Fraction Of High Angle Grain Boundaries Research Articles

Influence of scanning speed on microstructures and mechanical properties of SLM produced Hastelloy X: As-built and solution-treated

Abstract Hastelloy X (HX) alloys with ideal strength and ductility match can be obtained by selective laser melting (SLM) and a proper follow-up heat treatment. This work studies the influence of scanning speed on grain size, grain boundary distribution, recrystallization and mechanical properties of as-built HX. These influences are reevaluated after a solution treatment at 1175℃ for 4h. The results reveal that the average grain size decreases, while the aspect ratio, texture intensity and the proportion of high-angle grain boundaries (HAGBs) increases with the increase of scanning speed. A small amount of recrystallization has occurred in the as-built alloys due to the cyclic thermal effect of SLM scanning. The finer grains and larger aspect ratio imply the higher energy storage during SLM, which will increase the recrystallizing nucleation rate. Solution treatment eliminates the fiber texture of <100>//BD, significantly increases the HAGBs fraction and recrystallization fraction, reduces the grain aspect ratio, and coarsens the grains. With the increase of scanning speed, the strength of the Hastelloy X increases and the elongation decreases. The decrease of grain size is the main reason for the increase of yield strength.

Microstructure and corrosion behavior of a hot-rolled pure Mg after annealing at various temperatures

The microstructure and corrosion behavior of a hot-rolled pure Mg in the rolled state and after annealing at various temperatures was studied in this paper. It was found that the hot-rolled pure Mg after annealing at 250 °C, 350 °C, 400 °C, 500 °C and 550 °C showed homogeneous microstructures and the presence of a partially recrystallised tissue in hot rolled pure Mg. Corrosion of annealed Mg mainly occurred at grain boundaries, which stemmed from the relatively higher energy and chemical activation on the grain boundaries. Initial corrosion sites were provided by different proportions of High angle grain boundaries (HAGBs). The corrosion rate tends to increase and then decrease with increasing grain size. The Mg annealed at 450 °C showed a non-uniformed microstructure, with a lower fraction of the recrystallization grains and higher fraction of HAGBs, leaving to the severe localized corrosion and the highest corrosion rates. Corrosion rate measurement indicated that there existed a curvilinear relationship between the corrosion rate and grain size.

Simulation and Study of Influencing Factors on the Solidification Microstructure of Hazelett Continuous Casting Slabs Using CAFE Model.

The Hazelett continuous casting and rolling process represents a leading-edge production method for cold-rolled aluminum sheet and strip billets in the world. Its solidification microstructure significantly influences the quality of billets produced for cold rolling of aluminum sheets and strips. In this study, employing the CAFE (Cellular Automaton-Finite Element) method, we developed a coupled computational model to simulate the solidification microstructure in the Hazelett continuous casting process. We investigated the impact of nucleation parameters, casting temperature, and continuous casting speed on the microstructural evolution of the continuous casting billet. Through integrated metallographic analyses, we aimed to elucidate the controlling mechanisms underlying the Hazelett continuous casting process and its resultant microstructure. The results demonstrate that the equiaxed rate of grains increases with an increase in nucleation density, and the grain size decreases under constant cooling strength. With other nucleation parameters held constant, the grain size decreases as undercooling increases, and the columnar crystal zone expands. The nucleation density of the Hazelett continuous casting aluminum alloy has been determined to range between 1011 m-3 and 1013 m-3, and the undercooling ranges between 1 °C and 2.5 °C. The solidified grain structure can be controlled between 35 μm and 72 μm. The grain size of the continuous casting billet increases with an increase in pouring temperature and decreases as the casting speed increases. Elevating the pouring temperature positively impacts the fraction of high-angle grain boundaries and promotes the dendritic to equiaxed grain transition. Moreover, there exists potential for further optimization of continuous casting process parameters.

