NbC Carbides Research Articles

Corrosion resistance of niobium-added slag-free self-shielded flux-cored welding overlay in neutral solution

The niobium-added iron-based slag-free self-shielded flux-cored wire was developed and the effect of niobium addition on the corrosion resistance in neutral pH solution was investigated. The carbides, including NbC carbides, primary (Cr, Fe)7C3, and eutectic (Cr, Fe)3C carbides, remain intact after neutral corrosion, but the grain interface is corroded preferably for all the niobium-added welding overlays. The eutectic microstructure of 7.2 wt-% niobium-added welding overlay, which has more grain interface, leads to severe corrosion. When niobium addition reaches 14.4 wt-%, the welding overlay possesses the best corrosion resistance since more NbC carbides are formed, prophase austenite dendrite is precipitated, decreasing the grain interface, and chromium content is increased in the matrix of the hypoeutectic microstructure.

Quantitative Microstructural Characterization of Precipitates and Oxide Inclusions in Inconel 625 Superalloy Additively Manufactured by L-PBF Method

In this study, we perform quantitative characterization of precipitates and oxide inclusions in Inconel 625 additively manufactured by the laser powder-bed fusion (L-PBF) process. The application of different microscopy techniques allowed us to characterize the microstructure at micro- and nano-scale in the as-built and stress-relieved condition and correlate the features of grains and cellular substructure with parameters of particles along the planes parallel and perpendicular to the build direction. The optimized imaging conditions and image analysis procedure allowed easily distinguishing precipitates and oxide inclusions and performing their quantitative analysis. The results showed that intercellular areas are the preferential sites of precipitation of the Laves phase and NbC carbides with diameters in the range of 10 to 440 nm. Moreover, aluminum oxide inclusions with diameters in the range of 30 to 300 nm are randomly distributed. Regardless of the processing conditions of the examined samples, the influence of the stress-relief annealing on the secondary phases was not observed. In both the as-built and stress-relieved samples, the size of precipitates is in submicrometer scale. The analysis provided detailed information about the parameters of particles depending on the orientation versus the build direction. It was demonstrated that despite the tendency for columnar grain morphology and the anisotropy of the cellular substructure, the particle distribution is almost uniform throughout the volume of the additively manufactured L-PBF Inconel 625.Graphical

Effect of TiC content on the carbide particle growth of NbC-Ni cemented carbide

The current study reports the influence of TiC addition on carbide particle growth of NbC-Ni cemented carbides. Microstructural analysis after liquid phase sintering in vacuum at 1420 °C for 40 min showed a decrease in NbC grain size as TiC content increased, associate to a change in NbC carbide morphology from rounded to equiaxed. Chemical composition from EDS analysis revealed a mixed carbide (Nb,Ti)C formation, with a gradient of Ti in carbide region for samples with TiC above 15%. Dictra® simulations of spherical particles immersed in Ni matrix evidenced a retardation in carbide dissolution and a decrease in the volume dissolved as TiC is added. It was also noted that liquid phase composition for 140 min showed less Nb in liquid phase available for reprecipitation during cooling process, promoting carbide particle growth inhibition. Ti profiles from Dictra® and carbide composition from EDS suggest a thin Ti rich layer formation in carbide/binder interface, which can be related to the carbide particle growth inhibition mechanism. TiC addition promote an increase in hardness and toughness values, and the highest values for both properties was found for 7%TiC-NbC-12%Ni composition (mass percent).

Mechanical and microstructural characterizations of friction stir welded dissimilar butt joints of Inconel 718 and AISI 204Cu austenitic stainless steel

