Steel Grain Research Articles

Tailoring hetero-grained austenite via a cyclic thermomechanical process for achieving ultrahigh strength-ductility in medium-Mn steel

In this study, a bimodal-grained austenite, composed of approximately 30% coarse grains and about 70% refined grains in (α+γ) medium Mn steel, was designed and developed based on a novel and simple cyclic thermomechanical process. The austenite stability was adjusted using these hetero-structures to regulate the strain-induced martensite transformation kinetics to balance the strength, ductility, and fracture resistance could be balanced. By coupling the transformation induced plasticity (TRIP) effect, hetero-deformation induced (HDI) strengthening and twinning-induced plasticity (TWIP) effect, the hetero-structured steel achieved an ultra-high tensile strength of nearly 1260 MPa and an elongation of approximately 70%, i.e., the product of strength and elongation of over 88 GPa%. This study presents a novel insight into an austenite-stability-guided microstructural design to address the strength-ductility/toughness trade-off in medium Mn steel.

Microstructure and Corrosion Resistance of Fusion Welding Zone for Duplextubes Welded with Q345R Tube Sheet under Different Welding Currents

Duplextubes are widely used in oil and gas storage and transportation, the nuclear industry, and other fields, but the welding quality of metals is an important factor affecting the use of equipment. In order to study the welding quality of S10C steel/Incoloy 825 duplextubes and Q345R tube sheet based on gas tungsten arc welding technology with a filler of ER50-6 carbon steel welding wire, the microstructure and grain size of fusion welding zone of duplextubes and tube sheet under welding currents of 150 A, 160 A, and 170 A were studied by optical microscopy and scanning electron microscopy. At the same time, the corrosion behavior of fusion welding zone after the welding was investigated in 3.5 wt.% NaCl solution by potentiodynamic polarization and electrochemical impedance spectroscopy. The results show that the metallurgical structure of the fusion welding zone was mainly composed of δ-ferrite and retained austenite. The grain size in the fusion welding zone increased with the increase of the welding current. The corrosion resistance of the fusion welding zone welded with a high welding current of 170 A was better than that with low welding currents. However, the pitting corrosion resistance of fusion welding zone with the lowest welding current of 150 A was better than that with high welding currents. This study can provide a preliminary exploration for the manufacture and applicability of duplextubes air coolers.

Effect of surface nanocrystallization produced by laser shock processing on the corrosion fatigue behavior of 300M steel

The effect of surface nanocrystallization produced by laser shock processing (LSP) on the corrosion fatigue behavior of 300M steel was systematically investigated. The surface integrity, microstructure evolution and residual stresses before and after corrosion fatigue were characterized. The results showed that afte LSP the surface grain size of 300M steel was sufficiently refined down to nanoscale after LSP, and a large residual compressive stress formed. Under the same loading stress level, the corrosion fatigue life of LSP-treated 300M steel was improved significantly. This improvement increased with increase of LSP pulse energy. After corrosion fatigue, the surface grain of LSP-treated 300M steel remained at the nanoscale, showing a good stability. The dislocation density and the number of deformation twins in the subsurface layer became higher with increase of LSP pulse energy to accommodate deformation. Surface residual compressive stress induced by LSP has a relaxation during corrosion fatigue, depending on the laser pulse energy and fatigue loading force.

Determination of Steel Losses and Optimization of the Thickness of the Transformer Magnetic Conductor Sheets

