Baker-Nutting Orientation Relationship Research Articles

Isothermal precipitation kinetics of carbides in undercooled austenite and ferrite of a titanium microalloyed steel

A new method for isothermal precipitation kinetics of carbides in undercooled austenite and ferrite has been established based on measuring the strength increments. Precipitation-time-temperature (PTT) diagrams of carbides were determined with this method performed on a Gleeble simulator, for a low-carbon titanium microalloyed steel in a temperature range from 530°C to 670°C. Fine precipitates were studied by using high resolution transmission electron microscope (HRTEM), and their volume fractions together with their attributions to strength increments were calculated and analyzed. Results show that the strength increments are mainly attributed to the γ/α interphase precipitation whose nano-scale TiC precipitates obey the Baker-Nutting orientation relationship with respect to the ferrite matrix: (100)TiC ǁ (100)α-Fe and [011]TiC ǁ [001]α-Fe. The PTT curves obtained have a classic C-shape with their noses at about 600°C, where the strength increment reaches 114MPa after isothermally holding for 1h. Additionally, the corresponding precipitation kinetics curves are presented in an S-shape, and a modified-form of Avrami formula is introduced to describe their growth periods. The presented mathematical expression fits the experimental data quite well. Consequently, the proposed new method is intuitive, effective and convenient, particularly suitable for detecting the precipitation kinetics in low temperature range.

The mechanism of intragranular ferrite formed on Ti-rich (Ti,V)(C,N) precipitates in the coarse heat affected zone of a V–N–Ti microalloyed steel

The effect of Ti-rich (Ti,V)(C,N) precipitates on the evolution of intragranular ferrite in the coarse grain heated effected zone (CGHAZ) of V–N–Ti steel was systematically investigated via transmission electron microscopy. The coarse grain boundary ferrite, bainite, and ferritic side-plates in simulated CGHAZ were replaced by polygonal ferrite and acicular ferrite, when the nitrogen content was increased from 0.0044wt% to 0.0094wt%. In addition, the undissolved Ti-rich core acted as a nucleation site for the V-rich cap; the cap reached the critical size at which nucleation of intragranular ferrite occurs nucleation size of intragranular ferrite with the aid of the core. Owing to a cap-induced increase in the nucleation potency, the intragranular ferrite have formed on the Ti-rich (Ti,V)(C,N) hold the Baker-Nutting orientation relationship ((001)IGF//(001)(Ti,V)(C,N), [110]IGF//[100](Ti,V)(C,N)).

Nitriding of ternary Fe–Cr–Mo alloys; role of the Cr/Mo-ratio

Abstract Ternary Fe–Cr–Mo alloys, all containing about 2 at.% total alloying element content (Cr + Mo) but with atomic Cr/Mo-ratios ranging from 1.0 to 7.2, were nitrided at 580 °C, 550 °C, and 520 °C. The nitrided zone was characterized by light microscopy, hardness measurements, electron probe microanalysis and X-ray diffraction. The alloying element nitrides forming in the nitrided zone were characterized by scanning and transmission electron microscopy. Initially, the continuous precipitation of finely distributed, “mixed”, cubic NaCl-type (Cr,Mo)N y nitride platelets, exhibiting a Baker–Nutting orientation relationship with the ferrite matrix, occurs. The developing N-concentration-depth profiles indicate that alloys with a lower Cr/Mo-ratio show a slightly slower nitride-precipitation rate. The attainable surface hardness and the residual surface macrostress are of similar magnitudes for all Cr/Mo-ratios. Upon prolonged nitriding (= aging of the nitrided microstructure), a discontinuous transformation takes place leading to the formation of a coarsened lamellar microstructure. The nature of this discontinuous reaction changes from a discontinuous coarsening of the initial (Cr,Mo)N y nitride platelets for alloys of high Cr/Mo-ratios to a discontinuous precipitation of hexagonal CrMoN 2 for alloys of low Cr/Mo-ratios. The latter reaction, as compared to the first one, is driven by a larger driving force and thus the extent of the discontinuous transformation increases considerably with decreasing Cr/Mo-ratio. A summarizing discussion on precipitation of “mixed” vs. separate nitrides is presented.

