Austenite Size Research Articles

Achieving superior ductility for laser directed energy deposition 300 M steel through isothermal bainitic transformation

The microstructure evolution and tensile properties of laser directed energy deposition (LDED) 300 M steel with isothermal heat treatment at 320 ℃ for different holding time were investigated in this paper. The results showed that the microstructure of the as-deposited LDED 300 M steel consisted of martensite and bainite. After heat treatment, the microstructure of the LDED 300 M steel is still the mixture of martensite, bainite and retained austenite, while the size of the lower bainite significantly decreased. As the increasing holding time, the content of martensite gradually decreased, while the content of bainite and austenite gradually increased. When the holding time of LDED 300 M steel increased from 20 min to 40 min, the number and size of austenite increased slightly. However, when the holding time was 60 min, the number and size significantly increased. As holding time continuously increase, it contributes little to the number and size of austenite. With regard to the hardness of the LDED 300 M steel, it was 464 HV when the holding time was 20 min. With the increment of holding time, the hardness gradually decreased until 428 HV when the holding time was 80 min. While holding time had less effect on the tensile strength and yield strength. To be specific, the tensile strength decreased from 1348 MPa to 1332 MPa, and yield strength decreased from 853 MPa to 759 MPa. The elongation gradually increased, and it increases to 33 % when the holding time was 80 min, which was higher than that of the forged 300 M steel standard (15 %). Effect of various factors, like microstructure, dislocation density and ε-carbides precipitated in the bainite on the ductility of the LDED 300 M steel with isothermal heat treatment was also studied.

Microstructural Characteristics and Impact Fracture Behaviors of a Novel High-Strength Low-Carbon Bainitic Steel with Different Reheated Coarse-Grained Heat-Affected Zones

To obtain the correlation of microstructural characteristics and toughness in a novel high-strength low-carbon bainitic structural steel with medium and heavy plate after multipass welding, a welding thermal simulation experiment was conducted to simulate different subregions in the reheated coarse-grained heat-affected zones (CGHAZ). The microstructure evolution was then analyzed and factors that influence the fracture behavior were studied. The results show that the brittle zone appeared in subcritical reheated CGHAZ, and the fractured morphology was cleavage fracture. Supercritical reheated CGHAZ had the highest impact toughness, and the fractured morphology was primarily the ductile fracture with dimples formed via the micropore polycondensation mechanism. With an increase in the secondary pass welding thermal cycle peak temperature (tp2), the average length size of martensite and austenite (M-A) decreased from 9 to 2 μm. The coarsening of M-A constituents was the main reason for decrease in the crack initiation absorbed energy. A large number of retained austenite and cementite precipitates in subcritical reheated CGHAZ clearly worsened the impact toughness, and the massive austenite and cementite precipitates more than offset the beneficial effects of high-angle boundaries. This phenomenon led to disappearance of the effect of high-angle grain boundary of prior austenite and lath bainite on arresting crack propagation. In supercritical reheated CGHAZ, crack propagation absorbed energy was increased because of grain refinement, fine precipitates, lamellar residual austenite at corners, and high-angle grain boundary.

Microstructure-mechanical property evaluation and deformation mechanism in Al added medium Mn steel processed through intercritical rolling and annealing

The present work investigates the microstructure evolution, mechanical properties and deformation mechanism in medium Mn high Al steel processed through intercritical rolling and subsequent intercritical annealing treatment at different temperatures. The annealed samples possessed a multi-phase microstructure consisting of intercritical ferrite/martensite, austenite and δ-ferrite. However, the morphology of the phases varied with annealing temperature. The result shows that annealing at a temperature of 730° and 780 °C led to the development of bimodal grain structure consisting of fine laths and equiaxed ultra-fine-grains (UFG) of ferrite-austenite; whereas annealing at 830 °C led to fully equiaxed coarse ferrite-austenite grains. An excellent combination of strength and ductility (product of ultimate tensile strength and elongation) as high as 56 GPa% was obtained in the 780 °C annealed sample. The chemical composition and grain size of austenite was found to be critical factors governing its stability. A mixture of lath and equiaxed austenite grains, having appropriate stability, in 780 °C annealed sample led to sustained four-stage strain hardening during deformation. Multiple work hardening mechanisms involving transformation induced plasticity (TRIP) effect, twin induced plasticity (TWIP) effect and discontinuous TRIP effect were found to occur sequentially in the equiaxed and lath-type austenite during the deformation that led to the extraordinary strength ductility combination.

