Higher Volume Fraction Of Martensite Research Articles

On the critical role of martensite hardening behavior in the paradox of local and global ductility in dual-phase steels

The ductility of sheet metal is typically limited by either localized necking or by damage and fracture. A recent ductility classification refers to these failure modes respectively as “global” and “local” formability. The forming limit curve (FLC) and the uniaxial tensile test assess global formability, whereas the fracture forming limit (FFL), the true thickness fracture strain, hole expansion ratios (HER), etc. are indicators of local formability. Experimental hole expansion data in the literature for different dual-phase (DP) steel grades of similar strength and composition presents a paradox: grades which are found to be ductile in a tensile test and/or FLC show a low ductility in hole expansion, whereas other grades with a low ductility in conventional tests perform surprisingly well at cut edges. In this work, an in-depth systematic statistical analysis of idealized artificial two-phase microstructures is carried out to unravel the underlying mechanisms of the observed paradoxical trends. This is done by scaling the hardness of martensite and its volume fraction to generate virtual DP steels of the same strength but different strain hardening and mechanical phase contrast (hardness difference). The proposed micromechanical model adequately reproduces the experimentally observed trends. The results show that in DP steels, a higher global (necking-driven) ductility is obtained upon delaying martensite plasticity by increasing the mechanical contrast of the two phases. Consequently, the stress–strain distributions becomes more heterogeneous, resulting in a lower fracture limit of one of the phases or interfaces in shear loading, thereby, reducing the local ductility. Global ductility is improved by higher mechanical phase contrast and lower martensite volume fraction, whereas local ductility is improved by low phase contrast and higher martensite volume fraction. The hardening behavior of martensite is the key to avoiding the above trade-off between local and global ductility. It is shown that if the strain hardening capacity of martensite in the later stages of deformation (in high strains) can be increased, this would result in removing the observed paradoxical trends of local and global ductility in DP steels.

An Optimization Study on Resistance Spot Welding of DP600 Sheet Steel via Experiment and Statistical Analysis

Resistance Spot Welding (RSW) is widely used in many automotive, boiler, and ship manufacturing industries. Therefore, the optimization and effectiveness of the RSW process are very useful and cost-efficient processes in addition to improving weld quality. This study used various welding parameters to investigate the optimization of the RSW process of Advanced High Strength Steel (AHSS). RSW was carried out using different welding times, currents, and electrode pressure values. Microstructural analysis, nugget formation, and tensile shear test were performed on samples. This study applied the Taguchi method and ANOVA to set up the welding process and optimize its results. Microstructural characterization showed that the weld nugget microstructures had a high-volume fraction of martensite. Using the Minitab software, Taguchi (DOE) and ANOVA on the tensile shear test results showed that welding current is more effective than clamping pressure on the tensile shear load-bearing capacity (TLBC). However, clamping pressure affects the weld current effects on weld performance. Additionally, the TLBC of RSW samples increases, whereas increasing the clamping pressure leads to a decrease in TLBC and nugget dimensions. Consequently, a detailed analysis of the RSW coefficients and their effects led to optimized results.

The Combined Effects of Sliding Velocity and Martensite Volume Fraction on Tribological Behavior of a Dual-Phase Steel

The focus of this study was the combined effects of sliding velocity (SV) and martensite volume fraction (MVF) on the tribological behavior of dual-phase (DP) steel. Dry frictional tests were realized on a ball-on-disk tribometer by unidirectional sliding at 0.1 m/s, 0.6 m/s, and 1 m/s. Considering the increase in SV, both the friction coefficient (COF) and wear rate decrease due to the increase in oxidative wear. At 0.1 m/s, the major wear mechanism is abrasive wear, accompanied by partial material flaking and less oxidation wear; at 0.6 m/s, the wear mechanism is in the transition between abrasive wear and oxidation wear; at 1 m/s, oxidation wear becomes the dominant wear mechanism. For the same SV, specimens with lower MVF exhibited lower wear compared to specimens with higher MVF; this phenomenon is especially evident at low SV. As the SV increases, the effect of MVF on the wear decreases, and the velocity becomes the dominant factor.

