Initial Martensite Research Articles

Relation of martensite-retained austenite and its effect on microstructure and mechanical properties of the quenched and partitioned steels

A two-step quenching and partitioning (Q&P) treatment was applied to low-carbon alloy steels. The relation of initial martensite - retained austenite - fresh martensite and its effect on microstructure and mechanical properties were investigated by experiments. The results reveal that the volume fraction of retained austenite can reach the peak value of 17%, and the corresponding volume fractions of initial martensite and fresh martensite are 40% and 43%, respectively, when the tested steel is treated by initial quenching at 330°C, partitioning at 500°C for 60s and final quenching to room temperature. Moreover, the micromorphologies of austenite and martensite become finer with the increasing of initial martensite fraction. The elongation is the highest when the volume fractions of initial martensite and retained austenite are 70% and 11%, respectively, meanwhile, the yield strength increases and tensile strength decreases gradually with the increase of initial martensite fraction, which proves that the mechanical properties including elongation, yield strength and tensile strength are based on the comprehensive effect of the retained austenite fraction, the finer microstructure and austenite stability.

Analysis of the Electric and Thermal Effects on Mechanical Behavior of SS304 Subjected to Electrically Assisted Forming Process

Significant improvements in deformation resistance and ductility of metals are observed in the electrically assisted forming (EAF) process. Both electroplastic effect (EPE) induced by electric current and thermal effect associated with Joule heating have been proposed to explain the phenomenon. However, there are still arguments in the contribution of the EPE in EAF process. In this paper, both electrically assisted tension tests (EAT) and thermally assisted tension tests (TAT) were conducted on SS304 specimens at the same temperature. The existence of EPE is investigated, and the contribution of EPE is also distinguished with thermal effect numerically by considering the initial yield stress, dislocation hardening, and martensite phase transformation. It is shown when the temperature is around 34 °C, the electric current of 50 A/mm2 in EAT induces additional stress reduction of 16% in the short-range internal stress (effective stress) involved in the initial yield stress and volume reduction of 45.2% in martensite formation compared with results in TAT. However, the effect is not obvious for the cases of 100 A/mm2 and 150 A/mm2 when the temperature is above 100 °C. By comparing the storage coefficient and recovery coefficient of dislocation in EAT and TAT, it indicates that electric current has no additional activation effect on dislocation movement of SS304.

Influence of warm deformation on the formation of a fragmented structure in low-carbon martensitic steels

Methods of optical metallography and scanning and transmission electron microscopy were used to investigate the structure of low-carbon steels of martensitic classes VKS-7 and VKS-10 subjected to warm rolling or upsetting at temperatures of 600 and 700°C (in the α state) and 800°C (in the γ state). It has been shown that the deformation by rolling at 600°C to degrees of 40 and 60% does not lead to the destruction of the lath structure of the initial martensite; an increase in the rolling temperature to 700°C and of the degree of deformation to 80% favors the development of recrystallization in situ. It has been found that, upon warm deformation by upsetting, recrystallization occurs at lower temperatures than in the case of the warm rolling. It has been shown that warm deformation by upsetting at a temperature of 700°C leads to the formation of a fragmented structure with a high fraction of ultrafine grains with a size less than 2 μm.

Irreversibility and transformation randomness of thermoelastic martensitic transformation in Ni–Ti alloys

In this work, the in situ optical observation was carried out in complete and incomplete transformation cycles of Ni–Ti alloys. In complete transformation cycles, initial martensite plates nucleate randomly in austenite. However, in a partial transformation cycle, the existing martensite plates have an influence on guiding the formation of subsequent martensite plates. And the randomness decreases with the decrease in transformation volume involved in the partial cycle. It is suggested that the randomness of transformations contributes to the introduction of defects, and the irreversibility associates with transformation randomness of martensite plates. For instance, a higher randomness in transformations could introduce more defects and more obvious irreversibility. On the other hand, defects generated in thermoelastic martensitic transformation are responsible for the hysteresis of transformations. Therefore, the randomness of transformations also contributes to the transformation hysteresis. These results could help further understanding on some martensitic transformation phenomena of shape memory alloys, such as the nonlinear and history-dependent characteristic.

