Kinetics of Martensite Formation in Substitutional Fe-Al Alloys: Dilatometric Analysis

High-resolution differential dilatometry was employed to study the kinetics of the martensite formation upon isochronal cooling/quenching of substitutional Fe-(0.5, 0.7, and 1.0) at. pct Al alloys at fast cooling/quenching rates in the range of 17 K (17 °C) through 100 K (100 °C) s^?1, with an emphasis on the as-yet unexpected influence of cooling/quenching rate. The martensite transformation initiated at nearly the same temperature (i.e., the $ M_{\text{S}} $ temperature) in the ferrite-phase region for all cooling/quenching rates applied, which indicates athermal nucleation: the chemical driving force governs the initiation of the nucleation of the martensite plates. Variation of the cooling/quenching rates revealed two principal kinetic features: both the temperature ranges passed during transformation and the grain size of the product martensite increase with the increase of cooling/quenching rates. A modular phase-transformation model, incorporating a classic partitioning analysis for nucleation and anisotropic growth for impingement, has been employed to extract the velocity of the migrating martensite/austenite interface from the dilatometric data. The thus obtained velocity of the martensite/austenite interface as function of temperature indicates a thermally activated growth governed by relatively lower activation energy, as determined by evaluation of the martensite-formation-rate maximum as function of cooling/quenching rate.     

In situ observation of growth behaviour of acicular ferrite in low-carbon steel containing 13 ppm magnesium

SS400 steel with 13?ppm magnesium was prepared to study the effects of supercooling degree on the formation behaviour of acicular ferrite (AF) and microstructures of Mg-containing low-carbon steel. The inclusions characterised using an automated inclusion analyzer, ASPEX, were mostly MnS, MgAl₂O₄ and MgO with sizes of 1–2?μm. The growth behaviour of AF during the cooling process for austenite transformation to ferrite and continuous cooling temperature diagram was investigated using in situ observation with high-temperature confocal laser scanning microscopy. The initial formation temperature of AF decreased with increasing cooling rate. The temperature of AF transformation was in a narrow range of around 100–150?K. The probability of AF nucleation from inclusions and the average AF lath growth rate increased with increasing cooling rate due to the larger driving force and thermal strain energy around the inclusions during the cooling processes.     

Effect of Boron on the Hot Ductility Behavior of a Low Carbon Advanced Ultra-High Strength Steel (A-UHSS)

This research work studied the effect of boron additions (14, 33, 82, 126, and 214 ppm) on the hot ductility behavior of a low carbon advanced ultra-high strength steel. For this purpose, specimens were subjected to a hot tensile test at different temperatures [923 K, 973 K, 1023 K, 1073 K, 1173 K, and 1273 K (650 °C, 700 °C, 750 °C, 800 °C, 900 °C, and 1000 °C)] under a constant true strain rate of 10^?3 s^?1. The reduction of area (RA) of the tested samples until fracture was taken as a measure of the hot ductility. In general, results revealed a marked improvement in hot ductility from 82 ppm B when the stoichiometric composition for BN (0.8:1) was exceeded. By comparing the ductility curve of the steel with the highest boron content (B5, 214 ppm B) and the curve for the steel without boron (B0), the increase of hot ductility in terms of RA is over 100 pct. In contrast, the typical recovery of hot ductility at temperatures below the Ar₃, where large amounts of normal transformation ferrite usually form in the structure, was not observed in these steels. On the other hand, the fracture surfaces indicated that the fracture mode tends to be more ductile as the boron content increases. It was shown that precipitates and/or inclusions coupled with voids play a meaningful role on the crack nucleation mechanism, which in turn causes hot ductility loss. In general, results are discussed in terms of boron segregation and precipitation on austenitic grain boundaries during cooling from the austenitic range and subsequent plastic deformation.     

