Nb Carbonitride Research Articles

Nb–Ti composite precipitation behaviour and its effect on the growth of austenite grains in peritectic steel

ABSTRACT Refinement of solidification austenite grains is an important way to control the transverse corner cracks. In the present work, the effect of Ti–Nb carbonitride on the refinement of solidification austenite grains is studied by laboratory experiments. Then, the precipitation temperature of Nb–Ti carbonitride is analysed by establishing a thermodynamic model. The results show that the austenite diameter decreases slowly from 1.45 to 1.16 mm (without Ti addition) and significantly decreases from 0.84 to 0.21 mm (with 0.02 wt-% Ti addition) with the Nb content varying from 0.03 to 0.09 wt-%. The pinning pressure provided by composite precipitation greatly increases with 0.02 wt-% Ti addition. The thermodynamic model of Nb–Ti carbonitride shows that the precipitation temperature of (Nb, Ti)(C, N) increases by approximately 20°C compared to Nb(C, N), which is the main reason for the increase in the pinning pressure.

The Pitting Behavior of Newly Modified 17-4 Precipitation Hardened Stainless Steel with Different Nb, N, and Mo Contents

The effect of the addition of Nb, N, and Mo to 17-4 precipitate hardening stainless steel (17-4 PH steel) on its pitting behavior in a simulated geothermal environment was investigated using a cyclic polarization test, and the pitting potentials and the repassivation potentials of the materials were acquired to evaluate their pitting resistance. Nb and N were found to lower the pitting potential of 17-4 PH steel, while Mo increased it. In the case of repassivation potential, the addition of all these elements seemed to have a slightly negative impact. Cr-rich oxide, Nb carbonitride (Nb (C, N)), and reversed austenite were found in the materials. The element distribution of/around the Cr-rich oxide inclusion and the Nb (C, N) precipitate of the material were studied. A Cr-depleted zone was found around the inclusion/precipitate, which rendered the surrounding area to be more susceptible to pitting corrosion. Pit embryos and well-developed pits were observed after the cyclic polarization tests. Pits tended to initiate in the vicinity of the inclusion/precipitate, and the dissolution of the oxide around the pit embryo was seen, which was believed to cause the local deterioration of the electrochemical environment. Lacy cover with narrow strip-shaped holes was found above the well-developed pit, and the forming mechanism of these holes was believed to be the dissolution of martensitic.

Formation of Nb(C,N) Carbonitride in Cast Austenitic Heat-Resistant Steel during Directional Solidification under Different Withdraw Rates

It is recognized recently that primary “Chinese-script” Nb(C,N) carbonitride is critical to the development of cast austenitic heat-resistant steels for ultra-high temperature applications. In this paper, the precipitation behavior of Nb(C,N) carbonitride in a novel creep and fatigue resistant steel was investigated by the use of the liquid metal cooling directional solidification (LMC-DS) method under different withdraw rates. Thermodynamic calculations were also performed to aid in the understanding of the solidification behavior. Microstructural characterization and thermodynamic calculation agreed that the alloy solidified in the path of primary austenite, eutectic Nb(C,N) carbonitride, and secondary ferrite, regardless of the withdraw rate. However, the primary and secondary dendrite arm spacing decreased significantly with an increase in the withdraw rate, and a quantitative relationship was established. Furthermore, the eutectic reaction range increased at a higher withdraw rate, due to the rapid increase of the solid phase fraction and the accumulation of solutes in the interdendritic liquid phase. This gave rise to a decline in the interlamellar spacing of primary Nb(C,N) carbonitride sheets and rods for the higher withdraw rate. Therefore, a fine “Chinese-script” Nb(C,N) carbonitride in this type of alloys can be achieved through increasing the withdraw rate or the cooling rate during casting.

