Boundary Allotriomorphic Ferrite Research Articles

Effect of quenching and tempering treatments on microstructure and mechanical properties of 300M ultra-high strength steel fabricated by laser powder bed fusion

To achieve high-performance 300 M steel parts through the Laser Powder Bed Fusion (LPBF) additive manufacturing process, post-heat treatments are indispensable. Herein, the effects of quenching and tempering treatments on the microstructure and mechanical properties of LPBF-fabricated 300M ultra-high strength steel were investigated, and the relevant strengthening and toughening mechanisms were analyzed. The microstructure of quenched samples primarily consisted of quenched martensite with minor amounts of bainite and some grain boundary allotriomorphic ferrite. The quenched samples presented significantly higher strength than the 300M forgings, primarily attributed to the contributions of solid solution strengthening, grain boundary strengthening and dislocation strengthening. After following tempering treatment, the microstructure of LPBF-fabricated samples mainly consisted of tempered martensite with minor amounts of bainite and residual austenite. Compared to the quenched samples, the tempered samples exhibited a decrease in strength while demonstrating a significant increase in ductility. The samples treated with the optimal heat treatment process (900 °C/1 h/Oil Cooling for quenching + two times of 300 °C/2 h/Air Cooling for tempering) demonstrated a remarkable strength–ductility balance, with a yield strength (YS) of 1834 MPa, an ultimate tensile strength (UTS) of 2099 MPa, and an elongation (EL) of 10.2%, which are all beyond or comparable to those of 300M forgings.

Effect of the Austenitization Route on the Bainitic Reaction Kinetics and Tensile Properties of an Alloyed Austempered Ductile Iron

The effect of the austenitization route on the bainitic reaction kinetics of an alloyed (3.2C–2.8Si–1.8Ni–1.4Cu–0.4Mn–0.2Mo–0.1Cr) austempered ductile iron was studied. Two-step, conventional and rapid austenitization heat treatments were employed to produce different austenite grains sizes (94, 39, and 15 μm, respectively) and, in one case, secondary graphite precipitation. The overall bainitic transformation kinetic at 350 °C was described using the Johnson-Mehl-Avrami-Kolmogorov equation, and the values of the Avrami's adjustable parameters were discussed. The austempering reaction of the coarser austenite microstructure with secondary graphite precipitation featured the fastest kinetics, while the one derived from the medium austenite grain showed the slowest reaction rate. The increase in the graphite/austenite interfacial area reduced the half-transformation time (t50-value) by one magnitude compared to the austenite grain boundary area. The austempered samples from the rapid austenitization route comparatively featured the best tensile properties and the highest ISO and ASTM standards grades despite the considerable proportion of grain boundary allotriomorphic ferrite.

Effect of Mn content on low-cycle fatigue behaviors of low-carbon bainitic steel

The microstructures and low-cycle fatigue behaviors of steels with different Mn contents subjected to continuous cooling heat treatment processes (from Ms +10°C to Ms −20°C) are examined. Phase transformation testing results show that the steel without Mn exhibits short transformation time and short incubation period. When Mn content increases from 1.8% to 3.2%, bainite transformation time is prolonged, especially incubation period. The microstructure of the steel without Mn is mainly grain boundary allotriomorphic ferrite, while the other three steels with Mn consist of bainitic ferrite plate and retained austenite. Mn is an essential element used to obtain lower bainite. The low-cycle fatigue of the steels undergoes three stages, namely, cyclic hardening, saturation or cyclic softening, and fracturing. Higher elongation of the steel without Mn is beneficial to improve fatigue life under plastic strain amplitudes. The Coffin–Manson formula and damage hysteresis model are then used to evaluate the fatigue performances of the steels. It shows that the steel with 2.3% Mn exhibits higher fatigue damage capacity (W0) with a considerable damage transition exponent (β) than the other three steels. This result attributes to the largest distribution of high-angle misorientation of in the bainitic steel with 2.3% Mn.

