Search Results (64)

This study addresses the constitutive relationship of the welded coarse-grained heat-affected zone (CGHAZ) in 80-mm-thick DH36 marine steel plates. By integrating quasi-static tensile testing, digital image correlation (DIC) technology, and metallographic analysis, we systematically investigated the mechanical property differences and underlying mechanisms between the CGHAZ and base metal (BM). High-precision DIC technology enabled strain field characterization at the microscale in the CGHAZ, while the Ramberg-Osgood model was adopted to establish a dual-material constitutive equation. The results demonstrate that grain coarsening induced by welding thermal cycles significantly influenced the mechanical responses: the CGHAZ exhibited enhanced tensile strength but reduced plastic compatibility due to decreased grain boundary density. Notably, gradient differences in elastic modulus (CGHAZ: 184 GPa vs. BM: 213 GPa) and yield strength (CGHAZ: 363 MPa vs. BM: 373 MPa) between the BM and CGHAZ necessitate strict differentiation in engineering design. This work overcomes the limitations of oversimplified CGHAZ properties in conventional design approaches, providing a novel methodology for strength assessment and lightweight design of marine structures. The findings offer critical theoretical insights and practical guidelines for enhancing the reliability of offshore engineering equipment. Full article

The martensitic transformation mechanism in the heat-affected zone of DP1180 steel plays a decisive role in the strength of welded joints. In this work, the nucleation and growth kinetics of martensite laths in the coarse grain heat-affected zone (CGHAZ) are analyzed by a high-temperature laser scanning confocal microscope (LSCM). The grain distribution and stress distribution of the samples after in situ observation are analyzed by electron backscatter diffraction (EBSD). The results reveal that austenite grain growth is realized by continuous grain boundary annexation and grain boundary migration of small grains by large grains during the heating process. Seven growth modes of CGHAZ martensitic laths under laser welding conditions are proposed. Additionally, the end growth of martensitic laths is mostly attributed to the collision with grain boundaries or other laths to form a complex interlocking structure. The results of this study could provide important data support for the development of dual-phase steel materials and improvement of welding quality. Full article

This study investigates the effects of molybdenum (Mo) and boron (B) on the microstructures and impact properties in the coarse grain heat-affected zone (CGHAZ) of a low-carbon V-Ti-N steel. The results demonstrate that, at a heat input of up to 75 kJ/cm, the addition of Mo alters the microstructure of the CGHAZ, transforming it from a polygonal ferrite (PF) + degraded pearlite (DP) composition to a granular bainite (GB) + a small amount of acicular ferrite (AF). This transformation increases the impact energy from 20 J to 35 J. Furthermore, with the Mo/B composite addition, the CGHAZ microstructure was refined due to the formation of a large number of acicular ferrites, and the mean equivalent diameter (MED_MTA≥15°) decreased from 7.9 μm to 4.2 μm. Consequently, the impact toughness of the CGHAZ increased from 35 J to 111 J. The correlation between the Mo/B elements, microstructure and impact toughness was investigated in detail. The findings of this study have the potential to generate novel ideas for the advancement of steel materials in the context of high heat input welding. Full article

In order to find the optimal heat input for simulating the welding of the coarse-grained heat-affected zone (CGHAZ) of a novel Q690 MPa V-N microalloyed medium and heavy plate, the study investigated the precipitation of V (C, N), microstructural changes, and impact toughness under five different heat inputs (E). The results show that in the CGHAZ, as the heat input increases, the dominant microstructure changes from intragranular acicular ferrite (IGAF) and lath bainitic ferrite (LBF) to polygonal ferrite (PF) and a small amount of IGAF. At the same time, the area fraction of the brittle phase martensite/austenite (M/A) constituents increased from 4.96% to 7.95% as heat input increased, and the microhardness difference between the M/A constituents and the matrix significantly increased. In addition, with the E increases, the fraction of high-angle grain boundaries (HAGBs), which can hinder crack propagation, increases from 59.2% to 62.2% and then decreases from 62.2% to 49.3%. Moreover, the impact toughness of the simulated CGHAZ of the Q690 MPa V-N microalloyed medium and heavy plate first increases from 62 J to 100 J and then decrease to 20 J. Full article