Tensile Behavior of Intercritically Annealed Ultra‐Fine Grained 8% Mn Multi‐Phase Steel

The room temperature tensile behavior of intercritically annealed hot‐rolled Fe‐8%Mn‐0.4%C‐3%Al‐2%Si‐(0–0.2)%V steel, with an ultra‐fine grained austeno‐ferritic microstructure was investigated. An initial athermal martensite microstructure provided a high density of ferrite nucleation sites during intercritical annealing, resulting in an ultra‐fine grained microstructure. Carbon and manganese were partitioned to austenite during intercritical annealing. The austenite phase exhibited the plasticity‐enhancing TWIP + TRIP effect, i.e., deformation twinning and strain‐induced martensite transformation occurring in succession, which resulted in a superior work hardening rate. The intercritically annealed hot‐rolled Fe‐8%Mn‐0.4%C‐3%Al‐2%Si‐0.2%V steel exhibited a total elongation of 40%, a yield strength of 920 MPa, and tensile strength of 1200 MPa.

Isothermal transformations in advanced high strength steels below martensite start temperature

The transformation products in advanced high strength steels have been studied during the isothermal decomposition of austenite, subsequent to initial martensite formation. Rapid cooling to various temperatures below martensite start was carried out in a dilatometer with the intention to form controlled volume fractions of initial martensite and austenite, followed by isothermal holding. The transformation kinetics was monitored by means of dilatometry and microstructural characterisation by scanning electron microscopy, electron backscatter diffraction and X-ray diffraction. Hardness measurements of the resulting microstructures were analysed. The results revealed that the microstructures formed below MS are mainly composed of different fractions of tempered martensite, isothermal bainite with carbide precipitation and retained austenite.

The effects of the initial martensite microstructure on the microstructure and tensile properties of intercritically annealed Fe–9Mn–0.05C steel

The effects of the initial microstructure of α′ martensite on the microstructural evolution during intercritical annealing and the tensile properties of annealed specimens were investigated for Fe–9Mn–0.05C (wt.%) steel. The hot-rolled specimen with fully α′ martensitic microstructure showed a mixed microstructure of lath-shaped ferrite (αL) and austenite (γL) after intercritical annealing. The αL grains had a high density of dislocations due to inactive recovery, and also had a low Mn concentration. The γL grains had a low density of dislocations and high Mn and C concentrations. The αL and γL grains were deformed simultaneously during the tensile test because the αL grains were as hard as the γL grains due to their high dislocation density, resulting in continuous yielding. The cold-rolled specimen with a deformed α′ martensite microstructure exhibited a mixed microstructure of globular-shaped ferrite (αG) and austenite (γG) after intercritical annealing. The αG grains had a low dislocation density due to active recovery, and also had a low Mn concentration. The γG grains had a low dislocation density and high Mn and C concentrations. The soft αG grains with a low dislocation density were easily deformed at the early stage of the tensile test, resulting in discontinuous yielding and a large yield point elongation.

Phase transformations during the decomposition of austenite below Ms in a low-carbon steel

The purpose of this study is to investigate and understand the phase transformations during the decomposition of austenite, which occurs during isothermal treatments below the martensite start temperature (Ms) in a low-carbon steel. Isothermal holding treatments after rapid cooling to various temperatures (forming a controlled volume fraction of initial martensite) were carried out in a dilatometer. Results obtained by dilatometry, microstructural characterization and hardness were analyzed. This combination of results shows that the microstructures formed below the Ms temperature are mainly bainitic, mixed with tempered martensite. The kinetics of isothermal bainite formation was described by a nucleation-based transformation model. The complex competition and interactions between their transformation mechanisms during the isothermal holding at different temperature regimes are discussed.

The Effects of Cooling Rate on Retained Austenite Characteristics of a 0.2C-1.5Si-1.5Mn-1.0Cr-0.05Nb TRIP-Aided Martensitic Steel

With the aim of increasing the volume fraction and stability of the retained austenite characteristics in a transformation-induced plasticity (TRIP)-aided steel with wider lath-martensite structure matrix, the effects of varying the post-hot-working cooling rate of a 0.2%C-1.5%Si-1.5%Mn-1.0%Cr-0.05%Nb (mass%) steel on the retained austenite characteristics were investigated. When, after hot-working at 950°C, the steel was cooled to room temperature from 430°C above the martensite-start temperature using cooling rates lower than 3°C/s, the steel attained a higher volume fraction of metastable retained austenite and lower volume fractions of a finely dispersed martensite-austenite complex phase, carbide, and pro-eutectoid ferrite, although the volume fraction of bainitic ferrite increased. This was associated with a marked carbon-enrichment in the untransformed austenite, which was mainly due to the promoted bainitic ferrite, the initial lath martensite, and the refined prior austenitic grain.