Microstructural evolution and mechanical properties of 9Mn steel during warm/cold rolling and subsequent intercritical annealing

The microstructural evolution of 9Mn steel during warm rolling in the intercritical region and cold rolling, both followed by subsequent intercritical annealing was studied. A significantly different microstructure was obtained by warm rolling and cold rolling, the former contained austenite and deformed ferrite/martensite and the latter consisted of deformed ferrite and transformed martensite, which significantly impacted the microstructure after intercritical annealing. The existence of austenite in warm-rolled sample reduced the elemental content in the deformed matrix, weakening the kinetics of austenite formation during subsequent annealing. In addition, the occurrence of dynamic recovery during warm rolling led to a low storage energy of recrystallization, resulting in a partial recrystallization of ferrite during IA. However, a slightly larger volume fraction as well as more heterogeneous austenite morphology with a wide austenite size distribution was obtained by combining both warm rolling and annealing. These austenite grains could transform to martensite in a more sustainable way, increasing the TRIP-related strain. Consequently, the warm-rolled + annealed steel showed a better combination of ultimate tensile strength (1380 MPa) and total elongation (29.11%).

FORMATION OF CARBON STEEL STRUCTURE DURING HOT PLASTIC DEFORMATION

Purpose. The main purpose of the work is to determine the peculiarities of the development of recrystallization processes of carbon steel austenite depending on the degree of hot plastic deformation and to develop proposals for improving the structural state of the metal of the railway solid-rolled wheel. Methodology. Two carbon steels of a railway wheel with a minimum and maximum carbon content of 0.55 and 0.65 % and other chemical elements within the grade composition of the steel 60 were used as research material. Samples in the form of cylinders with a diameter of 20 mm and a height of 40 mm were heated in a muffle furnace, exposed for a certain time to equalize the temperature across the cross section of the sample. After that, the samples were subjected to hot compression on Instron type test machine. The temperature interval of hot compression of the samples was 950–1100 ºС, with deformation degrees in height in the range of 10–40%. The strain rate was 10-3–10-2sec-1. A standard etching was used to detect the boundaries of the austenite grains. Structural studies were performed using Epikvant type light microscope at magnifications sufficient to determine the structure of austenite grains. The grain size of austenite was determined by the methods of quantitative metallography. Findings. In the case of hot compression of the railway wheel blank, increasing the concentration of carbon atoms only within the grade composition of the steel is sufficient to increase the average austenite grain size, which confirms the proposals to limit the carbon content in the metal of railway wheels. The formation of a certain degree of austenite structural heterogeneity at the cross section of the rim or hub of the railway wheel is due to a change in the development mechanism of recrystallization processes depending on the deformation value. Under conditions of the same degree of hot plastic deformation, the replacement of one-time compression by fractional one is accompanied by a violation of the conditions of formation of the recrystallization nucleus. As a result of the specified replacement of the scheme of hot plastic deformation we obtain reduction in the austenite grain size. Originality. Based on a study of the development of collective recrystallization processes during the hot compression of carbon steel of the railway wheel, it was determined that the increase in carbon content contributes to the austenite grain increase. After hot compression of the wheel blank, the structural inhomogeneity of austenite that occurs is determined by a change in the mechanism of recrystallization processes development. During deformations above the critical degree, the recrystallization nuclei are formed and successively grow, which leads to the structure refinement. In the case of deformations below the critical value, the growth of austenite grains occurs according to the coalescence mechanism, according to which fragments of boundaries with large disorientation angles consistently disappear. Practical value. For austenite grain refining in massive elements of solid-rolled railway wheel we offer to replace one-time hot compression by fractional one.