Thermal process influence on microstructure and mechanical behavior for friction taper plug welding in structural steel S355

The present work reports the influence of thermal process on microstructure and mechanical behavior for friction taper plug welding (FTPW) in structural steel S355. The results indicate that there is a significant decrease in total energy input and cooling time in temperature interval from 800 to 500 °C (Δt8/5) with increasing the axial force. However, the varying axial force has little effect on the peak temperature. The microstructure of weld zone contains acicular ferrite, polygonal ferrite, grain boundary allotriomorphic ferrite, and lath bainite. With decreasing total energy input and Δt8/5, the formation of lath bainite is promoted as well as the precipitation and growth of grain boundary allotriomorphic ferrite is inhibited. The hardness tends to increase with decreasing total energy input and Δt8/5. Charpy impact energy at the bonding line is almost 35 J ± 7 J for all specimens. On the other hand, Charpy impact energy at the weld zone increases gradually with decreasing total energy input and Δt8/5.

Microstructure characterization and mechanical behavior of laser additive manufactured ultrahigh-strength AerMet100 steel

Ultrahigh-strength AerMet100 steel thick plate was fabricated by laser additive manufacturing process. The as-deposited microstructures of the test steel were characterized using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The mechanical properties were then examined using vickers-hardness test and tensile test. Results indicate that the as-deposited microstructures of the steel mainly consist of grain boundary allotriomorphic ferrite (GBA), grain interior irregular proeutectoid ferrite, plate-like upper bainite, needle-like lower bainite and retained austenite, which result in a good strength and some ductility anisotropy. The low deformation compatibility of specimen at the transverse direction (perpendicular to the deposition direction) mainly ascribes to the poor cracking resistance of the prior-austenite columnar grain boundary with coarse GBA phases. Compared to the almost intergranular cracking taken place in the transverse tensile specimen, the fracture mode of the longitudinal tensile specimen is a mixed mode of the predominant transgranular cracking and minor intergranular cracking.

Microstructures and Properties of HAZ of JB800 Bainite Steel

JB800 steel has grain boundary allotriomorphic ferrite and granular bainite (FGBA/BG), and it is a kind of high strength low alloy steels, which has simple produce procedure, lower cost and excelled property. The law of microstructure transformation in CGHAZ, hardness, and impact toughness in HAZ of JB800 steel were studied by means of thermal simulation. The test results show that under the general condition of welding process (t8/5=5~50s), microstructure of CGHAZ is composed of mixture microstructure of Martensite and Bainite and with the increase of cooling rate, the content of Martensite will decrease, but that of Bainite will increase; when t8/5 is 20s, CGHAZ zone have better impact toughness, which is composed of 95% Martensite and 5% Bainite. Therefore t8/5 should be controlled at about 20s to get better impact toughness.

Charpy Impact Properties of Grain Boundary Allotriomorphic Ferrite and Granular Bainite Duplex Microstructure

A new type of high strength and low cost bainitic steel with ultra-low carbon content and high Si content has been developed on the basis of Mn-series air-cooling bainitic steels. The tensile properties of YS>690MPa and the impact toughness of AKV>60J at-40°C were obtained by controlling the processing parameters. This was attributed to the formation of the grain boundary allotriomorphic ferrite (FGBA) and the granular bainite (GB) with different shape of M/A islands. The high strength due to the inter-lath lamellar M/A islands or retained austenite companying with high dislocated bainitic ferrite laths of average 300nm width. The effect of microstructure on the impact crack initiation and propagation was studied. The results showed that crack initiation occurred in two different types of sites: at interphase boundaries of bainite ferrite (BF) and M/A islands, at grain boundaries. The FGBA and bainite ferrite (BF) both had blunting effect on microcrack tip to reduce the crack propagation path. Because of the presence of FGBA, the unit crack path was short, at less than 5μm. The blunting effect of BF could be enhanced by the M/A islands, which force the cracks change the propagation path and reduce the unit crack path to less than the size of bainite packets. The mechanism of low temperature microcrack origin of the ultra-low carbon bainitic (ULCB) steel with the microstructure of the FGBA and GB was also discussed.

