Fraction Of High Angle Grain Boundaries Research Articles

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.

Topological characteristics of grain boundary networks during severe plastic deformations of copper alloys

The grain boundary character distribution strongly affects the properties of polycrystalline materials. Grain boundaries of similar characters form networks, whose topological invariants can be considered as distribution descriptors. Understanding the evolution of such descriptors during severe plastic deformations (SPD) can elucidate the evolution of properties and underpin the design of processing routes for target behaviour. For topological analysis of grain boundary networks, polycrystalline materials are considered here as polyhedral complexes with grain boundaries classified into two types — low-angle and high-angle. Changes of grain boundary types are calculated using sub-grain rotations, which reflects the physical mechanism of microstructure evolution during SPD. A non-physical approach, by direct conversions of low-angle to high-angle boundaries, is also explored as a reference to demonstrate the impact of the physical constraint imposed by rotations. Reported is the discovery of topological phase transitions in the grain boundary networks which might take place during severe plastic deformations of different copper alloys. Depending on the evolution approach, the transitions correspond to zeros of the Euler characteristic, or of the logarithm of the inverse connectivity, of the grain boundary network. The relations between these transitions and the fraction of high-angle grain boundaries, and between the fraction of high-angle grain boundaries and plastic strain obtainable experimentally, provide new perspectives for grain boundary engineering and network design. Determining the dominant evolution mechanism and critical accumulated strain for a given material and processing route requires further experimental studies of triple junction evolution.

Evolutions of Microstructure and Impact Toughness of Submerged Arc Weld Metal via Introducing Varied Si for Weathering Bridge Steel

In this paper, the influence of the silicon (Si) content on microstructure and impact property of submerged arc weld metals (WMs) for weathering bridge steel was clarified. Actual submerged arc welding (SAW) was carried out to produce WMs with 0.18 wt.%, 0.36 wt.%, 0.51 wt.%, and 0.60 wt.% of Si. The low temperature impact property of weld metal was detected, and the weld microstructures were characterized by optical microscopy (OM), scanning and transmission electron microscope (SEM and TEM), and electron backscatter diffraction (EBSD). The results indicate that WMs consist of polygon ferrite (PF), acicular ferrite (AF), granular bainitic ferrite (GBF), and martensite/austenite (M/A) constituents in each Si content. With increasing Si, the proportion of PF increased, while AF and GBF coarsened, the area fraction, fM/A, and the mean size, dM/A, of M/A constituents increased, the number of inclusions decreased, but the size increased. Further, the fraction of high-angle grain boundaries (HAGBs) with the misorientation tolerance angles (MTAs) greater than 15° reduced, while the mean equivalent diameter, MEDMTA≥15°, of ferrite grains with HAGBs increased. Accordingly, the impact toughness of WM was degraded from 108.1 J to 39.4 J with the increase in Si. The increase in M/A constituents and inclusions size reduced the critical fracture stress, thereby formation of larger microcracks and cleavage planes occurred. The reduced HAGBs exhibited a low hindering effect on crack propagation, and, consequently, the impact toughness decreased with increasing Si content.

The role of reverted transformation in hydrogen embrittlement of a Cu-containing low carbon high strength steel

This work presents an investigation on the role of reverted transformation in hydrogen embrittlement, aiming at revealing the influencing mechanism of austenite and grain boundary characteristics on hydrogen embrittlement (HE) susceptibility during reverted transformation. The results showed that HE susceptibility decreased with the annealing temperature increasing from 680 °C to 720 °C, which attributed to a combined effect of austenite and grain boundary distribution. Both the fractions of austenite and high angle grain boundary (HAGB) increased with the annealing temperature increasing from 680 °C to 700 °C, which resulted in a lower HE susceptibility at 700 °C due to strong H storage of austenite and HAGB. As the annealing temperature further increased to 720 °C, the fraction of austenite exhibited a decline, but the fraction of HAGB monotonously increased. As a result, a lower HE susceptibility was achieved at 720 °C. This indicated that grain boundary distribution played a determining role in the resistance to HE. We ascribed this to the variant selection and Bain/CP grouping.

Deposition current density-induced grain boundary engineering of electrodeposited cobalt coatings for enhanced electrochemical stability

ABSTRACT Cobalt coatings were electrodeposited at different deposition current densities (10 mA·cm−2, 20 mA·cm−2, 30 mA·cm−2, 40 mA·cm−2, 60 mA·cm−2) over mild steel substrate. The coating deposited at 30 mA·cm−2 showed the highest corrosion resistance (Rp = 7515Ω·cm2) due to a higher fraction of Low energy grain boundary constitution (i.e. coincidence site lattices (CSLs) and low angle grain boundaries (LAGB)) and exhibited higher fraction of stabler Co3O4 passive oxide layer. The coatings electrodeposited at 60 mAcm−2 exhibited lower corrosion resistance (Rp = 1304 Ω·cm2) due to a higher fraction of high-angle grain boundaries and a higher fraction of relatively lesser stable Co(OH)2.

