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This study investigates the distribution characteristics of residual ferrite in 304L austenitic stainless steel continuous casting slab and the impact of heat treatment processes on its content. Through optical microscopy (OM), thermodynamic calculation software (Thermo–Calc) and heat treatment experiments, it is found that the residual ferrite content along the thickness direction at the width center of the slab exhibits an “M”-shaped distribution—lowest at the edges (approximately 3%) and highest near the center (approximately 13%). Within the triangular zone of the slab, the residual ferrite content varies between 1.8% and 12.2%, with its average along the thickness direction also showing an “M”-shaped distribution; along the width direction, the average residual ferrite content is lower at the edge positions, while within the internal triangular zone, it ranges between 8% and 10%. The ferrite morphology changes significantly across solidification zones: elongated in the surface fine-grain zone, lath-like and skeletal in the columnar grain zone and network-like in the central equiaxed grain zone. Thermodynamic calculations indicate that the solidification mode of the 304L continuous casting slab follows the FA mode. Heat treatment experiments conducted across the entire slab thickness demonstrate effective reduction in residual ferrite content; the optimal reduction is achieved at 1250 °C with a 48 min hold followed by air cooling while preserving the original “M”-shaped distribution characteristic after treatment. Increasing the heat treatment temperature, prolonging the holding time and reducing the cooling rate all contribute to reducing residual ferrite content.

The objective of this study is to investigate the effect of the solidification microstructure of CJ5L Recycled Stainless Steel in the cast state on its thermoplasticity. Therefore, the residual ferrite, solidification structure, and high-temperature thermoplasticity in both Recycled and Non-Recycled steel ingots are examined. The principal experimental techniques employed include SEM, OM, EPMA, and EDS. It was observed that the solidification microstructure underwent a gradual transformation from a dendritic structure with a skeletal shape to a worm-like dendrite as the thickness increased. This resulted in the formation of large equiaxed grains at the center of the steel ingots. The cooling rate decreased from 3~16 °C/s at the surface to below 0.8 °C/s at the center. The residual ferrite gradually transformed from a skeletal to granular and rod-like form with increasing depth, eventually forming a ferrite network at the center of the casting. In the Recycled steel, the composition segregation resulted in the formation of a network ferrite aggregation at the center of the steel ingots. The analysis of microstructure changes in conjunction with thermodynamic calculations revealed that the solidification mode of CJ5L stainless steel underwent a transition from the ferritic–austenitic (FA) mode to the austenitic–ferritic (AF) mode with increasing casting thickness. This resulted in an increase in the amount of residual ferrite from the surface to the center. The high-temperature thermoplasticity analysis of CJ5L stainless steel showed that at temperatures between 800 °C and 900 °C, the casting displayed optimal properties, minimizing crack formation during subsequent processing.

Fracture toughness is an important index related to the service safety of marine risers, and weld is an essential component of the steel catenary risers. In this paper, microscopic structure characterization methods such as scanning electron microscopy (SEM) and electron back scatter diffraction (EBSD), as well as mechanical experiments like crack tip opening displacement (CTOD) and nanoindentation, were employed to conduct a detailed study on the influence of the microstructure characteristics of multi-wire submerged arc welded seams of steel catenary riser pipes on CTOD fracture toughness. The influence mechanisms of each microstructure characteristic on fracture toughness were clarified. The results show that the main structure in the weld of the steel catenary riser is acicular ferrite (AF), but there is also often side lath plate ferrite (FSP) and grain boundary ferrite (GBF). With the increase in the proportion of FSP and GBF in the weld microstructure, the CTOD fracture toughness of the weld decreases gradually. The weld AF is a braided cross arrangement structure, and most of the grain boundary orientation difference is higher than 45°. The effective grain size refinement of AF can effectively prevent crack propagation and significantly improve fracture toughness. GBF is distributed along proto-austenitic grain boundaries PAGB, and the large hardness difference between the GBF and the AF matrix weakens the grain boundary. Cracks can easy be initiated at the interface position of the two phases and can propagate along the GBF grain boundary, resulting in the deterioration of toughness. Although the hardness of FSP is between that of GBF and AF, it destroys the continuity of the overall weld microstructure and is also unfavorable to toughness.

