Coarse Prior Austenite Grain Research Articles

Crystallographic characteristics and mechanical behavior of laser welded joints of hot roll bending parts

In this work, laser welding was performed on the hot roll bending parts obtained by local-induction-heating bending forming. The influence of the crystallographic characteristics of the phase transformation products in different regions of the laser welded joint on the mechanical properties was systematically studied, and the reasons for the performance degradation were discussed in detail. The results show that upper-critical HAZ (UCHAZ) forms coarse prior austenite grain (PAG) and bainite, resulting in the lowest dislocation density of 2.5 × 1014 m−2 and the lowest impact toughness value of 46 J/cm2. Sub-critical HAZ (SCHAZ) forms tempered martensite, causing the joint to soften and the tensile strength to decrease by 18%. The number of variants in SCHAZ is the lowest, where short laths with smaller misorientations merge into long laths, and precipitates nucleation and growth occurs. The uniform orientation of the variants in SCHAZ forms low-angle grain boundaries (LAGBs), which reduces the hindrance to crack propagation, so the mechanical properties of SCHAZ decrease. In addition, the texture analysis found that UCHAZ has the highest 〈001〉//RD orientation volume fraction, indicating that UCHAZ is most prone to rapid expansion of brittle cracks on the {001} cleavage plane, so the toughness of UCHAZ is reduced. SCHAZ has the highest {001}〈110〉 texture volume fraction and the lowest proportion of high-angle grain boundaries (HAGB), indicating that the deformed grain structure with the {001} cleavage planes distributed along the LAGB after welding heat tempering is the main reasons for the delamination of SCHAZ.

Microstructure development of modified 9Cr-1Mo steel during laser powder bed fusion and heat treatment

Herein, the effect of laser powder bed fusion (LPBF) parameters on the microstructure development of modified 9Cr-1Mo steel and the relationship between the as-built and heat-treated microstructures were characterized using electron backscattered diffraction (EBSD). The optimal energy density for the steel was determined to be 100–150 J/mm3. The microstructure of the as-built steel was found to be mainly composed of coarse and columnar δ-ferrite grains and fine martensite grains. δ-ferrite was formed through rapid solidification, while martensite was formed in the heat-affected zone during LPBF. A higher energy density and scan strategy with a 67 deg. rotation in each layer were found to be suitable for obtaining a higher area fraction of martensite because they can provide a larger heat-affected zone compared with a lower energy density and a 90 deg. rotation. Tempering reduced the hardness via recovery of martensite and enhanced the precipitation of M23C6 carbide at martensite, while the microstructural morphology in the as-built condition was maintained. The typical as-built microstructure disappeared after the normalizing treatment, while the size of the prior austenite grains was affected by the as-built microstructure. In-situ EBSD observation of the phase transformation behavior during normalization revealed that martensite became fine prior austenite grains, while δ-ferrite became coarse prior austenite grains. From these results, the effects of energy density and scan strategy on microstructure development were identified not only for LPBF but also during the normalizing and tempering heat treatments.

