Baker-Nutting Orientation Relationship Research Articles

Role of Precipitates on the Grain Coarsening of 20CrMnTi Gear Steel during Pseudo-Carburizing

The carburizing period for tool steel could be significantly shortened by operating at a higher carburizing temperature. However, grain coarsening happens during the carburizing process, and then results in the deteriorated surface properties in 20CrMnTi gear steel, especially at an elevated carburizing temperature. The relationships between grain coarsening and the precipitates in the developed 20CrMnTi gear steel during pseudo-carburizing were established by microstructure characterization, precipitate analysis and in-situ observation to clarify the coarsening mechanism. The results manifested the Baker–Nutting orientation relationship between the (Ti, Mo)(C, N) particles and the matrix, and then testified to the redissolution and ripening of the (Ti, Mo)(C, N) precipitates pre-formed in the α phase during the carburizing. Coarsening in austenite grain during the carburizing process was mainly caused by the rapid redissolution and ripening of the (Ti, Mo)(C, N) precipitates, although this occurred in a very short pseudo-carburizing time. The area density of the dispersed unripe (Ti, Mo)(C, N) particles markedly decreased from 0.389% in as-hot rolled gear steel to 0.341%, and then from 0.279% in carburized steels at 970 and 980 °C, respectively. Additionally, the redissolution and ripening of the (Ti, Mo)(C, N) precipitates were accelerated by the elevated carburizing temperature of 980 °C, at which time the growing rate in austenite grains was 2.34 μm/min during the prior 1 min (0.79 μm/min during the prior 3 min at 970 °C). The temperature then decreased to 0.003 μm/min in the subsequent carburizing process. The results obtained our current work reflected that the particles with excellent thermal stability should play important roles in the limitation of grain coarsening during the carburizing process.

Nucleation of equiaxed δ-ferrite by accelerating TiN formation controlled by oxide during welding of ferritic stainless steel

The accelerating effect of oxides on TiN formation to promote the nucleation of equiaxed ferrite was investigated under simulated gas tungsten arc (GTA) welding conditions. This revealed that the formation of oxides such as Ti2O3, (Ti, Al)2O3, and MgAl2O4 is essential for the solidification of equiaxed δ-ferrite. MgAl2O4 significantly accelerates the formation of TiN and produces a fine-grained equiaxed structure. Minor alloying with Ti, Al, and Mg significantly influenced the composition and crystal structure of the oxides. Ti2O3 and (Ti, Al)2O3 were identified as corundum structures with the same orientation relationship as TiN on the {0001} crystal plane. Ti2O3 and (Ti, Al)2O3 could act as heterogeneous nuclei for TiN because low lattice misfits of 0.6% and 0.6–8.1% could be reached for Ti2O3 and (Ti, Al)2O3 for {0001}oxide//{111}TiN and [10–10]oxide//[110]TiN, respectively. Spinel type MgAl2O4 was found to have a fully paralleled orientation relationship with TiN, resulting in a 4.8% misfit with TiN in each crystal plane. This indicates that all the crystal planes of MgAl2O4 could accelerate the formation of TiN. Baker-Nutting orientation relationship was confirmed for TiN and δ-ferrite. Therefore, the TiN formed by oxide acceleration could nucleate δ-ferrite. The multiple accelerating planes of MgAl2O4 and a high quantity of MgAl2O4-TiN as nucleation sites showed an optimal effect on equiaxed solidification under a high-temperature gradient.