Effect of Final Cooling Temperature on Mechanical Properties of a Water Cooled Mn-Series Low Carbon Bainitic Steel as 8Mn2SiNb

The effect of final cooling temperature on the mechanical properties of a water cooled Mn-series low carbon bainitic steel as 8Mn2SiNb has been investigated in this paper. The results indicate that the optimum final cooling temperature is 450 °C, followed by air cooling to room temperature. Compared with air cooling, the condition of water cooling to 450 °C increases the tensile strength and yield strength about 13.3% (From 805MPa to 929MPa) and 59.0%(From 464MPa to 741MPa) respectively, remaining 21.5% elongation and 151J toughness. SEM observation reveals that the microstructure of the steel after water cooling to 450 °C is mainly granular bainite +lath martensite +refined grain boundary allotriomorphic ferrite (FGBA). Compared with air cooling, the condition of water cooling to 450 °C increases the volume fraction of strengthening phase (M-A island) from 28.2% to 38.1%.

Effect of 0.06%Nb on the Microstructure and Mechanical Properties of Mn-Series Low Carbon Air-Cooling Bainitic Steels

The effect of 0.06%Nb on the microstructure and mechanical properties of grain boundary allotriomorphic ferrite (FGBA) / granular bainite (Bg) air-cooling bainitic steels has been investigated in this paper. The results indicate that the steel acquires superior mechanical properties by adding 0.06%Nb. Compared with Non-Nb steel, the addition of 0.06%Nb increases the tensile strength and yield strength about 37.1% (From 780MPa to 1070MPa)and 26.6%(From 557MPa to 705MPa) respectively, remaining 18.3% elongation and 97J toughness. The addition of 0.06%Nb not only promotes the nucleation of intragranular ferrite but also refines the allotriomorphic ferrite grain , both of which in turn contribute to the refinement of granular bainite cluster including its ferrite platelets and M-A islands. Under the synthetic roles of the microstructure refinement and precipitation strengthening, 148MPa yield strength improvement has been acquired in the low carbon air-cooling bainitic steel by the adding of 0.06%Nb.

Effects of Nb on the Microstructure and Mechanical Properties of Water-Quenched FGBA/BG Steels

In order to reduce the alloying cost, Mn-series low carbon water-quenched grain boundary allotriomorphic ferrite (FGBA)/granular bainite (BG) steels have been developed. The effect of 0.06 wt.% Nb on microstructure and mechanical properties of FGBA/BG steel was investigated. The result showed that the addition of 0.06 wt.% Nb improved the hardenability of the FGBA/BG steel, refined the grain size of FGBA, promoted the granular bainitic transformation, and refined the granular bainite including its bainitic ferrite and martensite/austenite (M/A) constituents. With the addition of 0.06 wt.% Nb, the yield strength increased from 560 to 741 MPa, and the impact energy increased from 93 to 151 J, respectively, for 30-mm thickness steel plates. It is supposed that the addition of 0.06 wt.% Nb could improve the mechanical properties of the FGBA/BG steel by refining the microstructure and increasing the amount of strengthening phases.

Microstructure and mechanical properties of FGBA/BG air cooling bainitic steels containing niobium

The effect of different Nb additions on the mechanical properties and microstructure evolution of grain boundary allotriomorphic ferrite (FGBA) / granular bainite (BG) air cooling bainitic steels was investigated. The results indicate that the tensile strength and yield strength increase by 157 and 97 MPa, respectively with the addition of 0.02wt% Nb. The steel acquires superior strength and toughness with the addition of 0.06wt% Nb. The results of scanning electron microscopy and transmission electron microscopy reveal that the addition of Nb not only refines the size of granular bainite but also increases the volume fraction of granular bainite in FGBA/BG steels. The refinement effect of granular bainite is improved with the increase of Nb content.

