Microstructure Of Steel Research Articles

Using tensile and compressive stress superposition during incremental forming to manipulate martensitic transformation in SS304

Single point incremental forming (SPIF) is a flexible manufacturing process that has applications in industries ranging from biomedical to automotive. In addition to rapid prototyping, which requires easy adaptations in geometry or material for design changes, control of the final part properties is desired. One strategy that can be implemented is stress superposition, which is the application of additional stresses during an existing manufacturing process. Tensile and compressive stresses applied during SPIF showed significant effects on the resulting microstructure in stainless steel 304 truncated square pyramids. Specifically, the amount of martensitic transformation was increased through stress superposed incremental forming. Finite element analyses with advanced material modeling supported that the stress triaxiality had a larger effect than the Lode angle parameter on the phase transformation that occurred during deformation. By controlling the amount of tensile and compressive stresses superposed during incremental forming, the microstructure of the final component can be manipulated based on the intended application and desired final part properties.

Impact of niobium addition and non-metallic inclusions’ characteristics on the microstructure and mechanical properties of low-carbon CrNiMnMoB ultrahigh-strength steel: A comprehensive investigation

The effect of cooling rate and Nb addition on the phase transformation, microstructures, and mechanical properties of thermomechanically controlled hot-rolled low-carbon CrNiMnMoB ultrahigh-strength steel were studied. The aspects impeding Nb's benefits on hardenability, phase transformation, and mechanical properties were investigated. The hot rolling process was simulated, and a continuous cooling transformation diagram was constructed for the studied steels. Microstructures were analysed using laser scanning confocal microscopy and field emission scanning electron microscopy, with the latter also used for in-depth investigations of the non-metallic inclusions. Transmission electron microscopy was employed to investigate the precipitate characteristics. The concentrations of Nb in the solution and precipitates were quantified for the martensitic and granular bainitic structures. The effect of Nb in solution on phase transformation and strengthening is diminished, and the hardness is reduced by Nb precipitation at an early processing stage. Decreasing the cooling rate promotes the formation of granular bainitic structures rather than martensite. After being subjected to hot rolling followed by water quenching and air cooling, the microstructures are autotempered martensite and fully granular bainite, respectively. Various non-metallic inclusions, including Al2O3, MnS, and Al2O3–MnS, MnS·BN, Al2O3·BN and MnS·Al2O3·BN were observed. The number density and area fraction of non-metallic inclusions in the Nb-containing steel and the air-cooled samples are higher than those observed in the Nb-free and water-quenched samples. Adding 0.04 wt% of Nb slightly increased the strength, but the toughness properties dropped, mainly due to the higher number density and area fraction of non-metallic inclusions and primary large precipitates.

EBSD-based image quality analysis of in-situ tempered martensitic steel generated by L-PBF

Microstructures resulting from additive manufacturing technologies, such as laser powder bed fusion (L-PBF), can exhibit both intended and unintended gradation. Intended in form of functionally graded materials (FGM) providing improved material performance. Un-intended in form of part specific local changes due to variations in thermal boundary conditions resulting from the geometry at hand (e.g. slender lattices or downskin areas). To characterize, avoid and optimize such gradations, vast spacial measurements are necessary. This work investigates if electron backscatter diffraction (EBSD) can provide such information, to characterize the microstructural gradation of low alloy steel. Central to this work is the analysis of the image quality (IQ), since the tempering of martensitic steel microstructures results in a reduction of dislocation density, changes in carbide precipitation as well as crystal structure and thus varying IQ. Based on the multi-peak IQ approach and information about the thermal history, the microstructure is considered as purely martensitic. The analysis of single laser track experiments reveals that the impact of cooling rates as well as in-situ tempering effects can be measured, which present the two major effects regarding microstructural formation in L-PBF. Experiments on in-situ and conventionally heat-treated samples show correlating trends regarding tempering states, hardness and IQ distribution. Vast measurements over a sample size of 10 × 10 mm2 demonstrate the possibility to analyze FGM. Furthermore, micro-gradations are observed within each laser track within the material. Consistent trends and correlations indicate that microstructural changes dominate changes in IQ values, while the impact of EBSD parameter and sample preparation appears to be minor. The results promote the suitability of IQ analysis for the spatial characterization of low alloy steel generated by L-PBF.

