Martensitic Structure Research Articles

Microstructural Changes and Mechanical Properties of Precipitation-Strengthened Medium-Entropy Fe71.25(CoCrMnNi)23.75Cu3Al2 Maraging Alloy.

Metal alloys with enhanced mechanical properties are in considerable demand in various industries. Thus, this study focused on the development of nanosized precipitates in Fe71.25(CoCrMnNi)23.75Cu3Al2 maraging medium-entropy alloy (MEA). The Fe-based alloying design in the MEA samples initially formed a body-centered cubic (BCC) lath martensite structure. After a subsequent annealing process at 450 °C for varying durations (1, 3, 5, and 7 h), nanosized precipitates (B2 intermetallic) enriched with Cu and with a diameter of approximately 5 nm formed, significantly increasing the hardness of the alloy. The highest Vickers microhardness of 597 HV, along with compressive yield strength and ultimate compressive strength of 2079 MPa and 2843 MPa, respectively, was achieved for the Aged_7h sample. Therefore, the BCC lath martensite structure with B2 intermetallics leads to remarkable mechanical properties.

Characterization of carburized layer on low-alloy steel fabricated by hydrogen-free carburizing process using carbon ions

Hydrogen-free carburized layer was fabricated by irradiating the carbon ions onto a heated low-alloy steel JIS SCM420 substrate using a coaxial arc plasma gun and controlling the diffusion of carbon atoms. The maximum hardness of carburized layer was 733 HV, and the hardness decreased reflecting the carbon concentration distribution in the depth direction of the substrate. The carburized layer was formed to depth of 200 μm. A martensitic structure was observed in the cross-sectional structure observation. Surface elemental analysis by SEM-EDS did not show segregation of chromium or oxygen to grain boundaries. Hydrogen concentration analysis by thermal desorption spectroscopy revealed that the concentration of diffusible hydrogen, which contributes to hydrogen embrittlement, was 0.0064 wt. ppm. Susceptibility to hydrogen embrittlement test was conducted using the three-point bending slow strain rate tensile test. The hydrogen-free carburized layer exhibited about 2.7 times the strength and about 2.3 times the elongation to the hydrogen charged layer after hydrogen-free carburizing. These results led us to conclude that the hydrogen-free carburizing method is notably effective in preventing hydrogen embrittlement.

Terahertz-induced martensitic transformation in partially stabilized zirconia

Martensitic crystal structures are usually obtained by rapid thermal quenching of certain alloys, which induces stress and subsequent shear deformation. Here, we demonstrate that it is also possible to intentionally excite a suitable transverse acoustic phonon mode to induce a local shear deformation. We irradiate the surface of a partially stabilized zirconia plate with intense terahertz pulses and verify martensitic transformation from the tetragonal to the monoclinic phases by Raman spectroscopy and the observed destructive spallation of the zirconia microcrystals. We calculate the phonon modes in tetragonal zirconia and determine the decay channel that triggers the transformation. The phonon mode required for the martensitic transformation can be excited via the Klemens process. Since terahertz pulses can induce a specific local shear deformation beyond thermal equilibrium, they can be used to elucidate phase transformation mechanisms with approaches based on nonlinear phononics.

Excellent mechanical properties of a ZrTi-based alloy via laser surface remelting

In recent years, more and more attention has been paid to the research and development of high strength Zirconium-Titanium (ZrTi) based alloy system. However, like almost all metallic materials, increasing the strength of ZrTi alloy leads to a significant decrease in plasticity. In this paper, high strength and toughness ZrTi alloy with gradient structure was prepared by laser surface remelting. The metastable β phase with high dislocation density was obtained by laser local heating and rapid cooling, and a large amount of acicular martensite structure was formed in the heat affected zone, combined with the dual phase of the matrix, a gradient structure was formed from the surface to the matrix. The tensile strength was increased to 1550 MPa and the tensile elongation was kept above 10%. The composite deformation mechanisms of dislocation slip, twinning and stress-induced martensitic transformation of gradient structure have also been discussed.

