Solidification Mode Research Articles

Characterization of the microstructure and mechanical properties of activated tungsten inert gas welded and hot-wire tungsten inert gas welded super austenitic stainless steel AISI 904L

Super austenitic stainless steel AISI 904L is a premium stainless steel variant known for its superior properties and ability to retain the same in highly challenging conditions. This study investigates the feasibility of welding 10-mm thick plates of AISI 904L through activated tungsten inert gas welding and hot-wire tungsten inert gas welding methods. The fabricated joints are then subjected to solidification mode study, ferrite number analysis, microstructural characterization, and mechanical tests. Results of the solidification mode study indicated primary austenitic solidification in both the weld joints. The ferrite number measurement revealed that the hot-wire tungsten inert gas weld joint had a higher δ-ferrite content of 1.58 over 0.99 of the activated tungsten inert gas weld joint. A microstructure study confirmed the presence of refined equiaxed grains in the fusion zone of the hot-wire tungsten inert gas weld joint, while the activated tungsten inert gas weld joint displayed equiaxed grains crammed between the columnar grains and dendrites. Liquid penetrant inspection of bend-tested samples proved the soundness and quality of both weld joints. The hot-wire tungsten inert gas weld joint excelled over the activated tungsten inert gas weld joint with a higher microhardness value. During tensile tests, activated tungsten inert gas and hot-wire tungsten inert gas weld joints displayed average ultimate tensile strength values of 521 and 514 MPa, respectively. The impact toughness of the activated tungsten inert gas weld joint was 248 J, while it was 220 J for the hot-wire tungsten inert gas weld joint. Field emission scanning electron microscope analysis equipped with energy-dispersive spectroscopy and elemental color mapping provided details about weld microstructure and the reasons for the material's failure. The study proved the efficiency and soundness of the 10-mm thick joints of AISI 904L fabricated by activated tungsten inert gas and hot-wire tungsten inert gas welding techniques.

Microstructure and mechanical properties of Nb-alloyed austenitic CrNi steel fabricated by wire-feed electron beam additive manufacturing

We investigated the phase composition, microstructure, mechanical properties, and fracture mechanisms of Nb-alloyed austenitic stainless steel, produced by electron beam additive manufacturing (EBAM), after different regimes of the post-EBAM solid-solution treatments. As-build EBAM-produced steel has a predominantly austenitic structure, contains vermicular δ-ferrite (Vδ = 5–7%) and Nb-enriched precipitates (Laves phase Fe2Nb, FeNb and NbC). Niobium-alloying assists the stabilization of austenitic phase but does not suppress the columnar grain growth during EBAM-fabrication of austenitic stainless steels. Solid-solution treatments provide a change in morphology of δ-ferrite, a decrease in the ferrite volume fraction and the dissolution of intermetallic phases (Fe2Nb, FeNb), which all affect the tensile properties. Post-EBAM treatments contribute a decrease in the yield strength of the steel and improve the elongation-to-failure. All EBAM-produced specimens have good mechanical properties, which are comparable with those for steels produced by conventional methods. Irrespective of treatment regime, EBAM-fabricated specimens exhibit ductile transgranular fracture. For the EBAM-produced Nb-alloyed steel, the phase composition is described from the point of view solidification mode, and the effects of the precipitate hardening and solid-solution strengthening on the mechanical properties and fracture mechanisms were discussed.

Elimination of δ-ferrite in N50 steel and its effect on cryogenic mechanical properties

δ ferrite serves as a detrimental phase in TF (Toroidal Field) superconducting magnet jacket pipe materials. The distribution features of ferrite in N50 casting and forging were investigated, and the cryogenic mechanical properties with different ferrite content were studied accordingly. The results indicated that the non-equilibrium crystallization leads to the solidification mode gradually changing from AF (Austenitic-ferritic) at the surface to the FA (Ferritic-austenitic) at the center of the casting. The skeleton δ ferrites induced by interdendritic segregation could disappear almostly after annealing while the tangled lath δ ferrite at the center of casting caused by the more serious segregation seems to be stable. Based on thermodynamics calculations and through changing the Cr and Ni content according to the Espy diagram, the stable ferrites can be eliminated when the Creq/Nieq is less than 1.25. From fractography, the cryogenic impact toughness of N50 steel would be deteriorated because more secondary cracks were taken place by the mismatched deformation between the harder residual δ ferrites and matrix. Though the elimination of δ ferrites caused a slight decrease in cryogenic tensile strength, it greatly improved the cryogenic impact toughness.