High-strength low-alloy (HSLA) steels have garnered significant attention owing to their widespread applications across various industries, with weldability being a particularly critical aspect. However, the impact toughness of the coarse-grained heat-affected zone (CGHAZ) remains a notable challenge under high-heat-input welding conditions. Despite existing research acknowledging the beneficial effects of micro-alloying elements on steel properties, there are still numerous uncertainties and controversies regarding the specific influence of these elements on the microstructure and impact toughness of the CGHAZ under specific welding conditions. To address this issue, this study presents a comprehensive review of the impact of common micro-alloying elements on the microstructure and toughness of the CGHAZ during high-heat-input welding. The results indicate that elements such as cerium, magnesium, titanium, vanadium, nitrogen, and boron significantly improve the toughness of the CGHAZ by promoting intragranular nucleation of acicular ferrite and inhibiting the coarsening of austenite grains. In contrast, the addition of elements such as aluminum and niobium adversely affect the toughness of the CGHAZ. These findings offer crucial theoretical guidance and experimental evidence for further optimizing the welding performance of HSLA steels and enhancing the impact toughness of the CGHAZ. Full article

Electron beam welding (EBW) is one of the most highly precise methods that is gaining more importance in high-strength structural steel (HSSS) thicker plate application in various vehicles, construction industries, etc. Since it offers particular advantages over arc welding processes like narrow welds, reduced heat-affected zone (HAZ), and low distortion, it inherits lower linear heat input characteristics. The main purpose of this study is to analyze and compare the effect of localized electron beam–post-weld heat treatment (LEB-PWHT) with that of an as-welded EB-welded S960QL joint of a thickness of 12 mm for various joint and HAZ properties. LEB-PWHT can be beneficial in terms of time saving, more local treatment, higher flexibility, energy saving, greater efficiency, increased productivity, etc. In this study, LEB-PWHT was applied to an autogenous EB-welded S960QL joint using a defocused beam. Microstructural characteristics were observed through light optical and scanning electron microscopy (SEM) while mechanical properties, including microhardness, tensile strength, bending, and Charpy V-notch (CVN) impact test, are compared in as-welded and LEB-PWHT joints. The microstructural results showed that the EBW coarse-grain heat-affected zone (CGHAZ) consists of martensite, while the PWHT weld metal contains tempered martensite with carbide precipitates. The fine-grain heat-affected zone (FGHAZ) of EBW exhibits a martensitic and bainitic microstructure, whereas the FGHAZ of the PWHT joint exhibits equiaxed grain with finely dispersed carbides. The hardness decrease after LEB-PWHT in the weld metal and HAZ was approximately 23% and 21%, respectively. An increase in tensile strength (3%) was observed in the LEB-PWHT joints (1082 MPa) compared to the EBW joint (1051 MPa). Both tensile and bending tests demonstrated improved ductility behavior after PWHT. However, the impact test at −40 °C indicated a reduction in toughness in the weld metal of LEB-PWHT (27 J) compared to EBW (63 J). Full article

To reduce manufacturing costs, energy companies aim to maximize the deposition rate during welding operations by increasing the interpass temperature (IT), thereby minimizing the cooling time. However, IT can significantly affect weldment performance, particularly its Charpy V-notch (CVN) impact energy (toughness). The present study investigates the effect of increasing IT beyond the limit specified by the ASME B31.3 (315 °C) on the CVN impact energy (−30 °C) of the simulated coarse-grained heat-affected zone (CGHAZ) of a 2.25Cr-1Mo steel submerged arc welded (SAW). The CGHAZ thermal cycles were obtained through finite element method simulations and physically replicated using a Gleeble machine. The increase in IT beyond the ASME-specified limit significantly reduces the CVN impact energy of the CGHAZ. However, the values obtained remained above the minimum required threshold (NORSOK M630, 42 J). The main effect of increased IT was grain coarsening. Additionally, an inverse linear relationship was observed between effective grain size (EGS) and CVN impact energy. The steel’s microstructure showed non-significant sensitivity to variations in IT within the studied range. These findings suggest that, under the conditions studied, increasing IT could be a viable option for optimizing production by reducing welding time and potentially lowering costs. Full article

