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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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DOE OSTI2012

Mesoscale Modeling and Validation of Texture Evolution during Asymmetric Rooling and Static Recrystallization of Magnesium Alloy AZ31B

The focus of the present research is to develop an integrated deformation and recrystallization model for magnesium alloys at the microstructural length scale. It is known that in magnesium alloys nucleation of recrystallized grains occurs at various microstructural inhomogeneities such as twins and localized deformation bands. However, there is a need to develop models that can predict the evolution of the grain structure and texture developed during recrystallization and grain growth, especially when the deformation process follows a complicated deformation path such as in asymmetric rolling. The deformation model is based on a crystal plasticity approach implemented at the length scale of the microstructure that includes deformation mechanisms based on dislocation slip and twinning. The recrystallization simulation is based on a Monte Carlo technique that operates on the output of the deformation simulations. The nucleation criterion during recrystallization is based on the local stored energy and the Monte Carlo technique is used to simulate the growth of the nuclei due to local stored energy differences and curvature. The model predictions are compared with experimental data obtained through electron backscatter analysis and neutron diffraction.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Role of deformation temperature on the evolution and heterogeneity of texture during equal channel angular pressing of magnesium

Investigations on texture evolution and through-thickness texture heterogeneity during equal channel angular pressing (ECAP) of pure magnesium at 200 °C, 150 °C and room temperature (RT) was carried out by neutron, high energy synchrotron X-ray and electron back-scatter diffraction. Irrespective of the ECAP temperature, a distinctive basal (B) and pyramidal (C{sub 2}) II type of fibers forms. The texture differs in the bottom 1 mm portion, where the B-fiber is shifted ~ 55° due to negative shear attributed to friction. - Highlights: • ECAP of magnesium was carried out at 200 °C, 150 °C and room temperature. • Microstructure and micro-texture evolution was examined using EBSD in FEG–SEM. • Bulk-texture was studied using neutron diffraction and compared with micro-texture. • Through thickness texture heterogeneity was observed by synchrotron radiation. • Changes in these parameters with respect to deformation temperature are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Epitaxial GaN films by hyperthermal ion-beam nitridation of Ga droplets

Epitaxial GaN film formation on bare 6H-SiC(0001) substrates via the process of transformation of Ga droplets into a thin GaN film by applying hyperthermal nitrogen ions is investigated. Pre-deposited Ga atoms in well defined amounts form large droplets on the substrate surface which are subsequently nitridated at a substrate temperature of 630 Degree-Sign C by a low-energy nitrogen ion beam from a constricted glow-discharge ion source. The Ga deposition and ion-beam nitridation process steps are monitored in situ by reflection high-energy electron diffraction. Ex situ characterization by x-ray diffraction and reflectivity techniques, Rutherford backscattering spectrometry, and electron microscopy shows that the thickness of the resulting GaN films depends on the various amounts of pre-deposited gallium. The films are epitaxial to the substrate, exhibit a mosaic like, smooth surface topography and consist of coalesced large domains of low defect density. Possible transport mechanisms of reactive nitrogen species during hyperthermal nitridation are discussed and the formation of GaN films by an ion-beam assisted process is explained.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Epitaxial Ni{sub 3}FeN thin films: A candidate for spintronic devices and magnetic sensors

A new type of epitaxial ferromagnetic nitride (Ni{sub 3} Fe N = permalloy nitride = 'PyN') compound films were grown on Al{sub 2}O{sub 3}(1120) substrates using reactive triode magnetron sputtering. The results of electron back-scattering diffraction and x-ray diffraction techniques indicate a high quality epitaxial crystalline structure with growth normal of (100). Magnetization measurements of epitaxial PyN films revealed several unique results. (1) A textbook square hysteresis loop that suggest existence of single magnetic domain in these films. (2) A coercive field is tunable from a few mOe up to a few Oe by changing the film thickness. (3) A magnetization that switches (rotate) over a very small field range of {delta}H{sub C} {<=} 0.05 Oe, independent of the film thickness. This small {delta}H indicates a very large resistive sensitivity ({delta}R/{delta}H) of the epitaxial PyN. (4) The epitaxial PyN thermal cycling through several cycles between '2 and 800 K' (-271 Degree-Sign C to +527 Degree-Sign C) shows much less degradation only about 2-5% compared to 40% degradation of a simple Py film. Four-probe transport measurements give an anisotropic magnetoresistance of Almost-Equal-To 6%, sufficiently higher than other known ferromagnetic materials. These interesting properties are ideal for a variety of spintronic devices and magnetic sensors.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of pre-cold rolling on the evolution of microstructure, microtexture, and mechanical properties of the mushy state rolled in-situ Al-4.5Cu-5TiB2 composite

