Original Austenite Research Articles

Influence of Solid Solution Treatment on Microstructure and Mechanical Properties of 20CrNiMo/Incoloy 825 Composite Materials

The utilization of 20CrNiMo/Incoloy 825 composite materials as high-pressure pipe manifold steel can not only improve the strength and hardness of the steel, but also improve its corrosion resistance. However, research on the heat treatment of 20CrNiMo/Incoloy 825 composite materials is still scarce. Thus, the aim of this study was to investigate the influence of solid solution treatment on the microstructure and properties of 20CrNiMo/Incoloy 825 composite materials. Firstly, the composite materials were subjected to solid solution treatment at temperatures ranging from 850 to 1100 °C with varied holding times of 1 h, 4 h, and 6 h. Microstructural analysis revealed that the solid solution treatment temperature had a more pronounced effect than the treatment time on the interface decarburization layer, carburization layer, and grain size. It was observed that the carburized layer thickness decreased while the decarburized layer thickness increased with an increase in the solid solution treatment temperature, oil cooling was found to enhance the hardness of the base layer of the composite materials, and the size of the original austenite grains of 20CrNiMo steel and Incoloy 825 increased with an increase in the solid solution treatment temperature. Secondly, the tensile properties, microhardness, and fracture morphology were evaluated after the composite materials underwent solid solution treatment at temperatures between 950 °C and 1100 °C for 1 h. The results indicated that increasing the solution temperature initially led to an increase in tensile strength and elongation after fracture, followed by a decrease; furthermore, the hardness of Incoloy 825 exhibited a declining trend, while the hardness of 20CrNiMo first decreased then increased. Thirdly, the shear properties and interfacial element diffusion of the composite materials were analyzed following solid solution treatment in a temperature range of 950 °C to 1100 °C for 1 h. The findings demonstrated that higher solid solution treatment temperatures induced full diffusion of Cr, Ni, and Fe atoms at the interface and softened the matrix, leading to an increase in the thickness of the diffusion layer and toughening of the composite interface. Therefore, the shear strength increased with an increase in the solid solution treatment temperature. Finally, the optimal solid solution treatment process for 20CrNiMo/Incoloy 825 composite materials was determined to be 1050 °C/1 h oil cooling, following which the composite materials had good comprehensive mechanical properties.

Synergistic regulation of nano-precipitates and reversed austenite in titanium-free maraging steel by low-temperature solution treatment and double aging treatment

To synergistically enhance the strength and toughness of titanium-free maraging steel, a multi-scale characterization method was used to illustrate the effects of low-temperature solution treatment and double aging treatment on the microstructure of titanium-free maraging steel in this paper. After the low-temperature solution treatment and the double aging treatment, the tensile strength of titanium-free maraging steel increased from 1954 MPa to 2160 MPa and the elongation increased by 8.95 %. By the low-temperature solution treatment, the original austenite grain size of the titanium-free maraging steel was refined to 0.69 μm. The double aging treatment promoted the diffusion of Mo and Ni elements, increased the volume fraction of ω phase, Ni3Mo nano-precipitation phase and reversed austenite, and refined the size of ω phase and Ni3Mo by 14.2 % and 7.9 %, respectively. The nanoparticles of titanium-free maraging steel mainly include the ω phase, Ni3Mo and Laves phase. The strengthening mechanism of nanoparticles was quantitatively evaluated from the shear mechanism and Orowan dislocation loop mechanism. The mechanism shows that the ω phase is the main contributor to the overall precipitation strengthening. Therefore, low-temperature solution treatment and double aging treatment provide a potential solution for achieving high strength and high toughness in maraging steel.

Synergistic twinning and deformation-induced martensite transition facilitating 3GPa duplex stainless steel wire prepared by drawing

3 GPa-strength stainless steel wire was prepared. Sub-20 nm martensite generated from original austenite which is metastable and possesses low stacking fault energy. Based on quantitative analysis of microstructural parameters from HEXRD and TEM, the sub-20 nm martensite leveraging the advantage of heterostructure props up the ultra-high strength of the wire.