INTERPHASE PRECIPITATION BEHAVIORS OF NANOMETER-SIZED CARBIDES IN A Nb-Ti-BEARING LOW-CARBON MICROALLOYED STEEL

High strength low alloy steels utilize chemical composition design of low carbon content and are microalloyed with Nb, V and Ti, or other additions, such as Mo and B, etc. The increase of strength is attributed to grain refinement strengthening, solid-solution strengthening, dislocation strengthening and precipitation hardening.Moreover, the precipitation hardening attracts more and more attentions. However, the detailed results on the sheet spacing, inter-particle spacing, crystallography, composition and the nucleation site of the interphase precipitation carbides in Nb-Ti containing steels have not been reported as yet. In this work, the microstructure, mechanical properties and precipitation behaviors in a low carbon Nb-Ti microalloyed steel were investigated using the dilatometer and TEM. The results show that the interphase precipitation can be observed for different isothermal temperatures and the sheet spacing, inter-particle spacing and size of the interphase precipitation carbides are refined by lowing isothermal temperature. The characteristic sheets of interphase precipitation carbides were identified as planar and curved. The planar sheets of interphase precipitation carbides have been analyzed and found to be parallel with{011},{012},{013}and {035} planes of ferrite. Moreover, the interphase precipitation carbides which have been determined to be(Nb, Ti)C have a Na Cl-type crystal structure with a lattice parameter of 0.434 nm and obey the Baker-Nutting(B-N) orientation relationship with respect to ferrite matrix. The contribution of the interphase precipitation hardening to the yield strength of the experiment steel has been estimated above 300 MPa, based on the Orowan mechanism.

Role of inclusions for formation of acicular ferit in low carbonsteel weld deposits

Acicular ferit was investigated from the viewpoint of orientation relationships among acicular ferit, prior austenite, and inclusions. The Baker-Nutting orientation relationship was observed between acicular ferit and a TiO layer forming at inclusion surfaces. Meanwhile, the Kurdjumov-Sachs orientation relationship was observed between acicular ferit and prior austenite. The results indicate that acicular ferit is considered to nucleate due to a low misfit with TiO layers; however, there is still doubt. In case the TiO layer form at molten steel, its orientation would be random with austenite. Thus, acicular ferit nucleating with the Baker-Nutting orientation relationship should have no orientation relationship with austenite. In order to address the issue, the inclusion formation process was investigated by using the liquid-tin quenching technique and the thermodynamic calculation. The results reveal that TiO form not at the high temperature such as molten steel temperature but at below the solid phase temperature such as austenite or ferit temperature. As the result of orientation analysis between prior austenite and TiO, TiO is not considered to form in austenite. It can be assumed that TiO layers form in order to match interface with acicular ferit. Additionally, the Mn depleted zone were also not observed. Therefore, acicular ferit is considered to nucleate not due to the low misfit or the Mn depleted zone but due to another mechanism.

Interrelationships of defects, nitride modification and excess nitrogen in nitrided Fe-4.75 at.% Al alloy

Abstract In order to investigate the influence of crystal-lattice defects (dislocations) and of a relatively large Al content on the development of different modifications of aluminium nitride in ferrite and on the associated uptake of excess nitrogen, recrystallised and cold rolled specimens of Fe-4.75 at.% Al alloy were subjected to gaseous nitriding treatments at 550 °C employing a nitriding potential of 0.104 atm−1/2. In contrast with earlier results for an alloy of relatively low Al content, in both the nitrided cold rolled and the nitrided recrystallised specimens nanosized AlN precipitates developed, both of the metastable, cubic, NaCl type modification and of the stable, hexagonal, wurtzite type modification, which exhibit with the ferrite matrix a Baker–Nutting orientation relationship and a Pitsch–Schrader orientation relationship, respectively. A high-temperature hydrogen reduction treatment confirmed the stoichiometry of the precipitated nitrides as AlN. The fraction of cubic, NaCl type, AlN is significantly higher in the nitrided cold-rolled specimens because of its preferential development along dislocations. As compared to the nitrided recrystallised specimens, the higher dislocation density and the higher amount of cubic AlN in the nitrided cold-rolled specimens leads to a larger amount of excess nitrogen in the nitrided cold-rolled specimen.

