FA Mode Research Articles

Predictive tools for the cooling rate-dependent microstructure evolution of AISI 316L stainless steel in additive manufacturing

This study presents a thorough evaluation of advanced predictive tools for the solidification microstructure of AISI 316L stainless steel, taking into account both chemical composition and cooling rate. In order to achieve a broad range of cooling rates comparable to prominent additive manufacturing processes, autogenous laser welds were carried out on the steel plate by means of powderless directed energy deposition (DED) and selective laser melting (SLM) machines. Computational analysis of cooling rates and metallographic investigations revealed that DED welds solidify at moderate rates, exhibiting a primary ferrite (FA) solidification mode. Conversely, SLM welds solidify at significantly higher rates, showing dual solidification modes: initial austenite (A) at the fusion boundary and subsequent ferrite (F) at the interior. The WRC-1992 constitution diagram predicts the FA mode in accordance with the findings in the DED welds. An implicated Schaeffler diagram, proposed for a comparable cooling rate, also predicts the same mode. An artificial neural network model, referred to as ORFN, consistently predicts the variation of ferrite content within the DED welds based on the cooling rate variation. The dual mode observed in the SLM welds aligns with established knowledge regarding the alteration of stainless steel solidification under extremely high cooling rates. While the occurrence of the A mode at the fusion boundary is anticipated according to implicated Schaeffler diagrams for extremely high cooling rates, the transition to the F mode is not addressed explicitly.

Understanding and Controlling the Weld Microstructure of Steels

In this article, selected studies are reviewed with a focus on the analysis of microstructure development in steel weld metals. In the study of austenitic stainless steel weld metals, microstructure development in the primary ferrite solidification mode (FA mode) was clarified and related to why FAmode welds are resistant to hot cracking. In studies of duplex stainless steel weld metals and high-Cr ferritic stainless steel weld metals, nitrogen-driven microstructure development and TiN-assisted grain refinement, respectively, were described, and discussions about the mechanism of equiaxed grain formation in the weld metals were added in the latter. In the study of low-alloy steel weld metals, the roles of titanium oxide and titanium nitride (TiN) inclusions on intragranular ferrite formation and the refinement of weld microstructure were described based on crystallographic analysis and the first principles calculation. At the end, the potential importance of the application of different multiscale, multiphysics simulations to welding research was pointed out.

Effect of Cooling Rate on Crystallization Behavior during Solidification of Hyper Duplex Stainless Steel S33207: An In Situ Confocal Microscopy Study

Hyper duplex stainless steel (HDSS) is a new alloy group of duplex stainless steels with the excellent corrosion resistance and mechanical properties among the existing modern stainless steels. Due to the incorporation of the high content of alloying elements, e.g., Cr, Ni, Mo, etc., the crystallization behavior of δ-ferrite from liquid is of vital importance to be controlled. In this work, the effect of the cooling rate (i.e., 4 °C/min and 150 °C/min) on the nucleation and growth behavior of δ-ferrite in S33207 during the solidification was investigated using a high-temperature confocal scanning laser microscope (HT-CLSM) in combination with electron microscopies and thermodynamic calculations. The obtained results showed that the solidification mode of S33207 steel was a ferrite–austenite type (FA mode). L→δ-ferrite transformation occurred at a certain degree of undercooling, and merging occurred during the growth of the δ-ferrite phase dendrites. Similar microstructure characteristics were observed after solidification under two different cooling rates. The variation in the area fraction of δ-ferrite with different temperatures and time intervals during the solidification of S33207 steels was calculated at different cooling rates. The post-microstructure as well as its composition evolution were also briefly investigated. This work shed light on the real-time insights for the crystallization behavior of hyper duplex stainless steels during their solidification process.