Highlights: • Microstructure-Microtexture evolution study of the mushy state rolled composite (MRC) and MRC with pre-cold rolling (PCMRC). • The Microstructure and microtexture were characterized using SEM, EBSD, and TEM. • Exactly opposite trends in micro-hardness variation across the thickness of MRC and PCMRC were discussed. • The PCMRC exhibited the best combination of hardness, strength, and ductility. • A proper microstructure-microtexture-property correlation was established with experimental evidence. The mushy state rolling has been found as a promising technique for near-net forming of the cast Al-4.5 wt% Cu-5 wt% TiB{sub 2} in-situ composite to obtain microstructural refinement and moderate mechanical properties. Cold rolling with 30% reduction in thickness was carried out before mushy state rolling, for achieving a relatively more homogeneous distribution of CuAl{sub 2}-TiB{sub 2} particles as well as an appropriate microstructure to enhance the mechanical properties. A comparative investigation on the microstructural and microtextural evolution, along with mechanical properties of both the mushy state rolled composite (MRC) and pre-cold rolled mushy-state rolled composite (PCMRC) was performed, and the microstructure-property correlation was established. The PCMRC exhibited the lowest homogenized lengthscale parameter (L{sub H} 29 μm) indicating the maximum uniformity of particle distribution, computed with the help of multi-scalar analysis of area fractions (MSAAF) technique. Electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM) studies revealed the presence of fine sub-grains within centrally located deformed Al grains due to dynamic recovery, as well as the particle stimulated nucleation of equiaxed fine Al grains by dynamic recrystallization or rapid solidification of liquid near edges for both the investigated composites. The PCMRC displayed a superior combination of hardness (96.82 ± 4.5 HV{sub 5kgf}), 40% higher ultimate tensile strength (301.4 ± 1.8 MPa), and 68% more uniform elongation (30.8% ± 0.5%), compared to the MRC. The improvement in mechanical properties is attributed to the uniform particle distribution and presence of both higher density of low angle boundaries and recrystallized grains adjacent to the particles, obtained from prior cold rolling.

36 MATERIALS SCIENCE↗
DOE OSTI2018

The effect of ball-milling time and annealing temperature on fracture toughness of Ni-3 wt.% SiC using small punch testing

Highlights: • The fracture energy of Ni-SiC is strongly dependent on the grain size of Ni matrix • The ball-milling time governs the fracture behaviour of Ni-3 wt.% SiC • A higher proportion of big SiC particles at grain boundary is responsible for a lower fracture toughness - Abstract: A set of dispersion strengthened Ni-3 wt.% SiC samples have been prepared via a powder metallurgy route for an application in molten salt reactor (MSR) systems. A mixture of 97 wt.% Ni and 3 wt.% SiC powder was prepared by varying the ball-milling time (8, 24, 36 and 48 h). The ball-milling process was followed by powder consolidation using spark plasma sintering (SPS) and rapid cooling to room temperature. The samples were then annealed at different temperatures (300 °C, 700 °C and 850 °C). The fracture behaviour of the Ni-3 wt.% SiC samples was investigated using the Small Punch Test (SPT). A scanning electron microscope (SEM) with energy dispersive spectrometer (EDS) and electron back-scatter diffraction (EBSD), as well as transmission electron microscopy (TEM) were employed to examine the microstructure of the tested Ni-3 wt.% SiC samples. The obtained results show that the SPT fracture energy is strongly dependent on the grain size of Ni matrix, which is affected by ball-milling times and annealing temperature. It has been further found that the segregation of large SiC particles at grain boundaries promotes intergranular fracture and thus significantly reduces the fracture energy. The results indicate that the distribution and size of SiC particles at grain boundary is mainly affected by the ball-milling time. Hence, the ball-milling time governs the fracture toughness of Ni-3 wt.% SiC samples.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Strain softening during tension in cold drawn Cu–Ag alloys

