Martensite Islands Research Articles

Microstructure Evolution and Precipitation Behavior in Nb and Nb-Mo Microalloyed Fire-Resistant Steels

Microstructure evolution and precipitation behaviors before and after tension at elevated temperature of Nb and Nb-Mo microalloyed fire-resistant steels were investigated by scanning electron microscopy and transmission electron microscopy, physical–chemical phase analysis and small-angle X-ray scattering. Results showed that the martensite and austenite (M/A) islands in a rolled state disappeared after tempering, and cementite and a large number of nanometer-sized carbide precipitated. Those nanosized particles were identified as NbC in Nb steel and (Nb, Mo)C in Nb-Mo steel. The precipitation amount of Nb is nearly equal for the two steels in a rolled state. However, after tempering treatment at 600 °C, the precipitation of Nb increased in Nb-Mo steel more obviously than that in Nb steel, indicating that Mo promotes the precipitation of Nb. Correspondingly, the amount of dissolved Nb was reduced, which results in decreased coarsening kinetics of (Nb, Mo)C in Nb-Mo steel as compared with that of NbC in Nb steel.

Tuning strength-ductility combination of AISI 1008 steel through large strain single-roll drive rolling and intercritical annealing

In this study, the influence of large-strain single-roll drive rolling and intercritical annealing on the microstructure evolution and mechanical behavior of AISI 1008 steel was investigated. The microstructures of all fabricated dual-phase (DP) steels exhibited equiaxed and homogeneously distributed fine ferrite grains and martensite islands. Compared to the as-received sample, the hardness of the produced dual-phase steels increased by up to 100%. As the temperature of intercritical annealing treatment increased, the hardness of the dual-phase samples increased from 233.2 HV to 258.4 HV. All fabricated DP steels showed much higher strength and toughness than the as-received sample. Overall, with increasing the temperature of intercritical annealing, the strength, ductility, and toughness of DP steels were improved due to increasing the martensite fraction. Also, by increasing the temperature of intercritical annealing from 770 to 860°C, the work hardening rate of the DP sample was enhanced. The hardness and strength of dual-phase steels had a good linear correlation to the fraction of martensite. The failure mechanism of the 80% rolled sample was mainly brittle while all DP samples exhibited a perfect ductile fracture characterized by many homogeneous deep dimples without any cleavage facets.

Influence of microstructure on hydrogen trapping and diffusion in a pre-deformed TRIP steel

Austenitic steels are known to exhibit a low hydrogen diffusion coefficient and hence a good resistance to hydrogen embrittlement. Therefore, it is an experimental challenge to investigate their hydrogen diffusion properties. In this study, the electrochemical permeation technique is used to determine the hydrogen diffusion coefficients in different pre-deformed states (φ = 0, 0.32, 0.39, 0.49) of the high-alloy austenitic TRIP steel X3CrMnNiMoN17-8-4 in a temperature range of 323 K–353 K. In combination with microstructural analysis, a correlation between phase transformation from γ-austenite to α′-martensite and dislocation density is shown. As a result of the lattice transformation from fcc to bcc, the diffusion rate of hydrogen is significantly increased (Dapp,φ = 0 = 3.6 × 10−12 cm2 s−1, Dapp, φ = 0.32 = 1.6 × 10−11 cm2 s−1at 323 K). With higher degrees of deformation, the dislocation density also increased in the martensite islands, resulting in a degressive growth of the diffusion coefficient (Dapp, φ = 0.39 = 5.3 × 10−11 cm2 s−1, Dapp, φ = 0.49 = 1.1 × 10−10 cm2 s−1at 323 K). Moreover, detailed calculations are performed to describe the way of hydrogen trapping and to give a possible mechanism of diffusion.

