Nb Carbides Research Articles

Influence of Heat Treatment Parameters on the Microstructure of 17-4 PH Single Tracks Fabricated by Direct Energy Deposition

Post-fabrication heat treatment (PFHT) is one of the most applied strategies for achieving the desired microstructure and mechanical resistance on additive manufactured components because of the non-equilibrium microstructural state of the material in the as-built condition. In particular, during PFHT, 17-4 PH martensitic stainless steel is mainly strengthened by the precipitation of Cu-rich nanometric particles and Nb carbides into the metal matrix. In this work, the influence of specifically designed PFHTs on the microstructural and mechanical properties of 17-4 PH single tracks fabricated via direct energy deposition was studied. Different solubilization and aging times, as well as a direct aging strategy, were considered. Optical microscopy, X-ray diffractometry, and transmission electron microscopy were used to investigate the microstructure evolution induced by the PFHTs. Moreover, Vickers microhardness measurements were performed to evaluate the increase in mechanical strength. In all cases, the heat-treated single tracks showed a mean microhardness higher than that of the depositions in the as-built condition. In the single tracks subjected to solution treatment, followed by aging for about 100 h, the presence of both Cu-rich precipitates and Nb carbides was assessed; conversely, when directly aged from the as-built condition, only Nb carbides were detected. In the latter case, the carbides were finer and closer to each other than those in the single tracks aged after the solution treatment.

Steel grades 91 and 92 microstructure and precipitate evolution atlas and life assessment tool

ABSTRACT P91 and P92 steels are widely used in power plants and petrochemical industry for long-term service components. Due to high resistance to creep, fatigue and corrosion, the use of grade 91 and grade 92 steels allows ultrasupercritical conditions (600 °C, 300 bar). To achieve this goal the alloy design was based on the following metallurgical concepts: stable tempered martensitic microstructure, precipitation strengthening through M23C6 (Cr, Fe, Mo or W carbides) and fine MX (V, Nb carbides), solution hardening through elements as Mo or Mo/W and high Cr content. This study is focused on the microstructure evolution of grade 91 and grade 92 steels under aging and creep conditions. Three sets of laboratory-aged specimens heated in oven at 550°C, 600°C and 650°C were examined. Furthermore, the influence of stress on the microstructure was evaluated. The microstructures were characterized by several means of investigations and the results were compared to literature.

Microstructural evolution and interfacial properties of explosively welded Nb/steel composite plate during post-heat treatment

In this work, Nb/steel composite plates were fabricated by explosive welding technique. Then post-heat treatments were conducted for large temperature range of 723 K–1273 K. Microstructural evolution and interfacial properties in welded interface were revealed by SEM, EBSD, XRD, microhardness, nanohardness and shear strength examinations. The effects of annealed temperature on interfacial microstructures and properties of Nb/steel composites were systematically investigated. The results demonstrated that microstructures and microhardness of Nb-side almost presented no changes with annealed temperature; Carbon elements from steel-side diffused easily into Nb-side forming a thin Nb carbide layer in which content of NbC increased as increasing annealing temperature; At 1273 K, formation of Nb2C was also found in carbide layers. In the steel-side, during low temperature (723 K–923 K) annealing, the deformed pearlite near interface recrystallized to form ferrite of which grains grew up with increasing annealed temperature; During high temperature (1073 K–1273 K) annealing, a large number of voids appeared at 1073 K, and typical normalizing structure generated at 1173 K–1273 K. Meanwhile, it was also found that with increasing of annealed temperature, shear strength firstly increased to maximum value of 326 MPa at 923 K and then decreased to 267 MPa at 1273 K. Failure mechanism of shear tests exhibited mixed ductile-brittle fracture at low temperature annealing; Then it converted into brittle fracture at high temperature annealing. Microhardness near interface of steel side firstly decreased to the minimum value of 106HV at 923 K and then increased to value of 220HV at 1273 K.

Wet Ball Milling Applied to Production of Composites and Coatings Based on Ti, W, and Nb Carbides

The paper demonstrates the potential of wet ball milling of metals for the synthesis of various carbides and carbohydrides. The work reports on multicomponent carbides formed in Ti-(Cu/Fe/Si)-C, W-Fe-C, and Nb-(Cu/Fe/Si/Al)-C systems, as well as metastable or high-temperature intermetallics formed in Ti-Si, Nb-Si, Nb-Al, and Nb-Cu-Fe systems, which are stabilized with interstitial carbon. The formation of phase composition of powders fabricated under mechanochemical synthesis and subsequent thermal treatment has been studied. The as-fabricated powders have been used to produce bulk compacts and to apply wear-resistant coatings on steel (iron).