Microstructure and mechanical properties of dissimilar friction stir welds in austenitic-duplex stainless steels

We produce defect-free dissimilar friction stir welds between 1.86 mm-thick 2205 duplex stainless steel (DSS) and 1.95 mm-thick 304 austenitic stainless steel (ASS). The refined grain structure in the stirring zone (SZ) has three typical regions. The duplex phase region (DPR) and the austenite phase region (APR) have similar microstructures as the SZ of 2205 DSS welds and 304 ASS welds, respectively. The mixed region (MXR) has a smaller ferrite volume fraction (less than 25%) than the 2205 base metal (BM, 46.0%), whereas the average grain size of ferrite (0.82 μm) and austenite (0.83 μm) in the MXR is significantly smaller than that of austenite grains (2.32 μm) in the APR. The evolution of the corresponding microstructure and the diffusion of elements within the typical regions are interpreted according to experimental observations and phase diagrams. Due to the grain refining, the MXR (287.5 Hv) and the DPR (298.5 Hv) both have a higher microhardness than 2205 BM (252.7 Hv). The APR (226.5 Hv) also has a higher microhardness than 304 BM (189.7 Hv). The tensile tests indicate that the dissimilar welds have a good ductility and a higher tensile strength (905 MPa) than the 304 BM (783 MPa).

Grain refinement and orientation alternation of 10 mm 316L welds prepared by magnetic field assisted narrow gap laser-MIG hybrid welding

The effect of vertical magnetic field on grain size and orientation of ferrite (δ) and austenite (γ) in welds of 10 mm 316L stainless steel were investigated during multilayer narrow gap laser-MIG hybrid welding. Results show that shapes of weld beads are changed by the ampere force from “T” to “V” and “▪”, which results from arc current and magnetic field. The optimized fusion boundary accelerates thermal diffusion and raises temperature gradient, which reduces duration of high temperature of δ-γ solid phase transition and heat affected zone (HAZ). As a result, δ-islands and reduction of HAZ are observed. At the meantime, the growth direction of γ grains are altered from preferred 〈100〉-crystal direction to maximum temperature gradient with the increase of temperature gradient. Moreover, vermicular, continuous and reticular δ dendrites are changed to be skeletal, deflected and seaweed-like in the interface and columnar zone under the effect of magnetic field. The occurrence of this phenomenon is associated with the joint effect of ampere force, viscous resistance and hot metal liquid. And the fragment of primary δ dendrites leads to the refinement of γ grains and increase of central zone.

Effects of prior austenite grain size on impression creep and microstructure in simulated heat affected zones of boron modified P91 steels

The induction of this paper was to simplify the exaggeration effects of prior austenite size (PAG) size on microstructure, impression creep, creep-rupture life, creep-damage, and precipitate size in the sub-heat affected zones (sub-HAZs) of boron modified P91 steels. Simulation of sub-HAZs caused the manipulation in PAG size and microstructure. This manipulation reduced precipitate sizes (~50%) in P91B samples that lead to a 20% reduction in the non-homogeneous distribution of size of precipitates. Evaluation of minimum creep-damage rate (MCR) suggested FG (type-IV) and B-CG (type-III) as weakest links having a critical PAG size of 17 μm. But, creep-performance of B-CG was still better than most of the P91 samples. While PAG hardening limit was observed in CF/B-CF that made them the strongest links. Activation energy (AE) calculation showed that an increase in PAG size for P91 samples motivated MCR, inviting PAG embrittlement. While vice versa observed in P91B samples, inviting PAG hardening. P91B samples exhibited maximum creep-performance by the shrinking span of the creep-damage and increasing creep-rupture life. Mutual dependency between creep-rupture life and creep-damage showed that their relation was independent of microstructural features, thermal history, amount of deformation, and boron modifications. Whereas, limited creep-damage was found for samples having normalized creep-rupture life ≥ 2. This paper also discussed various diffusion mechanisms for sub-HAZs by simplifying inconsistency between calculated and actual AE that suggested both interstitial and substitutional nature of boron. Small-grained samples (PAG ≤ 17 μm) had lower AE, while big-grained samples (PAG > 17 μm) had higher AE.

Bainite Transformation-Kinetics-Microstructure Characterization of Austempered 4140 Steel

Bainite transformation is a kinetic process that involves complex solid diffusion and phase structure evolution. This research systematically studies the bainite transformation of austempered 4140 steel in a wide range of isothermal temperatures, in which four bainite phases structures were generated: upper bainite; mixed upper bainite and lower bainite; lower bainite and mixed lower bainite and martensite. The kinetics of bainite transformation has been described with a linear trend using an Avrami n-value. It was found that the bainitic ferrite sheaves grow with widthwise preference. The sheaves are stable when half-grown and are variable in length, due to austenite size limit or soft/hard impingement, or autocatalytic nucleation, or these conditions combined. The full-grown upper/lower bainite sheaves were found to be 1.9 μm/1.2 μm in width under the conditions of this study. Each individual bainite sheave is lath-like instead of wedge-like. The upper bainite sheaves mostly appear as broad-short-coarse lath, while the lower bainite sheaves appear as narrow-long-fine lath. The overall bainite transformation activation energy ranges from 50–167 kJ/mol.

