Nb Microalloying Research Articles

Underlying mechanisms for the effect of Nb micro-alloying on the elemental distribution and precipitation behavior in the X70 weld metal

Niobium (Nb) is a widely recognized micro-alloying element due to its low cost and substantial impact on steel properties. While the effect of Nb in processed steels has been well investigated, studies on its elemental distribution and precipitation behavior in weld metal remain scarce. This study focuses on the weld metal of specially designed Nb-rich X70 pipeline steel by scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS) characterization, complemented with thermodynamic and kinetics modeling analysis. In the majority of the weld, Nb was essentially uniformly distributed. This suggests that Nb primarily exists in the solid solution form in the weld metal, which is also supported by precipitation kinetics modeling results. This is primarily due to the short thermal history associated with the welding process, which leads to insufficient time for the uniform precipitation of Nb. Two instances of Nb precipitates were observed at the weld centerline and reinforcement region. The low partition coefficient of Nb results in an elevated local concentration along the weld centerline. However, precipitation kinetics calculations suggest that this enhancement alone is not adequate to induce precipitate formation. The occurrence of MnS and the prior formation of Ti precipitates may provide heterogeneous nucleation sites for Nb, facilitating the nucleation of Nb precipitates in the weld centerline and reinforcement region.

Precipitation Behavior and Strengthening–Toughening Mechanism of Nb Micro-Alloyed Direct-Quenched and Tempered 1000 MPa Grade High-Strength Hydropower Steel

Faced with the rapid development of large-scale pumped-storage power stations, the trade-off between the strength and toughness of hydropower steels in extreme environments has been limiting their application. The effects of Nb micro-alloying and direct quenching and tempering processes on the strengthening–toughening mechanism of 1000 MPa grade high-strength hydropower steel are studied in this paper, and the precipitation behavior of Nb is discussed. The results showed that only the 0.025Nb steel using the DQT process achieved a cryogenic impact energy of more than 100 J at −60 °C. Under the DQT process, a large number of deformation bands and dislocations were retained, refining the prior austenite grains and providing more nucleation sites for the precipitation of NbC during the cooling process. The DQT process has a more obvious local strain concentration, mainly focusing on the refined lath boundary, which indicates that the refinement of the microstructure also promotes the stacking of dislocations. The improvement in fine grain strengthening and dislocation strengthening by the DQT process jointly led to an increase in strength, resulting in a better combination of strength and toughness.

Effect of N and Nb Microalloying on Corrosion Resistance of 309S Stainless Steel for Canister-Stored Nuclear Waste

Effect of N and Nb Microalloying on Corrosion Resistance of 309S Stainless Steel for Canister-Stored Nuclear Waste

Improving strength-toughness of low carbon bainitic microalloyed steel via tailoring isothermal quenching process and niobium microalloying

This study focuses on improving the strength-toughness properties of low carbon bainitic steel through the optimization of the isothermal quenching process and the adoption of a microalloying strategy. The impact of Nb microalloying on the microstructure and mechanical properties was thoroughly investigated. The optimal isothermal quenching process for achieving a balance between strength and toughness is as follows: the steels were austenitized at 900 °C for 30 min and then subjected to an isothermal treatment at 450 °C for 10 min. The resulting microstructure consisted mainly of granular bainite (GB) and lath bainite (LB), with GB accounting for 60 % of the structure. After the optimized isothermal quenching process, the ultimate tensile strength and yield strength of the Nb microalloyed steel increased to 1210 MPa and 693 MPa, respectively. The impact energy, KU2, was measured at 41.8 J. The study revealed that Nb microalloying refines the bainite structure by reducing activation energy and increasing the nucleation rate. This refinement was observed in the reduced length and thickness of the bainitic laths, as well as the increased dislocation density. Additionally, fine composite precipitated particles (Nb, V)C with an average diameter of 8.2 nm were dispersed throughout the microstructure. The addition of Nb significantly enhanced the effects of fine grain strengthening, precipitation strengthening, and dislocation strengthening in low carbon bainitic microalloyed steel. As a result, the strength was greatly improved without sacrificing toughness, thanks to the combined actions of these three strengthening mechanisms.