Microstructure and properties of Cr[sbnd]Nb carbide coatings on graphite via powder immersion reaction assisted coating

Chromium‑niobium (CrNb) carbide coatings were formed on graphite by powder immersion reaction assisted coating (PIRAC) method. In our experiments, a small amount of iodine was added to the mixture of Cr and Nb powder to lower the annealing temperature to as low as 1173 K. The resultant composite coatings comprised Cr7C3 and NbC phases, of which the Cr7C3 phase was found in the graphite's internal gaps and NbC was distributed on the surface of the coatings. The formation of the multilayer structure might be explained by thermodynamics of metal iodides and carbides. The optimal composite coating's hardness was close to that of CrC compounds coatings, and its toughness was close to that of NbC coatings. Thermal expansion coefficient of the coated sample was similar to the graphite matrix within the temperature range of 313-673 K. No cracks were observed even though stress mismatch was expected due to the large temperature change and large difference in mechanical properties between carbides and graphite. These results showed that the multilayer structure of CrNb composite coating synthesized by PIRAC was promising for improving the mechanical properties of the graphite.

Influence of Nb Addition on Microstructure and Creep Property of Heat-Resistant Cast Steel in Vacuum Carburizing and Quenching

The relation between microstructure and creep property of austenitic heat-resistant cast steels with and without Nb addition under the condition of repeated vacuum carburizing and quenching was investigated. Cr-carbide scale is formed on the sample surface by a carburizing reaction, resulting in the depletion of Cr in the matrix adjacent to the scale. A carburized layer consisting of various fine carbides is observed below the Cr-depleted layer, and the carburized layer depth is suppressed by Nb addition. When process of vacuum carburizing and quenching is repeated, formation of voids caused by heating and rapid cooling is more remarkable in primary Cr carbides than in primary Nb carbides. As the carburized layer depth increases, creep rupture time of both cast steels shifts to the shorter time side; however, Nb addition is effective for extending creep rupture time at 1303 K.

Layered Titanium Niobium Oxides Derived from Solid-Solution Ti–Nb Carbides (MXene) as Anode Materials for Li-Ion Batteries

Layered Titanium Niobium Oxides Derived from Solid-Solution Ti–Nb Carbides (MXene) as Anode Materials for Li-Ion Batteries

Effects of Mn and Microalloying Composition on Corrosion and Hydrogen-Induced Cracking of API 5L X65 Steels

The corrosion and hydrogen-induced cracking (HIC) resistance as well as hydrogen permeation behavior of two API 5L X65 steels with different amounts of Mn and microalloying elements were compared. The corrosion behavior of both steels, evaluated using electrochemical impedance spectroscopy and polarization curves in solution A of NACE TM0284-16 standard with and without H2S saturation (sour medium), showed no relevant differences in each medium, which can be ascribed to their similar microstructures; however, the corrosion resistance of both steels was lower in the sour medium. The investigation of resistance to hydrogen-induced failures disclosed better performance for the low Mn steel. This was confirmed using a harsher HIC test performed in an HCl acidified sour medium and was ascribed to the presence of Nb carbides nanoprecipitates, as revealed by the scanning transmission electron microsccopy analysis.

Microstructural evolution and stress relaxation cracking mechanism for Super304H austenitic stainless steel weld metal

• Stress relaxation cracking in Super304H austenitic stainless steel weld metal. • The pre-compressed CT technique combined with FEM analysis was successfully used. • The mechanism responsible for stress relaxation cracking formation was illustrated after short-term aging and long-term aging. The pre-compressed CT technique was used to quantitatively investigate the formation of stress relaxation cracks under different tensile residual stresses and aging time in Super304H austenitic stainless steel weld metal. The statistical results revealed that intergranular cracks could occur within 2000 h under 650 °C when the residual stress was applied with greater than 18 KN pre-compression force. Detailed grain interior and boundary analyses showed that the growth of intragranular Cu-rich particles could induce a strong grain interior, and the intergranular Nb(C, N) carbides were one of the causes to crack under short-term aging time. For long-term aging time conditions, the intergranular M 23 C 6 carbides were more susceptible to crack than intergranular Nb(C, N) carbides. Finally, the mechanism responsible for stress relaxation cracking formation was carefully illustrated for the weld metals after short-term aging and long-term aging, respectively.