On nitrogen diffusion during solution treatment in a high nitrogen austenitic stainless steel

The generation and microstructural characteristics of the surface layer formed during solution treatment at 1200°C of a high nitrogen austenitic stainless steel, have been investigated using optical microscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). Contrasting with a bulk microstructure of austenite and ferrite, a multi-layer surface structure is obtained, which consists of a white outer-surface layer which is ferrite and a grey subsurface layer with austenite. Formation of the multi-layer structure occurs through phase transformations between austenite and ferrite due to the various levels of nitrogen diffusion. With increasing solution treatment time, the thicknesses of both the surface and the subsurface layer increase. Hardness tests show that the microhardness increases from the surface layer, to the subsurface layer and then to the bulk. This is attributed to the combined effects of solid solution strengthening from nitrogen and the grain size of austenite or ferrite.

Effect of vibration frequency on primary phase and properties of grey cast iron fabricated by lost foam casting

The properties of gray cast iron (GCI) are affected by density of matrix, size of flake graphite and primary austenite. In this paper, the Y-type specimen of GCI was prepared by lost foam casting (LFC) with and without vibration, and the influence of vibration frequency on the density of matrix, size of primary phase, and properties of the GCI was studied. The results show that the length of the flake graphite and the size of the primary austenite in GCI firstly decrease and then increase with the increase of the vibration frequency. With a vibration frequency of 35 Hz, the length of the flake graphite is the shortest, the primary austenite is the finest and the density of the matrix is the highest. In addition, the tensile strength, elongation and hardness of the GCI firstly increase and then decrease with the increase of the vibration frequency, due to the refinement of the primary phase and the increase of the matrix density. In order to analyze the refinement mechanism of the primary phase of the GCI fabricated by the LFC with vibration, the solidification temperature fields of the GCI fabricated by the LFC with the vibration frequency of 0 and 35 Hz were measured. The results show that the vibration reduces the eutectic point of the GCI and increases the supercooling degree during the eutectic transformation. As a result, the length of the flake graphite and the size of the primary austenite in GCI fabricated by LFC with the vibration frequency of 35 Hz decrease.

Investigation on Microstructure Evolution and Strengthening Mechanism in Boron-Bearing Advanced High Strength Steel

Investigation on the casting, hot-rolling and heat treatment process of a high boron-bearing advanced high strength steel was conducted. The scanning electron microscopy (SEM), electron-probe micro-analyzer (EPMA), electron backscattered diffraction (EBSD) and X-ray diffraction (XRD) were employed to analyze the evolution of microstructure which includes the phase constitution and boride morphology during hot processing. Through the control of as-casted, heat treated microstructure and using interrupted tensile tests, the strengthening mechanism was revealed. Results showed that, the strain-induced ferrite transformation of austenite and grain size were the decisive factors which control the strength and ductility, while the defects and internal stress etc. were the secondary factors.

Effect of the austenitizing temperature on the microstructure evolution and mechanical properties of Q&P steel

Combining quenching and partitioning heat treatment (Q&P) with bainite phase transformation, the 0.24C-1.53Si-2.01Mn-0.019Nb high strength steel was heat treated at various austenitizing temperatures. The microstructure evolution and their effects on mechanical properties were analyzed by means of scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD). Using the discontinuous tensile test combined with in situ EBSD observation to explore the phase transformation behavior of retained austenite during tensile deformation, we found that diversified retained austenite size, morphology and distribution are the main reasons for the excellent mechanical properties of Q&P steel and the strength-plastic product of 25.5 GPa·%.