In this paper, the effects of carbon, Si, Cr and Mn partitioning on ferrite hardening were studied in detail using a medium Si low alloy grade of 35CHGSA steel under ferrite-martensite/ferrite-pearlite dual-phase (DP) condition. The experimental results illustrated that an abnormal trend of ferrite hardening had occurred with the progress of ferrite formation. At first, the ferrite microhardness decreased with increasing volume fraction of ferrite, thereby reaching the minimum value for a moderate ferrite formation, and then it surprisingly increased with subsequent increase in ferrite volume fraction. Beside a considerable influence of martensitic phase transformation induced residual compressive stresses within ferrite, these results were further rationalized in respect of the extent of carbon, Si, Cr and Mn partitioning between ferrite and prior austenite (martensite) microphases leading to the solid solution hardening effects of these elements on ferrite.

In the present research work, an effort has been made to examine the effect of the ERNiCrCoMo-1 filler on solidification mechanism, microstructural characterization, welded joint integrity, and residual stresses of the dissimilar welded joint (DWJ) of martensitic grade P92 steel and Ni-based superalloy Inconel 617 for advanced ultra-supercritical (A-USC) power plant application. Weld joints have been fabricated for V groove geometry by using the multipass gas tungsten arc welding (GTAW) process. The multiple aspects of the welded joint structural integrity have been tested by performing the tensile test, microhardness tests and Charpy impact test. The ERNiCrCoMo-1 weld solidified in austenitic mode with columnar and cellular dendrites in the interior region, while columnar dendrites were observed near the interface region. The unmixed zone (UZ) formation was noticed at the ERNiCrCoMo-1 filler weld and P92 steel interface, while the UZ gets eliminated at Inconel 617 interface. The microstructural observation near the interface showed that migrated grain boundaries were observed frequently near the lower region of the weld metal (WM), while at the interface of the P92 steel and ERNiCrCoMo-1 filler welds, higher density of soft δ ferrite patches for the capping and backing passes were observed. The energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD) results confirmed the presence of the Cr- and Mo-enriched M23C6 precipitates, Mo-enriched M6C and Ti-enriched Ti(C, N) precipitates in the WM. Acceptable mechanical properties were obtained at room temperature. The Charpy impact toughness (CIT) was observed 98 ± 5 J and 108 ± 3 J for WM with V notch at the top and root region, respectively. The dramatic reduction in CIT was after the postweld heat treatment (PWHT) was attributed to the evolution of the carbide particles in interdendritic areas. Tensile strength results of the cross-weld specimen showed the tensile strength value marginally lower than the P92 steel but significantly lower than the Inconel 617 base metal in both as-welded (AW) and PWHT condition along with fracture in the week region of P92 steel. The failure from the region of P92 steel instead of the ERNiCrCoMo-1 filler WM confirmed that the welded joint was safe for A-USC power plants boiler application. A significant heterogeneity in microhardness was seen along the weldments with a peak hardness of 445 ± 8 in P92 CGHAZ and a lower hardness of 181 HV in the peninsula. The increase in microhardness of the WM as a result of PWHT was attributed to the evolution of the carbide particles in the WM. Through thickness residual stresses variation was also measured for both WM and HAZ region and the effect of the PWHT on the magnitude and nature of the residual stresses were also performed. Hence the work provides insight into welding procedure development, microstructural evolution in the WM and HAZ, variation in mechanical properties, and residual stresses variation for the welded joint of P92 steel and Inconel 617 alloy.

In this paper, the effect of repair welding heat input on microstructure, residual stresses, and stress corrosion cracking (SCC) sensitivity were investigated by simulation and experiment. The results show that heat input influences the microstructure, residual stresses, and SCC behavior. With the increase of heat input, both the δ-ferrite in weld and the average grain width decrease slightly, while the austenite grain size in the heat affected zone (HAZ) is slightly increased. The predicted repair welding residual stresses by simulation have good agreement with that by X-ray diffraction (XRD). The transverse residual stresses in the weld and HAZ are gradually decreased as the increases of heat input. The higher heat input can enhance the tensile strength and elongation of repaired joint. When the heat input was increased by 33%, the SCC sensitivity index was decreased by more than 60%. The macroscopic cracks are easily generated in HAZ for the smaller heat input, leading to the smaller tensile strength and elongation. The larger heat input is recommended in the repair welding in 304 stainless steel.

Over the years, the high magnetic induction of industrial Mn-added electrical steel is assumed to be the enhancement of {100} texture derived from its austenite–ferrite phase transformation during hot rolling (phase transformation (PT) method). However, it is still undetermined without straightforward experimental evidence. The reason for {100} texture improvement of Mn-added electrical steel is experimentally confirmed due to the recrystallization induced by the austenite–ferrite phase transformation during hot rolling. Moreover, a more promising methodology to further improve {100} texture and formability of hot-rolled electrical steel is promoted by the control of hot rolling deformation condition (shear deformation (SD) method). The results show that the nucleation mechanisms of {100} oriented recrystallized grains are different in the samples by SD and PT methods, which are in-depth shear deformation and austenite–ferrite phase transformation, respectively. In this case, coarse {100} oriented recrystallized grains and low residual stress are obtained in the sample by SD method, which is responsible for its superior {100} texture and formability. In contrast, the sample by PT method forms fine recrystallized grains with random orientations and accumulates severe residual stress.

