Microstructure Of Lath Martensite Research Articles

Role of Different Kinds of Boundaries Against Cleavage Crack Propagation in Low-Temperature Embrittlement of Low-Carbon Martensitic Steel

The present paper investigated the relationship between low-temperature embrittlement and microstructure of lath martensite in a low-carbon steel from both microstructural and crystallographic points of view. The fracture surface of the specimen after the miniaturized Charpy impact test at 98 K (−175 °C) mainly consisted of cleavage fracture facets parallel to crystallographic {001} planes of martensite. Through the crystallographic orientation analysis of micro-crack propagation, we found that the boundaries which separated different martensite variants having large misorientation angles of {001} cleavage planes could inhibit crack propagation. It was then concluded that the size of the aggregations of martensite variants belonging to the same Bain deformation group could control the low-temperature embrittlement of martensitic steels.

Internal friction analysis of lath martensite in press hardened steel

Internal friction (IF) measurements were carried out on a press hardened steel (PHS) after continuous annealing, press hardening and bake hardening. The IF peaks of the PHS with a lath martensite microstructure were analysed by comparison with previously published data. This was supplemented by comparison with the IF spectra of the same steel with a ferrite–pearlite microstructure after deformation at room temperature, and after recrystallisation annealing and quenching. The relation between the IF peaks of PHS, and the γ-peak, Snoek peak and Snoek-Kê-Köster peak observed for ferritic steel is discussed.

Effects of Plastic Deformation, Heat Treatment and Molybdenum and Nickel Concentration on Lath Martensitic Microstructure

Lath martensitic microstructures were produced in steels containing 0.15% of carbon and various amounts of molybdenum and nickel. The behavior of the lath martensitic microstructure was studied during cold rolling in steels with six different chemical compositions. All materials have endured 75% deformation without damage. In addition, the effects of heat treatments on microstructures and hardness were investigated as well.

Microstructure and Mechanical Properties of Heat-treated T92 Martensitic Heat Resistant Steel

AbstractT92 samples were solutionized at 1,050 °C, 1,100 °C and 1,150 °C for 20 min and then tempered at 730 °C, 745 °C and 760 °C for 60 min. Optical microscopy studies were carried out to understand the microstructural evolution due to heat treatment. These heat-treated samples comprised of lath martensite microstructure in all the cases. Prior austenite grain size of the heat-treated samples increased with solutionizing temperature. Tensile properties were evaluated using micro-tensile samples. Hardness values of the heat-treated samples were estimated using Vickers hardness tester. Interestingly, for all the given tempering condition, the hardness values showed an increasing trend with solutionizing temperature while their tensile strength values tend to decrease. Fractograph analysis depicted that increasing the solutionizing temperature led to grain boundary decohesion.

A Two-Tilt Analysis of Electron Diffraction Patterns from Transition-Iron-Carbide Precipitates Formed During Tempering of 4340 Steel

Fine-scale transition-iron-carbide precipitates in a lath martensitic microstructure of 4340 steel tempered at 200 °C for 1 h were examined via imaging and electron diffraction techniques with a transmission electron microscope. Region-to-region variations were eliminated by analyzing a small volume of material (about 0.03 μm3) at two tilt conditions. Geometric analyses showed that measured interplanar spacings compared favorably with accepted values from both epsilon-carbide and eta-carbide phases (within 1%), whereas measured interplanar angles were within 1−2% of accepted values. Centered-dark-field imaging identified precisely which reflections were produced from a single group of small precipitates (each about 10 nm in diameter). Consistent indexing schemes are provided for epsilon- and eta-carbide, including the proper angular relationship between the two tilt conditions. An interzonal angle of 17.9o ± 0.1o was determined for both candidate phases. The orientation relationship between the observed transition-iron-carbide phase and martensite was determined, and confirmed previous results reported for both carbide phases. Diffracted intensities of several reflections were estimated and compared favorably with those calculated from structure factors derived from idealized crystal structures of both transition-iron-carbide phases. All results are shown to be near-equally consistent with a hexagonal epsilon-carbide phase and an orthorhombic eta-carbide phase.

