Refinement Of Prior Austenite Grains Research Articles

Cyclic quenching treatment doubles the Charpy V-notch impact energy of a 2.3 GPa maraging steel

A cyclic quenching treatment (CQT) succeeded in turning a 2.3 GPa maraging steel with a Charpy impact energy of 9 J into a new grade with the same strength but a Charpy impact energy of 20 J upon 4 cyclic treatments. The improvement of mechanical properties is attributed to the refinement and increased chemical heterogeneity of the martensitic substructure, rather than the refinement of prior austenite grain (PAG), as well as the Transformation-Induced Plasticity (TRIP) effect facilitated by small austenite grains. The role of local segregation of Ni during CQT in the formation of Ni-rich austenite grains, Ni-rich martensite laths and Ni-poor martensite laths, was investigated and verified by DICTRA simulations. This study highlights the important influence of Ni partitioning behavior during CQT, providing insights into microstructural evolution and mechanical properties.

Effect of heat treatment on mechanical compression properties of C250 maraging steel fabricated by directed energy deposition

The mechanical compression properties of C250 maraging steel fabricated by directed energy deposition (DED), in the as-deposited and post-fabrication heat-treated conditions, are investigated under quasi-static loading conditions. The microstructure of the material is studied using analytical transmission electron microscopy and a comparison is made between the DED fabricated and conventionally produced wrought materials. The results reveal that the DED fabricated material, in the as-deposited condition, exhibits lower yield and ultimate compressive strengths compared to the commercially heat-treated wrought alloy, due to the absence of strengthening precipitates and the presence of a retained austenite phase. However, post-fabrication thermal treatment significantly improves the compressive strengths of the as-processed material to levels comparable to those of the conventionally produced wrought material. This is attributed to the formation of nano-sized Ni3Mo strengthening precipitates, a reduction in the amount of austenite phase, and the refinement of prior-austenite grains during the thermal treatment. Moreover, the DED fabricated material, in the heat-treated condition, with the presence of an austenite phase shows higher ductility than the commercially heat-treated wrought material.

Refinement mechanism of large heat-input welding CGHAZ microstructure by N addition and its effect on toughness of a V-Ti-N microalloying weathering steel

The large heat input welding performance of uncoated weathering steel remains a great challenge since their coarse grain heat-affected zones (CGHAZs) are usually occupied by coarse and brittle microstructure consisting of granular bainitic ferrite (GBF) and martensite/austenite (M/A) constituents. In this article, a V-Ti-N microalloying weathering steel suitable for large heat input welding was achieved through the N addition, and the mechanism dominating the modified microstructure and the enhanced impact toughness of CGHAZ was deeply elucidated mainly by HRTEM. The results indicated that in the simulated CGHAZ with large heat input of 80 kJ/cm, the increase of N content from 0.0030 wt% to 0.0075 wt%, and 0.0115 wt% elevated the density of nano- and submicron-sized particles of (Ti, V)(C, N) containing VN, which in turn resulted in the refinement of prior austenite grains (PAGs) by the improved grain boundary pinning effect and the enhancement in heterogeneous nucleation of intragranular ferrite (IGF) by the decreased lattice disregistry (δ) between (Ti, V)(C, N) and IGF. The fraction of high-angle grain boundaries (HAGBs) with misorientation tolerance angles (MTAs) higher than 15° was thereby increased while the mean equivalent diameter (MED) of ferritic grains with HAGBs was decreased. Simultaneously, the N addition scattered the M/A constituents, promoted the internal substructure of dislocation-type M/A at the expense of twin-type M/A, and accordingly reduced the hardness difference between the M/A constituents and the matrix. All these factors retarded brittle crack initiation and propagation, and low-temperature impact toughness of simulated CGHAZ increased from 62.8 J to 141.2 J.

