Niobium-rich Carbides Research Articles

Effect of post-weld heat treatment on heat affected zone microstructure of additively manufactured Inconel 718

Laser powder bed fusion for metals is a method of producing end use components for industrial use. Powder bed fusion machines are relatively small, and are usually used to create only the critical part of the larger assembly. Therefore, L-PBF manufactured parts must be attached to each other for example by welding. The industrial world needs to be able to join the printed superalloy components to the traditionally manufactured components to reach better corrosion, wear and/or heat resistance in selected parts in an assembly. The problem is that there is limited amount of information about the suitable welding parameter values for these applications. This study examines how the standard heat treatment cycles affect to the quality of the weld, and if the post-heat treatment is reducing undesired phases in the heat affected zone. Test has shown that post heat treatment highlights cuboidal shaped niobium rich carbides throughout the material to the heat affected zone grain boundaries.

The effects of laser welding parameters on weldability and quality of the microstructure of additively manufactured Inconel 718-316L joints

Additive manufacturing of metals is a way of producing high-quality end-use parts. Technical alloys, for example Inconel 718, can be used to obtain a lot of benefits for example wear, corrosion, and heat resistance. Laser welding of Inconel 718 is a standard process, but there is rather limited amount of information of welding of additively manufactured nickel superalloys to the additively manufactured stainless steel. The process parameters need to be considered in laser welding. Undesired microstructure can occur due to wrong heat input during the welding process. This study examines laser welding of additively manufactured Inconel 718-316L parts and the effects of the welding parameters to the quality of the weld by analysing microstructure from the heat affected zone. This is done to achieve better part quality more cost efficiently compared to traditionally produced parts and to optimize the welding parameters. It is not feasible to manufacture the full large structure with IN718 and AM could be used to manufacture just the functional parts of the assembly. Tests have shown that welding heat input and cooling time affect to the quality. Inconel 718 hardness decrease across the fused zone because of the mixing of different elements in the molten weld pool. Laser welding highlights cuboidal shaped niobium rich carbides throughout the material to the heat affected zone grain boundaries on Inconel side.

Transformation During Aging of Niobium-Rich Carbides in Cast Heat-Resistant Alloys

Transformation During Aging of Niobium-Rich Carbides in Cast Heat-Resistant Alloys

Correlative statistical microstructural assessment of precipitates and their distribution, with simultaneous electron backscatter diffraction and energy dispersive X-ray spectroscopy

Modern engineering alloys have bespoke microstructures, where features such as precipitates are used to control properties. In many Ni-based alloys, carbo-nitride precipitates are introduced to strengthen and improve performance. These precipitates can be distributed throughout the microstructure and niobium rich carbides are often found at grain boundaries. In this work, we used combined energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) to characterise the population of these precipitates. Processing of the EDS signal is used to label the Mo/Nb-rich precipitates, and their size and location are measured from maps using a circular Hough transform. This label map is combined with the grain boundary network (from EBSD analysis). Statistical analysis, using ANOVA testing, reveals differences in chemistry between carbides found in Ni-rich matrix grain interiors, on random high angle boundaries and on special boundaries (Σ3 and Σ9). These results are compared between wrought and power metallurgy product forms. These distributions are discussed in the context of their performance within demanding environments, such as reactor core internals.

Investigation of microstructure and corrosion resistance of AISI 304 stainless steel joint with ER308 and ERNiCr-3 filler metals by GTAW

In this study, the effect of two different filler metals (ERNiCr-3 and ER308) on microstructure and corrosion resistance of gas tungsten arc welded AISI 304 has been investigated. The microstructure of the joints was investigated by optical and scanning electron microscope. The microhardness of samples was investigated to determine the mechanical properties of the welds. Also, the electrochemical corrosion resistance of the samples was studied by Tafel polarization and impedance analysis in 1 M hydrochloric acid. The results showed the niobium-rich carbides in the welded specimen by ERNiCr-3 filler, also this sample exhibited a higher hardness and corrosion resistance. The exchange current density and the polarization radius of the ERNiCr-3 filler metal were 0.069 mA and 244 Ω, and those of ER308 filler metal were 0.121 mA and 66 Ω, respectively. The higher hardness and corrosion resistance is attributed to the formation of hard niobium-rich carbides in the weld zone, which eliminates the formation of chromium carbides.

