Volume Fraction Of Acicular Ferrite Research Articles

Tuning heterogeneous microstructure and improving mechanical properties of twin-roll strip cast low carbon steels by on-line heat treatments

Coarse austenite grains are unavoidable in the low carbon steels processed by twin-roll strip casting. Unfortunately, the polygonal ferrite (PF) grains cannot be refined by conventional on-line cooling process, resulting in low strength. In this work, an on-line heat treatment including reaustenitizing and subsequent controlled cooling was performed in laboratory to form heterogeneous microstructures composed of PF and acicular ferrite (AF). It has been founded that increased prior austenite grain size and the cooling rate led to more AF laths, which inhibited the growth of PF grains. When the prior austenite grain size is less than 60 μm, the formation of AF is suppressed. The tensile tests indicate that the heterostructured steels exhibited higher yield strength than the samples without AF. The improvement of yield strength can be attributed to the grain boundary strengthening, dislocation strengthening and back stress strengthening. The higher AF volume fraction leads to higher dislocation density in PF grains, and more AF/PF interfaces may increase the amount of accumulated geometrically necessary dislocations. Therefore, the contributions of dislocation and back stress strengthening increase with the increase of AF volume fractions. High dislocation density and the generation of back stress contribute a high strain hardening rate to delay the necking and sustain good plasticity. Moreover, the heterostructured steels exhibit the two-stage work hardening behavior during tensile test, which indicates that plastic deformation starts in PF grains while the AF laths remain in elastic state, and then plastic deformation occurs in both strain hardened PF grains and AF laths.

Crack-Tip Opening Displacement of Girth Welds in a Lean X70 Pipeline Steel.

Crack-tip opening displacement (CTOD) tests were conducted on girth welds of two API 5L X70 pipeline steels (pipe A and pipe B) to investigate the influence of base metal composition on the fracture toughness of the joint. CTOD measurements across the weld showed that the weld fusion zone had the lowest CTOD values for both pipes, with pipe B having a higher CTOD value than pipe A. Detailed microstructure characterization of the multi-pass weld showed that the fusion zone in both pipes consisted of three distinct zones: the columnar zone, the coarse equiaxed zone, and the fine equiaxed zone. Both the columnar zone and coarse-grained equiaxed zone had acicular ferrite and grain boundary ferrite microstructures, whereas the fine-grained equiaxed zone had a finer ferrite microstructure compared to the other two zones. The main difference between the two pipes was the variation in ferrite grain sizes and the volume fractions of grain boundary ferrite and acicular ferrite. In comparison to pipe B, pipe A, with a higher concentration of Mo, Ni, and Cu in both the base metal and the weld fusion zones, consisted of a higher volume fraction of grain boundary ferrite and a lower volume fraction of acicular ferrite in the columnar and coarse-grained equiaxed zones. The lower concentration of Mo, Ni, and Cu in pipe B likely resulted in the formation of a predominantly acicular ferrite microstructure in the fusion zone, thereby improving the toughness of the weld joint in comparison to pipe A.

ENHANCING THE CRYOGENIC IMPACT TOUGHNESS OF FERRITIC STEEL BY MULTI-TYPE AND FINE-MICROSTRUCTURE DESIGN

Based on pure melting, multi-type and fine-microstructure design, a ferritic steel with an outstanding combination of cryogenic impact toughness and strength was fabricated. The microstructure was characterized by means of optical microscopy, electron back-scattered diffraction and transmission electron microscopy. The steels with different final cooling temperatures of 663 °C and 560 °C show ferrite and perlite microstructure, and ferrite grain size can be refined from 3.15 µm to 2.67 µm by lowering the final cooling temperature from 663 °C to 560 °C. Moreover, a higher volume fraction of acicular ferrite and polygonal ferrite can be obtained for a lower final cooling temperature of 560 °C. Hence, the cryogenic impact toughness can be enhanced, and the brittle failure can be suppressed, even at –110 °C, showing a higher impact toughness compared with other ferritic steels.