Corrosion Behavior of Q420qNH Plate and Welded Joint in Deicing Salt Medium by Cyclic Immersion + Infrared Drying

The corrosion behavior of Q420qNH steel plate and welded joint in deicing salt medium is investigated by cyclic immersion + infrared drying accelerated corrosion experiments. The results show that the corrosion products are mainly Fe2O3 and FeOCl under the influence of Cl− concentration differential corrosion microcells in the immersion corrosion anoxic stage, and the corrosion products are mainly β-FeOOH, γ-FeOOH, and Fe3O4 under the influence of oxygen concentration differential corrosion microcells catalyzed in the infrared drying stage. Throughout the corrosion process, the corrosion weight gain and average corrosion rate of the welded sample are always lower than that of the plate sample. The Iα-FeOOH/Iβ-FeOOH + FeOCl peak intensity ratio of the welded sample is always higher than that of the plate sample and the heat-affected zone (HAZ) sample. The corrosion types of the three samples are inhomogeneous total corrosion + pitting, and the depth of the pits in the plate is much larger than that in the weld and the HAZ. In the late stage of corrosion, the self-corrosion current density is: plate > HAZ > weld, and the resistance of the rust layer of the weld and HAZ samples is 1.66 times and 1.31 times that of the plate sample, respectively. This is because the microstructure of the weld and HAZ are composed of a large number of fine acicular ferrite and lath-shaped bainite generated inside the original austenite grains, and the small angle grain boundary is not conducive to the initiation and expansion of pitting corrosion. In addition, the higher content of Cu, Cr, Ni, and other elements in the weld can refine the rust layer products and effectively impede the intrusion of Cl−, contributing to corrosion resistance improvement. As the combined effect of Cl− and immersion anoxic environment inhibits the formation of α-FeOOH and promotes the formation of a large number of loose and unstable β-FeOOH and soluble FeOCl, the weathering steel is not well protected by the rust layer during cyclic immersion corrosion in deicing salt medium.

Enhancement of hydrogen embrittlement resistance in 310S austenitic stainless steel through ribbon-like δ-ferrite

In general, δ-ferrite phases, especially in the weld, are not conducive to the hydrogen embrittlement (HE) resistance of austenitic stainless steels (ASSs). However, it is interesting to note that if a ribbon-like δ-ferrite was introduced into ASS, its HE resistance can be improved. In this work, we take 310 S ASS as an example and form a ribbon-like δ-ferrite on the original austenite grain boundary by quenching at 1200℃. This newly formed ribbon-like δ-ferrite phase not only alters the hydrogen enrichment characteristic from austenite grain boundaries to austenite/δ-ferrite interfaces but also contributes to an increased resistance to HE.

Effect of Reheating Temperature on the Microstructure and Properties of Cu-Containing 440 MPa Grade Non-Tempered Ship Plate Steel.

This study investigated the effects of reheating temperature on the microstructure and mechanical properties of Cu-containing 440 MPa grade non-tempered ship plate steel. The mechanical properties test, thermodynamic simulation, optical microscopy, scanning electron microscopy, transmission electron microscopy, and other tests were performed. The results revealed that with increasing reheating temperature, the ferrite grain size of Cu-containing 440 MPa non-tempered ship plate steel increased. Also, with increasing reheating temperature, the size of copper particles and niobium-titanium composite precipitates in the original austenite decreased. Consequently, this led to a weakening of the pinning effect on the original austenite and an increase in the size of the transformed ferrite grains. Moreover, with increasing reheating temperature, the number of Cu precipitates in the test steel after air cooling and rolling increased, while the size of the precipitates decreased, thereby weakening the solid solution strengthening effect of Cu, and precipitation was enhanced. Additionally, as the reheating temperature increased, the tensile strength and yield strength of the air-cooled test steel after rolling increased, while the impact toughness decreased.

