Martensite Block Research Articles

Engineering mechanical properties by controlling the microstructure of an Fe–Ni–Mn martensitic steel through pre-cold rolling and subsequent heat treatment

Experiments were conducted to examine the effects of prior cold rolling and subsequent heat treatment on the microstructure, phase evolution and mechanical properties of an Fe–10Ni–7Mn martensitic steel. The results show that 70% cold rolling leads to deformation-induced austenite formation in the microstructure and reduces the size of the martensite blocks. Prior cold rolling increases the reversed austenite volume fraction under post-deformation intercritical annealing (PDIA) at 600 °C compared with the solution annealed condition. Electron back scattering diffraction and dilatometric studies showed that under PDIA there is a reverse transformation of martensite to austenite through a sequential combination of martensitic and diffusional mechanisms. There is also a precipitation of θ-NiMn particles in the PDIA specimen after subsequent ageing leading to an increase in the ultimate tensile strength in tensile testing. Cyclic tensile testing revealed pseudoelastic behavior in the cold-rolled specimen but this disappeared after PDIA and appeared again after subsequent ageing. • Cold rolling produces deformation-induced austenite formation and also refines the martensite blocks. • The prior austenite and the smaller martensite blocks are the primary potential sites for austenite nucleation during PDIA. • Ultimate tensile strength improves from ~695 MPa for the solution annealed sample to ~1215 MPa for the PDIA processed specimen followed by ageing. • Observed pseudoelasticity in the cold rolled specimen disappears after PDIA. • Pseudoelasticity appears again in the PDIA processed specimen followed by ageing.

Characteristics and mechanisms of hydrogen-induced quasi-cleavage fracture of lath martensitic steel

This study presents an in-depth characterization of the microstructures, crystallographic orientations, and dislocation characteristics beneath hydrogen-induced quasi-cleavage fracture features of an as-quenched, lath martensitic (α') 22MnB5 steel. The fracture surfaces of gaseous hydrogen-embrittled martensitic specimens were analyzed by a combination of multiple analytical tools: scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and transmission Kikuchi diffraction (TKD). The dominant fracture mode in the hydrogen-affected zones was quasi-cleavage fracture, which involved significant plasticity, evidenced by plastic zones near tear ridges and a high density of dislocations beneath the quasi-cleavage facets. The martensite constituent sizes, variant orientations, and boundaries influenced the quasi-cleavage surface morphologies. Fractography revealed the occurrence of {100}α'-type cleavage across martensitic laths, developing relatively “flat” quasi-cleavage surfaces, in addition to {110}α'-type cracking likely along lath and block boundaries, and fracture along non-cleavage planes. The likelihood of the formation of the relatively “flat” quasi-cleavage surfaces increased with increasing martensitic constituent sizes. Substantial dislocation bands formed on intersecting {112}α' slip planes within a martensite block below the hydrogen-induced cleavage fractures on {100}α' planes. River markings on the quasi-cleavage surfaces were found to originate from the complex, hierarchical lath martensitic microstructure. Steps and ridges on the quasi-cleavage facets generally linked with the various martensitic boundaries, suggesting that they were produced as a result of crack deviations at those boundaries. The fracture paths and martensite quasi-cleavage mechanisms are discussed in the context of the role of hydrogen and Cottrell cleavage model.

Effect of aging temperature on the austenite reversion and mechanical properties of a Fe–10Cr–10Ni cryogenic maraging steel