Currently, transformer manufacturers are tasked with creating energy-efficient devices with a reduction of steel losses of up to 44 %. Appropriate theoretical developments are needed for its implementation. With a reduction of eddy-current losses, caused, for example, by reduction of decrease in the thickness of the magnetic conductor sheets, hysteresis losses simultaneously increase. A similar effect is caused by changing the size of the crystal grain of steel, thermomagnetic treatment and other technological impacts. In this regard, the exact determination of the components of total losses in steel is an urgent problem, the solution of which would minimize total losses. The article analyzes the expressions that determine the specific losses for eddy currents and hysteresis through the parameters of the magnetic circuit, and states that this technique is too complicated for engineering calculations. Since eddy current losses are proportional to the square of the frequency, and hysteresis losses are proportional to the frequency in the first degree, simple calculated expressions of eddy current and hysteresis losses have been obtained using the wattmetric method. Based on the fact that the dependence of the magnetization loss on the thickness of the magnetic conductor plates is a decreasing linear function, and the eddy current loss is an ascending parabolic function, an expression of the optimal thickness of the plates has been found, the implementation of which makes the losses of steel minimal. This information will make it possible to minimize total steel losses more effectively by varying the design parameters and the material of the magnetic conductor. It is shown that the charted idling losses of transformers manufactured by different manufacturers differ by more than 30 % and can be rounded and underestimated; therefore it is advisable to obtain this parameter as a result of an experiment (idling experiment).

The microstructure and tensile property changes of S31042 steel after long time service

Abstract Ultra-supercritical power generation technology is one of the main solutions to the many contradictory problems of carbon emissions and environmental protection in coal-fired power plants. S31042 austenitic heat-resistant steel has become a typical material for high-temperature components used in ultra-supercritical units because of its good high-temperature creep properties and high-temperature corrosion resistance. In this study, typical characterization techniques such as SEM (Scanning Electron Microscopy), TEM (transmission electron microscopy), and EBSD (Electron Backscattered Diffraction) were used to observe the microstructure of S31042 austenitic heat-resistant steel, while the room-temperature tensile and high-temperature tensile properties of the material were tested under the corresponding conditions. The results show that the grain of S31042 heat-resistant steel after service is the same as that of the non-service samples, which is still twinned austenite. The room-temperature tensile properties and high-temperature tensile properties of S31042 after long service were almost unchanged.

Crystal Orientation Dependence of the Portevin–Le Chatelier Effect in Instrumented Indentation: A Case Study in Twinning-Induced Plasticity Steels

Instrumented indentation can be effectively used to investigate the Portevin–Le Chatelier (PLC) effect at small scales. It has been shown that the PLC effect in single crystals may depend on the crystal orientation, yet the underlying mechanism is unclear. Here, the orientation dependence of the PLC effect was systematically studied by conducting instrumented indentation tests in the [001]-, [101]- and [111]-oriented grains of a polycrystalline twinning-induced plasticity steel. It is found that the crystal orientation does not affect the PLC effect at relatively high indentation strain rates. In contrast, there is a strong orientation dependence at lower rates, with enhanced difficulty in the formation of serrations in the order of the [001], [111] and [101] orientations. This finding contradicts the previous proposals of the orientation effects, which are associated with the dislocation waiting time. On the basis of both the orientation and rate effects observed here, we proposed that the crystal orientation influences the occurrence of serrations in instrumented indentation by affecting the number of activated slip systems and, therefore, the probability of finding sufficient dislocation sources to accommodate the plastic avalanche.

Origin of fine equiaxed grains in industrial low-temperature grain-oriented silicon steel normalized sheet and their influence on magnetic properties