Gradient microstructure in Ni-Cr-Mo ultra-heavy steel plate after tempering

This paper studied through-thickness gradient microstructure of an 80mm ultra-heavy steel plate by multi-dimensional characterizations, meanwhile the cause of microstructure variability as well as its effect on strength was analyzed. Results show major lath-structure in the whole steel plate, while lath morphology and grain boundary distribution are different along the thickness direction. During phase transformation, the higher cooling rate in the surface enhances the forming ability of V1/V4 variant pairs and corresponding 5°-10° boundaries, meanwhile the higher cooling rate and smaller original austenite grain contribute to more nucleation sites and stronger strain incompatibility, which is corresponding to more low-angle boundaries, so the surface layer shows higher low-angle boundaries; Compared with the surface layer, the moderate cooling rate is beneficial for V1/V2 variant pairs, and Σ3 boundaries between the pairs are obtained, leading to higher high-angle boundary density with decreased low-angle grain boundary density. This boundary characteristic corresponds to the thinner and longer lath morphology. With respect to the relationship between mechanical properties and microstructure, more low-angle boundaries in the surface layer and more high-angle boundaries in inner layers contribute to stronger dislocation strengthening and grain boundary strengthening respectively, and realizes the coordinated through-thickness property.

The Influence of the Second Phase on the Microstructure Evolution of the Welding Heat-Affected Zone of Q690 Steel with High Heat Input.

Q690 steel is widely used as building steel due to its excellent performance. In this paper, the microstructure evolution of the heat-affected zone of Q690 steel under simulated high heat input welding conditions was investigated. The results show that under the heat input of 150-300 kJ/cm, the microstructures of the heat-affected zone are lath bainite and granular bainite. The content of lath bainite gradually decreased with the increase in heat input, while the content of granular bainite steadily increased. The proportion of large-angle grain boundaries decreased from 51.1% to 40.3%. Overall, the average size of original austenite increased, and the precipitates changed from Ti (C, N) to Cr carbides. During the cooling process, the nucleation position of bainitic ferrite was from high to low according to the nucleation temperature, and in order of inclusions at grain boundaries, triple junctions, intragranular inclusions, bainitic ferrite/austenite phase boundaries, twin boundaries, grain boundaries, and intragranular inclusions at the bainitic ferrite/austenite phase interface. The growth rate of bainitic ferrite nucleated at the phase interface, grain boundary, and other plane defects was faster, while it was slow at the inclusions. Moreover, it was noted that the Mg-Al-Ti-O composite inclusions promote the nucleation of lath bainitic ferrite, while the Al-Ca-O inclusions do not facilitate the nucleation of bainitic ferrite.

Effect of Co content on microstructure and mechanical properties of maraging steel

Effect of Co element on the microstructure and mechanical properties of maraging steel is studied and summarized. The microstructure and mechanical properties of five different compositions of maraging steels were characterized and tested in three heat treatment states, with the heat treatment schemes of homogenization treatment at 1300 °C × 10h, solution treatment at 800 °C × 2h, and aging treatment at 480 °C × 5h. The research has discovered that increasing the Co content in three different heat treatment states can lead to a reduction in the size of the original austenite grains and the width of martensite laths. In the aged state, higher Co content causes the precipitation phase to transform from an ellipsoidal Mo-rich phase to a needle-like Ni3(Mo, Ti) phase. These changes in the size, shape, and density of the precipitated phases result in a more pronounced effect of precipitation strengthening, leading to a significant increase in the material's strength.

Fatigue crack propagation of the gradient surface-modified layer of high-strength steel

A gradient modified layer can be produced on the high-strength steel surface by carburizing heat treatment. In this study, microstructure characterization and fatigue crack propagation test were carried out on the gradient surface-modified layer of the 18CrNiMo7-6 high-strength steel. With increasing depth of the surface-modified layer, the yield strength and the kernel average misorientation value decrease, and the equivalent grain size of the slatted martensite structure and the number of the large-angle boundaries increase. The orientation difference angle of the point to point pre- and post-fatigue at the crack of the transition layer and the matrix layer increases from less than 2° to approximately 10° and about 4°, respectively. With the increase of the depth of the surface-modified layer, the degree of deformation inside the martensitic packet decreases, while the extent of macroscopic deformation of the material increases. Deformation occurs inside the martensitic packet at the main crack, and there is essentially no deformation inside the martensite packets away from the main crack. On both sides of the main crack, a large number of extrusions parallel to the martensitic lath appear. The large-angle boundaries of the original austenite grain boundary and martensite boundary hinder the propagation of the fatigue crack.