Study on Laser Overlap Welding of Titanium/Aluminum Dissimilar Metals Based on Niobium Microalloying

Brittle intermetallic compounds, formed during the welding process of titanium/aluminum (Ti/Al), lead to a significant reduction in joint mechanical properties. The purpose of this study is to mitigate the formation of brittle phases during the laser welding of dissimilar Ti/Al metals, thereby enhancing the mechanical properties of the joints. In this investigation, an innovative approach is adopted, utilizing Nb foil as an interlayer to effectively minimize the formation of brittle intermetallic phases during dissimilar welding. A comprehensive analysis of the microstructure of the transition layer was conducted using material characterization methods, including scanning electron microscope equipped with an energy dispersive X-ray spectrometer. The mechanical performance of the welded joints was assessed using tensile testing. The results indicate that the effective welding width and joint penetration depth at the joint interface were reduced in Ti/Al dissimilar metals when Nb was added as an intermediate layer, under the same welding process parameters, when compared to unalloyed weld seams. Furthermore, the utilization of a 0.05 mm Nb foil as the intermediate layer results in a significant 25% increase in the average shear strength compared to the other condition, with the average shear strength of the joint reaching its peak value at 192 N/mm. The unalloyed Ti/Al weld joint usually fractured along the melting zone, displaying complete brittle fracture characteristics. After Nb microalloying, the joint typically fractures along the transition zone and interface, exhibiting both cleavage and ductile fracture characteristics, indicating the combination of a brittle and toughness fracture. This study provides experimental evidence and new insights for welding Ti/Al composite structures, with significant theoretical and practical applications.

Corrosion and abrasion behavior of high-temperature carburized 20MnCr5 gear steel with Nb and B microalloying

This paper investigates the effect of Nb and B microalloying on the wear and corrosion resistance of 20MnCr5 steel after high-temperature carburization at 950 °C. The abrasion and corrosion performance of the steel was compared using electrochemical experiments and dry sliding wear tests. The results show that Nb microalloying reduces the size of residual austenite, thereby improving the wear resistance of 20MnCr5 steel. Furthermore, the combination of Nb and B microalloying reduces the amount of residual austenite, leading to further improvements in wear resistance. Both microalloying methods reduce the wear scar depth and debris accumulation. The corrosion potentials of 20MnCr5, 20MnCr5Nb, and 20MnCr5NbB are 0.645 V, 0.627 V, and 0.675 V, respectively. The addition of Nb to 20MnCr5 steel results in higher corrosion resistance after carburization due to the refinement of needle-like martensite. However, 20MnCr5NbB steel exhibits low corrosion resistance due to the coarsening of needle-like martensite caused by B microalloying. In summary, this study provides new insights into the effects of microalloying elements on the properties of steel. The findings contribute to a deeper understanding of how to design and manufacture gears and other components with superior wear and corrosion resistance.

Study on thermal stability and biocompatibility of bimodal microstructure in Cr–Mn–N austenitic stainless steel

