Massive Martensite Research Articles

Enhanced strength-plasticity matching of lamellar 1 GPa-grade dual-phase steels via cyclic intercritical quenching

In this work, an innovative cyclic intercritical quenching process has been proposed for achieving excellent mechanical properties in terms of strength and plasticity, which results in a lamellar martensitic-ferritic dual-phase steel. The microstructural evolution under cyclic heat treatment was thoroughly investigated, and the mechanism that enables the lamellar structure to enhance its strength-plasticity matching was also explored. The experimental results revealed that the incomplete austenitization process inhibits the coarsening of the reversed austenite, and the cyclic intercritical quenching procedure finer and more evenly distributes the reversed austenite. The single intercritical quenching (C1) generated mostly massive martensite with traces of ferrite, whereas three-cyclic intercritical quenching (C3) yielded a fine and uniformly distributed lamellar structure. A comparison in terms of the mechanical properties between C3 and C1 steels indicates that uniform elongation, total elongation, and the product of strength and uniform elongation of the C3 steel increased by 85.4%, 47.3%, and 65.0%, respectively, while ultimate tensile strength was dropped by only 11.0%. In addition, the yield ratio of C3 steel is just 0.48. The slight decrease in strength is due to the fine lamellar microstructure. The high plasticity is attributed to the good deformation compatibility between lamellar ferrite and lamellar martensite with excellent plastic deformability. Therefore, cyclic mechanical heat treatment is very promising for the production of low-cost dual-phase steels.

Influence of Partitioning Temperature on Microstructure and Mechanical Performance of Medium Manganese Fe-C-Mn-Cu-Cr-Mo-Nb Steel

A hot-rolled medium manganese Q&P steel was annealed at an intercritical annealing temperature (670 °C) and quenched to four partitioning temperatures (300 °C, 280 °C, 260 °C, and 240 °C). The effect of the partitioning temperature on the microstructure and mechanical properties of the steel was studied. The microstructure evolution at different partitioning temperatures was characterized using scanning electron microscopy (SEM), a transmission electron microscope (TEM), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD). The results showed that, as the partition temperature increased, the fractions and the size of the fresh massive martensite (M2) increased, the initial lamellar martensite (M1) was coarsened, and the amount of M1 decreased. Moreover, the amounts of lath-like retained austenite (RA) decreased and the amounts of bulky austenite increased. The tested steel partitioned at 260 °C had relatively greater fractions of RA, in which there were sufficient contents of carbon diffused from M1. Therefore, the test steel partitioned at 260 °C showed fine comprehensive mechanical properties. The combination of tensile strength and elongation reached 30.3 GPa.%, and its work-hardening plateau was the longest, showing excellent ductility as it had sufficient fractions of the retained austenite with good stability.

An in-situ study of low thermal expansion and internal stress evolution in FeMn alloys

The mechanism of low thermal expansion phenomenon in antiferromagnetic FeMn alloys needs to be revealed. In this work, the evolution of lattice thermal expansion and internal stress in FeMn alloys was investigated by in-situ synchrotron X-ray diffraction during thermal cycling. Preceding the martensitic transformation, the coefficient of lattice thermal expansion (CLTE) decreased and the full width at half maximum (FWHM) of the diffraction peaks broadened abnormally. The evolution of the lattice thermal expansion and the FWHM were both reversible and isotropic. The additional reflection spots around the fundamental diffraction spots in the electron diffraction patterns of austenite and the dispersed nano-scale martensite embryos in austenite observed in the dark field TEM images proves the existence of premartensitic transformation. Similar phenomena were found in FeMn Invar alloy, which indicate that the Invar effect is closely related to the internal stress evolution caused by premartensitic transformation. • In this paper, many abnormal phenomena were observed in the Fe Mn alloys. The lattice thermal expansion behaviors and internal stress evolution in FeMn alloys was investigated by in situ synchrotron X-ray diffraction. The correlation between low thermal expansion property and premartensitic transformation were revealed by comparing the similar abnormal phenomena during Invar effect and the premartensitic transformation. The highlights are as follows: • During the Invar effect of Fe 27Mn alloy and preceding the martensitic transformation of Fe 24Mn alloy, the coefficient of thermal expansion decreased and the FWHM of diffraction peaks of austenite increased abnormally. The evolution of thermal lattice strain and FWHM was reversible and isotropic. • Transmission electron microscopy observations confirmed that there are massive martensite embryos embedded in austenite phase during Invar effect and premartensitic transformation. The existence of martensite embryos caused the abnormal increase of FWHM and may be related to the low thermal expansion behavior of FeMn alloy. • These similar phenomena imply a close relationship between the mechanism of Invar effect and the premartensitic transformation in FeMn alloy. • Our results may help to reveal the relationship between Invar effect and premartensitic transformation in antiferromagnetic Invar alloys, and lead to an in-depth understanding of the mechanism of low expansion property.