Precipitation Characteristics in a Low‐Carbon Vanadium–Titanium‐Bearing Steel

The precipitation characteristics in a low‐carbon vanadium–titanium‐bearing steel were investigated using transmission electron microscope. The analysis of selected area electron diffraction patterns and energy dispersive X‐ray spectrometer reveal that these nanometer‐sized particles are (Ti0.7,V0.3)C carbides with a NaCl‐type crystal structure with the lattice parameter of 0.436 nm. Most sheet planes are close to {211}α, {210}α, {140}α, or {111}α and only few sheet planes are close to {110}α. And note that none ferrite grain only occupied by interphase‐precipitated particles was observed. Moreover, the interphase‐precipitated nanometer‐sized particles can obey the Baker–Nutting orientation relationship or the Nishiyama–Wassermann orientation relationship.

Microstructure and Kinetics of Nitride Precipitation in a Quaternary Iron-Based Model Fe-2.82 at. pct Cr-0.13 at. pct Mo-0.18 at. pct V Alloy

Internal nitride development in iron-based quaternary Fe-Cr-Mo-V alloy, as a model alloy for 31CrMoV9 steel, was investigated by performing controlled gaseous nitriding experiments. The nitride-precipitation process starts with the development of nanosized platelets of, coherent, cubic NaCl-type nitride, along {100} lattice planes of the ferrite matrix, in association with matrix-lattice dilation. The development of nitride platelets having a NaCl-type crystal structure, satisfying the Baker–Nutting orientation relationship with the ferrite matrix, and the nitrogen content of the nitrided zone suggest the development of a quaternary “mixed” (Cr x ,V y , Mo1−x−y)N nitride, similar to the development of “mixed” ternary nitrides as reported for nitrided Fe-Cr-Al and Fe-Cr-Ti alloys. In a later stage, the nitride platelets undergo discontinuous coarsening resulting in the development of a lamellar microstructure consisting of nitride and ferrite lamellae. Kinetic analysis demonstrated that the thermally activated nature of growth of the diffusion zone is controlled with about equal weights, by the diffusion of nitrogen in the substrate matrix and the matrix lattice solubility of nitrogen.

Coherency strain and precipitation kinetics: crystalline and amorphous nitride formation in ternary Fe–Ti/Cr/V–Si alloys

Specimens of iron-based binary Fe–Si alloy and ternary Fe–Me–Si alloys (with Me = Ti, Cr and V) were nitrided at 580 °C in a NH3/H2-gas mixture applying a nitriding potential of 0.1 atm−1/2 until nitrogen saturation in the specimens was attained. In contrast with recent observations in other Fe–Me1–Me2 alloys, no “mixed” (Me1, Me2) nitrides developed in Fe–Me–Si alloys upon nitriding: first, all Me precipitates as MeN; and thereafter, all Si precipitates as Si3N4. The MeN precipitates as crystalline, finely dispersed, nanosized platelets, obeying a Baker–Nutting orientation relationship (OR) with respect to the ferrite matrix. The Si3N4 precipitates as cubically, amorphous particles; the incoherent (part of the) MeN/α-Fe interface acts as heterogeneous nucleation site for Si3N4. The Si3N4-precipitation rate was found to be strongly dependent on the degree of coherency of the first precipitating MeN. The different, even opposite, kinetic effects observed for the various Fe–Me–Si alloys could be ascribed to the different time dependences of the coherent → incoherent transitions of the MeN particles in the different Fe–Me–Si alloys.

Crystallographic analysis for acicular ferrite formation in low carbon steel weld metals