Determination of optimum blast furnace slag cooling rate for slag recycling in cement manufacture

Abstract

The crystallisation behaviour of molten blast furnace slag was observed in situ using the single hot thermocouple technique. Isothermal and non-isothermal experiments were conducted to construct the diagrams for time temperature transformation and continuous cooling transformation. The molten slag should be cooled at a minimum critical cooling rate of 10°C s^?1. During crystallisation, melilite is the main crystal phase and rankinite is the primary phase. The crystallisation mechanism of the melilite crystal phase involves one-dimensional direction growth with bulk nucleation, whereas the growth mechanism of the rankinite crystal phase is between the surface nucleation mechanism and the one-dimensional direction growth mechanism. The crystallisation activation energies of the melilite and rankinite crystal phases are 238.07±28.81 and 523.52±58.56 kJ mol^?1 respectively.     

Gas Atomization of Amorphous Aluminum: Part I. Thermal Behavior Calculations

In this article, the thermal history and cooling rate experienced by gas-atomized Al-based amorphous powders were studied via numerical simulations. Modeling simulations were based on the assumption of Newtonian cooling with forced convection, as well as an energy balance, which involves gas dynamics, droplet dynamics, and heat transfer between gas and droplet. To render the problem tractable, phase transformations, crystal nucleation, and growth were not taken into account in the analysis of the solidification of Al droplets; instead, an energy balance approach was formulated and used. The numerical results and associated analysis were used to optimize processing parameters during gas atomization of Al-based amorphous powder. The results showed that the cooling rate of droplets increases with decreasing powder size and can reach in excess of 10⁵ K/s for powder <20 μm in diameter. Gas composition has a more significant influence on cooling rate than gas pressure, and 100 pct He has the highest cooling effect. The results also showed that the cooling rate increases with increasing melt superheat temperature.     

凝固过程重轨钢中MnS粒子形核与长大动力学分析

利用经典形核理论和扩散控制长大模型计算分析了重轨钢中MnS粒子析出的动力学行为,计算结果表明,MnS粒子在重轨钢凝固过程以均匀形核和晶界形核为主,主要在凝固末期析出。在设定的重轨钢成分下,计算出MnS的有效形核温度为1 634K,即Mn、S实际浓度积等于平衡浓度积。降低S的质量分数小于5.0×10-5能够推迟MnS接近固相线析出,而对MnS的长大半径影响较小;提高冷却速率从0.14K/s到1.45K/s,连铸坯内柱状晶区中MnS的长大半径比中心等轴晶区的大1个数量级,但对MnS的析出时机无影响。S元素是MnS在凝固过程中粗化长大的控制性环节,在凝固过程冷却速率对MnS粒子长大半径起着决定性的作用。     

Fracture Behavior of High-Nitrogen Austenitic Stainless Steel Under Continuous Cooling: Physical Simulation of Free-Surface Cracking of Heavy Forgings

18Mn18Cr0.6N steel was tension tested at 0.001 s^?1 to fracture from 1473 K to 1363 K (1200 °C to 1090 °C, fracture temperature) at a cooling rate of 0.4 Ks^?1. For comparison, specimens were tension tested at temperatures of 1473 K and 1363 K (1200 °C and 1090 °C). The microstructure near the fracture surface was examined using electron backscatter diffraction analysis. The lowest hot ductility was observed under continuous cooling and was attributed to the suppression of dynamic recrystallization nucleation.     