Synergistic integration of laser shock peening and heat treatment for refined microstructure and enhanced mechanical properties in additively manufactured 17–4PH stainless steel

This study explores the combined effects of laser shock peening (LSP) and heat treatment (HT) on the microstructure, residual stress, and mechanical behaviour of selective laser-melted 17–4 precipitation-hardened (PH) stainless steel. Subsequently, laser shock peening (LSP) was performed on both as-printed (AP) and heat-treated (HT) sample surfaces, and their microstructure, residual stress, and mechanical behaviour were evaluated. The severe plastic deformation (SPD) resulted in significant compressive residual stresses (CRS) in the surface layer of the AP+LSP (-879 MPa) and AP+HT+LSP (-850 MPa) samples. Laser peening caused dislocation accumulation and micro-level grain refinement in the sample's deformed layers, improving its mechanical properties. The AP+HT+LSP sample shows significant improvements over the as-printed (AP) sample. The fraction of high-angle grain boundaries (HAGBs) and high misorientation angles were higher in the AP+HT+LSP sample, and its effect is revealed in the microhardness data. The collective contribution of precipitation strengthening, Hall-Petch, CRS, dislocation strengthening, and increased fraction of HAGBs is observed in mechanical and microstructural aspects of laser shock peened specimens.

Discrete model for discontinuous dynamic recrystallisation applied to grain structure evolution inside adiabatic shear bands

Discontinuous dynamic recrystallisation (DDRX) is a well-known phenomenon playing a significant role in the high-temperature processing of metals, including industrial forming and severe plastic deformations. The ongoing discussion on the Zener–Hollomon (Z–H) parameter as a descriptor of materials’ propensity to DDRX and a measure of microstructure homogeneity leaves more questions than answers and prevents its practical application. Most of the existing DDRX models are continuous, and so the geometry and topology of real grain microstructures cannot be considered. The present study uses a fully discrete representation of polycrystalline aluminium alloys as 2D/3D Voronoi space tessellations corresponding to EBSD maps. Such tessellations are geometric realisations of combinatorial structures referred to as polytopal cell complexes. Combining discrete models with FEM LS-Dyna simulations of shock-wave propagation in AA1050 and AW5083 aluminium alloys makes it possible to estimate for the first time the contribution of DDRX to the final material microstructure inside adiabatic shear bands. It is shown that the increase of the initial fraction of high-angle grain boundaries, caused by preliminary deformation, significantly increases the spatial homogeneity and decreases the clustering of DDRX grains. The obtained results contradict the conventional assumption that the microstructures obtained by severe plastic deformation under quasi-static and dynamic deformation conditions are similar due to the similar value of the Z–H parameter: competition between the two recrystallisation mechanisms leads to almost unpredictable final grain structures inside share bands that require further comprehensive experimental studies. This agrees with experimental evidence for high material sensitivity to the Z–H parameter.

Investigation on microstructural evolution and local mechanical performance of friction stir lap welded AA6061-T6/ AA7075-T6 joints

The current study uses the friction stir lap welding (FSLW) technique to successfully fabricate two layered dissimilar AA6061/AA7075 joints. A defect-free joint with superior interfacial properties was obtained at the optimized process parameters (tool rotational speed / transverse speed) of 850 RPM/ 55 mm/min. The detailed investigations of microstructure evolution, crystallographic texture, and strength correlation were performed using the field emission scanning electron microscope (FE-SEM), optical microscope (OM), electron backscattered diffraction (EBSD), Vickers microhardness, nano-indentation, and miniature tensile tests. EBSD analysis reveals the presence of a significant grain refinement at the bottom (3.65 µm), middle (4.58 µm), and top region (6.34 µm) of stir zone (SZ) compared to AA6061 (42.56 µm) and AA7075 (34 µm) base metals. An increasing trend in the fraction of high angle grain boundary (FHAGBs) and recrystallization fraction was noticed from top to bottom of the build within the SZ. Continuous dynamic recrystallization (CDRX) with the gradient in strain, strain rate, and temperature leads to these microstructural variations within the SZ. The microhardness study shows a decrease in microhardness in the SZ due to thermal softening and precipitate dissolution at higher temperatures. Appreciable interfacial material mixing leads to remarkable strength, ductility, and strain-hardening behaviour within the SZ, especially at the interface section. Pole figures (PF) reveal the presence of major shear texture components (B/B̅, and C) with some minor presence of A1*/A2*and A/A̅ that confirms the presence of higher strains within the SZ. The dominant presence of recrystallization textures components (like cube {001} 〈110〉, Goss {011} 〈100〉, and P {011} 〈122〉 ) within the SZ confirms the presence of the CDRX mechanism.