Impact of laser beam welding on mechanical behaviour of 2.25Cr–1Mo (P22) steel

The use of welding processes in the manufacturing and repair of structures intended for the energy industry plays a key role in the guarantee of a continuous supply of fossil fuels, which is the basic condition for ensuring energy security. A square butt joint of 10 mm thick plate of 2.25Cr–1Mo (P22) steel was fabricated by autogenous laser beam welding process and then post-weld heat treatment (PWHT) for two sets of process parameters (760 °C for 90 min–T1 and 730 °C for 180 min–T2). The joints were subjected to detailed metallographic investigations and hardness measurements, static tensile tests and impact toughness tests. The joint in as-welded state is characterized by significant heterogeneity of structural morphology and mechanical properties, while PWHT significantly improves uniformity in microstructure and mechanical properties along weldments. PWHT in particular causes tempering of the bainite and promotes the formation of carbide precipitates (M7C3, M23C6, and Mo2C) of globular and spherical shape along boundaries and bainite blocks. Laser welding caused a drastic increase in the weld metal hardness (388 HV) and coarse prior austenite grain bainite region (390 HV) compared to the hardness of base metal (BM) (below 200 HV). Both of the applied PWHT variants reduced the hardness of these areas of welded joints to the BM level. During the tensile tests, all the joints were fractured in BM. In the case of AW joints the tensile strength was higher than BM and it was 625 ± 10 MPa and after both PWHT, tensile strength was decreased by about 100 MPa. Due to over tempering of bainite in BM the elongation of the joints was several percentage points lower (27 ± 3 for AW joints) than the value for BM (35% ± 2), but slightly higher in the case of joints after PWHT (31 and 33 ± 1). Impact energy of the weld metal (259 ± 8 J) was lower than P22 base metal (320 ± 8 J), but PWHT causes a significant increase in impact energy of the weld metal (291 ± 6 J and 306 ± 10 J for T1 and T2, respectively). The analysis of these results shows that the proposed laser welding procedure with a high temperature gradient and PWHT enables the fabrication of P22 steel welded joints that meet the quality criteria.

Effects of prior austenite grain size on reversion kinetics of different crystallographic austenite in a low carbon steel

The effects of prior austenite grain (PAG) size on reversion kinetics of different crystallographic austenite (γ) with acicular and globular morphologies in a low carbon steel with the structure of pre-tempered martensite have been studied. It is unexpected that the coarse PAG (C-PAG) shows a strong inhibition on the reversion kinetics of globular γ, while a weak influence on that of acicular γ. Intragranular globular γ is hardly to be formed in the low carbon steel, and the grain boundary (GB) ones are dominantly formed. C-PAG provides low-density PAGBs for GB globular γ formation, leading to slower reversion kinetics. Theoretical analyses revealed that the low-density and fine cementite particles result in a low nucleation potency for globular γ thus less intragranular ones were formed. On the other hand, C-PAG leads to coarse martensite structure, which was expected to have a strong retardation on the reversion kinetics of acicular γ. However, acicular γ is mainly nucleated at block boundaries for the cases of (Fine) F-PAG, and it is almost equally nucleated at block and sub-block boundaries for C-PAG. This decreased the differences in nucleation density and growth kinetics of the acicular γ between the C-PAG and F-PAG, and the influences of PAG size on the reversion kinetics is weakened.

Tailoring Heterogeneous Microstructure in a High-Strength Low-Alloy Steel for Enhanced Strength-Toughness Balance

The attainment of both strength and toughness is of vital importance to most structural materials, although unfortunately they are generally mutually exclusive. Here, we report that simultaneous increases in strength and toughness in a high-strength low-alloy (HSLA) steel were achieved by tailoring the heterogeneous microstructure consisting of soft intercritical ferrite and hard martensite via intercritical heat treatment. The heterogeneous microstructure features were studied from the perspective of morphology and crystallography to uncover the effect on mechanical properties. Specifically, the volume fraction of martensite increased with increasing annealing temperature, which resulted in increased back stress and effective stress, and thereby an improved strength-ductility combination. The enrichment of carbon and alloying elements in the martensite was lowered with the increase in annealing temperature. As a result, the hardness difference between the intercritical ferrite and martensite was reduced. In addition, the globular reversed austenite preferentially grew into the adjacent austenite grain that held no Kurdjumov-Sachs (K-S) orientation relationship with it, which effectively refined the coarse prior austenite grains and increased the density of high angle grain boundaries. The synergy of these two factors contributed to the improved low-temperature toughness. This work demonstrates a strategy for designing heterostructured HSLA steels with superior mechanical properties.

