Ferrite Austenite Research Articles

Wire arc additive manufacturing of functionally graded carbon steel - stainless steel 316L - Inconel 625: Microstructural characterization and mechanical behavior

This work utilized a gradient method of joining plain carbon steel to stainless steel 316 L and then to Inconel 625 using wire arc additive manufacturing. The research investigated the quality of Functionally Graded Materials (FGM) structure, continuity, defect formation, microstructure, and mechanical properties of gradient regions. The investigation showed a strong, defect-free metallurgical bond between plain carbon steel and stainless steel 316 L and stainless steel 316 L and Inconel 625. The microstructure of stainless steel 316 L resulted from the solid-state transformation of ferrite-austenite (FA), with a significant presence of delta ferrite in the austenite matrix. In Inconel 625, the Laves intermetallic phase formed discontinuously between dendritic arms due to the microsegregation of alloy elements like niobium and molybdenum during solidification. The hardness values of Inconel 625, stainless steel 316 L, and plain carbon steel were 194–257 HV, 171–178 HV, and 159–170 HV, respectively. The ultimate tensile strength, yield strength, and elongation were achieved at 487 ± 10 MPa, 300 ± 6 MPa, and 40 % ± 0.15, respectively. The tensile test samples failed on the plain carbon steel side, indicating higher tensile strength at the interface and a well-bonded joint between the two alloys. Small, homogeneous dimples on the fracture surface confirmed the ductile fracture mode. The research demonstrates the use of wire-arc additive manufacturing (WAAM) to fabricate gradient materials with the required properties.

Microstructure Evolution and Mechanical Properties of Ferrite–Austenite Stainless Steel Bimetals Fabricated via Wire Arc Additive Manufacturing

A bimetallic structure is implemented by wire arc additive manufacturing from 316L austenitic stainless steel to 430 ferritic stainless steel. Microstructure, interface characteristics, and tensile properties of 316L‐430 bimetals are investigated. In 316L/430 bimetals, defects like porosity and cracks are not present. The compositional distribution and microstructure evolution of 316L‐430 bimetallic deposition are significantly impacted by the deposition sequence and heat input. As heat input increases, microstructure evolution of 430 deposited layer is single F → F+ diffusely distributed (Cr, Fe)23C6 → F+ grain internal (Cr, Fe)23C6 → F + M, and 316L deposited layer presents typical austenite + skeletal δ ferrite. Microstructure of 430 deposited layers presents heterogeneous, grain coarsening, and martensite along grain boundaries, resulting in high‐temperature embrittlement. The highest hardness of 316L‐430 bimetals appears on 430 sides near the interface. The tensile strength of 430/316L bimetals is greater than that of 316L/430, and the tensile specimens exhibit brittle fracture.

Studies on welding of thin stainless steel sheets with pulsed nanosecond fiber laser in butt joint configuration

In this work, we report full penetration welding of 1.6 mm thick AISI 304L stainless steel sheets in a butt joint configuration using a pulsed nanosecond fiber laser of an average power of 200 W. The welding was carried out by a focused laser beam oscillating in a circular path. The effects of beam oscillation parameters, e.g., amplitude, frequency, and weld speed, on weld morphology and microstructure were studied. Electron back scattered diffraction was used to characterize the weld microstructure and to map the distribution of austenite and ferrite phases in the weld. The solidification mode of the weld was found to change from the equilibrium FA (Ferrite-Austenite) to AF (Austenite-Ferrite) to A (Austenite) on an increase in the cooling rate with a concomitant drop in the fraction of δ-ferrite. The welds were found to be without any cracks with the sporadic presence of porosities. The welds were found to be mechanically strong.

Friction welding process with SFPC consumable pin applied on stainless steel duplex SAF 2205

The objective of this work is to investigate the use of Friction Hydro Pillar Processing (FHPP), in the Duplex Stainless Steel- Sandvik Austenite Ferrite 2205 (AID SAF 2205), with low Axial Force of application and to qualify it in a similar way to repairs by conventional welding processes, through standard tests foreseen in norms. In addition, it is intended to go beyond conventional tests and bring new information about precipitated phases during processing, their consequences on microstructural evaluation and corrosion performance. Through the FHPP there are the advantages of: being able to repair the structure with the plant and/or the equipment in operation, as it does not involve melting the material, thus allowing a low structural distortion in addition to being a typically surgical type of repair where little material needs to be removed to perform the repair.