Statistics of grain microstructure evolution under anisotropic grain boundary energies and mobilities using threshold-dynamics

This paper investigates the statistics of two-dimensional grain microstructures during grain growth under anisotropic grain boundary (GB) energies and mobilities. We employ the threshold dynamics method, which allows for unparalleled computational speed, to simulate the full-field curvature motion of grain boundaries in a large polycrystal ensemble. Two sets of numerical experiments are performed to explore the effect of GB anisotropy on the evolution of microstructure features. In the first experiment, we focus on abnormal grain growth and find that GB anisotropy introduces a statistical preference for certain grain orientations. This leads to changes in the overall grain size distribution from the isotropic case. In the second experiment, we examine the development of texture and the growth of twin boundaries for different initial microstructures. We find that texture development and twin growth are more pronounced when the initial microstructure has a dominant fraction of high-angle grain boundaries. Our results suggest effective GB engineering strategies for improving material properties.

The effect of heat input on weld-bead geometry, mechanical, phase transformation temperature, and corrosion properties of autogenous double pulse TIG welded nitinol sheets

This research aims to analyze the welding of 1 mm thick NiTinol sheets using an autogenous double pulse tungsten inert gas (DPTIG) welding process. The effect of heat input (HI) on bead geometry, microstructure, hardness, tensile strength, phase transformation temperature (PTT), and corrosion behavior was studied. The lower (111.25 J/mm), and higher (120.14 J/mm) HI produced an average grain size of 26 μm and 36 μm, respectively. The microstructure of the fusion zone (FZ) had coarser columnar grains with intermetallic phases such as Ni3Ti, and TiO2. The grain size in the FZ increased with the increase in HI. Sample P (111.25 J/mm) showed a higher hardness of 280.54 HV and tensile strength of 566 MPa due to a higher proportion of austenite phase (99.4%), the smaller grain size of 26 µm, a larger fraction of high angle grain boundary (HAGB) of 75.8%, and higher kernel average misorientation (KAM) value of 4.93. Compared to base metal (BM), sample P (111.25 J/mm), and sample S (120.14 J/mm) exhibited a reduction in tensile strength of 19.14% and 32.29%, respectively. The decline in hardness and tensile strength was attributed to the formation of intermetallic phases, a decrease in the Ti/Ni ratio, coarser grain formation, and a decrease in HAGB fraction and KAM values. The fractured tensile samples showed a mixed mode of fracture with dimples and cleavage facets. Compared to BM, Sample Q (118.05 J/mm) exhibited lesser variation in temperature hysteresis values for austenite and martensite temperatures, with a deviation of 0.4°C and 3.1°C, respectively. All the welded samples had better corrosion behavior than the BM due to a higher Ti/Ni ratio.

Discrete modelling of continuous dynamic recrystallisation by modified Metropolis algorithm

Continuous dynamic recrystallisation (CDRX) is often the primary mechanism for microstructure evolution during severe plastic deformation (SPD) of polycrystalline metals. Its physically realistic simulation remains challenging for the existing modelling approaches based on continuum mathematics because they do not capture important local interactions between microstructure elements and spatial inhomogeneities in plastic strain. An effective discrete method for simulating CDRX is developed in this work. It employs algebraic topology, graph theory and statistical physics tools to represent an evolution of grain boundary networks as a sequence of conversions between low-angle grain boundaries (LAGBs) and high-angle grain boundaries (HAGBs) governed by the principle of minimal energy increase, similar to the well-known Ising model. The energy is minimised by a modified Metropolis algorithm. The model is used to predict the equilibrium fractions of HAGBs in several SPD-processed copper alloys. The analysis captures non-equilibrium features of the transitions from sub-grain structures to new HAGB-dominated grain structures and provides estimations of critical values for HAGB fractions and accumulated strain at these transitions.