Correlation between Weld Metal Microstructure Evolution and Welding Process Parameters of Low Alloy Ferritic Steel Weldments

This study attempts to propose a convenient method to adjust the welding process parameters window with the aim of high volume fraction of acicular ferrite in the weld metal (WM). The samples are directly extracted from the deposition zone of two low alloy ferritic steel dissimilar welded joints (DWJs) that manufactured using different consumables, and their microstructural evolution is controlled using thermomechanical simulation technique. The results show that it is hard for the low‐alloyed WM to obtain high amount of acicular ferrite no matter how to control the cooling process, meaning that the welding consumable with only a small amount of alloying elements is not suitable for welding this DWJ. However, for the high‐alloyed WM, the acceptable window of the cooling process is determined within 10–22 °C s−1 to achieve aimed microstructure. In situ observation shows that the preexisting bainitic laths significantly reduce the growth space of acicular ferrite plates. The average lengthening rate of acicular ferrite plates is ranged from ≈5 to ≈68 μm s−1 at a cooling rate of 3 °C s−1.

Characterization of weld joints produced by continuous wave and double pulse gas metal arc welding in naval grade high-strength low-alloy steel

The current research examines gas metal arc welding (GMAW) joints generated in naval grade high-strength low-alloy (HSLA) steel using both continuous wave GMAW (CW-GMAW) and double pulse GMAW (DP-GMAW). The joint was created in four passes with DP-GMAW, whereas CW-GMAW required eight passes. Weld joints were characterized using various techniques, including optical microscopy (OM), scanning electron microscopy (SEM), residual stress analysis, and mechanical evaluation. The mechanical evaluation consisted of microhardness, impact, and tensile tests. The Charpy V-notch (CVN) impact test was performed at room temperature and −60°C. For both CW-GMAW and DP-GMAW, the heat-affected zone and the fusion zone were harder than the base metal. The fracture occurred in the base metal region in all the tensile test specimens containing the weld joints. The DP-GMAW joints showed higher impact toughness at both room temperature and −60°C. In comparison to the fusion zone of the CW-GMAW joint, the fusion zone of the DP-GMAW joint had a greater volume fraction of acicular ferrite, a finer grain size, and a higher percentage of high-angle grain boundaries. These factors have contributed, it is believed, to the higher impact toughness of the DP-GMAW joint.

Effects of Mo and Nb on Microstructure and Charpy Impact Properties of CGHAZ in HSLA Steels

In this study, the effects of Mo and Nb on the microstructure and Charpy impact properties of coarse-grain heat affected zone (CGHAZ) in high-strength low-alloy (HSLA) steels were investigated. The high-Mo steel had a lower volume fraction of acicular ferrite (AF) and higher volume fraction of granular bainite (GB) than those of the low Mo steel. The high-Mo CGHAZ specimen had a lower volume fraction of AF and higher volume fraction of GB compared to the low-Mo CGHAZ specimen. This is because, an increase in Mo lowers the bainite transformation temperature, induces the formation of GB and bainitic ferrite (BF), and reduces the diffusion velocity of carbon, thereby inhibiting the formation of ferrite. The Nb-added steel had a volume fraction similar to that of AF but smaller packet sizes of GB compared to the non-Nb steel. Compared to the non-Nb CGHAZ specimens, the Nb-added CGHAZ specimen had a lower volume fraction of AF, a higher volume fraction of GB, smaller packet sizes of GB and BF, and a smaller prior austenite grain size. This is because, Nb forms Nb(C,N) to inhibit the grain growth, thus inhibiting the formation of GB and BF. The Nb-added low-Mo CGHAZ specimen with the highest AF fraction and smallest average grain size had the highest Charpy-absorbed energy. However, articles in the Journal of Welding and Joining reported that the other CGHAZ specimens with high volume fractions of GB and BF with coarse packets showed low Charpy-absorbed energy.

Effect of forging ratio on tensile properties and fatigue performance of EA4T steel

In this study, EA4T axle steel was hot-forged using the forging ratios of 2.0, 3.0, 4.0, and 5.0. Effects of the forging ratio on microstructure, tensile properties, and fatigue performance of the steel were investigated. The microstructure was examined by laser scanning confocal microscopy. The fatigue fracture of the forged specimens was observed by scanning electron microscopy. The hardnesses of the billet and the forged samples were measured by a Vickers Indenter. Phase composition and grain size distribution were analyzed. The forged samples using the forging ratio of 4.0 exhibited excellent hardness, mechanical properties, and fatigue behavior. The effects of ferrite volume fraction, acicular ferrite volume, and polygonal ferrite grain size on the mechanical properties were discussed. The relationship between the mechanical properties and the ferrite volume fraction, acicular ferrite volume fraction as well as polygonal ferrite grain size was obtained. According to the analysis of the polygonal ferrite grain size, acicular ferrite, pearlite colonies, and insufficient plastic deformation zone affecting the mechanism of fatigue failure, the effect of the forging ratio on the fatigue life was explored.