Revealing the kinetics of non-metallic inclusion reactions in steel using in-situ high temperature environmental scanning electron microscopy

The kinetics of solid-state reactions involving non-metallic inclusions (NMIs) in steel is a topic of growing interest due to the need to understand the effects of heat treatments on NMI composition and stability. In this study, a High Temperature Environmental Scanning Electron Microscope (HT-ESEM) is utilized to conduct in-situ observations of various types of NMIs, including calcium aluminate complex, CaS, MgAl2O4-CaS, MnS, and TiN, in steels at temperatures of 800 °C and 900 °C for different holding times. The observed morphological variations in the NMIs indicate a decrease in the fractions of CaS and MnS, promoting void formation and ferrite–austenite phase transformation. To estimate the NMI reactions with the residual atmosphere in the sample chamber during experiments, HT-ESEM-driven thermodynamic calculations have been performed using FactSage 8.2 software. These calculations provide valuable insights into the interpretation of the experimental data, facilitating a better understanding of the complex NMI reactions in steel.

Mechanical Properties and Microstructure of Austenite—Ferrite Duplex Stainless Steel Hybrid (Laser + GMAW) and SAW Welded Joint

The purpose of the research was to develop a technology for producing thick-walled duplex steel welded joints. The material used in the research was X2CrNiMoN22 duplex steel in the form of a 15 mm thick plate. The welded joint was produced by the modern, high-performance Hybrid Laser Arc Welding (HLAW) method. The HLAW method involves welding a joint using a laser, the Gas Metal Arc Welding (GMAW) method and the Submerged Arc Welding (SAW) method. The HLAW method was used to make the root pass of the double butt welded joint, while the filler passes were made by the SAW method. The obtained welded joint was subjected to non-destructive and destructive testing. The non-destructive and macroscopic tests allowed the joint to be classified to the quality level B. Microscopic examinations revealed the presence of ferritic-austenitic microstructure in the base material and the weld, with different ferrite content in specific joint areas. The analysed joint had high strength properties (tensile strength (TS) ~ 790 ± 7 MPa) and high ductility of weld metal (~160 ± 4 J) heat-affected zone (~216 ± 26 J), and plasticity (bending angle of 180° with no macrocracks). At the same time, hardness on the cross-section of the welded joint did not exceed 280 HV10.

Delta ferrite formation and evolution during slab processing from an 80-ton industrial heat of AISI 304 austenitic stainless steel

Although delta ferrite is a very common phase in most austenitic stainless steels, studies about its formation and evolution during slab processing from industrial heats of tens of tons are scarce. The main objective of this research is to study the evolution of delta ferrite (quantity, chemical composition, morphology, and distribution) along the production route from the cast slab to the coil of an industrial heat of 80 tons of 304 stainless steel. Samples were extracted after the following processing steps: continuous casting, first and second hot-rolling pass, and solution-heat-treating, arriving at the final commercial condition. Sample analyses were carried out with several complementary microstructural characterization techniques: optical microscopy, scanning electron microscopy with energy dispersive spectroscopy (EDS), X-ray diffraction, and magnetic measurements of delta ferrite content (feritscope). Thermocalc® indicates that the present continuous cast slab solidifes according to the FA (ferrite-austenite) mode and the final microstructure should be completely austenitic in equilibrium conditions. Nevertheless, delta ferrite is detected along the processing steps, indicating that the steel is out of phase equilibrium. The ferrite content measured after solidification varies significantly across the as-cast slab thickness. Lower values are detected on the surfaces, followed by a gradual increase when moving into the slab, reaching a peak, and finally decreasing at the slab center. This pattern of delta ferrite content is named “M type” distribution. The average content of delta ferrite decreases after each subsequent processing step, namely the two hot-rolling passes and the solution heat-treating.

Investigation of microstructural and corrosion behavior of 316LSi structure developed by wire arc additive manufacturing

SS316LSi austenitic stainless steel thin wall was deposited using wire arc additive manufacturing technology, which is known for its high deposition rate and effective material utilization. The optical micrograph revealed the formation of vermicular δ-ferrite, dispersed in the austenite matrix in the deposited wall. The X-ray diffraction patterns confirmed the formation of γ-austenite, δ-ferrite, and chromium carbide (Cr23C6) phases. The SEM and energy dispersive spectroscopy study exhibited the formation of intergranular precipitates. In the magnetic study, ferrite number was found from 3 to 7 in a different portion of the wall which revealed the formation of the ferromagnetic δ-ferrite phase. The Cr and Ni equivalent ratio, 1.66, ferrite number, and presence of vermicular δ-ferrite in the microstructure showed that ferrite austenite can be the mode of solidification in the alloy. The preferred grain growth direction was determined by electron backscattered diffraction analysis to be < 001 > orientation along the build-up direction. In the potentiodynamic polarization and double loop electrochemical potentiodynamic reactivation study, the corrosion rate and intergranular corrosion were correlated to the amount of δ-ferrite and Cr23C6 respectively. The degree of sensitization confirmed that the amount of Cr23C6 formed is least in the lowermost and highest in the central portion of the deposited wall.