In Situ Thermo-magnetic Investigation of the Austenitic Phase During Tempering of a 13Cr6Ni2Mo Supermartensitic Stainless Steel

The formation of austenite during tempering of a 13Cr6Ni2Mo supermartensitic stainless steel (X2CrNiMoV13-5-2) was investigated using an in situ thermo-magnetic technique to establish the kinetics of the martensite to austenite transformation and the stability of austenite. The austenite fraction was obtained from in situ magnetization measurements. It was found that during heating to the tempering temperature 1 to 2 vol pct of austenite, retained during quenching after the austenitization treatment, decomposed between 623 K and 753 K (350 °C and 480 °C). The activation energy for martensite to austenite transformation was found by JMAK-fitting to be 233 kJ/mol. This value is similar to the activation energy for Ni and Mn diffusion in iron and supports the assumption that partitioning of Ni and Mn to austenite are mainly rate determining for the austenite formation during tempering. This also indicates that the stability of austenite during cooling after tempering depends on these elements. With increasing tempering temperature the thermal stability of austenite is decreasing due to the lower concentrations of austenite-stabilizing elements in the increased fraction of austenite. After cooling from the tempering temperature the retained austenite was further partially decomposed during holding at room temperature. This appears to be related to previous martensite formation during cooling.     

Homogeneous nucleation kinetics of Al3Sc in a dilute Al-Sc alloy

The homogeneous nucleation kinetics of the L1₂ A1₃Sc phase in the face-centered cubic (fcc) matrix of an A1-0.11 at. pct Sc alloy were measured at 561 and 616 K using the isothermal transformation technique and transmission electron microscopy (TEM). Applying classical homogeneous nucleation theory in conjunction with available thermodynamic data, an average interphase boundary energy of about 94 ±23 mJ/m² was estimated from the nucleation rate data. This energy was found to vary weakly with temperature. The effects of heat-treatment technique (isothermal transformationvs quench and age) on nucleation kinetics indicate that quenched-in vacancies may influence nucleation kinetics primarily by increasing the interdiffusion of Sc in Al. This paper is based on a presentation made in the “G. Marshall Pound Memorial Symposium on the Kinetics of Phase Transformations” presented as part of the 1990 fall meeting of TMS, October 8–12, 1990, in Detroit, MI, under the auspices of the ASM/MSD Phase Transformations Committee.     

The effect of cooling conditions on the microstructure of rapidly solidified Ti-6Al-4V

The effect of cooling conditions, giving estimated cooling rates in the range 10⁴ °C per second to 10⁷ °C per second, on the microstructure of Ti-6Al-4V has been evaluated. The microstructures of as-solidified particulates were martensitic, with the martensite lath length decreasing with beta grain size,L, which in turn decreased with increasing cooling rate. For material alpha + beta heat-treated or vacuum hot pressed, the alpha morphology was dependent on the prior cooling rate. For materials cooled at <5 × 10⁵ °C per second martensite transformed to lenticular alpha, while material cooled at >5 × 10⁵ °C per second developed an equiaxed alpha morphology. This change in morphology was explained in terms of high dislocation density or grain size refinement, both of which result from the high cooling rate. When the beta grain size (L) was plottedvs section thickness (z), and estimated cooling rate (T), power law relationships analogous to those reported for secondary dendrite arm spacing were found:L = 1.3 ± 0.4z^089±006 (thin, chill-substrate quenched),L = 0.17 ± 0.05z^0.86±0.01(thick, convection-cooled material), andL = 3.1 × 10⁶ T^{−0.93±0.12} (all material), whereL and z are in μm andT is in K/s. The last relationship is in agreement with the 0.9 exponent predicted using a model developed for the effect of grain size on cooling rate assuming classical homogeneous nucleation and isotropic linear growth during solidification. The first two relationships were rationalized by assuming that the two materials cooled under near-Newtonian conditions.     

Simulation of austenite decomposition in continuous cooling conditions: a cellular automata-finite element modelling

Transformation of austenite to ferrite under continuous cooling condition was investigated. The heat conduction problem was managed by finite element method while two-dimensional cellular automata modeling was simultaneously performed to predict the progress of austenite decomposition using a two-step algorithm to reduce surface-to-volume ratio. Continuous cooling experiments on low carbon steel were made and the ferrite structure was determined and compared with the simulation data. The predicted and the experimental results demonstrated an acceptable consistency and the activation energy for ferrite growth was determined as 171 kJ/mole. The rate of ferrite transformation increased under examined continuous cooling conditions owing to higher nucleation rate. Moreover, the initial austenite grain size has shown a significant impact on the rate of transformation e.g. in air-cooled samples as the austenite grain size decreased from 24 to 34 µm, the mean ferrite grain size decreased about 8 µm.     