Effect of ultra-fast heating on microstructure and mechanical properties of cold-rolled low-carbon low-alloy Q&P steels with different austenitizing temperature

We performed ultra-fast heating (150 °C/s) quenching and partitioning (Q&P) processes to a cold-rolled low-carbon low-alloy steel. Adjusting the annealing temperature, we obtained two types of ferrite-containing Q&P steels with different matrix structure, i.e., martensitic matrix and ferritic matrix. The effect of ultra-fast heating on microstructure evolution and mechanical properties was analyzed, compared with the samples treated with conventional heating rate (10 °C/s). We also investigated the strain partition and fracture behaviors, basing on in-situ tensile tests under SEM. Results show that ultra-fast heating can significantly increase the A c1 and A c3 temperatures, and extend the temperature span of intercritical zone. It can also delay the complete recrystallization of deformed ferrite and promote the austenite nucleation, which leads to the structure refinement of the sample with martensitic matrix, improving both the strength and ductility. The increase of austenite nucleation rate and the incomplete recrystallization of deformed ferrite jointly promote the austenite transformation, which results in a distinct increase of martensite islands fraction in the sample with ferritic matrix, leading to its obvious increase of tensile strength and decrease of ductility. Strain partition is an important factor affecting the TRIP effect in ferrite-containing Q&P steels. For the sample with a martensitic matrix, retained austenite mainly possesses a film-like morphology and exists between martensite laths. The relatively high mechanical stability of retained austenite films and low local strain in martensitic regions result in the weak TRIP effect. The retained austenite in the sample with a ferritic matrix mainly distributes among ferrite grains and exhibits a blocky morphology. The relatively low mechanical stability of blocky retained austenite and high local strain in ferrite jointly enhance the TRIP effect. • Ultra-fast heating delays ferrite recrystallization. • Ultra-fast heating restrains martensite tempering. • Ultra-fast heating results in grain refinement of martensitic matrix. • Strain partition is a crucial factor controlling TRIP effect. • Main ductility origin is compatible deformation of matrix structures.

A multi-scale framework to predict damage initiation at martensite/ferrite interface

Martensite/ferrite (M/F) interface damage largely controls failure of dual-phase (DP) steels. In order to predict the failure and assess the ductility of DP steels, accurate models for the M/F interfacial zones are needed. Several M/F interface models have been proposed in the literature, which however do not incorporate the underlying microphysics. It has been recently suggested that (lath) martensite substructure boundary sliding dominates the M/F interface damage initiation and therefore should be taken into account. Considering the computationally infeasibility of direct numerical simulations of statistically representative DP steel microstructures, while explicitly resolving the interface microstructures and the sliding activity, a novel multi-scale approach is developed in this work. Two scales are considered: the DP steel mesostructure consisting of multiple lath martensite islands embedded in a ferrite matrix, and the microscopic M/F interfacial zone unit cell resolving the martensite substructure. Based on the emerging microscopic damage initiation pattern, an effective indicator for the M/F interface damage initiation is determined from the interface microstructural unit cell response, along with the effective sliding in this unit cell. Relating these two effective quantities for different interface microstructural configurations leads to an effective mesoscale model relating the interface damage indicator to the sliding activity of the martensite island in terms of the mesoscopic kinematics. This microphysics-based M/F interface damage indicator model, which could not be envisioned a-priori, is fully identified from a set of interfacial unit cell simulations, thus enabling the efficient prediction of interface damage initiation at the mesoscale. The capability of the developed effective model to predict the mesoscopic M/F interface damage initiation is demonstrated on an example of a realistic DP steel mesostructure.

Study of microstructure-property correlations in dual phase steels for achieving enhanced strength and reduced strain partitioning

This study uses a computational modeling framework for studying the structure-property correlations in Dual Phase (DP) steels. The microstructure-sensitive, dislocation density-based, J2 plasticity finite element framework simulates deformation in the constituent ferrite and martensite phases and the interplay of various deformation mechanisms. The framework accounts for the effects of the input microstructure, specifically the spatial heterogeneity of phase distribution, strength differential, and grain size on the evolution of deformation in terms of the local and aggregate mechanical properties. Experimental characterization of the deformed microstructure has been performed using in-situ Digital Image Correlation (DIC). The model predictions are validated with the DIC measurements on DP 600 as well as available data from literature. The simulations reveal significant impact of varying martensite volume fraction and phase strength differential, but not that of martensite island size. New DP microstructures having enhanced strength and reduced strain partitioning are then predicted. Processing routes to achieve such microstructures are proposed.