Development of Desirable Fine Ferrite Grain Size and Random Second Phase Dual-Phase Steel Microstructures Using Composition and/or Processing Modifications

Microstructural morphology is known to have a significant impact on the mechanical properties of dual-phase steels. A fine ferrite grain size and random distribution of small second phase islands are desirable to provide superior isotropic properties compared to the banded second phase distribution that is typical for this type of steel. A rapid alloy prototyping (RAP) facility has been used to investigate three different DP 800 variants by systematically varying the compositions and/or process parameters compared to the ‘standard’ DP800 composition and processing that gives a banded microstructure. For Variant 1, the heating rate during the annealing cycle after cold rolling varied between 0.65 and 30 °C/s for the 45%, 60% and 75% cold reduction samples. It was found that a cold reduction of 75% and heating rate of 15 °C/s resulted in the microstructure that can give the best combination of strength and ductility because of the fine grain size and high martensite volume fraction. For Variant 2, the effect of changing the hot rolled (HR) microstructure (ferrite–pearlite, ferrite–bainite or martensite) on the final microstructure was investigated. Both the ferrite–50% bainite and fully martensite/bainite HR materials for all cold reductions resulted in annealed microstructures with necklace martensite morphology and finer ferrite grains compared to the ferrite–pearlite HR material, which gave a typical banded ferrite–martensite microstructure with a coarser ferrite grain size. For Variant 3, the Mn content was reduced, and increased Nb was used to achieve higher pancaking during the hot rolling stage, which refined ferrite grains in the HR condition with the same hardness. After annealing with the standard parameters only the 45% cold-reduced material produced a finer ferrite grain size than the standard material, whereas the 60% and 75% cold-reduced samples required a higher heating rate to achieve finer ferrite grain sizes due to rapid recrystallisation and growth kinetics.

Amplitude-dependent internal friction of dislocation interactions in dual-phase steel

Abstract Dislocation interactions in the ferrite matrix of a dual-phase steel have been studied as a function of martensite volume fraction (11 – 25%) and tensile straining (0 –5 %) by strain-induced amplitude-dependent (strain amplitude varying from 10– 7 to 10 –4) internal friction measurements at room temperature, using a high-frequency composite oscillator (40 kHz). Results showed that a higher volume fraction of martensite increases the internal stress in the softer ferrite matrix and restricted the dislocation motions, giving rise to a lower amplitude-dependent internal friction. Application of tensile straining further shortened the effective mobile dislocation length which was revealed by the internal friction measurements and this resulted in a high initial strain hardening rate of the high-martensite samples. Reduction in the martensite volume fraction and the mobile dislocations in the ferrite matrix through process control resulted in an increase in the amplitude-dependent internal friction, indicating a longer effective mobile length of dislocations which, in turn, delayed early necking of the dual-phase samples during tensile straining.

Laser welding of dual-phase steels with different silicon contents: Phase evolutions, microstructural observations, mechanical properties, and fracture behavior