Development of analysis method for hot cracking considering mechanical and metallurgical factors

Since welding hot cracking is one of the most harmful defects which may largely reduce the strength of welded joints and structures, it is very important to study the prediction and prevention of its generation. Hot cracking is caused by both mechanical and metallurgical factors. In this research, authors proposed a new computational method for evaluating the occurrence of hot cracking through considering both mechanical and metallurgical behavior. As the metallurgical behavior, strain due to solidification shrinkage and mechanical resistance of molten metal were modelled using the solidification ratio as a parameter. The mechanical behavior, an index for generation of hot cracking using the increment of plastic strain is proposed. In addition, authors proposed a simplified prediction method using the temperature gradient for the direction of columnar crystal growth, which affects the location of hot cracking, and is used to evaluate the associate angle of the columnar crystals. The developed method was applied to butt welding. Through the computation using proposed method, it was found that the predicted direction of columnar crystal growth by the proposed method is in good agreement with the experimental results. In addition, the generation of hot cracking in the same butt welding was predicted using the proposed method and the obtained cracking positions agreed well with those observed by experiment. The Influence of the strain of solidification shrinkage on hot cracking was investigated in relation to the surrounding constraints, the temperature gradient and solidification mode, which are presumed to be highly related to the occurrence of hot cracking. From the above results, it can be concluded that the proposed method can analyze the effects of mechanical and metallurgical factors on hot cracking.

Influence of ultrasonic impact treatment on microstructure and mechanical properties of nickel-based alloy overlayer on austenitic stainless steel pipe butt girth joint

Ultrasonic impact treatment (UIT) is carried out on the Ni-based alloy stainless steel pipe gas tungsten arc welding (GTAW) girth weld, the differences of microstructure, microhardness and shear strength distribution of the joint before and after ultrasonic shock are studied by microhardness test and shear punch test. The results show that after UIT, the plastic deformation layer is formed on the outside surface of the Ni-based alloy overlayer, single-phase austenite and γ type precipitates are formed in the overlayer, and a large number of columnar crystals are formed on the bottom side of the overlayer. The average microhardness of the overlayer increased from 221 H V to 254 H V by 14.9%, the shear strength increased from 696 MPa to 882 MPa with an increase of 26.7% and the transverse average residual stress decreased from 102.71 MPa (tensile stress) to −18.33 MPa (compressive stress), the longitudinal average residual stress decreased from 114.87 MPa (tensile stress) to −84.64 MPa (compressive stress). The fracture surface has been appeared obvious shear lip marks and a few dimples. The element migrates at the fusion boundary between the Ni-based alloy overlayer and the austenitic stainless steel joint, which is leaded to form a local martensite zone and appear hot cracks. The welded joint is cooled by FA solidification mode, which is forming a large number of late and skeleton ferrite phase with an average microhardness of 190 H V and no obvious change in shear strength. The base metal is all austenitic phase with an average microhardness of 206 H V and shear strength of 696 MPa.

Metastable coupled growth kinetics between primary and peritectic intermetallic compounds within the liquid Mo-37 wt% Co refractory alloy

