact energy of 15 J. Multi-scale microstructure characterization indicates that such superior property synergy arises from the unique microstructure: lath martensite matrix with high-density dislocations and interstitial solid soluble carbon atoms, embedded fine M2C, NiAl and Cu-rich nanoprecipitates. We present an in-depth discussion on the origins of superior strength/ductility/toughness combinations of the new steel to validate the design strategy and provide an accessible pathway exploiting high-performance ultra-high strength steels.

Effect of different heat inputs on the microstructure and mechanical properties of the laser welded joint of CLF-1 steel

Abstract In this study, we examined the effects of various heat inputs on the microstructure and mechanical properties of the laser-welded joints of CLF-1 low-activation ferritic/martensitic steel medium-thick plates, which are used in the Test Blanket Module (TBM). The results indicate that under four different welding heat inputs (2727 J/cm, 3000 J/cm, 3563 J/cm, and 4500 J/cm), the weld microstructure is primarily composed of a large amount of lath martensite and a small amount of δ-Fe. As the heat input increases, the average width of lath martensite in the weld microstructure progressively increases, while the average volume fraction and width size of δ-Fe experience a significant increase. When the heat input surpasses 3563 J/cm, δ-Fe exhibits a dense aggregation. The tensile properties of the joints created under different heat inputs are superior to those of the base material. At the lowest heat input, the maximum impact toughness of the weld reaches 40 J.

A unique kind of distribution band of martensitic laths formed along {332}〈113〉 twin boundary in the Ti[sbnd]42Nb alloy subjected to instable deformation

Multi-scale electron microscopy characterization of the {332}<113 > β twins produced in the necking region near fracture surface of the deformed β-type Ti42Nb (wt%) alloy have been carried out. A unique kind of band-shaped zone with plenty of laths of two α″ martensitic subvariants distributed in about 90° crossing directions has been found to be formed in the {332}<113 > β deformation twin along the twin boundary. This finding clearly reveals that intense local stress concentration and its complex accommodation process associated with the β-to-α″ and α″-to-β structural transitions can occur specially in the twinned region of stress-induced α″ martensite phase along its boundary for the β Ti alloy subjected to instable deformation. It is also clearly indicated that the local stress accommodation due to the α”-to-β structural transition can be unsymmetrical with respect to the twin boundary.

Development of Robust Steel Alloys for Laser-Directed Energy Deposition via Analysis of Mechanical Property Sensitivities

To ensure consistent performance of additively manufactured metal parts, it is advantageous to identify alloys that are robust to process variations. This paper investigates the effect of steel alloy composition on mechanical property robustness in laser-directed energy deposition (L-DED). In situ blending of ultra-high-strength low-alloy steel (UHSLA) and pure iron powders produced 10 compositions containing 10–100 wt% UHSLA. Samples were deposited using a novel configuration that enabled rapid collection of hardness data. The Vickers hardness sensitivity of each alloy was evaluated with respect to laser power and interlayer delay time. Yield strength (YS) and ultimate tensile strength (UTS) sensitivities of five select alloys were investigated in a subsequent experiment. Microstructure analysis revealed that cooling rate-driven phase fluctuations between lath martensite and upper bainite were a key factor leading to high hardness sensitivity. By keeping the UHSLA content ≤20% or ≥70%, the microstructure transformed primarily to ferrite or martensite, respectively, which generally corresponded to improved robustness. Above 70% UHSLA, the YS sensitivity remained low while the UTS sensitivity increased. This finding, coupled with the observation of auto-tempered martensite at lower cooling rates, may suggest a strong response of the work hardening capability to auto-tempering at higher alloy contents. This work demonstrates a methodology for incorporating robust design into the development of alloys for additive manufacturing.