0.02%Nb空冷仿晶界型铁素体/粒状贝氏体复相钢的相变及强韧性

In order to produce Mn-series low carbon steels using alloying elements with low cost and following the conventional production line without controlled cooling facilities and not need- ing special heat-treatment, Mn-series low carbon air cooling grain boundary allotriomorphic ferrite (FGBA)/granular bainite (BG) steels, alloying with Si-Mn-Cr, have been developed. The effect of 0.02%Nb on the phase transformation and strength-toughness of a FGBA/BG steel have been inves- tigated by using Gleeble-1500D machine and hot rolling test in this paper. Two specimens with and without Nb addition were deformed by 21% at 850 and then air cooling to room temperature. For Nb free steel, the average size of FGBA is 25 µm in length and 10 µm in width. The average size of intragranular ferrite is about 6—8 µm. For FGBA/BG steel with 0.02%Nb, the average size of FGBA is 10 µm in length and 4 µm in width. The average size of intragranular ferrite is less than 4 µm. Compared with Nb free steel, the size of bainitic ferrite and martensite/austenite (M-A) island de- crease and their corresponding volume fractions increase in the FGBA/BG steel with 0.02%Nb. The experimental results indicate that the addition of 0.02%Nb increases the hardenability of the FGBA/BG steel, suppresses the transformation of ! �, refines the size of grain boundary allotriomorphic ferrite, promotes the granular bainitic transformation, lowers the bainitic start temperature (Bs), and refines the granular bainite including its bainitic ferrite and M-A islands. It is suggested that the effects of

Mn-Series Low-Carbon Air-Cooled Bainitic Steel Containing Niobium of 0.02%

A new hot rolled low-carbon air-cooled bainitic steel containing Nb of 0.02% has been developed based on alloying design of the grain boundary allotriomorphic ferrite (F gba )/granular bainite (B g ) duplex steel. The as-rolled microstructure and mechanical properties of bainitic steel containing Nb of 0.02% were investigated by tensile test, optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results show that adding 0.02% of Nb obviously improves the strength without sacrificing toughness of the F gba /B g steel. Adding 0.02% of Nb not only refines the grain boundary allotriomorphic ferrite grains but also promotes the refinement of granular bainite including its bainitic ferrite and M/A island. Any Nb(C,N) has been hardly observed in the steel containing Nb of 0.02%. It is suggested that the strengthening mechanism of Nb of 0.02% can be mainly attributed to the effect of Nb in solution (solute drag-like effect) on the phase transformation rather than the precipitation strengthening of Nb(C,N) particles.

Effect of Small Addition of Niobium on the Mechanical Properties of F<sub>GBA</sub>/ B<sub>G</sub> Duplex Phase Air Cooling Bainitic Steel

The effect of small addition of niobium on the mechanical properties of grain boundary allotriomorphic ferrite (FGBA)/ granular bainite (BG) air cooling bainitic steel has been investigated by tensile test, Optical Microscopy (OM), and scanning electron microscopy (SEM) observation. The results show that (1) The small addition of niobium improves the mechanical properties of the FGBA/BG steel obviously. Compared with Non-Nb FGBA/ BG steel, 0.02% Nb increases the tensile strength and yield strength about 20% (From 780Mpa to 937Mpa)and 17%(From 557Mpa to 650Mpa) respectively, remaining 18% elongation. (2) With the addition of 0.02%Nb, both the size and the volume fraction of FGBA decrease. Meanwhile, the volume fraction of refined intragranular ferrite and M-A islands increases. Compared with Non-Nb steel, the volume fraction of M-A island in 0.02Nb steel increases from 21% to 31%, and the average size of M-A island decreases from 1.2μm to 0.95um.(3)It is indicated that the small addition of Nb(0.02%) not only refines the allotriomorphic ferrite grain but also promotes the nucleation of intragranular ferrite, both of which in turn contribute to the refinement of granular bainite cluster including its ferrite platelets and M-A islands. (4)It is suggested that the strengthening effect of 0.02%Nb can be mainly attributed to the segregation of Nb to γ/α phase boundaries(solute drag-like effect) rather than the precipitation strengthening of Nb(C,N). Nb enrichment in the α/γ interphase causes the decrease of C-activities and activity gradient in the austenite matrix around the α/γ interphase and thus causes the decrease of C-diffusion rate in austenite, and further the restraint to ferrite growth. The effects also depress bainite transformation temperature and driving force, as a result, the bainite cluster is refined.