Static Recrystallization Kinetics and Crystallographic Texture of Nb-Stabilized Ferritic Stainless Steel Based on Orientation Imaging Microscopy

In the present study, Nb-stabilized ferritic stainless steel was prepared with annealing (430-A) and without annealing (430-NA) annealing, and the microstructure of the resulting samples was examined. The steel was then subjected to cold rolling and isothermal annealing in order to analyze its recrystallization kinetics and texture evolution. Microstructural characterization was performed by scanning and transmission electron microscopies. Recrystallization kinetics were evaluated by measuring the microhardness of the samples, and analyzing their kernel average misorientation and grain orientation spread via electron backscatter diffraction. The Avrami exponent data revealed that one-dimensional grain growth occurred owing to the migration of high-angle grain boundaries. The mean activation energies for recrystallization for 430-NA and 430-A was found to be 365 and 419 kJ mol−1, respectively. The recrystallization texture was influenced by oriented nucleation and selected growth mechanisms, as well as by the Nb carbonitride distribution and grain boundary energy. The recrystallized and growing grains with the {554}〈225〉 orientation showed a dimensional advantage over the other recrystallized components. The coincident site lattice boundaries were attributed to the progression of recrystallization since the CSL numeric fraction increased as the temperature increased. The {554}〈225〉 component was associated with the ∑19a boundary, which exerted a significant control on the selective growth during the recrystallization.

Precipitation kinetics of Nb carbonitride in austenite and acicular ferrite and its effect on hardness of high-Nb steel

The dissolved Nb contents in four high-Nb steels were quantitatively measured after hot deformation at 925 °C and reheating at 600 °C. The Nb carbonitride precipitation kinetics in austenite and in acicular ferrite were studied, and the effects of Nb carbonitride on the precipitation hardness were discussed. The results show that the nucleation rate of Nb precipitation in austenite is strongly depended on the dissolved Nb content in matrix. The parameter n of the JMAK equation is increased with the increase of the dissolved Nb content in matrix. The n value of the factor of nucleation rate of Nb carbonitride precipitated in acicular ferrite is related to the dissolved Nb and C contents in matrix. Nb carbonitride precipitated in austenite decreases the hardness of the final products by accelerating the phase transformation. By contrast, Nb carbonitride precipitated in acicular ferrite exhibits high precipitation strengthening potency.

ORR Activity of Nb Oxide Based Catalyst Prepared from Nb Compound Including C and N

The development of a non-precious metal cathode catalyst is strongly required for Polymer Electrolyte Fuel Cells (PEFCs) because the cost of platinum and precious metal catalyst is very high. In our previous study, we have reported that oxide-based compounds prepared from group 4 and 5 metal carbonitrides had a high oxygen reduction activity. In this paper, the ORR activity of Nb oxide-based catalysts prepared by heat treatment of the Nb carbide, Nb nitride and Nb carbonitiride under low oxygen pressure was evaluated. Among this, the specimen prepared from Nb carbonitride had a highest ORR activity. We found amorphous carbon on the surface high active catalysts by raman spectroscopy and elemental analysis. Carbon on the surface might affect the ORR activity.

Nb Microalloyed Steel of Grain Coarsening Law

With the process parameters and processing conditions in the mill, a systematic experimental research were conducted to Q345 steel containing niobium austenite grain growth law, and analysis the dissolution and precipitation behavior of Nb carbonitride. The results shows that Nb-microalloyed steel heated to 1180 ~ 1230 °C can significantly improve strength and toughness of steel and performance.

The Effects of Nb Carbo-Nitride Precipitation Conditions on Abnormal Grain Growth in Nb Added Steels

In order to investigate the effects of Nb carbo-nitride precipitation conditions on abnormal grain growth behavior during high temperature carburizing, size of Nb carbo-nitride precipitates was controlled by precipitation treatment at 1173-1273K for 0.6-54ks, and the specimens were quasi-carburized at 1323K. Abnormal grain growth was enhanced when the size of Nb precipitates was fine or coarse, so there is a suitable size range in Nb precipitates to suppress abnormal grain growth. The reason why abnormal grain growth was enhanced is the lack of pinning force as the conventional theory proposed by Hillert or Gladman; however, it cannot be explained by this theory that small precipitates promote abnormal grain growth. It is considered that Ostwald ripening rate of precipitates is also an important factor in controlling abnormal grain growth in addition to the amount and size of precipitates and austenite grain size, which were parameters in the Gladman‘s theory on abnormal grain growth behavior.