In this work, the microstructure–property relationship of the heat-affected zone (HAZ) of a FH690 ultra-heavy marine steel plate was investigated based on insight of microstructure and crystallographic features. After multi-pass welding with a heat input of ~30 kJ/cm, an ~8 mm wide HAZ was obtained with a coarse grain HAZ (CGHAZ) of ~3.8 mm, fine grain HAZ (FGHAZ) of ~3.4 mm, and intercritical HAZ (ICHAZ) of ~1 mm. High impact toughness values of ~120 and 140 J at −60 °C were obtained for coarse grain HAZ and fine grain HAZ, respectively. The microstructure of the CGHAZ and FGHAZ was fine lath bainite. Although the average prior austenite grain size for the CGHAZ was ~75 μm, which was five times that of the FGHAZ (15 μm), a high density of high-angle grain boundaries (HAGBs) with misorientation higher than 45° was obtained in the CGHAZ. This is the underlying reason for the excellent low-temperature toughness of the HAZ. Thermo-dynamic calculations indicated that the high density of HAGBs in the CGHAZ was attributed to the decreased bainitic transformation temperature due to the reduced phase transformation driving force via the high nickel addition, leading to weak variant selection. In addition, the high nickel addition offered high hardenability for high hardness in the FGHAZ. The outcome of this study could provide an experimental and fundamental basis for designing high-strength ultra-heavy steel plates with excellent weldability. Full article

X80 steel plays a pivotal role in the development of oil and gas pipelines; however, its welded joints, particularly the heat-affected zone (HAZ), are susceptible to stress corrosion cracking (SCC) due to their complex microstructures. This study investigates the SCC initiation mechanisms of X80 steel welded joints under practical pipeline conditions with varying levels of cathodic protection. The SCC behaviors were analyzed through electrochemical measurements, hydrogen permeation tests, and interrupted slow strain rate tensile tests (SSRTs) conducted in a near-neutral pH environment under different potential conditions (OCP, −1.1 V_SCE, −1.2 V_SCE). These behaviors were influenced by microstructure type, grain size, martensite/austenite (M/A) constituents, and dislocation density. The sub-zones of the weld exhibited differing SCC resistance, with the fine-grain (FG) HAZ, base metal (zone), welded metal (WM) zone, and coarse-grain (CG) HAZ in descending order. In particular, the presence of coarse grains, low dislocation density, and extensive M/A islands collectively increased corrosion susceptibility and SCC sensitivity in the CGHAZ compared to other sub-zones. The SCC initiation mechanisms of the sub-zones within the X80-steel welded joint were primarily anodic dissolution (AD) under open-circuit potential (OCP) condition, shifting to either hydrogen-enhanced local plasticity (HELP) or hydrogen embrittlement (HE) mechanisms at −1.1 V_SCE or −1.2 V_SCE, respectively. Full article