In the industrial low-temperature grain-oriented (LTGO) silicon steel normalized sheet, there are a certain number of fine equiaxed grains distributed along the rolling direction. However, the formation mode of the fine equiaxed grains and the influence of them on magnetic properties have not yet been well understood. In the present work, the above issues were investigated using field emission scanning electron microscopy (SEM), field emission transmission electron microscopy (TEM), X-ray diffraction (XRD), high temperature confocal scanning laser microscopy (HTCSLM), etc. It was found that (1) the fine equiaxed grains could be divided into many grain colonies according to grain orientation, and within each colony, the grain orientations were similar; (2) during normalizing, the fine equiaxed grains were mainly generated inside the elongated deformed grains which contained a large number of subgrains; (3) the amount of α → γ phase transformation during the normalizing was very small. Therefore, it can be inferred that the fine equiaxed grains were mainly produced by subgrain coalescence, rather than by phase transformation. Moreover, γ-grain (〈111〉 // ND) colonies with ideal grain size could be formed in the fine equiaxed grain regions after ideal normalizing process. Due to the heredity of microstructure and texture, γ-grain colonies, which can facilitate the formation of the precise Goss texture ({110} 〈001〉 ) during high-temperature annealing, were formed in the primary recrystallized matrix after cold rolling, decarburizing and nitriding, as a result, the magnetic properties of the final product were significantly improved. The present work can provide help to further improve the understandings of the microstructural evolution in the LTGO silicon steel normalized sheet and the influence of normalized microstructure on magnetic properties.

Effect of titanium on the discontinuous growth of as-cast coarse columnar γ grain of peritectic carbon steel

ABSTRACT The effect of titanium on the discontinuous growth of coarse columnar γ grain (CCG) of peritectic steel during solidification was investigated by a fast-directional solidification device. Based on the experiments, the growth characteristic of CCG and Ti(C, N) precipitates were detected by optical microscope, transmission electron microscope (TEM) and scanning electron microscope (SEM), respectively. The results showed that the averages height of CCG region and the growth rate of CCG (VCCG ) decrease with the increase of titanium content. When the titanium content increases to 0.09 wt-%, the CCG completely disappears during the whole solidification process. The microscale Ti(C, N) particles formed in liquid would decrease the growth rate of CCG which provides the convenient condition for the growth of nanoscale Ti(C, N) in the γ phase. The pinning pressure provided by Ti(C, N) precipitates increases with the titanium content, which is the main reason of the decrease of VCCG .

Interface stability and fracture mechanism of Al/Steel friction stir lap joints by novel designed tool

A novel welding tool, characterized by the enlarged pin design was developed to solve the hook feature or insufficient interface deformation for Al/steel friction stir lap welded joints. The welding tool enhanced the interface deformation effect and eliminated hook feature. The thickness of the intermetallic compound (IMC) layer at the interface decreased from 3.3 μm to 0.46 μm, when the welding speed increased from 30 mm/min to 300 mm/min. The laminated structure composed of IMCs and fine steel grains gradually disappeared, with the interface gradually changing from serrated to straight as welding speed increased. The nanohardness value of the microstructure reached 9.4 ± 0.3 GPa at a distance of 10 μm from the interface layer. Due to the larger metallurgical bonding area, the best line load 499.4 N/mm reached 52 % of the 3 mm 6082-T6 alloy, which was obtained at the rotational speed of 1200 r/min and the welding speed of 50 mm/min. Four different interface failure modes were found and established during the shearing process. The strain concentration phenomenon of the successive occurrence, development and transfer of interface failure presented obvious interval characteristics in time and space.

Multi-scale characterisation of microstructure and texture of 316L stainless steel manufactured by laser powder bed fusion

316L stainless steel produced by additive manufacturing (AM) offers superior mechanical and corrosion properties compared to parts of the same steel processed by conventional methods. However, thorough understanding of the detailed microstructural evolution during various additive manufacturing techniques is lacking when compared to conventionally produced counterparts. As such, the present work contributes to filling this gap in knowledge by using advanced microscopy techniques to analyse the microstructure of 316L stainless steel builds produced by laser powder bed fusion. The results show that changes in processing parameters lead to variations in grain structures, texture, and hardness. For instance, the dominant texture and grain structure is heavily influenced by changes to the laser scanning rotation between individual layers, while increasing laser power reduces hardness. The current study also provides additional insights into how grains in AM 316L steel accommodate strain. Due to the complex strain fields in austenite grains, they are usually split into complex-shaped regions separated by dislocation boundaries with characters similar to deformation- and micro-band boundaries in conventionally deformed austenite. Through electron probe micro-analysis over a large area, the current study reveals the tendency of Mo and Si concentrations to fluctuate around the cell and melt pool boundaries.