Fe–10Cr–10Ni cryogenic maraging steel is one of the key candidate materials applied in harsh cryogenic condition due to its high strength and good cryogenic toughness. In this study, the effect of aging temperature on its microstructure and mechanical properties was systematically studied based on austenite reversion and nano-precipitation. It shows that a desirable combination of high strength (834 MPa, 25 °C) and excellent cryogenic impact toughness (164 J, −196 °C) can be obtained by aging treatment at 500 °C. Multi-scale characterizations were conducted to reveal the microstructure characteristics of the steel. It was found that obvious film-like reversed austenite nucleate and grow at the high-angle grain boundaries of martensite matrix in the steel aged at 500 °C, whereas the higher aging temperature resulted in a larger content of blocky reversed austenite in martensite blocks. Austenite reversion mechanism was proposed based on the double-spherical-cap model and diffusion kinetics of Ni element. Besides, it was found that the precipitation sites of Ti-rich particles are not only distributed in matrix but also located at the dislocations, and they were identified as the clusters of Ni3Ti precipitates. Finally, the origin of the above cryogenic toughness includes the transformation-induced plasticity (TRIP) effect from the soft film-like austenite, higher density of high angle grain boundaries of martensite and fine nanoscale precipitates. Moreover, the precipitation strengthening from the clusters of Ni3Ti precipitates contributes to the high strength of the steel aged at 500 °C.

Fatigue Crack Initiation and Propagation at High Temperature of New-Generation Bearing Steel

The new generation of bearing steel has good comprehensive properties, which can satisfy most of the requirements of bearing steel in a complex environment. In the presented work, fatigue properties of 15Cr14Co12Mo5Ni2 bearing steel have been investigated by means of rotating bending fatigue tests on smooth bar specimens after carburization and heat treatment. Optical microscope, scanning electron microscopy, electron backscatter diffraction, and Image-Pro Plus software were used to analyze the fracture, microstructure, and carbides. The results suggest that the fatigue strength at room temperature and 500 °C is 1027 MPa and 585 MPa, respectively. Scanning electron micrographic observations on the fracture surface of the fatigue specimens at 500 °C show that fatigue cracks usually initiate from voids in the carburized case and oxide layer on the surface of steel. The failure mode in the carburized case is a quasi-cleavage fracture, and with the increase of crack propagation depth, the failure mode gradually changes to fatigue and creep-fatigue interaction. With the increase of the distance from the surface, the size of the martensite block decreases and the fracture surface shows great fluctuation.

The Influence of Different Microstructure on Tensile Deformation and Acoustic Emission Behaviors of Low-Alloy Steel.

The effect of different microstructures, obtained under different tempering temperatures on acoustic emission (AE) characteristics and source mechanisms during tensile deformation, was investigated in this study. Different heat treatments were carried out on hot-rolled low-alloy steels to obtain different microstructures (ferrite/pearlite, tempered martensite and tempered sorbite) and the AE was used to monitor the deformation and fracture process of samples of different types (BM, 200 °C tempered and 600 °C tempered). The results showed that the microstructure had different influences on the high amplitude burst-type signals and low amplitude continuous-type signals during the deformation and fracture process of low-alloy steels. In the 200 °C tempered sample, the continuous-type signals were enhanced by the high yield stress and dislocation velocity induced by the block of the lath martensite whose substructure was high-density dislocation. On the other hand, the interaction of the precipitates with the local dislocations increased the intensity of AE events, thus generating burst-type signals with higher amplitude in the 600 °C tempered samples.

Microstructure-specific hardening of ferritic-martensitic steels pre and post 15 dpa neutron irradiation at 330 °C: A dislocation dynamics study

Abstract In this work, we used Dislocation Dynamics (DD) simulations to investigate the role of the hierarchical defects microstructure of ferritic-martensitic steel Eurofer97 in determining its hardening behavior. A Representative Volume Element (RVE) for DD simulation is identified based on the typical martensitic lath size. Material properties for DD simulations in b.c.c Eurofer97 are determined, including the dislocation mobility parameters. The dependence of material parameters on temperature is fitted to experimental yield strength measurements carried out at room temperature and 330 °C, respectively. Voids and precipitates observed in the microstructure, such as M23C6 and Tantalum-rich MX, are considered in our DD simulations as inclusions with realistic size distribution and volume density, while 〈1 1 1〉 -and 〈1 0 0〉 -type irradiation loops are included directly in the DD simulations. The lath structure, together with its typical precipitates arrangement and the different crystallographic orientation of the martensitic blocks can also be captured in the simulations. DD simulations are used to extract microstructure-specific hardening parameters, which can be used to simulate the properties of Eurofer97 at the engineering scale.