Laser welding Al–Si coated hot stamping steel in conduction mode: weld formation and Al-rich microstructure

The non-uniform mixture microstructure is always revealed in laser welds of Al–Si coated hot stamping steel. It is very necessary to understand aluminum-rich segregation on phase morphology evolution. At same time the uniform and high aluminum welds were tried to obtain by replacing keyhole mode with conduction mode. The uniform fusion zones contained 3–6 wt.% Al, higher than normal keyhole welds. The weld microstructure was mixed of delta ferrite, lath martensite and few discussed needle phases with different morphology. Lower Al region revealed high fraction of island martensite, when aluminum content w(Al) > 5 wt.%, the needle phase was precipitated from the matrix of delta ferrite. High resolution transmission electron microscopy confirmed it long periodical stacking ordered structure in body-cubic crystal matrix.

Study on Microstructural Evolution of DP Steel Considering the Interface Layer Based on Multi Mechanism Strain Gradient Theory.

A multi-mechanism constitutive model is proposed in this paper to better describe the effect of the local hardening behavior of the interface layer on the mechanical heterogeneity of dual-phase (DP) steel. The constitutive equations considering the geometrically necessary dislocations (GNDs) and back stress at grain level and sample level were established. Based on the finite element simulation results, the influences of local hardening and microstructure characteristics on the strain–stress evolution, statistical storage dislocations, GNDs, and back stress of DP steel were studied and discussed. Due to the local hardening effect, the ferrite phase was treated as an inhomogeneous matrix reinforced by some small islands of martensite in the simulation. The simulation results show that the thickness of the interface layer has a significant effect on the macroscopic hardening property of DP steel, while the number of interface layers has little effect. Meanwhile, the GNDs and back stress at the grain level also have little effect on the strengthening of DP steel. The contribution of GNDs at the sample level to the flow stress is about 47%.

On the Measurability and Predictability of HAZ Softening in GMAW of Automotive DP980 Steel

Dual Phase (DP) steel, composed of a ferrite matrix with dispersed islands of martensite, has become popular in auto-body car construction due to its outstanding mechanical properties (i.e., high strength and good ductility). DP steel softens at the sub-critical heat-affected zone (SC-HAZ) when subjected to welding thermal cycles, owing to the tempering of the martensite phase. In this work, DP980 steel was subjected to varied thermal cycles: (a) furnace-tempering treatment, (b) gas metal arc welding (GMAW), and (c) resistance spot welding (RSW), in order to characterize the tempering of martensite below the Ac1 critical temperature and at the sub-critical heat-affected zone (SC-HAZ) in the case of the welded specimens. The coarsening stage of cementite phase was characterized through microstructure observations and hardness measurements. As expected, the comparative results indicated an advanced stage of the martensite tempering in the furnace heat-treated specimens, followed by the GMAW and the RSW specimens. Further, developed softening kinetic models have been suitably employed and adjusted in order to predict the extent of softening along the SC-HAZ of the GMAW specimen. Finally, as the advanced stage of cementite coarsening is due to the influence of the arc welded thermal cycle, a reasonable estimation of the hardness profile was obtained, particularly for tempering temperatures above 400 °C.

Microstructure evaluation of an advanced high strength steel with superior results in terms of strength-ductility trade-off

Major improvements in new advanced high strength steels, especially related to microstructural features, have been made by the steel sector to access the trade-off between strength and ductility. In this context, the present study provides a detailed analysis of the microstructure of a cold rolled steel with a minimum tensile strength of 980 MPa, which possesses superior elongation when compared to other conceptions of steels from the same strength level. The annealing process was simulated in a Gleeble machine, and the microstructural characterization was done using optical and scanning electron microscopy, EBSD and XRD analysis. Austenite decomposition, using dilatometric test, and mechanical properties were also evaluated. The steel characterization revealed a microstructure consisting of ferrite matrix with martensite islands and retained austenite particles, in a fraction equivalent to that of conventional TRIP steels, dispersed throughout. The carbon content in the austenite, however, was less than 1.0% w/w, which results in a relatively low stability. Therefore, the increase in strain hardening capacity enabled by the deformation-induced transformation of austenite to martensite produces increased ductility during straining, distinguishing the analyzed material from other steels of the same strength level.