Development and analysis of a powder bed friction stir (PBFS) additive manufacturing process for aluminum alloys: A study on friction-stirring pitch (ω/v) and print location

Metallic components fabricated with powder bed fusion additive manufacturing (AM) processes are known to have defects arising from liquid-solid phase transformation. Friction stirring can be an effective alternative source of heat, as evidenced in some of the recently developed powder deposition-based solid-state AM processes. However, they possess practical issues such as making compacted powder feedstock beforehand, consolidation of powder filler inside the tool during deposition followed by necessary extrusion, and significant flash formation, among a few. This paper proposed a novel powder bed friction stir (PBFS) additive manufacturing process inside a specifically designed mould that could potentially mitigate such issues in the existing technologies. The process was successfully demonstrated by building single-track and parallel double-track 3D structures layer-by-layer from an AA6061 powder bed using a rotating non-consumable friction stirring tool. Furthermore, for a better understanding of the underlying process mechanics, deposits were analyzed for their surface topography, microstructural evolution, and mechanical performance under different heat inputs as well as different print locations. Different heat inputs were applied by varying the friction-stirring pitch (FS pitch) from ‘Low’ to ‘High’. The print locations analysis was carried out in a parallel double-track fabricated at the optimal FS pitch. The results revealed that the ‘High’ FS pitch yielded an increased percentage of recrystallized grains and a fraction of high-angle grain boundaries as well as a larger average grain size due to the high heat input, better material intermixing, and the dominance of continuous dynamic recrystallization. As a result, an improved mechanical performance, except for the microhardness, was observed. The ‘High’ FS pitch also exhibited an improved surface topography. The results also revealed that the relative dominance of the re-stirring-induced recrystallization and the reheating-induced plastic deformation was the key factor for the variation of the microstructure in the longitudinal, transverse, and short-transverse directions. A gradual decrease in the tensile strength was observed from the ‘Top’ to the ‘Bottom’ segments of the overlapped region, contrary to the central region of a track.

Shape Memory Properties and Microstructure of FeNiCoAlTaB Shape Memory Alloys

The three-point-bending shape memory properties, microstructure, and magnetic properties of Fe40.95Ni28Co17Al11.5Ta2.5B0.05 (at.%) alloys were investigated. The magnetic results showed a martensitic transformation in the samples that were aged at 700 °C for 6 and 12 h under the applied magnetic fields of 0.05 and 7 Tesla. The martensitic start temperature increased from −113 °C to −97 °C as aging times increased from 6 to 12 h. Increasing the magnetic fields from 0.05 to 7 Tesla, the transformation temperatures increased to a higher temperature. Both samples reach saturation magnetization (140 emu/g) under 7 Tesla. The 98.5% cold-rolled alloys that were annealed at 1250 °C for 0.5 h presented a strong <100> texture in the rolling direction with an average grain size of 360 μm. Increasing the annealing time to 1 h, the intensity of texture reduced from 31.61 to 23.19. The fraction of low angle grain boundaries (LABs) for the 98.5% CR samples after annealing at 1250 °C for 0.5 h and 1 h was about 24.6% and 16.1%, respectively. Three-point-bending results show that the sample aged at 700 °C for 6 h displayed 0.2% recoverable strain at a stress level of 800 MPa. Failure occurred before the 900 MPa cycle could be completed. The sample aged at 700 °C for 12 h shows no transformation before the applied stress level of 300 MPa. As the stress levels increase to 400 MPa, the sample shows the shape memory effect and displayed 0.8% recoverable strain at a stress level of 400 MPa. The samples are failures during the 500 MPa cycle. The observed recoverable strain values were lower than those that were theoretically predicted, which was possibly due to the larger volume fraction of high-angle grain boundary and the slightly lower than expected average grain size.

High Cycle Fatigue Behaviour of 316L Stainless Steel Produced via Selective Laser Melting Method and Post Processed by Hot Rotary Swaging.