Investigation of Martensitic Phase Transformation and Magnetic Properties of Fe-30%Ni-2.6wt%Mo-X%Co Alloys

In this study, the microstructure of thermal effective martensitic phase transformation observed in the Fe-30wt.%Ni-2.6wt.%Mo-Xwt.%Co (X = 0.8, 1.8) alloy was investigated morphologically and crystallographically by using Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). In SEM studies, it was observed that the morphology of the martensite phase changed from lath and lenticular structure to massive structure with the addition of Co to the alloy. In addition to this, nano martensite formation was also observed as another effect of Co addition (figure 2c, d). Properties of lenticular martensite, electron diffraction pattern of austenite and martensite structures were given by TEM studies. According to the Differential Scanning Calorimetry (DSC) analysis, it was determined that the martensitic transformation temperature (Ms) significantly increased with the increase of the Co amount in the alloys. Also, by using Mössbauer spectroscopy, it was shown that the amount of martensite increased with the increase of Co amount and the magnetic order of the alloy changed accordingly.

Geometrical Influence on Material Properties for Ti6Al4V Parts in Powder Bed Fusion

One major advantage of additive manufacturing is the high freedom of design, which supports the fabrication of complex structures. However, geometrical features such as combined massive volumes and cellular structures in such parts can lead to an uneven heat distribution during processing, resulting in different material properties throughout the part. In this study, we demonstrate these effects, using a complex structure consisting of three conic shapes with narrow cylinders in between hindering heat flux. We manufacture the parts via powder bed fusion of Ti6Al4V by applying a laser beam (PBF-LB/M) as well as an electron beam (PBF-EB). We investigate the impact of the different thermal regimes on the part density, microstructure and mechanical properties aided by finite element simulations as well as by thermography and X-ray computed tomography measurements. Both simulations and thermography show an increase in inter-layer temperature with increasing part radius, subsequently leading to heat accumulation along the build direction. While the geometry and thermal history have a minor influence on the relative density of the parts, the microstructure is greatly affected by the thermal history in PBF-LB/M. The acicular martensitic structure in the narrow parts is decomposed into a mix of tempered lath-like martensite and an ultrafine α + β microstructure with increasing part radius. The EBM part exhibits a lamellar α + β microstructure for both the cylindric and conic structures. The different microstructures directly influence the hardness of the parts. For the PBF-LB part, the hardness ranges between 400 HV0.5 in the narrow sections and a maximum hardness of 450 HV0.5 in the broader sections, while the PBF-EB part exhibits hardness values between 280 and 380 HV0.5.

Influence of thermo-mechanical treatment in austenitic and ferritic field condition on microstructural and mechanical properties of reduced activation ferritic-martensitic steel

Abstract 9Cr-1W-0.06Ta reduced activation ferritic-martensitic (RAFM) steel has been investigated in normalized and tempered (N + T) condition and in thermo-mechanically treated (TMT) in austenitic and ferritic field conditions. RAFM steel in N + T condition has a tempered martensitic structure (body-centred tetragonal, bct). It is unstable and will transfer into a stable body-centered cubic (bcc) in the structure during a long period subjected to high temperatures. To make it stable structure, the steel was subjected to austenitization at 1423 K for 10 min, and then cooled in still air (≈−1 K s−1) to 973 K and this temperature was maintained for 2 h in furnace to fully convert austenite into ferritic phase then the steel was subjected to 25% reduction in thickness by hot rolling. Optical, scanning and transmission electron microscopic investigations have been carried out to assess the microstructural changes of the steel N + T and TMT conditions. Hardness, tensile and creep studies are carried out and the results were correlated with the microstructural studies. TMT processed steel resulted in coarser prior austenite grains and exhibited ferrite in phase with fine distribution of M 23C6 and MX precipitates whereas N + T condition is subjected to tempered martensitic structure with coarser M 23C6 and MX precipitates. Even though ferrite phase present in TMT processed steel it exhibits higher tensile and creep strengths due to the presence of high dislocations and finer distribution of precipitates than the N + T condition.