Evaluation of solidification cracking susceptibility factors in carbon steel and availability of Ti addition based on theoretical analysis

This study conducted a basic investigation to improve the solidification cracking susceptibility of carbon steel. BTR is one of the susceptibilities to solidification cracking, and it is said that solidification cracking susceptibility is reduced when BTR is smaller. BTR of three commercial filler metals was evaluated by using Trans-Varestraint test. EPMA and EDS analysis of the weld metal microstructure revealed two compounds. The white structure was composed of silicon, manganese, and sulphur, and the black structure was Ti-based compound. Theoretical analysis of BTR was conducted by applying a two-phase peritectic solidification model in FA mode and considering MnS crystallization during solidification segregation. The analysis results indicated that BTR of carbon steel is attributed to the difference in solidification completion temperatures, and carbon is the most deleterious element. Also, MX compound (Ti, C, N) crystallized in addition to MnS at the end of solidification. Theoretical analysis was conducted for Ti virtual addition materials because MX compound (Ti, C, N) crystallization by Ti addition may lead to BTR reduction by trapping C in the liquid phase. The results showed that Ti addition reduced BTR. The amount of C in the residual liquid phase was reduced by an increase in MX compound (Ti, C, N) crystallization due to Ti addition. Based on theoretical considerations, it can be concluded that Ti addition to carbon steel is effective in reducing BTR.

Visible light modulation and anomalous thermal transport in two-dimensional <i>X</i>-AlN (<i>X</i> = C, Si, TC) semiconductor

Aluminum nitride (AlN) is of paramount importance in developing electronic devices because of excellent stability and thermal transport performance. However, lack of novel materials which can provide colorful physical and chemical properties seriously hinders further digging out application potential. In this work, we perform an evolutionary structural search based on first-principles calculation and verify the dynamic and thermal dynamic stability of porous buckled AlN and <i>X</i>-AlN (<i>X</i> = C, Si, TC) structural system, which constructs by introducing C, Si atoms and triangular carbon (TC) into the porous vacancy of AlN, by calculating phonon spectra and first-principles molecular dynamic simulations. Structural deformation becomes gradually serious with the increase of structural unit size and significantly influences structural, electronic, and thermal transport properties. Firstly, we point out that a flat energy band appears around the Fermi level in C-AlN and Si-AlN because of weak interatomic interaction between C/Si and the neighbor Al atoms. Unoccupied C-/Si-p<sub><i>z</i></sub> and Al-p<sub><i>z</i></sub> do not form <inline-formula><tex-math id="M1">\begin{document}$ {\rm{\pi }} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20230116_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20230116_M1.png"/></alternatives></inline-formula> bond and only a localized flat band near Fermi level arises, and thus the absorption peaks of structures are enhanced and the red shift occurs. Bonding state of <inline-formula><tex-math id="M2">\begin{document}$ {\rm{\pi }} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20230116_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20230116_M2.png"/></alternatives></inline-formula> bond from hybridized C-p<sub><i>z</i></sub> orbitals in triangular carbon of TC-AlN lowers the energy of conduction band at <i>K</i> point in the first Brillouin zone and the corresponding antibonding state raises the band at <i>Γ</i>, therefore transition from indirect bandgap of AlN to direct bandgap of TC-AlN appears. Secondly, porous buckled AlN shows the lowest thermal conductivity due to asymmetric Al—N bonds around the porous vacancy and vertically stacked N—N bonds. Introduced C and Si atoms both reduce structural anharmonicity, while the former has a relatively small distortion, and so it has a higher thermal conductivity. Triangular carbon in TC-AlN hinders phonon scattering between FA mode and other phonon modes and has the weakest anharmonicity because of the strongest bond strength, and obtains the highest thermal transport performance. Finally, we unveil the physical mechanism of anomalous thermal conductivity in <i>X</i>-AlN system by modulating the biaxial tensile strain. Enhanced vertical N—N bonds dominate thermal transport due to its weaker anharmonicity with a slightly strain, and when tensile strain is above the 4%, soften phonon modes reduce phonon velocity and thus hinders the thermal transport process. Therefore, occurs the anomalous thermal transport behavior, i.e. thermal conductivity first rises and then drops with applied biaxial strain increasing. Our work paves the way for modulating two-dimensional AlN performance and provides a new insight for designing promising novel two-dimensional semiconductors.