Effect of thermomechanical cyclic quenching and tempering treatments on microstructure, mechanical and electrochemical properties of AISI 1345 steel

Thermomechanical cyclic quenching and tempering (TMCT) can strengthen steels through a grain size reduction mechanism. The effect of TMCT on microstructure, mechanical, and electrochemical properties of AISI 1345 steel was investigated. Steel samples heated to 1050°C, rolled, quenched to room temperature, and subjected to various cyclic quenching and tempering heat treatments were named TMCT-1, TMCT-2, and TMCT-3 samples, respectively. Microstructure analysis revealed that microstructures of all the treated samples contained packets and blocks of well-refined lath-shaped martensite and retained austenite phases with varying grain sizes (2.8–7.9 µm). Among all the tested samples, TMCT-3 sample offered an optimum combination of properties by showing an improvement of 40% in tensile strength and reduced 34% elongation compared with the non-treated sample. Nanoindentation results were in good agreement with mechanical tests as the TMCT-3 sample exhibited a 51% improvement in indentation hardness with almost identical reduced elastic modulus compared with the non-treated sample. The electrochemical properties were analyzed in 0.1 M NaHCO3 solution by potentiodynamic polarization and electrochemical impedance spectroscopy. As a result of TMCT, the minimum corrosion rate was 0.272 mm/a, which was twenty times less than that of the non-treated sample. The impedance results showed the barrier film mechanism, which was confirmed by the polarization results as the current density decreased.

The study on martensite morphology in the stir zone and its influence to impact toughness during friction stir welding medium–Mn ultrahigh strength steel

A 6 mm thickness medium-Mn ultrahigh strength steel was friction stir welded (FSW) with various rotation speed at constant of welding speed in this study. The microstructural evolution and impact toughness of the stir zone were investigated in a systematic manner. The microstructural observation results suggested that the metastable austenite in base metal disappeared after FSW, and the stir zone of FSW samples contained various morphology of martensite (i.e., well tempered martensite, less tempered martensite and common martensite). The content of well tempered martensite and less tempered martensite in the stir zone was decreased with the increase of rotation speed, and the martensite blocks and prior austenite grain size were gradually coarsened. It is suggested that the plastic deformation and the small prior austenite grain size promoted the formation of well tempered martensite in the stir zone at low rotation speed. The well tempered martensite could effectively release the stress concentration by interfaces separation with other martensite of different morphology during impact test, which greatly benefited the impact toughness. Furthermore, the small martensite blocks could also improve the impact toughness. The impact toughness of the stir zone was decided by the martensite blocks size and the content of different morphology of martensite phase.

Martensite tempering kinetics: Effects of dislocation density and heating rates

In this study, the combined effect of dislocation density and rapid thermal cycling on the tempering kinetics of fully martensitic steel has been investigated by straining the sheets to different deformation levels. It was found that the strain-induced dislocations created more nucleation sites for carbide precipitation, and decreased carbide growth time. Strain-enhanced precipitation caused the carbide morphology to change from elongated to small quasi-spherical precipitates. Plastic deformation also accelerated the decomposition of carbides and film-like retained austenite in the as-received martensite laths. After tempering the deformed martensite had higher microhardness than tempered unstrained martensite, resulting from finer and semi-coherent precipitates, a high retained dislocation density, and the presence of untempered and partially tempered martensite blocks.

Investigation of Size Effects Due to Different Cooling Rates of As-Quenched Martensite Microstructures in a Low-Alloy Steel

Martensite transformation is a complex mechanism in materials that is classically initiated by a suitable heat treatment. This heat treatment process can be optimized based on a better understanding of the physical mechanisms on the length scale of several prior austenite grains. It is therefore appropriate to consider individual process steps of heat treatment in isolation. The aim of this study is to characterize the microstructural size changes caused by a variation of the cooling rate applied during the quenching process. For this purpose, individual martensitic microstructures from different heat treatments are analyzed using the electron backscatter diffraction (EBSD) method. With special orientation relationships between the parent austenite and martensite, the structure of the prior austenite grains and the close packet plane packets can then be reconstructed. The influence of the heat treatments on these characteristics as well as on the martensite blocks is thus quantified. No significant influence of the quenching rate on the sizes of martensite blocks and packets could be found.