Crystallization Control in Metallurgical Slags Using the Single Hot Thermocouple Technique

With the single hot thermocouple technique (SHTT) the solidification behavior of metallurgical slags has been studied by in situ observation, constructing time–temperature–transformation (TTT) or continuous‐cooling‐transformation (CCT) diagrams. The SHTT is a unique apparatus that enables measurement of the slag sample temperature using a thermocouple while the sample is heated or cooled simultaneously. Due to the low heat capacity of the system sample/thermocouple high heating or cooling rates can be easily obtained (>3000°C/min). The following findings are reported in the present paper: (i) For the CaO–Al₂O₃ slag – 44% CaO, 56% Al₂O₃ (wt%) – the CCT diagram shows large differences between liquidus and the temperature for first crystals precipitation, even at low cooling rates, for example, 168°C below the liquidus when cooling at a rate of 6°C min^?1. (ii) For the CaO–SiO₂ slag – % CaO/% SiO₂ (wt%) = 0.7 – no crystal is observed for continuous cooling, even at low cooling rates, such as 10°C min^?1. During isothermal experiments crystallization was observed only at 1000°C with an incubation time of 76 s (average of six experiments, standard deviation 27 s). However, crystallization becomes much more intense for the CaO–SiO₂ slag when increasing the temperature after reaching lower temperatures (<1000°C), where probably the conditions for nucleation are better.     

Characterization of the nucleation and growth behavior of copper precipitates in low-carbon steels

The nucleation and growth behavior of copper precipitates in ferrite was investigated both theoretically and experimentally for two low-carbon steels with and without niobium additions in samples cooled directly to the desired aging temperature from the austenitizing temperature. Theoretical nucleation and growth rate models were constructed using calculated thermodynamic data in conjunction with classical theories. The maximum nucleation and growth rates for Cu were experimentally determined to be 8.0 × 10²¹ nuclei/m³ s at 612 °C and 0.12 nm/s at 682 °C, respectively. Using an experimentally determined “effective” activation energy for the diffusion of copper, the theoretical nucleation rate curve compared very well with the hardness data for the first 5 minutes of aging. The growth behavior of the Cu precipitates was investigated through use of a conventional transmission electron microscope (TEM) for samples directly aged at 550 °C. For aging times up to 21 hours, the average precipitate size scaled with a time dependence of t ^1/2.     

Effect of Process Variables on the Grain Size and Crystallographic Texture of Hot-Dip Galvanized Coatings

A galvanizing simulator was used to determine the effect of galvanizing bath antimony (Sb) content, substrate surface roughness, and cooling rate on the microstructural development of metallic zinc coatings. Substrate surface roughness was varied through the use of relatively rough hot-rolled and relatively smooth bright-rolled steels, cooling rates were varied from 0.1 to 10 K/s, and bulk bath Sb levels were varied from 0 to 0.1 wt pct. In general, it was found that increasing bath Sb content resulted in coatings with a larger grain size and strongly promoted the development of coatings with the close-packed {0002} basal plane parallel to the substrate surface. Increasing substrate surface roughness tended to decrease the coating grain size and promoted a more random coating crystallographic texture, except in the case of the highest Sb content bath (0.1 wt pct Sb), where substrate roughness had no significant effect on grain size except at higher cooling rates (10 K/s). Increased cooling rates tended to decrease the coating grain size and promote the {0002} basal orientation. Calculations showed that increasing the bath Sb content from 0 to 0.1 wt pct Sb increased the dendrite tip growth velocity from 0.06 to 0.11 cm/s by decreasing the solid–liquid interface surface energy from 0.77 to 0.45 J/m². Increased dendrite tip velocity only partially explains the formation of larger zinc grains at higher Sb levels. It was also found that the classic nucleation theory cannot completely explain the present experimental observations, particularly the effect of increasing the bath Sb, where the classical theory predicts increased nucleation and a finer grain size. In this case, the “poisoning” theory of nucleation sites by segregated Sb may provide a partial explanation. However, any analysis is greatly hampered by the lack of fundamental thermodynamic information such as partition coefficients and surface energies and by a lack of fundamental structural studies. Overall, it was concluded that the fundamental mechanisms behind the microstructural development of solidified metallic zinc coatings have yet to be completely elucidated and require further investigation.     