FE-SEM Study of Fine Nb Precipitates in Carbon Extraction Replicas

Precipitation strengthening is an important parameter controlling the mechanical properties of low carbon steels. These precipitates are very fine and are normally analyzed using either thin foils or carbon extraction replicas under a transmission electron microscopy (TEM). In this work, field emission gun scanning electron microscope (FE-SEM) was applied successfully in the characterization of niobium (Nb) carbo-nitride (C,N) precipitates using carbon extraction replicas. FE-SEM observation of high strength linepipe steel replicas before and after aging at 400°C for 1 hr confirmed the presence of Nb(C,N) precipitates in ferrite. The FE-SEM could analyze small particles (below 50 nm) embedded in the steel but the analysis had to be carried out at low voltages to maximize spatial resolution resulting in a poor signal. However, carbon extraction replicas in the FE-SEM can be analyzed using high voltages, since the interaction volume effect is no longer a problem.

高Crフェライト鋼の大型鋼塊における共晶Nb(C,N)の生成予測

Nb addition is widely used for strengthening of low-alloy steel. In case of large ingots, however, where the cooling rate during solidification is low, eutectic Nb-carbide can easily coarsen. Nb addition can substantially reduce the ductility and toughness of the material by the influence of such large eutectic carbide. As the effect of Nb on the strengthening is advantageous in high Cr steel, 12% Cr rotor forgings generally contain about 0.05wt% Nb in order to improve the high temperature strength. In case of 12% Cr steel, Nb forms Nb carbo-nitride (Nb(C, N)), because 12% Cr steel usually contains N of several hundred ppm. This research shows a guideline of chemical composition to avoid the formation of Nb(C, N) for a large 12% Cr steel ingot manufactured by conventional method. Formation condition of eutectic Nb(C, N) in 12% Cr steel and low-alloy steel were compared. It is confirmed that Nb(C, N) is formed less easily in 12% Cr steel than in low-alloy steel. The experimental results were arranged such that the amount of eutectic Nb(C, N) was expressed in an experimental formula as a function of contents of Nb, C and N, and the cooling rate. The experimental formula was verified the investigation of a large 12% Cr steel ingot weighing 68 t, and its validity was confirmed.

Aging Hardening of Cu-Nb-Ni-Cr-Mo Steels

The hardness versus time curves during aging at 450-650℃ of Cu-Nb steels with various copper content have been measured. The precipitation of e-Cu, Nb carbonitfide and Cr carbide found during aging contributes to the hardening. The result indicates that the Cr carbide hardness peak emerges after long time aging. The more is the copper content the earlier the hardness peak of e-Cu occurs. In high copper steel the hardness peaks of e-Cu and Nb carbonitride separates and in low copper steel they overlaps.

A microstructural study of rapidly solidified and heat-treated austenitic Fe–Mn−Al–Mo–W–Nb–C alloys

The microstructural characteristics of melt-spun and heat-treated austenitic Fe−28Mn−8.6Al−0.5Mo−0.7W−0.5Nb−1.1C (in wt. %) alloys have been investigated by means of transmission electron microscopy. The melt-spun alloy contained fine austenitic cells and some intercelluar Nb(C, N) precipitates. Detailed observations revealed fine {100} modulations in the matrix of the cells, as well as a concomitant L′I2 atomic ordering arising from it. These observations indicate that the onset of decomposition of the initial austenite phase occurred during the rapid solidification process. Aging of the melt-spun alloy at 823–1173 K produced various microstructures, including a general precipitation of Nb(C, N) in the matrix. On isochronal annealing for 1 h, this matrix Nb(C, N) precipitation commenced at 1073 K with the formation of metastable coherent K-carbide (K′) near cell boundaries. On annealing at temperatures above 1123 K, only the Nb(C, N) precipitates were formed, on a fine scale, being accompanied by the formation of precipitate-free regions in the vicinity of cell and grain boundaries. Both intercellular and matrix Nb(C, N) precipitates obeyed a cube-to-cube orientation relationship with austenite. The general matrix precipitation of Nb(C, N) and formation of precipitate-free regions are discussed in terms of a vacancy (defect)-depletion effect. Finally, it was demonstrated that, by employing a double heat-treatment schedule of annealing at 1173 K followed by aging at 823 K, a novel microstructure consisting of fine dispersoids of Nb(C, N) carbo-nitride, distributed over the matrix of {100} modulated structure, could be produced.