Heterostructured austenitic stainless steels (ASS) are becoming a significant research area because of their outstanding mechanical properties and considerable potential for various applications. However, the thermal stability of heterogeneous microstructures, particularly the bimodal microstructure, has received limited attention and lacks systematic investigation. Additionally, there is a scarcity of reports regarding the biocompatibility of bimodal microstructures. Herein, the thermal stability and biocompatibility of the bimodal microstructure prepared by cold rolling and annealing in Cr–Mn–N series ASS (without and with Nb microalloying) are studied. The findings demonstrate that the bimodal microstructures of ASS are formed after annealing at 700 °C, 800 °C, and short-time annealing at 900 °C. Moreover, the addition of Nb significantly enhances the thermal stability of the bimodal microstructure and maintains the bimodal feature up to 1000 °C. The thermal stability of bimodal microstructures depends on the competition in coarse and fine grains growth. The good thermal stability of Nb(C, N) at high temperatures leads to consistently higher pinning force within the fine-grained zone. As a result, the growth of fine grains lags behind that of coarse grains, which leads to the persistence of bimodal microstructure at higher temperatures. The bimodal microstructure of ASS demonstrates superior biocompatibility, attributed to its ability to promote higher cell viability, exhibit stronger fibronectin intensity, and facilitate a wider fibronectin expression network in osteoblasts. These characteristics make the bimodal ASS more favorable for osteoblast attachment and proliferation compared to its coarse-grained counterpart. This study significantly enhances our understanding of the thermal stability and cellular functionality of bimodal ASS, highlighting its potential for various biomedical applications.

Study of an Economical and Effective Heat Treatment Method to Improve the Performance of Gear Steels

During heat treatment, gear steels undergo grain coarsening, so an appropriate heat treatment process that can efficiently control the austenite grain growth and ensure a good performance of gear steels is particularly important. Herein, the influence of a requenching treatment on the comprehensive behavior of low‐carbon alloy steel 20Cr2Ni4 is explored. The pristine steel samples with and without Nb microalloying subjected to conventional quenching and tempering (QT) treatment are compared with the requenched steel sample. The results show that different requenching temperatures considerably impact grain refinement and homogenization. The steel requenched at 830 °C exhibits the greatest tensile strength of 1444 MPa and excellent toughness, comparable to the mechanical properties of the steel upon Nb microalloying after the conventional QT treatment. The formation of twin martensite and uniform fine grains with a high dislocation density together with numerous low‐angle and high‐angle grain boundaries is responsible for the simultaneously improved strength and ductility of the requenched specimen; thus, the proposed requenching heat treatment is a promising, efficient, and reliable method to double‐refine prior austenite grains and their packet and block structures and improve the comprehensive properties with a combination of high strength and toughness, yielding high‐strength gear steel.

Effect of Coiling Temperature on Microstructure and Properties of a Ti–Nb Microalloyed High‐Speed Guardrail Steel

The effects of coiling temperature on microstructure and properties of a Ti–Nb microalloyed high‐speed guardrail steel are investigated. The results show that when the coiling temperature varied from 570 to 670 °C, the optimum comprehensive mechanical properties are obtained at the coiling temperature of 620 °C. The yield strength, tensile strength, and total elongation are 729, 813 MPa, and 11.8%, respectively. The difference in yield strength between the samples at varying coiling temperature is attributed to different strengthening effects of grain refinement, precipitation, and dislocation strengthening, among them the grain refinement and precipitation strengthening contributions are dominating, with contributions of 290 and 186 MPa, respectively, at 620 °C. Moreover, the fine nano‐scale (Ti, Nb)C particles are formed during and after the coiling stage.

Application Research on Nb Microalloying of High-Carbon Pearlite Bridge Cable Wire Rods

The application of Nb microalloying to high-carbon pearlite bridge cable wire rod steel has always been controversial, especially in the actual production process, which will be affected by the cooling rate, holding temperature and final bonding temperature. In this paper, the experimental characterization, finite element simulation and phase diagram calculation of the test steel were carried out, then the microstructure and properties of different parts of Nb microalloying of bridge cable wire rods were compared and analyzed. The phase transition interval of pearlite during the water-cooling process of bridge cable wire rods is increased due to the refinement of austenite grains, and the significant increase in the end temperature of the phase transition makes the average interlamellar spacing of pearlite increase. The cooling rate of different parts of bridge cable wire rods simulated by Abaqus has little difference. At the same time, Nb microalloying effectively increases the proportion of low-angle grain boundaries, so that the overall average misorientation representing the surface defects is reduced. This helps to reduce the surface energy and increase the stability of the microstructure. Combined with the mechanical properties of microtensile rods, it is found that the grain refinement effect of Nb is greater than that of coarsening interlamellar spacing during hot rolling deformation in actual production, which makes the tensile strength at the 1/4 section increase significantly. The overall tensile strength and area shrinkage of the steel wire have also been effectively improved.