Experiments were conducted on Cu–0.1wt.%Ag alloys to evaluate the influence of producing procedures and annealing conditions on microstructure evolution and mechanical properties of Cu–Ag alloys. Optical microscopy (OM), electron back-scattered diffraction (EBSD), X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used for microstructural evaluation and mechanical properties were characterized by tensile tests. The results indicated that hot-extruded Cu–Ag alloys had a typical dynamic recrystallized microstructure with equiaxed grains. Cold drawing at room temperature leaded to partial recrystallized microstructure with a mixture of coarse and fine grains. The dominate {001}<100 > cubic texture formed during hot extrusion was changed to be {112}<111 > copper texture by cold drawing. Strain softening occurred during room temperature tension of cold drawn Cu–Ag alloys with an average grain size of 13–19.7 μm. - Highlights: • Strain softening occurred during tension of Cu–Ag alloys with coarse grain size. • Work hardening was observed in hot-extruded and annealed Cu–0.1wt.%Ag alloys. • Strain softening was ascribed to dynamic recovery and dynamic recrystallization.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Evolution of twinning in extruded AZ31 alloy with bimodal grain structure

Twinning in extruded AZ31 alloy with a bimodal grain structure is studied under compression along the extrusion direction. This study has combined in-situ measurements during the compression tests by Synchrotron Radiation Diffraction and Acoustic Emission techniques and the evaluation of the microstructure and texture in post-mortem compression samples deformed at different strains. The microstructure of the alloy is characterized by the coexistence of large areas of fine dynamic recrystallized grains and coarse non-recrystallized grains elongated along extrusion direction. Twinning occurs initially in large elongated grains before the macroscopic yield stress which is controlled by the twinning in equiaxed dynamically recrystallized grains. - Highlights: • The AZ31 extruded at low temperature exhibits a bimodal grains structure. • Twinning takes place before macroscopic yielding in coarse non-DRXed grains. • DRXed grains controls the beginning of plasticity in magnesium alloys with bimodal grain structure.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Effect of Nd:YAG laser welding on microstructure and hardness of an Al-Li based alloy

Butt joints of 3.0 mm thick sheets of an Al-Li based alloy have been produced using Nd:YAG laser welding without filler metals. The hardness distribution and microstructure of the alloy and welded joints were investigated. The changes in the grain shapes, grain orientations, microtexture, and precipitates of the fusion zone were analyzed using optical microscope, electron back scattered diffraction (EBSD) and transmission electron microscopy (TEM). The results show that Nd:YAG laser welding leads to a change of the microhardness, grain shape, grain orientations, and a disappearance of the microtexture and precipitates. A narrow band of EQZ along the fusion boundary and a predominantly equiaxed dendritic structure are developed in the fusion zone. The formation of the predominately equiaxed dendritic grains is due to a heterogeneous nucleation mechanism aided by equilibrium A1{sub 3}Zr phases as well as the growth of pre-existing nuclei created by dendrite fragmentation, or by grain detachment resulted from Nd:YAG laser welding processes. In addition, Nd:YAG laser welding produces lower Vickers hardness than that of the base metal due to the decrease in the in quantity of {delta} Prime precipitates in the fusion zone. - Graphical Abstract: The grain shapes, grain orientations, microtexture, and precipitates of the solidified fusion zone were investigated and compared with the base metal using optical microscope, electron back scattered diffraction (EBSD) and transmission electron microscope (TEM). EBSD orientation map of laser welded joint in 5A90 alloys is presented in Fig. 3. It clearly shows that a narrow band EQZ along the fusion boundary and the predominantly equiaxed grains have been developed in the fusion zone of 5A90 alloys. Also, it is clear that the microstructure of the base metal is characterized by laminated grains with preferred orientations, whereas the fusion zone is predominately equiaxed grains in different colors having random orientations. Highlights: Black-Right-Pointing-Pointer The predominantly equiaxed dendritic structure is developed in the fusion zone. Black-Right-Pointing-Pointer The fusion zone with equiaxed grains shows random orientations and microtexture. Black-Right-Pointing-Pointer The loss in hardness in the fusion zone is due to the decrease in {delta} Prime precipitates. Black-Right-Pointing-Pointer The non-epitaxial growth occurs at fusion boundary. Black-Right-Pointing-Pointer The equilibrium A1{sub 3}Zr phases maybe the nuclei of new grains in the fusion zone.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Enhanced Strength of 304 SS-Ti6Al4V Laser-Welded Joints Containing Composite Interlayers