Inclusions contributing to acicular ferrite nucleation were investigated from a crystallographic point of view in low carbon low alloy steelweld metals. The samples from electro slag welding (ESW) and submerged arc welding (SAW) deposits with various cooling rates were prepared in this study. In those samples, intragranular acicular ferrite formation was observed from inclusions. The inclusions contributing to acicular ferrite formation were of multi-phase type consisting of amorphous phase, spinel type and MnS. They were surrounded by a Ti-enriched layer. It was confirmed by selected area diffraction patterns and energy-dispersive X-ray spectrometer analyses that the Ti-enriched layer was TiO. The acicular ferrite had a Baker–Nutting orientation relationship with the TiO layer on the inclusion surface. The misfit was 3.0% at the interface between the acicular ferrite and TiO. Therefore, it is considered that TiO on the inclusion surface contributes to the heterogeneous nucleation of acicular ferrite by small lattice misfit. However, themorphologies of ferrite growth which nucleated from inclusions were different in both samples. Whereas the growth of ferrites nucleated from TiO was enough in ESW, the size of nucleated ferrite in SAW was a few hundred millimetres in size. In the early stage of nucleation from TiO, ferrite had small deviation from Kurdjumov–Sachs orientation relationship (K–S relationship) in both ESW and SAW. However, there was a difference in the growth stage of ferrite. The ferrite orientations were gradually changed to fit to the K–S relationship in ESW. On the other hand, the nucleated ferrite in SAW stopped growing and the newly nucleated ferrite which had K–S relationship prior to austenite was formed adjacently because of large super cooling due to small heat input.

MICROSTRUCTURE, MECHANICAL PROPERTIES AND INTERPHASE PRECIPITATION BEHAVIORS IN V-Ti MICROALLOYED STEEL

The microstructure, mechanical properties and precipitation behaviors in a low carbon V-Ti microalloyed steel were investigated using thermal simulation. The microstructural characteristics of tested steel were analyzed using OM and TEM. The results show that the larger volume fraction of ferrite can be obtained for different isothermal temperatures. The ferrite volume fraction is increased and ferrite grain size is reduced as the isothermal temperature is lowered. The planar interphase precipitation can be observed for different isothermal temperatures, and both sheet spacing and precipitates size are refined by lowering isothermal temperature. Moreover, the nanometer-sized carbides have a NaCl-type crystal structure with a lattice parameter of about 0.436 nm and they can obey one variant of Baker-Nutting (B-N) orientation relationship of (100)(carbide)//(100)(ferrite) and [011](carbide)//[001](ferrite). The precipitation hardening for the specimen treated at 680 degrees C for 30 min can reach 360.6 MPa.

Molybdenum-Nitride Precipitation in Recrystallized and Cold-Rolled Fe-1 at. pct Mo Alloy

Nitriding of recrystallized and cold-rolled Fe-1 at. pct Mo alloy at 853 K (580 °C) in a NH3/H2 gas mixture leads to the formation of cubic nanometer-sized Mo2N-type precipitate platelets. These platelets obey a Baker–Nutting orientation relationship with the ferrite matrix. After prolonged nitriding, micrometer-sized colonies of lamellae consisting of a hexagonal MoN-type nitride and ferrite develop in a discontinuous precipitation reaction. These nitride lamellae have a Burgers-type orientation relationship with the ferrite lamellae. As compared to the recrystallized specimens, in the cold-rolled specimens, the precipitation of the initial Mo2N-type platelets occurs much faster and moreover leads to incoherently diffracting precipitates; upon continued nitriding, a much earlier but only partially occurring transition of Mo2N-type to MoN-type precipitates is observed. The results indicate that incorporation of iron in the nitrides can occur, if at all, only up till a negligible level, thereby invalidating earlier data.

Precipitation of Nb in Ferrite After Austenite Conditioning. Part I: Microstructural Characterization

While the role of Nb during the processing of austenite is quite clear, what happens in subsequent stages to the concentration of this element left in solution is subject to some debate. In particular, some uncertainty still subsists concerning the eventual homogeneous precipitation in Nb supersaturated polygonal ferrite. The present work was aimed at clarifying the precipitation sequence of Nb during coiling, through a systematic work and a careful selection of the processing conditions in order to produce different scenarios concerning the initial state of Nb. A Nb-microalloyed steel was thermomechanically processed in the laboratory followed by simulated coiling at different temperatures in the 873 K to 1023 K (600 °C to 750 °C) range. Transmission electron microscopy (TEM) showed interphase precipitation of NbC at high coiling temperatures, while at 873 K (600 °C), homogeneous general precipitation took place in ferrite and followed a Baker–Nutting orientation relationship.