The Effect of Adding V and Nb Microalloy Elements on the Bake Hardening Properties of ULC Steel before and after Annealing

Bake hardening (BH) is a vital part of special steel production. Studies in this field have focused on steels under homogeneous yielding, but until now, none have been conducted on the phenomena that occur for steels under heterogeneous yielding. In the current study, the effect of adding Nb and V alloying elements on the strength of ultra-low carbon (ULC) steel after bake hardening was investigated. The effects of pre-strain, grain size, and recrystallization annealing temperature were analyzed, as well as the effect of Nb and V on the yield stress caused by the bake hardening process. For this purpose, five types of alloys with different V and Nb contents were melted, cast in an induction furnace, and subjected to hot hammering and hot rolling. Then, cold rolling was applied to the samples by ~80%. To eliminate the effects of cold working, tensile samples were subjected to recrystallization annealing at 750 and 800 °C for 30 min, and the samples were quickly quenched in a mixture of a NaCl solution and ice. The annealed samples were subjected to a pre-tensile strain in the range of 2-12% and then aged in a silicone oil bath at 180 °C for 30 min. Then they were subjected to a tensile test. The obtained results showed that with the increase of the pre-strain and the annealing temperature, the values of baking hardness increased. The presence of V in the composition of steel reduced the annealing temperature.

Nb micro-alloying on enhancing yield strength and hindering intermediate temperature decomposition of a carbon-doped high-entropy alloy

Carbon-doped fine-grained high-entropy alloys (HEA) presented a good combination of yield strength and ductility. However, carbon containing HEAs inherently tend to decompose into intermetallic compounds during thermal exposure at intermediate temperatures. Therefore, selective micro-alloying with 0.2%Nb in CoCrFeMnNi-1.3%C alloy (Nb-HEA) has been attempted to enhance the room temperature mechanical properties as well as for hindering the thermal decomposition at an intermediate temperature. Nb micro-alloying in the carbon-doped high-entropy alloy induces NbC carbides precipitation at 700–900 °C, strengthening the Nb-HEAs. The Nb-HEA shows an excellent combination of yield strength of ∼1096 MPa and elongation of ∼12% after annealing at 700 °C for 1 h. Furthermore, Nb micro-alloying hinders the FCC matrix decomposition at an intermediate temperature (500 °C). Specifically, the brittle σ phase formation at intermediate temperature is dramatically suppressed, while the growth of the L10 and BCC/B2 is inhibited. The results of this work may serve as a guide for designing the Nb alloying HEAs.

Effects of Nb microalloying on the magnetic properties and microstructure of Fe-Si-B-P-Cu-Nb nanocrystalline alloys prepared with pure and industrial raw materials

Fe-based nanocrystalline alloys with high saturation magnetic flux density (Bs) and low coercivity (Hc) are highly desirable, especially if they can be prepared with industrial raw materials (IRMs). Here, Fe82.3Si4B9P4Cu0.7 and Fe82.3Si4B9P3Cu0.7Nb1 nanocrystalline ribbons were prepared with IRMs and pure raw materials (PRMs). It was found that Nb microalloying broadens the annealing process window of the ribbons due to the suppressed precipitation of Fe(B, P) phase. In addition, the soft magnetic properties of the Fe82.3Si4B9P3Cu0.7Nb1 ribbons prepared with IRMs are insensitive to impurities and surface-crystallized layer. Compared to the PRMs-prepared ribbons, the IRMs increase the Hc of the Nb-free ribbons, whereas having little effect on the Hc of the Nb-doped ribbons with dendritic-crystallites in the surface layer. These results originate from the former forming coarse α-Fe grains and narrow magnetic domains, while the latter mainly forming fine α-Fe grains and broad magnetic domains. This work provides a valuable reference for the development and industrialization of nanocrystalline alloys with high Bs and low Hc.