Microstructural Changes During Stress Relief Heat Treatment of Inconel 625–A106 Carbon Steel Joint

The present study investigates the effect of stress relief treatment at different temperatures (900, 1040, and 1200 °C) on the microstructure of Inconel 625 and A106 carbon steel weld joints. The results of the study show that, due to the generation of heat by welding, the bainite phase was formed in the heat-affected zone of A106 alloy. By applying stress relief treatment, even at the lowest temperature, the bainite phase was removed from the microstructure and the ferritic–pearlitic structure was formed. As the temperature of stress relief treatment increased, the grain size of the samples and the ratio of pearlite structure increased in the matrix. The morphology of the weld metal was converted from the columnar dendritic to an island state. Gradually, by increasing the temperature up to 1200 °C, austenitic grain boundary appeared in the microstructure. The SEM results confirmed the presence of complex molybdenum–niobium–rich carbides among secondary phase particles in the weld zone. In addition, an increase in the heat treatment temperature caused an increase in the concentration and dimensions of the carbides in the heat-affected zone, weld metal, and base metal of Inconel 625.

Microstructural analysis of Cr35Ni45Nb heat-resistant steel after a five-year service in pyrolysis furnace

Abstract To analyze the microstructure across thickness exposed to various environment of the pyrolysis furnace tubes, the sample of Cr35Ni45Nb heat-resistant steel from a five-year service tube was sliced into slices from inner to outer wall. Moreover, microhardness was examined to investigate the influence of the microstructure on the property of the heat-resistant steel. The results show that the primary carbides Cr 7 C 3 and NbC have transformed into chromium-rich Cr 23 C 6 and niobium-rich carbides G phase in the middle section of the aged pyrolysis furnace tube. The Cr 2 O 3 and SiO 2 layer covered the outer and inner surfaces, and the Cr-depleted zones were formed under the oxide layer of the inner and outer walls. The carbides of M 23 C 6 were transformed into M 7 C 3 type carbides in the carburization zone under the Cr-depleted zone of inner surface, while the M 7 C 3 type carbides also existed under the Cr-depleted zone of outer wall.

Dynamic precipitation of nickel-based superalloys undergoing severe deformation below the solvus temperature

Abstract The authors performed uniaxial compression tests of nickel-based superalloys: single crystal CMSX–4, also precipitation hardened; Inconel 718 and X750, at temperatures below the γ′ solvus, in order to study the effect of temperature and strain rate on their flow stress and microstructural development. On the basis of the obtained flow stress values, the activation energy of a high-temperature deformation process was estimated. Microstructural observations of the deformed samples at high temperatures, previously solution heat treated and aged CMSX–4 and Inconel alloys revealed non-uniform deformation effects. Distribution of either molybdenum- or niobium-rich carbides was found to be affected by localized flow within the investigated strain range at relatively low deformation temperatures, 720–850 °C. Microstructural examination of the alloys also showed that shear banding and cavity growth were responsible for the decrease in flow stress and a specimen fracture at larger strains.

Influence of chemical composition on microstructure and phase evolution of two HP heat resistant stainless steels after long term plant-service aging

The present work is aimed on the microstructure characterisation of two different HP steels after over 80 000 h of service in petrochemical plants of steam reforming. By means of optical and electron microscopy, as well as energy dispersive X-ray elements mapping and X-ray diffraction techniques, the alloys have been deeply investigated in order to make clear the influence of chemical composition, in particular titanium and tungsten additions, on the microstructural evolution after long service aging. The most evident features of aged materials can be summarised in terms of coalescence and coarsening of interdendritic precipitates, precipitation of secondary carbides in the austenite matrix and transformation of niobium-rich carbides in the G-phase silicide. We have observed that titanium and tungsten additions have played a synergistic role in preventing and reducing chemical evolution of the secondary phases present in the matrix.