On the grain coarsening behavior of 20CrMnTi gear steel during pseudo carburizing: A comparison of Nb-Ti-Mo versus Ti-Mo microalloyed steel

Increasing carburizing temperature of gear steel will tremendously shorten the carburizing time, thus reducing energy consumption, and improving workpiece efficiency. Carbonitride particles plays an important role in ensuring the fine grains of the gear steel with an elevated carburizing temperature. In the present study, the relationships between grain size and pseudo-carburizing parameters in Nb-Ti-Mo and Ti-Mo 20CrMnTi steel were established. Evolution of carbonitride particles under the different pseudo-carburizing parameters in the two steels were elaborately characterized. Besides, in-situ observation on the nucleation and growth of austenite grains in the two steels were creatively accomplished to simulate the evolution of austenite during carburizing. Results reveal that the carburizing temperature and time corresponded to the grain coarsening both improved in the Nb-Ti-Mo steel by 0.02 wt% Nb addition with a synchronous reduction of 0.02 wt% Ti and 0.02 wt% Mo. And the Nb-Ti-Mo steel, with increased volume fraction and thermostability of (Nb, Ti, Mo)(C, N) particles, yielded excellent resistance to grain coarsening at higher temperature although the carburizing temperature played a crucial role in the grain coarsening. The grain coarsening was ascribed to the redissolution and ripening of carbonitride precipitates, especially the former. Besides, these carbonitrides mainly precipitated in the α phase during cooling process after hot rolling based on the immediate Baker-Nutting orientation relationship between the matrix and carbonitrides. In addition, the martensite transformation proceeded in a gradual rather than burst transformation pattern with an instantaneous formation of martensite lath.

Strengthening Mechanisms of Tempered Martensite in Vanadium-Added Steels

To clarify the effects of vanadium additions on the strengthening mechanisms of tempered martensitic steel, the microstructures, precipitates, dislocation densities, and tensile properties of water-quenched and tempered Fe–0.2C–0.5Si–2.5Mn–xV (mass%; x = 0–0.82) steels were analyzed. The vanadium carbide precipitates were plate-shaped and had a Baker–Nutting orientation relationship with the ferrite matrix. The size and shape of the vanadium carbide precipitates on the slip plane were considered when evaluating the contribution of precipitation strengthening. The increase in yield strength upon adding 0.82 mass% vanadium to the tempered steel was mainly caused by precipitation strengthening owing to the vanadium carbide precipitates and dislocation strengthening owing to the high dislocation density. This study demonstrates that the contribution of precipitation strengthening might be overestimated if it is assumed that the precipitates all hinder dislocation motion by the Orowan mechanism.

Segregation of alloying elements at the TiC/V interface: A first-principles study

For precipitation/matrix coherent interface, the interfacial segregation behavior of solute atom plays a vital role in controlling the effectiveness of precipitation strengthening of the alloy. In this study, first-principles calculations based on density functional theory are used to figure out the interfacial structure of precipitates/vanadium and explain how solute atoms segregate at the interface and affect the interfacial strengthening. Firstly, according to the Baker-Nutting orientation relationship, the equilibrium interface structures are obtained. Meanwhiles, the segregation behavior of solute atoms at the different interfaces in vanadium alloys was studied. It is found that elements Sc, Ti, Y, Zr, Hf, and Ta, in the TiC/V(100) interface, tend to segregate at the interface, while other ones are not easy to segregate. The alloy elements show the same segregation trend as the TiC/V(100) interface, except for Nb, which also tends to segregate at the TiC/V(110) interface. According to the calculation, it is found that the alloying element with the larger atomic size and Voronoi volume is more likely to segregate into the interface, indicating that the atomic size effect dominates alloying element segregation at the two interfaces. The findings of this work can be used to develop theoretical approaches for future alloy designs by controlling alloying element doping.

Interphase and fibrous precipitation in a thermomechanically processed V–Nb–Mo microalloyed steel

A novel V–Nb–Mo steel was thermomechanically processed to promote ferrite grain refinement and precipitation during the austenite to ferrite transformation. Transmission electron microscopy and three-dimensional atom probe tomography confirmed the formation of two main types of precipitates: nano-scale interphase precipitates and fibrous precipitates (FP). Both the precipitates had a Baker–Nutting orientation relationship with the ferrite matrix, but with different variants. Interphase precipitates exhibited a disc shape with an average distance between precipitate rows of 15 ± 2 nm and an average Guinier radius of 3.9 ± 1.7 nm, whereas FP had a rod-shape 2.5 ± 0.5 nm in radius and 8–25 nm in length. It appeared that the FP are formed along the plane perpendicular to the interphase precipitates plane.