Microstructure and Mechanical Properties of Dual‐Phase Steels by Combining Adjusted Initial Microstructures and Severe Plastic Deformation

Herein, the effects of initial microstructure in combination with the severe plastic deformation on the developed microstructures and mechanical properties of dual‐phase (DP) steels are investigated. DP steels are produced by an intercritical annealing from the preliminarily heat‐treated samples with different initial microstructures. The constrained groove pressing (CGP) process is additionally applied before the final intercritical annealing step. It is found that the martensitic and tempered martensitic microstructures lead to DP steels with a good balance of strength and elongation because of finer and more uniformly dispersed martensitic islands. The ferrite/pearlite banded initial microstructure is not a proper choice for intercritical annealing as all mechanical properties become worse. The CGP can be employed before intercritical annealing for refining microstructure and enhancing damage tolerance of DP steels. Nevertheless, the impacts of the CGP process on the resulted strength of DP steels are obviously different depending on the initial microstructures. The amounts, sizes, and distributions of dimples and cleavage facets on the fracture surfaces of DP steels correlate well with the corresponding microstructures and tensile characteristics.

Effect of laser welding on microstructure and mechanical behaviour of dual phase 600 steel sheets

Dual Phase steels are one of the most used advanced high-strength materials in the industry, due to its combination of a ductile ferritic matrix and disperse hard martensite islands, which provide outstanding mechanical properties for components to be cold stamped. This work investigated fiber laser welding applicability in Dual Phase 600 1.6 mm thick steel sheets, evaluating potential welding impacts on properties of the material for industrial applications. A first set of bead-on-plate welds was generated to define best parameters for subsequent tests. A second set was prepared, consisting of butt joints welded in the optimized condition. Weld microstructure was characterized as 100% martensitic at fusion zone (FZ), with growing fractions of ferrite at Heat Affected Zone (HAZ) as one moves away from fusion line. Hardness is around 60% higher at FZ than at BM, being maximum at supercritical HAZ due to its highly refined microstructure and HAZ softening was not observed. Tensile and Erichsen cupping tests presented similar strength results between welded and non-welded specimens, with slight ductility reduction. Finally, numeric simulations based on Finite Element Analysis were carried out to estimate temperature evolution, phase proportions, residual stresses and distortion levels, achieving excellent agreement with experimental results.

Microstructural Analysis of Strain Rate Dependence for Commercial Dual Phase Steel DP590 Undergoing Finite Strain Deformation

This paper analyses the correlation of strain rate dependency of the material with sheet metals manufactured orientation that would influenced the microstructural changes in terms of martensite-ferrite and voids characteristics. The investigation on the deformation behaviour of Advanced High Strength Steel (AHSS) DP 590 was conducted using a uniaxial tensile test along the two principal directions that is through longitudinal (0°) and transverse (90°) through the strain rates of 1x10-1s-1to 1x10-4s-1. This allows for a strain rate dependency analysis conducted via stress-strain curve development. Scanning Electron Microscope (SEM) is used to examine the deformed microstructure for micro structural behaviour and for the void analysis on the number of voids and the average void sizes. Referring to the stress-strain curve, it is shown that the mechanical properties of the specimen behaved differently at those preferred orientations and are sensitive to the changes in the loading conditions. This could be observed as the DP590 at 0⁰ of rolling direction have higher stiffness value as compared to the 90° of transverse direction, the variant of the response of the material could be observed to be mildly an isotropically inclined instead of behaves as an isotropically behaved material. The response is also observable within the deformed microstructure. For instance, the higher strain rate indicates a higher number of voids, with a larger average void sizes, from this response it is observed the changes of the strain rate of the material would affect the material behaviour. As for the ferrite-martensite analysis, it could be observed in both different material orientations that as the lower strain rate was applied to the specimen, the strength of the material tends to become weaker since the martensite island becomes smaller at smaller strain rate levels.