Effect of heat treatment and defects on the tensile behavior of a hot work tool steel manufactured by laser powder bed fusion

AbstractMicrostructure and tensile properties of a hot work tool steel manufactured via laser powder bed fusion (LPBF) were investigated. Specimens were built under two different orientations and subjected to two quenching and tempering heat treatments, featuring different austenitizing and tempering temperatures and the eventual presence of a sub‐zero step. Microstructural analyses revealed a homogeneous tempered martensite structure after both heat treatments, with the only distinction of a higher alloying segregation at a sub micrometric scale length in samples subjected to the highest tempering temperatures. Hardness and tensile tests indicated a negligible effect of building orientation on mechanical properties, but a significant influence of heat treatment parameters. The treatment featuring the lower tempering temperatures and the sub‐zero step resulted in higher hardness, tensile strength, and elongation, attributed to a lower martensite tempering and alloying segregation. Tensile fracture occurred via crack initiation and unstable propagation from large LPBF defects in all the investigated conditions.

Impact Fracture Surfaces as the Indicators of Structural Steel Post-Fire Susceptibility to Brittle Cracking.

The results of experimental research on forecasting post-fire resistance to brittle failure of selected steel grades used in construction are presented and discussed in this paper. The conclusions are based on detailed analysis of fracture surfaces obtained in instrumented Charpy tests. It has been shown that the relationships formulated based on these tests agree well with conclusions drawn based on precise analysis of appropriate F-s curves. Furthermore, other relationships between lateral expansion LE and energy Wt required to break the sample constitute an additional verification in both qualitative and quantitative terms. These relationships are accompanied here by values of the SFA(n) parameter, which are different, depending on the character of the fracture. Steel grades differing in microstructure have been selected for the detailed analysis, including: S355J2+N-representative for materials of ferritic-pearlitic structure, and also stainless steels such as X20Cr13-of martensitic structure, X6CrNiTi18-10-of austenitic structure and X2CrNiMoN22-5-3 duplex steel-of austenitic-ferritic structure.

Effect of paint baking on the halo ring and mechanical behavior of 30MnB5 hot-stamped steel resistance spot welding joints

The present work investigates the effect of paint baking (PB) on the resistance spot welding (RSW) joints of 30MnB5 (1.8 GPa grade) hot-stamped (HS) steel, with focus on the halo ring and fracture behavior under cross-tension loading condition. Microstructural and elemental distribution analyses reveal that severe segregation of alloying elements occurred in the fusion zone (FZ) and that a halo ring with a martensitic structure and a width of about 78 μm was formed in the as-welded joint. The PB process resulted in uniform distribution of alloying elements and recarburization in the halo ring region, thereby drastically decreasing the halo ring width to about 33 μm, which changed the fracture behavior of the joint. Therefore, the as-welded joint failed through the halo ring, while in the PB joint, the fracture propagated partly through the FZ and then redirected through the thickness direction. The PB joints exhibited superior mechanical performance to the as-welded joint, especially the energy absorption.

Microstructure evolution and mechanical properties of TiB2/18Ni300 composite material produced by laser additive manufacturing

As a reinforcing phase, TiB2 exhibits exceptional wettability and stability when mixed with steel matrices, making it an ideal reinforcement for steel matrix composites. In this paper, we add TiB2 for additive manufacturing of 18Ni300 steel for weight reduction and reinforcement. TiB2 reinforced 18Ni300 matrix composites were prepared by laser cladding. SEM, EDS, and XRD were used to observe the joints' interface and fracture characteristics and analyse the substance composition. Tissue analysis was then performed using EBSD and TEM. According to the TEM and XRD results, there is a martensite structure within the cell, and the cell boundary is an austenite structure. After adding TiB2, a reinforcing phase ((Mo, Fe)3B2) is distributed at the boundary of the cell structure. The high hardness of boride increases the microhardness of the composite material. The yield strength, modulus, and elongation of the samples prepared with 18Ni300 powder are lower than that of forged 18Ni300, but the tensile strength reaches 1134 MPa. The addition of TiB2 increases the composite material's yield strength and elastic modulus significantly. This strengthening technique, which employs the addition of ceramic particles to martensitic steel, is currently not widely used. This experiment provides a theoretical basis and process reference for preparing high-stiffness and lightweight ceramic particle reinforced metal matrix composites.