Microstructure and Mechanical Properties of P21-STS316L Functionally Graded Material Manufactured by Direct Energy Deposition 3D Print

Functionally graded materials (FGMs) have a characteristic whereby the composition and structure are gradually changed according to the location, and the mechanical properties or chemical properties are gradually changed accordingly. In this study, using a multi-hopper direct energy deposition 3D printer, an FGM material whose composition changes gradually from P21 ferritic steel to stainless steel 316L austenitic steel was fabricated. From optical microscope, scanning electron microscope, and X-ray diffraction analysis, columnar, cell, and point type solidified micro-structure and precipitations were observed depending on the deposited compositions. Electron probe microanalysis and electron backscatter diffraction analysis confirmed the component segregation, ferrite austenite volume fraction and phase distribution behavior according to compositions. In the FGM specimen test, the ultimate tensile strength of STS316L, which was the most fragile, was measured, and the toughness was measured for the notch area, which did not represent the FGM characteristics. Hardness showed changes according to FGM position and was suitable for FGM analysis. The maximum hardness was measured in the FGM duplex area, which was caused by grain refinement, precipitate strengthening, and solid solution strengthening. In nuclear power plant welds high strength can cause adverse effects on stress corrosion cracking, and caution is needed in applying FGM.

Constitutive Flow Equation for Cast Austenite–Ferrite Stainless Steels

Constitutive Flow Equation for Cast Austenite–Ferrite Stainless Steels

Effect of Microstructure Morphology of Q&amp;P Steel on Carbon and Manganese Partitioning and Stability of Retained Austenite

In this study, we used 0.2C-1.7Si-1.9Mn wt% cold-rolled sheet as the experimental material to prepare the Q&amp;P sample with blocky microstructures and the QQ&amp;P sample with lath-shaped microstructures through the Q&amp;P and QQ&amp;P processes, respectively. The partitioning behavior of carbon and manganese in the two samples after intercritical annealing and partitioning were studied. During the intercritical annealing, the partitioning of carbon and manganese in the Q&amp;P and the QQ&amp;P samples occurred, resulting in the contents of carbon and manganese being significantly higher than those in the ferrite. Meanwhile, due to the migration of the ferrite–austenite interface during the formation of the austenite, the distributions of carbon and manganese in the lath-shaped and blocky austenite were both homogenous. The morphology of the microstructures had little influence on the distribution of carbon and manganese in metastable austenite during intercritical annealing. In the partitioning, the migration of the ferrite–austenite interface and diffusion of manganese can be ignored. Carbon first diffuses from the ferrite grains to the ferrite–austenite interface and then diffuses in the austenite grains. The morphology of the microstructures has a great effect on the homogenization of carbon in austenite grains. Compared with coarse blocky austenite, lath-shaped austenite can shorten the diffusion path of carbon in austenite grains and increase the homogeneity of carbon in austenite grains, thereby improving the thermal stability of lath-shaped austenite. Compared with the Q&amp;P sample, the QQ&amp;P sample has higher content of retained austenite (14.74% vs. 13.96%), better elongation (25.9% vs. 19.2%), and higher product of strength and elongation (27.5 GPa% vs. 24.4 GPa%).

Effects of Rare Earth on Austenite–Ferrite Phase Transformation in a Low-Carbon Fe–C Alloy

Effects of Rare Earth on Austenite–Ferrite Phase Transformation in a Low-Carbon Fe–C Alloy

Direct observation of solidification behaviors of Fe-Mn-Si alloys during TIG spot welding using synchrotron X-ray

To perform a detailed investigation of the solidification behaviors of alloys during welding, it is crucial to observe the dendrite-scale microstructural evolution. Therefore, by utilizing high-spatial resolution X-ray imaging and diffraction, in this study, a successful technique was developed for the time-resolved and in situ observation of δ- and γ-Fe dendrite growth and solidification cracking behaviors of Fe-Mn-Si alloys during TIG spot welding. For the specimen solidifying in the austenite (A) mode, solidification cracking initiated at the center of the weld pool and propagated toward the weld pool boundary through high-angle dendrite boundaries. However, for the specimen solidifying in a unique ferrite-austenite (FA) mode, many equiaxed γ-Fe dendrites nucleated in front of the growing columnar δ-Fe dendrites. Solidification cracking propagation was suppressed by the numerous equiaxed γ-Fe dendrites that developed at the center of the weld pool. Therefore, the solidification mode significantly influenced the solidification cracking susceptibility.