Abstract This paper reports an investigation into the effect of silicon content on phase transformations, microstructural evolutions, and mechanical behavior of Nd:YAG laser welded dual-phase steels. The experimental investigation was conducted on laboratory-processed 0.15 C-1.7 Mn steels with different silicon contents. The heat treatment cycle to develop a dual-phase structure consisted of austenitization at 790 °C for 15 min followed by water quenching. The fabricated steels were welded in a butt joint configuration to obtain a fully-penetrated weld joint. Optical microscopy, scanning electron microscopy, and uniaxial tensile test were performed to evaluate microstructural changes, phase transitions, and mechanical properties, respectively. The fracture behavior of the welded joints was also investigated using a scanning electron microscope. The results showed that the volume fraction of martensite was increased with an increase in silicon content. The samples with higher silicon content also had a finer structure, which was due to the higher driving force created during previous cold working. All welded joints were found to consist of a nearly fully martensitic structure, and the heat-affected zone (HAZ) contained a low-temperature region (with partially or fully tempered martensite) and a high-temperature region (with newly-formed martensite). The specimen with the lowest silicon content had the minimum elongation and toughness modulus owing to an incomplete tempering process in HAZ. The specimens containing higher amounts of silicon were fractured in the base metals because of the presence of the martensite phase as a major micro-constituent in the fusion zone. The fracture mechanism of the laser-welded dual-phase steels with a low-volume fraction of martensite and small ferrite grain size was micro-void formation whilst the separation and de-cohesion were observed in dual-phase steels that had a high-volume fraction of martensite.

Study on mechanical properties and microstructure of DP590 steel with different annealing process

Annealing process is critical to mechanical properties and microstructure of DP steel. For DP steel with strength grade of 590MPa, experiments with different combinations of intercritical annealing temperature and over-aging temperature were carried out during annealing. The tensile tests of the final products and nanohardness tests of different phases were made, and the microstructures were analyzed. The results indicated that higher intercritical annealing temperature was favorable to higher volume fraction of martensite and lower nanohardness of martensite with lower C density. The ultimate tensile strength increased monotonously with the volume fraction of martensite increasing. Higher over aging temperature would make martensite islands be partially resolved and carbides precipitate, which made lower volume fraction of martensite, and resulted in lower nanohardness of martensite. The revealed rules could provide important guide to control material properties of DP steel by manipulating annealing process.

Effects of cooling rate on the mechanical properties and precipitation behavior of carbides in H13 steel during quenching process

The effects of cooling rate (CR) on the mechanical properties and precipitation behavior of carbides in H13 steel during quenching process were investigated. The retained austenite tends to be more unstable with increasing CRs, while the martensite increases gradually, based on XRD analyses and EBSD results. The values of hardness are increased, and the elongation along with impact energy is decreased, respectively, at higher CRs. Tensile strength remains above 2.0 GPa. Work hardening rates increase considerably in three samples, suggesting that transformation-induced plasticity effect may take place during the tensile test. Moreover, an increase in yield strength is observed when CR exceeds 15 K s−1, possibly due to a high volume fraction of martensite, decline in average grain size and precipitation of fine carbides. Types of the precipitates acquired were identified by electrolysis and XRD analyses. The results indicate the predominant existence of MC, M6C and M7C3, which are confirmed by SEM-EDS analyses and FactSage thermodynamic calculations. The size, volume and distribution of the carbides were also scrutinized under SEM. It is found that the volume fraction and size of the precipitates both decrease with increasing CRs. Based on these experimental data, an optimum CR for the quenching process could be determined to achieve the desired distribution of carbides, which in turn leads to the enhanced mechanical behaviors.

Microstructural Evolution During Deformation of a QP980 Steel

Quench and partitioning steels offer a valuable combination of high strength and ductility. Here we report on microstructural evolution in a QP980 steel during deformation. This is measured using a range of techniques including in-situ tensile tests coupled with scanning electron microscopy, ex-situ interrupted tensile tests coupled with electron back scattered diffraction, and X-ray diffraction measurements. Microstrain partitioning among ferrite, martensite, and retained austenite is quantified using microscopic digital image correlation. The average true strain in ferrite is approximately two and three times that in martensite and blocky retained austenite, respectively, consistent with nanohardness measurements of each phase. The combination of high strength and ductility of this steel is attributed to co-deformation of ferrite and tempered martensite. Some of the retained austenite blocks located at ferrite and martensite interfaces are almost fully transformed to martensite through transformation-induced plasticity, also contributing to ductility. Cracking of large blocky retained austenite in regions with more intense strain localization starts at relatively lower global strains. However, this appears to have little impact on the final failure process. Rather, it is the formation of large cavities in regions with higher martensite volume fraction that provides the primary mechanism of damage and failure.