High Temperature Deformation of Superalloy Inconel 718

Experimental results on hot deformation and dynamic structural processes of nickel based alloy Inconel 718 are reviewed. The focus is the analysis of dynamic precipitation processes which operate during hot deformation of these materials at elevated temperatures. Hot compression tests were performed on the solution treated precipitation hardenable nickel based superalloy Inconel 718 at 720-1150°C with a constant true strain rates of 10-4 and 4x10-4s-1. True stress - true strain curves and microstructure analysis of the deformed nickel based superalloy is presented. The properties and dynamic behaviour are explained through observation of the microstructure using standard optical, scanning and transmission electron microscopy. Structural observations of solution treated Inconel 718 deformed at high temperatures, reveal non uniform deformation effects. The distribution of niobium-rich carbides were affected by localized flow within the strain range investigated at relatively low deformation temperatures 720 - 850°C.

Crystallography and Morphology of Niobium Carbide in As-Cast HP-Niobium Reformer Tubes

The microstructures of two as-cast heats of niobium-modified HP stainless steels were characterized. Particular attention was paid to the interdendritic niobium-rich carbides formed during solidification of these alloys. At low magnifications, these precipitates are grouped in colonies of similar lamellae. Higher magnifications revealed that the lamellae actually obtain two distinct morphologies. The type I morphology exhibits broad planar interfaces with a smooth platelike shape. Type II lamellae have undulating interfaces and an overall reticulated shape. To provide further insight into the origin of these two different morphologies, the microstructure and crystallography of each have been studied in detail using high resolution scanning electron microscopy, transmission electron microscopy, various electron diffraction methods (electron backscatter diffraction (EBSD), selected area diffraction (SAD), and convergent beam electron diffraction (CBED)), and energy dispersive X-ray spectroscopy.

Segregation Effects and Phase Developments during Solidification of Alloy 625

The solidification behaviour of pure Alloy 625, and Alloy 625 enriched respectively in iron and carbon, was investigated in situ by hot-stage light optical microscopy. Using this technique planar front solidification for distances of several hundred microns was facilitated. After solidification, the material that experienced planar front solidification corresponded accurately to that of dendrites tens of microns in width adopting an equivalent dendrite arm width approach. Eventually, the planar solidification front broke down, where after the residual liquid solidified eutectic-like. This material contained γ-phase, Laves phase and, if carbon was dissolved in the liquid, niobium rich carbides formed. Molybdenum and niobium showed strong tendencies to segregate. Their segregation was balanced by inverse segregation of nickel and iron. The chromium concentration remained almost constant in γ in the entire matrix material. Addition of carbon did not cause detectable alterations of the material that experienced planar front solidification. However, it promoted the formation of niobium rich carbides in the material that solidified eutectic-like. Thus, this material differed from that of the pure sample in constitution, and consequently in γ-phase composition. Niobium rich carbides formed prior to Laves phase; in carbon rich volumes only the carbides form. As compared to the pure sample, the sample enriched in iron had decreased global minimal solute concentration in the material that experienced planar front solidification. However, once the concentrations were corrected with respect to the dilution simply caused by the presence of iron, the solidification behaviour in this material was identical to that of the pure sample. The constitution in the material that solidified eutectic-like was γ and Laves phase. As compared to the pure sample, the Laves phase was enriched in iron.

Replacement of vanadium by niobium in 86–5–2 high-speed tool steels

The microstructure and mechanical properties of a series of experimental high-speed steels based on the S6–5–2 (M2) composition have been investigated. In these alloys the 2% vanadium component of the S6–5–2 composition has been partially or completely replaced by niobium additions of up to 3%. It has been shown that niobium-replacement steels have superior cutting properties, which are attributed to the formation of a high volume fraction of niobium-rich carbides. X-ray energy analysis indicates that NbC replaces VC as the MC phase, and niobium changes the chemical composition of the M6C and/or M23C6 carbides. The niobium-replacement grades undergo secondary hardening on tempering, and niobium raises the liquation temperature allowing higher hardening temperatures to be used. Since niobium-rich carbides replace carbides containing the more expensive tungsten, molybdenum, and vanadium alloying elements, it is suggested that niobium could be used to reduce the amounts of these elements needed, and so to improve the performance/cost ratio of S6–5–2.