Metallurgical microstructure classification using CNN: A comprehensive study on heat treatment analysis for steel

Predicting metallurgical microstructures is challenging due to the intricate features that emerge during manufacturing. This research utilizes Convolutional Neural Networks (CNNs) to recognize microstructure images of mild steel alloys, representing diverse microstructures. The study focuses on forecasting steel microstructure properties, specifically identifying and predicting Pearlite (P) and Bainite (B) microstructures after steel heat treatment, using a CNN approach. This comprehensive workflow leads to robust microstructure classification, achieving a model training accuracy rate of 98.95 %. While its final validation accuracy of 98.8 %, highlighting its capability to generate precise predictions across both classes seamlessly. Valuable insights from confusion matrices highlight the model’s performance in distinguishing between “Bainite” and “Pearlite” microstructures. Additionally, the study underscores the significance of CNNs in metallurgical microstructure analysis, with insightful confusion matrices revealing the model’s capability to differentiate between microstructures. This is useful for innovative microstructure analysis in materials science, emphasizing the transformative potential of AI in enhancing our understanding of processing-microstructure relationships in industrial applications.

Mechanical properties and microstructure of high-strength and high-ductility steel at elevated temperature

This paper aims to investigate the mechanical behavior and microstructure of a novel high-strength and high-ductility (HSHD) steel at different temperatures ranging from 25 to 600 °C. The testing machine conducted uniaxial tensile tests with seven different temperatures. The true plastic stress-strain curves of HSHD steel were fitted using the Ludwigson model based on the experimental results, and the microstructure of HSHD steel was analyzed using Electron Backscatter Diffraction (EBSD). The experimental results revealed an obvious temperature-mechanical response relationship in HSHD steel, showing a gradual decrease in tensile strength and yield strength with increasing temperature, while the uniform and post-necking elongation exhibit a trend of decreasing and then increasing. Compared to other steels, HSHD steel exhibited significant advantages of strength-ductility balance over a wide range of temperatures. A Ludwigson model coupled with temperature parameters was established, which provided a good fitting effect to the true plastic stress-strain curves of HSHD steel. Microstructure analysis shows significant Brass and Copper textures near the fracture surface of HSHD steel at all temperatures. Additionally, a large number of deformation twinning was observed within the grain of HSHD steel. The excellent high-temperature mechanical properties of HSHD steel can be attributed to the strong thermal stability of deformation twins.

Effect of applied stress on bainite transformation, microstructure, and properties of 15CrMo steel

The effect of external stress on the isothermal bainite transformation kinetics and microstructure of 15CrMo steel was studied using dilatometer, optical microscopes (OM), electron back-scatter diffraction (EBSD), scanning electron microscopes (SEM) and X-ray diffraction (XRD). The results show that the additional mechanical driving force provided by the external stress promotes the transformation of bainite, resulting an increase volume fraction of bainite-ferrite (BF) and causing the BF laths to become slender. The application of external stress can also induce selective orientation during the bainite transformation process, and the orientation relationship between the prior austenite and the BF under stress is closer to the Kurdjumov-Sachs (K-S) relationship. There is also an incomplete transformation of bainite transformation under stress. At the same time, the Vickers hardness of 15CrMo steel would increase after applying external stress. The mechanical driving forces calculations demonstrate that when tensile stress and compressive stress are of similar magnitudes, they provide comparable mechanical driving forces. As a result, they exert similar effects on the kinetics of bainite transformation, leading to no discernible differences in the resulting bainite microstructure.