Microstructure and Strengthening Mechanisms in an HSLA Steel Subjected to Tempforming

An effect of tempforming on the microstructure, the carbide precipitation, and the strengthening mechanisms of high-strength low-alloyed steel has been analyzed. The quenched steel was subjected to 1 h tempering at a temperature of 873 K, 923 K, or 973 K followed by plate rolling at the same temperature. Tempforming resulted in the formation of an ultrafine grained lamellar-type microstructure with finely dispersed carbides of (Nb,V)C, Fe3C and Cr23C6. A decrease in tempforming temperature resulted in a reduction of the transverse grain size from 950 nm to 350 nm. Correspondingly, the size of Fe3C/Cr23C6 particles decreased from 90 nm to 40 nm while the size of (Nb,V)C particles decreased from 17 nm to 4 nm. Refining the tempformed microstructure with a decrease in thetempforming temperature provided an increase in the yield strength from 690 MPa to 1230 MPa.

Insight into the effect of Nb microalloying on the microstructure–property relationship of a novel wire rod

The microstructure–property relationship of a novel wire rod was investigated after Nb microalloying. The tensile properties were significantly improved. The impact of Nb on pearlitic transformation, residual stress, residual strain, and microstructure uniformity was discussed in this paper. By changing the alloy phase diagram, Nb microalloying improved the undercooling of the pearlitic transformation, resulting in finer lamellar spacing. Residual stresses and residual strain as different kinds of deformation energy storage satisfied the water-bed effect after Nb microalloying. After hot rolling, the residual stresses of the Nb-bearing wire rod were reduced while the residual strains increased. The increase in deformation between the transverse and longitudinal sections (TS and LS) during hot rolling led to an increase in residual strain and a decrease in residual stress in wire rods. Furthermore, the uniform grains and more random texture observed after Nb microalloying increased the microstructure uniformity of wire rods. This phenomenon was thought to be caused by the precipitation of Nb carbides during hot rolling.

Peculiarities of Structure and Physical-Mechanical Properties of High-Strength Cr-Mo Pipe Steel Applicative Mainly in a Sour Environment

This paper demonstrates the results of the study of microstructure and physical-mechanical properties of the high-strength economically alloyed Fe-Cr-Mo steel, developed by RosNITI JSC for the production of the oil country tubular goods (OCTG) (casing and tubing). The main requirement for this steel is to provide simultaneous increased strength and resistance to sulfide stress cracking (SSC). It was shown that this problem could be solved by special heat treatment. As a result, the structure of this steel consists of a secondary sorbite with a lower dislocation density. Hardening is provided by dispersion-strengthened V, Nb carbides.

Formation of carbonitrides and precipitation of alloying elements during thermomechanical treatment of high-strength low-alloy steels. Atomistic modeling

Understanding the processes that control the formation of precipitates of alloying element, their composition and morphology is important for the choosing of optimal regimes of thermomechanical treatment, providing the required structural state and properties of low-alloyed steels. The use of modern methods of atomistic modeling and ab initio calculations to study the mechanisms and conditions of precipitation depending on the steel composition and temperature is discussed. The enthalpy of formation of Ti, V, Nb carbides and nitrides in γ-Fe was calculated using the methods of the theory of the electron density functional and the temperatures of their dissolution were determined in dependence on the composition of the steel. The results obtained agree with the experimental data and obtained by the CALPHAD method. Calculations of the electronic structure, enthalpy of mixing, energy of effective interactions between atoms of alloying and impurity elements have been performed. It was shown that an essential factor determining the nature of the interaction between alloying elements was the magnetic state of iron. It has been established that the enthalpy of mixing, which expresses the tendency of the system to decomposition, increases with the transition of α-Fe from the ferromagnetic to the paramagnetic state. In ferromagnetic α-Fe energetically, the formation of precipitates of Cu, as well as particles of a mixed composition of Cu‒Al, Cu‒Ni and an ordered phase of NiAl are preferable. It has been shown that precipitates Cu-based can be formed during technological times. The transition of α-Fe to the paramagnetic state results in increase of interactions between alloying elements (the exception is the interaction of Cu‒Cu and Cu‒Ni). As a result, additional combinations of alloying elements appear, the interaction between which lead to the formation of precipitates. At the same time, in γ-Fe, precipitates can be formed only based on Mo and Nb. The role of magnetism in the formation of precipitates and their influence on the γ→α-transformation is discussed. It has been shown that the results obtained make a base for enhancement technologies of high-strength low-alloyed steel production.