Refinement mechanism of large heat-input welding CGHAZ microstructure by N addition and its effect on toughness of a V-Ti-N microalloying weathering steel

The large heat input welding performance of uncoated weathering steel remains a great challenge since their coarse grain heat-affected zones (CGHAZs) are usually occupied by coarse and brittle microstructure consisting of granular bainitic ferrite (GBF) and martensite/austenite (M/A) constituents. In this article, a V-Ti-N microalloying weathering steel suitable for large heat input welding was achieved through the N addition, and the mechanism dominating the modified microstructure and the enhanced impact toughness of CGHAZ was deeply elucidated mainly by HRTEM. The results indicated that in the simulated CGHAZ with large heat input of 80 kJ/cm, the increase of N content from 0.0030 wt% to 0.0075 wt%, and 0.0115 wt% elevated the density of nano- and submicron-sized particles of (Ti, V)(C, N) containing VN, which in turn resulted in the refinement of prior austenite grains (PAGs) by the improved grain boundary pinning effect and the enhancement in heterogeneous nucleation of intragranular ferrite (IGF) by the decreased lattice disregistry (δ) between (Ti, V)(C, N) and IGF. The fraction of high-angle grain boundaries (HAGBs) with misorientation tolerance angles (MTAs) higher than 15° was thereby increased while the mean equivalent diameter (MED) of ferritic grains with HAGBs was decreased. Simultaneously, the N addition scattered the M/A constituents, promoted the internal substructure of dislocation-type M/A at the expense of twin-type M/A, and accordingly reduced the hardness difference between the M/A constituents and the matrix. All these factors retarded brittle crack initiation and propagation, and low-temperature impact toughness of simulated CGHAZ increased from 62.8 J to 141.2 J.

Effect of arc mode on laser-arc hybrid additive manufacturing of Al–Cu alloy: Pore defects, microstructure and mechanical properties

Laser-arc hybrid additive manufacturing (LAHAM) of Al–Cu alloy was carried out to investigate the effects of different arc modes on pore formation, microstructure characteristics, and tensile properties. The introduction of high-frequency pulse current and cyclic polarity reversal in the CMT-PADV mode was found to be beneficial for reducing pores. Compared with CMT and CMT-P modes, the CMT-PADV mode exhibited a transformation from columnar grains to equiaxed grains, with the smallest grain size of 26.6 μm, the highest fraction of high-angle grain boundaries of 84.2 %, and the longest length of grain boundaries reaching 14.9 cm. Moreover, the eutectic structure along the grain boundaries was observed in the samples fabricated by CMT and CMT-P modes, while it appeared disrupted and intermittently distributed in the CMT-PADV mode. In this case, the excellent mechanical properties were obtained for CMT-PADV mode, with tensile strength and elongation 276.6 MPa and 23.82 % in horizontal directions, 262.9 MPa and 20.9 % in vertical directions, respectively.

Effect of temperature on superplastic deformation behavior of 2198 Al–Cu–Li alloy

Al–Li alloy possesses remarkable advantages such as low density, high specific strength and specific elastic modulus, making it an exceptional aerospace structural material with considerable application potential. However, Al–Li alloy suffers from poor formability, which restricts its broader engineering applications. Alternatively, superplastic forming at high temperatures is widely applied for producing such components. Temperature is a key factor affecting the thermal deformation properties of Al–Li alloy. In this study, tensile tests of 2198 Al–Cu–Li alloy were conducted at different temperatures, and dedicated characterizations were performed to reveal the underlying mechanisms responsible for the hot deformation behavior. Results reveal that three distinct stages are observed in the superplastic deformation process of this alloy. Specifically, in the initial low-temperature range (400 °C–450 °C), incomplete dynamic recrystallization (DRX), accompanied by grain rotation and elongation plays a dominant role in the hot deformation mechanism. In the middle temperature stage (450 °C–500 °C), both DRX and grain boundary sliding (GBS) occur. At the high-temperature stage (500 °C–550 °C), diffusion creep emerges as the prominent mechanism. Moreover, optimum superplasticity is achieved at 500 °C, which results from a synergistic combination of several factors, including grain refinement by DRX, an increased fraction of high-angle grain boundaries (HAGBs), reduced dislocation density and weak texture.