Enhanced Strength and Toughness of Low‐Carbon Bainitic Steel by Refining Prior Austenite Grains and Austempering Below Ms

Austempering is conducted below and above martensite start temperature (Ms) at the fine and coarse prior austenite grain size (PAGS), respectively, in a low‐carbon bainitic steel, and the relationship between multiphase microstructure and mechanical properties is investigated. The study indicates that refinement of bainite laths can be obtained by decreasing the PAGS and/or austempering temperature from above to below Ms. The former reduces bainite content, and increases the volume fraction of retained austenite (RA), thereby obtaining higher uniform elongation and strain hardening exponent. However, the latter effectively decreases the size and amount of blocky RA, as well as bainite content and conversely increases the ratio of volume fraction of film RA to that of blocky RA, which leads to significant improvement in yield strength and impact toughness. In addition, the variant selection is relatively weak with the decrease in the PAGS and austempering temperature, which has a beneficial effect on toughness. Furthermore, an outstanding combination of mechanical properties (1070 MPa yield strength, 1466 MPa tensile strength, total elongation of 11.68%, and V‐notch impact toughness at 20 and −40 °C of 78 and 44 J cm−2, respectively) is achieved by austempering below Ms in the finer PAGS.

Stress corrosion crack growth rate of welded joint used for low-pressure rotor of nuclear turbine in oxygenated pure water at 180 °C

Stress corrosion cracking (SCC) growth behavior of NiCrMoV steels welded joint of nuclear steam turbine was systematically investigated, which was conducted in pure water with 8 ppm oxygen at 180 °C. Crack growth rate (CGR) was used to evaluate the susceptibility of SCC for WM with fine bainites and HAZ with coarse prior austenite grains, simultaneously the cracking mechanisms for both zones were explored. WM has lower CGR than HAZ under the similar stress intensity factor, showing it had higher resistance to SCC. Oxygen promoted the nucleation and propagation for cracks. The oxygen diffusion zone (ODZ) formed in front of inner oxide layer through continually diffusion of oxygen in grain, and caused the embrittlement and transgranular cracking for WM. However, fine grains in WM could bear crack driving force and reduce stress concentration to increase the resistance for transgranular crack propagation. As for HAZ, the chromium-rich carbides along the prior austenite grain boundaries (PAGBs) accelerated the intergranular oxidation and facilitated the formation of intergranular oxide zone (IOZ). Thus, the crack could easily propagate in PAGB, and lead to higher CGR for the HAZ.

On the heterogeneous microstructure development in the welded joint of 12MnNiVR pressure vessel steel subjected to high heat input electrogas welding

Microstructure features of 12MnNiVR pressure vessel steel welded joint deposited by the high heat input electrogas welding have been systematically investigated. It is revealed that the welded joint is featured by a heterogeneous juxtaposition. The coarse grained heat-affected zone (CGHAZ) primarily consists of lath bainites and minor granular bainites. The fine grained heat-affected zone (FGHAZ) is dominated by polygonal ferrites, pearlites, and fine cementite particles. Moreover, electron backscatter diffraction results further demonstrate that the CGHAZ is populated by coarse prior austenite grains (PAGs) with high frequency (61.3%) of low angle grain boundaries (LAGBs). On the other hand, the FGHAZ is filled with fine PAGs with a lower frequency (19.6%) of LAGBs. Such microstructural differences may likely contribute to differed mechanical properties for samples tested at designed positions.

Study of Ferrite During Refinement of Prior Austenite Grains in Microalloyed Steel Continuous Casting