New insights to the metallurgical mechanism of niobium in high-carbon pearlitic steels

Two series of high-carbon pearlitic steels (68C-xNb and 75C-xNb) with different C and Nb contents were produced by hot-rolling. The effects of Nb on the microstructure and impact toughness of high-carbon pearlitic steels were investigated using scanning electron microscope, transmission electron microscope, electron backscattering diffraction and impact toughness tests. In particular, the nucleation and growth of pearlite were investigated through in-situ observations. It was found that the average pearlitic nodule was markedly refined from 10.2 μm to 6.0 μm with the addition of 0.026 wt% Nb in 68C steels, and it was refined from 13.7 μm to 6.7 μm with the addition of 0.014 wt% Nb in 75C steels. In situ observations determined that the refinement of the pearlitic nodule was due to the refinement of prior austenite grains and retardation of pearlite growth by Nb addition. The impact toughness of high-carbon steels was significantly improved by Nb addition without a decrease in strength because a finer pearlitic nodule was obtained. The maximum improvement ratios for 68C and 75C steels were 81% and 26%, respectively. In addition, according to the experimental results and theoretical calculations of the dissolution of Nb-containing precipitates, the refinement of prior austenite was mainly caused by the solute-dragging effect of Nb during isothermal holding at 1180 °C. Moreover, there was an optimum addition of Nb to 75C steels (less than 0.015 wt%); above this threshold, neither the prior austenite grains or pearlitic nodules were further refined, nor was the impact toughness further improved.

A novel strategy for preparing high-performance powder metallurgical low alloy ultrahigh strength steel

In this study, a combined process of hot isostatic pressing and hot rolling (HIP-rolling) is developed to prepare high-performance powder metallurgical (PM) low alloy ultrahigh strength steel. The HIP-rolled 30CrMnSiNi2A steel achieves ultrahigh ultimate tensile strength of 1823.6 MPa, high elongation of 17.6% and superior impact energy of 29.4 J. The formation of lower bainite/martensite duplex structure, the refinement of prior austenite grains and blocks, and the disruption of prior particle boundaries are the critical factors to achieve the excellent strength-toughness balance. This study provides a novel insight for the development and application of PM low alloy ultrahigh strength steel.

Enhanced Toughness of High‐Temperature Carburizing Gear Steel via Refining Twin Martensite and Retained Austenite by Nb Microalloying

High‐temperature carburizing technology is replacing traditional carburizing technology due to energy savings and emission reductions. However, further effort is required to understand the mechanical properties for practical high‐temperature carburizing. Herein, the microstructures and mechanical properties of the 20MnCr5 gear steel with different Nb contents after carburizing at the temperature from 900 to 950 °C and the corresponding holding time from 8 to 5 h are investigated. For a given steel, the toughness slightly decreases with the carburizing temperature because of the increase of prior austenite grain size. For high‐temperature carburizing, the impact energy increases from 45.9 to 84 J after Nb microalloying. The toughening mechanism is analyzed through theoretical discussion and various characterizations of fracture morphology and microstructure. It is indicated that the refinement of prior austenite grain caused by the solute drag effect of Nb leads to the refinement of acicular martensite and retained austenite. The combined effect of microstructure refinement results in the significant enhancement of impact toughness and accompanies by coordinated plastic deformation. The fracture spreads along the martensite blocks due to the decrease in the thickness of twin lamellas in acicular martensite, which also contributes to the enhancement of toughness.

The Impact of Prenitriding on the Microstructure and Mechanical Properties of the Carburized Surface Layer in a Martensitic Stainless Steel

Herein, it is proposed to obtain a favorable combination of microstructure and mechanical properties of martensitic stainless steel 14Cr14Co13Mo5 by prenitriding method before carburizing and appropriate heat treatment processes. The result indicates that the effective hardened layer depth is increased by 32% to 0.66 mm, and the hardness is significantly improved by prenitriding. The confocal laser scanning microscope and electron backscattered diffraction show that the prenitriding can effectively refine the martensite substructure of the carburized layer after quenching due to the refinement of prior austenite grain. In addition, plenty of second‐phase particles M2N with finer size appear at the same case layer depth instead of coarse M23C6 in the specimen prenitrided, which optimize the microstructure of the case layer.