Effect of cooling methods on mechanical and corrosion properties of Inconel 625 during solution treatment

Inconel 625 alloy is a nickel-based deformation superalloy which is widely used in aviation, ocean engineering, chemical engineering and other important fields. In recent years, with the continuous expansion in application field of Inconel 625 alloy, the requirements of mechanical properties, corrosion resistance and other comprehensive properties of the alloy are constantly improved. In this paper, hardness tester, tensile machine, electrochemical workstation, optical microscope, scanning electron microscope and other equipment are used to study Inconel 625 alloy, and the microstructure evolution law and mechanical properties and corrosion properties of the alloy under different heat treatment conditions are studied. The experimental results show that the as-cast Inconel 625 alloy is mainly composed of austenite which is displayed high hardness and poor corrosion resistance. Heat treatment is a common way for tunning the comprehensive properties of Inconel 625 alloy. After 1090°C heat treatment, different cooling method were used to tune the corrosion resistance of Inconel 625 alloy. The cooling rate has a great influence on the microstructure and properties of the alloy. After water cooling, the precipitation of Nb carbide at the grain boundary is retained, which improves the corrosion performance of the alloy. After air cooling or furnace cooling, chromium carbide precipitates in the alloy again, which leads to the decrease of corrosion properties of the alloy. The special treatment can effectively improve the corrosion resistance of the alloy, and improve the plasticity of the material without reducing the strength. This paper provides an effective basis for optimizing the heat treatment process of Inconel 625 alloy in production to improve the comprehensive properties of the material.

Analysis of Precipitates in Welded Joint of TP347H Stainless Steel

The precipitates in welded joint heat affected zone (HAZ) and base metal of TP347H stainless steel were observed and analyzed by SEM, TEM and EPMA. The results show that precipitate of NbC can be observed in welded joint HAZ in as-welded, and more NbC precipitate in high temperature service. The size of primary NbC in base metal ranges from tens of nanometers to hundreds of nanometers, but only tens of nanometers of the precipitated NbC. The fine precipitate of NbC is mainly distributed in the crystal, however, continuous distribution of NbC is also observed on the grain boundary of the service joint. Cr carbide precipitates at the grain boundary, and some of particles distribute continuously on the grain boundary after service. The fine NbC precipitated in the crystal strengthens the matrix, and the continuous distribution of Nb and Cr carbides on the grain boundary reduces the interfacial bonding strength, then both of which result decrease of the material plasticity.

Enhancement of Properties of Micro-alloyed Low-Carbon Ni-Added Steel by Thermomechanical Treatment

The concurrent use of micro-alloying additions and thermomechanical treatment is a common practice for obtaining high-quality grades of forged steels. Judicious use of chemistry, heat treatment schedule, and controlled processing can lead to a wide range of microstructure and thus final properties suitable for the production of different parts for the automobile industry. In this paper, thermomechanical treatment of Ni-added micro-alloyed steel containing Nb, V, Ti has been carried out with different deformation schedules (50-70%) at high temperatures followed by the varying cooling rate (i.e., forced air cooling, oil quenching, and water quenching). The cooling rates for three conditions were measured as 5 °C/s for forced air cooling, 40 °C/s for oil quenched, and 110 °C/s for water quenched samples. Thermomechanical and post-tempering treatment cycles were employed for the steel with 1150 and 900 °C as upper and lower forging temperatures. The best combination of properties viz; YS: 1044 MPa, UTS: 1308 MPa, % El: 16.7, %RA: 57 and Impact Toughness: 50 J/cm2 could be achieved for 70% deformed samples, followed by forced air cooling. The reasons have been attributed to the segregation of Ti & Nb carbides at grain boundaries as revealed by microstructural analysis. This has led to restriction of grain boundary coarsening at high temperatures. Further, fine vanadium carbide and the presence of retained austenite have contributed to improving the elongation and toughness values.