Understanding the protective ability of the native oxide on an Fe-13 at% Cr alloy at the atomic scale: A combined atom probe and electron microscopy study

We have performed high-resolution structural and chemical characterization of the native oxide films formed on a ferritic Fe- 13 at% Cr alloy at different stages of growth. The steady-state protective oxide film was found to be 5–6 nm thick, epitaxially grown, with a cubic spinel-structure ((Fe, Cr)3O4) and a Baker-Nutting orientation-relationship with the underlying alloy. Composition evolution within the oxide during growth, measured by atom probe tomography, has been explained on the basis of the low solid-state mobility of Cr3+ cations. We propose that the miscibility gap in the Fe-Cr spinel oxide solid-solution can be used to explain the reduced driving force for oxide-layer thickening with time.

Surface Defects on Semicoherent and Incoherent NaCl-Type Carbides Dispersed in Hot-Rolled Ferritic Steel

The interface structure of semicoherent titanium carbide (TiC) nanoparticles and incoherent micron-sized TiC precipitates in Fe–C–Ti alloy hot-rolled steel sheets was studied by transmission electron microscopy (TEM) and positron annihilation spectrometry (PAS). The TiC nanoparticles were formed by interphase precipitation during the γ-to-α transformation, and the coarse micron-sized TiC precipitates remained in the matrix as undissolved carbides in the slab reheating process before hot rolling. The semicoherent TiC nanoparticles have coherent planes with the Baker–Nutting orientation relationship and incoherent surface almost parallel to {001}α planes. The present investigation revealed that positrons are not annihilated inside TiC particles but rather are trapped at open volume misfit defects located at the incoherent interface between TiC particles and the matrix. The positron lifetime related to misfit defects associated with the semicoherent TiC nanoparticles and incoherent micron-sized TiC precipitates was around 240 ps and 500 ps, respectively. Considering the results of TEM observations and the theoretical positron lifetime, it can be concluded that the positron annihilation site related to TiC nanoparticles is located at the incoherent interface and consists of some vacancies.

Effect of grain orientation and interface coherency on the hydrogen trapping ability of TiC precipitates in a ferritic steel

The way of capturing hydrogen has always been the focus of research in steels. Through the in-situ SKPFM study, a great Volta potential difference has been found between the TiC precipitates (∼0.82 μm) and the matrix after hydrogen charging, especially at the phase interface, which is demonstrated as an effective hydrogen trapping site. Moreover, the TiC carbides precipitated in the ferrite (111) plane show a higher average potential increase than that in the (110) plane. The difference of hydrogen trapping ability can be attributed to the deviation of Baker-Nutting orientation relationship of the precipitate/matrix interface, since the interface coherency on the (111) plane is destroyed more severely due to a lower Schmid factor of (111) orientation during the coiling process.

Study on Diversified Carbide Precipitation in High‐Strength Low‐Alloy Steel during Tempering

Herein, the precipitation and mechanical properties of high‐strength low‐alloy steel (Q960E) under different tempering temperatures from 560 to 640 °C are investigated. Diversified precipitations (including MC, M3C, M23C6, and M7C3) are formed during tempering. The size of the M3C, M23C6, and M7C3 precipitates increases, as the temperature increases, and the precipitates are gradually spheroidized with an average size of less than 100 nm. MC‐type precipitates (where M = Nb and Ti) are produced at different tempering temperatures. When tempered at 600 °C, Q960E steel exhibits excellent mechanical properties, including a high strength and elongation with a good impact performance. At 600 °C, a large number of fine MC carbides with an average size of less than 25 nm are formed. MC and matrix exhibit the Baker–Nutting orientation relationship. Edge dislocations exist in the transition region between the MC nanophase and the matrix. These dislocations reduce the mismatch degree between the MC nanophase and the matrix and promote MC nucleation. Meanwhile, the steel retains many low‐angle grain boundaries when tempered at 600 °C, which is beneficial to its high strength.