This study explored the feasibility of friction stir welding (FSW) of dissimilar butt joints of 3 mm thick Inconel 718 Ni-based superalloy and AISI 204Cu stainless steel. The effect of operating parameters on microstructure and mechanical properties of the weldments were investigated with varying traverse speeds of 40, 70, 90 and 140 mm/min while keeping the rotational speed constant, i.e. 300 rpm. The microstructural characterization was performed using optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results revealed that the formation of M23C6 carbide particles and fine equiaxed recrystallized austenite and ferrite grains in the nugget zone led to an increase in mechanical properties. The atomic diffusion region at the joint interface of the Inconel 718 and AISI 204Cu was an average thickness of 1.5 μm showed an fcc microstructure with refined MC carbide particles (average size 40 nm) and NbC carbides (average size 50 nm). In addition, a dragged material from Inconel 718 on the advancing side of the stainless steel 204Cu was found, which was directly affected by the traverse speed and related to the strength of the joint. Grain refinement in the nugget zone caused by dynamic recrystallization was observed at all conditions, improving mechanical properties. Based on the obtained results, the medium traversed speeds, i.e. 70 and 90 mm/min at a rotational speed of 300 rpm, were recommended for the sound weld.

Elimination of Primary NbC Carbides in HSLA Steels for Oil Industry Tubular Goods

Two high-strength low-alloyed steels alloyed with various Nb contents were prepared using continuous casting (CC) technology. In the round Steel 30Nb CC billet, it was the segregation of C and Nb in the central equiaxed zone that promoted the precipitation of primary NbC carbides at grain boundaries. After processing into seamless tubes, micron-sized primary NbC carbides were located in the middle of the segregation bands, surrounded by many nano-sized NbC carbides formed during the post-solidification cooling and/or hot-piercing rolling. Even when the Nb content was decreased down to 0.006 wt.%, there were still coarse primary NbC carbides in the segregation bands. However, no primary NbC carbide was found in the round Steel 30Nb billet prepared using the electroslag remelting technology. Therefore, this study revealed that a rapid solidification rate was relatively effective in eliminating primary NbC carbides compared with merely decreasing the Nb content.

The potential significance of microalloying with Nb in enhancing the resistance to hydrogen-induced delayed fracture of 1300-MPa-grade high-strength bolt steel

The present work aims to explore the potential significance of microalloying with Nb in enhancing the resistance to hydrogen-induced delayed fracture (HIDF) of a novel 1300-MPa-grade V-microalloyed high-strength bolt steel 42CrNiMoV using constant load tensile test. The results show that ∼ 82% of the added 0.03 wt% Nb is consumed to form NbC carbides which causes significant grain refinement and thus an overall strengthening of ∼ 60 MPa. The effective hydrogen diffusion coefficient is reduced by ∼ 39% and the corrosion resistance is significantly improved through the Nb addition. The Nb-added steel has enhanced tolerance of diffusible hydrogen content and its resistance to HIDF is enhanced by ∼ 13%. The mechanism of Nb in enhancing the resistance to HIDF was discussed. It is suggested that microalloying with Nb is an effective approach to further improve the HIDF resistance for high-strength bolt steels.

Improving surface quality and superficial microstructure of LDED Inconel 718 superalloy processed by hybrid laser polishing

In the present research, a hybrid laser polishing technology combining pulsed laser and continuous wave laser was applied to polish the surface of laser directed energy deposition (LDED) Inconel 718 superalloy components. The surface morphology, microstructure evolution and microhardness of the as-fabricated, the single pulsed laser polishing (SPLP) and the hybrid laser polishing (HLP) processed samples were investigated. The results revealed that the as-fabricated sample has a rough surface with sintered powders. In the matrix, the NbC carbide and Cr2Nb based Laves phase array parallel to the build direction and the small γʺ-Ni3Nb particles precipitate in matrix uniformly. The surface roughness of the as-fabricated sample is 15.75 μm which is decreased to 6.14 μm and 0.23 μm by SPLP and HLP processing, respectively. The SPLP processing refines the grains and secondary phase significantly in the remelted layer which is reconstructured with the cellular structure and plenty of substructures. The HLP processing also refines the grain and secondary phase but the secondary phases still exhibit array distribution. In addition, the tangled dislocations pile up along the interface of secondary phases. Compared with the as-fabricated sample, the SPLP processing decreases the surface microhardness but the HLP processing increases the surface microhardness, and the Young's elasticity modulus of surface layer is improved by SPLP and HLP processing to 282 ± 5.21 GPa and 304 ± 5.57 GPa, respectively.