Influence of oxide inclusion compositions on microstructure and toughness of weld metal for high-strength steel

The effect of oxide inclusion compositions on microstructure and notch toughness of weld metal for HT780MPa class high-strength steel was investigated. Three different types of weld metals were prepared by varying deoxidant elements. Intragranular bainite microstructure and dispersed oxide inclusions were observed and quantified by an optical microscope and by electron backscattered diffraction analyses to reveal the affectors of notch toughness. It has been found that fracture appearance transition temperature of the weld metal depends on the size of intragranular bainite (acicular ferrite) nucleated on oxide inclusions and that the upper shelf energy is affected by the density of dispersive oxides inclusions. It has also been clarified that all oxide inclusions, which contribute to intragranular transformations, consisted of titanium-rich crystalline phases and manganese-rich amorphous phases. By detailed finite element (FE)-TEM observation, the crystalline phases were identified as MnTi2O4 having Baker–Nutting orientation relationship with the intragranular bainite. It is considered that the reason for the bainite to have the orientation relationships of both MnTi2O4 and austenite is that the MnTi2O4 is generated at the interface between the amorphous phases and austenite previous to the intragranular transformation.

Influence of thermal and radiation effects on microstructural and mechanical properties of Nb–1Zr

The microstructural changes and corresponding effects on mechanical properties, electrical resistivity and density of Nb–1Zr were examined following neutron irradiation up to 1.8 dpa at temperatures of 1073, 1223 and 1373 K and compared with material thermally aged for similar exposure times of ∼1100 h. Thermally driven changes in the development of intragranular and grain boundary precipitate phases showed a greater influence on mechanical and physical properties compared to irradiation-induced defects for the examined conditions. Initial formation of the zirconium oxide precipitates was identified as cubic structured plates following a Baker–Nutting orientation relationship to the β-Nb matrix, with particles developing a monoclinic structure on further growth. Tensile properties of the Nb–1Zr samples showed increased strength and reduced elongation following aging and irradiation below 1373 K, with the largest tensile and hardness increases following aging at 1098 K. Tensile properties at 1373 K for the aged and irradiated samples were similar to that of the as-annealed material. Total elongation was lower in the aged material due to a strain hardening response, rather than a weak strain softening observed in the irradiated materials due in part to an irregular distribution of the precipitates in the irradiated materials. Though intergranular fracture surfaces were observed on the 1248 K aged tensile specimens, the aged and irradiated material showed uniform elongations >3% and total elongation >12% for all conditions tested. Cavity formation was observed in material irradiated to 0.9 dpa at 1073 and 1223 K. However, since void densities were estimated to be below 3 × 10 17 m −3 these voids contributed little to either mechanical strengthening of the material or measured density changes.

First-principles study of structure, vacancy formation, and strength of bcc Fe/V4C3 interface

Voids are representative of the damage process in both creep and ductile fractures. Although the matrix/precipitate interface has been considered the preferential nucleation site for voids, the relationship between the atomic structure of this interface and the nucleation mechanism of a void has never been sufficiently investigated. In this study, the bcc Fe/V4C3 interface is selected as a model interface between a matrix and precipitate. The vacancy formation energy and intrinsic mechanical strength at this interface are investigated using a first-principles calculation because they should be related with the nucleation of creep and ductile voids, respectively. Within the considered interface, the Fe vacancy is found to be dominant. When the Baker–Nutting orientation relationship is satisfied at the interface, the calculated intrinsic mechanical strength of the interface is 23.8 GPa. However, when the geometric coherence at the interface is low as compared to that of the Baker–Nutting orientation relationship, it is found that the interfacial mechanical strength is significantly weakened. At each interface, it is found that the back-bond of the interface determined the interfacial strength because of the strongly bonded Fe–C on the interface. The nucleation mechanism of a void at the matrix/precipitate interface is discussed based on the present findings. It is suggested that local decohesion at the matrix/precipitate interface should be the origin of the nucleation of a ductile void.