The formation of coarse prior austenite grain is a key factor to promote transverse crack, and the susceptibility to the transverse crack can be reduced by refining the austenite grain size. In the present study, the high-temperature confocal laser scanning microscope (CLSM) was used to simulate two types of double phase-transformation technologies. The distribution and morphology of ferrites under different cooling conditions were analyzed, and the effects of ferrite distribution and morphology on the double phase-transformation technologies were explored to obtain the suitable double phase-change technology for the continuous casting process. The results indicate that, under the thermal cycle TH0 [the specimens were cooled down to 913 K (640 °C) at a cooling rate of 5.0 K/s (5.0 °C/s)], the width of prior austenite grain boundaries was thick, and the dislocation density at grain boundaries was high. It had strong inhibition effect on crack propagation; under the thermal cycle TH1 [the specimens were cooled down to 1073 K (800 °C) at a cooling rate of 5.0 K/s (5.0 °C/s) and then to 913 K (640 °C) at a cooling rate of 1.0 K/s (1.0 °C/s)], the width of prior austenite grain boundary was thin, and the dislocation density at grain boundaries was low. It was beneficial to crack propagation. After the first phase change, the developed film-like ferrite along the austenite grain boundaries improved the nucleation conditions of new austenitic grains and removed the inhibition effect of the prior austenite grain boundaries on the austenite grain size.

The microstructural evolution and impact toughness of nugget zone in friction stir welded X100 pipeline steel

The X100 pipeline steel was successfully friction stir welded at rotation speed of 600 rpm and welding speed of 50 mm/min using W-Re stirring tool. Microstructural analysis and Charpy v-notch impact-tests of the welds were carried out in nugget zone (NZ). In this case, the NZ exhibits inhomogeneous microstructure. Fine granular bainite was observed on the advanced side (AS) of the NZ as a result of mechanical stabilization effect, whereas the coarse lath-bainite with few granular bainite was observed on the retreating side (RS), which resulted from coarse prior austenite grains due to the lower cooling rate. Furthermore, a transition region containing intersecting bainite laths appeared between the two zones above. Generally, it was found that boron, which dissolved into the nugget zone during FSW, contributed the formation of classical lath-bainite in hard zone when FSWing pipeline steel using the PCBN stirring tool. The Charpy v-notch impact-tests results show that the whole NZ achieved good impact toughness and the poor impact toughness microstructure was successfully avoided in our study.

Microstructures and mechanical properties of TRIP steel produced by strip casting simulated in the laboratory

Conventional transformation induced plasticity (TRIP) steel (0.17C-1.52Si-1.61Mn-0.03Al, wt%) was produced via strip casting technology simulated in the laboratory. Effects of holding temperature, holding time and cooling rate on ferrite formation were studied via analysis of the continuous cooling transformation diagram obtained here. A typical microstructure for conventional TRIP steels consisting of ~ 0.55 fraction of polygonal ferrite with bainite, retained austenite and martensite was obtained. However, coarse prior austenite grain size of ~80µm led to large polygonal ferrite grain size of ~17µm, coarse second phase regions of ~21µm size, small amount of retained austenite (0.02–0.045) and the presence of Widmanstätten ferrite. Optimisation of the microstructure-property relationship was reached via a variation in the isothermal bainite transformation temperature. The highest retained austenite fraction of 0.045±0.003 with medium carbon content of 1.23±0.01wt% was obtained after holding at 400°C, resulting in the highest ultimate tensile strength of 590±35MPa and largest total elongation of 0.27±0.05. The presence of TRIP effect in the studied steel was revealed through the analysis of strain hardening exponent and modified Crussard–Jaoul model. Effect of processing parameters on retained austenite retention and stress-strain behaviour was discussed.

Effect of γ→α Phase Transformation on Refining Austenite Grains of Microalloyed Steel in Continuous Casting by Simulation