Short Circuiting Transfer, Formation, and Microstructure of Ti-6Al-4V Alloy by External Longitudinal Magnetic Field Hybrid Metal Inert Gas Welding Additive Manufacturing.

In this work, the external longitudinal magnetic field hybrid metal inert gas welding (M-MIG) additive manufacturing method is employed to produce the Ti-6Al-4V alloy part. The effect of process parameters on the droplet transfer formation and microstructure of the part was studied by a high-speed camera, optical microscope, and electron backscattered diffraction. The results showed that a typical short-circuiting transfer was obtained with the wire feeding speed of 2 m/min-4 m/min. An external longitudinal magnetic field had an obvious effect on the arc shape. The uniform formation of the deposition layer was obtained with the wire feeding speed of 4 m/min. The width of M-MIG deposition layer was greater than that of the MIG, and the width of M-MIG deposition layer was increased with the increase of the magnetic excitation current. The microstructure of the deposition layer was mainly comprised of acicular martensite α' and massive martensite αm. In addition, the β grain size in the M-MIG was less than that of the MIG. The average microhardness of the MIG deposition layer was 281.6 HV, which was less than that of M-MIG.

Effect of heat treatment on microstructures and properties of electron beam welding CLF-1 steel

Vacuum electron beam welding was adopted for the reduced activation ferritic martensitic steel (CLF-1 steel) with a thickness of 32 mm, and a well-formed butt joint without metallurgical defects was obtained under the optimum technological parameters. After welding, the specimens were subjected to high temperature tempering (710 °C/120 min/AC (air cooling), abbreviation HTT) and incomplete annealing (840 °C/120 min/AC, abbreviation IA) heat treatment, and the microstructure and mechanical properties of the as-welded state and two different heat treatment states were compared. The results showed that the as-welded weld metal (WM) was composed of coarse lath martensite, and the heat affected zone (HAZ) was composed of a large amount of massive martensite and dispersed fine MX and M23C6. Compared with the welded state, the martensite structure of HTT state WM and HAZ were refined, nano-scale M23C6 was precipitated along the boundary of tempered martensite in WM and nano-scale MX was precipitated in tempered martensite lath structure, both of which played a role of dispersion strengthening. Compared with the HTT state, the martensite structure of the WM and HAZ in the IA state became coarser, and both M23C6 and MX tend to grow together. The fracture position of the tensile specimens of as-welded state, HTT state and IA state welded joints were all located on the base metal (BM), and the tensile strength decreased successively, while the yield strength and elongation increased successively. The average impact absorbed energy of the BM, as-welded state, HTT state and IA state welds were 236 J, 42 J, 241 J and 51 J, respectively. The impact performance of HTT state welds surpassed the BM.

Formation of equiaxed grains in selective laser melted pure titanium during annealing

Commercially pure titanium (CP-Ti) has seen major applications in biomedical equipment. The use of selective laser melting (SLM) has become more common concerning the fabrication of CP-Ti components with a complicated shape in biomedical implants, where high strength and ductility are required. However, SLM manufactured CP-Ti often exhibits high strength and low ductility, as well as mechanical anisotropy because SLM process typically results in the formation of long columnar grains comprising of fine acicular α′ martensite. Heat treatment must be preceded to transform the acicular α′ to equiaxed α grain. This study demonstrated that annealing at 650 °C of SLM CP-Ti can create an equiaxed structure, resulting in a weakened texture. The formation of equiaxed grains occurred at two types of microstructural features, acicular α′ and irregular massive (α′m) martensite, which exhibited different sizes, morphologies, and contained different types of substructures under SLM as-fabricated condition. The formation mechanism of equiaxed grains in the α′ region is dominated by the coalescence of acicular α′ of the same variant and the dissolution of α′ into the matrix, whereas the formation mechanism in the α′m region is primarily dominated by the growth of the preexisting subgrains. A small number of equiaxed grains with new orientations were formed in the α′m region. The majority of equiaxed grains were formed in the acicular α′ region and inherited the grain orientations of the preexisting α′ or matrix. Therefore, a similar but weakened texture was inherited from the microstructure of the SLM CP-Ti after annealing.