Influence of Heat Treatment on Microstructure and Mechanical Properties of Direct-Quenched Fe-0.06C-0.2Si-2.0Mn Steel

In this study, the effect of subsequent heat treatment applied to high-strength low-alloy steel (HSLA) on the structure–property relationships was investigated. Tempering and intercritical annealing processes are introduced to elucidate the influence of subsequent heat treatment on mechanical properties of direct-quenched Fe-0.06C-0.2Si-2.0Mn steel from a microstructural perspective. The tempering process results in a typical tempered martensite with uniformly dispersed cementite, whereas the intercritical annealing process forms a dual-phase microstructure composed of soft ferrite and hard martensite for the direct-quenched steel. In the intercritical annealed steel, a number of mobile dislocations at the interphase (martensite/ferrite) boundary significantly decrease the yield strength, and the large difference in strength between ferrite and martensite enhances work hardening. Charpy V-notch impact test results indicate that the tempering and intercritical annealing processes improved the absorbed energy by more than 100 J compared to the direct-quenched steel at room temperature, and at −50 °C, the intercritically annealed steel exhibited the highest absorbed energy of approximately 140 J. Additionally, the high fraction of high-angle grain boundaries and fine grains of the intercritically annealed steel increase the resistance to cleavage crack propagation, thereby reducing the ductile-to-brittle transition temperature.

Effect of Cyclic Loading on the Surface Microstructure Evolution in the Pearlitic Rail

The effects of cyclic loading on the surface microstructure evolution of different contact locations in a used pearlitic rail were studied. Microstructures were analyzed using Scanning Electron Microscopy (SEM). Meanwhile, grain boundaries and crystallographic orientations were explored via Electron Backscatter Diffraction (EBSD). At last, wheel–rail contact probabilities and forces were calculated using rail profiles. The results indicate that the side wear region located in the gauge face was 71.5% in the high-angle grain boundaries (HAGBs) fraction, 0.88 in the Kernel Average Misorientation (KAM) value, 36% in the recrystallization (REX) fraction, and had a predominant orientation in grains. The rolling contact fatigue (RCF) region situated at the gauge corner was 66.3% in the HAGBs fraction, 0.92 in the KAM value, 33% in the REX fraction, and was mis-orientated in grains. The region located at the edge of the running band was 60.7% in the low-angle grain boundaries (LAGBs) fraction, 0.97 in the KAM value, 12% in the REX fraction, and was mis-orientated in grains. Continuous dynamic recrystallization (cDRX) took place in wear and RCF regions during the cyclic rolling contact loading, creating ultra-fine grains with a transformation from LAGBs to HAGBs, lower KAM values, and more REX. Grains oriented along [111] parallel to the vertical direction in the wear region were influenced by the dominant normal force, while grains in the RCF region were non-oriented, which was attributed to large lateral and vertical forces of similar magnitudes.

Innovation in thermal cycling aging compared to isothermal aging for precipitation hardening stainless steel

The cyclic thermal process can assist and accelerate the kinetics of phase transformation. Conventional UNS S17400 grade stainless is characterized by a martensitic microstructure. After solution treatment, the steel was aged by thermal cycling between 600 °C and 25 °C and quenched in water in each cycle, completing under the self-designed system. The nano precipitates of very fine copper particles and larger NbC particles were found by using transmission electron microscopy (TEM). The fraction and quantity of high angle grain boundaries (HAGBs) after 36 cycles were the highest among the three numbers of thermal cycles. The peak hardness also occurred after 36 cycles and was attributed to the finest grains, high fraction of HAGBs, and the largest local microstrain. The microtwins and the reverted γ were formed by the thermal cycling process. The estimated fraction value of reverted γ was very low, below 0.1, with a calculated precipitation rate about 12.6 s−1 at t0.5.