Abstract The formation of coarse prior austenite grain is a key factor to promote transverse crack, and the transverse crack susceptibility can be reduced by refining the austenite grain size. In the present study, the high-temperature confocal laser scanning microscope (CLSM) was used to simulate and study the effect of two γ→α phase transformation conditions on the refinement of the prior austenite grains. Under the condition of the uniform distribution of the second phase precipitation, the effect of the distribution of the proeutectoid ferrite at different cooling rates and refinement of prior austenite grain were investigated. The results indicate that, at a cooling rate of 5.0°C s–1, the austenite grain size undergoing TH1 thermal cycle was 31% smaller than the austenite grain undergoing TH0 thermal cycle. Under TH0 cooling system, the proeutectoid ferrite was uniformly distributed in the austenite matrix; under TH1 cooling, the proeutectoid ferrite precipitated and mainly concentrated along the austenite grain boundaries to form developed-film-like ferrite, which is favorable to break the prior austenite grain boundaries. After the first phase transformation, the film-like ferrite improved the nucleation conditions of new austenite grains, thus more new austenite grains splitted the prior austenite grains, ultimately refining the prior austenite grains.

Microstructural effects on high-cycle fatigue properties of microalloyed medium carbon steel 38MnVS

High-cycle fatigue properties of medium carbon microalloyed (MA) steel 38MnVS with different microstructural characteristics were studied by rotating bending fatigue test. Low alloy steel 40Cr with quenched and tempered (QT) microstructure was also used for comparison. The results show MA steel in the as-forged condition has the best fatigue property mainly owing to the significant precipitation strengthening of fine V(C,N) precipitates and much fine ferrite–pearlite microstructure, which is even better than that of the QT steel 40Cr. The formation of film-like ferrite along coarse prior austenite grain boundary and coarse bainitic type microstructure as well as the coarsening of the initial fine precipitates significantly deteriorated the MA steel's fatigue properties. SEM examination of fatigue fracture surfaces revealed that the fatigue fracture mechanism of MA steel is different from that of QT steel. It is concluded that microstructure of ferrite–pearlite type MA steels should be carefully controlled after rolling and/or forging to obtain improved fatigue properties.

Integrity Assessment of Grade 92 Welded Joint under Creep Condition

Integrity of tempered martensitic grade 92 (9Cr-1.8W-0.5Mo-VNb) steel welded joint has been assessed by carrying out creep test at 923K over a wide stress range. The failure location in the joint was found to shift from base metal to heat affected zone (HAZ) of the weld joint with decrease in applied stress. The HAZ of the weld joint consisted of coarse prior-austenite grain (CGHAZ), fine prior-austenite grain (FGHAZ) and intercritical (ICHAZ) regions in a sequence from the fusion boundary to unaffected base metal. The failure was associated with localized creep deformation coupled with creep cavitation. To have a better insight to the creep failure behaviour of the joint, the different microstructural constituents of HAZ of the joint has been simulated. Creep and tensile tests were carried out on the simulated HAZs specimens. The steel suffered degradation in creep rupture strength on subjecting heating in the intercritical (between Ac1 and Ac3) (ICHAZ) as well as at temperature just above the Ac3 (FCHAZ). The extensive creep cavitation has been observed in the simulated FGHAZ of the joint. Similarly, localized extensive creep cavitation and precipitation of tungsten rich Laves phase has been observed in the FGHAZ of the weld joint. This caused the type IV failure in the FGHAZ of the weld joint. The integrity of grade 92 steel joint under creep condition is governed by the above microstructural modifications of the steel in HAZ by weld thermal cycle.

Creep rupture behavior of 9Cr–1.8W–0.5Mo–VNb (ASME grade 92) ferritic steel weld joint