Theory and practice of diagnostics of fire hazardous modes of operation of catalytic converters

Introduction. The wide-scale use of catalytic converters and particulate filters in automobile engines has aggravated the problem of their ignition and updated the research and methodological framework for the examination of causes of fire emergency modes (FEMs) of operation of fuel catalytic units (FCUs). The relationship between the FEMs of the FCU operation and failures of the fuel equipment, wear of the cylinder-piston group of engines and deviations in fuel compositions was confirmed. The goal was to develop a diagnostic method for fire hazardous modes of operation of FCUs of vehicles.Methodology. A model of oxidative catalysis underway in the FCU has been proven rational. The model is used to calculate the thermo-catalytic efficiency and heat generation in the active layer of the γ-Al2O3 platinum catalyst depending on the temperature of exhaust gases (EG), concentrations of CO, CH and soot. It has been found out that catalysis can theoretically develop in four limit domains: internal kinetic domain, internal diffusion domain, external diffusion domain, and external kinetic domain.Results and discussion. Experimental and computational studies have shown the probability of emergence of breakdown vehicles with a multiple excess of soot emissions and thermal stresses. A 10‑fold increase in CO, CH and soot in EG rises the thermal performance of the catalytic reaction from 17,282 to 491,907 kJ/h, creating a fire hazard in a KamAZ engine. To identify a FEM, the diagnostic method based on the «free acceleration» (FA) mode according to GOST 33997–2016 is proposed. The procedure is supplemented with maximum revolutions and restrictions (0.5 s) of the FA mode time. The latter is necessary for the guaranteed operation of the engine in the «full load mode». The method was applied in the course of the fire engineering studies on a Ford Mondeo car having a TDCi (Common Rail System) diesel engine and a catalytic particulate filter. Laboratory examination and analytical studies have found that the main reason for the operation of FCU in emergency (due to environmental and fire hazards) modes is the corrosion of precision parts of the fuel equipment accumulated during its long-term operation. Progressive corrosion is caused by excessive sulfur and moisture content in fuel and oil.Conclusions. It’s been proven that the emergency heating of a catalytic converter causes a sharp rise in the car combustion risk. The authors have proposed an original method for the diagnostics of fire-hazardous modes of operation of catalytic converters based on procedures set in GOST 33997–2016 (ТР ТС 018/2011).

Microstructure and mechanical properties of wire-powder hybrid additive manufacturing for high nitrogen steel

A novel wire-powder hybrid arc additive manufacturing (WPHAAM) was applied for melting zone nitridizing to avoid the deterioration effect caused by the loss of nitrogen in high nitrogen steel. A designed Cr-Mn-N wire with 0.79 wt% of nitrogen (HNS6) was used to build a high nitrogen wall body by introducing WPHAAM method. The influence of nitrogen content on solidification behavior, microstructure, and tensile properties were studied in the as-built conditions. With the increase of amount of feeding powder, the solidification mode of the deposited wall changes from primary ferrite with second-phase austenite (FA mode) to primary austenite with second-phase ferrite (AF mode), and the mechanical properties are enhanced. When the powder feeding rate reaches 0.33 g/min, the solidification changes to A mode, and the deposited metal is austenite. The ultimate tensile strength and elongation of the deposited metal are 1014 MPa and 50.8% respectively.