Role of prior austenite grain boundary and retained austenite in strain localization of medium-carbon high-strength steels

Exploring the micromechanical behavior of lath-martensitic quenching and tempering (Q&T), as well as quenching and partitioning (Q&P) steels presents a significant challenge due to their complex multi-phase constituents and hierarchical structures. This study conducts a comprehensive comparative analysis using medium-carbon Q&T and Q&P steels with identical chemical compositions. Atom probe tomography (APT) was used to investigate the distribution of chemical elements, while high-resolution digital image correlation (HR-DIC) provided insights into nanoscopic strain localization and partitioning. Both Q&T and Q&P steels exhibit significant strain localization at prior austenite grain boundaries (PAGBs) due to substantial carbon segregation. Strain localization in Q&T steel at PAGBs and packet boundaries strongly depends on preferred geometric orientations, with boundaries inclined 45° to the loading direction experiencing severe deformation. Conversely, in Q&P steel, PAGBs adjacent to large clusters of fine lath martensite, lacking a packet with a longitudinal direction parallel to boundaries and a feasible in-lath slip system, primarily undergo deformation. Carbon partitioning from martensite to retained austenite (RA), along with coexisting carbon and manganese segregation at phase interfaces, stabilizes the RA, resulting in a neighborhood-dependent deformation behavior of RA. The existence of RA not only absorbs carbon, maintaining low strength and high deformability, but also uniformly distributes and induces stable interfaces to hinder the formation of large packets with easily activated in-lath slip transmission.

Strengthening mechanisms in vanadium-microalloyed medium-Mn steels

In this work, the impact of adding vanadium, ranging from 0 to 2 wt%, on the microstructure and mechanical properties of as-cast medium manganese steel was explored. A dual phase microstructure consisting of martensite and retained austenite was observed in the 0 V, 0.05 V and 0.8 V conditions. The 2 V condition contained retained austenite, lath martensite, and δ-ferrite bands. The retained austenite fraction and prior austenite grain size initially increased at lean vanadium concentrations but significantly dropped at the highest vanadium concentration. The element distribution in the constituent phases was investigated in detail. Mn, C and Si partitioning to austenite was observed in the 0 V, 0.05 V and 0.8 V conditions. V and C segregation to the grain boundaries and significant grain refinement were evident in the 2 V condition. The findings also revealed that increasing the vanadium content led to an increase in the hardness of the steel. This assessment was validated by tensile testing, which showed an improvement in yield and tensile strength of the steel with increasing vanadium content, and were supported by reconstruction of the parent austenite grains employing martensitic structures. Finally, the influence of different strengthening mechanisms on the yield strength of as-cast, microalloyed medium-manganese steels was also discussed in terms of simulated stacking fault energy, as well as the quantitative contributions from solid solution, grain boundary, and precipitation strengthening mechanisms.

Effect of α‐Ferrite on Plasticity and Fracture Mechanisms of Dual‐Phase Ultrastrong Heterolamellar Steels

Lamellar heterostructure design is widely recognized as a potential method to improve strength and ductility simultaneously of high‐performance steel. However, the high mechanical contrast among soft/hard phase and inharmonic plastic deformation lead to complex fracture mechanisms and limit the improvement of plasticity. Herein, the plasticity and tensile fracture mechanisms of two types of δ‐ferrite and lath martensite heterolamellar steels with and without α‐ferrite are investigated. The sample without α‐ferrite shows a combination of cleavage in δ‐ferrite and dimple zones in the martensite. The microcracks initiate in the martensite early and the then the cleavage fracture occurs in the δ‐ferrite due to the stress concentration. In contrast, the sample with α‐ferrite has lower plasticity. Failure of the samples with α‐ferrite is govern by the microcracks initiating in the interface of martensite/α‐ferrite. Eliminating the impact of the effect of α‐ferrite, the total elongation increases from 8.9% to 11.6%, and the ultimate tensile strength (UTS) increases from 1567 to 1652 MPa. This study investigates the critical factor in plasticity control of dual‐phase heterostructure and promotes the development and application of lamellar structure.

Effects of Tempering on Microstructure and Properties of Additive Manufacturing Cu-Bearing AISI 431 Steel.