Fe-P based nanocrystalline alloys with good soft-magnetic properties via Nb microalloying

The demand for nanocrystalline alloys with excellent soft magnetic properties as well as satisfactory processibility leads to the development of Fe-P based nanocrystalline alloys. In this work, Fe-P based Fe85.5-xP11C2B1Cu0.5Nbx (x=0, 0.1, 0.3, 0.5, 1.0 and 1.5) nanocrystalline alloys with high saturation magnetization (Bs), low coercivity (Hc), wide annealing temperature range, large annealing duration interval and good bending ductility are successfully prepared through Nb microalloying. This kind of nanocrystalline alloy exhibits high Bs and low Hc in the range of 1.41-1.81 T and 2.41-12.03 A/m. Especially, Fe85.4P11C2B1Cu0.5Nb0.1 alloy shows optimal soft magnetic properties, with high Bs of 1.81 T and low Hc of 5.4 A/m. All alloys maintain low Hc after ten-minute heat treatment at various temperatures of up to Tx2, proving high stability during heat treatment, as evidenced by microstructure characterization, domain observation and thermal analysis. The newly developed Fe85.4P11C2B1Cu0.5Nb0.1 alloy, with high Bs, low Hc, good processibility and low-cost advantage, provides a promising candidate for high-performance soft magnetic applications.

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

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

Revealing the effects of Nb micro-alloying on microstructure and mechanical properties of FeCrAl thin films: Experimental and computational investigations

FeCrAl-based materials have attracted ever-increasing attentions due to their excellent resistances against high-temperature oxidation and stress-corrosion cracking, while enhancement of their mechanical properties can expand their potential applications. In this work, the micro-alloying method by adding Nb was applied to enhance the mechanical property of FeCrAl. The FeCrAl thin films with different Nb contents (Fe-16Cr-5Al, Fe-14Cr-5Al-1Nb, Fe-12Cr-5Al-2Nb, Fe-13Cr-3Al-7Nb, in wt%) were synthesized using magnetron co-sputter deposition. Benefiting from the refined grain size, the as-deposited Fe-13Cr-3Al-7Nb thin film exhibited the highest hardness (3.3 GPa), followed by Fe-14Cr-5Al-1Nb (1.5 GPa) and Fe-12Cr-5Al-2Nb (1.2 GPa), whereas the hardness of Nb-free film was the lowest (0.8 GPa). The fracture resistance (which is related to the ratio of hardness and elastic modulus) also enhances with the addition of Nb. After annealing (873 K for 6 h), the hardness of the Fe-13Cr-3Al-7Nb thin film further increased to ~6.7 GPa, in contrast to the as-annealed Nb-free thin film with no obvious hardness change. The CALPHAD (CALculation of PHAse Diagram) results indicate that with adding Nb the original single BCC (body centered cubic) phase tended to decompose into the Fe-rich and Cr-rich BCC domains, which is consistent with the transmission electron microscopic (TEM) observations. The observed strengthening of the annealed Nb-containing samples is therefore attributed to the phase separation. The first-principles calculations also support this notion. Thus, the current work has delineated the effect of Nb addition on both deposited and annealed FeCrAl-based thin films, which shows the usefulness of combined experimental and computational methods in interpreting microstructure and mechanical property evolutions of engineering materials.