Significant Improvement in Strength and Toughness of Nanoscale Precipitate–Strengthened Steel by Direct Quenching and Tempering Process

Herein, the microstructural evolution and mechanical properties of a low‐carbon Nb–Ti–Mo‐bearing steel subjected to hot rolling followed by four different processing schedules (air cooling, direct quenching, low‐temperature tempering, and high‐temperature tempering) are studied to elucidate the significance of structural refinement and nanoscale precipitates for obtaining the optimized combination of strength and toughness. The microstructure of air‐cooled steel composed of coarse polygonal ferrite and granular bainite, and minimum yield strength of 473 MPa and lowest impact energy at −40 °C of 40 J are obtained. The microstructure of steel direct quenched and tempered at 520 °C consists of sub‐micron tempered martensitic and bainitic laths with a high relative fraction of high misorientation boundaries and high density of 3–5 nm (Nb, Ti, Mo)C precipitates, and maximum yield strength of 726 MPa and highest impact energy at −40 °C of 213 J are obtained. The formation of nanoscale precipitates coupled with fine tempered martensitic and bainitic blocks contributes to 253 MPa improvement of yield strength and more than five times enhancement of impact energy. The small blocks of tempered martensite and bainite can deviate and finally arrest the crack propagation, whereas the coarse polygonal ferrite and bainite provide inadequate resistance for crack propagation.

Internal residual stress originated from Bain strain and its effect on hardness in Fe–Ni martensite

To further understand the internal residual stress that is microscopically generated via martensitic transformation in steels, the origin of internal strain attributed to Bain correspondence between face-centered cubic (fcc) and body-centered cubic (bcc) was evaluated from macro- and micro-viewpoints and its effect on hardness was investigated in an interstitial free Fe-16%Ni martensite. Neutron diffractometry and electron backscatter diffraction analysis showed that body-centered cubic (bcc) crystal structure of as-quenched martensite contained elastic distortions leading to small tetragonality, even in martensitic steels without solute carbon, and that the extended [001]bcc of martensite tended to be parallel to <001>fcc of prior fcc austenite. In addition, the combination of micro-scale focused ion beam (FIB) and high-precision digital image correlation techniques revealed that a micropillar fabricated by FIB processing within a martensite block was anisotropically deformed by the release of the residual strain distributed in as-quenched martensite in correspondence with the tetragonal distortions of the bcc crystal structure. These results prove that a small part of the Bain strain remained as an internal elastic residual strain and was microscopically distributed among Bain groups in lath martensite after martensitic transformation. Furthermore, the residual strain generated a hydrostatic internal stress, and therefore, the nanohardness decreased considerably by the micropillar fabrication accompanied by the release of the internal stress. This means that the internal residual stress in martensite among Bain groups influences the mechanical properties of martensitic steel.

Influence of Segregation on Microstructure and Hot Workability of Grade 250 Maraging Steel

The optimal process parameters of grade 250 maraging steel for hot forging were investigated with a process map and microstructure analysis. To generate the process map, hot compression tests were performed at 800–1200 °C and strain rates of 0.01–5 s−1. The flow stress–strain curves were calibrated by Bayesian artificial neural network (ANN) modeling to compensate the heat generated by dynamic deformation. The Ni and Mo segregation during the solidification of the ingot caused alternating layers of Ni- and Mo-rich and -lean bands, which affected the recrystallization behavior during hot compression. According to the calculated process map, 1100–1200 °C × 0.01–0.7 s−1 and 1000–1200 °C × 0.01–0.2 s−1 are favorable process conditions that ensure wide process windows in terms of strain rate and temperature, respectively. The common features of the microstructure deformed under both conditions were relatively coarse martensite blocks and low values in the electron backscattered diffraction (EBSD) kernel average misorientation (KAM) results (i.e., low residual stresses), which were attributed to a low fraction of fine martensite block region.