Microstructure Observation around Hardness Transition Area of Repeated Induction Heated S45C Shaft

It had been reported that repeated induction heating process decreased the prior austenite grain size and refined the martensitic structure of JIS SUJ2 samples. In the present work, we compared hardness values and structures of hard surface and soft core of three-times repeated induction heated and once tempered (IH Q3T1) JIS S45C steel bar specimen and once induction heated and once tempered (IH Q1T1) bar. Structures consist of three areas: Hard; Transition; and Soft Core Areas. Martensite, ferrite and martensite, and ferrite and pearlite were observed at the Hard, Transition, and Soft Core Areas. From these observations, it was found that ferrite ratio increased with depth from the surface, and repeated induction heating had little effect on the hardness distribution and internal structure.

Investigation of the microstructure and mechanical behaviour of resistance spot-welded CR210 steel joints using graphene as an interlayer

The present work investigated the resistance spot weldability of 0.85 mm thick galvanised CR210 steel sheets employing graphene nanoplatelets (GNPs) as an interlayer. The GNPs were drop-casted on the steel surface and resistance spot welding (RSW) was carried out at optimum welding current range of 6–9 kA with a constant weld time of 0.7 s. The lap-shear and cross-tensile tests were conducted to assess the mechanical behaviour of the weldments obtained with GNPs as an interlayer. An enhancement of ∼124% in the lap shear strength was observed in the specimen welded at 9 kA. However, the incorporation of GNPs led to the deterioration of the cross-tensile strength of the welded joints. The analysis of the different zones formed after welding was done using SEM and EBSD techniques which revealed the martensitic structure and finer grains obtained in the fusion zone. The presence of dislocation pileups, nano-precipitates, and interfacial shear stress transfer at the GNP-Fe interface was observed by TEM. Raman spectroscopy was carried out to get an insight into the stresses generated in the GNPs owing to RSW at different welding currents. Fracture surface analysis of lap shear specimens revealed the presence of shear dimples at the nugget zone welded at 7 kA and a mixed mode of fracture was observed in the specimen welded at 8 kA. However, a complete nugget pull-out was consistently found during the lap shear test of the specimen welded at 9 kA. Microhardness study revealed an increase in the fusion zone hardness at different welding parameters owing to the existence of GNPs and martensitic structure.

Strengthening mechanism and three-body impact abrasive wear behavior of the hot-rolled air-cooling martensitic wear resistant steel

The microstructure, mechanical properties and three-body impact abrasive wear behavior were investigated. The results indicated that martensite transformation occurred even under air-cooling, and fully martensitic structure consisted of lath martensite and a small amount of twinned martensite was obtained. And the martensite auto-tempering occurred during the subsequent air cooling process, resulting in precipitation of fine ellipsoidal V(C, N) particles and acicular η-Fe2C cementite in the matrix. Meanwhile, the tensile strength, yield strength, Brinell hardness and impact energy (−40 °C) of experimental steel were 1526 MPa, 973 MPa, 454.3 HB and 28 J, respectively. The strengthening mechanism analysis showed that solution strengthening was still the dominant strengthening method even though a large number of carbides were precipitated in the matrix. The dislocation strengthening was limited due to lower dislocation density, and the hot rolled recrystallization led to considerable grain refinement strengthening. Moreover, the results of three-body impact abrasive wear tests indicated that the wear resistance of experimental steel reached a same level as commercial Hardox450 steel. The wear mechanisms of experimental steels were plastic deformation fatigue, abrasive embedded and cutting. The mass loss rates of experimental steel and Hardox450 steel were 0.0033 g/min and 0.0032 g/min under the impact energy of 2.5 J, respectively.