The Effects of Prior Austenite Grain Refinement on Strength and Toughness of High-Strength Low-Alloy Steel

The effects of prior austenite grain (PAG) refinement on the mechanical properties of bainitic/martensitic steels not only come from itself, but also have more complex effects by affecting the substructure formed by coherent transformation. In this study, the samples of a low-alloy steel were water quenched from different austenitizing temperatures and the bainitic/martensitic microstructures with different PAG sizes were obtained. Electron back-scattered diffraction was used to characterize the microstructure and different types of boundaries were identified and quantitatively analyzed. The tensile tests and series temperature Charpy impact tests of different heat treatment were also carried out and comprehensively analyzed with microstructure characterization works. The results show that the uniform refinement of prior austenite grain can increases the density of packet boundary and block boundary, which leads to microstructure refinement with higher density of high-angle grain boundaries with misorientation >45°. The contribution of this microstructure refinement to toughness is significant, but its contributions to strength and elongation are relatively limited. Compared to uniform refined PAG, if the PAGs are mixed crystal, the density of block boundary will be reduced, which leads to a lower density of the high-angle boundary with misorientation >45° and the positive effects of microstructure refinement on toughness improvement are weakened. The observation of fracture surface of impact specimens indicates that refining the PAG can delay the tendency of brittle fracture with the decrease in test temperature, and even in the case of brittle fracture, the cleavage facet of the fracture surface is relatively smaller. This result also verifies that PAG refinement can effectively improve toughness by inhibiting cleavage fracture.

N-induced microstructure refinement and toughness improvement in the coarse grain heat-affected zone of a low carbon Mo–V–Ti–B steel subjected to a high heat input welding thermal cycle

In the coarse grain heat-affected zone (CGHAZ) of a low carbon Mo–V–Ti–B steel subjected to a simulative welding thermal cycle with a heat input as high as 75 kJ/cm, the influence of an increase in nitrogen content from 0.0085 to 0.0144 wt% on the microstructures and impact properties of the steel was comparatively estimated using Gleeble-3800 simulation, microstructure characterization and impact testing. With a rise in the nitrogen content, the number of fine and coarse particles also increased. An increase in the number of fine precipitates can effectively pin the prior austenite grain (PAG) boundary and inhibit the grain growth, resulting in the refinement of PAG in the CGHAZ. The intragranular and intergranular V-containing coarse particles effectively promoted the nucleation of intragranular acicular ferrite (IGAF) and grain boundary polygonal ferrite (GBPF). Accordingly, the contents of heterogeneously-nucleated IGAF and polygonal ferrite (PF) increased; however, the bainite content decreased with a rise in N content, leading to a prominent microstructure refinement in this CGHAZ with a high heat input. Furthermore, the nitrogen addition led to the formation of finer and more dispersed martensite-austenite (M/A) constituents in the CGHAZ. Therefore, the impact toughness of this CGHAZ was enhanced. This can be attributed to the microstructure refinement of the CGHAZ, which in turn was ascribed to an increasing nitrogen.

Effects of Nb or (Nb + Mo) alloying on Charpy impact, bending, and delayed fracture properties in 1.9-GPa-grade press hardening steels

In this study, effects of Nb or (Nb + Mo) alloying into a 1.9-GPa-grade 32MnB5 press hardening steel on Charpy impact, bending, and delayed fracture properties were investigated. All the properties were enhanced in the Nb- or (Nb + Mo)-alloyed steels, especially in the (Nb + Mo)-alloyed steel. The Nb alloying worked for the precipitation of nano-sized complex carbides, thereby leading to the refinement of prior austenite grains by a Zener pinning effect. Mo also contributed to the grain refinement, while Mo affected it mostly in a solid-solution state by a solute drag effect. This grain refinement resulted in the more deviated crack propagation path and consequently in the more improved Charpy impact energy. The size reduction of Ti(C,N) inclusion due to the attachment of a small amount of Nb and Mo also improved the impact toughness. As well as microstructural effects, the lower residual stress due to the lower peak bending load by the Nb or (Nb + Mo) alloying contributed to the increase in H-induced delayed cracking time. Therefore, this study suggests that synergic interactions of Nb and Mo alloying would provide promising solutions for improving various properties, which should be more importantly considered under actually using environments such as mechanically forming and crashing for press hardening components, while satisfying the 1.9-GPa-strength 32MnB5 specification.