The Microstructure and Mechanical Properties of Ferritic-Martensitic Steel EP-823 after High-Temperature Thermomechanical Treatment

The effect of high-temperature thermomechanical treatment (HTMT) with plastic deformation by rolling in austenitic region on the microstructure and mechanical properties of 12% chromium ferritic-martensitic steel EP-823 is investigated. The features of the grain and defect microstructure of steel are studied by Scanning Electron Microscopy with Electron Back-Scatter Diffraction (SEM EBSD) and Transmission Electron Microscopy (TEM). It is shown that HTMT leads to the formation of pancake structure with grains extended in the rolling direction and flattened in the rolling plane. The average sizes of martensitic packets and ferrite grains are approximately 1.5–2 times smaller compared to the corresponding values after traditional heat treatment (THT, which consists of normalization and tempering). The maximum grain size in the section parallel to the rolling plane increases up to more than 80 µm. HTMT leads to the formation of new sub-boundaries and a higher dislocation density. The fraction of low-angle misorientation boundaries reaches up to ≈68%, which exceeds the corresponding value after HTMT (55%). HTMT does not practically affect the carbide subsystem of steel. The mechanical properties are investigated by tensile tests in the temperature range 20–700 °C. It is shown that the values of the yield strength in this temperature range after HTMT increase relative to the corresponding values after THT. As a result of HTMT, the elongation decreases. A significant decrease is observed in the area of dynamic strain aging (DSA). The mechanisms of plastic deformation and strengthening of ferritic-martensitic steel under the high-temperature thermomechanical treatments are also discussed.

Observation of Retained γ Grains in TRIP Steels Using SEM-FIB/EBSD Method and Examination of Stability Evaluation Method

Morphologies of retained γ grains in TRIP steels are a very important factor for understanding the relationship between the microstructure and mechanical properties of TRIP steels. We have investigated a serial sectioning technique using a scanning electron microscope (SEM) equipped with an electron backscatter diffraction (EBSD) and a focused ion beam (FIB) to reconstruct 3D microstructure of TRIP steels. In this paper, FIB fabrication condition dependences of reconstructed 3D structures are presented in TRIP steels and γ-grains stabilities are discussed in terms of ion beam effects. Retained γ-grains are not recognized by EBSD map from a FIB fabricated surface using normal ion accelerating voltage of 30 kV, because γ grains are easily transformed into martensite due to ion irradiation. It was found that the transformation was suppressed by using a low acceleration voltage of 7 kV or less, and the three-dimensional morphology could be observed using low acceleration voltage serial sectioning technique. It becomes possible to evaluate morphology of each phase and we proposed evaluation method of γ stabilities by ion irradiation transformation.

Special Features of Formation of Austenite Grains in Steel 55

Special Features of Formation of Austenite Grains in Steel 55

Computational and experimental studies on the efficiency of Sonchus arvensis as green corrosion inhibitor for mild steel in 0.5 M HCl solution

In this work, the corrosion inhibiting properties of Sonchus arvensis extract (SAE) for mild steel corrosion in 0.5 M HCl solution has been examined by employing weight loss measurement, surface morphologies such as Scanning Electron Microscopy (SEM), UV–vis spectroscopy and Contact angle measurement. The maximum inhibition efficacy of 97% was attained at 298 K temperature at an optimum SAE concentration of 500 ppm. SEM findings demonstrated that a defensive layer was developed on the metallic surface as a result of the validated experimentation, showing the significant inhibitory action of SAE in the corrosion prevention of metal by preventing HCl attacks at steel grain boundaries.