Influence of Inclusions on the Nucleation of Acicular Ferrites in a Ti–Zr‐Bearing Steel

Laboratory experiments are conducted to study the influence of inclusions on the nucleation of acicular ferrite in Ti–Zr‐bearing steels. The ferrite transformation in steels with different Ti–Zr contents is observed using a high‐temperature confocal laser scanning microscope (HT‐CLSM). The average composition and size of inclusions and the information about the nucleation of acicular ferrite (AF) are analyzed using a scanning electron microscope. When the content of titanium in the steel increased from 70 to 370 ppm, the main composition in the inclusion transformed from Al2O3 to Ti2O3, the volume fraction of AF increased from 24.4% to 52.5%. The average content of ZrO2 in inclusions is 0%, 18.51%, and 90.39%, when the Zr content of steel is 0 80, and 140 ppm, the volume fraction of AF is 52.5%, 76.9%, and 56.8%. The relationship between the cosine of the contact angle at the inclusions‐ferrite interface and the diameter of inclusions is established based on experimental results, which is consistent with the results calculated based on the classic nucleation theory.

Effect of Ti content on the inclusions, microstructure and fracture mechanism of X100 pipeline steel laser-MAG hybrid welds

Laser-MAG hybrid welding has a wide range of application prospects for pipeline steel welding, because it combines the advantages of arc welding and laser welding. However, the brittle microstructure of bainite or martensite in hybrid welding results in low impact toughness, and thus it cannot be used in pipeline laying. Hence, in this study, we induced acicular ferrite (AF) into laser-MAG hybrid welds, because of its good impact toughness. We prepared X100 steel welds with different Ti content using laser-MAG hybrid welding. Then, the microstructure evolution, impact toughness and fracture mechanism of the welds with different Ti content were investigated. The results illustrated that with increasing Ti content, the outer layer inclusion composition evolved from mostly Al2O3 to Ti2O3 and finally to TiC. Thus, Ti2O3 induced AF nucleation by forming an Mn-depleted zone, which caused the volume fraction of AF to initially increase and then decrease. The impact toughness also changed accordingly (16.3 → 21.9→8.7J) and the cracks nucleated and propagated in granular bainite during impact fracture. When they encountered AF, the cracks deflected and even stopped propagating. Therefore, by controlling the Ti content in the laser-MAG hybrid weld, the impact energy at −40 °C was higher than in the base metal. In addition, the higher quantity of AF with a high effective grain boundary density (0.57/μm) and low kernel average misorientation value could effectively hinder crack propagation and consequently improve impact toughness.

Microstructural and crystallographic characterization of 455 MPa sour service plates with low Mn high temperature processed alloy concept and its association with drop weight tear test performance

The aim of this study was to analyze the mechanical properties of a X65 low Mn and high Nb steel for sour service application, particularly the drop-weight-tear test (DWTT) performance in order to establish correlation with the microstructural and crystallographic features of a steel with complex microstructure. Samples were analyzed at 55%, 65% and 75% of deformation during the finishing stage of rolling. The characterization of the samples involved electron backscatter diffraction (EBSD) technique for the determination and volume fraction quantification of the microconstituents and crystallographic texture analyses. The results showed that the plate with 55% of deformation displayed the lowest level of strength and the lowest DWTT shear area, while the plate with 65% of deformation exhibited the highest strength and DWTT toughness levels. EBSD results indicated that the plate produced with 65% of deformation had the largest dislocation density among all three samples and showed the highest intensity of the texture components concentrated along the α-fiber, particularly in the vicinity of the {113}<110> and {112}<110> components, responsible for activating ductile fracture, while samples 55% and 75% had a broader spread towards {001}<110>, and an overall higher intensity of the rotated cube components that favor cleavage fracture. For the plate with 75% of deformation, the mean flow stress (MFS) result suggested that partial recrystallization occurred during the finishing stage of hot rolling which was also proved by EBSD.These findings have proven that low Mn steels processed at high temperatures have a crucial dependency on Nb for the development of deformation textures that will help sustain high levels of fracture toughness, particularly when dealing with heavy plates for offshore applications. It has been observed that this steel concept is inclined to experience loss of toughness due to the softening of austenite during the finishing stage of rolling, even though the temperatures were below the no-recrystallization temperature, and a proper rolling schedule must be selected to mitigate this effect.The overall results showed that a rolling scheduled aiming at increasing the volume fraction of acicular ferrite and reducing the polygonal ferrite together with a higher level of strain and crystallographic texture consisting of high intensity deformation components, will result in a better combination of strength and toughness.