Mechanical Response of Ni-Based CU5MCuC Alloy to Different Stabilization Thermal Treatments

The Ni–Fe–Cr system is the basis of a series of commercial alloys featuring chemical–physical characteristics that allow them to be used in a variety of fields where excellent resistance to aggressive environments is required. In this scenario, the CU5MCuC alloy, the foundry counterpart of Alloy 825, is proving successful in the petrochemical field thanks to its good corrosion resistance in acidic and highly oxidizing environments. Intergranular corrosion resistance, critical for this material, is ensured by the stabilization treatment that allows precipitation of Nb carbides. Strengthening of this alloy takes place only via a solid solution. Therefore, its mechanical properties depend on the solution annealing treatment: often this treatment alone does not make it possible to reach the UTS imposed by the ASTM-A494 standard. In this work, the possibility of using stabilization treatment to increase mechanical strength as well was considered. Treatments, with different combinations of time and temperature, were carried out in order to modify the material’s microstructure. After the thermal treatments, microstructural analyses, mechanical tests and (pitting and intergranular) corrosion and resistance tests were carried out to identify optimal treatment parameters in order to promote the evolution of microstructural constituents capable of improving mechanical strength without decreasing corrosion resistance. The treatment that achieves the best compromise between mechanical properties and corrosion resistance is stabilization at 970 °C for 4 h.

Effect of undissolved Nb carbides on mechanical properties of hydrogen-precharged tempered martensitic steel

Nb carbides have attracted significant attention to enhance the resistance of tempered martensitic (TM) steel to hydrogen embrittlement (HE). However, previous studies have elucidated the role of Nb carbides in HE resistance without categorizing their types (i.e., undissolved and newly precipitated). This study focuses on the effect of “undissolved” Nb carbides on the tensile and fatigue properties of hydrogen-precharged TM steels. It validated the following two factors for the HE resistance of the TM steels containing undissolved Nb carbides: hydrogen-trapping by the carbides and refinement of prior austenite grain. The former factor rarely affected the HE resistance owing to the interfacial incoherency between the undissolved carbides and ferritic matrix. Such results are distinguished from previous studies focusing on the newly precipitated carbides. In contrast, the latter factor contributed significantly to the HE resistance via the decrease in hydrogen contents per unit surface of prior austenite grain boundaries.

Relationship between the microstructure and local corrosion properties of weld metal in austenitic stainless steels

The purpose of this study is to evaluate the corrosion resistance of weld metal by electrochemical methods and discuss the relationship between microstructure and corrosion resistance. Intergranular and pitting corrosion resistances were measured using electrochemical potentiokinetic reactivation (EPR) test and pitting potential measurement respectively. The reactivation ratio and pitting potential corresponded to its chemical composition. The specimens containing more Cr and Mo showed higher resistance. In the EPR test, the dendrite core with a relatively low Cr content was corroded. In the pitting corrosion test, Nb carbide became the initiation site of pitting corrosion which propagated along the cell structure.

Effects of Nb and Mo alloying on resistance to hydrogen embrittlement in 1.9 GPa-grade hot-stamping steels

Ultra-high-strength (over 1.8 GPa) of hot-stamping steels can be achieved by increasing C content of conventional hot stamping steel (1.5 GPa) or controlling precipitation behavior, but the enhanced strength often leads to the deteriorated resistance to hydrogen embrittlement. The complex alloying of Nb and Mo improves the resistance; however, the underlying microstructural evolutions and synergistic mechanisms are not understood yet. In this study, Nb- or (Nb + Mo)-alloyed 1.9 GPa-grade hot-stamping steels were fabricated in the laboratory, and their resistance to hydrogen embrittlement was evaluated by slow strain-rate tensile (SSRT) tests without and after hydrogen charging. Most of Nb consumed to form Nb carbides, hindering the migration of prior austenite grain boundaries mainly by a common Zener's pinning effect. The reduced grain size resulted in the decreased amount of diffusible hydrogen per unit grain boundary area. In addition, coherent or semi-coherent precipitates provided stable hydrogen trapping sites, leading to the low diffusivity and the consequently high resistance to hydrogen embrittlement. In the (Nb + Mo)-alloyed steel, Mo carbides preferred to nucleate at the surface of the pre-existing Nb carbides or to form needle-shaped isolated Mo2C carbide, but a considerably large amount of Mo remained inside the matrix as a solute. The prior austenite grain size was thus further reduced mainly by an additional solute drag effect. The solute Mo also enhanced the grain boundary cohesion, thereby leading to the absence of intergranular fracture and the sufficient post elongation even after the hydrogen charging.