Dislocation-Assisted Particle Dissolution: A New Hypothesis for Abnormal Growth of Goss Grains in Grain-Oriented Electrical Steels

Dislocation-Assisted Particle Dissolution: A New Hypothesis for Abnormal Growth of Goss Grains in Grain-Oriented Electrical Steels

Effect of Prior Austenite Grain-Size on the Annealing Twin Density and Hardness in the Austenitic Stainless Steel

The present study examined the effect of prior austenite grain size on twin density and hardness of austenitic stainless steels (ASS). The 253 MA and 316L ASS were subjected to multi-pass cold rolling to reduce thickness up to 2.3 mm. Subsequently, the rolled steels were heat treated at 1100°C at 0, 900, 1800, 2700, and 3600 seconds in a tubular furnace in a hydrogen atmosphere. At the end of the annealing time, the rolled steel was quenched in the cooled zone of the tubular furnace until it reached room temperature in a hydrogen atmosphere. Then, microstructure observation of ASS was done to identify the austenitic grain size and annealing twin, and a hardness test was performed using the micro-Vickers hardness scale. The line intercept method was used to measure the changes in 253 MA and 316L austenitic grain sizes. ImageJ software was used to measure grain size and twin length. The results showed that austenite grains of both steels grew normally; 253 MA ASS had a lower SFE and K value than 316L ASS, which indicated that 253 MA ASS had sluggish grain growth, smaller grains, more easily formed annealing twins, and higher twin density. The Hall–Petch coefficient, K’, of 253 MA ASS was higher than 316L ASS, which resulted in a higher hardness value. The Sellars, Pande and Hall-Petch models were shown to predict austenite grain sizes, twin density, and hardness in 253 MA and 316L ASS.

Methods for obtaining a dispersed structure and increasing the strength of silicon-manganese steels

For high-strength structural steels, the problem of grinding grain and increasing strength is solved by the use of highly efficient technologies, the development of new steel compositions and the development of rational thermomechanical processing. Goal. The aim of the work is to transform the structure, study the methods of grain grinding and increase the strength properties of structural steels 09Г2, 09Г2С as a result of modification by nanodisperse compositions, heat treatment and intense plastic deformation. Methodology. The research material was structural low-carbon steels 09G2, 09G2S. The process of modifying the steel parameters of the geometric shape of the melts was carried out by smelting steels 09G2 and 09G2C in an induction furnace. The modified workpieces were subjected to intensive plastic deformation and heat-treating treatment according to the mode: heating temperature 1050 °C, exposure 5 min; cooled medium: water and 20 % solution of NaCl in water. Then – a rest at temperatures of 500 °C; 600 °C, exposure time – 30 minutes. Metallographic studies of the structure of steels before and after modification and mechanical testing of standard samples were performed. Results. The study of the structure grains of steels 09Г2 and 09Г2С in the initial state showed the presence of large grains up to 30 μm, reduced microhardness and yield strength. Originality. The substantiation of the choice of type and fraction of nanodisperse modifier was carried out. The use of plasma-chemical synthesis to obtain nanopowders based on titanium was substantiated. Nanopowders of titanium carbonitride Ti (CN) fraction 50 ... 100 nm were obtained by the method of plasma chemical synthesis. Practical value. The following methods were proposed for grinding grain and increasing the strength properties of steels: nanomodification, intensive plastic deformation in combination with heat-strengthening treatment.

Influence of nanomodication on structure for-mation and properties of structural steel