Multiple mechanisms of lath martensite plasticity

The multi-scale complexity of lath martensitic microstructures requires scale-bridging analyses to better understand the deformation mechanisms activated therein. In this study, plasticity in lath martensite is investigated by multi-field mapping of deformation-induced microstructure, topography, and strain evolution at different spatial resolution vs. field-of-view combinations. These investigations reveal site-specific initiation of dislocation activity within laths, as well as significant plastic accommodation in the vicinity of high angle block and packet boundaries. The observation of interface plasticity raises several questions regarding the role of thin inter-lath austenite films. Thus, accompanying transmission electron microscopy and synchrotron x-ray diffraction experiments are carried out to investigate the stability of these films to mechanical loading, and to discuss alternative boundary sliding mechanisms to explain the observed interface strain localization.

Study on High-Temperature Flow Behavior and Substructure and Texture Evolution of TA15 Titanium Alloy

The hot deformation behaviors of TA15 titanium alloy were investigated by isothermal compression experiments on Gleeble-3500 thermal simulation machine. The results indicate that the flow stress curves of TA15 titanium alloy in the two-phase region are dynamic recrystallization (DRX) type while in the β single-phase region are main dynamic recovery (DRV) type. The evolution of microstructure and substructure (grain boundary misorientation and dislocation) under different process parameters were studied by using electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). Microstructure analysis shows that a large number of recrystallized α grains and martensite α′ phase appear as the strain rate decreases under the condition of two-phase region. However, lath martensite microstructure is replaced by lamellar martensite microstructure at low strain rate in β single-phase region. Grain boundary misorientation analysis indicates that low angle boundaries (LABs) transform into high angle boundaries (HABs) sufficiently by reducing strain rate or increasing deformation temperature. Texture evolution analysis shows that the degree of preferred orientation after deformation weakens and the intensity of texture decreases with strain rate increasing in the two-phase region. However, more potential slip systems are activated in the β single-phase region. TEM analysis suggests that microscopic deformation bands with high density of parallel arrangement dislocations evolve into subboundaries or boundaries. As the deformation continues, dislocations are accumulated around the subboundaries, and they promote the transformation of subgrains with LABs into new grains with HABs.

Characterization of (Nb,Ti,Mo)C Precipitates in an Ultrahigh Strength Martensitic Steel

A study on ultrahigh strength steel plate subjected to novel thermo-mechanical control process was presented. The mechanical properties examination showed that the investigated steel exhibited excellent combination of ultra-high strength (2200 MPa) and toughness (26 J). The microstructure of the experimental steel was observed by scanning electron microscope and transmission electron microscope. Desired martensitic lath with width of about 180–230 nm was obtained. Nanostructured carbide precipitates with sizes of 20–50 nm, which contained Nb, Ti and Mo, were observed in the lath martensitic microstructure, and confirmed to be MC-type carbides with B1 structure by means of selected area electron diffraction. The compositional characteristics revealed by energy dispersive X-ray spectrometer mapping implied that the carbide forming elements Nb, Ti and Mo distributed in the precipitates evenly. Three-dimensional atom probe tomography reconstruction further indicated that Mo incorporated into the precipitates without enrichment in the carbide-matrix interface and probably substituted for Nb and Ti to form the (Nb,Ti,Mo)C carbides.

Simulation of dislocation recovery in lath martensite steels using the phase-field method

Simulation of dislocation recovery using the phase-field method is used to clarify the collapse process of the lath martensite phase in high-Cr ferritic steels, in which densely packed and tangled dislocations exist. Edge dislocations move more rapidly than screw dislocations at elevated temperatures, and when stress is applied, the tendency becomes more pronounced. Dislocations form sub-boundaries, a so-called polygonization structure, in order to decrease the total energy. The size of the sub-boundary is estimated to be smaller than the lath width. As a result, it is found that collapse of the lath martensite microstructure starts with the division of laths caused by dislocation rearrangement.