Distribution feature of specific misorientation angle in a bainitic steel

The effects of grain refinement on the properties of materials are largely reflected in the misorientation of specific crystallographic planes. For bainitic and martensitic steels, the distribution of specific misorientation angle of crystallographic {100}-plane ({100}-SMA) and {110}-plane ({110}-SMA) greatly influences the cleavage fracture and slip behavior respectively. In this study, the overall misorientation angle and specific misorientation angles of boundaries in a bainitic steel were characterized using electron back-scattered diffraction and analyzed by statistic. The obtained results indicate that prior austenite grain boundaries mostly have high overall misorientation angles, meanwhile they have high {100}-SMAs and relatively high {110}-SMAs. Thus, the refinement of prior austenite grain is an effective grain refinement way with respect to both cleavage fracture and slip behavior. Not all variant boundaries have high {100}-SMA or {110}-SMA, and the distribution features of {100}-SMA and {110}-SMA of variant boundaries are different. Therefore, though coherent transformation in steel can refine the microstructure with respect to both cleavage fracture and slip behavior, it would not be effective for both of them at the same time.

Effect of undissolved Nb carbides on mechanical properties of hydrogen-precharged tempered martensitic steel

Nb carbides have attracted significant attention to enhance the resistance of tempered martensitic (TM) steel to hydrogen embrittlement (HE). However, previous studies have elucidated the role of Nb carbides in HE resistance without categorizing their types (i.e., undissolved and newly precipitated). This study focuses on the effect of “undissolved” Nb carbides on the tensile and fatigue properties of hydrogen-precharged TM steels. It validated the following two factors for the HE resistance of the TM steels containing undissolved Nb carbides: hydrogen-trapping by the carbides and refinement of prior austenite grain. The former factor rarely affected the HE resistance owing to the interfacial incoherency between the undissolved carbides and ferritic matrix. Such results are distinguished from previous studies focusing on the newly precipitated carbides. In contrast, the latter factor contributed significantly to the HE resistance via the decrease in hydrogen contents per unit surface of prior austenite grain boundaries.

Effects of Nitrogen Content and Hot Forming Temperature on Prior Austenite Grain Size and Mechanical Properties for Normalizing Cryogenic Pressure-Vessel Steels

The effects of nitrogen content and hot forming temperature on prior austenite grain size for normalizing cryogenic pressure-vessel steels were described. The evolution of V(C,N) precipitates during thermal cycle of hot forming was analyzed. The optical microscopy displays that as increasing N content from 0.0094 to 0.0198 wt.% the refinement of prior austenite grain was seen at each hot forming temperature of 870, 910 and 950 °C. The transmission electron microscopy presents that much finer V(C,N) precipitates were found in sample with 0.0198 wt.% N content than in sample with 0.0094 wt.% N content. Thermo-Calc calculation reveals the precipitation temperature of V(C,N) was increased by increasing N content. While the Ostwald ripening rate of V(C,N) was decreased by increasing N content. TEM result and Thermo-Calc calculation indicate that increasing N content enhanced the thermal stability of V(C,N) precipitates. The stable finer V(C,N) precipitates contribute to the finer prior austenite grain and good mechanical properties for sample with 0.0198 wt.% N content even at higher temperature of 950 °C during hot forming process.