Combined Martensite and Bainite Formation from Austenite Decomposition in HSLA Steel

Recent studies have shown the possibility to induce time-dependent phase transformations during isothermal treatment between the martensite start (MS) temperature and martensite finish (Mf,) temperature, i.e. after initial martensite formation. Such treatments result in specific complex microstructures consisting of bainite, martensite and retained austenite, depending on the holding temperature and time. However, the nature of the isothermal transformations below MS is not completely understood and issues like isothermal formation of martensite and bainite formation are still under discussion. The purpose of this study is to investigate the phase transformations from austenite, subsequent to initial martensite formation, during isothermal treatments at different temperatures of HSLA steel. The microstructure development was monitored by means of dilatometry and microstructural characterization of the transformation products by Optical Microscopy, Scanning Electron Microscope, Electron Backscatter Diffraction and X-ray diffraction. The phase transformations and complex competition and interactions between the different transformation mechanisms are discussed.

Effect of Initial Microstructure on Impact Toughness of 1200 MPa-Class High Strength Steel with Ultrafine Elongated Grain Structure

A medium-carbon low-alloy steel was prepared with initial structures of either martensite or bainite. For both initial structures, warm caliber-rolling was conducted at 773 K (500 °C) to obtain ultrafine elongated grain (UFEG) structures with strong 〈110〉//rolling direction (RD) fiber deformation textures. The UFEG structures consisted of spheroidal cementite particles distributed uniformly in a ferrite matrix of a transverse grain size of about 331 and 311 nm in samples with initial martensite and bainite structures, respectively. For both initial structures, the UFEG materials had similar tensile properties, upper shelf energy (145 J), and ductile-to-brittle transition temperatures 98 K (500 °C). Obtaining the martensitic structure requires more rapid cooling than is needed to obtain the bainitic structure and this more rapid cooling promote cracking. As the UFEG structures obtained from initial martensitic and bainitic structures have almost identical properties, but obtaining the bainitic structure does not require a rapid cooling which promotes cracking suggests the use of a bainitic structure in obtaining UFEG structures should be examined further.

Ultrafine Structure and High Strength in Cold-Rolled Martensite

Structural refinement by cold rolling (10 to 80 pct reductions) of interstitial free (IF) steel containing Mn and B has been investigated from samples with different initial structures: (a) lath martensite, (b) coarse ferrite (grain size 150 μm), and (c) fine ferrite (22 μm). Unalloyed IF steel with a coarse grain size (120 μm) has also included based on a previous study. Deformation microstructures and structural parameters have been analyzed by transmission electron microscopy and electron backscatter diffraction, and mechanical properties have been characterized by hardness and tensile testing. At low to medium strains, lath martensite transforms into a cell block structure composed of cell block boundaries and cell boundaries with only a negligible change in strength. At medium to large strains, cell block structures in all samples refine with increasing strain and the hardening rate is constant (stage IV). A strong effect of the initial structure is observed on both the structural refinement and the strength increase. This effect is largest in lath martensite and smallest in unalloyed ferrite. No saturation in structural refinement and strength is observed. The discussion covers the transformation of lath martensite into a cell block structure at low to medium strains where the driving force is suggested to be a decrease in the dislocation line energy. Medium to large strain-hardening mechanisms are discussed together with structure-strength relationships assuming additive stress contributions from dislocations, boundaries, and elements in solid solution. Good agreement is found between flow stress predictions and stress values observed experimentally both in the initial undeformed martensite and in deformed samples.