Improving strength-ductility synergy in 301 stainless steel by combining gradient structure and TRIP effect

Here we investigate the microstructure and mechanical properties of gradient-structured 301 stainless steel synthesized by surface mechanical attrition treatment. Microstructure gradient in both martensite content and grain size, from nanostructured (NS) surface with high volume fraction of martensite to coarse-grained (CG) austenite interior, is identified in the mechanical gradient surface layer. Gradient samples exhibit excellent strength-ductility synergy due to the structure gradient-induced synergistic hardening and phase transformation induced plasticity (TRIP), i.e., combination of gradient and TRIP effects. Comparative analysis reveals that mechanical gradient can promote the phase transformation process, i.e. encouraging the TRIP effect.

Study of Martensitic Transformation in 304L Austenitic Stainless Steel after Tensile and Low Cycle Fatigue Tests

There are many studies on austenitic stainless steels with transformation induced plasticity (TRIP). Basically, in these steels, there is a significant increase in strength and toughness with the transformation of austenite to martensite. 304L steel finds extensive application in industry. Studies relating to martensitic transformation with plastic deformation are quite common. Many studies involve monotonic loading relating to the martensite formed. In practice, 304L steels are subject to distinct types of loading and possibly with stress concentrators. Thus, also in smaller quantities, it is possible to find in the literature studies involving cyclic loading with the TRIP effect. To contribute to the literature on the analysis of the TRIP effect on these steels, 304L steel samples with stress concentrators underwent interrupted monotonic tensile tests. Optical microscopy (OM) and x-ray diffraction (XRD) technique characterized the martensitic transformation. Other 304L steel samples with a stress concentrator underwent a low cycle fatigue test. The martensitic transformation, in this case, was possible to follow with the electron backscatter diffraction technique (EBSD). The samples after the interrupted monotonic tests show a high martensite volume fraction formed 1mm away from the notch (30% to 50%), due to the plastic deformation suffered. From 5.5mm of the notch, the samples again display a microstructure like that of the as-received (AR) sample. For the low cycle fatigue tested sample, the high concentration of deformation-induced martensite was within 15mm of the discontinuity. Approximately 0.5mm from the circular discontinuity, the sample again has a microstructure like the initial sample (IS).

Effect of tempering time on microstructure, mechanical, and electrochemical properties of quenched–partitioned–tempered Advanced High Strength Steel (AHSS)

The quenched–partitioned–tempered (QPT) steel is one of the novel steel in third–generation advanced high strength steels. Herein, an attempt has been made to investigate the microstructure, mechanical, and electrochemical properties of QPT steel as a function of tempering time. Microstructural analysis reveals that a high volume fraction of martensite with low fractions of retained austenite and bainite were achieved after 30 s of tempering. Mechanical tests validated that this triplex composite microstructure resulted in an 18% improvement in Vickers hardness, 73% in tensile strength (Rm) with little reduction of 12% elongation (εT) in comparison to non–heat treated steel sample. Tempering for 90 s increased the volume fractions of stable retained austenite, bainite and decreased the fractions of martensite by carbon diffusion to austenite. Further increase in tempering time stabilized retained austenite at an expense of bainite and martensite phases, resulting in reduced hardness and improved strength and elongation. Electrochemical analysis of the QPT heat–treated samples performed in 3.5% NaCl solution showed that the high corrosion rate was yielded from the steel sample tempered for 30 s, while other samples tempered for 90 to 150 s showed much lower corrosion rate than non–heat–treated steel sample.