Precipitation of Nb(C,N) carbo-nitride in an austenitic Fe-Mn-Al-C alloy prepared by the rapid solidification process

The age-hardenable austenitic Fe-Mn-Al-C alloys have been extensively investigated by many researchers as a high strength material for structural applications and as a sideband alloy that decomposes into a [100] modulated structure on aging at lower temperatures. Little is known about the precipitation of Nb(C,N) carbo-nitride in Nb-containing austenitic Fe-Mn-Al-C alloys, and occurrence of general matrix precipitation in those alloys has not been reported. On the other hand, there have been many studies of the precipitation of NbC from Nb,C-containing austenitic Fe-Cr-Ni steels. In particular, it has been documented in detail that rapid solidification could facilitate a dense precipitation of NbC carbide in the alloy matrix. Inspired by these earlier works, a study was undertaken to investigate precipitation reactions, especially of Nb(C,N), in Nb-containing austenitic Fe-Mn-Al-C alloys prepared by the melt-spinning process. In this brief paper, the authors present some results of the work, demonstrating that fine precipitates of Nb(C,N) carbo-nitride can be produced in the matrix of an austenitic Fe-Mn-Al-C alloy. Also, a discussion as to the observed mode of the matrix Nb(C,N) precipitation is presented.

9Cr-1Mo-VNbN鋼のクリープ破断強度への圧延および焼ならし条件の影響とV,Nb炭窒化物の役割

9Cr-1Mo-VNbN鋼のクリープ破断強度への圧延および焼ならし条件の影響とV,Nb炭窒化物の役割

Continuous-cooling-precipitation kinetics of Nb(CN) in high-strength low-alloy steels

Although isothermal precipitation has been frequently studied with respect to industrial hot deformation processing, the temperature decreases continuously under these conditions so that isothermal data cannot be applied directly to predict the precipitation kinetics. This study therefore was concerned with the continuous-cooling-precipitation (CCP) behavior of Nb carbonitride in austenite. In the present work, the Liu-Jonas (L-J) model was used to calculate the precipitation start (Ps) time at a given temperature from experimental data. A new calculation method for predicting the precipitation finish (Pf) time, based on reaction kinetics and classical nucleation and growth theory, was also developed. The additivity rule was then used to calculate the extent of precipitation during continuous cooling. Isothermal precipitation rates for 0.04 pct Nb steels were measured experimentally by the stress relaxation method. The CCP behavior was then calculated from the model, and the accuracy of the predictions was evaluated by carrying out continuous-cooling tests using a deformation dilatometer. The precipitate size distributions were determined by the transmission electron microscopy of specimens quenched after increasing intervals of cooling at various cooling rates. ThePs andPf times estimated from the particle size data show good agreement with the calculated CCP behavior.

The non-equilibrium segregation of boron during the recrystalization of Nb-treated HSLA steels

The effect of boron segregation on recrystallization was investigated in two Nb and two Nb-free steels. The results indicate that the addition of B to a Nb steel retards the recrystallization of austenite after high temperature deformation to a significant degree. By contrast, the effect of B when added alone is not of commercial importance. At deformation temperatures above 950°C, the influence of boron is mainly due to grain boundary segregation and solute drag; below 900°C, the retardation results from the precipitation of Nb carbonitride. During isothermal holding, there are two types of non-equilibrium segregation on boundaries. Strain-induced segregation occurs on the original boundaries; it increases and attains a maximum rapidly, then decreases and disappears after 60 s of holding at 1000°C. A second type of non-equilibrium segregation occurs on newly formed boundaries during recrystallization. It develops quickly as the new boundaries are formed and begin to move, and can persist until grain impingement occurs and the motion is halted. The enhanced effect of B in the presence of Nb is attributed to the formation of (Nb, B) complexes, which appear to increase the solute drag force and decrease the boundary velocity.