Effect of cooling route on microstructure and mechanical properties of twin-roll casting low carbon steels with an application of oxide metallurgy technology

The oxide metallurgy was employed in the fabrication of twin-roll strip casting low carbon steels. The quantitative dependences of the microstructure and mechanical properties on cooling parameters were revealed and emphatically investigated. After twin-roll strip casting, a two-step cooling route closed to the actual process was designed and the cooling rates exhibited great influences on the proportion of polygonal ferrite (PF) and acicular ferrite (AF). With increasing cooling rates in both steps, the AF volume fraction raised from 0.04 to 0.75 because the rapid cooling restrained the formation of PF. The AF laths were induced by fine Al–Si–Ti–Mn composite oxides (0.2–2 μm), leading to a heterogeneous microstructure consisting of PF and AF clusters with high proportion of large-angle (≥15°) grain boundaries. It was found that the AF laths kept K–S relationship with austenite and AF laths with collinear growth direction belong to the same codirectional variants group in one AF cluster. With the increase of AF volume fraction, the yield strength (YS) and the ultimate tensile strength (UTS) were improved, while the strip retained excellent elongation. The improved YS was mainly ascribed to the grain boundary strengthening and dislocation strengthening.

Effect of inclusions on microstructure and mechanical behavior of multi-pass welded naval grade steel

This paper assesses the metallurgical characteristics and mechanical properties of multi-pass gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) of micro-alloyed steel DMR 249-A. The prime objective was to carry out a comparative study of the microstructure and mechanical properties of the joints produced by the two types of welding. A high volume of larger sized inclusions in GMAW contributed to inferior mechanical properties. The coarse-grained part of the heat-affected zone (CGHAZ) showed a lower microhardness. Fracture always occurred in the heat-affected zone, and it is believed that it is associated with CGHAZ. GTAW joints showed low tensile residual stress, higher hardness, and tensile strength. GTA weldment also showed superior impact toughness at sub-zero temperature (–60 °C). Mn-containing inclusions were seen in GTA weldments; it is believed that they promote the formation of acicular ferrite. This is believed to be responsible for the superior mechanical properties of GTA weldments. The microstructural analysis of the two weldments revealed the presence of a higher volume fraction of acicular ferrite in the GTAW. All in all, GTAW joint was found to perform better than GMAW joint.

Correlation between Microstructure and Mechanical Properties of Base Metal and HAZ of 500 MPa Steel Plates for Offshore Platforms

In this study, two types of thick steel plates are prepared by controlling carbon equivalent and nickel content, and their microstructures are analyzed. Tensile tests, Vickers hardness tests, and Charpy impact tests are conducted to investigate the correlation between microstructure and mechanical properties of the steels. The H steel, which has high carbon equivalent and nickel content, has lower volume fraction of granular bainite (GB) and smaller GB packet size than those of L steel, which has low carbon equivalent and nickel content. However, the volume fraction of secondary phases is higher in the H steel than in the L steel. As a result, the strength of the L steel is higher than that of the H steel, while the Charpy absorbed energy at -40 °C is higher than that of the L steel. The heat affected zone (HAZ) simulated H-H specimen has higher volume fraction of acicular ferrite (AF) and lower volume fraction of GB than the HAZ simulated L-H specimen. In addition, the grain size of AF and the packet sizes of GB and BF are smaller in the H-H specimen than in the L-H specimen. For this reason, the Charpy absorbed energy at -20 °C is higher for the H-H specimen than for the L-H specimen.

Coupling Effect of Prior Austenite Grain Size and Inclusion Characteristics on Acicular Ferrite Formation in Ti-Zr Deoxidized Low Carbon Steel

The coupling effect of prior austenite grain size and inclusion characteristics on acicular ferrite (AF) formation was investigated in Ti-Zr deoxidized low carbon steel by utilizing the high temperature confocal laser scanning microscope (HT-CLSM), optical microscope (OM), and scanning electron microscope (SEM) equipped with energy-dispersive spectrometer (EDS). The results indicated that with the target heating temperature increased from 1100 °C to 1350 °C, the average size of prior austenite grain varied from 58.22 to 237.40 μm, and the average grain size increased rapidly when the temperature was above 1200 °C. For inclusion characteristics, different target heating temperatures had no obvious effect on inclusion types, but had a great influence on the average size and number density of each, especially for the intragranular effective inclusions. In addition, as the increase of target heating temperature, the types of microstructure were identical, but both AF volume fraction and AF relative nucleation ability increased first and then decreased. When the target heating temperature of sample was 1250 °C, the AF volume fraction reached the maximum of 49.48 pct. However, the AF relative nucleation ability reached the maximum of 474.5 at the target heating temperature 1200 °C, at this time, the AF volume fraction was 47.92 pct, only 1.56 pct smaller than that at 1250 °C. Therefore, considering the AF volume fraction and AF relative nucleation ability, the optimal target heating temperature for AF formation in this study was 1200 °C, and the corresponding prior austenite grain size was 69.58 μm.