Complex isothermal precipitation behavior of V(C, N) in V-, N-added low carbon steel

Precipitation behavior of carbide/carbonitride in high-strength low-carbon steel is significantly affected by the ferrite transformation modes, which may change from the negligible partitional local equilibrium mode to the partitional local equilibrium mode as holding temperature increases or holding time increases at a certain temperature for ferrite phase transformation. Herein, the isothermal precipitation behavior of V(C, N) under negligible partitional local equilibrium mode of ferrite transformation in V-, N-added low carbon steel has been investigated. Not only the complex precipitation coexisting phenomenon of random precipitation and fibrous precipitation but also that of random precipitation and interphase precipitation was observed and certificated by the selected area electron diffraction patterns. In addition to Baker–Nutting orientation relationship, the Kurdjumov–Sachs OR [111]α//[110]P, $$\left( {1\overline{1}0} \right){\upalpha }//\left( {1\overline{1}\overline{1}} \right)$$ P between ferrite and interphase precipitation were found. Due to big variation in the movement velocity of austenite/ferrite interface, non-uniform distribution of precipitates occurred, resulting in hardness of ferrite grains changing from 185 to 215 HV.

Acicular ferrite nucleation and growth in API5L-X65 steel submerged arc welded joints

In the present study, acicular ferrite nucleation and growth were investigated from a crystallographic point of view in API5L-X65 steel weld metals. It was observed that acicular ferrite was nucleated on inclusions in weld metal. The titanium-enriched layer around inclusions was characterised as the TiO phase by selected area diffraction patterns. A scanning electron microscope equipped with an electron backscatter diffraction technique was applied to investigate the crystallographic orientation relationship. The results showed that acicular ferrite lathes had a Baker–Nutting orientation relationship with the TiO layer on the inclusion surface in the nucleation of acicular ferrite. However, it is confirmed that the growth of acicular ferrite lathes depends on the deviation from the Kurdjumov–Sachs relationship between lathes and primary austenite.

Formation mechanism of (TiNb)C surface reinforced layer by in-situ solid-state carbonization reaction

To investigate the formation mechanism of the (TiNb)C in Ti-Nb system during the surficial carburization, in-situ solid-state carbonization reaction was adopted to fabricate (TiNb)C surface reinforcement layer (SRL) in TiNb alloy. The experimental results have been employed to research a relationship between (TiNb)C formation mechanism and carbon diffusion mechanism. The research results confirmed that the carbon diffusion flux mainly governs the SRL formation mechanism and changes the one-step carbide formation, i.e., TiNb →C (TiNb)CI, into two-step process, i.e., TiNb →C (TiNb)2C →C (TiNb)CII + Nb. The outer layer is formed by the one-step reaction, where the micro-sized particles comply with the Baker-Nutting orientation relationship and have higher average hardness of 33.10 GPa. However, the inner layer is formed by the two-step process, which consists of submicro-sized particles presented a random orientation relationship, and the average hardness of inner layer is 30.97 GPa. Moreover, the X-ray diffraction analysis and EDS results indicate that the Ti/(Ti + Nb) ratios of Nb-rich and Ti-rich particles in the outer layer are ~62.26% and >73.41%, respectively. Whilst, all the particles in the inner layer are Ti-rich and the Ti/(Ti + Nb) ratios range from 81.62% to 83.48%.

Effect of Mn Content on the Characteristics of Inclusions in Ti-Containing Steel Welds

Steel weld metals containing Ti were prepared with Mn contents of 0.35%–2.98%. The inclusion characteristics in the weld metals were investigated by means of thermodynamic calculations coupled with electron probe microanalyses and transmission electron microscopy. The results show that with an increase in Mn content, the primary constituent phases of the inclusions are changed from pseudobrookite to a combination of pseudobrookite and spinel and eventually to a mixture of (Mn–Si–Al)-oxide, spinel and ilmenite. Ti3O5, MnTi2O4 and MnTiO3 are the primary constituents of the pseudobrookite, spinel and ilmenite solid solutions, respectively. Spinel and ilmenite have higher amounts of Mn but lower amounts of Ti compared with that of pseudobrookite. The formation of large amounts of ilmenite and spinel leads to the development of a Mn-depleted zone (MDZ) in the vicinity of the matrix/inclusion interface. Acicular ferrite has a Baker–Nutting orientation relationship with MnTi2O4.