The metastable coupled growth between primary σ-Mo3Co2 and peritectic ε-Co7Mo6 phases for hypoperitectic Mo-37 wt% Co refractory alloy was investigated by electromagnetic levitation (EML) and drop tube (DT) methods. The highest alloy undercooling attained 261 K (0.14 TL) under EML conditions, which reached up to 311 K (0.17 TL) in DT experiments. At small undercoolings, the typical peritectic solidification prevailed and the corresponding double recalescence processes were in-situ recorded for levitated alloys. In this case, the primary σ-Mo3Co2 phase preferentially nucleated from liquid alloy and its growth velocity monotonously increased as a power function with the rise of undercooling. When the liquid undercooling increased beyond the threshold of 162 K, the competitive nucleation between σ-Mo3Co2 and ε-Co7Mo6 compounds was facilitated, leading to a novel lamellar eutectic-like microstructure for peritectic-type alloys. Meanwhile, the measured growth velocity exhibited a sudden drop at first and then slightly increased with the bulk undercooling. Once the alloy undercooling exceeded 295 K (D = 226 µm), only the metastable coupled growth mechanism could occur within the liquid alloy. The final microstructure of hypoperitectic Mo-37 wt% Co alloy even transformed from lamellar to anomalous eutectic-like morphology in a sufficiently undercooled state. Furthermore, as the undercooling increased, the random orientation relationship between σ-Mo3Co2 and ε-Co7Mo6 phases in typical peritectic solidification mode was replaced by the parallel orientation, i.e. {111}σ//{101̅0}ε for metastable coupled growth mechanism.

A kinetic transition from peritectic crystallization to amorphous solidification of rapidly quenched refractory Nb-Ni alloy

In this study, the kinetic transition of solidification modes for refractory peritectic Nb54Ni46 alloy was revealed using melting spinning experiments combined with molecular dynamics (MD) simulation and classical nucleation theory. This begins with a shift from the usual peritectic solidification to the direct precipitation of the peritectic Nb7Ni6 phase when the roller speed reaches 10 m/s. Once the roller speed exceeds 20 m/s, the nucleation of the Nb7Ni6 phase is suppressed and the amorphous phase forms. The high-resolution transmission electron microscopy images indicate that the stacking fault and twining model of the peritectic Nb7Ni6 phase are {101¯2}/<1¯011> with a misorientation angle of 35.65°. The results of an individual local crystallization zone in amorphous matrix imply that the chemically disordered crystalline structure is more likely to form during the early stages of nucleation and growth, whereas the chemically ordered crystalline structure may evolve during the growth of the complex intermetallic compound Nb7Ni6 phase. Furthermore, the MD simulation of the glass transition temperature agrees well with the experiment, and the Voronoi polyhedron analysis suggests that the Ni-centered 〈0,0,12,0〉, 〈0,2,8,2〉 as well as Nb-centered 〈0,1,10,4〉 icosahedron-like clusters and their medium-range ordered structures play an important role in the formation of amorphous Nb54Ni46 alloy. In addition, the predictions made using classical nucleation theory and the time-temperature-transformation diagram propose that the phase selection rules are consistent with the experimental results as the undercooling and cooling rate increase, with the critical cooling rate for the formation of amorphous phase determined to be 1.71 × 107 K/s.

Rapid crystal growth kinetics of tungsten dendrites under electrostatic levitation state

The rapid crystal growth kinetics of tungsten dendrites within extremely refractory W60Re40 liquid alloy was examined under electrostatic levitation state. Tungsten dendrite growth through two solidification modes was revealed through modulating the active nucleation and spontaneous nucleation within the liquid undercooling range between 145 and 470 K, leading to the displacement of normal peritectic transition by metastable isomorphous solidification. The tungsten dendrites kept the priority growth direction of 〈100〉 and their growth velocity versus undercooling showed a power law function. The liquid alloy would solidify through normal peritectic transition if assisted with active nucleation from bottom Mo electrode.

Numerical model of PCM heat exchange based on dynamic boundary

Using phase change material (PCM) to store solar energy can effectively alleviate energy and environmental problems. The accurate numerical models are the guarantee to promote the practical application of hydrated salt PCM. However, the phenomenon of supercooling and phase change hysteresis makes the PCM present various operating states under the actual dynamic inlet boundary conditions, which makes it difficult to solve the temperature field. In this study, a numerical model is established, which can independently determine whether each computing node is in the heating mode, the cooling mode, the heating after partial solidification mode, or the cooling after partial melting mode at each moment. The calculation codes for the first two modes in the model have been verified experimentally, and the latter two modes are modeled using the curve scale method proven to be the most accurate. To test the running process of the numerical model, a dynamic inlet boundary condition was constructed and input. The operation results show that the numerical model can smoothly switch the mode of each node and then solve it.