This paper deals with a study of additively manufactured (by the Selective Laser Melting, SLM, method) and conventionally produced AISI 316L stainless steel and their comparison. With the intention to enhance the performance of the workpieces, each material was post-processed via hot rotary swaging under a temperature of 900 °C. The samples of each particular material were analysed regarding porosity, microhardness, high cycle fatigue, and microstructure. The obtained data has shown a significant reduction in the residual porosity and the microhardness increase to 310 HV in the sample after the hot rotary swaging. Based on the acquired data, the sample produced via SLM and post-processed by hot rotary swaging featured higher fatigue resistance compared to conventionally produced samples where the stress was set to 540 MPa. The structure of the printed samples changed from the characteristic melting pools to a structure with a lower average grain size accompanied by a decrease of a high fraction of high-angle grain boundaries and higher geometrically necessary dislocation density. Specifically, the grain size decreased from the average diameters of more than 20 µm to 3.9 µm and 4.1 µm for the SLM and conventionally prepared samples, respectively. In addition, the presented research has brought in the material constants of the Hensel-Spittel formula adapted to predict the hot flow stress evolution of the studied steel with respect to its 3D printed state.

Correlation between local mechanical properties and corresponding microstructures in a friction stir processed Monel alloy

Friction stir processing (FSP), an innovative local microstructural modification technique for bulk material, has been used to process Monel alloy. A detailed microstructural analysis was conducted by acquiring high-resolution electron backscatter diffraction (EBSD). The local tensile properties and hardness nanoindentation were obtained for the different FSP area zones. The coarse equiaxed grain structure of 18.9 μm average grain size at the base material (BM) has been dynamically recrystallized refined equiaxed grains of an average grain size of 3.6 μm in the stir zone (SZ) where the thermomechanically affected zone (TMAZ) has an average grain size of 10.7 μm. The grain structure at the interface of the SZ and TMAZ consisted of mixed fine recrystallized grains and coarse deformed grains. The SZ is dominated by significantly refined grains, with the highest fraction of high-angle grain boundaries (HAGBs) of about 71%. The point-to-point misorientation measured by Kernel Average Misorientation (KAM) indicates low values in the SZ compared with the highest values at the interface between SZ and TMAZ. In terms of tensile properties, the yield strength of TMAZ and SZ improved to 380 and 440 MPa, respectively. The ultimate tensile strength (UTS) also increased by 18% from 499 to 593 MPa in TMAZ and 37% from 499 to 699 MPa in the SZ. The maximum nano hardness found in the TMAZ of about 7.13 ± 1.92 GPa due to the high density of the substructure. However, the SZ slightly increased to 3.88 ± 0.22 GPa from 3.33 ± 0.17 GPa of the BM.

Enhanced strength and ductility in friction stir processed Cu–Mn alloys

In this study, the ultrafine-grained (UFG) Cu–5Mn alloys were prepared by friction stir processing (FSP) to investigate the influence of short-range ordering (SRO) on the tensile deformation behavior. Due to the dynamic recrystallization, the uniform and equiaxed UFG microstructures were obtained in FSP Cu–5Mn alloys with low density of dislocations, high fraction of high-angle grain boundaries, high Taylor factor and high degree of recrystallization. The introduction of Mn atoms into the Cu matrix cannot change the stacking fault energy apparently but increase the SRO degree. Although the dominant mechanism was still the wavy slip of dislocation, the planar slip of dislocations was motivated and many twin layers were observed in the ultrafine grains of FSP Cu–5Mn alloy. The change of SRO degree can significantly improve the dislocation storage capacity during the tensile deformation process, and promote the improvement of the strain-hardening rate, resulting in an excellent strength-ductility match in FSP UFG Cu–5Mn alloy.

Plastically heterogeneity-driven fracture in additive manufactured steels

Tuning the fracture evolution to resist crack propagation by engineering microstructures is effective for enhancing the toughness of metals. However, many microstructures produced through existing traditional technologies cannot overcome the dilemma known as the strength-toughness trade-off in metals. Here, we report a plastically heterogeneity-driven fracture manner affected by the unique microstructures in additive manufactured (AM) 316L stainless steels that endows a superior combination of strength and toughness, surpassing the conventional 316L steels. The good toughness is due to the natural heterogeneous microstructural arrangement of coarse-grain (CG) and fine-grain (FG) zones in the AM steel. Plasticity is typically initiated within FG zones with a larger fraction of high-angle grain boundaries (HAGBs). In contrast, the CG zones are less susceptible to plasticity localization before the full strain hardening capacity of the FG zones is exploited. It results in strong strain/stress partitioning and regional hardening processes, providing mesoscale soft-hard barriers to crack propagation. In addition, the low-angle grain boundaries (LAGBs) and dislocation cells at the microscale are demonstrated to strengthen the matrix uniformly without introducing stress concentration and damage incidents detrimentalto toughness. This work demonstrates the extensive engineering application prospect of AM steels and the potential of the line-by-line or layer-by-layer build strategies for developing more fracture tolerate AM steels.