Characterization of Y-bearing particles in CNS-I-ODS steel fabricated by vacuum induction melting & casting technique

Vacuum induction melting (VIM) & casting is not a common but a promising approach to produce ODS steel with large volume and high throughput. Meanwhile, the Y-bearing particles in the cast ODS steel presented various chemical compositions and wide size distribution. Thus it is important and necessary to clarify the Y-bearing particle characteristics in the cast ODS steel. To determine the distribution characteristics, chemical compositions, crystal structure and size distribution of the Y-bearing particles, CNS-I-ODS was fabricated by adding 1.0 wt.% metallic yttrium into the molten China Nuclear Steel (CNS) during VIM. The results indicated that some banding structures comprised of yttrium oxides and Y-bearing intermetallic compounds formed along rolling direction. XRD, SEM, TEM and EDS demonstrated that yttrium oxides with the size of 550–900 nm were consisted of Y2O3 and Y(O, C) phases while Y-bearing intermetallic compounds with the size of 26–500 nm were consisted of Fe-Y and Y-W-Ta phases. Besides, Y addition also facilitated the formation of δ-ferrite, refinement of prior austenite grains and precipitation of carbides. Furthermore, it is acceptable that the CNS-I-ODS exhibited similar elongation of 22.7–23.5%, slightly lower strength of 622 MPa and higher ductile-brittle transition temperature of −42 °C compared to the Y-free steel, although some large yttrium oxides were distributed in the former matrix.

Nanoengineered Hypereutectoid Steel with Superior Hardness and Wear Resistance

Hypereutectoid SAE 52100 steel is extensively used in bearing applications. Microstructure modification in terms of dislocation martensite and carbide refinement was achieved for SAE 52100 steel through a simple duplex heat treatment. Refinement of prior austenite grains to less than 5 μm resulted in the conversion of conventional high-carbon twinned martensite to dislocation martensite. The concurrent refinement of austenite grains and carbide precipitates was accomplished by high-temperature austenitization followed by low-temperature tempering. This resulted in nanoscale nonstoichiometric e-carbides within a heavily twinned martensitic structure. These nanoscale carbides acted as grain boundary pinning agents after their transformation to θ-carbides during the final austenitization process. The resulting microstructure was characterized by a fine dispersion of θ-carbides within dislocation martensite and showed roughly 24% increase in tensile strength and 30% better wear resistance compared to conventional structure.

New insights from crystallography into the effect of refining prior austenite grain size on transformation phenomenon and consequent mechanical properties of ultra-high strength low alloy steel

Based on new insights from crystallography, this study aims to establish the relationship between prior austenite grain size and mechanical properties and enhance our understanding of Hall-Petch relationship. The refinement of prior austenite grains was achieved by decreasing the austenitizing temperature (from 920 ℃ to 880 ℃) and quenching. In addition, samples subjected to 880 ℃ heat treatment and quenching produced a significantly higher percentage of martensite. Electron backscattered diffraction (EBSD) used to characterize the crystallographic characteristics indicated that the steel subjected to 920 ℃ heat treatment and quenched had larger prior austenite grains, belonging to the transformation of Bain group. After 880 ℃ heat treatment and quenching, the prior austenite grains were smaller and more uniform, which belonged to the transformation dominated by CP (close-packed plane) group. The transformation from Bain group to CP group was related to transformation driving force, and resulted in increase in the density of high angle grain boundaries (DHAGBs). Using thermal expansion approach to measure the initial martensite transformation temperature (Ms temperature), the samples heat treated and quenched at 920 ℃ and 880 ℃ showed Ms temperature of 400 ℃ and 427 ℃, respectively, implying that the phase transformation driving force was increased by refining the prior austenite grain. Charpy impact energy test at −40 ℃ suggested that after 880 ℃ heat treatment and quenching, the Charpy energy increased from 46 J to 92 J, consistent with the results of EBSD.

Microstructures and mechanical properties of friction stir welds on 9% Cr reduced activation ferritic/martensitic steel

In this study, the microstructures and mechanical properties of 9%Cr reduced activation ferritic/martensitic (RAFM) steel friction stir welded joints were investigated. When a W-Re tool is used, the recommended welding parameters are 300rpm rotational speed, 60mm/min welding speed and 10kn axial force. In stir zone (SZ), austenite dynamic recrystallization induced by plastic deformation and the high cooling rates lead to an obvious refinement of prior austenite grains and martensite laths. The microstructure in SZ contains lath martensite with high dislocation density, a lot of nano-sized MX and M3C phase particles, but almost no M23C6 precipitates. In thermal mechanically affect zone (TMAZ) and heat affect zone (HAZ), refinement of prior austenite and martensitic laths and partial dissolution of M23C6 precipitates are obtained at relatively low rotational speed. However, with the increase of heat input, coarsening of martensitic laths, prior austenite grains, and complete dissolution of M23C6 precipitates are achieved. Impact toughness of SZ at −20°C is slightly lower than that of base material (BM), and exhibits a decreasing trend with the increase of rotational speed.