Continuous Dynamic Recrystallization during Warm Deformation of Tempered Lath Martensite in a Medium Carbon Steel

To Understand the Mechanisms of Accelerated Dynamic Recrystallization Behavior during the Warm Deformation of Martensites, the Tempered Lath Martensite of 0.4C Steel (Fe-0.399%C-1.96%Mn in Mass %) Was Deformed at 650 °C in Compression to Different Reductions, and Microstructural Evolution Was Investigated. During the Deformation, an Initial Lath Martensite Structure with a Complicated Morphology Was Gradually Changed into More Equiaxed Structure. After 50% Reduction and above, an Equiaxed, Fine Grained Structure Mainly Surrounded by High-Angle Boundaries Was Uniformly Formed with Dislocation Substructures, where the Dislocation Density in the Grains Is Relatively Low. Since there Was No Significant Boundary Migration during this Process, this Microstructural Evolution Can Be Termed as Continuous Dynamic Recrystallization.

Microstructural evaluation of austenite reversion during intercritical annealing of Fe–Ni–Mn martensitic steel

Akin to the binary Fe–Ni alloys, ternary Fe–Ni–Mn alloys show lath martensite structure which transforms to austenite during annealing at high temperatures. The transformation is accompanied by precipitation and reversion either by diffusion or by displacive mechanisms which leads to the structural and property refinement. This study was aimed to clarify microstructural changes during reversion of an Fe–10Ni–7Mn alloy which is suffered from severe grain boundary embrittlement after aging. The alloy was annealed at 600 °C for various holding times, where a mixed structure of α and γ phases are expected to be stable thermodynamically. Microstructure changes were investigated by means of electron back scattering diffraction (EBSD), X-ray diffraction (XRD), dilatometer and differential scanning calorimeter (DSC). In the solution-annealed sample only diffraction lines belonging to the bcc iron are found. However, after intercritical annealing bcc-martensite in combination with fcc austenite were revealed as the latter retained after subzero-cooling subsequently. Volume fraction of retained austenite was indicated to increase remarkably with holding time. It was found that austenite particles are nucleated at block and packet boundaries in the lath martensite structure. After annealing for prolonged times, austenite grows further as a result of which the initial lath martensite structure is subdivided, leading to fine (α + γ) dual phase structures.

Compatibility of reduced activation ferritic martensitic steel JLF-1 with liquid metals Li and Pb–17Li

The corrosion of reduced activation ferritic martensitic steel, JLF-1 (Fe–9Cr–2W–0.1C), in high-purity Li was quite small. However, carbon in the steel matrix was depleted by the immersion to the Li. The depletion caused the phase transformation of the steel surface in which the morphology of the steel surface changed to ferrite structure from initial martensite structure. The phase transformation degraded the mechanical property of the steel. However, the carbon depletion and the phase transformation of the steel were suppressed in carbon doped Li. The carbon in the steel was chemically stable and did not dissolve into the Li when the carbon potential in the Li was high. The concentration of nitrogen and oxygen must be kept as low as possible because the corrosion was larger when the concentration of oxygen or nitrogen in the Li was higher. The chemical reaction between the steel and the Li compounds of Li3N and Li2O was also investigated. The corrosion of the JLF-1 steel in Pb–17Li was summarized as the dissolution of Fe and Cr from the steel into the melt. The corrosion of the specimen with Er2O3 coating fabricated by metal organic decomposition process in the Li and the Pb–17Li was investigated. The coating was deformed, cracked and partially exfoliated in the liquid metals, though the oxide itself was chemically stable in the liquid breeders. The damage was probably made by the stress, which was generated by a large difference of the thermal expansion ratio between the solidified Li or Pb–17Li and the coating during a heat up and a cool down process of the corrosion test.

Insitu neutron diffraction study of micromechanical interaction and phase transformation in dual phase NiTi alloy during tensile loading

It is well known that the shape memory effect of NiTi alloy is closely related to the micro-structural characteristics. Neutron diffraction method can used to explore the changes of the phase transformation, lattice strain and twining reorientation of bulk NiTi alloy during deformation caused by the applied stress. In this paper, combining the four types of deformation characteristics in the macro stress-strain curves of dual phase NiTi alloy and using in-situ neutron diffraction measurement, the micromechanical interactions and phase transformation are determined. The volume fraction of the initial austenite before deformation is about 22%. The contrast transformation, which is corresponding to the lattice strain rapid decreasing of (110)B2 and increasing of (002)B19', reveals that the stress-induced transformation from austenite to martensite phase appears with the volume fraction of austenite decreasing rapidly and 011 II type twinning increases at the low strain hardening stage. At the same time, the initial martensite grains change their orientation to a favorable direction and the new {201} type martensite twinnings induced with the increase of applied stress cannot recover after unloading. At the high strain hardening stage, the twinning deformation is considered to be the main mechanism from the observing of the changes in the full width at half maximum (FWHM). Meanwhile, the slipping caused by dislocation is the main deformation mechanism corresponding to the obvious increas of the FWHM at the statured stage of the strain hardening.

