Martensite Grains Research Articles

Hybrid structure of white layer in high carbon steel \u2013 Formation mechanism and its properties

This study identifies for the first time, the hybrid structure of the white layer in high carbon steel and describes its formation mechanism and properties. The so-called ‘white layer’ in steel forms during high strain rate deformation and appears featureless under optical microscopy. While many researchers have investigated the formation of the white layer, there has been no definitive study, nor is there sufficient evidence to fully explain the formation, structure and properties of the layer. In this study, the formation, morphology and mechanical properties of the white layer was determined following impact testing, using a combination of optical and SE- microscopy, HR-EBSD, TKD and TEM as well as nano-indentation hardness measurements and FE modelling. The phase transformation and recrystallization within and near the white layer was also investigated. The microstructure of the steel in the white layer consisted of nano-sized grains of martensite. A very thin layer of austenite with nano sized grains was identified within the white layer by HR-EBSD techniques, the presence of which is attributed to a thermally-induced reverse phase transformation. Overall, the combination of phase transformations, strain hardening and grain refinement led to a hybrid structure and an increase in hardness of the white layer.

Symmetrical tilt grain boundary engineering of NiTi shape memory alloy: An atomistic insight

The role of austenitic symmetric tilt grain boundaries (STGBs) on the martensitic transformation in NiTi alloy during thermal cycling between 500K and 10K are studied from molecular dynamics simulations. Four austenitic STGBs including Σ3[11¯0](111), Σ3[11¯0](112), Σ9[11¯0](114) and Σ9[11¯0](221) have been considered. Through martensitic transformation, the austenitic STGBs transform to martensitic grain boundaries (GBs) and martensitic twin boundaries form in the interior of the simulation models. After austenitic transformation, the martensitic GBs recover to the austenitic STGBs and the twin boundaries disappear. We find that the roles of STGBs on the martensitic transformation can be divided into three groups based on the martensite start temperature (Ms). (1) The Σ3[11¯0](112) and Σ9[11¯0](221) STGBs retard the martensitic transformation, i.e., decreases Ms, by raising the potential energy of the grain interior. (2) The Σ9[11¯0](114) STGB promotes the martensitic transformation by lowering the potential energy of the grain interior. (3) The effects of the Σ3[11¯0](111) STGB depend on whether twins form during cooling. The evolutions of the microstructures during thermal loadings are analyzed using the von Mise shear strain; insight obtained in this work paves the way for engineering NiTi shape memory alloy through grain boundaries.

Microstructure Optimization of Dual-Phase Steels Using a Representative Volume Element and a Response Surface Method: Parametric Study

Dual-phase (DP) steels have received widespread attention for their low density and high strength. This low density is of value to the automotive industry for the weight reduction it offers and the attendant fuel savings and emission reductions. Recent studies on developing DP steels showed that the combination of strength/ductility could be significantly improved when changing the volume fraction and grain size of phases in the microstructure depending on microstructure properties. Consequently, DP steel manufacturers are interested in predicting microstructure properties and in optimizing microstructure design. In this work, a microstructure-based approach using representative volume elements (RVEs) was developed. The approach examined the flow behavior of DP steels using virtual tension tests with an RVE to identify specific mechanical properties. Microstructures with varied martensite and ferrite grain sizes, martensite volume fractions, carbon content, and morphologies were studied in 3D RVE approaches. The effect of these microstructure parameters on a combination of strength/ductility of DP steels was examined numerically using the finite element method by implementing a dislocation density-based elastic-plastic constitutive model, and a Response surface methodology to determine the optimum conditions for a required combination of strength/ductility. The results from the numerical simulations are compared with experimental results found in the literature. The developed methodology proves to be a powerful tool for studying the effect and interaction of key microstructural parameters on strength and ductility and thus can be used to identify optimum microstructural conditions.