Femtoamp and picoamp modes of electrospray and paper spray ionization

Sub nanoampere ionization was investigated using glass emitters and paper spray substrates. An electrometer was set up to measure ionization current as low as 20 fA, while the ion trap mass spectrometer was operated to analyze ion current as low as 1 ion per second. Femto amp (100 s fA) ionization currents were generated using glass emitters with 90 nm tip or 4 μm tips. Using the 4 μm emitter tips, both the fA mode and the regular nanoESI mode were generated. Mass spectra produced by the fA mode ESI were >3 order of magnitude less intense than those produced by nanoESI mode. Beyond generating ESI type of ions, fA mode ESI generates native cytochrome c charge states without using any buffering agent. Femto amp ionization was also observed when conducting paper spray ionization, as well as when ionizing dry paper substrate without solvent. Ultra-low current ionization allowed direct analysis of undiluted sample matrices without carryover contamination. Samples collected and stored on a dried paper substrate were analyzed with no sample preparation.

Influences of cooling rates on delta ferrite of nuclear power 316H austenitic stainless steel

High temperature confocal melting was conducted to study the volume fraction variation of as-cast δ-ferrite at cooling rate from 5 °C/min to 6000 °C/min of 316H austenitic stainless steel (316H). The results demonstrate that the solidification mode of 316H was ferrite-austenite type (FA mode) regardless of the cooling rate. The peritectic reaction occurs between ferrite and liquid phase during solidification. The higher the cooling rate is, the higher the δ-ferrite volume fraction is. The δ-ferrite fraction variation can be explained by the combination of FA solidification mode during solidification and ferrite to austenite solid-state transformation during cooling after solidification.

Microstructural Infarct Border Zone Remodeling in the Post-infarct Swine Heart Measured by Diffusion Tensor MRI.

Introduction: Computational models of the heart increasingly require detailed microstructural information to capture the impact of tissue remodeling on cardiac electromechanics in, for example, hearts with myocardial infarctions. Myocardial infarctions are surrounded by the infarct border zone (BZ), which is a site of electromechanical property transition. Magnetic resonance imaging (MRI) is an emerging method for characterizing microstructural remodeling and focal myocardial infarcts and the BZ can be identified with late gadolinium enhanced (LGE) MRI. Microstructural remodeling within the BZ, however, remains poorly characterized by MRI due, in part, to the fact that LGE and DT-MRI are not always available for the same heart. Diffusion tensor MRI (DT-MRI) can evaluate microstructural remodeling by quantifying the DT apparent diffusion coefficient (ADC, increased with decreased cellularity), fractional anisotropy (FA, decreased with increased fibrosis), and tissue mode (decreased with increased fiber disarray). The purpose of this work was to use LGE MRI in post-infarct porcine hearts (N = 7) to segment remote, BZ, and infarcted myocardium, thereby providing a basis to quantify microstructural remodeling in the BZ and infarcted regions using co-registered DT-MRI.Methods: Chronic porcine infarcts were created by balloon occlusion of the LCx. 6–8 weeks post-infarction, MRI contrast was administered, and the heart was potassium arrested, excised, and imaged with LGE MRI (0.33 × 0.33 × 0.33 mm) and co-registered DT-MRI (1 × 1 × 3 mm). Myocardium was segmented as remote, BZ, or infarct by LGE signal intensity thresholds. DT invariants were used to evaluate microstructural remodeling by quantifying ADC, FA, and tissue mode.Results: The BZ significantly remodeled compared to both infarct and remote myocardium. BZ demonstrated a significant decrease in cellularity (increased ADC), significant decrease in tissue organization (decreased FA), and a significant increase in fiber disarray (decreased tissue mode) relative to remote myocardium (all p < 0.05). Microstructural remodeling in the infarct was similar, but significantly larger in magnitude (all p < 0.05).Conclusion: DT-MRI can identify regions of significant microstructural remodeling in the BZ that are distinct from both remote and infarcted myocardium.