Low-carbon steel fatigue behavior after pack carburizing with buffalo bone charcoal and barium carbonate media

The pack carburizing method is a technique that can be employed to enhance the surface hardness of Low-Carbon Steel (LCS). This method can potentially improve the hardness of the material while maintaining its strength. This study aims to investigate how different carburizing media, specifically varying percentages of Buffalo Bone Charcoal (BBC) powder as a carbon source, impact the mechanical properties of LCS. The temperature in the carburizing process ranges from 850°C and 950°C while maintaining a holding time of 2 hours. The carbon derived from buffalo bone charcoal is finely ground and mixed with barium carbonate (BaCO3) as the energizer during this process. This study used different ratios of BBC powder and BaCO3 as carburizing mediums. The ratios tested were 60% BBC + 40% BaCO3, 70% BBC + 30% BaCO3, and 80% BBC + 20% BaCO3, based on the weight of the BBC powder used in the carburizing process. The steel will be combined with BaCO3 and powdered BBC for this research. Next, the fatigue test was examined. BBC in the pack carburizing process increases LCS carbon content by 0.735% from 0.268%. This change alters the steel's microstructure, possibly increasing its hardness and wear resistance. The direct link between BBC concentration and LCS carbon enrichment shows the process's efficiency. Pack carburizing also improves LCS fatigue strength. This improvement is due to higher BBC concentration, smaller carburizing medium particles, and higher processing temperatures. After carburizing, the LCS can resist 36,625 to 61,435 cycles.

How the multi-phase microstructure of a novel tool steel determines its corrosion behaviour in sulphuric acids

The corrosion behaviour of a Fe81Cr15V3C1 steel was analysed compared to commercial X90CrMoV18 (1.4112) in 0.01–1 mol/L H2SO4 solutions. The rapidly cooled steel comprises martensite dendrites, carbide networks (M7C3, MC) and austenite interdendritic areas. Electrochemical measurements with surface analysis (SEM, EDX, AES, GD-OES) revealed the phase impact on corrosion. Active corrosion of austenite occurs at martensite-carbide boundaries leading to narrow bands. A unique oxidation between passivity and transpassivity with dissolution along lamellar carbides and vanadyl(IV) cation release enhances with increasing acid concentration. Both steels exhibit similar corrosion resistance, a superior mechanical performance of Fe81Cr15V3C1 explains its potential for tool manufacturing.

Microstructure and Mechanical Properties of a New TWIP Steel under Different Heat Treatments.

The effects of solution treatment and annealing temperature on the microstructure and mechanical properties of a new TWIP steel that was alloyed from aluminum (Al), silicon (Si), vanadium (V), and molybdenum (Mo) elements were investigated by a variety of techniques such as microstructural characterization and room tensile testing. The austenite grain size grew slowly with the increase in annealing temperature. The relatively weak effect of the solution treatment and annealing temperature on the austenite grain size was attributed to the precipitation of MC and M2C, which hindered the growth of the austenite grain. The plasticity of the TWIP steel in cold rolling and annealing after solution treatment was obviously higher than that in cold rolling and annealing without solution treatment. This was because the large-size precipitates redissolved in the matrix after solution treatment, which were not retained in the subsequently annealed structure. Through cold rolling and annealing at 800 °C after solution treatment, the prepared steel exhibited excellent strength and plasticity simultaneously, with a yield strength of 877 MPa, a tensile strength of 1457 MPa, and an elongation of 46.1%. The strength improvement of the designed TWIP steel was mainly attributed to the grain refinement and precipitation strengthening.

Effect of quenching and tempering treatments on microstructure and mechanical properties of 300M ultra-high strength steel fabricated by laser powder bed fusion

To achieve high-performance 300 M steel parts through the Laser Powder Bed Fusion (LPBF) additive manufacturing process, post-heat treatments are indispensable. Herein, the effects of quenching and tempering treatments on the microstructure and mechanical properties of LPBF-fabricated 300M ultra-high strength steel were investigated, and the relevant strengthening and toughening mechanisms were analyzed. The microstructure of quenched samples primarily consisted of quenched martensite with minor amounts of bainite and some grain boundary allotriomorphic ferrite. The quenched samples presented significantly higher strength than the 300M forgings, primarily attributed to the contributions of solid solution strengthening, grain boundary strengthening and dislocation strengthening. After following tempering treatment, the microstructure of LPBF-fabricated samples mainly consisted of tempered martensite with minor amounts of bainite and residual austenite. Compared to the quenched samples, the tempered samples exhibited a decrease in strength while demonstrating a significant increase in ductility. The samples treated with the optimal heat treatment process (900 °C/1 h/Oil Cooling for quenching + two times of 300 °C/2 h/Air Cooling for tempering) demonstrated a remarkable strength–ductility balance, with a yield strength (YS) of 1834 MPa, an ultimate tensile strength (UTS) of 2099 MPa, and an elongation (EL) of 10.2%, which are all beyond or comparable to those of 300M forgings.