The significant effect of non-recrystallization zone reduction on microstructure and mechanical properties in multi-phase steel from the perspective of crystallographic structure and variant pairing

We elucidate here the significant effect of non-recrystallization zone reduction on microstructure and mechanical properties in multi-phase steel from the perspective of crystallographic structure, particularly, digitization and visualization of variant pairing. The volume fraction of deformation induced ferrite increased with the increase of non-recrystallization zone reduction, accompanied by the increase of acicular ferrite. This behavior is related to suppression of upper bainite by occupying the prior austenite grain boundaries by allotriomorphic ferrite. The increase of non-recrystallization zone reduction led to a transition from upper bainite to acicular ferrite, and consequently influenced the mechanical properties. The multi-phase steel with a microstructure consisting of ultra-fine ferrite, bainite and a large volume fraction of acicular ferrite had an excellent combination of high yield strength (561 MPa), low yield-to-tensile ratio (0.81), high uniform elongation (11.4%) and remarkable cryogenic toughness of 212 J tested at −100 °C. There was a dominance of V1/V8 pairing in upper bainite, resulting in low density of high angle grain boundaries (HAGBs). While the density of HAGBs in acicular ferrite/bainite was significantly higher because of microstructure refinement by acicular ferrite and the transition in variant pairing from V1/V8 to V1/V2. The V1/V2 pair was primarily related to the boundaries between acicular ferrite and bainite. Moreover, the brittle crack propagated straightly in upper bainite. In contrast, the high density of HAGBs in acicular ferrite/bainite impeded and arrested microcracks before they grew to a critical Griffith crack length, and subsequently suppressed the initiation of cleavage crack and decreased the ductile–brittle transition temperature.

Effect of Chemical Compositions on Microstructure and Mechanical Properties of Base Metal and HAZ of Bainitic Steel Plates

In this study, three kinds of bainitic steel plates are manufactured by varying the chemical compositions and their microstructures are analyzed. Tensile and Charpy impact tests are performed at room and low temperature to investigate the correlation between microstructure and mechanical properties. In addition, heat affected zone (HAZ) specimens are fabricated by a simulation of welding processes, and the HAZ microstructure is analyzed. The base steel that has the lowest carbon equivalent has the highest volume fraction of acicular ferrite and the lowest volume fraction of secondary phases, so the strength is the lowest and the elongation is the highest. The Mo steel has a higher volume fraction of granular bainite and more secondary phases than the base steel, so the strength is high and the elongation is low. The CrNi steel has the highest volume fraction of the secondary phases, so the strength is the highest and elongation is the lowest. The tensile properties of the steels, namely, strength and elongation, have a linear correlation with the volume fraction of secondary phases. The Mo steel has the lowest Charpy impact energy at -80 oC because of coarse granular bainite. In the Base-HAZ and Mo-HAZ specimens, the hardness increases as the volume fraction of martensite-austenite constituents increases. In the CrNi-HAZ specimen, however, hardness increases as the volume fraction of martensite and bainitic ferrite increases.

Microstructure and mechanical properties correlation of weld joints of a high strength naval grade steel