Study of Ferrite During Refinement of Prior Austenite Grains in Microalloyed Steel Continuous Casting

The formation of coarse prior austenite grain is a key factor to promote transverse crack, and the susceptibility to the transverse crack can be reduced by refining the austenite grain size. In the present study, the high-temperature confocal laser scanning microscope (CLSM) was used to simulate two types of double phase-transformation technologies. The distribution and morphology of ferrites under different cooling conditions were analyzed, and the effects of ferrite distribution and morphology on the double phase-transformation technologies were explored to obtain the suitable double phase-change technology for the continuous casting process. The results indicate that, under the thermal cycle TH0 [the specimens were cooled down to 913 K (640 °C) at a cooling rate of 5.0 K/s (5.0 °C/s)], the width of prior austenite grain boundaries was thick, and the dislocation density at grain boundaries was high. It had strong inhibition effect on crack propagation; under the thermal cycle TH1 [the specimens were cooled down to 1073 K (800 °C) at a cooling rate of 5.0 K/s (5.0 °C/s) and then to 913 K (640 °C) at a cooling rate of 1.0 K/s (1.0 °C/s)], the width of prior austenite grain boundary was thin, and the dislocation density at grain boundaries was low. It was beneficial to crack propagation. After the first phase change, the developed film-like ferrite along the austenite grain boundaries improved the nucleation conditions of new austenitic grains and removed the inhibition effect of the prior austenite grain boundaries on the austenite grain size.

Effect of large-size M23C6-type carbides on the low-temperature toughness of martensitic heat-resistant steels

The relationship between the low-temperature toughness of and the microstructures found in a martensitic heat-resistant steel 10Cr12Ni3Mo2VN was studied using heat treatment and microscopic microstructure analysis. This study discusses the effect of large-size M23C6 type carbides on the low-temperature toughness. Recent results show that the large-size M23C6-type carbides that remain undissolved during austenitizing have a notably adverse effect on the low-temperature toughness after quenching and tempering. The large-size M23C6-type carbides form during normalization and can be eliminated by austenitizing at temperatures of 1050 °C and above. Refinement of prior austenite grains can markedly improve the low-temperature toughness. To meet the requirement of FATT50≤−35 °C in 10Cr12Ni3Mo2VN steel, the average size of prior austenite grains should be less than 65 μm. No large-size M23C6 type carbides should be present after quenching, and the tempering temperature should be 690 °C.

Effect of γ→α Phase Transformation on Refining Austenite Grains of Microalloyed Steel in Continuous Casting by Simulation

Abstract The formation of coarse prior austenite grain is a key factor to promote transverse crack, and the transverse crack susceptibility can be reduced by refining the austenite grain size. In the present study, the high-temperature confocal laser scanning microscope (CLSM) was used to simulate and study the effect of two γ→α phase transformation conditions on the refinement of the prior austenite grains. Under the condition of the uniform distribution of the second phase precipitation, the effect of the distribution of the proeutectoid ferrite at different cooling rates and refinement of prior austenite grain were investigated. The results indicate that, at a cooling rate of 5.0°C s–1, the austenite grain size undergoing TH1 thermal cycle was 31% smaller than the austenite grain undergoing TH0 thermal cycle. Under TH0 cooling system, the proeutectoid ferrite was uniformly distributed in the austenite matrix; under TH1 cooling, the proeutectoid ferrite precipitated and mainly concentrated along the austenite grain boundaries to form developed-film-like ferrite, which is favorable to break the prior austenite grain boundaries. After the first phase transformation, the film-like ferrite improved the nucleation conditions of new austenite grains, thus more new austenite grains splitted the prior austenite grains, ultimately refining the prior austenite grains.