Effect of microstructure on sag resistance of ultra-high strength spring steel evaluated by hysteresis loop

Sag refers to the reduction in the free height of coil springs during actual usage, hence sag resistance is one of the crucial properties of spring steel and its improvement is essential for prolonging the service life of springs. However, limited research has been conducted on the effect of microstructure on sag resistance in recent years. This study aims to investigate the relationship between microstructure and sag resistance in 65SiCrV6 spring steel. The steel was quenched from various temperatures to obtain microstructures with varying sub-block size, and tempered at different temperatures to obtain microstructures with distinct tempered carbides morphology. The microstructures were characterized and the sag resistance was evaluated by hysteresis loop effect formed during cyclic tensile test. The results revealed that smaller sub-block sizes of martensite and higher proportions of high-angle grain boundary (HAGB) are beneficial for improving sag resistance. Incoherent granular cementite precipitated in the tempered microstructure improves sag resistance by enhancing interactions with dislocations, and coarsened cementite does not reduce the sag resistance due to the generation of back stress in heterogeneous microstructure. The hysteresis loop effect resulting from heterostructure as well as interactions between dislocations and tempered carbides increases with rising tempering temperature until it reaches a saturation state. This study highlights the importance of optimizing the microstructure with both small sub-block size and incoherent cementite precipitation through appropriate heat treatment process to enhance sag resistance, meanwhile provides valuable insights into understanding the micromechanism of sag resistance in spring steel.

Directed energy deposition of 18NiM300 steel: effect of process and post processing conditions on microstructure and properties

ABSTRACT This current study investigates the effect of Direct Energy Deposition (DED) process conditions on the properties and microstructure of M300 maraging steel samples. The investigation centers on two key factors: laser power and deposition environment. The microstructure of this tool steel is analyzed by computing the Primary Cellular Arm Spacing. The findings revealed a significant influence of both inert atmosphere and laser power on cooling conditions. These different cooling rates influence the phase content as demonstrated by X-Ray Diffraction and Electron Backscatter Diffraction measurements. It was demonstrated the presence of different content of residual austenite at cell boundaries. These distinct microstructural features caused variations in the hardness values of the printed samples. Furthermore, a direct aging heat treatment was implemented, that was chosen from Differential Scanning Calorimetry measurements results. This heat treatment proves effective in achieving consistent hardness increases and eliminated the differences among samples built in different process conditions. This outcome suggests the possibility of selecting the most economically viable DED parameters for optimal results.

Electrochemical insight into the passivity and corrosion of 316 L stainless steel fabricated through wire arc additive manufacturing

This study explores the microstructure of AISI 316L stainless steel fabricated by wire arc additive manufacturing (WAAM) and establishes correlations with its passivity and corrosion characteristics in a 0.9 wt% NaCl solution, while making comparisons with the wrought alloy counterpart. The WAAM-fabricated alloy demonstrates a favorable chemical composition in comparison to the wrought alloy, exhibiting a heterogeneous microstructure comprised of residual delta ferrite within the austenitic matrix. The pitting vulnerability of the WAAM-printed alloy is observed to be lower than that of the wrought counterpart, a phenomenon further investigated through passive film behavior analysis. Mott-Schottky analysis and the point defect model (PDM) revealed the development of a less defective passive layer on the WAAM-fabricated alloy, enhancing overall passivity and electrochemical response, attributed to the interplay between microstructural features and chemical composition.

