Ferrite Transformation Temperature Research Articles

Ultrastrong and ductile steel welds achieved by fine interlocking microstructures with film-like retained austenite

The degradation of mechanical properties caused by grain coarsening or the formation of brittle phases during welding reduces the longevity of products. Here, we report advances in the weld quality of ultra-high strength steels by utilizing Nb and Cr instead of Ni. Sole addition of Cr, as an alternative to Ni, has limitations in developing fine weld microstructure, while it is revealed that the coupling effects of Nb and Cr additions make a finer interlocking weld microstructures with a higher fraction of retained austenite due to the decrease in austenite to acicular ferrite and bainite transformation temperature and carbon activity. As a result, an alloying design with Nb and Cr creates ultrastrong and ductile steel welds with enhanced tensile properties, impact toughness, and fatigue strength, at 45% lower material costs and lower environmental impact by removing Ni.

The Effect of Cooling Rate on the Banded Structure of 20CrMnTi Gear Steel Based on C Diffusion and Partitioning Analysis

This study, through experimental analysis and numerical calculations, not only identifies the critical cooling rate for eliminating banded structures in 20CrMnTi gear steel but also analyzes the impact of cooling rate on banded structures, including the mechanisms related to element distribution and interface migration. The results indicate that the fundamental cause of banded structure formation is the presence of chemical segregation bands, and increasing the cooling rate leads to a reduction in the grade of ferrite/pearlite banded structures, with a critical cooling rate of 0.55 °C s−1. Bainite and martensite banded structures are observed in samples cooled at rates of 3 and 5.8 °C s−1, respectively. As the cooling rate increases from 0.07 to 1 °C s−1. The ferrite phase transformation time in the solute‐poor zone is shortened from 171 to 12 s within the ferrite phase transformation temperature difference (Δ) between the poor and rich solute zones. The concentration difference of carbon (C) on both sides of the α/γ phase interface increases from 0.350 mass% to 0.431 mass%, and the diffusion distance of C decreases from 34.38 to 7.57 μm. When the diffusion distance of C is less than half of the width of the solute‐rich banding (12.5 μm), the ferrite banded structure disappears.

Machine learning interphase precipitation behavior of Ti micro-alloyed steel guided by physical metallurgy principle

Nano-sized interphase precipitates, which form in ordered rows, are critical for high strength low alloy (HSLA) steels, in achieving the desired strength needed for downgauging for light-weighting automotive structures. The occurrence of interphase precipitation depends on many interrelated parameters, such as alloy chemical composition, temperature, processing parameters and crystallography. In this paper, we use data analysis based on machine learning algorithm: decision tree to predict whether interphase precipitation can occur. Due to the high strength of interphase precipitation and the minimum alloy content is one of the important goals pursued of iron and steel enterprises. Therefore, under the condition that interphase precipitation occurs, the chemical composition of the alloy can be reduced by using interphase precipitation. Guided by the physical metallurgy principle, this paper transforms the processing parameters into physical metallurgy parameters such as grain size, stored deformation energy, ferrite phase transformation temperature, and decision tree (DT) algorithm was used to model and verify whether interphase precipitation can occur. Under the condition of interphase precipitation, the support vector machine (SVM) models of interphase precipitation characteristic values were established. Based on the DT model and the SVM model, the particle swarm optimization (PSO) algorithm was used for the reduction design of the alloy, and the chemical composition can be reduced according to the target strengthening strength. The results of alloy reduction have been verified by experiments, which shows that the alloy can be reduced by using interphase precipitation.

On the role of chemical banding and austenite grain size in microstructure evolutions and phase transformation kinetics of gear steels