Ultrastrong gradient nanostructured CSS-42L bearing steel and its enhanced wear resistance at elevated temperature

A gradient nanostructured surface layer was successfully fabricated on the CSS-42L(15Cr14Co12Mo5Ni) aerospace bearing steel by ultrasonic shot peening (USP) technology at room temperature. The original austenite in the as-received (AR) CSS-42L bearing steel was fully transferred into lath martensite by quenching and two cryogenic tempering cycles. The lath martensitic structure at the surface of the bearing steel was refined into nanoscale martensite phases with an average size of 33.9 nm after USP treatment. The microhardness of the gradient heterogeneous nanostructured CSS-42L bearing steel increased by approximately 39 % compared to that of the tempered bearing steel. The mechanisms of martensite refinement during the USP process were analyzed and discussed using advanced materials characterization methods. Dry sliding wear tests at elevated temperatures (500 °C) were conducted on the tempered and USPed CSS-42L bearing steel samples, respectively. The wear resistance of the USPed sample was significantly enhanced compared to that of its untreated coarse-grained counterpart. The wear rate coefficient of the USPed CSS-42L was reduced by 80.42 % compared to the AR sample. The superior wear resistance of the USPed CSS-42L bearing steel was attributed to the formation of an ultrastrong surface layer with a gradient heterogeneous nanostructure fabricated by USP technology.

Study on periodic immersion and infrared aging corrosion behavior of bridge weathering steel Q420qNH welded joint and plate in NaHSO3 solution

AbstractThe periodic immersion corrosion behavior and corrosion mechanism of Q420qNH steel weld, heat‐affected zone and plate sample in simulated industrial atmospheric medium were studied. The corrosion type of the three samples is the non‐uniform total corrosion, and the corrosion products at different stages are different. Ferric oxide and lepidocrocite (γ‐FeOOH) are generated in the immersion stage and the initial stage of infrared drying, respectively. However, lepidocrocite (γ‐FeOOH), ferrosoferric oxide (Fe3O4) and goethite (α‐FeOOH) generated in the later stage of infrared drying. The infrared drying process is conducive to the formation of goethite (α‐FeOOH). The corrosion rate of weld is always lower than that of plate, and the Iα‐FeOOH/Iγ‐FeOOH ratio of the weld is always higher than that of plate and heat‐affected zone sample. In the later stage of corrosion, the self‐corrosion current density of the three samples from small to large is weld, heat‐affected zone and plate, and self‐corrosion potential from large to small is weld, heat‐affected zone and plate. The resistance of the rust layer of the weld and heat‐affected zone sample is 1.81 times, 1.29 times of that of the plate, respectively. This is because the weld is composed of a large number of small angular grain boundary structures dominated by tiny acicular ferrite in the original austenite grain, which is conducive to the improvement of corrosion resistance. In addition, the higher chromium/carbon (Cr/C), copper and nickel elements of the weld can promote the conversion from lepidocrocite (γ‐FeOOH) to goethite (α‐FeOOH), and improve the density and stability of weld rust layer.

Enhancing corrosion resistance of additive manufactured heterogeneous martensite stainless steel by hot isostatic pressing

The effect of hot isostatic pressing (HIP) on microstructure, corrosion resistance and passive film characteristics for heterogeneous martensite stainless steel (MSS) manufactured by laser powder bed fusion (LPBF) was studied. The microstructure was mainly typical original austenite grains and martensite laths and the high content of austenite disappeared after HIP. The average grain size (about 14.09 μm) was larger than that of sample before HIP (about 4.33 μm). When the laser power was 170 W and the scanning speed was 550 mm·s−1, the density of the samples after HIP increased to >99.99%. When the scanning speed increased to 1250 mm·s−1, the density of samples increased from 97.82% to 99.89%. After HIP treatment, the passivation trend of low density (97.8%) samples was more obvious and the pitting potential increased from −44 mVSCE to 179 mVSCE. The pitting potential of high density (99.77%) samples increased from 265 mVSCE to 291 mVSCE and the electrochemical impedance value increased. The sensitivity of the samples to chloride ions significantly decreases after HIP. The donor density for sample before HIP was 2.5546 × 1020 cm−3, which was higher than that of sample after HIP (2.001 × 1020 cm−3). Therefore, the corrosion resistance of LPBF high strength stainless steel was enhanced after HIP.