Internal Residual Stress Originated from Bain Strain and its Effect on Hardness in Fe-Ni Martensite

To further understand the internal residual stress that is microscopically generated via martensitic transformation in steels, the origin of internal strain attributed to Bain correspondence between face-centered cubic (fcc) and body-centered cubic (bcc) was evaluated from macro- and micro-viewpoints and its effect on hardness was investigated in an interstitial free Fe-16%Ni martensite. Neutron diffractometry and electron backscatter diffraction analysis showed that the as-quenched martensite had a body-centered tetragonal (bct) crystal structure with small tetragonality, even in martensitic steels without solute carbon, and that the [001]bct of bct martensite tended to be parallel to fcc of prior fcc austenite. In addition, the combination of micro-scale focused ion beam (FIB) and high-precision digital image correlation techniques revealed that a micropillar fabricated by FIB processing within a martensite block was anisotropically deformed by the release of the residual strain distributed in as-quenched martensite in correspondence with the orientation of the bct crystal structure. These results prove that a small part of the Bain strain remained as an internal elastic residual strain and was microscopically distributed among Bain groups in lath martensite after martensitic transformation. Furthermore, the residual strain generated a hydrostatic internal stress, and therefore, the nanohardness decreased considerably by the micropillar fabrication accompanied by the release of the internal stress. This means that the internal residual stress in martensite among Bain groups influences the mechanical properties of martensitic steel.

Effects of Cyclic Heat Treatment on Microstructure and Mechanical Properties of 13%Cr-4%Ni Martensitic Stainless Steel

The present work investigates the effects of thermal cycling heat treatment (TCHT) on microstructure and mechanical properties of 13%Cr-4%Ni martensitic stainless steel (13-4MSS). As-received 13-4 MSS was subjected to three different thermal cycling schedules by using a thermo-mechanical simulator (Gleeble 3800). The evolved microstructures were studied at three cycles for each thermal cycling schedule with the help of optical, scanning electron microscopy, x-ray analysis, and transmission electron microscopy. Hardness and notched tensile tests were conducted to further characterize the as-received and processed specimens. This cyclic treatment raised the hardness to 413 HV (for TCHT at 950 °C) from 274 HV (for as-received). A 42% (for TCHT at 950 °C) and 39% (for TCHT at 1050 °C) hike in ultimate tensile strength (UTS) with a slight decline in ductility was observed as compared to the UTS and ductility of as-received steel. The evolved microstructure, dislocation density, and the refinement of martensitic blocks attributed to the enhanced hardness and UTS. The coarsening of laths (observed at 1050 °C) and bimodal lath structure (observed at 950 °C) were also found to control the mechanical properties of the present steel.

The effect of TiN inclusions on the fracture mechanism of 20CrMnTi steel with lath martensite

This study aims to investigate the effect of TiN inclusions on the fracture mechanism of 20CrMnTi steel with martensite. Size of martensite packets, blocks and TiN inclusions were characterized, and the room-temperature tensile properties, impact toughness and fracture toughness were tested of 20CrMnTi steel quenched at different temperatures. The effects of TiN inclusions on the impact toughness and fracture toughness were investigated according to the Hall-Petch relationship. The results show that, TiN inclusions are high temperature stable phases which insoluble to the matrix, mostly squared in shape and dispersed. The impact toughness and fracture toughness of 20CrMnTi steel decrease with increasing sizes of the initial austenite grains, martensite packets and blocks as the quenching temperature increases. Interestingly, the TiN inclusions strongly affect the toughness of the 20CrMnTi steel in the fracture and the fine-grained sample has a better toughness. Under high-stress concentrations, TiN inclusion particles can initiate cleavage cracking.