Effects of Heating and Cooling Rates on Phase Transformations in 10 Wt Pct Ni Steel and Their Application to Gas Tungsten Arc Welding

10 wt pct Ni steel is a high-strength steel that possesses good ballistic resistance from the deformation induced transformation of austenite to martensite, known as the transformation-induced-plasticity effect. The effects of rapid heating and cooling rates associated with welding thermal cycles on the phase transformations and microstructures, specifically in the heat-affected zone, were determined using dilatometry, microhardness, and microstructural characterization. Heating rate experiments demonstrate that the Ac₃ temperature is dependent on heating rate, varying from 1094 K (821 °C) at a heating rate of 1 °C/s to 1324 K (1051 °C) at a heating rate of 1830 °C/s. A continuous cooling transformation diagram produced for 10 wt pct Ni steel reveals that martensite will form over a wide range of cooling rates, which reflects a very high hardenability of this alloy. These results were applied to a single pass, autogenous, gas tungsten arc weld. The diffusion of nickel from regions of austenite to martensite during the welding thermal cycle manifests itself in a muddled, rod-like lath martensitic microstructure. The results of these studies show that the nickel enrichment of the austenite in 10 wt pct Ni steel plays a critical role in phase transformations during welding.     

The impact of cooling rates on the microstructure of Al-U alloys

The impact of cooling rates on the microstructure of Al-U alloys was studied by optical, scanning electron, and transmission electron microscopy. A variety of solidification techniques were employed to obtain cooling rates ranging between 3 × 10⁻² and 10⁶ K/s. High-purity uranium (99.9 pct) and high-purity aluminum (99.99 pct), or “commercially pure” type Al-1050 aluminum alloys were used to prepare Al-U alloys with U concentration ranging between 3 and 22 wt pct. The U concentration at which a coupled eutectic growth was observed depends on the cooling rates imposed during solidification and ranged from 13.8 wt pct for the slower cooling rates to more than 22 wt pct for the fastest cooling rates. The eutectic morphology and its distribution depends on the type of aluminum used in preparing the alloys and on the cooling rates during solidification. The eutectic in alloys prepared from pure aluminum was evenly distributed, while for those prepared from Al-1050, the eutectic was unevenly distributed, with eutectic colonies of up to 3 mm in diameter. Two lamellar eutectic structures were observed in alloys prepared from pure aluminum containing more than 18 wt pct U, which solidified by cooling rates of about 10 K/s. One structure consisted of the stable eutectic between UAl₄ and Al lamella. The other structure consisted of a metastable eutectic between UAl₃ and Al lamella. At least three different eutectic morphologies were observed in alloys prepared from Al-1050.     