EFFECT OF AUSFORMING ON MECHANICAL PROPERTIES OF 12%Cr FERRITIC/MARTENSITIC STEEL

Effects of ausforming through upsetting at 890 °C and following tempering on microstructure and mechanical properties under uniaxial tension of 12CrWMoNbVB ferritic/martensitic steel are studied. Electron microscopic investigations along with elemental analysis of the steel samples in various states provided detailed information about subgrain structure and typical precipitates, the latter mainly М23С6 and NbC carbides. Strength and plasticity parameters (σ0, σb, δ) are examined and shown to enhance under applied thermomechanical treatment as compared to standard characteristics of the steel. Preferred treatment of the steel is preliminarily evaluated.

Effect of niobium and austenitizing temperature on the microstructures and properties of spray-deposited AISI M3:2 high speed steel

The microstructure and properties of niobium-containing AISI M3:2 high speed steels (HSSs) fabricated by spray forming and traditional casting have been investigated. The results show that fine and uniformly-distributed grains without macrosegregation appeared in the as-deposited HSSs that differ from those of as-cast HSSs. Nb mostly appears in primary MC carbides, whereas it contributes less to the formation of M6C carbides. The high stabilization of Nb-rich MC carbides can pin the grain boundaries during high-temperature austenitizing process, thus conferring a fine grains and raising the content of dissolved alloying elements. Enhanced precipitation strengthening and fine dispersion of NbC carbides throughout the matrix contribute to the high hardness and red hardness of Nb-containing HSS.

Structure Formation in Equimolar Mixture of HfC–ZrC–TiC–NbC Carbides

Structure Formation in Equimolar Mixture of HfC–ZrC–TiC–NbC Carbides

Effects of the Primary NbC Elimination on the SSCC Resistance of a HSLA Steel for Oil Country Tubular Goods.

Sulfide stress corrosion cracking (SSCC) has been of particular concern in high strength low alloyed (HSLA) steels used in the oil industry, and the non-metallic inclusions are usually considered as a detrimental factor to the SSCC resistance. In the present work, continuous casting (CC) and electroslag remelting (ESR) were adopted to fabricate a 125 ksi grade steel in order to evaluate the effect of microstructure with and without primary NbC carbides (inclusions) on the SSCC resistance in the steel. It was found that ESR could remove the primary NbC carbides, and hence, slightly increase the strength without deteriorating the SSCC resistance. The elimination of primary NbC carbides caused two opposite effects on the SSCC resistance in the studied steel. On the one hand, the elimination of primary NbC carbides increased the dislocation density and the proportion of high angle boundaries (HABs), which was not good to the SSCC resistance. On the other hand, the elimination of primary NbC carbides also induced more uniform nanosized secondary NbC carbides formed during tempering, providing many irreversible hydrogen traps. These two opposite effects on SSCC resistance due to the elimination of primary NbC carbides were assumed to be offset, and thus, the SSCC resistance was not greatly improved using ESR.

Effect of microstructure on hydrogen embrittlement susceptibility in quenching-partitioning-tempering steel

Hydrogen embrittlement (HE) restricts the application of high strength steel in sustainable energy productions. As one type of efficient hydrogen trap sites, the NbC carbide precipitation is a superior approach to mitigate the HE susceptibility. The microstructure, mechanical properties and HE susceptibility were investigated in different high strength steels treated by quenching and partitioning (Q&P), quenching-partitioning-tempering (Q-P-T) and intercritical annealing quenching and partitioning (IAQP) processes in this study. The results show that the NbC particles can significantly improve the HE resistance of high strength steel treated by Q-P-T process. The NbC carbide precipitation has four different morphologies with varied sizes, which can effectively trap a large amount of diffusible hydrogen atoms. The retained austenite phases with different morphologies and locations have different hydrogen trap ability, but ferrite phase does not show strong hydrogen trap ability. Meanwhile, the NbC carbide precipitation can enhance the yield strength of steel effectively through precipitation strengthening, but has little effect on its ductility.