Microstructure based modelling of stress–strain relationship on dual phase steels

ABSTRACTThe deformation in microstructures of DP600, DP800 and DP1000 commercial advanced high strength steels have been researched through using the representative volume element method. For this purpose, deformation analyses have been carried out by transferring geometrical models and mechanical properties of phases of each material to the finite element software. Deformation relation between ferrite and martensite phases was determined. According to the loading direction, Shear bands have been observed to occur in the range of approximately 40°–45°. It has been understood that the failure mode is shear for DP600, DP800 and DP1000 steels. Crack propagation has been observed to occur in the ferrite phase trapped at the martensite grain boundary.

Effect of Tailoring Martensite Shape and Spatial Distribution on Tensile Deformation Characteristics of Dual Phase Steels

The authors simulated the industrially used continuous annealing conditions to process dual phase (DP) steels by using a custom designed annealing simulator. Sixty-seven percentage of cold rolled steel sheets was subjected to different processing routes, including the conventional continuous annealing line (CAL), intercritical annealing (ICA), and thermal cycling (TC), to investigate the effect of change in volume fraction, shape, and spatial distribution of martensite on tensile deformation characteristics of DP steels. Annealing parameters were derived using commercial software, including thermo-calc, jmat-pro, and dictra. Through selection of appropriate process parameters, the authors found out possibilities of significantly altering the volume fraction, morphology, and grain size distribution of martensite phase. These constituent variations showed a strong influence on tensile properties of DP steels. It was observed that TC route modified the martensite morphology from the typical lath type to in-grain globular/oblong type and significantly reduced the martensite grain size. This route improved the strength–ductility combination from 590 MPa–33% (obtained through CAL route) to 660 MPa–30%. Finally, the underlying mechanisms of crack initiation/void formation, etc., in different DP microstructures were discussed.

Heat Treatment of Low-Alloyed Steel up to Grade Q125

This work investigates the problem of how to reduce the prime cost of Grade Q125 Group 4 casing and tubing according to API 5CT and proposes process solutions to resolve it, consisting in the enhancement of series of mechanical properties without chemical alloying of steel with expensive elements such as chromium, molybdenum, vanadium, niobium and boron. During the investigation the process parameters of 9MnSi5 low-alloy steel heat treatment were developed, which confirmed the efficiency of this technology providing high values of strength and yield strengths which are minimum 931 MPa and 862 MPa respectively; at the same time maintaining the required cold resistance and resistance to hydrogen cracking of pipe. The suggested process parameters were based on heat cycle quenching with heating in induction furnace up to 1050–1100°С, 880–900°С and 740–780°С accordingly at each cycle and cooling rate in each case equal to 50°С/sec, during which the uniform martensite grain size ranging from 5 to 10 micron was obtained.

Characterisation of electric current-treated austenite using misorientation angle distributions in martensite

ABSTRACTThe angle misorientation distribution of martensite formed from electric current treated and non-treated austenite samples are studied using electron backscatter diffraction. The electric currents are pulsed with a loading width of 80 μs, at a frequency of 100 Hz and current density of 4.21 A/mm2 supplied by a DC Voltage. The majority of angle misorientations are < 5° and exist within the martensite grains, while only a minority of all misorientations are derived from the prior austenite grain boundary and were found to lie in the range 20–50°. Distinct textures develop in both electric current treated and non-treated samples with increased quenching temperature. Analysis of the prior austenite shows reduced grains in the electric current treated samples at all quenching temperatures considered.This paper is part of a themed issue on Materials in External Fields.

Case study: Formation of white etching layers in a failed rolling element bearing race

The aim of this investigation was to study the formation of white etching layer (WEL) induced by rolling contact fatigue (RCF) in a bearing steel. Through sectioning a failed bearing, the microstructure of the WEL was investigated using optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and micro indentation test. The result showed that the WEL is harder than the matrix. It consists of refined grains of martensite, austenite and carbides. In contrast to the thermally-induced WELs, where the austenite is the result of phase transformation due to austenitization, the austenite in the WELs of the current study comes from the original retained austenite. No phase transformation occurred, indicating that the WEL was formed under deformation induced the mechanism. WELs and white etching areas (WEAs) were compared in detail. The result suggested that WELs and WEAs are actually the similar responses to large shear deformation in RCF of martensitic bearing steels.