An Attempt to Understand Stainless 316 Powders for Cold-Spray Deposition

Cold gas dynamic spray (CS) is a unique technique for depositing material using high-strain-rate solid-state deformation. A major challenge for this technique is its dependence on the powder’s properties, and another is the lack of standards for assessing them between lots and manufacturers. The motivation of this research was to understand the variability in powder atomization techniques for stainless steel powders and their subsequent properties for their corresponding impacts on CS. A drastic difference (~30%) was observed in the deposition efficiencies (DEs) of unaltered, spherical and similar sized stainless steel (316) powders produced using centrifugal (C.A) and traditional gas atomization (G.A) techniques. The study highlights more the differences on a precursor level. Using recent advancements in large scale statistical measurements, such as laser diffraction shape analysis and µCT scanning; and traditional methods, such as EBSD and nanoindentation, an attempt was made to understand the powder’s properties. Insights on powder size and shape were documented. Significant differences were observed between C.A and G.A powders in terms of grain size, fraction of high-angle grain boundaries (HAGBs) and nanohardness. The outcomes of this study should be helpful for understanding the commercialization of the cold-spray process for bulk manufacturing of powder precursors.

Influence of Aging Temperature on Mechanical Properties and Structure of M300 Maraging Steel Produced by Selective Laser Melting.

This paper deals with the study of high-strength M300 maraging steel produced using the selective laser melting method. Heat treatment consists of solution annealing and subsequent aging; the influence of the selected aging temperatures on the final mechanical properties-microhardness and compressive yield strength-and the structure of the maraging steel are described in detail. The microstructure of the samples is examined using optical and electron microscopy. The compressive test results show that the compressive yield strength increased after heat treatment up to a treatment temperature of 480 °C and then gradually decreased. The sample aged at 480 °C also exhibited the highest observed microhardness of 562 HV. The structure of this sample changed from the original melt pools to a relatively fine-grained structure with a high fraction of high-angle grain boundaries (72%).

Research on the mechanism of Cu addition on the grain boundary characteristics and toughness in CGHAZ of low carbon low alloy steel

In this study, the coarse grained heat affected zone (CGHAZ) of Cu-containing/free steels was simulated at various heat inputs by thermal simulator. A comparative study was conducted to reveal the role of Cu addition and heat input in toughness variation by microstructural observation and crystallographic analysis. The results showed that the toughness exhibited a plateau-like variation with heat input increasing, which was mainly determined by the distribution of high angle grain boundary (HAGB). Also, a considerably higher toughness was achieved in Cu-containing steel irrespective of heat input. This attributed to the grain boundary distribution and variation selection. Cu addition weakened the variation selection and promoted the formation of more Bain groups, which helped to increase the fraction of HAGB and improve the impact toughness. In addition, Cu addition resulted in a higher fraction of boundary length in V1/V2, V1/V3, V1/V5 variant pair, which was beneficial to accommodate the transformation strain and release the residual stress, leading to a higher toughness in Cu-containing steels.

Effect of Mo content on impact toughness of CGHAZ in offshore engineering steels at different low temperatures

The effect of Mo content on impact toughness of the coarse-grained heat-affected zone (CGHAZ) in 420 MPa offshore engineering steels after 15 kJ/cm welding simulations is studied at low temperatures of −40, −60 and −80 °C. With increasing Mo content from 0.08 to 0.16 wt.%, the average size of the prior austenite grain in the CGHAZ is decreased from 150 to 70 µm, which mainly consists of lath bainite (LB). The fraction of high-angle grain boundaries (HAGBs) increases from 45.3% to 58.2%. The EBSD characterizations on the propagation and deflection of the main and secondary cracks in the CGHAZ indicate that the cleavage planes of the main crack mainly comprise of {100}, {110} and {112}. The way of fracture is changed from ductile fracture to quasi-cleavage fracture and cleavage fracture with decreasing the impact test temperature. As the Mo content is increased from 0.08 to 0.16 wt.%, the impact toughness values at −40, −60 and −80 °C are changed from 137, 101 and 40 J to 195, 117 and 88 J, respectively. With increasing the Mo content, the absorbed energies of the CGHAZ is improved because the sizes of PAG (Prior Austenite Grain) and LB are refined, and the fraction of HAGBs is increased to provide larger resistance for propagation of crack in CGHAZ.