Creep rupture behavior of 9Cr–1.8W–0.5Mo–VNb (ASME grade 92) ferritic steel weld joint fabricated by activated TIG (A-TIG) welding process have been investigated at 923K over a stress range of 80–150MPa. The weld joint was comprise of fusion zone, heat affected zone (HAZ) and base metal. The HAZ consisted of coarse prior-austenite grain (CGHAZ), fine prior-austenite grain (FGHAZ) and intercritical (ICHAZ) regions in an order away from the fusion zone to base metal. A hardness trough was observed at the outer edge of HAZ of the weld joint. TEM investigation revealed the presence of coarse M23C6 precipitates and recovery of martensite lath structure into subgrain in the ICHAZ of the weld joint, leading to the hardness trough. The weld joint exhibited lower creep rupture lives than the base metal at relatively lower stresses. Creep rupture failure location of the weld joint was found to shift with applied stress. At high stresses fracture occurred in the base metal, whereas failure location shifted to FGHAZ at lower stresses with significant decrease in rupture ductility. SEM investigation of the creep ruptured specimens revealed precipitation of Laves phase across the joint, more extensively in the FGHAZ. On creep exposure, the hardness trough was found to shift from the ICHAZ to FGHAZ. Extensive creep cavitation was observed in the FGHAZ and was accompanied with the Laves phase, leading to the premature type IV failure of the steel weld joint at the FGHAZ.

Effect of Boron on Creep Behaviour of Inter-Critically Annealed Modified 9Cr-1Mo Steel

Abstract Two different heats of modified 9Cr-1Mo steel, one without boron (P91) and the other with controlled addition of boron (P91B) and very low nitrogen were used for the present study. Microstructure of P91 steel, annealed at 875 °C (inter-critical region) consisted of very fine prior austenite grains without lath martensite. Grain size increased with increase in heat treatment temperature and showed coarse prior austenite grains with clearly defined lath after 900 °C heat treatment. In contrast, the microstructures of P91B steel subjected to identical heat treatment did not vary much with heat treatment temperature, prior austenite grain size remaining similar to that observed in the as received material. Significant improvement in the creep properties of P91B steel compared to that of P91 steel subjected to inter-critical temperature is explained based on the difference in microstructures observed in these two steels after the heat treatment.

Analysis of martensite–austenite constituent and its effect on toughness in submerged arc welded joint of low carbon bainitic steel

Martensite–austenite (M–A) constituent formed during welding is generally recognized as an important factor to decrease the toughness of welded joint. In this article, the morphology and chemical composition of M–A constituent in the low carbon bainitic steel welded joint was analysed in detail by means of optical microscope, transmission electron microscope and scanning electron microscope with electron probe microanalysis. The experimental results show that the M–A constituent formed in the different sub-zones presents different morphologies and different amounts. The maximum amount of M–A constituent occurs in the coarse grained heat affected zone (HAZ). It is evident that the carbon atoms segregate on the M–A constituent and carbon concentration on the slender M–A constituent is higher than that on the massive M–A constituent. Meanwhile, the distribution profile of silicon on the M–A constituent shows an obvious inhomogeneity. Most of M–A constituents have a twinned structure and/or a high dislocation density. According to impact testing results, the crack initiation energy in the HAZ specimens deteriorates significantly because the large M–A constituent can assist the formation of cleavage crack. On the other hand, the coarse prior austenite grain in the HAZ lowers the crack propagation energy.

Hydrogen Embrittlement of a 1500-MPa Tensile Strength Level Steel with an Ultrafine Elongated Grain Structure

A deformation of a tempered martensitic structure (i.e., tempforming) at 773 K (500 °C) was applied to a 0.6 pct C-2 pct Si-1 pct Cr steel. The hydrogen embrittlement (HE) property of the tempformed (TF) steel was investigated by a slow strain rate test (SSRT) and an accelerated atmospheric corrosion test (AACT). Hydrogen content within the samples after SSRT and AACT was measured by thermal desorption spectrometry (TDS). The tempforming at 773 K (500 °C) using multipass caliber rolling with an accumulative are reduction of 76 pct resulted in the evolution of an ultrafine elongated grain (UFEG) structure with a strong 〈110〉//rolling direction (RD) fiber deformation texture and a dispersion of spheroidized cementite particles. The SSRT of the pre-hydrogen-charged notched specimens and the AACT demonstrated that the TF sample had superior potential for HE resistance to the conventional quenched and tempered (QT) sample at a tensile strength of 1500 MPa. The TDS analysis also indicated that the hydrogen might be mainly trapped by reversible trapping sites such as grain boundaries and dislocations in the TF sample, and the hydrogen trapping states of the TF sample were similar to those of the QT sample. The QT sample exhibited hydrogen-induced intergranular fracture along the boundaries of coarse prior-austenite grains. In contrast, the hydrogen-induced cracking occurred in association with the UFEG structure in the TF sample, leading to the higher HE resistance of the TF sample.