Effects of Welding Speed on Microstructure and Mechanical Property of Fiber Laser Welded Dissimilar Butt Joints between AISI316L and EH36

Fiber laser welding of dissimilar materials between AISI316L austenitic stainless steel and EH36 ship steels were conducted. Then the effects of welding speed on microstructure and mechanical characterization of the welded joint were investigated. Optical microscopy, Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD) were used to analyze the microstructure. Microhardness testing, transverse tensile strength, and impact tests at the temperature of −40 °C were performed to study the mechanical properties. The martensite phase formed due to the rapid cooling rate during laser welding and low Creq/Nieq ratio. The coarse martensite grains in the center seam are transformed to finer martensite grains as the welding speed increases resulting in the higher cooling rate. The microhardness of joints was about 350 HV0.3, which was twice that of the base metal because of the formation of the martensite phase. When the welding speed was 0.6 m/min, fewer defects were found, and tensile testing indicated overmatching of the weld metal relative to the base metal. In addition, the joints also exhibited better ductility and impact toughness.

Revealing the mechanical and microstructural performance of multiphase steels during tensile, forming and flanging operations

The mechanical performance of Dual Phase (DP) and Complex Phase (CP) steels was investigated by SEM analysis, tensile testing, Forming Limit Curve investigation and flange formability testing. The alloys of interest were Dual Phase (DP) untempered, Dual Phase (DP) tempered and Complex Phase (CP) steels. Phase content analysis showed that the distribution of the ferrite and martensite phases was the same for the two DP alloys, but the grain size and condition (tempered/untempered) for the martensite islands was much different in the two alloys. In the tempered DP steel, the smaller grain size for the martensite and the tempering process resulted in increased elongation, more formability and ability to form a flange (flangeability). In CP steels the soft ferrite phase is replaced by harder bainite, yielding a bainitic-martensitic microstructure. Bainite reduced the total elongation of the alloy during tensile testing, reduced the formability (especially under plane strain conditions) of the alloy but improved the flangeability of the alloy. Under flanging conditions, CP steels deformed to higher strains, at tighter radii with minimum springback. Microstructural inspections at the outer radius of the flanged specimens revealed that in CP steels bainite deforms similarly to martensite, therefore the strain partitioning is smaller in CP steels in comparison to DP steels. Plastic deformation in CP steels upon flanging occurs with the formation of strong slip bands in both martensite and bainite. In contrast, the martensite and ferrite grains in DP steels deform quite differently leading to strong strain localisations. Void nucleation and cracking occurred at the martensite islands or within the soft ferrite phase next to the martensite islands. In CP steels no voids or damage was observed within the matrix. A special case study was done with a thicker and stronger alloy, a Martensitic 1400 steel to reveal the flangeability limits for advanced high strength steels. Neither cracks nor damage were observed visually on the flanged specimens. However SEM observations at the outer radius of the flanged samples revealed significant void growth at inclusion sites and cracks nucleating within the matrix adjacent to the inclusions.

In-situ observation of martensitic transformation in Cu-Al-Mn cryogenic shape memory alloy

A low temperature metallographic in-situ observation apparatus was designed to research the martensitic phase transformation of Cu-Al-Mn cryogenic shape memory alloy. Some martensitic transformation rules were summarized. In the processing of martensite phase transition, martensite grains gradually grow up as the temperature decreases and shrink as warming up. Many martensites phase with different orientation formed in a grain (self-accommodation of martensite variants). The result also suggested an interaction between martensites. Appearance of a new martensite will make another martensite disappear. When continue cooling the sample, the martensite which is disappeared will reappear. The nucleation site of martensite can be everywhere. However, nucleation rate in the grain boundaries and sample boundaries is higher than that of other sites. Martensites can grow cross the grain boundaries into another grain. The crossing point can be a new nucleation point.