AISI 431 martensitic stainless steels (MSS) with 2.5 wt% Cu were fabricated via laser-directed energy deposition additive manufacturing followed by single-step tempering treatment. The influences of different tempering times at 600 °C on microstructure and mechanical properties of the as-deposited 431-2.5Cu MSS have been explored and analyzed. The as-deposited MSS specimen primarily consisted of lath martensite, austenite and M23C6 carbide. After the single-step tempering treatment at 600 °C, Cu-enriched (ԑ-Cu) nano-precipitates and reverse austenite can be formed and promoted by extending the tempering treatment. The microhardness, strength and elongation can be improved with increasing the tempering time up to 1.0 h, and subsequently reduced with the tempering time prolonging to 2.0 h. Compared to 431 MSS that requires a multiple-step heat treatment for excellent performance, the 431-2.5Cu MSS specimen presented superior tensile properties after single-step tempering at 600 °C for 1.0 h in the present work. The ultimate tensile strength (UTS), yield strength (YS) and elongation (EL) of one-hour tempered MSS were 1611 MPa, 1334 MPa and 16.3%, respectively. This study provides a quantitative theoretical reference and experimental basis for realizing short-process fabrication of the Cu-bearing MSS with high strength and ductility.

The Microstructure, Mechanical Properties, and Precipitation Behavior of 1000 MPa Grade GEN3 Steel after Various Quenching Processes

This study examines the microstructure, mechanical properties, and precipitation behavior of 1000 MPa grade GEN3 steel when subjected to various quenching processes, with a focus on the quench and partition (Q&amp;P) technique. The Q&amp;P-treated samples achieved 1300 MPa tensile strength and demonstrated superior yield strength, attributed to their refined substructure and their large amounts of precipitates. The quenched samples exhibited the thinnest martensite laths due to the highest martensite volume. Despite the as-annealed samples having the smallest grain size, the Q&amp;P treatment resulted in optimal microstructural refinement results and a high dislocation density, reaching 1.15 × 1015 m−2. Analysis of the precipitates revealed the presence of V8C7, M7C3, M2C, and Ti(C, N) across various heat treatments. The application of the McCall–Boyd method and the Ashby–Orowan correction model indicated that quench and tempered (Q&amp;T) samples contained the largest volume of fine precipitates, contributing to their high yield strengths. These findings offer valuable insights for optimizing heat treatment processes to develop advanced high-strength steels for industrial applications.

Analyzing the effect of different shielding atmospheres on fiber laser welding parameters for modified 9Cr-1Mo steel

The present research aims to investigate the influence of and the relationship between welding speed ( v), focal position ( FP), and laser power ( LP) as input parameters for fiber laser to weld P91 under pure argon and CO2 shielding atmospheres using response surface methodology and using the desirability approach; optimization of input parameters has been performed to maximize the depth of penetration ( DP) while minimizing the top bead width ( TW) and heat-affected zone width ( HW). The results indicate that laser power ( LP) has the most significant impact on depth of penetration ( DP), whereas welding speed ( v) affects top bead width ( TW) and heat-affected zone width ( HW). The optimal combination of parameters under argon shielding results in a significantly low heat input of 48 J/mm (0.048 kJ/mm) and 50 J/mm (0.050 kJ/mm) pure argon and CO2 shielding atmospheres, respectively. Despite the difference in heat input being only 4.08%, an 11.618% increase in DP is observed under the CO2 shielding atmosphere. Due to the high-speed nature of the welding process (argon: 82.65 mm/s and CO2: 79.283 mm/s) combined with the significant reduction in heat input, the heat-affected zone is limited to a mere width of 0.203 mm under an argon shielding atmosphere and 0.263 mm under the CO2 shielding atmosphere compared with that reported in the other welding processes without the dissolution of precipitates in the fusion boundary of heat-affected zone. Microstructural characterization of the fusion zone has revealed the presence of an untempered lath martensite structure with precipitate dissolution. The precipitate dissolution has resulted in grain coarsening of 47.76% in argon weld and 39.65% in CO2 weld compared with the base metal grain size of 6.07 µm.