This paper investigates the microstructural characterization of the fracture propagation edge during laser beam welding of Ti6Al4V and 304 stainless steel using Cu, V, and Ni interlayers as a single Cu interlayer and composite interlayer structures with different Cu interlayer thicknesses. This study attempts to increase the Ti-SS joint strength through the use of the composite interlayers. X-ray diffraction characterization based on a 0.5-mm-thick Cu interlayer showed that the SS/Cu/Ti joint contained considerable brittle IMCs; however, the fracture surface of the SS/Cu-V/Ti composite interlayer sample did not contain any Fe-Ti IMCs or Cr{sub 2}Ti. EBSD observation indicated that the crack propagated transgranularly from solidified CuTi in the interdendritic regions of the FeTi compounds in the composite interlayer sample; however, the crack propagated intergranularly along Fe{sub 2}Ti grain boundaries in the single interlayer sample. The investigation based on a 1-mm-thick Cu interlayer showed that the SS/Ni-Cu1/Ti sample was stronger than the SS/Cu1/Ti sample due to a lack of CuTi at 3.5 kW. The tensile strength of the SS/Cu1-V/Ti joint was greater than that of the SS/Ni-Cu1/Ti joint due to the formation of ductile NiTi instead of brittle NiTi{sub 2}. The microstructures of the fracture edges after tensile testing illustrated that the eutectic CuTi + CuTi{sub 2} fractured at the SS/Cu1/Ti joint after tensile testing, while CuTi{sub 2} and eutectoid αTi + CuTi{sub 2} compounds fractured during the tensile experiment at the SS/Ni-Cu1/Ti and SS/Cu1-V/Ti joints, respectively. Investigations based on both the 0.5- and 1-mm-thick Cu interlayers showed that the tensile strength and elongation of the composite interlayer joints at a laser power of 3.5 kW were greater than those of the single interlayer joints.

36 MATERIALS SCIENCE↗
DOE OSTI2017

A New Grain Refiner for Ferritic Steels

A new grain refiner, LaB{sub 6}, was identified for ferritic steels based on the crystallographic calculation using the edge-to-edge matching model. Addition of 0.5 wt pct LaB{sub 6} led to a reduction of the average grain size from 765 to 92 μm and the proportion of the columnar structure from 35 to 8 pct in an as-cast Fe-4Si ferritic alloy. Although LaB{sub 6} was supposed to act as an active inoculant for δ-ferrite, thermodynamic calculation indicated that LaB{sub 6} is not thermodynamically stable in the melt of the Fe-4Si alloy. It was subject to decompose into La and B solutes. Consequently, both La and B reacted with Fe, O and S, forming different compounds. Microstructural examination at room temperature observed La{sub 2}SO{sub 2} and La{sub 2}O{sub 3} particles within the ferrite grains and Fe{sub 2}B along the grain boundaries in the samples. Through EBSD analysis, a reproducible orientation relationship between ferrite and La{sub 2}SO{sub 2} was identified. In addition, the edge-to-edge matching calculation also predicted the high potency for La{sub 2}SO{sub 2} to be an effective nucleant for δ-ferrite. It was considered that the grain refinement of LaB{sub 6} was attributed to the enhanced heterogeneous nucleation of δ-ferrite by La{sub 2}SO{sub 2}, and the solute effect of B due to the high Q-value in ferrite.