Cracking Behavior and High-Temperature Thermoplastic Analysis of 09CrCuSb Steel Billets

This paper characterizes the surface crack morphology and elements of 09CrCuSb steel continuous casting slabs and studies the high-temperature thermo-plasticity of continuous casting slabs using a Gleeble 3500 thermal simulator. Combining the macro/micro fracture morphology and microstructure characteristics, the formation rules of slab cracks are discussed. The research results show that the increase in Ae3 temperature caused by changes in alloy elements results in the precipitation of a thin, film-like coexisting ferrite along the grain boundaries at a certain temperature, as well as the element segregation behavior of low-melting-point alloy elements at the original austenite grain boundaries, which are the main factors inducing cracks in 09CrCuSb steel continuous casting billets. The plasticity of 09CrCuSb steel at high temperatures is poor at 800~900 °C. In continuous casting process control, it is necessary to try to avoid long-term stay of the billet at this temperature range.

Microstructure Evolution and Mechanical Response of a Direct Quenched and Partitioned Steel at Different Finishing Rolling Temperatures.

The effects of finishing rolling temperature on the microstructure and mechanical properties of a direct quenched and partitioned (DQ&P) steel were investigated by a thermal simulation machine, a field emission scanning electron microscope (FE-SEM), electron backscattering diffraction (EBSD), and a transmission electron microscope (TEM). The results show that the original austenite grain size was refined by 31% as the finishing rolling temperature decreased from 920 °C to 840 °C, leading to the formation of the finest martensite lath at 840 °C. At the same time, the lower finishing rolling temperature resulted in a higher dislocation density, and consequently improved the stability of the retained austenite. Moreover, compared to the conventional Q&P process, the comprehensive mechanical properties of a steel with similar chemical composition can be enhanced by DQ&P processing. With the decrease of finishing rolling temperature from 920 °C to 840 °C, the strength and total elongation increases. The yield strength, tensile strength, and total elongation reach the maximum values of 1121 MPa, 1134 MPa, and 11.7%, respectively, at 840 °C.

Effects of Nb and Y on the Solidification Microstructure and Mechanical Properties of S321 Austenitic Stainless Steel

δ‐Ferrite plays a controversial role in austenitic stainless steel and controlling its morphology and distribution is essential for the optimization of the properties of stainless steel. Herein, the solidification microstructure of S321 stainless steel with the addition of Nb and Y is studied by electron probe microanalysis, nanoindentation, and transmission electron microscopy. The results show that the addition of Nb promotes the precipitation of (Nb, Ti)(C, N) and coarsens the grain boundary carbides in S321 stainless steel, resulting in a decrease in the solute carbon content near the grain boundaries and a delay in the δ → γ phase transformation. The addition of Y inhibits the segregation of Ni along the original austenite grain boundaries and promotes the δ → γ phase transformation. Furthermore, the morphology of δ‐ferrite changes from lathy δ to skeletal δ and the grain boundary carbide network is broken. The higher density dislocation due to the phase transformation leads to an increase in the hardness of S321 steel containing Y.

Microstructure characteristics at different depths of 40CrNiMo steel after laser hardening

This study focuses on laser-hardened 40CrNiMo steel, which meets the requirements for use in sprag clutch wedges; specifically, the microhardness characteristics as well as the phase and element characteristics and formation mechanism at different depths are investigated after laser hardening. The results show that the hardness of the arc-shaped specimen with a laser-hardened layer fluctuates significantly after reaching a certain depth. With increasing depth, the original austenite grain size decreases exponentially, it becomes easier for acicular martensite to form near the surface, and the martensite bulge phenomenon near the surface is more pronounced. The proportion of small-angle grain boundaries decreases, while that of large-angle grain boundaries increases after laser hardening. The angle of the small-angle grain boundaries of the adjacent grains of the hardened layer is mostly below 5°, while that of the large-angle grain boundaries is predominantly between 50° and 60°. After laser hardening, the preferred orientation of the grains in the hardened layer is not clear. The aggregation of C atoms is an important factor determining the structural characteristics of the hardened layer.