Microstructure, Micro-hardness and Impact Toughness of Welded Austenitic Stainless Steel 316L

The objective of the paper is to study the mechanical behaviour of two welded joints that differ in their filler metals in regards with the microstructure. Gas tungsten arc welding process was employed to perform the welded joints between AISI 316L as a base metal and either ER316LN or ER308LN as filler metal. For both the welded joints, the microstructure of the fusion zone was predominantly austenitic containing a small amount of residual dendritic δ-ferrite in skeletal morphology formed during solidification (FA mode) and whose transformation to austenite upon cooling was incomplete. However, XRD analysis pointed out a relatively higher amount of δ-ferrite in the fusion zone of the welded joint (316L/ER308LN) than that in the welded joint (316L/ER316LN), as it could be expected from the calculated (Creq/Nieq) ratio of Schaeffler formula. The composite microstructure of the fusion zones exhibited higher values of micro-hardness than the heat-affected zone which contained coarse grains. Charpy impact toughness was higher for the welded joint realized using ER308LN as a filler metal but both types of welds fractured in a ductile mode.

Characterization of microstructure of A508III/309L/308L weld and oxide films formed in deaerated high-temperature water

The microstructure of A508III/309L/308L weld clad and the properties of the oxide films formed in simulated pressurized water reactor primary water at 290 °C were characterized. The A508III heat-affected zone (HAZ) consisted primarily of a decarburization zone with ferrite near the fusion line and a following pearlite structure with fine grains. A high hardness region in the HAZ could be the result of C-enrichment. M23C6 and M7C3 precipitates were observed in element transition zone. 308L stainless steel (SS) containing ∼ 12% ferrites exhibited both ferritic-austenitic solidification mode (FA mode, δ→γ) and austenitic-ferritic solidification mode (AF mode, γ→δ), whereas 309L SS containing ∼ 9% ferrites exhibited only FA mode. The A508III surface oxide film was mainly Fe3O4 in deaerated high-temperature water. The coarse grain zone covered with few oxide particles was different from other types of film on the other region of HAZ and the bulk zone. More pitting appears on 309L SS after immersion in deaerated high-temperature water due to the dissolution of inclusions. SS surface oxide films consisted primarily of spinels. The oxide film on SS was divided into two layers. Ni was concentrated mainly at the oxide/substrate interface. The oxide film formed on 309L was thicker than that on the 308L. The ferrite in the stainless steel could improve the oxidation resistance.

Effect of Ru Addition on the Mechanical Properties and Microstructure of 316L Austenitic Stainless Steel Weld Metal

The addition of Ru to the weld metal of 316L austenitic stainless steel (ASS) by Gas tungsten arc welding (GTAW) button welds has shown superior mechanical properties. In the present investigation, the effect of Ru additions on the physical properties of 316L weld was evaluated by investigating the microstructural and mechanical properties of weld metal. The microstructure and mechanical properties of the alloyed button welds were analyzed using optical microscope, scanning electron microscopy (SEM), EDX and Vickers hardness test respectively at 0.1%, 0.5%, 1% and 2% Ru addition. Primary ferrite (FA mode) solidification resulted in primary δ-ferrite and eutectic γ-austenite in the button welds, while hardness values increased to 198 HV with increasing Ru addition up to 2% Ru.

Solidification microstructure formation in HK40 and HH40 alloys

The microstructure formation processes in HK40 and HH40 alloys were investigated through JmatPro calculations and quenching performed during directional solidification. The phase transition routes of HK40 and HH40 alloys were determined as L → L + γ → L + γ + M7C3 → γ + M7C3 → γ + M7C3 + M23C6→ γ + M23C6 and L → L + δ → L + δ + γ→ L + δ + γ + M23C6 δ + γ + M23C6, respectively. The solidification mode was determined to be the austenitic mode (A mode) in HK40 alloy and the ferritic–austenitic solidification mode (FA mode) in HH40 alloy. In HK40 alloy, eutectic carbides directly precipitate in a liquid and coarsen during cooling. The primary γ dendrites grow at the 60° angle to each other. On the other hand, in HH40 alloy, residual δ forms because of the incomplete transformation from δ to γ. Cr23C6 carbide is produced in solid delta ferrite δ but not directly in liquid HH40 alloy. Because of carbide formation in the solid phase and no rapid growth of the dendrite in a non-preferential direction, HH40 alloy is more resistant to cast defect formation than HK40 alloy.