Effects of Nb microalloying on properties of Zr-Al-Fe-Cu glassy alloy

The effects of Nb-microalloying on glass forming ability, thermal properties and mechanical properties of (Zr0.6032Cu0.2256Fe0.0995Al0.0717)100-xNbx (x = 0, 1, 2, 3, 4) alloys were investigated. The best glass former was obtained for (Zr0.6032Cu0.2256Fe0.0995Al0.0717)97Nb3, which could be fabricated into full glass with diameter up to 6 mm at least. In addition, the origin of enhancing GFA of ZrAlFeCu amorphous alloy by means of the minor addition of Nb, Gd and Hf, was also discussed from the aspects of clusters and mixing entropy, which might provide a method of understanding the mechanism of enhancing glass-forming ability via microalloying, and choosing minor alloying element with an aim of enhancing glass-forming ability. It was found that the thermal stability reduced as the content of Nb increased along with the supercooled liquid region decreased. Nb-microalloying decease the fracture strength. However, moderate Nb microalloying could enhance the room temperature plastic strain.

Investigation of Thermomechanical Processing of Nb Microalloyed Steel Produced by Powder Metallurgy

One of the most important properties of microalloyed steels is the different microstructures which are obtained by changing the thermomechanical processing parameters and steel composition. The control of microstructure such as grain size, precipitates, dislocation structure and inclusions are highly effective to determine the mechanical properties of microalloyed steels. The strengthening of mechanical properties can be done by thermomechanical processesing due to increase of nucleation sides via maximizing the austenite boundary and density of the deformation band. In this study, the unalloyed steel and microalloyed steel containing 0.15% Nb were produced by powder metallurgy and hot deformed to reduce the thickness by 40% and 75%. EDX analyzes, density, hardness and grain size measurements were also performed on specimens. According to the SEM microstructure results, the grain size gradually decreased with respect to the deformation rate in both steels. It was observed that small grains occurred due to a complete recrystallization process at 75% deformation rate which were supported by grain size analysis. The density and hardness values increased by the increase in deformation rate. While the density and hardness of microalloy steel under sintered conditions was 89.46% and 75 Hv, respectively, 75% deformed condition was 98.51 % and 231 Hv.

Effects of V–Nb microalloying on the microstructure and properties of spring steel under different quenching-tempering times

The effects of V–Nb microalloying on the microstructure and mechanical properties of ultra-high-strength spring steel at various austenitizing and tempering times were investigated. To achieve this, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, electron backscatter diffraction, and tensile tests were performed. The equations for austenite grain coarsening at 900 °C and (V,Nb)C carbide growth at 400 °C were obtained. The results indicated that the tensile properties of the investigated steel significantly improved after V–Nb microalloying with the same quenching-tempering treatment. With increasing austenitization or tempering time, the tensile properties of the experimental steels dramatically declined owing to the coarsening of the grain size or of the martensite lath width and carbide size. Because V–Nb microalloying increased the densities of the grain boundaries and carbides, the full width at half maximum 211α, microstrain, and dislocation density decreased slowly with prolonged tempering time. The strength increment among the investigated steels mainly resulted from the grain refinement strengthening, dislocation strengthening, and most importantly, precipitation strengthening of (V,Nb)C carbide.

Effect of Nb micro-alloying on austenite nucleation and growth in a medium manganese steel during intercritical annealing

The third-generation advanced high strength medium manganese steels aim at high-performance automobile sheet steel applications. Micro-alloying of steels is considered an effective method to enhance mechanical properties through grain refinement and precipitation hardening. However, in the context of cold-rolled, intercritically annealed medium-manganese steels, the effect of micro-alloying is not well understood. In the present work, we studied the influence of 0.06 wt.% Nb micro-alloying in medium manganese steel of composition 10Mn-0.05C-1.5Al (wt.%). Firstly, using atom probe tomography we discuss the mechanism of NbC precipitation. Subsequently, the effect of NbC precipitation on the driving force for austenite nucleation is explained. We observe that the NbC precipitation pins the austenite-ferrite phase boundary during austenite growth, and the corresponding Zener pinning force was calculated. Furthermore, the energy dissipation during phase boundary migration caused by intrinsic friction and solute drag effect was discussed. These forces hindering the austenite growth were then compared with the chemical driving force for phase transformation. Lastly, the mechanical properties of the NbC microalloyed steels were explained based on the effect of NbC precipitation on austenite nucleation and growth.