Structure and properties of H-beams after accelerated water cooling

The structure and properties of the surface of DP155 H-beams made of 09G2S low-carbon steel are determined on the basis of materials physics, before and after thermomechanical strengthening—that is, accelerated water cooling. Such H-beams are used in monorail tracks. Highly defective structure in the surface layer is created by accelerated cooling of the beam in the line of the 450 bar mill at AO EVRAZ Zapadno-Sibirskii Metallurgicheskii Kombinat, in the following conditions: rolling speed 6 m/s; water pressure in the crosspiece-cooling section 0.22–0.28 MPa; temperature before cooling about 800°C. As a result, the hardness, wear resistance, and scalar dislocation density are higher than in the steel without strengthening. Without thermal strengthening, the microhardness of the samples is 2.70 ± 0.33 GPa, while the Young’s modulus is 269.2 ± 27.1 GPa. Thermomechanical strengthening increases its microhardness to 3.30 ± 0.29 GPa, and decreases the Young’s modulus to 228.2 ± 25.7 GPa. In addition, the microhardness range is increased from 2.20–3.80 GPa to 2.64–4.60 GPa, while the Young’s modulus range is reduced from 208.0–403.0 GPa to 184.1–278.2 GPa on thermomechanical strengthening. It is found that thermomechanical strengthening increases the wear resistance of the steel’s surface layer by a factor of ~1.36 (decrease in wear rate from 5.3 × 10–5 to 2.9 × 10–5 mm3/N m) and increases the frictional coefficient by a factor of 1.36 (from 0.36 to 0.49). Without thermal strengthening, the structure observed is dislocational chaos; the scalar density of the dislocations is (0.9–1.0) × 1010 cm–2. High-temperature rolling and subsequent accelerated cooling of the samples produces dislocational substructure of band type in the ferrite grains and of reticular type in the martensite grains: the mean scalar density of the dislocations in the surface layer is 4.5 × 1010 cm–2. Possible explanations for such behavior are discussed.

Influence of Microstructure on the Corrosion Resistance of AISI 317L (UNS S31703)

The AISI 317L stainless steel is an austenitic grade with at least 3%Mo. Recently, this steel has replaced AISI 316L in many applications in chemical and petrochemical industries, due to its higher pitting corrosion resistance. The microstructure of the hot rolled and annealed material studied in this work consists of austenitic grains and 4.0% of delta ferrite (δ) with elongated islands morphology. This microstructure was modified by three processes: cold rolling with 87% of reduction, aging at 450oC for 400 h, and welding by gas tungsten arc welding (GTAW) process. The corrosion resistance was evaluated by anodic polarization tests (ASTM G-61) and critical pitting temperature tests (ASTM G-150). Cold rolling produced a microstructure consisting of elongated grains of austenite and martensite α', high dislocation density and the elongated islands of delta ferrite present in the annealed material. Welding produced a dendritic microstructure with 7.0% of delta ferrite and some σ precipitated in the ferrite. Finally, the aging at 450oC for 400 h provoked the decomposition of ferrite. The results show that these microstructural changes affected the pitting corrosion resistance, as detected by ASTM G-61 and ASTM G-150 tests.

Microstructure, magneto-structural transformations and mechanical properties of Ni50Mn37.5Sn12.5-xInx (x=0, 2, 4, 6 % at.) metamagnetic shape memory alloys sintered by vacuum hot pressing

Structural, magnetic and mechanical properties of Ni50Mn37.5Sn12.5-xInx (x = 0, 2, 4, 6% at.) alloys sintered by vacuum hot pressing were investigated. The porosity of sintered alloys increases with increasing of In concentration. The addition of In leads TCA to decrease while Ms is shifted to higher temperatures. Annealing sintered samples at 1020 K for 24 h allowed for lowering of the thermal hysteresis and narrows the temperature range of the forward and reverse martensitic transformations. Ternary Ni–Mn–Sn alloy exhibits the best mechanical properties and it can be plastically deformed up to about 5% while maintaining about 2000 MPa strength. TEM observations revealed unique microstructure observed in the case of polycrystalline Heusler alloys consisting of equiaxed grains of martensite and austenite with size of about 2 μm.