Effect of Laser Welding on Microstructure and Mechanical Behaviour of Dual Phase 600 Steel Sheets

Dual Phase steels are one of the most used advanced high-strength materials in the industry, due to its combination of a ductile ferritic matrix and disperse hard martensite islands, which provide outstanding mechanical properties for components to be cold stamped. This work investigated fiber laser welding applicability in Dual Phase 600 1.6 mm thick steel sheets, evaluating potential welding impacts on properties of the material for industrial applications. A first set of bead-on-plate welds was generated to define best parameters for subsequent tests. A second set was prepared, consisting of butt joints welded in the optimized condition. Weld microstructure was characterized as 100% martensitic at fusion zone (FZ), with growing fractions of ferrite at Heat Affected Zone (HAZ) as one moves away from fusion line. Hardness is around 60% higher at FZ than at BM, being maximum at supercritical HAZ due to its highly refined microstructure and HAZ softening was not observed. Tensile and Erichsen cupping tests presented similar strength results between welded and non-welded specimens, with slight ductility reduction. Finally, numeric simulations based on Finite Element Analysis were carried out to estimate temperature evolution, phase proportions, residual stresses and distortion levels, achieving excellent agreement with experimental results.

Compositional heterogeneity in multiphase steels: Characterization and influence on local properties

Local variations in chemical composition, i.e., compositional heterogeneities, play an important role in the thermodynamic stability and spatial distribution of phases in multiphase steels. The influence of heterogeneous elemental distribution on the resulting local microstructure and mechanical properties has been investigated in the present work. Two steels, one with dual-phase (DP) and one with complex-phase (CP) microstructure, were fabricated from the same cast but differently heat treated in the intercritical annealing regime to yield variations in local microstructure formation. The heterogenous distribution of C and Mn that exists on different length scales are quantified by a characterization approach correlating local chemistry, microstructure, and hardness by using electron probe microanalysis (EPMA), electron backscattered diffraction (EBSD), and nanoindentation. Mn was alternately enriched and depleted in bands parallel to the rolling direction caused by segregation during continuous casting. The local C content was closely associated with the spatial phase distribution resulting from intercritical annealing. In the DP steel, the heterogeneity of C distribution was more pronounced and martensite inherited from the Mn banding due to the lower intercritical annealing temperature. On the other hand, the CP steel, heat treated at higher intercritical annealing temperature, exhibited a more homogenous C distribution resulting in a homogeneous distribution of martensite. In addition to their varying partitioning into the present phases, C and Mn in large martensite islands tended to concentrate in the vicinity of phase interfaces and deplete in the center. Furthermore, chemical boundaries with the smooth transition in local properties were observed in the CP steel. The Mn segregation bands and their impact on martensite morphology and location is reflected by the local hardness distribution. The correlation between compositional heterogeneity, microstructure formation, and microscopic as well as macroscopic mechanical properties is discussed in detail.

Realistic morphology-based Representative Volume Elements for dual-phase steels

We present a framework for the digitization of Dual-Phase (DP) steel microstructures into three-dimensional Representative Volume Elements (RVEs). Using a stack of microscopy images 6-μm high, separated by approximately 0.3 μm, in a microstructure analysis software, we obtain solid models, and subsequently finite element meshes of RVEs for three DP steels with distinct microstructures. No approximations with regards to the shape, size, and distribution of the second phase particles are required. A comprehensive mean-field model due to (Allain et al., 2015) obtains flow curves for the martensite and ferrite; ferrite grain size, average martensite island size, and steel composition are the inputs. It bears emphasis that no additional parameter adjustments to the flow curves was required. The finite element model was solved with an implicit integration scheme to obtain uni-axial stress–strain curves that agree with the experimental curves to within 8%. We use our framework to propose some guidelines for the development of RVEs. We also show that the plastic strain between the martensite islands, at the onset of necking, is approximately 2.5-3 times the global necking strain. Furthermore, we make a case for the iso-work assumption made in one-dimensional mean-field models that treat DP steel as a two phase composite.