Microstructure and coordination mechanism of interface of stainless steel/carbon steel cladding plate prepared by vacuum diffusion bonding

Vacuum diffusion bonding (VDB)-prepared 304 austenite stainless steel (SS)/Q235 carbon steel (CS) clad plates were the focus of this study. Specifically, the metallurgically bonded interfaces, which are known to have good mechanical properties, were investigated. The microstructures of the interfaces were extensively investigated using transmission electron microscopy (TEM). The characterization results revealed the existence of annealing twins and lath martensite precipitates at the interface; the high strength and plasticity at the interface were determined to be attributed to these features. In addition, the distribution and Burgers vector of dislocations at the ferrite–austenite interfaces were investigated by using two-beam method of TEM electron diffraction contrast image (EDCI) technology and high-resolution TEM imaging. As a result, the coordination mechanisms at the ferrite–austenite interfaces during the deformation of the SS/CS clad plates were clarified; the observations include: (1) the perfect dislocation with the Burgers vector a2⟨111⟩ in the ferrite moved across the interface and formed a perfect dislocation with the Burgers vector a2⟨110⟩ in the austenite; (2) the ferrite lattice was joined to the perfect dislocation with the Burgers vector a2⟨110⟩ in the austenite; and (3) the ferrite lattice continuously extended across the interface, and was connected to the austenite lattice. According to the results on the Burgers vector of dislocations, two types of dislocation reactions occurred at the ferrite–austenite interfaces during the deformation. Lastly, the transfer matrix of the orientation relationship (OR) between austenite and ferrite was calculated. The results revealed that the OR differed from that of the conventional OR between body-centered cubic and face-centered cubic structures; thus, it can be concluded that the preparation of SS/CS clad plates via VDB technology may lead to the formation of a unique OR between ferrite and austenite.

Copper and its effects on microstructure and correlated tensile properties of super duplex stainless steels

In the last few years, Cu is handled as a promising element to improve the corrosion resistance of duplex steels. Although there is a very limited number of studies in the literature describing the influence of Cu on the microstructure of duplex steels in the aged condition, to the authors' knowledge, no comprehensive study has been presented so far that describes in detail its influence on the microstructure and correlated mechanical properties in the solution-annealed condition. Consequently, this work is intended to fill that gap to provide a fundamental base for material design of novel duplex steels. Microstructural investigations showed a preferred formation of austenite combined with an elemental redistribution of Cr and Mo. Especially at the highest Cu content investigations revealed precipitation of Cu particles causing – so far unknown – an intragranular austenite in ferrite. It is proposed that the enrichment of austenite forming elements, i.e. Ni, at their phase boundaries as well as a low misfit between Cu particles and the intragranular austenite nuclei play a significant role during its nucleation. Due to the identified microstructural changes triggered by Cu an increased imbalance of the flow stress between ferrite and austenite, and thus a decrease of the macroscopic yield strength can be proposed. In turn an increased work hardening with Cu addition causes an unaffected ultimate tensile strength.

Study on phase transformation of CMnSi steel when heat treatment

After manufacturing, if the CMnSi steel was heat treatment, it would create the multi-phase microstructure consists of highly ductility ferrite matrix, martensite, bainite and amounts of austenite. Thereby, the strength and ductility of the steel were improved. In the process of improving the quality of steel, there will be two processes: the plastic deformation process and the heat treatment process. In this paper, we present the study on the microstructure and mechanical properties of CMnSi steel which was heated. The heat treatment process of CMnSi steel is a special heat treatment process including the process of heating the steel to austenite temperature at 900 °C then keeping the heat to ensure uniformity of steel. This steel was cooled quickly from austenite temperature to phase transformation temperature which had bainite transformation equal to about 400 °C (this temperature is determined by CCT diagram). The results of microstructure analysis show that by the heat treatment process, the microstructure of steel is included three main phases: ferrite, bainite, and residual austenite. The results of mechanical tests show that after the heat treatment, the strength limit of steel is 1141 MPa, the elastic limit is 943 MPa and the elongation is 36%.