Low alloy steels of higher strength grades are conventionally used for pressure hull application. Strength required range from 560 MPa to 800 MPa and plate thickness requirement is often higher (40 mm or more). Welding of high strength steels with higher thicknesses is often a challenge. From the view point of structural integrity challenge is not only to obtain defect free weld joint but also to meet the strength and low temperature toughness requirement in both weld and HAZ. In hull construction, different types of welding processes are used depending upon the requirement, accessibility and criticality of the weld joints to be produced. Conventionally, welding processes include Shielded Metal Arc Welding (SMAW), Gas Metal Arc Welding (GMAW), Submerged Arc Welding (SAW) & Gas Tungsten Arc Welding (GTAW). Number of weld consumables indigenously developed for a higher strength steel (YS~800 MPa), were used to develop the procedure, for producing weld joints, meeting the critical structural property requirements.Present study was focused on structural property correlation of weld joints made using these indigenous consumables. Weld joints were characterized in terms of impact toughness, Ductile Brittle Transition Temperature (DBTT), tensile properties and quantitative metallography. DBTT of weld joint is important from the aspect of structural integrity at sub-zero temperature. Impact toughness at -40° C varied from 39 J for SMAW process to 191 J for GTAW process. Yield strength of weld joints varied from 534 MPa for SMAW process to 907 MPa for GTAW process. Quantitative metallography was carried out to determine volume fraction of acicular ferrite (AF) in different welds. Heat input (HI) to produce a weld joint varied with welding processes used. HI for SMAW was 1.3 kJ/mm while it was 2.2 kJ/mm for SAW process. At HI of 1.85 kJ/mm (GTAW), a higher % fraction of AF (~77%) having Impact toughness of 191 kJ/mm at -40° C.

Application of LBW and LAHW for fillet welds of 12 and 15 mm structural steel

Laser beam welding (LBW) and laser-arc hybrid welding (LAHW) are attractive joining methods of medium thickness (10-20 mm) plates in a single pass welding technique. Both utilize high energy process in order to achieve high penetration depth and high welding speeds offering substantial increase in productivity. Deep and narrow welds were produced by LBW and LAHW suffered from high hardness (> 350 HV), primarily in the coarse-grained heat affected zone in joined 12 mm and 15 mm thick plates. In addition, weld imperfections like porosity, cracks, undercuts, or humping may occur. Mainly bainite and lath martensite were formed in LBW joints. By contrast, LAHW provided high volume fraction of acicular ferrite in the upper area. Filler wire was not transported to the root zone in fillet welds with flat position since standard arc mode caused unstable weld pool formation. A numerical model was developed to predict cooling rates and correlated to hardness and microstructure evolution.

Effect of Final Cooling Temperature on Microstructure and Mechanical Properties of a Cr-Ni-Mo-V Bainite Steel

The microstructural evolution and mechanical properties of a low-carbon high-strength bainitic steel under different final cooling temperatures were studied. The microstructures of the experimental steel at different final cooling temperatures were composed of acicular ferrite and granular bainite. A decrease in final cooling temperature resulted in an increase in volume fraction of granular bainite and a decrease in volume fraction of acicular ferrite. The specimen with the lowest final cooling temperature (459 °C) exhibited the highest yield strength, tensile strength, and yield ratio, and a lower total elongation than the specimens with the highest final cooling temperatures (501 and 535 °C) because of the higher volume fraction of granular bainite and finer bainitic ferrite lath. The specimen with the lowest final cooling temperature had a lower absorbed energy than that of the highest final cooling temperature, because the strip-like martensite–austenite (M-A) constituents that existed between the bainitic ferrite or in prior austenite grain boundaries reduced its toughness. At an intermediate final cooling temperature of 501 °C, the experimental steel exhibited excellent mechanical properties with a yield strength, tensile strength, and absorbed energy of 825 MPa, 1232 MPa, and 102 J, respectively.

Effect of Heat Input on the Microstructure and Mechanical Properties of Low Alloy Ultra‐High Strength Structural Steel Welded Joint

Microstructural evolution and mechanical properties of an ultra‐high strength structural steel welded joint at different heat inputs are investigated by metal active gas arc (MAG) welding technique. It is found that the microstructure of weld metal (WM) contained high volume fraction of acicular ferrite regardless of heat input. However, the acicular ferrite changes from bainite acicular ferrite (B‐AF) to Widmanstätten acicular ferrite (WF‐AF) with the increase in heat input. The microstructure of coarse grain heat affected zone (CGHAZ) is characterized by complete martensite, a mixture microstructure (lath martensite and bainite), and granular bainte, respectively, with increasing heat input from 7.5 to 10.5 and then to18.5 kJ cm−1. The acicular ferrite in WM nucleated at complex oxide inclusions and the number density of inclusions with the size of ≈0.6–1.2 µm significantly increases with the increase in heat input. The maximum drop of strength (113 MPa) occurs when the heat input is up to 18.5 kJ cm−1. High volume fraction of acicular ferrite in WM improves the impact toughness because of increased proportion of high angle grain boundaries and interlocking structure. The heat input should be higher than 7.5 kJ/cm to avoid the formation of welding cold crack and twinned martensite in CGHAZ.