Effect of annealing temperature on the evolution of microstructure, texture, and mechanical properties of hot-rolled 12Cr-ODS steel

To achieve high thermal efficiency, today's nuclear reactor structures are exposed to higher temperatures and pressures, which requires the use of high-strength steels with specific properties, such as ODS steels. There is a need to clarify the evolution of the microstructure and properties of steels at elevated temperatures. This study systematically investigates the evolution of microstructure, texture, and mechanical property variations of 12Cr-ODS steels after hot-rolling and subsequent annealing at 1000 °C and 1200 °C. The investigation utilizes optical microscopy, electron backscatter diffraction technique, and mechanical property measurements. The microstructure of hot-rolled samples shows a layered alternating distribution, which is distinguished by a notable presence of the α-fiber texture. Following annealing at 1000 °C, the martensite grains are smaller with a reduced hardness, while maintaining a strong α-fiber texture. After annealing at 1200 °C, there is a rapid increase in the growth of martensite grains, a significant rise in hardness, a reduction in the α-fiber texture characteristics, and an improvement in the γ-fiber texture characteristics. Moreover, the maximum intensity of the α-fiber texture diminishes as the annealing temperature increases. The mechanical properties of the samples deteriorated after annealing at 1200 °C, which can be attributed to the coarse martensite grains and the texture components containing the {001} cleavage plane dominating the occurrence of brittle cleavage fracture. The 0.1Y sample after annealing at 1000 °C exhibits an excellent combination of strength (1458 MPa) and ductility (20.3%), which is owing to the unique heterogeneous grain structure and the evolution of favorable texture.

Corrosion Study on Duplex Stainless Steel UNS S31803 Subjected to Solutions Containing Chloride Ions.

In the present work, the influence of a corrosive environment and temperature on the corrosion resistance properties of duplex stainless steel S31803 was evaluated. The corrosive process was carried out using solutions of 1.5% HCl (m/m) and 6% FeCl3 (m/m), at temperatures of 25 and 50 °C. The microstructure of UNS S31803 duplex stainless steel is composed of two phases, ferrite and austenite, oriented in the rolling direction, containing a ferrite percentage of 46.2% in the rolling direction and 56.1% in the normal direction. Samples, when subjected to corrosive media and temperature, tend to decrease their mechanical property values. It was observed, in both corrosive media, that with increasing test temperature, there is an increase in the corrosion rate, both uniform and pitting. The sample in HCl solution obtained a uniform corrosion rate of 0.85% at 25 °C and 0.92% at 50 °C and pitting rates of 0.77% and 1.47% at the same temperatures, respectively. When tested in FeCl3 solution, it obtained uniform corrosion of 0.0006% and 0.93% and pitting of 0.53% and 18.5%, at the same temperatures. A reduction in dissolution potentials is also noted, thus characterizing greater corrosion in the samples with increasing temperature.

Effect of boron on microstructure and mechanical properties of Al-killed high-strength low-alloy steel

Through the application of experimental analysis and mechanical characterisation, the influence of boron on the microstructure, strength and impact toughness of Al-killed high-strength low-alloy steel was examined. The results showed that with increasing boron content, the area fraction and average size of martensite–austenite constituents increased by 11.8% and 2.04 μm, respectively. Owing to the strengthening and combining effects of boron, the proportion of low angle grain boundaries in the boron-containing steel increased from 67.94% to 79.41% in comparison to the boron-free steel. After adding 0.0014% boron, the tensile and yield strength increased by 14.65% and 3.66%, respectively, due to the increasing content of martensite–austenite constituents. The long strip and large-sized martensite–austenite constituents of the steel matrix with 0.003% boron easily became the source of cracks and facilitated crack propagation, resulting in a decrease in strength. The toughness tested at −20 °C of the steel was severely worsened after adding boron, and the impact absorbed energy decreased from 105 to 7.4 J. This was because boron increased the martensite–austenite constituents, low angle grain boundaries and random boundaries, which favoured crack generation and propagation during the impact process, resulting in a sharp deterioration of toughness.