Defects in the banded structure of low-carbon gear steel significantly limit the application of gears in the construction industry. Therefore, the banded structure in the gear steel must be eliminated. In this study, the content and segregation banding width of Si, Mo, Cr, Mn, and Ni, phase distribution, austenite grain size, and hardness were characterized using electron probe microanalysis, laser scanning confocal microscopy, electron backscatter diffraction, and Vickers indenter. Element diffusion and α/γ interfacial migration were calculated using the Thermo-Calc analysis and simulation methods. The results showed that the best mechanical properties experimental steels without banded structures can be obtained during heating at 1300 °C for 2 h, 1250 °C for 1 h, and 1150 °C for 2 h for 20CrNiMo, 22CrMo, and 20CrMnTi, respectively. Under the critical hot-rolling process of eliminating the banded structure, the ferrite transformation temperature difference ΔAr3 and ferrite transformation interval (Ar3-Bs) of the 22CrMo sample were the largest. Si and Mo promoted interfacial migration, whereas Cr, Mn, and Ni inhibited interfacial migration. The interfacial migration speed of 22CrMo is the highest. The interfacial migration distance of the 22CrMo sample was the longest, and the final interfacial migration distance of 20CrMnTi was the shortest. When the migration position of the α/γ interface was less than 65 % of the system's total size, the ferrite-banded structure disappeared. The critical austenite grain sizes of 20CrNiMo, 22CrMo, and 20CrMnTi were 31.6, 35, and 22.2 μm, respectively. The banded structure is eliminated when the austenite grain size is smaller than the element segregation banding width. When eliminating the banded structure, the interfacial migration distance of 22CrMo was the longest and its critical austenite grain size was the largest.

Effect of B Segregation at Prior Austenite Grain Boundaries on Microstructures and Toughness in the Heat‐Affected Zone of Mg‐Deoxidized Shipbuilding Steel Plates

The effect of B segregation at prior austenite grain boundary (PAGB) on the microstructure and toughness in the heat‐affected zone (HAZ) after high heat input welding at kJ cm−1 for Mg‐deoxidized shipbuilding steel plates is clarified. With 0.0022 wt.% B addition, B segregation occurs at PAGBs in two forms: solute B and fine Fe23(BC)6 precipitates. B segregation with a concentration of about 4 at.% decreases boundary energy by 0.13 J m−2, which reduces the nucleation density of boundary ferrites by increasing the critical activation energy of ferrite nucleation. Thus, coarse grain boundary ferrites (GBFs) and ferrite side plates (FSPs) are suppressed with film‐like allotriomorphic ferrites formed at PAGBs, and ferrite transformation temperature reduces from 712–543 to 679–541 °C. In B‐free steels, (Fe, Mn)3C particles form at PAGBs with the Mn‐depleted zone (MDZ) at around (Fe, Mn)3C, promoting GBFs and FSPs. However, (Fe, Mn)3C precipitates and MDZ are not observed at PAGBs with B addition, because B segregation delays Mn diffusion to PAGBs. Thus, GBFs and FSPs reduce, and intragranular acicular ferrites (IAFs) increase with B addition, making the dislocation density and high‐angle grain boundary density increase, which improve HAZ toughness at −40 °C from 40 to 141 J.

Irreversible loss of hot ductility following simulated primary cooling of a C–Mn steel to temperatures above the ferrite transformation temperature

A hot ductility study has been performed on a low carbon peritectic steel grade with the view to simulate the effect of a temperature drop in the region just below the mould on the reduction of area during simulated unbending of a continuous cast thick broad slab. A Gleeble 1500D thermomechanical simulator was used to subject cylindrical specimens to various thermal histories, which included a fast primary cooling cycle to various minimum temperatures, Tmin, followed by a temperature rebound cycle and a slow secondary cooling step up to the unbending temperature prior to straining at a constant strain rate. Poor ductility was found when primary cooling was terminated at 70 °C above the ferrite start transformation temperature for primary cooling. This loss of ductility was not recovered during the rebound heating or secondary cooling steps. Dilatometry was performed to investigate this observation and it was shown that a small amount of ferrite formed at low temperatures, during the rebound heating. The deterioration in ductility was determined to be due to the formation of these thin ferrite films on the prior austenite grain boundaries. Dilatometry was used to show that the ferrite films later transformed back to austenite and the irreversible loss of the hot ductility was attributed to finely dispersed second phase particles remaining in position where the ferrite films had initially formed on the austenite grain boundaries.

Features of Austenite Formation in Low-Carbon Steel during High Speed Heating Induced by High-Speed Deformation

An experiment is performed for explosive convergence (collapse) of a cylindrical shell of low carbon steel with a ferrite-perlite structure. It is revealed that during ultra-rapid heating caused by high-speed deformation austenite formation occurs in an unusual sequence: first, free ferrite is converted, then pearlite. The decrease in free ferrite transformation temperature is explained by action of high pressure, as well as by varying degrees of heating steel structural constituents. An effect of barothermal quenching is observed, as a result of which a pearlite-martensite structure forms.