In situ delta ferrite estimation and their effects on FCPR at different orientations of multipass shielded metal arc welded SS304L

Solidification and re-solidification with subsequent phase transformation in different welding zones of multipass weldment play a vital role to define mechanical properties of joint structures. To avoid from penetration defects and inclusion contamination in molten metal pool, multipass shielded metal arc welding (SMAW) of SS304L is performed in this research using E308L filler metal. The objective of this research is to investigate out the multiple welding passes effects on microstructural evolution, in situ delta ferrite percentage and different mechanical properties. A metallurgical microscope is used to examine the delta ferrite phase in austenite. Instead of depending on general ferrite number (FN) obtained through rigorous physical attempts, a MATLAB® code for image processing is developed to estimate the local percentage of delta ferrite, within the austenitic phase. To reveal delta ferrite effects, its local percentage is discussed in relation to various properties. With the help of this technique, a good estimation is also made to detect the localized delta ferrite morphologies. The fusion zone (FZ) solidified in ferritic-austenitic mode (FA mode) comprises two basic delta ferrite morphologies; lacy and vermicular. Grain growth behavior and ferritic percentage of base metal (BM), heat affected zone (HAZ) and fusion zone (FZ) are discussed in single, double and triple pass weldments. Effects of delta ferrite percentage and morphology on fatigue crack initiation as well propagation are also investigated in different zones of each pass weldment. Fatigue crack propagation rate (FCPR) in different orientation with respect to welding zones is checked and it is noticed that propagation rate is much lower in perpendicular direction as compared to parallel one. Due to specific microstructural pattern, HAZ has greater fatigue crack propagation rate (FCPR) in comparison to BM and FZ. Fatigue crack propagation found highest resistant in triple pass weldment due to high concentration of lacy ferrites. However, in some cases it is found that crack mechanics is actually responsible instead of microstructural effects.

Microstructure evolution and mechanical properties of multiple-layer laser cladding coating of 308L stainless steel

Multiple-layer laser cladding of 308L stainless steel was obtained by a fiber laser using a way of wire feeding to repair the surface scrapped or erosive parts of 316L stainless steel. The microstructure of the coating was measured by a metallographic microscope, and phase composition was determined by X-ray diffraction. The results show that good metallurgical bonding can be obtained between the 308L stainless steel coating and 316L stainless steel substrate. The coating is mainly composed of columnar dendrites, and there are also a few planar crystals and cellular dendrites distributed in the bonding zone. Meanwhile, some equiaxed grains and steering dendrites are distributed in the apex of the coating. Grains incorporate in epitaxial columnar dendrite's growth between different layers and tracks. It has been proved using XRD that the coating basically consists of austenite and a small amount of δ ferrite. The coating solidifies in FA mode according to the Creq/Nieq ratio and metallurgical analysis results. The average content of δ ferrite is about 10.48% and morphologies of the ferrite are mostly vermicular, skeletal and lathy. Due to heat treatment and different cooling rate, the δ ferrite content generally increases as the number of laser cladding layers increases. The coating and the substrate have equivalent microhardness, and softening zone does not appear in the heat affected zone. The tensile strength and elongation of the coating are 548MPa and 40%, about 86% and 74% of the substrate, respectively. Ductile fracture is proved by the emergence of obvious dimples in the fracture surface.