Experimental Investigation on Microstructure and Mechanical Properties of 15CDV6 High Strength Low Alloy Steel Welded Using Spin Arc Gas Metal Arc Welding Process

Experimental Investigation on Microstructure and Mechanical Properties of 15CDV6 High Strength Low Alloy Steel Welded Using Spin Arc Gas Metal Arc Welding Process

Effect of Heat Treatment on Magnetic Properties of EN8D Steel Used in Automotive Applications

The present study focuses on achieving the optimum combination of mechanical and magnetic properties for EN8D steel. SAE 1010 and EN8D steels are the candidate materials to manufacture the fixed nucleus of the disk-type electro-mechanical horn. Both materials have either good magnetic or mechanical properties. The main function of fixed nucleus is to get magnetized and pull the mobile nucleus towards it. However, as magnetization and demagnetization take place in the range of 300-500 Hz, fixed and mobile nucleus continuously strikes each other at high frequency. Due to such high frequency working condition, fixed nucleus is prone to failure under wear and tear or under fatigue. For this kind of application, a combination of magnetic and mechanical properties is required in the material used to manufacture fixed nucleus. Thus, in current paper, the effects of chemical composition, microstructure, hardness, strength and toughness of EN8D steel before and after austenizing and tempering heat treatment are evaluated and compared with as-rolled EN8D and SAE 1010 steels, which are mostly used for manufacturing fixed nucleus. Magnetic permeability was evaluated using JMatPro software and compared. Austenizing treatment is performed at 900 °C for 1 hour, followed by water quenching. Tempering treatment of the same was performed at 550 °C for 2.5 hours. The Ultimate Tensile Strength (UTS) and hardness of EN8D after heat treatment decreased from 917 to 781 MPa and 269 to 233 HV0.1 respectively. Charpy V-Notch (CVN) toughness of EN8D steel after heat treatment increased from 12 to 45 joules. It indicates that the decrease in strength and hardness resulted into increase in toughness. Microstructure changes from ferrite and pearlite to combination of tempered martensite, ferrite and bainite. Magnetic permeability evaluated by JMatPro software shows increment from 450 to 564 Gauss/Oersted in EN8D steel after heat treatment.
 Keywords: Electro-mechanical horn, fixed nucleus, magnetic permeability, JMatPro, SAE1010 steel, EN8D steel

Effect of Ce content on the microstructure and TiN precipitation behaviour of high-titanium steel during slow cooling solidification

ABSTRACT This study used high-temperature melting experiments, optical microscopy, and scanning electron microscopy to investigate the effects of various Ce contents (0 wt.%~0.0165 wt.%) on the microstructure and TiN precipitation behavior in high-titanium steel during slow cooling solidification. The findings show that the microstructure of the solidified steel in each experiment are equiaxed grains at a cooling rate of 0.17 ℃/s. TiN mainly precipitates in the Liquid + δ two-phase region in this experimental steel. CeAlO3 and Ce2O2S precipitate before TiN, and their small lattice misfit with TiN (7.55% and 7.90%, respectively) are the primary cause of their propensity to act as the cores of TiN heterogeneous nucleation. The TiN area density in each experimental steel increases from 38.6 no./mm2 → 61.4 no./mm2 → 89.3 no./mm2 → 105.8 no./mm2 when the Ce content in the steel rises from 0 wt.% → 0.0015 wt.% → 0.0058 wt.% → 0.0165 wt.%.

Comparison in microstructure, mechanical properties, and fatigue behavior of 9Cr3W3Co turbine rotor steel and its welded joint

AbstractThe microstructure, mechanical properties, and high/very‐high cycle fatigue behaviors of 9Cr3W3Co turbine rotor steel and its welded joint were investigated. The welded nugget zone (WNZ) exhibited the highest microhardness value, followed by the heat‐affected zone (HAZ), while the base material (BM) showed the lowest hardness value. The WNZ exhibited a higher yield strength compared to the BM with reduced elongation, while maintaining similar ultimate tensile strength. The fatigue strength of the BM at 5 × 107 cycles is roughly 53.7% of the yield strength, whereas that of the WNZ increases to approximately 55.0%. In comparison to the fatigue strength of the BM at 5 × 107 cycles, the WNZ exhibits a notable increase of 10.5%. Smaller grain size and higher dislocation density were observed in the WNZ, which contributed to improved microhardness, yield strength, and fatigue resistance through grain boundary strengthening and dislocation hardening.