Effect of boron on intragranular ferrite nucleation mechanism in coarse grain heat-affected zone of high-nitrogen steel

Abstract The effect of boron on intragranular ferrite (IGF) nucleation mechanism in the simulated coarse grain heated-affected zone (CGHAZ) of a high-nitrogen steel was investigated. When boron (B) was added to the steel, the ferrite transformation temperature increased from 775 °C to 800 °C and the amount of polygonal ferrite increased from 63.2% to 78.6%. Consequently, the ductile-to-brittle transition temperature decreased from −30 °C to −50 °C. (Ti,V)(C,N)-MnS-BN, MnS-BN, Al2O3-MnS-BN, and BN served as IGF nucleation sites. The core of the complex precipitates was formed by undissolved (Ti,V)(C,N)-MnS, Al2O3-MnS, and MnS, with a BN cap formed on the undissolved precipitates, which increased the IGF nucleation potency.

Investigation of massive ferrite in an interstitial-free steel

The formation of massive ferrite in an interstitial-free (IF) steel with three different austenitization conditions (at 1200, 1100 and 1000 °C respectively for 3 min) has been investigated. To conduct the rapid continuous-cooling transformations at a cooling rate of 500 °C/s, hollow rod specimens were employed in a dilatometer. Thermal arrests, which were associated with the phase transition, were detected in the cooling curves, and thereby the massive ferrite transformation temperatures were determined. The corresponding treated samples were studied by optical metallography and transmission electron microscopy. The features of massive ferrite and its transformation characterization are discussed.

Effect of Transformation Temperature on the Ferrite–Bainite Microstructures, Mechanical Properties and the Deformation Behavior in a Hot-Rolled Dual Phase Steel

The ferrite–bainite microstructures and mechanical properties in a hot-rolled dual phase steel with different transformation temperatures were investigated in this paper. The deformation behavior of bainite and ferrite was analyzed by using in situ scanning electron microscope. The results showed that the ferrite and bainite can be refined and the volume fraction of bainite can be slightly increased by 5% by decreasing the ferrite transformation temperature from 690 to 635 °C and increasing the bainite transformation temperature from 400 to 450 °C. The strengthening mechanism was dislocation difference in polygonal and acicular ferrite and then the grain size difference of ferrite. The hole expansion ratio increased from 52.9 to 83.7% with the ferrite transformation temperature decreasing. The cracks or voids were smaller in size and also the number in steel with the lower ferrite transformation temperature after expanding. The microstructure separation along the boundary between bainite and ferrite and the slip band in the ferrite appeared at the non-uniform deformation stage. However, fewer appeared in the microstructure of steel with lower ferrite transformation temperature. Cooperative deformation of ferrite and bainite and finer microstructure in steel with lower ferrite transformation temperature were the mechanisms why the cracks could not easily nucleated and propagated, respectively. Thus this was also why there was a better hole expansion ratio. The ferrite–bainite microstructures and mechanical properties in a hot-rolled dual phase steel with different transformation temperatures were investigated. The deformation behavior of bainite and ferrite was analyzed by using in situ scanning electron microscope. The strengthening mechanism was dislocation difference in polygonal and acicular ferrite and then the grain size difference of ferrite. The hole expansion ratio increased from 52.9 to 83.7%. Cooperative deformation of ferrite and bainite and finer microstructure in steel with lower ferrite transformation temperature were the mechanisms why the cracks could not easily nucleated and propagated, respectively. Thus this was also why there was a better hole expansion ratio.

Effect of Hot Rolling Process Parameters on Microstructure Transformation and Microstructure of 45MnSiV Steel

The effects of final rolling temperature, cooling rate and deformation on phase transition point, the duration of the phase transition and the pearlite laminar layer of non-quenched and tempered steel 45MnSiV were studied by simulating the process of rolling and post-rolling cooling on Gleeble-3500 thermal simulator and thermal expansion tester. The results show that: the ferrite and pearlite transformation temperature ranges from 510 °C to 700 °C, and the bainite transformation temperature ranges from 400 °C to 500 °C when the steel is continuously cooled at a final rolling temperature of 950 °C, and the martensite transforming temperature is 300 °C under high cooling rate (> 10 °C/s); The pearlite laminar spacing decreases with the decrease of final rolling temperature. It can be seen that the rolling deformation increases the temperature at which the test steel undergoes a phase change at each cooling rate by comparing the results of deformation and no-deformation test at 950 °C. The effect of time advance on the phase transition zone of ferrite and pearlite is particularly obvious, but the effect on the phase transition temperature and time of the bainite and martensite phase transition is not obvious. When the final rolling temperature remains constant, the Rockwell hardness value of the test steel gradually increases, and the pearlite layer spacing decreases with the decrease of ferrite transformation temperature gradually and the increase of the cooling rate.