The state of the problem of grinding the grain structure and improving the mechanical properties of low-alloy structural steels has been studied. The state of the problem of grain structure refinement and improving the mechanical properties of low-alloy structural steels has been studied. The role of nanodispersed additives is reduced to the creation of additional artificial crystallization centers in the melt. They must be consistent with the critical radiuses of the embryos. According to our calculations, for the grinding of primary austenite grains in castings, the size of the introduced particles should be 40–50 nm. Output and modified castings of 09G2 and 09G2S steels were subjected to severe plastic deformation by equal-channel angular pressing followed by low-temperature annealing at 350 °C. In the initial state, cast steels 09G2 and 09G2S had a ferrite-pearlite structure with an average primary austenite grain size of 30 μm; after modification and deformation, the grain size was 10 μm. After quenching and cooling in water, the structure has changed insignificantly - ferritic-reed, with an average grain size of ~ 8...10 microns. After cooling the quenched samples in a solution of 20 % NaCl in water, the structure of packet martensite was obtained. In the initial state, the studied steels have insufficiently high property values: microhardness Нμ up to 3000 MPa, yield point σ 0,2 up to 800 MPa. When quenching in water, the hardness somewhat increases, the most significant increase is observed when the samples are cooled in a NaCl solution. Due to the significant grinding of martensite crystals, accelerated cooling provides a greater increase in hardness. A nanodispersed powder of titanium carbonitride Ti (CN) with a fraction of 50...100 nm was obtained by the method of plasma-chemical synthesis, the process technology was developed. Intensive plastic deformation of 09G2 and 09G2S steel castings was carried out. The structure and properties of steels before and after treatments have been studied. As a result of the combination of hardening methods, the grain size of the steels was reduced by 3 times and the yield strength increased from 3000 to 4000 MPa. Nanodispersed powder of titanium carbonitride Ti (CN) with a fraction of 50...100 nm was obtained by the method of plasma chemical synthesis, and a process technology was developed. Intensive plastic deformation of castings of 09G2 and 09G2S steels was carried out. The structure and properties of steels before and after treatments were studied. As a result of a combination of hardening methods, grinding of steel grains by 3 times and increasing the yield strength from 3000 to 4000 MPa was achieved

Formation of {100} Subgrain Variants and Σ3 Variants During Phase Transformation of Columnar Grains in Electrical Steel: Texture Memory and Variant Selection

The columnar grains are composed of near‐{100} subgrain variants, Σ3 variants, and untransformed coarse columnar grains. As the Si content increases from 0.35% to 1%, the untransformed columnar grains are remarkably reduced, but the {100} subgrain variants still exist significantly. The phase transformation strain induced by {100} subgrain variants is large, while that induced by small equiaxed grains, which conform to Σ3 relationship with columnar grains, is lower. During the twice‐phase transformations, even if the variants appear randomly, the probability of occurrence of the {100} variants is 33.3%. Moreover, the {100} subgrain variants mainly appear as single variants because the phase transformation strain of the single variant, which is independent of orientation, is higher than that of the variant pair. Although the strain energy of the {100} subgrains has no advantage, the low‐angle grain boundary interface energy is lower than other high‐angle grain boundaries except for Σ3, which contributes to the appearance of the {100} subgrain variants. The appearance of the Σ3 relationships is to reduce the overall energy, rather than the simple strain accommodation behavior between variants. Beyond that, the actual phase transformation does not follow the strict K−S relationship.

Effect of martensite morphology and volume fraction on the low-temperature impact toughness of dual-phase steels

The influence of martensite morphology and volume fraction on low-temperature impact toughness of dual-phase (DP) steels was investigated by impact tests, nanoindentation tests, atom probe tomography (APT) and microstructural examination. The APT results of medium-C martensite show that many small-sized C clusters are formed at the twin grain boundaries inside the lath, which increase nanohardness and decrease impact toughness. The impact test results indicate that the low-temperature impact toughness of lath martensite is significantly superior to ferrite. Refining the lath martensite substructure can improve the low-temperature impact toughness of DP steels. However, martensite twins obviously reduce the low-temperature impact toughness of DP steels. It is further found that the impact toughness of DP steel is affected by the toughness of ferrite and martensite, and conforms to rule of mixtures. Finally, the low-temperature impact fracture mechanism of DP steels was observed, and it is found that the large-size ferrite grains in DP steels preferentially occur cleavage fracture. As the impact temperature decreases, the fracture mode of DP steel changes from dimple plus quasi-cleavage fracture to brittle cleavage fracture.