The effect of soaking time after ultrafast heating on the microstructure and mechanical behavior of a low carbon steel

The main objective of this study is to understand the effect of the soaking time during the ultrafast heat treatment of a low carbon steel on its complex multi-phase microstructure, tensile mechanical behavior and properties of individual microconstituents. Tensile tests were performed to determine the macro-mechanical properties. Nanoindentation testing was carried out on individual microconstituents (martensite, recrystallized ferrite and non-recrystallized ferrite) identified a priori via EBSD analysis to measure their properties. It is shown that ultrafast heating combined with short soaking times results in improved macro-mechanical properties due to finer grain size and higher fraction of non-recrystallized ferrite, that has a higher nanohardness than recrystallized ferrite. Prolonged soaking times eliminate the advantages of the ultrafast heat treatment. This occurs because, even though a long soaking time promotes a higher volume fraction of martensite than a short one, it also induces substantial grain growth and complete recrystallization of the ferritic matrix. On the micro-scale, the ferritic grains show two different types of mechanical response. The recrystallized ferritic grains are prone to show pop-in events on the nanoindentation curves that are associated to dislocation nucleation events as a consequence of their low dislocation density, while non-recrystallized ferritic grains demonstrate a continuous response. The relationship between microstructure and mechanical properties on the macro- and micro-scales is discussed with respect to the microstructure, which in turn strongly depends on the applied heating rate and soaking time. A general recipe for microstructural design to improve the tensile mechanical behavior of low carbon steels implementing controlled heating and soaking conditions is outlined.

A high-strength heterogeneous structural dual-phase steel

The strengthening and ductilization of steels are of great importance to weight reduction of vehicles. In this work, the strength–ductility synergy of dual-phase (DP) steels was obtained by properly tailoring the structural heterogeneity, including the distribution and fraction of constituent phases. It was demonstrated that heterogeneous structural DP steels with high volume fraction of martensite (from 64 to 83%) led to a good combination of strength and ductility. Compared with the cold-rolled sheets, the tensile strength and uniform elongation of heterogeneous DP steel increased by 700 MPa (from 1.06 to 1.76 GPa) and 2.7% (from 1.3 to 4%), respectively. The contributions from back stress and effective stress were analyzed by cyclic loading–unloading experiments. The underlying deformation mechanism was discussed based on the results of mechanical test and microstructure observation.

Importance of Martensite Spatial Distribution at Large Volume Fractions in Imparting Ductility in High-Strength Dual-Phase Steel

Controlled austenite decomposition in a cold-rolled steel was carried out in a hot-dip process simulator with different inter-critical annealing process parameters to produce dual-phase steel consisting of martensite phase fraction above 50%. The evolving microstructure was characterized by using scanning electron microscope and electron backscattered diffraction techniques. Macro- and micro-texture was characterized by using x-ray diffraction and electron backscattered diffraction techniques, respectively. Uniaxial tensile deformation showed a very good combination of ultra-high strength above 900 MPa with uniform elongation of above 7%. An anomalous increase in ductility was observed for steel with high martensite volume fraction of 62.2% compared to that of lower fraction of 52.3%, indicating the influence of size and spatial distribution arrangement of ferrite and martensite phases. The enhancement in the ductility with high strength was attributed to the improved plastic deformability of hard fine martensite phase along with fine ferrite surrounding it. The nature of micro- and macro-texture was also found to affect strain partitioning compatibility between ferrite and martensite in this study.

The hot rolled duplex laminated steel presenting both high strength and high toughness

In this study, a new steel with the chemical composition of Fe-0.15C-5Mn-3Al (wt-%) was designed. Its microstructure and mechanical properties were examined by optical microscopy, scanning electron microscopy and mechanical tests. It was found that hot rolling in the dual phase region of ferrite and austenite at high temperature resulted in micro-laminated dual phase microstructure. An excellent combination of high strength (∼1200MPa) and ultrahigh impact toughness (∼400J) of the micro-laminated dual phase steel was obtained. Based on the measurement of the phase volume fraction and the analysis of the fractography, it was shown that the high strength was mainly dependent on the high martensite volume fraction, while the ultrahigh impact toughness was attributed to the micro-laminated microstructure.