Performance of Carbide Alloy Compounds in Carbon Doped MoNbTaW

In this work, the performance of the carbon doped compositionally complex alloy (CCA) MoNbTaW was studied under ambient and high pressure and high temperature conditions. TaC and NbC carbides were formed when a large concentration of carbon was introduced while synthesizing the MoNbTaW alloy. Both FCC carbides and BCC CCA phases were detected in the sample compound at room temperature, in which the BCC phase was believed to have only refractory elements MoNbTaW while FCC carbide came from TaC and NbC. Carbides in the carbon doped MoNbTaW alloy were very stable since no phase transition was obtained even under 3.1 GPa and 870 °C by employing the resistor-heating diamond anvil cell (DAC) synchrotron X-ray diffraction technique. Via in situ examination, this study confirms the stability of carbides and MoNbTaW in the carbon doped CCA even under high pressure and high temperature.

Effect of NbC and VC carbides on microstructure and strength of high-strength low-alloyed steels for oil country tubular goods

High-strength low-alloyed (HSLA) steels for oil country tubular goods (OCTGs) are usually microalloyed with niobium (Nb) and vanadium (V). In this work, the microstructure and strength of HSLA steels with different Nb and V contents were studied with the purpose of investigating the role of NbC and VC carbides. The results showed that NbC carbides were responsible for restraining the growth of prior austenite grains, and when the austenitizing temperature was below 910 °C, it has been found that the prior austenite grain size was similar in steels with 0.02 and 0.006 wt% Nb addition. VC carbides could impede dislocation annihilation and refine Cr-rich carbides and martensitic blocks during tempering. Moreover, VC carbides mainly contributed to the tempering softening resistance so that the V-bearing steels exhibited a higher strength than the V-free steel after tempering at 690 °C–730 °C. To illustrate the high tempering softening resistance of V-bearing steels, the changes in various strengthening contributions induced by V addition were quantitatively evaluated. The calculation results indicated that the main reason for the high tempering softening resistance of V-bearing steels was precipitation strengthening by nanosized VC carbides, in contrast to dislocation strengthening, substructure strengthening, and precipitation strengthening due to the refinement of Cr-rich carbides.

Interaction between WC and Inconel 625 under Solid and Liquid State Sintering Conditions

Cobalt is the most used metal binder in hard metals since its extraordinary wetting, adhesion and mechanical properties. Nevertheless, it has been recognized genotoxic and cancerogenic with higher toxicity in combination with WC. To substitute Co with an alternative binder, the interaction between the binder and WC must be taken into account. In this work, IN625 is considered as a binder alternative due to its desirable combination of high-temperature strength and corrosion/oxidation resistance. A characterization of the interaction between WC and IN625 was carried out by means of Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDXS) and X-Ray Diffraction (XRD). Depending on the sintering temperatures, different phases were evidenced at the WC–IN625 superalloy interface. From 1250 °C to 1300 °C, where solid-state sintering takes place, (Cr,Mo)23C6, W2C and (Cr,W) solid solutions were detected. At a sintering temperature of 1350 °C, IN625 melts and the formation of additional phases, such as an intermetallic Ni4W phase and (Mo,W) and (Mo,Nb) solid solutions, were observed. The precipitation of NbC and (Mo,Cr)23C6 carbides in IN625 was also detected.

Effect of microstructural evolution in thermal exposure on mechanical properties of HP40Nb alloy

The phase transformation of HP40Nb alloy at various temperatures and times was investigated and the microstructural evolution in aging process was characterised. Additionally, the mechanical tests were implemented for aged specimens. Experimental results indicate the microstructure of as-cast alloy consists of the austenite matrix with skeletal-shaped primary carbides. NbC carbide changes into G phase from 700 °C to 990 °C, and the nose temperature of transformation ranges from 900 °C to 950 °C. M7C3 carbide tends to transform into M23C6 carbide at higher temperature. Meanwhile, numerous secondary carbides are precipitated in the initial aging stage and dissolves gradually in further process. The interdendritic precipitates coalesce and coarsen continuously in aging treatment. The tensile strengths and the micro-hardness increase slightly followed by the continuous reduction in exposure, while the elongation and impact toughness decrease continuously. The microstructural evolution during aging process is consistent with the variation of mechanical properties.