Overcoming the trade-off between strength and ductility in austenitic stainless steel using a high-density pulsed electric current

This paper presents a method to overcome the trade-off between the strength and ductility in metals and clarifies its mechanism via a crystal structure analysis on type 316 austenitic stainless steel subjected to high-density pulsed electric current (HDPEC) treatment. Because of the HDPEC treatment, the fractions of low-angle grain boundaries (LAGBs) and high-angle grain boundaries (HAGBs) increased, while those of ∑ 3 twin boundaries (TBs) decreased significantly. An increase in the number of HAGBs and the formation of new LAGBs, which leads to achieving refined grains, was achieved by virtue of the change of misorientation angles and dislocation formation. Furthermore, the dissolution of Cr-rich precipitates occurred in the grain surroundings, which led to an increase in ductility of the material. The dislocation density, measured by the mean kernel average misorientation values, decreased because of the dislocation motion induced by HDPEC. The strengthening of the material was attributed to the grain refinement, and the increased ductility was attributed to the decreased dislocation density and decreased content of Cr-rich precipitates, which prevented precipitation hardening. The combination of these advantages contributed to overcoming the trade-off between strength and ductility.

Microstructure evolution, strength-ductility synergy and fracture mechanisms of Cu-18Sn-0.3Ti alloy prepared using Direct Current assisted Hot Pressed sintering and conventional solution treatment

Based on micron atomized spherical Cu-18Sn-0.3Ti alloy powders, the Cu-18Sn-0.3Ti alloy with an elongation of 23.4%, an ultimate tensile strength of 507.2 MPa and a yield strength of 229.4 MPa was prepared using Direct Current assisted Hot Pressed sintering (DCHP) and conventional solution treatment. The microstructure evolution mechanism of alloy during DCHP, the strength-ductility synergy and fracture mechanisms of solution treated DCHP (ST-DCHP) samples were systematically investigated. The results show that, under the synergistic effect of Joule heat, electron wind force and external load of DCHP, dislocations were consecutively generated and consumed as the driving force of dynamic recrystallization, fine equiaxed grains and a large number of annealing twins were formed, the prior particle boundaries (PPBs) of alloy powders gradually disappeared and good sintering necks were formed, which played key roles in improving the strength of alloy. Simultaneously, the δ phase was segregated from grain boundaries to PPBs during DHCP. During the subsequent solution treatment, the phase transformation of δ phase occurred, the β, γ and ε phases which were conducive to the plastic deformation of alloy were generated inside δ phase, resulting in a change in the fracture mechanism inside powders and at sintering necks. The high fraction of high-angle grain boundaries (HAGBs) was considered as the main reason for inducing the formation of high-density deformation twins with different directions inside fine α grains, which greatly improved the ductility of alloy. The strengthening contributions of microstructure were also evaluated, and the calculated yield strength was well consistent with the experimental ones. The results provide the theoretical guidance for the preparation of Cu-Sn-Ti alloy with higher Sn content and lay a foundation for the preparation of Nb3Sn superconducting wires with higher critical current density.

Precipitation Behavior and Corrosion Properties of Stirred Zone in FSWed AA5083 Al-Mg Alloy after Sensitization