Phase transformation and mechanical behaviour of thermo-mechanically controlled processed high-strength multiphase steel

A novel low-alloy high-strength steel [Fe–0.20C–1.65Mn–1.40Si–1.50Al–1.30Cu–1.05Ni–1.07Co (wt%)] has been thermo-mechanically processed with a finish rolling temperature of 850 °C, followed by air cooling and water quenching in order to obtain a good combination of strength and ductility. Phase transformations of the above steel at different cooling rates have been studied and continuous cooling transformation (CCT) diagram has been constructed using data, obtained from dilatometric study. The phase field of CCT diagram indicates microstructure changes from a mixture of ferrite and bainite to fully martensite accompanied with the enhancement of hardness with increasing cooling rate. The microstructural investigation at lower cooling rate (≤5 °C/s) suggests the possibility of achieving pearlite-free microstructure by direct air cooling from the austenite region. Directly air-cooled steel has demonstrated primarily ferrite–bainite microstructure, which shows attractive tensile strength (>1050 MPa) and ductility (>15 %). On the other hand, directly water-quenched steels reveal predominantly lath martensitic microstructure with high dislocation density which exhibits higher tensile strength (>1600 MPa) and lower ductility (~12 %). The multiple stages of strain hardening behaviour of the investigated steel under different cooling conditions have been examined with respect to microstructural evolution.

Transformation-rate maxima during lath martensite formation: plastic vs. elastic shape strain accommodation

Recently, a modulated formation behaviour of lath martensite in Fe–Ni(-based) alloys was observed, exhibiting a series of transformation-rate maxima. This peculiar transformation behaviour was explained on the basis of the hierarchical microstructure of lath martensite, minimising the net shape strain associated with martensite formation, by a block-by-block formation of martensite packages occurring simultaneously in all packages. In the present work, the martensitic transformation upon slow cooling of two Fe–Ni alloys, containing 22 and 25 at.% of Ni, respectively, was investigated by high-resolution dilatometry with the aim of identifying the influence of alloy composition on the modulated transformation behaviour. The differences observed for the two alloys, a more rapid sequence of the transformation-rate maxima and a narrower temperature range in case of Fe-25 at.% Ni, can be explained consistently as a consequence of the lower transformation temperatures in Fe-25 at.% Ni, highlighting the role of temporary accommodation of the shape strain during formation of the lath martensite microstructure: the depression of the transformation toward lower temperatures leads to a higher strength of the austenite, hence resulting in a more elastic (less plastic) temporary accommodation of the shape strain upon block formation and thereby in a more effective mutual compensation of the shape strain by neighbouring blocks. A kinetic model on the basis of energy-change considerations is presented which is able to describe the observed modulated transformation behaviour.

Effect of substructure on mechanical properties and fracture behavior of lath martensite in 0.1C–1.1Si–1.7Mn steel

The purpose of this study was to analyze the microstructure of lath martensite in 0.1C–1.1Si–1.7Mn (wt.%) steel and its effect on mechanical properties and fracture behavior. The microstructure was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and electron back scattering diffraction (EBSD). Charpy V-notch impact samples and compact tension (CT) samples were used to investigate the Charpy impact properties and fatigue crack growth behavior of the steel, respectively. The propagation of cleavage crack and fatigue crack were analyzed to figure out the effective grain size. The results showed that the typical hierarchical lath martensite structure contained prior austenite grains, packets, blocks and laths; packet size and block width were positively correlated to prior austenite grain size, while lath width was maintained at about 0.29 μm. Yield strength was related to prior austenite grain size, packet size and block width, and obeyed Hall–Petch relationship. Grain refinement was effective in improving the resistance to cleavage fracture by introducing barriers to crack propagation; packet boundaries and block boundaries hold similar ability to impede the propagation of crack. Paris model can well describe the FCG behavior of the investigated steel. Block width governs the effective grain size for strength, toughness and fatigue crack propagation.