Microstructure characteristics of the directionally solidified TiAl-Nb alloys with different solidification modes

Microstructure characteristics are investigated for the TiAl-Nb alloys with four typical solidification modes, namely, single β, hypo-peritectic, hyper-peritectic and single α solidification. The fully lamellar structures is formed in both the dendrite and interdendritic regions in single β, hypo-peritectic, and hyper-peritectic solidification alloys, and the order of lamellar thickness is hyper-peritectic, hypo-peritectic and single β solidification alloys. Some γ islands originating from the prior interdendritic regions are formed by the chemical segregation in hyper-peritectic solidification alloy, but in single α solidification alloy, some fine lamellar structure only exist in the dendritic core and the granular γ grains are observed in the primary α interdendritic region. For hypo-peritectic solidification alloy, lamellar grain boundaries are almost located in the core of primary β phases. Lamellar orientation can be changed along either the boundaries of dendrites or the ridge-like Nb-segregations. However, for hyper-peritectic solidification alloy, the lamellar boundary locates in both the interdendritic region and the core of primary β dendrite. The lamellar orientation is changed mainly along the dendritic Nb-segregation, not along the dendrite boundary. Not every α2 in lamellar grain has Burgers orientation relationship with the prior β dendrite for both hypo- and hyper-peritectic solidification alloys. Several macrosegregation regions consisting of the curved coarsened lamellae are produced only in hypo-peritectic alloy. These regions may be induced by relatively weak thermo-gravitational convection (TGC) effect, which do not change the lamellar grain orientation but make a small orientation angle deviation of the α2 phase from each other in the grain.

Strongly out-of-equilibrium growth morphologies in fast solidifying eutectics

In metal printing processes such as laser-based additive manufacturing, typical solidification velocities yield a strongly out-of-equilibrium (SOE) growth regime that is intermediate between the slow solidification in conventional metal casting and rapid solidification regimes characterized by severe solute trapping. Using phase-field simulations supported by experimental observations, we provide evidence for an inherent growth mode of the SOE regime in slightly hypereutectic Al-Ni, leading to the destabilization of the planar eutectic coupled growth front and the development of a protruded state. We interpret this scenario theoretically in terms of a concomitant growth of different solidification modes, here the coupled eutectic together with $\ensuremath{\alpha}$ primary phase. As another example of concomitant growth under SOE conditions, we present simulation results for single-phase solidification of a Ni fcc-phase exhibiting dendrites and doublons.

Effect of Ta microalloying on joint performance by tailoring the microstructure during laser beam welding of Ti-22Al-27Nb

The development of Ti2AlNb based alloys has set a strong competition between conventional Ni-based superalloys and titanium aluminides for aerospace industry due to their compatible/superior mechanical properties. With advent of time, progressive manufacturing technologies are being worked out to transform this novel alloy system into tangible engineering products. Laser beam welding (LBW) of Ti2AlNb based alloys is a provocative task due to evolution of metastable B2 phase in the fusion zone (FZ). In the present study, a comparative analysis on LBW of Ti-22Al-27Nb was carried out by preparing the joints with/without an interlayer of Tantalum (Ta) microfilm. The evolution of microstructure in the weld zone (WZ) was analyzed through Electron probe microanalysis (EPMA), Electron backscattered diffraction (EBSD) analysis, High-angle annular dark-field (HAADF), Bright field (BF) and Selected area electron diffraction (SAED) imaging. The effect of Ta on laser beam welded joint (LBWJ) of Ti-22Al-27Nb was established by correlating the room temperature mechanical properties (tensile properties and hardness) with the evolution of phases, grains and grain size distribution and inter/intra grain misorientations in the weld zone (WZ). The addition of Ta into FZ triggered heterogeneous nucleation, causing epitaxial as well as non-epitaxial mode of solidification in the weld pool. A substantial increase in tensile strength of Ta adulterated LBWJ of Ti-22Al-27Nb was the provenance of grain refinement and greater fraction of high angle grain boundaries (64.89%). The tensile strength and percentage elongation of LBWJ prepared with 0.01 mm thin interlayer of Ta was measured up to 1049 MPa (i.e. 99.15% of that of the base material) and 9.53% respectively. Further dilution of the LBWJ with Ta contributed no significant increase in tensile strength due to nucleation of Intermetallics along the periphery of FZ and caused an early failure.