Design and Characterization of a Heat-Resistant Ferritic Steel Strengthened by MX Precipitates

The long-term performance of current heat-resistant ferritic steels used in steam generators is primarily limited by microstructure degradation as a result of coarsening or transformation of precipitates. To improve the microstructural stability, a heat-resistant ferritic steel was designed via computational thermodynamics to exclusively contain metal carbonitride (MX) precipitates as the primary means for strengthening at elevated temperatures. The volume fraction of precipitates is 0.35 vol pct, about one-seventh of P91. These MX precipitates are either V-rich or Nb-rich with plate-like or spheroidal morphology, respectively. The precipitate size remains almost constant at 973 K for up to 3000 hours aging. Electron diffraction analysis revealed a Baker-Nutting orientation relationship between the precipitate and the matrix. Consistent with the thermodynamics-based design, M23C6-, Laves-, and Z phase were not detected. The creep threshold stress, derived from high-temperature compressive creep tests, are evaluated to be 63 ± 1 and 43 ± 2 MPa at 923 K and 973 K, respectively, on par with or slightly better than P91. This study reveals that MX precipitates in ferritic steels coarsen slowly at temperatures up to 973 K and that a relatively small volume fraction of MX precipitates can provide effective long-term creep performance at elevated temperatures.

Precipitate/vanadium interface and its strengthening on the vanadium alloys: A first-principles study

The precipitates of second phases with coherent or incoherent interfaces can interact with dislocations to act as slip obstacles that can significantly improve the mechanics properties of alloys. In this work, the first-principles calculations based on density functional theory were performed to characterize the precipitate/Vanadium interface in the vanadium alloys and uncovering how structure of precipitations and interfacial defects (vacancy or solutes) distribution influence on the effectiveness of precipitation strengthening. Based the Baker-Nutting orientation relationship, the equilibrium stable precipitate/matrix interface structure are obtained. It is found that the so-called layer buckling exists in the interfacial region. Meanwhile, considering the equilibrium interface structure, the compressive strain is taken up by the Ti-based precipitation in the interface. For V-based precipitations, on the contrary, the interfacial strain is reversed that the tensile strain is taken up by the V-based precipitations. Under the uniaxial tensile loading, the stress-strain relations of heterostructures with different kinds of precipitates MX (M = Ti, V and X = C, N) are obtained to characterize the precipitation strengthening in the Vanadium alloys. The ideal tensile strength for heterostructure is sensitive to the size of precipitation. Furthermore, it is interesting to note that Ti-based precipitations not only improve the tensile strength, but also meliorate the ductility of the V alloys, which pointing to the inference that Ti-based precipitations are more effective to strengthen the V alloys than V-based precipitations. Our findings from this study can be implemented into providing the theoretical strategy for further design of the new style alloys.

New orientation relationship with low interfacial energy in MC/ferrite system observed in Nb-Ti bearing steel during isothermal quenching process

Here we report a new crystallographic orientation relationship (OR) between MC-type carbide (M = metal) and ferrite matrix, observed in the specimen held isothermally at 600 °C for 20 min, named NEU-OR. The 〈110〉carbide//〈110〉 ferrite alignment is the main difference from the well-known Baker–Nutting (BN) OR. NEU-OR is related to BN OR by the relative rotation of 45° about the zone axis of 〈110〉carbide. The interfacial energy between carbide and ferrite with NEU-OR is lower than that with BN OR based on semi-coherent dislocation theory, which benefits increases in the number density of precipitates.