A model for the microstructure behaviour and strength evolution in lath martensite

A new model describing the microstructure and strength of lath martensite is introduced. The packet and block size were found to linearly depend on the prior-austenite grain size when introducing relevant crystallographic and geometric relationships of their hierarchical arrangements. A mechanism for the lath boundary arrangement within a block is postulated to ensure complete carbon redistribution to the lath boundaries. Accordingly, the dislocation density is obtained by considering the lattice distortion energy within a lath being equal to the strain energy of the dislocation density at the lath boundaries. Tempering effects are introduced by estimating the extent of carbon diffusing away from the lath boundaries; this mechanism relaxes the Cottrell atmospheres of lath dislocations and coarsens the boundaries. The yield stress as well as the microstructure evolution during tempering are successfully predicted by combining these results. The model is further extended to describe the yield stress in dual-phase steel microstructures by employing the iso-work principle. The model predictions are validated against experimental data in seven martensitic and five dual-phase steels, where the prior-austenite grain size, carbon content, tempering conditions and martensite volume fraction are employed as input. These results cover wide composition, initial microstructure and tempering conditions.

Microstructure and Mechanical Properties of 22MnB5 Steel with Different Heat Treatment

Microstructure and mechanical properties of 22MnB5 Steel were analysis with different heat treatment experiment. The result show that the martensite lath with water cooling become smaller than that with metal die cooling, The lath martensitic microstructure is disappeared with die cooling & tempered specimen. The tensile strength reaches the highest 1645.34MPa by water cooling, but the plastic strength product is lowest. Q&P and die cooling & tempering process can improve the elongation of 22MnB5 with a small amount of tensile strength decrease. The plastic strength product reaches the highest of 20312.74MPa•% by die cooling & tempering method.

Microstructure and Mechanical Properties of GMAW Weld Metal of 890 MPa Class Steel

The relationship between microstructure and mechanical properties of gas metal arc weld metal with strength over 890 MPa is discussed. The microstructure of the weld metals is characterized with OM, SEM, TEM and EBSD. The microstructure of the weld metals is mainly composed of martensite and bainite. Experimental results show that the microstructure with predominant fine lath bainite possesses good toughness of 77 J, while its yield strength is less than 800 MPa. The microstructure of coarse lath martensite and bainite has the lowest toughness of 43 J and its yield strength is 820 MPa. The mixed microstructure with fine martensite, bainite and retained austenite films bears good combination of toughness and yield strength (62 J and 880 MPa, respectively). It is concluded that fine effective grain size and ductile phase of austenite films are two main factors to achieve good mechanical properties.

The microstructure of lath martensite in quenched 9Ni steel

Many of the most useful structural steels have dislocated lath martensitic structures. The microstructures of these steels are complex since each prior austenite grain contains as many as 24 different crystallographic variants of the γ(fcc)–α′(bcc) transformation. Recent research, using electron backscatter diffraction (EBSD), has significantly clarified the “block-and-packet” structure of lath martensite in low-carbon steel. The blocks are bivariant composites of two transformation variants, with the three distinct blocks stacked so that all six of the possible variants of the packet are used. The present work was undertaken to complete the description of this structure and identify its underlying causes. We address these issues in two steps. First, we present an EBSD characterization of lath martensite in low-carbon 9Ni steel. The results show that all packets have the bivariant block structure, and, with the proper notation, the full hierarchical structure has a simple pattern that is easily described and visualized. The results are readily explained on the basis of two assumptions: (i) the bivariant block, in contrast to a single-variant block, has an α′–γ invariant plane very near {011}α″||{111}γ, which permits the plate-shaped blocks to stack without significant strain to form a packet; (ii) the transformation goes to completion, with the consequence that the net strain in the prior austenite grain is a simple dilatation, and polygranular bodies can transform martensitically without significant residual stress. In a companion paper we use an appropriate modification of the crystallographic theory of lath martensite to validate the first of these assumptions.

Effect of Cooling Rate During Hot Punching on Property of High-performance 22MnB5 Steel Parts

The effect of cooling rate during punching process on the microstructure and mechanical property of the 22MnB steel was investigated by three different processing conditions i.e. the processes with and without argon protection as well as a simulated industrial process. The results show that the cooling rates of all the hot punched parts with the three different processing conditions are higher than the critical cooling rate of 22MnB5 steel, thus the hot punched steels with a microstructure of lath martensite exhibit tensile stresses higher than 1500 MPa. When the temperature of hot punch tools is higher, an oxide scale appeared on the punched workpiece surface, thereby, the cooling rate and the mechanical property of the steel become lower, and the martensitic plate becomes thicker. The hot punched part with tensile strength about 1600 MPa and strength multiplied ductility c.a. 20, 000 MPa·% was available by heating with argon protection while reducing the initial temperature of punch tools to ambient temperature. This is because the cooling rate of the punched part was high and thereby the martensite plates of the steel became fine for the process with protective gas.