Experimental investigation on dissimilar weld between super duplex stainless steel 2507 and API X70 pipeline steel

This work investigates the microstructure and mechanical properties of 2507 super duplex stainless steel and API X70 high strength low alloy steel weld joint. This joint finds application in offshore hydrocarbon drilling riser and oil–gas pipelines. Coated shielded metal arc welding electrodes have been designed and extruded on 309L filler and their performance compared with a commercial austenitic electrode E309L. Filler 309L solidifies in ferrite-austenite (F-A) mode with a resultant microstructure comprising skeletal ferrites with austenite distributed in the interdendritic region. Results of tensile and impact tests indicate that weld fabricated with laboratory-developed electrodes has higher ductility and impact energy than the commercial electrode. The tensile strength and weld hardness of commercial electrodes are superior. The laboratory-made electrode’s microhardness is lower than the commercial electrodes, making the former less prone to failure. An alternative welding electrode coating composition has been suggested through this work and found to be performing satisfactorily and comparable to the commercially available electrodes.

Comparisons of Different Oxide Fluxes in Activated Gas Tungsten Arc Welding of Duplex Stainless Steels for Improved Depth of Penetration and Pitting Corrosion Resistance

ABSTRACT The present work analyses the effect of five types of oxide fluxes – SiO2, TiO2, CrO3, MnO2 and MoO3 – on the depth of penetration and pitting corrosion behaviour of UNS S32205 duplex stainless steel produced with the activated tungsten inert gas welding (A-TIG) process. Five bead-on-plate (BoP) weld samples prepared autogenously with constant weld parameters. Depth of penetration (DP), depth-to-width (D/W) ratio and weld bead width (BW) were measured and compared with reference to the weld thermal cycle. The test results of ferrite measurement and micro-hardness were compared with normal – without flux added TIG-welded sample. Macro analysis of all samples indicates that SiO2 and TiO2 flux offer enhanced penetration depth and D/W ratio, in contrast, to use of other fluxes. Possibly, under the effect of either reversed Marangoni or arc constriction or both, however, all TIG BoP weld samples exhibited poor pitting corrosion resistance. Enhanced heat input due to flux and thus resultant unbalanced ferrite austenite proportion affected pitting corrosion resistance. ASTM A 923 microstructures did not show any presence of nitride formation. Results of energy-dispersive X-ray spectroscopy and scanning electron microscopy analysis of corrosion pits showed no abnormality with the chemistry of standard 2205 DSS.

Enhancement of Strength–Ductility Balance of the Laser Melting Deposited 12CrNi2 Alloy Steel Via Multi-step Quenching Treatment

A ferrite–austenite 12CrNi2 alloy steel additively manufactured by laser melting deposition (LMD) was heat treated by direct quenching (DQ) and tempering inter-critical quenching (TIQ) at 800 °C for enhancing its strength–ductility balance. Both heat-treated alloy steels have the martensite–ferrite dual-phase (DP) microstructures. The volume fractions of martensite in the two treated alloy steels are nearly similar (~ 85 vol%), while the sizes of the prior austenitic grain for martensite are different. The martensite-dominated DP microstructure resulted in an obvious improvement in strength–ductility balance of the alloy steel. Compared with the DQ treatment, the multi-step TIQ treatment caused the strength–ductility balance of the alloy steel to be enhanced due to its higher total elongation. The better ductility of the TIQ-treated alloy steel can be attributed to the optimization of the microstructure. The preferred orientation of ferritic grain in the as-deposited alloy steel which was adverse to plastic deformation through dislocation slip was eliminated via the multi-step TIQ treatment. Moreover, the TIQ treatment promoted the formation of finer-grained martensite with larger areas of grain boundaries and twinning boundaries which resulted in the enhancement of the coordinated deformability of the martensite with the ferrite.

Micro Plastic Deformation Behavior of Carbide-Free Bainitic Steel during Tensile Deformation Process.

The microstructure of carbide-free bainitic steel has been widely reported because of its excellent mechanical properties. However, no work has explored the deformation behavior of each phase in the microstructure, namely, retained austenite (RA) and bainitic ferrite (BF). In this paper, the deformation behavior of RA and BF during tension was investigated through the in situ electron backscatter diffusion method. The results showed that the RA surrounded by BF with a high Schmid factor (SF) was accelerated towards being transformed into martensite, but the surrounding BF with low SF value retarded the transformation. The kernel average misorientation (KAM) findings revealed that the strain of the RA and BF rapidly increased initially, slowed down after the critical points of 0.25 and 0.54, respectively, and then increased again. The calculated result for the KAM fraction indicated that the contribution of RA to the total plastic deformation gradually decreased throughout the total tensile process, whereas that of BF increased in the presence of RA.