Effect of microstructure on the critical strain to onset of serrated flow in modified 9Cr–1Mo steel

The influence of microstructure on the strain to onset of serrated flow in modified 9Cr–1Mo steel has been studied at 573 K. The different microstructures have been developed by soaking the steel at different temperatures starts at a temperature below Ac 1 to temperature above Ac 4 followed by oil quenching and tempering at 1033 K for 1 h. Soaking the steel in the intercritical temperature range (between Ac 1 and Ac 3) reduced the hardness and tensile strength of the steel whereas soaking at temperatures above Ac 3 increased the hardness and tensile strength until the occurrence of relatively soft δ-ferrite for soaking at temperatures above Ac 4. The δ-ferrite formation reduced the hardness of the steel. The steel showed serrated flow in the load-elongation curves at 573 K in all the microstructural conditions except for those having coarse prior austenite grain. The critical plastic strain to the onset of serrated flow was found to increase with hardness of the steel, as influenced by different microstructures. The variation of hardness, tensile strength and the critical plastic strain to onset of serration flow had been rationalized on the basis of the inter-barrier spacing to dislocation motion in the steel.

Differences in impact values of core and rim in electroslag weld metal: Improvement of toughness properties in ESW weld metal in constructional steels (2nd Report)

Summary The purpose of this paper is to investigate the impact properties of weld metal produced under different welding conditions with special reference to electroslag welding (ESW) as a high heat input welding process generally applied in the fabrication of four‐sided thick‐plate box columns for general multi‐storey buildings. The paper focuses in particular on the impact properties of ESW weld metal at its centre (core (C)) and periphery (rim (R)). The results obtained may be summarised as follows: The vE value of the weld metal core is lower than that of the weld metal rim. The absorbed energy transition temperature is also higher. The vE values of the weld metal core and rim altered with a change in the welding heat input Q (varied at six levels in the 10.1 ∼ 126.7 kJ/mm range), generally decreasing with an increasing heat input. The vE value of the core is lower than that of the rim. In the weld metal produced at the maximum heat input (126.7 kJ/mm), however, the core and rim have much the same vE values. The vE value of the weld metal core shows little change with a change in the steel type (one type of TMCP and two types of SM490A), and remains at a low value. When there is a change in the surrounding gas (oxygen (O2), air, and argon gas), the vE value decreases in an Ar, air, O2 sequence. The weld metal core vE value is little dependent on any change in the flux type (three types of fused flux) and weight of flux used (0.5 or 0.7 N). The microstructural observations suggest that grain‐boundary ferrite (GBF) occurs at the coarse prior austenite grain boundaries of the rim, with fine acicular ferrite (AF) being mainly found trans‐granularly. In the core, grain‐boundary ferrite is formed at high density at the fine prior austenite grain boundaries, with massive polygonal ferrite (PF) being formed at high density transgranularly. There are thus distinct differences in the coarse ferrite morphology of the rim and core. The SEM observations of the fracture surfaces suggest that the core weld metal has a large grain size, a feature corresponding to its low vE value. The observations made at the fracture surface periphery indicate that numerous secondary cracks are initiated in the grain‐boundary ferrite of the core, suggesting that the ferrite has a low toughness. The results of the SEM simultaneous fractographic‐microstructural observations suggest that fracture selectively propagates along the grain‐boundary ferrite. This indicates that the low vE value of the core is due to the presence of high‐density grain‐boundary ferrite and massive transgranular polygonal ferrite.