Austenite – ferrite transformation temperature regression equations for low carbon steels with cooling rate account

The austenite-ferrite transformation temperatures evaluated by dilatometry using thermo-mechanical simulator Gleeble 1500D are investigated in this paper. The effect of cooling rates 1, 5, 10 and 15°C/s on the upper and lower critical transformation temperatures was evaluated for 30 specimens of six material groups. Considering the cooling rate from dilatometry tests and chemical composition (C ≤ 0.2%, Mn ≤ 2%, Si ≤ 0.26%) of particular specimens, the regression equations for both transformation temperatures were derived. These relations have to be satisfied to avoid the crack formation during continuous casting, as well as to provide the hot rolling control. The proposed regression equations are compared with 32 similar ones adopted from 1961 to 2017 and exhibit a good conformity and accuracy.

Effect of microalloying elements on microstructure and properties of quenched and tempered constructional steel

The effects of microalloying elements Nb, V and Ti on microstructure and properties of quenched and tempered steel were studied. Results showed that the addition of microalloying elements led to the formation of bainite and increased strength, while the austenization and ferrite transformation temperature was barely affected, i.e. 10°C. Microalloying elements shortened the incubation time for bainite transformation by refinement of austenite grain, and decreased the hardenability by forming carbides and therefore reducing the carbon content of super-cooled austenite. Either of them promoted the bainite transformation. The better tempering stability was ascribed to the as hot-rolled bainite microstructure and secondary carbide precipitation during tempering.

Modeling Static and Dynamic Kinetics of Microstructural Evolution in Hot Deformation of Fe–0.15C–0.2Si–1.4Mn–0.03Nb Alloy

To achieve microstructural refinement of ferrous alloys by using severe plastic deformation (SPD) methods, investigation of the microstructural evolution during hot deformation near the austenite–ferrite transformation temperature of ferrous alloys is necessary. In this study, a series of single and double‐compression tests of cylindrical Fe–0.15C–0.2Si–1.4Mn–0.03Nb specimens are conducted in the temperature range from 750 to 900 °C. On the basis of experimental results, inverse analysis, micrographic investigation, finite element method (FEM) analysis, and regression analysis are carried out to obtain the material genome for Fe–0.15C–0.2Si–1.4Mn–0.03Nb alloy, which describes the static and dynamic kinetics of microstructural evolution of this alloy. The validity and feasibility of the proposed material genome are verified by compression tests of hourglass‐shaped specimens and subsequent micrographic investigation.

Significant influence of carbon and niobium on the precipitation behavior and microstructural evolution and their consequent impact on mechanical properties in microalloyed steels

Carbon and niobium (Nb) play an important role in influencing the ultimate microstructure and mechanical properties. In this regard, we elucidate here the impact of carbon and Nb on the microstructural evolution and precipitation behavior during continuous cooling of industrially processed microalloyed steels with varying carbon and Nb-content. The microstructure and precipitation evolution was studied via electron microscopy and related to the outcome of thermodynamic simulation. The increase of carbon content in steel increased the precipitation temperature of (Nb, Ti)(C, N), which led to relatively larger size (Nb, Ti)(C, N) precipitates. Furthermore, high carbon content contributed to stabilization of austenite and delayed the transformation of ferrite and bainite, such that martensite/austenite content (M/A) was obtained. The M/A islands in high carbon-containing steel contributed to highest strength and intermediate elongation. The high degree of (Nb, Ti)(C, N) precipitation in steel contributed to refinement of prior austenite grain size and strain accumulation, which increased ferrite and bainite start transformation temperature, resulting in higher volume fraction of polygonal ferrite. Polygonal ferrite in steel with high Nb-content was responsible for relatively low strength in comparison with steels with higher carbon or intermediate carbon-Nb contents. Granular bainite and lath bainite in steel with intermediate C and Nb-contents was characterized by best combination of strength and elongation. The outcomes of the thermodynamic simulations were consistent with the experimentally observed microstructure.