36 MATERIALS SCIENCE↗
DOE OSTI2019

High-Temperature Properties and Microstructural Stability of the AISI H13 Hot-Work Tool Steel Processed by Selective Laser Melting

The microstructural stabilities, softening resistance, and high-temperature tensile properties of the H13 hot-work tool steel by selective laser melting (SLM) were systematically studied. A series of tempering procedures were performed on the as-SLMed specimens. Afterwards, the mechanism of softening resistance behavior was discussed based on the XRD, SEM, EBSD observations, hardness measurements, and high-temperature tensile tests. It was found that the as-SLMed H13 consisted of α-iron and γ-iron. The carbide-stabilizing elements aggregated as the cell-like substructures for the rapid solidification of the SLM process. After the softening resistance treatment, the retained austenite transformed to ferrite and carbide mixtures. The cell-like substructures dissolved slowly into the matrix when the temperature was below 550 °C. These factors increased the hardness and retarded the softening of the material. When the temperature was 600 °C, the microstructural constituents transformed to soft ferrite and globular carbides, which lead to a considerable decrease of the hardness. Due to the grain refinement, solid solution strengthening, and residual stress, the as-SLMed H13 exhibited better mechanical properties than that of the wrought counterparts.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Mechanical Properties and Microstructure Evolution During Bending–Unbending Deformation of Pure Titanium Sheet

In this study, the pure titanium sheets with thickness of 1 mm are deformed 1-5 times in the form of bending–unbending cyclic deformation. The bending angles before and after unbending are 120° and 180°, respectively. Mechanical properties of samples during cyclic deformation, including M/(BH){sup 2}-surface strain curve and spring back in each bending cycle, have been investigated. Moreover, the twinning/detwinning behavior and texture evolution in three typical bending layers (i.e., the tensile layer, neutral layer and compressive layer) in cyclic deformation have been tested by EBSD and TEM, and analyzed systematically. The results indicate that the most intensive twins, consist of similar proportion of tensile and compressive ones, distribute in the inner layer after 1st bending. While they disappear almost entirely after 1st unbending. The twins in outer layer could also be detwinned, and the new twins could be activated during 1st unbending. Besides, the twin density increases obviously and the residual twins could not be detwinned clearly after multiple passes of cyclic deformation. Moreover, the Schmid factor along the σ{sub 2} direction functions in the activation of twin variants regardless of the three-direction stress states.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Corrosion Behavior of 316L Stainless Steel Fabricated by Selective Laser Melting Under Different Scanning Speeds

This work discussed the effects of different scanning speeds (800, 1083, 1200 and 1400 mm/s) on the microstructure of 316L stainless steel manufactured by selective laser melting (SLM) and the related corrosion behavior. Results showed that there were more voids with faster scanning speeds, and there were oxide powder and non-melt silicon inside the defects. The SLM 316L exhibited a full gamma austenite phase filled with sub-grains, and the average grain size of SLM-1083 mm/s 316L was approximately 42 μm, three times larger than that of the quenched 316L. The pitting potentials for the SLM 316L were all approximately 300 mV higher than that of the quenched due to the modification of inclusions in SLM, but the corrosion rate for the SLM 316L was faster, which was attributed to the voids and unstable passive film. The number of pitting sites increased with the scanning speed, and the pits occurred preferentially at the voids in SLM 316L.

36 MATERIALS SCIENCE↗
DOE OSTI2019

EBSD Study on Processing Domain Parameters of Oxide Dispersion Strengthened 18Cr Ferritic Steel

This paper presents the results of an experimental study aimed to identify hot working domains in oxide dispersion strengthened (ODS) 18Cr ferritic steel over a wide range of temperatures (1323-1473 K) and strain rates (0.01-10 s{sup −1}). The experimental data were obtained by uniaxial compression test using the Gleeble-1500D simulator in this range of temperature and strain rate. An inverse relationship with temperature and positive strain rate sensitivity associated with dynamic recovery and recrystallization, which is influenced by temperature and strain rate, was derived from the flow stress. Based on the processing map generated at 0.5 true strain, using rate dynamic material model (DMM) approach and the calculated instability parameter (ξ(ϵ́))>0, the optimum processing domain has been determined for this steel. The most favorable processing parameters are found in the temperature ranges of 1350-1450 K with a strain rate of 0.01 s{sup −1} and 1473 K with a strain rate 0.1 s{sup −1} with peak efficiency of 30 and 35%, respectively. The material flow behavior was studied using scanning electron microscopy (SEM)-based EBSD microstructural characterization. The steel subjected to 1323 K at high strain rate 10 s{sup −1} in the low-efficiency workability region showed low aspect ratio as compared to the elongated bamboo-like initial microstructure; however, minimum strain rate (0.01 s{sup −1}) showing that localized slip/shearing is the key mechanism and fiber texture studied from the intensity distribution of inverse pole figure showed the presence of significant amount of θ-fibers. In contrast, dynamic recrystallization dominated at higher efficiency region in the safe domain of processing map and γ-fiber texture was evident in the specimen deformed at 1373 and 1473 K with strain rate of 0.01 and 0.1 s{sup −1}, respectively, which is responsible for the change in the initial 〈1 1 0〉//ED α-fiber texture.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Superior Corrosion Resistance Properties of TiN-Based Coatings on Zircaloy Tubes in Supercritical Water