This paper investigated the Mg-rich phase precipitation behavior and the corrosion performance throughout the thickness direction within the stirred zone (SZ) of friction stir welded (FSW) AA5083 alloy after 175 °C/100 h sensitization. For the as-welded SZ, the recrystallized grain size gradually decreased from the top surface (5.5 μm) to the bottom (3.7 μm). The top and bottom of the SZ maintained relatively high levels of deformed grains and accumulated strain induced by either shoulder pressing or pin stirring. After 175 °C/100 h sensitization, 100 nm thick β′-Al3Mg2 precipitates were present along the grain boundaries (GBs) in the SZ. The bottom of the SZ exhibited more continuous precipitates along GBs due to the fine grain size and the large fraction of high-angle grain boundaries (0.724%). Although the as-welded SZ exhibited excellent corrosion resistance, it became extremely vulnerable to intergranular cracking (IGC) and stress corrosion cracking (SCC) after sensitization. The large SCC susceptibility indices of the SZ samples ranged from 66.9% to 73.1%. These findings suggest that sensitization can strongly deteriorate the corrosion resistance of the Al-Mg FSW joint, which is of critical importance for the safety and reliability of marine applications.

Quasi-In Situ Localized Corrosion of an Additively Manufactured FeCo Alloy in 5 Wt Pct NaCl Solution

FeCo alloys are important materials used in pumps and motors in the offshore oil and gas drilling industry. These alloys are subjected to marine environments with a high NaCl concentration, therefore, corrosion and catastrophic failure are anticipated. So, the surface dissolution of additively manufactured FeCo samples is investigated in a quasi-in situ manner, in particular, the pitting corrosion in 5.0 wt pct NaCl solution. The local dissolution of the same sample region is monitored after 24, 72, and 168 hours. Here, the formation of rectangular and circular pits of ultra-fine dimensions (less than 0.5 µm) is observed with increasing immersion time. In addition, the formation of a corrosion-inhibiting surface layer is detected on the sample surface. Surface dissolution leads to a change in the surface structure, however, no change in grain shape or grain size is noticed. The surface topography after local dissolution is correlated to the grain orientation. Quasi-in situ analysis shows the preferential dissolution of high-angle grain boundaries (HAGBs) leading to a change in the fraction of HAGBs and low-angle grain boundaries fraction (LAGBs). For the FeCo sample, a potentiodynamic polarisation test reveals a corrosion potential (Ecorr) of − 0.475 V referred to the standard hydrogen electrode (SHE) and a corrosion exchange current density (icorr) of 0.0848 A/m2. Furthermore, quasi-in situ experiments showed that grains oriented along certain crystallographic directions are corroding more compared to other grains leading to a significant decrease in the local surface height. Grains with a plane normal close to the langle {1}00rangle direction reveal lower surface dissolution and higher corrosion resistance, whereas planes normal close to the langle {11}0rangle direction and the langle {111}rangle direction exhibit a higher surface dissolution.

Effect of heat input on the microstructure and impact toughness of submerged arc weld metal for high-performance weathering steel

This study aims to elucidate the appropriate heat input (E j ) range for submerged arc welding (SAW) of high-performance weathering steel. Generally, by increasing E j , the welding efficiency can be improved, but the toughness of the weld metal may be deteriorated. Therefore, SAW was employed to produce the weld microstructure under varying E j from 20 to 50 kJ cm−1. The Charpy V-notch impact tests were conducted at −40 °C, and the weld microstructures were characterized by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD). The results indicate that the weld microstructures consist of polygonal ferrite (PF), acicular ferrite (AF), granular bainitic ferrite (GBF), and martensite/austenite (M/A) constituents under each E j . With the increase in E j , the proportion of PF increases, while AF and GBF are coarsened, and the area fraction (f M/A) and mean size (dM/A) of M/A constituents increase monotonically. Further, the fraction (f MTA>15°) of high-angle grain boundaries (HAGBs) with the misorientation tolerance angles (MTAs) greater than 15° is reduced, while the mean equivalent diameter (MEDMTA≥15°) of ferrite grains with HAGBs increases. Accordingly, with the increase in E j , the impact toughness of weld degrades from 128.4 to 47.6 J. The higher degree of micro-strain concentration caused by the increase in M/A size and area leads to the formation of larger microcracks under small plastic deformation, while the reduced HAGBs have a lower inhibition effect on crack propagation. Finally, the impact toughness decreases with the increase of E j . Overall, the findings suggest that the E j of SAW should not exceed 40 kJ cm−1 in the construction of high-performance weathering steel.