The effect of molybdenum on interphase precipitation and microstructures in microalloyed steels containing titanium and vanadium

Despite much research into steels strengthened through interphase precitation, there remains much that is not clear, such as the role of a range of elements, particularly Mo, in the interphase precipitation process. Four steels were manufactured with identical composition, but with variations in Ti, V, Mo and N content to investigate the effect of composition on interphase precipitation. Alloys were rapidly cooled from the single austenite phase field and isothermally transformed at 630 °C and 650 °C for 90min. The addition of Mo was found to significantly reduce the austenite to ferrite transformation kinetics, particularly for the V steel. Interphase precipitation was observed in all alloys at both transformation temperatures. For the Ti bearing steel, the two types of precipitate were observed throughout the sample, namely TiC (finer) and Ti2C (coarser), while for the V bearing steels, VC (finer) and V4C3 (coarser) were observed. Where Mo was present in the alloy, it was found dissolved in all carbide types. The (Ti,Mo)C and (V,Mo)C formed by classical planer interphase precipitation (PIP) while the (Ti,Mo)2C and (V,Mo)4C3, that had a much wider row spacing, formed through curved interphase precipitation (CIP). Each adopted one variant of the Baker-Nutting orientation relationship. The Ti-microalloyed steels exhibited the smallest precipitates of all the steels, which were approximately the same size irrespective of whether Mo was present in the alloy and irrespective of the transformation temperature. However, the addition of Mo to the V bearing steels resulted in a significant increase in precipitate volume fraction and a reduction in precipitate size. The mechanisms of interphase precipitation leading to the coincident production of two different precipitate types is considered and the role of Mo on the interphase precipitation process is discussed. The resultant effect on strength is considered.

Inclusion Characteristics and Acicular Ferrite Nucleation in Ti-Containing Weld Metals of X80 Pipeline Steel

X80 steel weld metals with Ti contents of 0.003 to 0.13 pct were prepared by the single-pass submerged-arc welding process. The effects of Ti content in weld metals on the constituent phases of inclusions and chemical compositions of the constituent phases, as well as the potency of acicular ferrite (AF) nucleation on the inclusions were investigated. Moreover, the crystallographic orientation relationship between the AF and inclusion was examined. The results show that with an increase in Ti content, the primary constituent phases of the inclusions change from the (Mn-Al-Si-O) compound to a mixture of spinel and pseudobrookite solid solutions, and eventually to pseudobrookite. The spinel solid solution is characterized by the MnTi2O4 constituent. Compared to pseudobrookite, spinel has a lower Ti concentration, but a significantly higher Mn content. In the case of the presence of a considerable amount of spinel, the Mn element is enriched strongly in the inclusions, resulting in the development of a Mn-depleted zone (MDZ) in the matrix around the inclusions, which enhances the driving force for AF formation. AF shows the Baker–Nutting orientation relationship with MnTi2O4. The formation of MDZ and the presence of the Baker–Nutting orientation relationship promote the ability of inclusions to nucleate the intragranular AF.

Extending the boundaries of mechanical properties of Ti-Nb low-carbon steel via combination of ultrafast cooling and deformation during austenite-to-ferrite transformation

We underscore here a novel approach to extend the boundaries of mechanical properties of Ti-Nb low-carbon steel via combination of ultrafast cooling and deformation during austenite-to-ferrite transformation. The proposed approach yields a refined microstructure and high density nano-sized precipitates, with consequent increase in strength. Steels subjected to ultra-fast cooling during austenite-to-ferrite transformation led to 145 MPa increase in yield strength, while the small deformation after ultra-fast cooling process led to increase in strength of 275 MPa. The ultra-fast cooling refined the ferrite and pearlite constituents and enabled uniform dispersion, while the deformation after ultra-fast cooling promoted precipitation and broke the lamellar pearlite to spherical cementite and long thin strips of FexC. The contribution of nano-sized precipitates to yield strength was estimated to be ~247.9 MPa and ~358.3 MPa for ultrafast cooling and deformation plus ultrafast cooling processes. The nano precipitates carbides were identified to be (Ti, Nb)C and had a NaCl-type crystal structure, and obeyed the Baker-Nutting orientation relationship with the ferrite matrix.