Thin films of TiN and Ti0.35Al0.65N nanocomposite were deposited on polished Zircaloy-4 tubes. After exposure to supercritical water for 48 h, the coated tubes are remarkably intact, while the bare uncoated tube shows severe oxidation and breakaway corrosion. X-ray diffraction patterns, secondary electron images, backscattered electron images, and energy dispersive X-ray spectroscopy data from the tube surfaces and cross-sections show that a protective oxide, formed on the film surface, effectively prevents further oxidation and corrosion to the Zircaloy-4 tubes. This result demonstrates the effectiveness of thin film ceramics as protective coatings under extreme environments.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Microstructure characterization of recycled IN718 powder and resulting laser clad material

Highlights: • A simple and efficient recycling procedure of IN718 powder has been defined for the laser cladding process. • Characterization of wasted and cleaned powder and comparison with new IN718 powder pointed out a slight diameter increase. • Microstructure characterization of clad builds showed differences between builds with new and recycled powders. • Clad microstructure and porosity increase remain constant static properties but decrease rupture strain after 3 reuses. • Without special precautions for cladding, the recycled powder can be reused twice without modifying the properties of IN718. - Abstract: The possibility to reuse the metal powder wasted in Laser Material Deposition (LMD) process has been evaluated and a simple procedure developed. LMD uses metal powder which is fed through a nozzle into the focal point of a laser, where it melts the powder and the substrate material. During the process, a high ratio of particles hits against an unmelted area and directly bounces off the deposited area. The efficiency ratio of deposition can drop to 40% depending on the configuration and spot size. This work deals with the design of a procedure to recollect and reuse the wasted powder of a nickel based superalloy IN718. After usage, powder is recollected, undesired fractions are magnetically segregated and aggregates are removed by sieving. The particles are mixed again and ready for reuse. In order to study the effectiveness of the process, no new powder has been added to the recovered fraction, and this procedure has been repeated five times. Experimental tests show that deposited material present similar properties than those obtained with new powder grains. But, after 3 reuses, the porosity content increases consequently and the rupture strain decreases strongly. The implementation of this process allows the improvement of the final efficiency, reducing costs and decreasing the hazardous powder amount.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Ni content on the microstructure and mechanical properties of weld metal with both-side submerged arc welding technique

Highlights: • This manuscript primarily studied the effects of Ni content on the K65 weld metals; • Increase Ni content could significantly improve the strength and low temperature impact toughness of the weld metal. • The beneficial effect of Ni is attributed to its positive effect on phase transformation. • GBF keeps K-S or N-W relations only with one austenite and the variants belong to the same bain group are two reasons. • The M/A islands are considered to be another reason for the reduced toughness. - Abstract: The effects of nickel (Ni) content on the mechanical properties and microstructure evolution in the weld metals of commercial K65 oil and gas pipeline steel were studied. Increase Ni content could significantly improve the strength and low temperature impact toughness (−40 °C and −60 °C) of the weld metal. The above influences of Ni were attributed to the formation of predominant acicular ferrite (AF), at the expense of grain boundary ferrite (GBF), ferrite side plates (FSP) and martensite/austenite (M/A) constituent in weld metal. EBSD results indicated that GBF keeps K-S or N-W relations only with the prior austensite grain that belongs to one side of the grain boundary and the variants belong to the same bain group which always forms low angle boundary. Moreover, the retained austenite as one part of M/A constituent was mainly found in the weld metal with lower Ni content. It indicated that the transformation process could be promoted more completely by the increase of Ni content. The mechanism of the toughness improvement brought by Ni increasing was ascribed to the relatively uniform transformation to final microstructure, which reduces the appearance of GBF and M/A constituents.

36 MATERIALS SCIENCE↗