Microstructure of Martensite in Fe–C–Cr and its Implications for Modelling of Carbide Precipitation during Tempering

The microstructure of as-quenched martensite in four Fe–C–Cr alloys (0.15C-1Cr, 0.15C-4Cr, 1C-1Cr, 1C4Cr, mass%) has been investigated. Moreover, the microstructures served as input for setting up modeling of carbide precipitation during tempering of martensite. The modelling was conducted using the Langer-Schwartz approach and the software TC-PRISMA, which retrieves thermodynamic data from the Thermo-Calc databank. It was found that the martensite in the low carbon steels is predominantly lath martensite with units arranged parallel to each other. On the other hand, the plate martensite dominates the microstructure in the high carbon steels. The ratio of high-angle to low-angle grain boundaries was found to increase with increasing Cr in the low carbon steels, which indicates that Cr has a similar effect as C on the lath martensite microstructure, however, the micro-hardness remained unaffected by the addition of Cr. Finally, the precipitation modeling clearly demonstrates the importance of proper definition of the initial microstructure for predictive modelling. Parameters such as dislocation density and frequency of high-angle grain boundaries have a drastic effect on e.g. the mean size of carbides.

Processing, Microstructures and Mechanical Properties of Ultra-high Strength Steel Sheet

An ultra-high strength steel sheet with a tensile strength over 1500MPa and excellent low-temperature impact toughness was obtained by either using controlled rolling plus air cooling (CR+AC) or laboratorial thermo-mechanical controlled processing (TMCP). The effects of CR+AC and the TMCP processes were then examined on the microstructures and mechanical properties of this steel sheet. The results showed that by using CR+AC technology a uniformly distributed bainitic ferrite microstructure could be produced in this steel sheet under the appropriate reheating and rolling condition. By using TMCP technology, a lath bainite and martensitic microstructure with superior mechanical properties could also be generated. A microstructural analysis was made to interprete the strengthening-toughening mechanisms in association with the processes.

Effect of Austenizing and Tempering Heat Treatment Temperatures on the Fatigue Resistance of Carburized 16MnCr5 (ASTM 5117) Steel

The present investigation deals with the study of the effect of austenizing and tempering heat treatment temperatures on the fatigue resistance of carburized 16MnCr5 steel. Rotating bending fatigue specimens were machined from 16MnCr (ASTM 5117(steel rod, and pack carburized at 900°C for 2 hours soaking time. Carburized specimens were then austenized at 900°C for one hour, water quenched, reaustenized at temperatures 750°C, 800°C and 900°C for one hour, then tempered at 200°C temperature. Other carburized specimens were tempered by heating to 760°C temperature, water quenched to room temperature, then tempered at temperatures 200°C, 300°C, and 400°C for one hour. Austenized and tempered steel specimens after carburization as well as uncarburized steel specimens were then tested by rotating bending fatigue machine up to fracture under different stress levels (200, 250, 300, 350, 400) Mpa. Experimental results showed that fatigue resistance of austenized steel specimens after carburization process has been increased, and the crack length developed on the specimen surfaces was decreased with an increase in austenizing temperature up to 800°C, due to lath martensitic microstructure formation, beyond this temperature fatigue resistance was decreased and crack lengths were increased due to the grain coarsening of the lath martensite. It was also concluded that fatigue resistance of steel specimens that have been tempered after carburization process was increased, while crack lengths due to fatigue have been decreased with an increase in tempering temperatures due to the formation of tempered martensite and troostitic microstructure. The results also revealed that uncarburized steel specimens showed a lower fatigue resistance and a higher crack lengths than those austenized and tempered specimens after carburization.