Effect of microstructural factors on void formation by ferrite/martensite interface decohesion in DP980 steel under uniaxial tension

The effect of microstructural factors on ferrite/martensite (F/M) interface decohesion in DP980 steel under uniaxial tension was investigated using ex-situ tensile testing and the crystal plasticity finite element method (CPFEM). Ex-situ tensile testing of a miniaturized specimen with a 100 μm-thick grooved gauge region revealed that the main ductile failure mechanism in DP980 steel was F/M interface decohesion. First, the effect that the crystallographic orientation of ferrite exerted on the ductile fracture by F/M interface decohesion was investigated using the representative volume elements (RVEs) of single-crystalline ferrite matrix containing a hard martensite particle. Then, the effects of martensite morphology and grain boundary alignment on the heterogeneity of strain-stress partitioning, in-grain orientation gradient for the ferrite matrix, and the corresponding void formation by F/M interface decohesion were investigated using three types of RVEs with a bi-crystalline ferrite matrix that contained one of the followings: (i) a circular martensite particle, (ii) an elliptical martensite particle, and (iii) two adjacent elliptical martensite particles at the F/F grain boundary. To capture the void formation by F/M interface decohesion, a cohesive zone model based on traction-separation law was introduced along the interfaces between the ferrite and martensite phases. The simulation revealed that the crystallographic orientation for ferrite, the martensite morphology, and the grain boundary alignment all significantly affected the heterogeneity of strain-stress partitioning, the in-grain orientation gradient for the ferrite matrix, and the void formation by F/M interface decohesion in DP980 steel under uniaxial tension.

Effect of Curved Surface Shape and Feed Velocity on Microstructure and Mechanical Performance of Gray Cast Iron After Spot Continual Induction Hardening

Spot continual induction hardening (SCIH) is a surface heat treatment process, which can strengthen more than one small area or relative large area on complicated component surface. In order to investigate the microstructure and mechanical properties of gray cast iron with curved surface after SCIH, the microstructure, microhardness and residual stresses were analyzed under different process conditions. The results showed that the martensite grain in hardened region of concave surface was larger than that of convex surface. The domain sizes of concave and convex surfaces were smaller than that of matrix region due to the high heating rate in SCIH process. The phase transformation depth increased with the increasing of convex surface radius but decreased with the increasing of concave surface radius. The maximum values of residual tensile and compressive stresses increased with the increasing of feed velocity for convex and concave surfaces, respectively. The appearance positions of maximum tensile and compressive stresses were closer to center for convex and concave surfaces, respectively, when feed velocity increased from 1 to 5 mm/s. The achieved results indicated that the SCIH with relatively low feed velocity was more suitable for improving the mechanical properties of gray cast iron. Compared with convex surface, the concave surface of workpiece can obtain better mechanical properties under the same feed velocity of inductor.

Atomic-scale investigation of carbon atom migration in surface induced white layers in high-carbon medium chromium (AISI 52100) bearing steel

The microstructure and chemical composition of white layers (WLs) formed during hard turning of AISI 52100 steel were studied using atom probe tomography (APT) and transmission electron microscopy (TEM). APT analyses revealed a major difference in the re-distribution of the carbon (C) atoms between WLs formed above and below the Ac1 temperature, i.e. T-WL and M-WL, respectively. In T-WL, the C-atoms segregate to grain boundaries (GBs) forming interconnected or isolated C-rich clusters, ∼5 nm, with a concentration of 9.8 ± 0.3 at.%C. Apart from the GB segregation, in M-WLs, large C-rich regions were found with 24.8 ± 0.4 at.%C. Owing to the chemical composition (stoichiometry) and element partitioning of such regions, they were assigned as θ-carbides (cementite). The APT results reveal that the original θ-carbides remain un-dissolved in the M-WLs, but might be plastically deformed due to the excessive strain that exists in hard machining process. The obtained results are in good agreement with the temperatures that are reached during formation of M-WLs. The isolated nano-sized C-clusters were assigned as off-stoichiometric carbides whereas the interconnected C-rich clusters were attributed to Cottrell atmospheres, evident by the linear shape of the C-enrichment as observed in the APT reconstructions. The C-contents in the nano-sized martensitic and ferritic grains were estimated to 0.50 ± 0.06 at.%C and ∼0.46 ± 0.02 at.%C, respectively. The C-content in the ferritic grains, beyond the C-solubility limit in ferrite (<0.1 at.%) is governed by the high dislocation density inside the grains, supported by the favorable binding energy between dislocations and C-atoms compared to C-atoms and Fe in carbides. No other evidence of redistribution of the substitutional alloying elements was observed. TEM analyses showed that T-WLs comprises of an equiaxed and nano-sized grains with well-defined cell boundaries, whereas the structure in the M-WLs comprised of elongated sub-grains formed via re-orientation of the original martensite followed by breakage/partitioning into elongated sub-grains.