Ferrite Formation Behaviors from B1 Compounds in Steels

The effect of chemical composition of B1 compounds on the formation of intragranular ferrite during isothermal austenite-to-ferrite transformation was investigated by embedding single crystalline grains of chemically pure B1 compounds in low-alloy steel. Two kinds of B1 compounds having similar misfit against ferrite, TiO and TiN, were adopted in this study. Isothermal transformation experiments were carried out both above and below the bainitic transformation temperature. The orientation relationships between compound and ferrite and between ferrite and prior austenite were characterized by scanning electron microscope equipped with electron backscatter diffraction analyzer.It was found that the Baker-Nutting (B-N) orientation relationship played a dominant role in ferrite formation from B1 compounds at higher transformation temperature, but some low-index orientation relationships other than the Baker-Nutting orientation relationship were also observed as the transformation temperature decreased, which led to an increase in ferrite formation having the Kurdjumov-Sachs (K-S) orientation relationship with prior austenite. Ferrites from TiO had larger misorientation angles from the exact B-N orientation relationship and thus tended to have the K-S orientation relationship with prior austenite at lower transformation temperatures.

In-situ observation of phase transformation during rapid cooling processes

Inclusions contributing to acicular ferrite nucleation were investigated from a crystallographic point of view to understand the formation mechanism for acicular ferrite microstructure in low alloy steel laser weld metals. The sample was low carbon Ti-B weld metals with an oxygen content of 480 ppm. In this sample, intragranular acicular ferrite formation was observed from some inclusions and acicular ferrite had Kurdjumov-Sachs orientation relationship with austenite matrix. The multi-phase inclusions contributed to nucleation of acicular ferrite. They were surrounded by a Ti-enriched layer. It was confirmed by selected area diffraction and EDS analysis that the Ti-enriched layer was TiO. The acicular ferrite had Baker-Nutting orientation relationship with TiO layer on the inclusion surface. The lattice misfit was 3.0 % at the interface between the acicular ferrite and TiO.Therefore, it is considered that the TiO on the inclusion surface contributes to the heterogeneous nucleation of acicular ferrite by small lattice misfit.

Interphase Precipitation of VC and Resultant Hardening in V-added Medium Carbon Steels

Interphase precipitation of vanadium carbide (VC) accompanying ferrite and pearlite transformations and its effect on hardness have been examined by using medium carbon steels containing 0.1, 0.3 and 0.5 mass%V. Specimens transformed in a temperature range between 873 and 973 K consist of pearlite and small amount of proeutectoid ferrite. Ferrite fraction increases with raising transformation temperature or with increasing the V content. In addition to proeutectoid ferrite and pearlite, bainite is formed below 853 K, whose fraction is increased by the V addition. Hardening is significant in the V-added alloy between 873 K and 973 K and becomes larger by increasing V content in this temperature range. Meanwhile the alloying effect of V on the hardness remarkably decreases at 823 K where bainite transformation takes place partly. TEM characterization has revealed that VC are precipitated in both of proeutectoid and pearlitic ferrite with holding Baker-Nutting (B-N) orientation relationship with ferrite in the manner of fine rows parallel to the austenite / ferrite interphase boundary. Single variant of VC, whose habit plane is closer to ferrite / austenite boundary than the other two B-N variants, tends to be formed. The size of VC decreases and its number density increases by lowering transformation temperature, corresponding to the larger hardness increase.

Mechanism of Acicular Ferrite Formation in Low Carbon Steel Ti–B Weld Metals with Different Oxygen Levels

Inclusions contributing to acicular ferrite nucleation were investigated from a crystallographic point of view to understand the formation mechanism for acicular ferrite microstructure in mild and low alloy steel weld metals.The samples with different oxygen content in Ti–B weld metal were prepared in this study. In those samples, intragranular acicular ferrite formation was observed from some inclusions and acicular ferrite had Kurdjumov–Sachs orientation relationship with austenite matrix. The inclusions contributing to acicular ferrite nucleation were multi-phase type consisting of amorphous phase and MnS. They were surrounded by a Ti-enriched layer. It was confirmed by selected area diffraction patterns and analysis of EDS that the Ti-enriched layer was TiO. The acicular ferrite had Baker–Nutting orientation relationship with TiO layer on the inclusion surface. The misfit was 3.0% at the interface between the acicular ferrite and TiO.Therefore, it is considered that the TiO on the inclusion surface contributes to the heterogeneous nucleation of acicular ferrite by small lattice misfit.