Production and characterization of dual-phase steels from an AISI 8620 steel with high Mn content

Dual phase steels are materials whose microstructure is composed of a ferrite matrix with martensite islands. Ferrite provides excellent ductility, while martensite increases the strength of steel, this provides a special appeal in the automotive industry. The main objective of this research is to obtain dual phase steels from AISI 8620 steel with a high Mn content, performing heat treatments in the intercritical range to obtain martensite percentages of 27, 33, 41, and 48%, respectively. Microstructural characterization is performed using optical microscopy and scanning electron microscopy techniques, the mechanical characterization is carried out using hardness, tension and charpy impact tests. The highest mechanical resistance is achieved in steel with 41% martensite phase, while the highest ductility is given for the material with 27% martensite, a fractographic analysis of all materials allowed to determine that the type of fracture presented is ductile. When the martensite fraction increases, the impact energy exhibits a decreasing behavior, while the hardness behaves in an increasing way.

Low-carbon cast microalloyed steel intercritically heat-treated at different temperatures: microstructure and mechanical properties

In this study, dual-phase (DP, ferrite + martensite) microstructures were obtained by performing intercritical heat treatments (IHT) at 750 and 800 °C followed by quenching. Decreasing the IHT temperature from 800 to 750 °C leads to: (i) a decrease in the volume fraction of austenite (martensite after quenching) from 0.68 to 0.36; (ii) ~ 100 °C decrease in martensite start temperature (Ms), mainly due to the higher carbon content of austenite and its smaller grains at 750 °C; (iii) a reduction in the block size of martensite from 1.9 to 1.2 μm as measured by EBSD. Having a higher carbon content and a finer block size, the localized microhardness of martensite islands increases from 380 HV (800 °C) to 504 HV (750 °C). Moreover, despite the different volume fractions of martensite obtained in DP microstructures, the hardness of the steels remained unchanged by changing the IHT temperature (~ 234 to 238 HV). Applying lower IHT temperature (lower fraction of martensite), the impact energy even decreased from 12 to 9 J due to the brittleness of the martensite phase. The results of the tensile tests indicate that by increasing the IHT temperature, the yield and ultimate tensile strengths of the DP steel increase from 493 to 770 MPa, and from 908 to 1080 MPa, respectively, while the total elongation decreases from 9.8 to 4.5%. In contrast to the normalized sample, formation of martensite in the DP steels could eliminate the yield point phenomenon in the tensile curves, as it generates free dislocations in adjacent ferrite.

The fracture toughness of martensite islands in dual-phase DP800 steel

In situ microcantilever bending tests were performed on martensite islands in a dual-phase (DP) steel to extract the fracture toughness of martensite at the microscale and to understand damage initiation during forming of DP steels. All microcantilevers were produced through FIB milling. The martensite islands do not exhibit linear elastic brittle fracture; instead, significant ductile tearing is observed. The conditional fracture initiation toughness extracted by definition and by Pippan’s transfer criterion is Ki = 6.5 ± 0.4 MPa m1/2 and Ki,2% = 10.1 ± 0.3 MPa m1/2, respectively. The obtained value is well-represented by the strength-toughness trend of other ferritic steel grades. Considering the yield stress of the same martensite island, we found that crack initiation can occur only in very large martensite islands or in a banded or agglomerated martensite structure.Graphic abstract

Effect of cooling rate on the microstructure and mechanical properties of a low-carbon low-alloyed steel

Heavy plate steels with bainitic microstructures are widely used in industry due to their good combination of strength and toughness. However, obtaining optimal mechanical properties is often challenging due to the complex bainitic microstructures and multiple phase constitutions caused by different cooling rates through the plate thickness. Here, both conventional and advanced microstructural characterization techniques which bridge the meso- and atomic-scales were applied to investigate how microstructure/mechanical property-relationships of a low-carbon low-alloyed steel are affected by phase transformations during continuous cooling. Mechanical tests show that the yield strength increases monotonically when cooling rates increase up to 90 K/s. The present study shows that this is associated with a decrease in the volume fraction of polygonal ferrite (PF) and a refinement of the substructure of degenerated upper bainite (DUB). The fine DUB substructures feature C-rich retained austenite/martensite-austenite (RA/M-A) constitutes which decorate the elongated micrograin boundaries in ferrite. A further increase in strength is observed when needle-shaped cementite precipitates form during water quenching within elongated micrograins. Pure martensite islands on the elongated micrograin boundaries lead to a decreased ductility. The implications for thick section plate processing are discussed based on the findings of the present work.