Effect of Austenite Deformation on Continuous Cooling Transformation Microstructures for 22CrSH Gear Steel

The effect of compressive deformation of austenite on continuous cooling transformation microstructures for 22CrSH gear steel has been investigated using a Gleeble 1500 thermal simulator. The experimental results show that the deformation of austenite promotes the formation of proeutectoid ferrite and pearlite, and leads to the increase of critical cooling rate of proeutectoid ferrite plus pearlite microstructure. The grain boundary allotriomorphic ferrite occupies the austenite grain surfaces when the prior deformation takes place or the cooling rate is decreased, which causes a transition from bainite to acicular ferrite. The deformation enhances the stability of transformation from austenite to acicular ferrite, which results in an increase of M/A constituent.

Cleavage Initiation in Ti-V-N and V-N Microalloyed Forging Steels

The effects of matrix microstructure and features of non-metallic inclusion (morphology, type, volume fraction and size) on cleavage initiation in medium-carbon Ti-V-N and V-N microalloyed resulphurized forging steels have been determined by examining fracture surfaces produced in room temperature Charpy impact tests. The steels were generally Al-deoxidised but one V-N steel was Si-deoxidised. It has been found that, in the Ti-treated steel, having a ferrite-pearlite microstructure, brittle fracture initiation occurred at cracked coarse (Ti,V)(C,N) single phase or [(Ti,V)(C,N)/Al2O3/MnS] multi-phase inclusions. In the Ti-free steels, cleavage initiation was dependent on matrix microstructure and non-metallic inclusions. In the low strength Ti-free steels, with a ferrite-pearlite microstructure, the absence of a continuous grain boundary ferrite layer led to initiation from interfacing pearlite colonies. For the bainitic microstructure, cleavage initiated close to the notch, but the microstructural feature responsible could not be identified. For the ferritepearlite microstructure in the Si-deoxidised V-N steel, cleavage initiated at cracked Mn-Al-(Ca) silicate inclusions. The higher matrix strength and more continuous nature of grain boundary allotriomorphic ferrite in the V-N steel deoxidised with Al was associated with cleavage initiation from V-rich (V,Ti)(C,N)-containing inclusions. These were generally of smaller size than those in the Ti-treated steels.

Microstructural evolution and tensile behaviour of medium carbon microalloyed steel processed through two thermomechanical routes

A multiphase microstructure was obtained in a medium carbon microalloyed steel using two step cooling (TSC) from a lower than usual finish forging/rolling temperature (800–850°C). A low temperature anneal was then used to optimise the tensile properties. A multiphase microstructure (ferrite–bainite–martensite) resulted from forging as well as rolling. These were characterised using optical and scanning and transmission electron microscopy. X-ray diffraction, transmission electron microscopy and hardness measurements were used for phase identification. Tensile properties and work hardening curves were obtained for both the forged and the rolled multiphase variants. A Jaoul–Crussard (J–C) analysis was carried out on the tensile data to understand the basic mode of deformation behaviour. Rolling followed by the TSC process produced a uniform microstructure with a very fine grain boundary allotriomorphic ferrite, in contrast to the forged variety, which contained in addition coarse idiomorphic ferrite. The volume fraction of ferrite and its contiguity ratio in the rolled microstructure were greater than in the forged grade. The rolled microstructure exhibited a better combination of strength and toughness than that of the forged material. The rolled steel work hardened more than the forged variety owing to its fine, uniform (bainite–martensite and ferrite) microstructure. Retained austenite present in these steels underwent a strain induced transformation to martensite during tensile deformation. The J–C analysis of the work hardening rates revealed typical three stage behaviour in both varieties during tensile deformation.