Influence of NbC Content on the Wear Resistance of Alumina/Niobium Carbide Tools

Wear resistance is a fundamental property which defines the lifetime of cutting tools, but the investigation of wear performance of alternative hard materials for traditional WC-Co composites are recent. The present work evaluated the pin-on-disk wear behavior and mechanical properties, i.e., hardness and fracture toughness of spark plasma sintered Al2O3 matrix composites with additions of 5, 15, 25 or 30%wt of niobium carbide NbC. The wear resistance was observed to increase as a function of the NbC content, even though the hardness reached a plateau at 25%wt NbC. The composite behavior was compared to that of other alumina composite tool materials proving to be a promising material for applications such as ceramic cutting tools. The composition A95N5 presented the best combination of values of wear rate: 8.9 mm3/N.m, hardness equal to (17.36±1.72) GPa and fracture toughness of (3.2±0.6) MPa. m1/2.

Effect of Nb addition and Pre-strain on Hydrogen Embrittlement of Low-carbon Steels with Ferrite-pearlite Structure

The effect of pre-strain on the hydrogen embrittlement of Nb-free and Nb-added low-carbon steels with ferrite-pearlite structure was investigated in this study. After the steels were electrochemically charged with hydrogen, slow-strain rate tensile (SSRT) tests were conducted on them to examine hydrogen embrittlement behavior. The SSRT test results revealed that the Nb-added steel had a lesser decrease of elongation and reduction of area than the Nb-free steel. The formation of NbC carbide and grain refinement caused by the Nb addition improved resistance to hydrogen embrittlement. The loss of elongation and the reduction of area after hydrogen charging occurs when pre-strain is increased. The pre-strain increases dislocation density and thus increases the amount of reversible hydrogen trap sites associated with hydrogen embrittlement. 10% pre-strained specimens exhibited a significant loss in elongation and reduction of area, regardless of Nb addition. Based on the results of electron back-scatter diffraction, fractographic, and silver decoration analyses for Nb-free and Nb-added steels, the hydrogen embrittlement mechanism in low-carbon steels with different amounts of pre-strain is discussed in terms of dislocation density and hydrogen distribution.

Microstructural Properties of Fe–Cr–C/NbC Composite Coating Produced on Medium Carbon Steel Surface by TIG Coating Process

In this study, Fe-based composite coatings were produced on a medium carbon steel surface by the Tungsten inert gas method. The microstructure and phase structure of the coatings were analyzed by an optical microscope with clemex image analysis, scanning electron microscope equipped with an energy-dispersive spectroscopy and X-ray diffractometer. The results show that the coating is metallurgically bonded to the substrate when it is produced with sufficient energy input. In the microstructure of Fe–Cr–C coatings, hexagonal-shaped M7C3 carbides solidified in γ-(Fe,Cr) eutectic matrix, while in Fe–Cr–C/NbC composite coatings, quadrangular-shaped NbC carbides solidified. As the amount of FeNb melted increased, the amount of NbC in the structure increased.

Influence of Hot Isostatic Pressing on the Hot Ductility of Cast Alloy 718: The Effect of Niobium and Minor Elements on the Liquation Mechanism

The influence of two hot isostatic pressing (HIP) treatments on liquation behavior was investigated and compared with regard to the extent of heat-affected zone liquation cracking in cast Alloy 718. The extent of liquation was seen to increase after HIP treatment at 1190 °C due to solute changes caused by the homogenization of Nb, which contributed to extensive grain boundary melting. The HIP treatment at 1120 °C exhibited lower liquation with contributions from particle liquation of the Laves phase and constitutional liquation of NbC carbides. This was also reflected in a lower ductility recovery temperature, with slower recovery for the former due to the extensive liquation. Interestingly, the nil ductility temperatures were both below the predicted equilibrium solidus of the alloy, which suggests that the ductility drop is related to liquation caused by solute segregation at the grain boundaries.