Microstructure and mechanical properties of a low-carbon HDQ&P steel

ABSTRACTThe microstructure and mechanical properties of Fe-0.19C-1.46Mn-1.50Si, a low-carbon hot-rolling direct quenching and partitioning steel, were investigated, and particular emphasis was placed on the effects of the rolling temperature and the subsequent cooling method. Once rolling occurred above the recrystallization temperature, the grain size of prior austenite could be reduced from ∼88.2 to ∼11.8 μm. The corresponding packet and block sizes of martensite could be reduced from ∼17.6 to ∼4.3 μm and from ∼9.0 to ∼1.8 μm, respectively, after a typical two-step quenching and partitioning (Q&P) treatment following rolling immediately. Once rolling occurred below the recrystallization temperature, deformed prior austenite with a grain length and width of ∼150 and ∼60 μm, respectively, were obtained. Consequently, only the block size would be reduced to ∼1.9 μm. The volume fraction of retained austenite (RA), which was ∼11% in the specimens, was not significantly affected by the rolling temperature. Further, if the typical two-step Q&P treatment was replaced by air cooling after the rolled specimen was quenched to the martensitic start temperature, the volume fraction of RA in the specimen was reduced to ∼7.4%, and the corresponding carbon depletion degree in the martensitic matrix was decreased as well. Tension and impact tests indicated that the cooling method was more critical than the rolling temperature for determining the mechanical properties. The presence of sufficient amounts of RA and carbon depletion in the refined martensitic matrix contribute to the strong strain hardening and energy absorption capabilities of the steel together. As a result, rolling at a temperature that was slightly higher than the recrystallization temperature combined with the typical two-step Q&P treatment could be an optimized process to obtain excellent comprehensive mechanical properties. In particular, the ductile-to-brittle transition temperature (DBTT) could be reduced from −10°C to −30°C.

Effect of prior austenite on reversed austenite stability and mechanical properties of low carbon medium manganese steel heavy plate

AbstractThe effect of prior austenite on reversed austenite stability and mechanical properties of Fe‐0.06C‐0.2Si‐5.5Mn‐0.4Cr (wt.%) annealed steels was elucidated. With the decrease of austenitizing temperature from 1250 °C to 980 °C, the prior austenite changed from complete recrystallization to partial recrystallization, and the average austenite size was reduced. The volume fraction of reversed austenite was increased from 26.32 % to 30.25 % because of high density of grain boundaries and dislocations. The martensite transformation temperature of annealed steels was increased from ∼115 °C to ∼150 °C, and both of thermal and mechanical stability of reversed were reduced. There was no significant different in tensile properties, however, the impact toughness was enhanced from 100 J to 180 J at −60 °C. The excellent impact toughness in annealed steel (austenitized at 980 °C) was obtained because of higher density of high misorientation grain boundaries, more volume fraction of reversed austenite and reduced segregation at grain boundaries.

Microstructure and mechanical properties of two-step Cu-alloyed ADI treated by different second step austempering temperatures and times

Austempering ductile iron (ADI) is an attractive material due to its excellent comprehensive mechanical properties. However, the deficit in elongation and toughness always threatens its security application. Two-step austempering process is an effective way to improve elongation and toughness simultaneously. In the present work, the influence of the amount, morphology and distribution of ferrite and austenite on mechanical properties of ADI under different second-step austempering parameters has been analyzed. Results show that the amount of austenite and its carbon content decrease with increasing of second-step temperature. Carbide begins to precipitate as second-step austempering temperature reaches 380 °C. These factors together influence the mechanical properties of two-step Cu-alloyed ADI. Impact energy and fracture toughness are strongly affected by second-step austempering temperature, and are dramatically decreased with increase of second-step austempering temperature. Elongation remains constant when the second-step temperature is below 360 °C, and then it is rapidly decreased with further increase of second-step temperature. Strength is slightly influenced by second-step temperature. Ferrite morphology is not influenced by second-step austempering duration, while blocky retained austenite size is slightly decreased with the increasing of second-step austempering time. The amount of retained austenite is decreased while the carbon content of retained austenite is increased with the extending of second-step austempering time. The substructure of austenite is transformed from dislocation to twin when second-step austempering time exceeds 60 min. Strength and elongation are improved slightly with extending of second-step time. Impact energy and fracture toughness initially decrease with the extending of second-step time, and then remain constant when the time is longer than 60 min. This is a result of austenite content decreasing and carbon content of austenite increasing. The second-step austempering time mainly influences austenite content and its carbon content, which is a result of carbon diffusion behavior variation.