Microstructure control and wear resistance of grain boundary allotriomorphic ferrite/granular bainite duplex steel

Abstract The abrasive wear resistance of a hot-rolled and air-cooled high strength low alloy (HSLA) steel with grain boundary allotriomorphic ferrite/granular bainite (F GBA /Bg) duplex microstructure was investigated by comparison with the granular bainitic steel with the same composition and other two commercial HSLA ferrite/pearlite steels. The results from the wet sand/rubber wheel abrasion test show that the F GBA /Bg duplex steel possesses remarkably higher wear resistance than the two ferrite/pearlite steels, even higher than the granular bainitic steel with a higher hardness. The cross-sectional morphology of the worn surfaces reveals that the higher resistance to abrasive wear is mainly ascribed to the dispersive hard martensite/austenite (M/A) islands. The qualitative analysis on the work hardening characteristics and the microcrack initiation mechanisms indicates that in the F GBA /Bg duplex steel, the existence of the grain boundary allotriomorphic ferrite and the retained austenite in the self-tempered M/A islands improves the accommodated deformation capacity of the subsurface microstructure, and prevents the brittle exfoliation occurring in the granular bainitic steel.

Three-dimensional Morphology and Growth Kinetics of Intragranular Ferrite Idiomorphs Formed in Association with Inclusions in an Fe-C-Mn Alloy.

The three-dimensional (3-D) morphology and growth kinetics of intragranular ferrite idiomorphs formed in association with MnS(+VN) inclusions were studied in an Fe-C-Mn alloy. Ferrite idiomorphs began to be formed at a temperature ∼40°C lower than the grain boundary allotriomorphs, and the highest temperature of the formation of intragranular ferrite plates was ∼60°C below the Widmanstatten start temperature of grain boundary sideplates (W s ). The 3-D shape of ferrite idiomorphs was more equiaxed than grain boundary allotriomorphs. Measured growth rates were smaller than those calculated assuming spherical growth, similar to those of grain boundary allotriomorphs previously reported. The possible reasons for the retardation of growth, e.g. deprivation of carbon supersaturation by grain boundary ferrite allotriomorphs and solute drag etc., are discussed.

Growth kinetics of grain boundary allotriomorphs of proeutectoid ferrite in Fe-C-Mn-X2 alloys

The parabolic rate constant for the thickening of grain boundary ferrite allotriomorphs at the faces of austenite grain boundaries was measured as a function of isothermal transformation temperature in three Fe-C-X1-X2 alloys where X1 is Mn and X2 is successively Si, Ni, and Co. The results were compared with the predictions of the local equilibrium model for multi-component systems and with those derived from the theory of growth under paraequilibrium conditions. The distribution of Mn and Si in ferrite and austenite in the Fe-C-Mn-Si alloy was also measured as a function of reaction temperature with transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX). The observed temperature below which alloying element partition ceased was in good agreement with the local equilibrium model. Whereas the parabolic rate constant for thickening was considerably larger than the amount predicted by this theory in the alloying element diffusion-controlled regime, the opposite was true in the carbon diffusion-controlled regime. Similarly, the calculated paraequilibrium constant was usually considerably larger than that measured experimentally. Synergistic enhancements of the effects of Mn and X2 in diminishing thickening kinetics were observed for each X2. The time-temperature-transformation (TTT) curves for the beginning of transformation were calculated from a modified Cahn analysis for the overall kinetics of grain-boundary-nucleated reactions using values of the nucleation rate and the parabolic growth rate constant computed from various models and compared with experimentally determined TTT curves. Substantial discrepancies between the calculated and measured curves were ascribed to synergistic effects of Mn and X2 upon nucleation and growth kinetics.