The Solidification Mode of Fe-Mn-Al-C Lightweight Steel

Solidification behavior and solid-state transformation were investigated in Fe-Mn-Al-C lightweight steel. The solidification mode of the Fe-9.3Mn-5.6Al-0.2C (wt.%) lightweight steel was predicted to be the F mode (liquid → liquid + δ-ferrite → δ-ferrite → δ-ferrite + γ-austenite), according to the classification of the solidification modes of stainless steels. However, the microstructures of an ingot of the lightweight steel showed that the solidification occurred by the FA mode (liquid → liquid + δ-ferrite → liquid + δ-ferrite + γ-austenite → δ-ferrite + γ-austenite). To examine the difference between predicted and actual solidification modes, some specimens were annealed at various temperatures ranging from 1200°C to 1450°C for 10 min and then quenched. The microstructures of the annealed specimens exhibited that there were the narrow ranges of single δ-ferrite and solidification sections. This result indicates that the solidification mode of the present steel was the F mode, matching well with the predicted solidification mode. The reason for the FA mode, which was observed in the ingot, was because the liquid passed by a narrow solidification section and a single δ-ferrite region, and it directly entered the dual-phase region of δ-ferrite and γ-austenite during cooling.

Effect of Manganese on Microstructures and Solidification Modes of Cast Fe-Mn-Si-Cr-Ni Shape Memory Alloys

We investigated microstructures and solidification modes of cast Fe-(13-27)Mn-5.5Si-8.5Cr-5Ni shape memory alloys to clarify whether Mn was an austenite former during solidification. Furthermore, we examined whether the Creq/Nieq equations (Delong, Hull, Hammer and WRC-1992 equations) and Thermo-Calc software® together with database TCFE6 were valid to predict the solidification modes of cast Fe-(13-27)Mn-5.5Si-8.5Cr-5Ni shape memory alloys. The results have shown that the solidification modes of Fe-(13-27)Mn-5.5Si-8.5Cr-5Ni alloys changed from the F mode to the FA mode with increasing the Mn concentration. Mn is an austenite former during the solidification for the cast Fe-Mn-Si-Cr-Ni shape memory alloys. The Delong, Hull, Hammer, and WRC-1992 equations as well as Thermo-Calc software® together with database TCFE6 are invalid to predict the solidification modes of cast Fe-(13-27)Mn-5.5Si-8.5Cr-5Ni SMAs. To predict the solidification modes of cast Fe-Mn-Si-Cr-Ni alloys, a new Creq/Nieq equation should be developed or the thermodynamic database of Thermo-Calc software® should be corrected.

Microstructures of Austenitic Stainless Steel Produced by Twin-Roll Strip Caster

The microstructures of austenitic stainless steel strip were studied using color metallographic method and electron probe micro analysis (EPMA). In the cast strips, there are three kinds of solidification structures: fine cellular dendrite in the surface layer, equiaxed grains in the center and fine dendrite between them. The solidification mode in the surface layer is the primary austenite AF mode because of extremely high cooling rate, with the retained ferrite located around the primary cellular austenite. In the fine dendrite zone, the solidification mode of molten stainless steel changes to FA mode and the residual ferrite with fish-bone morphology is located at the core of the dendrite. The retained ferrite of equiaxed grains in the center is located in the center of broken primary ferrite dendrite with vermicular morphology.

Effect of Cr/Ni equivalent ratio on ductility-dip cracking in AISI 316L weld metals

The ductility-dip cracking (DDC) susceptibility of AISI 316L stainless steel weld metals was examined. Modified flux cored arc welding (FCAW) filler wires were fabricated with various chromium and nickel contents. The solidification mode and delta-ferrite content were determined from the chromium and nickel equivalent ratios (Cr eq/Ni eq). Ductility-dip cracking occurred through a grain boundary sliding mechanism in the reheated region of the weld metal in the ferrite at cell and dendrite boundaries (AF mode), and the primary course of DDC formation was associated with the straight migrated grain boundary (MGB) morphology. No DDC was observed in the tortuous MGB due to the pinning effect of delta-ferrite in the continuous network of vermicular type of ferrite (FA mode)/acicular ferrite and continuous austenite network (F mode) weld metals. The DDC at the triple point or the intersection of the MGB showed a creep-like morphology. Severe localized and thermal plastic deformation was observed through the formation of micro-voids when grain boundary sliding was generated in the ductility-dip temperature range under strong restraint conditions.