The relation between aging temperature, microstructure evolution and hardening of Custom 465® stainless steel

The microstructure and hardness of Custom 465® precipitation hardened stainless steel were characterized following 4 hours aging at temperatures ranging from 482°C to 648°C. Dilatometry measurements and thermodynamic calculations were used to understand the mechanism of martensite-to-austenite reversion. Three major stages of aging were observed. The first stage is the peak hardening at aging temperatures of 480–510°C, where both η-Ni3Ti precipitation and austenite reversion begin independently of each other. The second stage is the initial stage of overaging observed above 538°C as reduction in hardness is due to a slight increase in both the precipitate dimensions and the reverted austenite volume fraction. The third, progressive overaging stage occurs above 593°C in which a dramatic hardness decrease is caused by a significant increase in both precipitate dimensions and reverted austenite volume fraction. Transmission Kikuchi diffraction orientation mapping revealed that globular austenite randomly nucleates at martensite grain boundaries, while acicular austenite nucleates along martensite lath boundaries. Thermodynamic calculations indicated that the dissolution of η-Ni3Ti precipitates does not govern the austenite reversion, although both phases are in competition for Ni. While the thermodynamic calculation predicts increase in the fraction of austenite with increase in aging temperature, the fraction of austenite found at room temperature in aged samples does not vary when the aging temperature, in which austenite reversion occurs, is increased above 593°C. This is explained by the change in composition of the austenite as a function of aging temperature.

Softening Mechanisms of the AISI 410 Martensitic Stainless Steel Under Hot Torsion Simulation

This study investigated the softening mechanisms of the AISI 410 martensitic stainless steel during torsion simulation under isothermal continuous in the temperature range of 900 to 1150 °C and strain rates of 0.1 to 5.0s-1. In the first part of the curves, before the peak, the results show that the critical (ec) and peak (ep) strains are elevated for higher strain rate and lower temperatures contributing for higher strain hardening rate (h). Moreover, this indicated that dynamic recrystallization (DRX) and dynamic recovery (DRV) are not effective in this region. After the peak, the reductions in stresses are associated to the different DRX/DRV competitions. For lower temperatures and higher strain rates there is a delay in the DRX while the DRV is acting predominantly (with low Avrami exponent (n) and high t0.5). The steady state was reached after large strains showing DRX grains, formation of retained austenite and the presence of chromium carbide (Cr23C6) and ferrite δ at the martensitic grain boundaries. These contribute for impairing the toughness and ductility on the material. The constitutive equations at the peak strain indicated changes in the deformation mechanism, with variable strain rate sensitivity (m), which affected the final microstructure.

Nanoscale martensitic phase transition at interfaces in shape memory materials

In polycrystalline shape memory materials, mechanical interactions between martensitic transformation and grain boundaries at small scales play a critical role. Using a cobalt-based shape memory alloy, instrumented nanoindentation that probes nanoscale behavior reveals that grain boundary regions are resistant to transformation and have an adverse effect on shape memory possibly because an increase in strain energy outweighs reduction in interface energy. When grain boundaries are replaced by a thin, intergranular layer of a ductile and more malleable phase, grain boundary constraints are greatly alleviated, and transformation nearby can be well accommodated. Statistical analysis of results from a large number of nanoindents shows a decrease in shape recovery near grain boundaries and an increase in shape recovery near the new grain boundary phase, compared to grain interior. This is corroborated by analysis of nanoscale hardness and energy dissipation. Nanoscale martensitic transformation near interfaces depends largely on how the material across the interface accommodates transformation displacement. Engineering interfaces and enhancing local compatibility could drastically alter the energetics for phase transition at interfaces favorable for shape memory.