The effect of pre-weld heat treatment on the microstructure and low-temperature impact toughness of the coarse-grained heat-affected zone (CGHAZ) after simulated welding was systematically investigated through the utilization of scanning electron microscopy (SEM) and electron back-scattering diffraction (EBSD). The Charpy impact test validated the presence of an optimal pre-weld heat treatment condition, resulting in the highest impact toughness observed in the CGHAZ. Three temperatures for pre-weld heat treatment (690, 720 and 750 °C) were used to obtain three different matrices (Steel 1, Steel 2, Steel 3) for simulated welding. The optimal pre-weld heat treatment is 720 °C for 15 min followed by water quench. Microstructure characterization showed that there is an evident microstructure comprising bainite (B) in Steel 1 and Steel 2 after pre-weld heat treatment, while the addition of martensite (M) with the pre-weld heat treatment temperature exceeds Ac₁ by almost 60 °C (Steel 3). These differences in microstructures obtained from pre-weld heat treatment influence the refinement of high-temperature austenite during subsequent simulated welding reheating processes, resulting in distinct microstructural characteristics in the CGHAZ. After the optimal pre-weld heat treatment, Steel 2 subjected to single-pass welding thermal simulation demonstrates a refined microstructure characterized by a high density of high-angle grain boundaries (HAGBs) within the CGHAZ, particularly evident in block boundaries. These boundaries effectively prevent the propagation of brittle cracks, thereby enhancing the impact toughness. Full article

Single-pass-welding thermal cycles with different peak temperatures (T_p) were reproduced by a Gleeble 3800 to simulate the heat-affected zone (HAZ) of a Fe-24Mn-4Cr-0.4C-0.3Cu (wt.%) high manganese austenitic steel. Then, the effect of T_p on the microstructure and mechanical properties of the HAZ were investigated. The results indicate that recrystallization and grain growth play dominant roles. Based on this, the HAZ is proposed to categorize into three zones: the recrystallization heat-affected zone (RHAZ) with a T_p of 700~900 °C, the transition heat-affected zone (THAZ) with a T_p of 900~1000 °C, and the coarse grain heat-affected zone (CGHAZ) with a T_p of 1000~1300 °C. The recrystallization fraction was 29~44% in the RHAZ, rapidly increased to 87% in the THAZ, and exceeded 95% in the CGHAZ. The average grain size was 17~19 μm in the RHAZ, slightly increased to 22 μm in the THAZ, and ultimately increased to 37 μm in the CGHAZ. The yield strength in the RHAZ and THAZ was consistent with the change in recrystallization fraction, while in the CGHAZ, it satisfied the Hall–Petch relationship with grain size. In addition, compared with the base material, the Charpy impact absorbed energy at −196 °C decreased by 22% in the RHAZ, but slightly increased in the CGHAZ. This indicates that the theory of fine grain strengthening and toughening is not entirely applicable to the HAZ of the investigated high-Mn steel. Full article

This article uses scanning electron microscopy (SEM) and electron back-scattering diffraction (EBSD) to study the effect of C and Mn segregation on the microstructure and mechanical properties of high-strength steel with 20 mm thickness used for wind power before and after simulated welding. A Gleeble-3500 (GTC, Dynamic Systems Inc., Poestenkill, NY, USA) was used to study the microstructure evolution of the simulated coarse-grained heat-affected zone (CGHAZ) of experimental steel under different welding heat inputs (10, 14, 20, 30 and 50 kJ/cm) and its relationship with low-temperature impact toughness (−60 °C). The results indicate that alloy element segregation, especially Mn segregation, significantly affects the impact toughness scatter of the steel matrix, as it induces the formation of low-temperature martensite or hard phase, such as M/A (martensite/austenite) constituent. In addition, segregation also reduces the low-temperature impact toughness of the simulated welding samples and increases the fluctuation range. For high-strength steel with yield strength higher than 460 MPa used for wind power generation, there is an optimal welding heat input (~20 kJ/cm), which enables the simulated coarse-grained heat-affected zone (CGHAZ) to obtain the highest impact toughness due to the formation of lath bainite (LB) and the finest crystallographic block units. Excessive or insufficient heat input can induce the formation of coarse granular bainite (GB) or lath martensite (LM), leading to a larger size of crystallographic block units, reducing the hindering effect of brittle crack propagation and deteriorating low-temperature impact toughness. Full article