The influence of annealing treatments at 600°C, 750°C, and 900°C on the alloy structure and memory properties was investigated in this study using a powder-core wire fusion-deposited Fe-14Mn-6Si-9Cr-5Ni alloy, accompanied by a preliminary analysis of the effects of annealing temperature. The results show that compared with the alloy annealed at 600°C, the alloy annealed at 750°C has less crossover of cooling martensite, resulting in improved ordering of stress-induced martensite during pre-deformation, and consequently, enhanced memory properties. At 900°C, ferrite almost completely decomposed, promoting martensitic growth through the crystal, which sharply decreased the memory properties, only slightly better than those of the unannealed alloy. It can be seen that annealing at the appropriate temperature can regulate the morphology and amount of cooling martensite in the additively manufactured Fe-Mn-Si-Cr-Ni alloy, thus improving the memory properties. However, excessively high annealing temperatures can cause excessive decomposition or even disappearance of δ-ferrite, which stabilizes the grain boundaries, thus adversely affecting the memory properties of the alloy.

In the present paper, magnetic Barkhausen noise (MBN) measurements have been carried out to evaluate the hardness and ferrite grain size of API X70 steel. All samples were austenitized at 900–1200 °C for 0.5 h followed by air-cooling identically to develop different ferrite grain size. The microstructure examinations were determined by Scanning Electron Microscope (SEM). The average ferrite grain size in each sample was estimated using ImageJ open-source software. Hardness measurements were performed using durometer device. Measurements of MBN were conducted using MikroMach (Micromagnetic Materials Characterization) system. The microstructure observation shows that the increase in the austenization temperature (AUT) causes an increase in the ferrite grain size as well as their change in shape from polygonal to acicular. The results of mechanical tests showed that the increase in the austenization temperature leads to an increase in the hardness of the X70 steel. Actually, MBN method can be used to evaluate the changes in hardness and ferrite grain size in ferromagnetic materials. The sample with the lowest austenitic temperature has the highest Barkhausen noise amplitude (BNA); in contrast, the sample which contains the highest austenitic temperature has the lowest BNA; furthermore, when the austenization temperatures increases, the signal of the coercive field Hc shifts to the higher values of magnetic field. Additionally, BNA decreases, and Hc increases whenever hardness and ferrite grain size increases. In this way, a good correlation was found between MBN parameters, ferrite grain size, and hardness values. The realized experimental setup can be used for online evaluate steel microstructures and quality control of ferromagnetic materials in some industrial applications.

This study investigates the effects of arc length, current intensity, travel speed, gas flow rate, and pulse time on surface hardness to better understand the arc quenching of S45C steel with a curved shape. With the standard examination method, increasing the current intensity, Travel speed, and arc length causes the surface hardness to decrease. The surface hardness varies depending on the gas flow rate and pulse time. The Travel speed factor appears to have the greatest effect, followed by the gas flow rate and current intensity. Pulse time and arc length are ranked fourth and fifth, respectively, indicating a smaller impact on surface hardness. The microhardness diagram is divided into four stages: improving, rapid dropping, moderate dropping, and stable. The greatest hardness was 576 HV, with a case depth of 1200 μm. The structure of the arc-hardened sample is composed of hardening zones, HAZ, and base metal. The base metal is composed of ferrite and pearlite, which are the original microstructures of medium-carbon steel. The HAZ is made up of two phases: a brown bainite phase and a brighter ferrite phase. Ferrite, bainite, martensite, and residual austenite phases make up the hardening with a high hardness value area. These phases’ diversity results from their rapid heating and cooling rates as well as the significant variations in cooling rates among depths. The findings of the study on the optimum values of factors such as current intensity at 150 A, Travel speed at 150 mm/min, arc length at 2.5 mm, pulse time at 0.6 s, or gas flow at 10.5 l/min can help engineers to have a closer look at the parameters of this arc tempering technology affecting the surface hardness and their applications. Moreover, the hardness measurement value according to the Taguchi method investigation also shows that the highest value of surface hardness achieved is 42.6 HRC compared to 18 HRC of unhardened surface hardness. In addition, the findings on microstructure also help the applicator to better understand and evaluate the quality of this electric arc method for quenching the surface of S45C steel, thereby making it more useful in the industry.