Mechanical Surface Treatments of AISI 304 Stainless Steel: Effects on Surface Microrelief, Residual Stress, and Microstructure

The surface roughness, residual stress, and microstructure of AISI 304 stainless steel specimens after laser shock peening (LSP), water jet cavitation peening (WjCP), water jet shot peening (WjSP), and multi-pin ultrasonic impact treatment (UIT) were studied in this work. Compared to the initial state, the surface roughness (Ra) was, respectively, decreased by approx. 5.5, 7.8, 38.2, and 91.1% after the LSP, WjCP, WjSP, and UIT processes. The volume fraction of e-martensite of ~ 3-5% was observed in all treated specimens except for the LSP-treated ones. The volume fraction of α′-martensite was increased in the following sequence: WjCP (~ 5%), LSP (~ 5%), WjSP (~ 25%), UIT (~ 50%). The studied mechanical surface treatments promote a significant reduction in grains size of both austenite (~ 15-20 nm) and martensite (~ 20-37 nm) leading to essential hardening. All studied processes result in the formation of compressive residual stresses (− 377…693 MPa) and the improvement in the bearing curve parameters. The microhardness estimated accounting for the contributions of different hardening mechanisms to the yield strength magnitude correlates well with the experimental data. The grain boundary hardening and dislocation hardening are concluded to be the most influential mechanisms.

Austenite reversion kinetics and stability during tempering of an additively manufactured maraging 300 steel

Reverted austenite is a metastable phase that can be used in maraging steels to increase ductility via transformation-induced plasticity or TRIP effect. In the present study, 18Ni maraging steel samples were built by selective laser melting, homogenized at 820 °C and then subjected to different isothermal tempering cycles aiming for martensite-to-austenite reversion. Thermodynamic simulations were used to estimate the inter-critical austenite + ferrite field and to interpret the results obtained after tempering. In-situ synchrotron X-ray diffraction was performed during the heating, soaking and cooling of the samples to characterize the martensite-to-austenite reversion kinetics and the reverted austenite stability upon cooling to room temperature. The reverted austenite size and distribution were measured by Electron Backscattered Diffraction. Results showed that the selected soaking temperatures of 610 °C and 650 °C promoted significant and gradual martensite-to-austenite reversion with high thermal stability. Tempering at 690 °C caused massive and complete austenitization, resulting in low austenite stability upon cooling due to compositional homogenization.

Effect of deep cryogenic pretreatment on microstructure and mechanical properties of warm-deformed 7 Mn steel after intercritical annealing

Microstructure and mechanical properties of 7Mn steel undertaken different intercritical annealing (IA) with or without deep-cryogenic treatment (DCT) were investigated in this work. It is found that the additional DCT process can lead to the changes in the size and composition of austenite of the steel after IA. With the decrease of the IA temperature, the improvement effect of the DCT on mechanical properties is more remarkable. The product of ultimate tensile strength (UTS) and total elongation (TE) of the pre-DCT specimen IA at 600°C (DCT + IA600) is nearly twice as much as that of the non-DCT specimen. Austenite and ferrite grains exhibit different morphologies, lamellar and equiaxed, and bimodal size distribution at different IA temperatures. With the decrease of IA temperature, the occurring of serration behavior on engineering stress-strain curves is postponed, and the density of serration as well as the mean value of work hardening (WH) rate decrease, both of which are caused by the increasing stability of austenite. The volume fraction, morphology and composition of austenite can be tailored by pre-DCT and IA process to achieve the optimum combination of the retained austenite (RA) volume fraction and stability. The DCT + IA645 specimen exhibits an outstanding combination of 1167 MPa UTS and 35.4% TE.

Carbon partitioning and microstructure evolution during tempering of an Fe-Ni-C steel

Partitioning of C during tempering of quenched Fe-9.6Ni-0.5C-0.6Mn-0.6Mo-0.7Cr-0.1V (at.%) steel is determined by atom probe tomography and the resulting microstructure is described. The precipitated austenite size, together with its C and Ni content control its thermal stability and these can vary differently with tempering time and temperature. Thus, both austenite and strong carbide formers compete for the available C early in the process. Due to widely different transport kinetics, C likely plays a dominant role early but is either fully consumed or its role diminishes by dilution, and Ni partitioning eventually takes over as the austenite stability-controlling species.