Mechanical Properties Of Pack Carburized AISI 4340 With Variation Energizer Composition of Barium Carbonate (BaCO3) And Sodium Carbonate (Na2CO3)

<div><table cellspacing="0" cellpadding="0" align="right"><tbody><tr><td align="left" valign="top"><p>The aim of this research was to determine the mechanical strength of AISI 4340 steel after pack carburizing with variation of barium carbonate (BaCO<sub>3</sub>) and sodium carbonate (Na<sub>2</sub>CO<sub>3</sub>) as energizer. The various ratios of energizer: 40/60, 50/50 and 60/40 w/w%. Mechanical test was conducted to determine tensile (ASTM E-8), impact charpy strength (ASTM E-23), rockwell hardness number (ASTM E-18) and microstructure characterization (ASTM E-3). Both type specimens were temperature pack carburized is 950ºC and holding time is 3 hours. Results showed that specimen C has a lowest ultimate tensile strength mean values than the other specimens, which is 333.43±30.22 MPa. The results of the impact test showed that the lowest impact energy value is found in specimen C, which is 4.32 joules and the highest impact energy value is found in specimens without treatment, which is 15.80 joules. Based on microscope observation indicated that microstructure of specimen was martensite structure increase and the results of the hardness test was influenced by martensite phase, specimen C has the highest hardness compared to other specimens, which is 80.70 HRC while the untreated specimen is 56.90 HRC.</p></td></tr></tbody></table></div>

Structure and properties of TiNi shape memory alloy after low-temperature ECAP in shells

The effect of low-temperature equal-channel angular pressing (ECAP) in shells on the structure formation and mechanical and functional properties of a near-equiatomic titanium nickelide (TiNi) shape memory alloy is studied. ECAP with channel intersection angles of 120° and 110° is carried out at temperatures in the 20 °С to 330 °С range after 1 pass. The optimum material for shells is pure iron, which is not destroyed after ECAP down to room temperature. The optimum deformation temperature of ECAP in a shell is determined to be 200 °С. Deformation at low temperatures leads to the premature destruction of TiNi samples. ECAP of the TiNi core in a shell at 200 °С after 1 pass leads to the formation of an elongated, banded martensite structure with a bandwidth of approximately 100 nm. The best combination of mechanical and functional properties is found after ECAP of a TiNi sample with a diameter of 12 mm in core-shell mode at 200 °С followed by the addition of post-deformation annealing at 400 °С for 1 h.

Study of Laser Welding Process Parameter on the Microstructure and Mechanical Properties of Dissimilar Joining of Dual-Phase DP780 and Cold-Rolled CR340 Steel

Abstract Laser welding was performed in dual-phase steel (DP780) and cold-rolled steel (CR340) at a series of weld parameters (series I up to series VII): power input, heat input, weld velocity, and focus offset. The study was aimed at improvement of joint performance on the laser welding process parameter settings on the weld bead morphology and thereby evaluating their microstructure and mechanical properties. Optical and scanning electron microscopy and uniaxial tensile tests were performed to observe the microstructural changes, phase formation, and evaluate the mechanical properties, respectively. Tensile strength, microhardness, and strain localization using a digital image correlation technique were investigated for acquainting the local strain mapping associated with the loading. Microstructure characterization revealed that the fusion zone consisted of martensitic structure, and the heat affected contained newly formed martensite in both steels. Vickers hardness revealed that during welding, there was a softening phenomenon at the CR340 end. The DP/CR welds show a mechanical behavior that is driven by both the DP780 and CR340 steel. The joints showed strength up to ∼410 MPa and fracture strain up to ∼16 %, however the weld join failed from CR340 steel end weld during tensile tests. The larger heat input at series VI guarantees joints with an adequate strength but wider weld seams and a lower energy efficiency.