Simultaneously Improving Mechanical Properties and Stress Corrosion Cracking Resistance of High-Strength Low-Alloy Steel via Finish Rolling within Non-recrystallization Temperature

The effect of hot rolling process on microstructure evolution, mechanical properties and stress corrosion cracking (SCC) resistance of high-strength low-alloy (HSLA) steels was investigated by varying the finish rolling temperature (FRT) and total rolling reduction. The results revealed granular bainite with large equiaxed grains was obtained by a total rolling reduction of 60% with the FRT of 950 °C (within recrystallization temperature Tr). The larger grain size and much less grain boundaries should account for the relatively lower strength and SCC resistance. A larger rolling reduction of 80% under the same FRT resulted in the formation of massive martensite–austenite (M/A) constituents and resultant low ductility and SCC resistance. In contrast, a good combination of strength, ductility and SCC resistance was obtained via 80% rolling reduction with the FRT of 860 °C (within non-recrystallization temperature Tnr), probably because of the fine grain size and M/A constituents, as well as a high density of grain boundary network.

Ultrahigh-strength titanium gyroid scaffolds manufactured by selective laser melting (SLM) for bone implant applications

Commercially pure titanium (CP–Ti) gyroid scaffolds with interconnected pores and high porosities in the range of 68–73% and three different unit cell sizes of 2, 2.5, and 3 mm were manufactured by selective laser melting (SLM) for bone implant applications. The microstructure and mechanical properties of the scaffolds with different unit cell sizes and sample orientations were evaluated. The microstructure of as-built struts was dominated by massive martensite and the average microhardness of the struts was 2.27 GPa, which is ∼50% higher than that of dense cast CP–Ti. The elastic modulus and yield strength of the as-built scaffolds ranged from 1465 to 2676 MPa and from 44.9 to 56.5 MPa, respectively, values which are close to the elastic modulus of trabecular bone and presumably strong enough to bear the physiological loading of implants. The as-built scaffolds exhibited excellent ductility up to 50% strain and no sign of fracture up to 20–30% strain under compression. The dominant compressive response of the scaffolds was observed by formation of a plastic hinge which led to rotation of the struts about the plastic hinges followed by development of local shear bands in struts in the long plateau region. These SLM-manufactured gyroid CP–Ti scaffolds with significantly enhanced hardness and compressive strength exhibited an elastic modulus close to that of trabecular bone and offer a promising improvement on CP–Ti scaffolds for bone implant applications.

Microstructure and mechanical properties of laser-welded dissimilar DP780 and DP980 high-strength steel joints

The purpose of this paper is to research the effect of heat input on the microstructure and mechanical properties of DP780 and DP980 dissimilar steel welded joints by using laser welding and analyze the transformation mechanism from austenite to martensite in laser welding. The distribution of martensite volume fraction in each region was similar to the hardness value distribution. The hardness of the fusion zone was significantly improved by the laser welding, but the highest hardness value of the dissimilar steel welded joint appeared in the hardening zone on the DP980 side. At the same time, there was an obvious softening phenomenon at the side of DP980, but the minimal hardness value of DP780 and DP980 dissimilar steel welded joints appeared in the sub-critical HAZs on the side of DP780. This was why the DP780 and DP980 dissimilar welded joints were broken at the DP780 side of the sub-critical HAZ. Compared with the DP780 base metal and DP980 base metal, the plastic deformation and strength-ductility of DP780 and DP980 dissimilar welded joints were significantly reduced. The largest deformation zone of the dissimilar steel welded joints occurred in the heat-affected zone of the DP780 side. The elongation of the heat-affected zone of the DP780 side decreased with decreasing heat inputs. In addition, through the analysis of the transformation mechanism of austenite to martensite for DP780 and DP980 steels, when the peak temperature was lower than Ac1, the tempered martensite led to DP steels softening. When the peak temperature was higher than Ac1 but lower than Ac3, the decrease of martensite content of samples resulted in DP steels softening. When the peak temperature was higher than Ac3, the DP steels began to harden, and the microstructure was dominated by massive martensite and lath martensite.

Effect of Partitioning Time on the Microstructure and Mechanical Properties of Q&P Steels

The microstructure and properties of Q&P steel were studied by means of tensile test, OM and SEM after simulating heat treatment process in salt bath furnace. The results showed that the main microstructure of Q&P steel was lath martensite and retained film austenite. With the increase of partitioning time, the morphology of the parallel martensite lath became clear and ordered. With the trivialness and disorder with massive martensite appearing, the yield strengths and tensile strength decreased initially and then increased. On the other hand, the elongation increased initially and then decreased. This was because of that the retained austenite is unstable at the beginning for low carbon content, and the carbide precipitated after a long partitioning time. Therefore, there was an optimum partitioning time to obtain the best properties combination. Under 250 quenching temperature and 350 partitioning temperature, partitioning time was 60s, the tensile strength and elongation were 1027MPa and 27%, respectively. The product of strength and elongation was up to 27729MPa·%.