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.

Microstructural and mechanical analysis of double pass dissimilar welds of twinning induced plasticity steel to austenitic/duplex stainless steels

Double pass dissimilar welds between the twinning induced plasticity (TWIP) steel and austenitic stainless steel (ASS) AISI 304 L and duplex stainless steel (DSS) 2205 were studied theoretically and experimentally. A finite element (FE) thermal model linked to the liquid fraction calculation was used to estimate the optimal heat input level. The metallographic characterization of dissimilar weldments TWIP-SS 304 L and TWIP-DSS 2205 disclosed a priori the lack of hot cracking in the fusion zone (FZ) and liquation cracks in the heat-affected zone (HAZ). Later, the root bend test results corroborated a lack of fusion in the FZ-HAZ interface of the TWIP-DSS 2205 weld (TWIP steel side). The FE model predicted the heat accumulation in the SS 304 L and DSS 2205, which produced variation in the thermal gradient. The thermal gradient changes induced the ferritic-austenitic (FA) solidification mode in TWIP steel and SS 304 L and the massive austenite formation by epitaxial growth in the FZ-HAZ interface of the DSS 2205 side. Microhardness profiles corroborated the formation of homogenous FZs in both dissimilar welded joints. The microhardness increase/decrease was related to the solidification mode change in the FZ-HAZ interface. The mechanical strength and ductility of the TWIP-SS 304 L weld decreased 26% and 40%, respectively, compared to the reference autogenous similar weldment SS 304 L. The planar solidification growth detected in the FZ-HAZ interface was responsible for mechanical properties loss.

Solidification Simulation of Al-Si Alloys with Dendrite Tip Undercooling

A novel solution approach is proposed for the numerical simulation of the solidification process of binary Al-Si hypoeutectic alloys during upward and downward solidification modes. Undercooling is always observed during solidification, but the phenomenon could not be considered in the present-day numerical solution. In this approach, the temperature distribution in the mushy zone was used to define the fraction of solid, which enabled the evaluation of the effect of dendrite tip undercooling on the characteristics of the binary alloy solidification. The present numerical algorithm was found to significantly reduce the computation time. Transient temperature distribution and solidification time from the numerical analysis, with consideration of natural convection due to temperature and concentration gradients, have been successfully simulated and validated with experiment results. Numerical results with consideration of dendrite tip undercooling have better agreement with experimental results. The effect of dendrite tip undercooling on the fluid flow (velocity profile), G, R and λ1 for both upward and downward solidification modes of Al-Si alloys have been investigated and discussed. Consideration of undercooling was found to increase G and reduce R in both solidification modes. During downward solidification, considering undercooling significantly increased flow velocity and decreased λ1. The primary dendrite arm spacing could be validated with results from uni-directional solidification experiments only when dendrite tip undercooling was considered.

A comprehensive characterization of virgin and recycled 316L powders during laser powder bed fusion

The primary constraints hindering the widespread adoption of laser powder bed fusion (LPBF) are expensive raw materials, internal defects, difficulty in controlling the quality and stability, and lack of consistency in the powder feedstock. The spatters induced by the violent interaction between the laser and powder can be deposited on the unmelted powder bed, and if not effectively separated by sieving, they would pose a significant detriment to the characteristics and consistency of the recycled powders. In this work, a systematic investigation of the evolution mechanism of 316L powders after 10 and 30 successive recycling in the LPBF process was presented. Firstly, the variation mechanism of particle size distribution and morphology of 316L powder and the generation mechanism of heterogeneous particles were studied. Then, changes in microstructure and hardness trends were characterized. Finally, we emphasized the changes of chemical composition, phase composition, magnetic properties, and the formation mechanism of oxide spots. The results show that the circulation had significant effects on the physical properties, phase transformation, mechanical properties, and magnetic properties of the 316L powders, but had minor effects on the chemical composition, surface and cross-sectional microstructure. In addition, a large number of oddly shaped, large-diameter spatters were identified in the recycled powders, which could not be separated effectively by sieving, and the formation mechanism of the spatters were elaborated. Based on the comprehensive high-temperature oxidation thermodynamics and experimental results, it was concluded that the circular oxide spots on the spatter surface were a composite of multiple oxides of Mn and Si. Further, it was also shown that the ferrite content, XRD peak characteristics, and magnetic properties in the recycled powders were governed by the droplet solidification modes, which in turn is determined by the chemical composition and cooling rates.