Improvement of tensile properties of low-carbon steels via short-time intercritical annealing

The effect of low volume fraction formation of martensite on the tensile properties of low carbon steel was evaluated. First, steel samples with ferrite-cementite microstructure were produced. The thermomechanical treatment used included austenitizing at 1000 °C and then quenching in ice brine solution, tempering the obtained martensitic structure for 1 h at 650 °C, 80% cold rolling, and re-tempering for 2 h at 650 °C. In order to form a low volume fraction of martensite, steel samples with ferrite-cementite microstructure were intercritically annealed for 30 seconds at 740 °C. As a result of intercritical annealing treatment, 6.2% martensite was formed. The results of tensile test showed that the formation of 6.2% martensite led to the elimination of yield point phenomenon and Lüders banding, decrease of yield stress and increase of true stress at maximum load, while true uniform strain did not change significantly. The work hardening rate also increased significantly. Based on the results of modeling of the flow behavior with the Holloman equation, the work hardening capability of the steel sample including ferrite-cementite decreased after a certain plastic strain, while the work hardening capacity remained constant with the formation of a low volume fraction of martensite in the microstructure.

Enhanced strength and ductility of the low-carbon steel with heterogeneous lamellar dual-phase structure produced by cyclic intercritical rolling

Steels with outstanding mechanical properties are desired in industrial applications. Martensitic structure significantly increases the strength in low-carbon steels, but usually sacrifices their ductility. In order to improve the combination of strength and ductility, a full martensitic steel was further processed by cyclic intercritical rolling to produce a novel heterogeneous lamellar structured dual-phase (HLSDP) structure. This processing route constructs a small amount of lamellar ferrite surrounded by high volume fraction of refined martensite, which is a promising heterostructure for hetero-deformation induced (HDI) hardening. The HLSDP steel shows a simultaneously improved strength and ductility (ultimate tensile strength, UTS: 1.85 GPa; uniform elongation, UE: 5.6%), comparing to the lath martensite steel (UTS: 1.68 GPa and UE: 4.1%). The superior tensile properties of HLSDP steel are attributed to the higher strain hardening ability, which is related to the significant HDI hardening. Due to the deformation incompatibility between the soft zone (ferrite) and hard zone (martensite), strain partitioning is occurred during plastic deformation. In addition, micro digital image correlation (μ-DIC) analysis reveals the evolution of strain field during deformation. A significant strain gradient is produced near the interface of ferrite and martensite in the HLSDP steel.

Enhancement on the low-pressure shot peened process of monotonic plastic deformation behaviors of Ni–4Cu–3Mo alloy/super austenite stainless steel dissimilar joints using laser beam welding

This research paper mainly discusses the low-pressure shot-peened (LPSP) process of 2 mm thick sheet plates of monotonic mechanical behaviors of Ni–4Cu–3Mo alloy/super austenite stainless steel (SASS) dissimilar materials joint by laser beam welding. Three types of butt joint (Sample-1, Sample-2, and Sample-3) configuration methods of the laser beam welding process were implemented at the Ni–4Cu–3Mo alloy/SASS dissimilar weldments and LPSP weldments. The microstructure image of the fusion zone revealed that the high heat inputs were distributed at the grain boundary and the prior coarse grain structures were converted into NiCr and C–Cr with significant improvement in the α'-martensite structure. To achieve continuous improvement in yield strength of LPSP weldments compared to the dissimilar weldments, the plastic deformations rate was decreased in the shot-peened weldments. The base metals of tensile samples length were prepared as ±5 mm; however, samples clearly indicated that the dissimilar weldments and LPSP weldments showed the failure in the fusion zone. The average elongation of dissimilar LPSP weldments was increased by 13%; however, the heat inputs collapsed the grain boundaries growth and the structure was converted into twin boundaries and α-martensite structures. The hardness measurement at the fusion zone of dissimilar and LPSP weldments was performed and the average hardness value of 290 HV at the fusion zone and 211 HV at the heat-affected zone was observed for LPSP weldments. The fracture morphology revealed that neither major cracks nor minor voids at the fusion zone of the tensile sample were formed.