Microstructure and Properties of Mo Microalloyed Cold Rolled DP1000 Steels

Two kinds of ultra-high strength cold rolled dual phase steels have been developed by designing C-Si-Mn-Cr and C-Si-Mn-Cr-Mo alloy systems. Tensile strength and elongation of both steels exceed 1100 MPa and 10%, respectively. The microstructures of both steels consist of massive martensite and ferrite. And the massive martensite of Mo-free steel disperses in the ferrite with volume fraction of 64%. However, the massive martensite of Mo-containing steel is connected or closed by small martensite islands each other, and martensite volume fraction is 69%. As to Mo-free steel, the yield strength, yield ratio, and work hardening exponent n are 548 MPa, 0.49, and 0.26, respectively. As for Mo-containing one, the yield strength, yield ratio, and n value are 746 MPa, 0.66, and 0.33, respectively. Besides, the ferrite of Mo-free steel is deformed at the initial stage of plastic deformation. However, for Mo-containing one, Mo solution strengthened ferrite and small overaged martensite islands are deformed preferentially at small strain, which causes the yield strength to reach approximately 200 MPa higher than that of Mo-free steel.

Fatigue crack growth behavior of the simulated HAZ of 800 MPa grade high-performance steel

The present study focuses on the fatigue properties in the weld heat-affected zone (HAZ) of 800MPa grade high-performance steel, which is commonly used in bridges and buildings. Single- and multi-pass HAZs were simulated by the Gleeble system. Fatigue properties were estimated using a crack propagation test under a 0.3 stress ratio and 0.1 load frequencies. The microstructures and fracture surfaces were analyzed by optical microscopy, scanning electron microscopy, and transmission electron microscopy. The results of the crack propagation test showed that the fatigue crack growth rate of coarse-grained HAZ (CGHAZ) was faster than fine-grained HAZ (FGHAZ), although both regions have identical fully martensite microstructures, because FGHAZ has smaller prior austenite grain and martensite packet sizes, which can act as effective barriers to crack propagation. The fatigue crack growth rate of intercritically reheated CGHAZ (ICCGHAZ) was the fastest among local zones in the HAZ, due to rapid crack initiation and propagation via the massive martensite–austenite (M–A) constituent.

New method to realise superplastic deformation of ultrahigh strength steel

The superplastic behaviour of an ultrahigh strength steel (Fe–0·47C–1·98Mn–1·37Si–0·91Cr–0·25Mo) after a proper predeformation has been studied. The predeformation mentioned above was conducted with true strain of 1·2 at strain rate of 10−3 s−1 at 973 K. It was found that water cooled specimen after the predeformation exhibits superplastic characteristics under suitable conditions although the microstructure of the specimen contains massive martensite, which is commonly thought to be unbeneficial for superplasticity. Further study shows that the microstructure transforms to ferrite+spheroidised carbide dual phase under a very small strain during the early stage of superplastic deformation.

First stages of martensitic growth studied by TEM in a Cu-Al-Ni single crystal and associated mechanical spectroscopy instabilities

The martensitic transition of a Cu-Al-Ni single crystal has been studied by isothermal mechanical spectroscopy and in situ transmission electron microscopy (TEM) observations. A damping instability (with time) has been found at low frequency at the beginning of the transition. At the same time, TEM study has evidenced the presence of thin martensite plates which appear just at the beginning of the (massive) martensite formation and growth. The martensite plate growth rate is quite low and non continuous. While high internal friction in the transition domain is usually associated with the mobility between martensite/austenite interfaces, the present internal friction anomaly has been ascribed to the unstable motions of the first martensite plates.

Structural changes in martensite foils at the tips of microcracks arising as a result of stress relaxation

Changes in the structure at the tips of microcracks in structural materials with a structure of massive lath martensite are studied by diffraction electron microscopy. It is shown that changes at the tip of a crack occur as a result of successive stress relaxation. At first slip bands form, then reorientation zones (RZ) form, and finally zones free from dislocations (DFZ) form. The final stage is occurrence in DFZ of submicrocracks which then merge with a main crack. The dislocation structure at the tips of cracks is studied and the mechanism of submicrocrack generation is analyzed.