On the local solute redistribution equation of macrosegregation, remelting and the formation of channel segregates

In this paper, we reassess the local solute redistribution equation (LSRE) of macrosegregation which, since it first appeared in 1960s, has served as a cornerstone for understanding the composition variations that occur in the solidification of alloys. We highlight some anomalies in earlier literature, in particular as regards the prediction of remelting as a precursor to the formation of channel segregates (freckles, A-segregates and V-segregates) in casting processes. Also, we suggest extensions to the LSRE for situations where solute diffusion in the solid phase is not negligible, as well as when the mode of solidification is unconsolidated equiaxed dendritic, rather than columnar/consolidated equiaxed dendritic. In addition, the significance of the equation for latter-day numerical computations of macrosegregation is also discussed.

Plasma spheroidization of gas-atomized 304L stainless steel powder for laser powder bed fusion process

Particles of AISI 304L stainless steel powder were spheroidized by the induction plasma spheroidization process (TekSphero-15 spheroidization system) to assess the effects of the spheroidization process on powder and part properties. The morphology of both as-received and spheroidized powders was characterized by measuring particle size and shape distribution. The chemistry of powders was studied using inductively coupled plasma optical emission spectroscopy for evaluation of composing elements, and the powder&amp;rsquo;s microstructure was assessed by X-ray diffraction for phase identification and by electron backscattered diffraction patterns for crystallography characterization. The Revolution Powder Analyzer was used to quantify powder flowability. The mechanical properties of parts fabricated with as-received and spheroidized powders using laser powder bed fusion process were measured and compared. Our experimental results showed that the fabricated parts with plasma spheroidized powder have lower tensile strength but higher ductility. Considerable changes in powder chemistry and microstructure were observed due to the change in solidification mode after the spheroidization process. The spheroidized powder solidified in the austenite-toferrite solidification mode due to the loss of carbon, nitrogen, and oxygen. In contrast, the as-received powder solidified in the ferrite-to-austenite solidification mode. This change in solidification mode impacted the components made with spheroidized powder to have lower tensile strength but higher ductility.

SCC behaviour of laser and hybrid laser welded stainless steel weld joints

In this study, stress corrosion cracking (SCC) resistance was evaluated for type 316LN stainless steel weld joints fabricated by advanced welding techniques such as Laser, Hybrid Laser – Tungsten inert gas (HLT) and Hybrid Laser – Metal inert gas (HLM) welding. 316 LN stainless steel base metal exhibited significantly better resistance to SCC. The solidification mode, secondary dendritic arm spacing (SDAS) and tensile residual stress distribution in the weld metal was found to strongly influence SCC resistance of 316LN stainless steel weld joints. The weld joint processed by HLM process exhibited the highest time to failure during SCC testing and hence better SCC resistance due to primary ferritic mode of solidification, finer SDAS and lower width of tensile residual stress distribution.

A FEM Analysis on the Influence of Manganese on Carbon and Chromium Macrosegregation in Large Size Steel Ingot

In this study, 3D numerical simulations were performed to study the effect of Mn on the macrosegregation behaviors of carbon and chromium in a 40 MT steel ingot using Finite Element Modeling (FEM). Two Mn contents of 0 and 5 wt.% were investigated. Thermophysical properties such as specific heat, density and phase fractions were determined using thermodynamic software Thermo-Calc®. Simulation results indicated that higher Mn content increases the carbon macrosegregation while it tends to lower the one of chromium. Moreover, it changes the solute poor band into rich one in the case of chromium and no bands were obtained for carbon. These results are analyzed in terms of the changes of thermophysical properties, interactions between alloying elements and the change in the primary solidification mode from δ-ferrite to austenite resulting from the increase of Mn concentration.