Phase-field modeling of austenite grain size effect on martensitic transformation in stainless steels

A 2D elastoplastic phase-field model is developed to study the effect of prior austenite grain size on martensitic microstructure evolution in stainless steel. The effects of strain hardening and strengthening by grain size reduction (Hall-Petch effect) have been included in the model. The results show that martensite units form in different packets oriented in different crystallographic directions in simulated coarse grains, whereas uni-directional martensitic growth is observed in simulated fine grains. The number of packets and martensite block width increase with increasing grain size. With a decreasing grain size the martensitic transformation start temperature decreases, indicating strengthening of austenite. Once the transformation is initiated, at a given time, simulated fine grains give rise to higher volume fraction of martensite compared to simulated coarse grains. The von Mises equivalent stress and plastic strain are large in simulated fine grains compared to those in simulated coarse grains. The simulation results are in good agreement with experimental results.

Martensite transformation behavior and mechanical properties of cold-rolled metastable Cr-Mn-Ni-N austenitic stainless steels

In order to understand the mechanism of substitution for nickel with manganese in metastable austenitic stainless steels 14Cr10Mn and 16Cr6Mn, the transformation behavior of ε- and α′- martensite and the corresponding mechanical properties were investigated in this study. The microstructures and tensile and hardness properties of the cold-rolled steels under different rolling reductions at ambient temperature were measured using X-ray diffraction (XRD), scanning electron microscope (SEM) together with an electron back scatter diffraction (EBSD) and transmission electron microscope (TEM). The results show that α′- martensite is not easy to be transformed from austenite, and the martensite transformation γ → ε → α′ in 14Cr10Mn and 16Cr6Mn metastable stainless steels is found during cold rolling. The results also reveal that ε- martensite acts as an hardness reinforcing phase, and high total martensite volume fraction and ratio of ε- and α′- martensite content are helpful to achieve higher tensile strength with sigmoidal shape stress-strain curves. In addition, a model considering cold rolling reduction was developed to predict the contents of ε- and α′- martensite in the steels.

Microstructural evolution and deformation behavior of fiber laser welded QP980 steel joint

Advanced high strength steels for the auto industry are in high demand regarding the development of new type of steels, reliable and economic welding techniques, etc. In this study, the third generation advanced high strength steels, cold-rolled quenching and partitioning steels (QP980), were butt-welded using fiber laser to evaluate the microstructure, mechanical properties and formability of joint. The fusion zone composed of lath martensite with high microhardness (499 HV). Martensite tempering and carbide precipitation were found at the sub-critical heat affected zone, resulting in a slight decrease of microhardness (~ 37 HV drop). The presence of joint had no effect on tensile strength and all welded samples failed at the base metal with 98.8% joint efficiency. The formability of welded blanks in Erichsen cupping test was dependent on the weld line position. The formability ratio was only 68.08% of base metal when weld line offset was 0mm, and increased as the weld line moving away from the blank center because more deformation was transferred to base metal. Stretch-flange-formability of the joint in hole expanding test was inferior to that of base metal due to the high volume fraction of coarse martensite in fusion zone.

Development of Ultrahigh Strength TRIP Steel Containing High Volume Fraction of Martensite and Study of the Microstructure and Tensile Behavior

A new transformation induced plasticity (TRIP) steel containing high volume fraction of martensite was produced by austempering heat treatment cycle. Microstructure and tensile properties of this TRIP steel were investigated and compared to those of a dual phase (DP) steel with high martensite volume fraction. Microstructural analysis showed a mixture of ferrite, bainite, retained austenite and about 25–30 vol% of martensite in the TRIP steel. As a result of the strain induced transformation of retained austenite to martensite, the TRIP steel showed a strength elongation balance of 86% higher than that for the DP steel. In comparison to the commercial TRIP780 steel, the current TRIP steel showed a 15% higher ultimate tensile strength value while maintaining the same level of ductility. TRIP steel also had a larger work hardening exponent than DP steel at all strains.