A contribution to the study of the transformation characteristics of type 15%Cr6%NiMo steel under tempering in the intercritical temperature range

The study is concerned with the process of austenitizing type 15%Cr6%NiMo steel. According to the morphology of austenite formed on tempering in the intercritical temperature interval, according to its stability and transformation mechanism on cooling, the tempering process was divided into the following stages: (a) formation of austenite, stable on cooling, during tempering at temperatues up to 600°C, (b) formation of austenite on tempering in the temperature interval 625 to 650°C, the austenite being partially transformed to martensite on cooling, (c) formation of continuous austenite zones at temperatures exceeding 650°C; the austenite is transformed to massive martensite on cooling, (d) complete austenitizing on heating above the Ac 3 temperature and retransformation of austenite to lath martensite.

γ → α transformation in Fe, Fe–Ni, and Fe–Cr alloys

Evidence is presented to show that five transformation products are possible in pure iron and dilute substitutional alloys: equiaxed ferrite, massive ferrite, bainitic ferrite, lath martensite (massive martensite), and twinned martensite. The morphology and transformation temperatures of these products are discussed. In conclusion, the chemical driving force for each of the transformations is calculated as a function of composition.

Kinetics of the massive martensitic transformation in commercial carbon steels

1. Raising the quenching temperature of commercial carbon steels to 1180–1200°C produces a mixture of massive and twin martensite. 2. The temperature range in which massive martensite is formed is higher than the range in which twin martensite is formed and coincides with the range in which bainite is formed. 3. The limit concentration of carbon corresponding to a shift from the massive to twin martensite transformation depends on the cooling rate-with increasing cooling rates the carbon concentration at which twin martensite is formed decreases. 4. The critical cooling rate for formation of massive martensite is lower than for twin martensite. 5. The nature and kinetics of the formation of bainite and massive martensite are similar. Differences in the structures are due to the carbon content, since diffusional redistribution of carbon occurs in the process of the transformation. Thus, it is expedient to use the term bainite for carbon-containing alloys and massive martensite (isothermal) for carbon-free alloys (<0.1% C).

Ordering transformations and mechanical properties of Ti3Ai and Ti3Al-Nb alloys

Phase transformations and the kinetics of domain growth were studied in near stoichiometric Ti3Al and in a similar alloy containing about 5 at. pct Nb (Cb). The alloys were quenched from the β and from the α+ β fields and were subsequently annealed in the α2 field to study the ordering transformation. The critical temperature (T c) for ordering was found to be between 1125 and 1150° for both alloys. When quenched from aboveT c the microstructure of the stoichiometric compound contained massive martensite with small antiphase domains of average size 8 × 10— μm. On annealing the quenched structures in the range 700 to 1000°, domain coalescence occurred, the domains growing approximately as the square root of the annealing time. The activation energy for the domain growth process was found to be 64.6 ± 6 Kcal/mole (2.68 ± 0.25 × 105 J/mole). On quenching the alloy containing Nb the β transforms to a fine acicular martensite. On annealing, antiphase domain coalescence within the martensite plates and the simultaneous recrystallization of the martensite resulted in a fine subgrain structure even after annealing at 900° for up to 3 h. The mechanical properties and the fracture modes of the two alloys tested at 700° were correlated with the observed microstructural changes. The effects of Nb in this alloy are to slow the domain growth kinetics, to reduce the planarity of slip, and to increase nonbasal slip activity.

Microstrucure of explosively welded 0.1 Pct carbon mild steel tubes to 0.2 Pct carbon mild steel plate

The microstructures developed within trial explosive welds between 16 mm diam, 1.2 mm wall thickness tubes of 0.1 pct C 0.4 pct Mn mild steel and a 0.2 pct C 0.8 pct Mn mild steel base plate have been examined. A weld zone typically ∼20 μm wide is formed in which solid-phase bonding is interrupted by pockets of localized melting. The complex microstructure develops as a result of both severe plastic deformation and rapid cooling. Plastic deformation, limited to regions close to the weld interface and the internal surface of the tube, was confirmed by the high density of dislocations in the α phase. Twinning on {112} planes occurs within discrete regions of the tube plate and is discussed in terms of the geometrical arrangement of the tubes in the plate. The morphology of the product in the fusion pockets has been compared with the massive and acicular martensite which is typical of quenched low-carbon steels. The high-pressure shock waves that develop during the collision of the tube and tube plate result in pronounced local deformation adjacent to the weld junction. The closely interwoven microstructure produced has been interpreted as the result of a pressure-induced transformation.