Effects of the Phase Interface on Spallation Damage Nucleation and Evolution in Dual‐Phase Steel

The dual‐phase steel sample obtained by step quenching (SQ) of AISI 1020 steel is shock loaded using one‐stage light‐gas gun experiments. The free surface particle velocity is measured by Doppler pin system (DPS) during the loading experiment. The effect of phase interface on spallation damage nucleation and evolution in dual‐phase steel is investigated with optical microscopy (OM), nanoindentation, and electron backscatter diffraction (EBSD) techniques. The evolution mechanism of spallation damage in dual‐phase steel is systematically analyzed. The results show voids nucleated within martensite rather than at the martensite/ferrite interface under dynamic loading. The nucleation of voids in martensite was not random, the martensite block boundaries with a great Taylor Factor (TF) mismatch and a large angle to the impact direction were the preferred nucleation positions of the voids. Voids nucleate, grow and further coalesce into microcracks. Most of the microcracks propagate across the phase interface and the propagation direction hardly deflects. Due to the existence of many martensite blocks with high‐angle grain interface boundaries inside the martensite, the resistance of microcracks propagation in martensite is greater than that in ferrite. Furthermore, the size of microcracks is related to the angle between the direction of martensite blocks and shock direction.

Effect of Stress on Martensitic Transformation in Warm Worked and Quenched MediumCarbon Steel

As for ausforming of medium-carbon steel (Fe-0.56C-0.2Si-0.71Mn-0.017P-0.01 mass%S), it is investigated whether compressive stress during quenching changes the hardness of martensite. The result of hardness measurement indicates that the hardness of martensite is independent of compressive stress. In addition, whether some changes occur in the martensitic microstructure is investigated. There is no change in the size of martensite blocks and no change in the longitudinal direction of the martensite blocks. There is a slight tendency for specific variants to form preferentially in stressed test pieces. It is considered that the hardness does not change because the microstructure does not change significantly.

Simulation of Austenite Formation During Continuous Heating from Low Carbon Martensite with Poly-dispersed Cementite

The nucleation and growth of austenite during continuous heating in plain carbon martensite is simulated using classical nucleation and diffusion growth theories assuming that austenite is nucleated on cementite particles at prior austenite grain boundaries and martensite packet, block, and interlath boundaries. A critical nucleus model on the spherical substrate was modified to take into account the influence of the boundary energy on which cementite particles formed. Simulations were carried out using the particle size distribution of cementite measured in an Fe-0.2 mass pct C alloy heated to near eutectoid temperature (Ae1). Austenite nucleation stopped in a very short time regardless of boundary site or particle size of cementite due to the fast decrease in carbon supersaturation and the depletion of nucleation sites. The fraction of austenite nucleated on cementite at prior austenite boundaries and martensite packet boundaries etc was much greater than that nucleated on cementite at interlath boundaries. While cementite particles dissolved quickly after austenite was nucleated, a large proportion of cementite particles at lath boundaries remained undissolved until they disappeared at 30 °C to 40 °C above Ae1. The evolution of austenite grain size was also simulated after austenitization was completed, and compared with experiment.

In situ observation of high temperature microstructure and reverted austenite evolution in 13Cr supermartensitic stainless steel

In situ observation of high-temperature microstructure in 13Cr supermartensitic stainless steel was investigated by the high temperature laser scanning confocal microscopy (LSCM), and the effects of tempering temperature and tempering cooling rate on the morphology and contents of reverted austenite were characterized via transmission electron microscopy (TEM), x-ray diffraction (XRD) and electron backscattered diffraction (EBSD). The results indicate that only α phase appeared in the XRD diffraction pattern after quenching at 1050 °C, there was no retained austenite in the quenched microstructure. The martensite transformation starting temperature (Ms) was around 177.6 °C according to the results of LSCM. The amount of reverted austenite increased first and then decreased with the tempering temperature increasing, and it reached a maximum value when the sample tempered at 620 °C. With the cooling rate increasing, the amount of reverted austenite showed a sharp decrease after tempering at 620 °C. The reverted austenite precipitated on martensite laths, blocks and prior austenite grain boundaries, while the misorientation angles ranged from 0° to 60° in the tempered martensite structure.