This study investigates the impact of a high-entropy alloy filler metal coated with copper (Cu) and post-weld heat treatment (PWHT) on the weldability of a ferrous medium-entropy alloy (MEA) in gas tungsten arc welding. The addition of 1-at% Cu had an insignificant effect on the microstructural behaviour, despite a positive mixing enthalpy with other elements. It was observed that a small amount of Cu was insufficient to induce phase separation into the Cu-rich phase and refine the microstructure of the as-welded specimen. However, with an increase in the PWHT temperature, the tensile strength remained mostly consistent, while the elongation significantly increased (elongation of as welded, PWHT700, PWHT800, and PWHT 900 were 19, 43, 55 and 68%, respectively). Notably, the PWHT temperature of 900 °C yielded the most desirable results by shifting the fracture location from the coarse-grained heat-affected zone (CGHAZ) to base metal (BM). This was due to significant recrystallisation and homogenised hardness of the cold-rolled BM during PWHT. However, the CGHAZ with coarse grains induced by the welding heat input remained invariant during the PWHT. This study proposes a viable PHWT temperature (900 °C) for enhancing the weldability of cold-rolled ferrous MEA without additional process. Full article

The low-temperature fracture toughness of double-V weld seams is a well-known challenge due to the essential increased heat input for heavy-wall pipelines. A thorough investigation was conducted to explore the impact of the heat input on the grain size and precipitate coarsening, correlating the microstructure with the heat-affected-zone (HAZ) toughness. The results indicated that the actual weldments showed a toughness transition zone at −20 °C, with considerable scattering in Charpy V-notch (CVN) tests. Gleeble thermal simulations confirmed the decreased toughness of the coarse-grained HAZ (CGHAZ) with increasing heat input and prior austenite grain size (PAGS). A specially designed thermal treatment demonstrated its potential for enhancing the toughness of the CGHAZ, with the recommended thermal cycle involving peak temperatures of 700 and 800 °C, holding for 1 s, and rapid cooling. The toughness of the intercritically reheated CGHAZ (ICCGHAZ) improved with higher intercritical reheating temperatures and the removal of necklace-type M–A constituents along the PAG. Despite various thermal treatments, no significant improvements were observed in the toughness of the ICCGHAZ. Future work was suggested for optimising the use of tack welds to reduce the effective heat input (HI) associated with double-sided submerged arc welding (SAW). Full article

The mechanical properties of a coarse-grained heat-affected zone (CGHAZ) are affected by welding thermal cycling with varied heat input (E_j), but its effect on tensile properties is rarely studied. In the present work, E_j = 15, 35, 55, 75 kJ/cm CGHAZ samples were prepared via Gleeble^TM (St. Paul, MN, USA) for a novel V-Ti-N microalloyed weathering steel. The tensile properties of CGHAZ with varied E_j were evaluated. The results indicated that mixed microstructures dominated by lath bainitic ferrite (LBF) and granular bainitic ferrite (GBF) were obtained at E_j = 15 and 35 kJ/cm, respectively, while a mixed microstructure composed of GBF, intragranular acicular ferrite (IGAF), and polygon ferrite (PF) formed at E_j = 55 and 75 kJ/cm, apart from martensite/austenite (M/A) constituents in each E_j condition. The above variation tendency in the microstructure with the increase in E_j led to coarsening of low-angle grain boundaries (LAGBs) and a decrease in dislocation density, which in turn resulted in a yield strength (YS) decrease from 480 MPa to 416 MPa. The mean equivalent diameter (MED), defined by the misorientation tolerance angles (MTAs) ranging from 2–6°, had the strongest contribution to YS due to their higher fitting coefficient of the Hall–Petch relationship. In addition, the increase in the average size (d_M/A) of M/A constituents from 0.98 μm to 1.81 μm and in their area fraction (f_M/A) from 3.11% to 4.42% enhanced the strain-hardening stress. The yield strength ratio (YR) reduced as the E_j increased, and the lower density and more uniform dislocations inside the ferrite led to a uniform elongation (uE) increase from 9.5% to 18.6%. Full article