Microstructure and mechanical properties of 9Cr18Mo martensitic stainless steel fabricated by strengthening-toughening treatment

Microstructure and mechanical properties of strengthening-toughening treated (1020–1060 °C argon quenching + −73 °C isothermal quenching + 140–240 °C tempering) 9Cr18Mo martensitic stainless steel (9Cr18Mo steel for short) were investigated by using Thermo-Calc software, microstructure characterization and mechanical tests. It was found that the size range of carbides in the 9Cr18Mo steel was wide and approximately 0.2–15.0 μm. Most of micron-carbides (>1 μm) and submicron-carbides (<1 μm) were located at the grain boundary and in the grain interior of original austenite, respectively; and the 9Cr18Mo steel with higher volume fraction of submicron-carbides showed a better combination of strength and impact energy. It was detected that the carbides in the 9Cr18Mo steel mainly consisted of M23C6 and M7C3 with primary element of Cr, Fe, and secondary element of Mo. There were core-shell (M7C3-M23C6) structured carbides with two morphologies of coarse irregular (>10 μm) and micron near-spherical, as well as individual micron-M7C3, micron- and submicron-M23C6 with near-spherical morphology. Quasi-equiaxed grains with body-centered cubic (bcc) structure formed via recovery and recrystallization, {112}<111> bcc twins accompanied with hexagonal ω-Fe phase at the twin boundaries, retained lath- and blocky-austenite were observed in the tempered martensitic matrix of 9Cr18Mo steel. Moreover, the twins and retained austenite were prone to decomposition at higher tempering temperature. It was indicated that the yield strength of 9Cr18Mo steel was mainly correlated with subgrain block size of tempered martensite and dispersion parameter of carbides via the analysis of statistical data, and the subgrain block contributed more to the yield strength of 9Cr18Mo steel. The M7C3-M23C6 core-shell structured carbides and {112}<111> bcc twins associated with hexagonal ω-Fe phase found in this study were also helpful to understand the correlation between microstructure and mechanical properties of other high carbon chromium martensitic steels.

Effect of Trace Rare-Earth Element Ce on the Microstructure and Properties of Cold-Rolled Medium Manganese Steel

Rare-earth elements have been widely used in the field of functional materials, but their effects on the cold-stamping formability of high-strength automotive steels have rarely been studied. In this paper, the effect of the trace rare-earth element Ce on the microstructure and properties of cold-rolled medium manganese steel after ART (austenite-reverted transformation, ART) annealing was studied. The microstructure of the experimental steel was observed using SEM, and the mechanical properties were tested using a universal tensile testing machine. The volume fraction of the retained austenite and the texture of the steel were measured using XRD. The results showed that the original austenite grain size of the experimental steel was smaller after adding the trace rare-earth element Ce. After ART annealing, the grain size distribution of the experimental steel with rare-earth Ce was more uniform, and the comprehensive mechanical properties were better. Under the conditions of quenching at 800 °C for 5 min and annealing at 645 °C for 15 min, the maximum product of tensile strength and elongation was 28.47 GPa·%.

Effects of Vanadium on the Kinetics of Austenitization from Pearlite in Fe–V–C Alloys

Vanadium effects on the kinetic characteristics of the austenit transformation in Fe-V-C ternary alloys were investigated by microstructure analysis, differential scanning calorimetry (DSC) and the Johnson-Mehl-Avrami-Kolmogorov model (JMAK). The results show that the vanadium gives rise to the refinement of pearlite lamellar spacing before transformation, the original austenite grain size decreases after transformation, and and the increase of the amount of carbide at the grain boundarsy. Studies on kinetics showed that the austenization of the Fe-V-C ternary alloys is characterized by JMAK model involving site saturation nucleation, diffusion-controlled growth, and impingement correction. V increases the diffusion activation energy and decreases the pre-exponential factor for diffusion.