Cr4Mo4V钢等离子体基碳氮离子升温注渗层微观结构

本文采用XRD、SEM、TEM和HRTEM等方法表征了Cr4Mo4V钢升温碳氮离子注渗层的微观结构，结果表明升温注渗处理层中生成了铁的碳化物和氮化铬析出相，析出相弥散分布于晶内和晶界。马氏体发生细化，原始板条马氏体边界消失，形成了微晶、微孪晶和细小片层等结构。HRTEM在马氏体中观察到非晶区和层错等亚结构。 XRD, SEM, TEM and HRTEM are used to investigate the microstructure of Cr4Mo4V steel after Plasma- base ion implantation (PBII) carbonitriding treatment. The results indicate the carbide and chromium nitride phases precipitate in the carbonitrided layer within the grain or along grain boundary. The grain refining and boundary disap- pearing in initial martensite occur. The micro-crystal, micro-twin and micro-lath martensite structure are observed. The substructure of amorphous zones and stacking faults are found by HRTEM.

Dynamic polygonization in 9%Cr heat resistant steel

Structural changes in 9%Cr martensitic steel during creep were examined. The grip section of the crept specimen was characterised by a lath martensite structure, which hardly changed during the test. In contrast, quite different microstructure developed in the necking portion of the specimen. The structural changes were characterized by the evolution of relatively large equiaxed subgrains with remarkably lowered density of interior dislocations at places of initial martensite laths. The development of the well-defined subgrains in the necking portion was accompanied with a coarsening of second phase precipitations. The structural mechanism responsible for the microstructure evolution during creep is considered as a dynamic polygonization.

Enhanced mechanical compatibility of submicrocrystalline Ti–13Nb–13Zr alloy

This study aimed to achieve enhanced mechanical compatibility of Ti–13Nb–13Zr alloy by producing submicrocrystalline microstructure without imposing severe strains. In order to find the optimum processing conditions, a series of compression tests was performed for initial martensite microstructure in strain ranges up to 0.8 and 1.4, the strain rate range of 10−3 to 1s−1 and the temperature range of 500–700°C. Based on the microstructural analysis, the submicrocrystalline (∼0.4μm) alloy consisting of high-angle grain boundaries was produced via dynamic globularization at temperature of 600°C, equivalent strain rate of 10−1s−1 and strain of 1.4, which showed about 25% enhanced mechanical compatibility as compared to the conventionally produced ones. The formation of submicrocrystalline microstructure at relatively low strain was investigated by examining the microstructure before and after dynamic globularization.

Characterization of microstructure obtained by quenching and partitioning process in low alloy martensitic steel

Microstructure of low alloy martensitic steel treated by quenching and partitioning (Q&P) process was characterized by means of SEM, TEM, EBSD and XRD. Mechanical properties of steel processed by Q&P and the same steel processed by quenching and tempering (Q&T) were measured by uniaxial tensile test. The study suggests that microstructure is mainly composed of three phases, i.e. initial martensite, fresh martensite and retained austenite. The initial martensite formed at the first quenching step is easily etched; the fresh martensite is formed at the final quenching step and looks like ‘blocky’ type phase with size about 0.2–3 μm, and the retained austenite is mainly located on the packet boundary and initial austenite grain boundary. The measured volume fractions and carbon contents of these phases were slightly different from those predicted by the constrained paraequilibrium (CPE) model proposed by Speer, which was interpreted by the effects of the different grain sizes of the untransformed austenite after first quenching. Mechanical properties of steels processed by Q&P assume much higher strength and ductility than those processed by Q&T. It is concluded that Q&P process is a promising approach to control the multiphase structure with hard matrix and a ductile retained austenite, which gives an excellent combination of strength and ductility.