Experimental Simulation of Thermally Induced Stresses during Cooling of Continuously Cast Steel Slabs

Continuous casting is an established production route for steel slabs. The solidifying slab suffers a combination of internal stresses caused by thermal gradients, phase transformation, and external bending. The complex superimposed stress condition may produce local concentrations of tensile stresses that initiate cracks during cooling and deformation. Samples prepared from low-carbon steel slabs with and without Ni-content are compared and transformation stress phenomena are analyzed. In situ neutron diffraction experiments are carried out to reveal the stress condition within sections of cast steel slabs during cooling while passing the austenite–ferrite transformation temperature. The austenite–ferrite phase transformation is accompanied by an increase of volume of ≈1% between austenite-f.c.c. and ferrite-b.c.c., which causes internal stresses within the thermal stress gradients by cooling from the surface. The superposition of these thermal and transformation stresses is determined during cooling by an additional in situ diffraction experiment at constant macroscopic strain. The crack sensitivity of the Ni-containing slab is attributed to higher internal stresses within the austenite caused by the lower transition temperature than for the Ni-free steel.

Microstructural Characterization and Mechanical Properties Analysis of Weld Metals with Two Ni Contents During Post-Weld Heat Treatments

This study designed post-weld heat treatments, including reheating and tempering, associated with hot bending to investigate the microstructures, toughness, and hardness of two weld metals with different Ni contents (<1 wt pct level). The results indicated that a high Ni content decreased the ferrite transformation temperature and increased the proportion of acicular ferrite (AF). Furthermore, a high Ni content promoted the martensite/austenite (M/A) constituent formation after reheating. The promotion of the M/A formation increased the number of cementite particles, and accelerated cementite coarsening during tempering. The large-angle grain boundary density from the AF improved the toughness despite the negative effect of cementite. The strengthening contributions were calculated, and the grain refinement was the greatest. The high Ni content decreased the effective grain size with a 2 deg tolerance angle, thus enhancing the grain refinement contribution.

Hot Ductility and Deformation Behavior of C-Mn/Nb-Microalloyed Steel Related to Cracking During Continuous Casting

Hot ductility studies have been performed on C-Mn and C-Mn-Nb steels with an approach to simulate the effect of cooling conditions experienced by steel in secondary cooling zone during continuous casting. Thermal oscillations prior to tensile straining deteriorate hot ductility of steel by deepening and widening the hot ductility trough. C-Mn steels are found to exhibit ductility troughs in three distinct zones whereas C-Mn-Nb steel shows drop in ductility only at low temperature in the vicinity of ferrite transformation temperatures. Start of ferrite transformation in steels causes yield ratio to increase while work hardening rates and strength coefficient decrease with decrease in test temperature in presence of thermal oscillation prior to tensile testing. Inhibition of recrystallization due to build-up of AlN particles along with the presence of MnS particles in structure and low work hardening rates causes embrittlement of steel in austenitic range. Alloying elements enhancing work hardening rates in austenitic range can be promoted to improve hot ductility. The presence of low melting phase saturated with impurities along the austenitic grain boundaries causes intergranular fracture at high temperature in C-Mn steels.

Research on CCT Curve of 12Cr2Mo1R Pressure Vessel Steel

The continuous cooling transformation (CCT) curve of 12Cr2Mo1R pressure vessel steel was measured on Gleeble-1500 hot simulator. The transformation and structures were observed and analyzed. Test results show that with the addition of Cr and Mo, ferrite and perlite transformation temperature increases while bainite transformation temperature decreases, resulting in bainite zone separating from ferrite and pearlite zone. The critical cooling rate for martensite is 30 °C/s.

On-line spheroidization process of medium-carbon low-alloyed cold heading steel

Conventionally manufactured 35CrMo cold heading steel must undergo spheroidization annealing before the cold heading process. In this paper, different types of deformation processes with various controlled cooling periods were operated to achieve on-line spheroidal cementite using the Gleeble-3500 simulation technique. According to the measured dynamic ferrite transformation temperature (Ad3), the deformation could be divided into two types: low temperature deformation at 810 and 780°C; “deformation-induced ferrite transformation” (DIFT) deformation at 750 and 720°C. Compared with the low temperature deformation, the DIFT deformation followed by accelerated cooling to 680°C is beneficial for the formation of spheroidal cementite. Samples subjected to both the low-temperature deformation and DIFT deformation can obtain granular bainite by accelerated cooling to 640°C; the latter may contribute to the formation of a fine dispersion of secondary constituents. Granular bainite can transform into globular pearlite rapidly during subcritical annealing, and the more the disperse phase, the more homogeneously distributed globular cementite can be obtained.