In Situ Observation of Phase Transformation and Structure Evolution of a 12?pct Cr Ferritic Stainless Steel

This work focuses on an in situ observation of phase transformation of a 12?pct Cr ferritic stainless steel using high-temperature laser scanning confocal microscopy. ?????????? phase transformation temperatures are determined to be approximately 1073?K and 1423?K (800?°C and 1150?°C), respectively. The onset of phase transformation is found to occur at grain boundaries. When the temperature is beyond 1518?K (1245?°C), the grain growth rate suddenly becomes very high, and the grain growth is related to the self-organizing of adjacent grains. ?????? phase transformation has been mostly restrained when cooling rates are in the range of 22.4?K/s to 13.3?K/s (22.4?°C/s to 13.3?°C/s) except for at grain boundaries. Martensitic phase transformation, rather than ?????? phase transformation, occurs when the cooling rates are in the range of 8.5?K/s to 2.2?K/s (8.5?°C/s to 2.2?°C/s). The starting temperature of martensitic phase transformation is approximately 697?K to 728?K (424?°C to 455?°C) for specimens heated to 1373?K (1100?°C) (i.e., ?? phase field), which is 50?K to 100?K (50?°C to 100?°C) higher than that of specimens heated to 1723?K (1450?°C) (i.e., ?? phase field). Many bulges remain on the surfaces of the specimen heated to 1723?K (1450?°C), and their formation mechanism is analyzed.     

An Investigation of the Massive Transformation from Ferrite to Austenite in Laser-Welded Mo-Bearing Stainless Steels

A series of 31 Mo-bearing stainless steel compositions with Mo contents ranging from 0 to 10 wt pct and exhibiting primary δ-ferrite solidification were analyzed over a range of laser welding conditions to evaluate the effect of composition and cooling rate on the solid-state transformation to γ-austenite. Alloys exhibiting this microstructural development sequence are of particular interest to the welding community because of their reduced susceptibility to solidification cracking and the potential reduction of microsegregation (which can affect corrosion resistance), all while harnessing the high toughness of γ-austenite. Alloys were created using the arc button melting process, and laser welds were prepared on each alloy at constant power and travel speeds ranging from 4.2 to 42 mm/s. The cooling rates of these processes were estimated to range from 10 K (°C)/s for arc buttons to 10⁵ K (°C)/s for the fastest laser welds. No shift in solidification mode from primary δ-ferrite to primary γ-austenite was observed in the range of compositions or welding conditions studied. Metastable microstructural features were observed in many laser weld fusion zones, as well as a massive transformation from δ-ferrite to γ-austenite. Evidence of epitaxial massive growth without nucleation was also found when intercellular γ-austenite was already present from a solidification reaction. The resulting single-phase γ-austenite in both cases exhibited a homogenous distribution of Mo, Cr, Ni, and Fe at nominal levels.     

Investigation of Degree of Undercooling of Fe-O Alloy on Al2O3 Substrate

An Fe-O drop of ~0.4 g in an alumina crucible was quenched from ~2200 K (~1927 °C) at a cooling rate of ?87 K/s by flowing He gas at a flow rate of 25 L/min STP. A deep undercooling of approximately 200 K was obtained regardless the oxygen content in the sample. According to the classical nucleation theory, the contact angle between a solid Fe nuclei and the Al₂O₃ substrate was estimated to be 53 (±4) deg.     

Thermal dependence of austempering transformation kinetics of compacted graphite cast iron

The evolution of the relative fraction of high-carbon austenite with austempering time and temperature was analyzed in a compacted graphite (CG) cast iron (average composition, in wt pct: 3.40C, 2.8Si, 0.8Mn, 0.04Cu, 0.01P, and 0.02S) at five different austempering temperatures between 573 and 673 K. Samples were characterized by Mössbauer spectroscopy, hardness measurements, and optical microscopy. During the first stage of transformation, the kinetics parameters were determined using the Johnson-Mehl’s equation, and their dependence with temperature in the range from 573 to 673 K indicates that the transformation is governed by nucleation and growth processes. The balance between growth-rate kinetics and nucleation kinetics causes the kinetics parameter (k) to have a maximum at ≈623 K of 3.9×10^?3(s^?1). The evolution of the C content in the high-carbon austenite was found to be controlled by the volume diffusion of carbon atoms from the ferrite/austenite interface into austenite, with a dependence of t ^0.40±0.05 on the austempering time (t).