Acicular Ferrite Microstructure Research Articles

Unravelling the impact of Al-Ti content on the microstructure-toughness relationship in the coarse-grained heated-affected zone of super-high-strength pipeline steels

This study aims to investigate the role of Al-Ti addition on particle modification in super-high-strength pipeline steels, and its following effect on microstructural evolution and impact toughness in the coarse-grained heated-affected zone (CGHAZ) is also explored. The steel plates with low and high Al-Ti content (0.009 wt-% Al + 0.006 wt-% Ti vs. 0.033 wt-% Al + 0.025 wt-% Ti), respectively, are subjected to gas metal arc welding with 6–7 kJ/cm heat input. The results reveal that the second-phase particles including micron-scale inclusion of Al-Mg oxide covered with CaS and nano-scale precipitate of titanium nitride are obtained in low Al-Ti steel, which are modified to the Al-Mg-Ti oxide core surrounded by CaS and TiN precipitate in high Al-Ti steel. The nano-scale TiN precipitate and micron-scale inclusion can restrict the austenite grain growth at high temperatures and induce acicular ferrite nucleation at medium temperatures, respectively, leading to the fine-grained mixed microstructure of bainite and acicular ferrite in CGHAZ of both steels. Because of the variant selection during the decomposition of austenite to bainite and acicular ferrite, those fine-grain structures with amounts of high-angle grain boundaries exhibit excellent CGHAZ toughness in both specimens. Compared with low Al-Ti steel, more precipitates induce smaller austenite grains, and the inclusions containing titanium increase the ability of inclusion to promote acicular ferrite nucleation in CGHAZ of high Al-Ti steel. Both of them resulted in smaller crystallographic grain and toughness improvement in CGHAZ of high Al-Ti steel than that in low Al-Ti steel.

Microstructure Refinement via Nucleation of Intragranular Acicular Ferrite Stimulated by Ti-Containing Core-Shell Structured Particles in Low-Carbon Steel.

This study investigated the microstructure, mechanical properties, and nucleation mechanism of acicular ferrite (AF) present in hot-rolled Ti deoxidized steel. In our experiments, the impact toughness of Ti deoxidized steel is significantly increased to 144 J at -20 °C, while those Mn and Al deoxidized steels are only 9 J and 18 J, respectively. Interlocked AF is the primary microstructure of Ti deoxidized steel. The second-phase particles of the core-shell-type structure, in which Ti2O3 is the nucleus and TiO is the outermost shell, act as effective nucleating agents to stimulate AF nucleation. The low lattice disregistry between TiO and AF is the main factor contributing to the production of AF. It is also revealed that Ti2O3 and MnS fulfill the particular orientation relationship, contributing to the formation of an Mn-depleted zone (MDZ) adjacent to MnS, proposed to be one of the possible mechanisms for promoting AF nucleation.

Nucleation Stages Onset and Intra-granular Ferrite Morphologies in Titanium Free Micro-Alloyed Steel

The aim of this work is focused on nucleation stages with emphasis on the development of intra-granular ferrite morphologies during isothermal austenite transformation in titanium free micro-alloyed steel. Isothermal treatment was carried out in the temperature range 350 to 600οC. These treatments were interrupted at different times between 2 and 1800 s in order to analyze the evolution of the microstructure. Metallographic evaluation was done by using optical and scanning electron microscopy (SEM) enabled determination of the nucleation onset at all treatments and subsequent on the development of intra-granular ferrite of isothermally treated Ti free micro-alloyed steel. The results show that at high temperatures ( 500  C) polygonal intra-granularly nucleated ferrite idiomorphs, combined with grain boundary ferrite and pearlite were produced and followed by an incomplete transformation phenomenon, At intermediate temperatures (450 and 500  C) an interlocked acicular ferrite (AF) microstructure is produced, and at low temperatures (400 and 350  C) the sheave of parallel acicular ferrite plates, similar to bainitic sheaves but intra-granularly nucleated were observed. In addition to sheaf type acicular ferrite, the grain boundary nucleated bainitic sheaves are observed.

Effect of Wire Preheat and Feed Rate in X80 Steel Laser Root Welds: Part 2 — Mechanical Properties

In Part 1 of this study, the influences of feed rate and preheat of the wire on the fusion zone microstructure of X80 steel laser root welds were investigated. Part 2 of this study reveals the effect of cold versus hot wire feed on mechanical properties. Although strength overmatching with the X80 base metal was maintained, the wire alloying elements reduced the hardness of the upper region of the laser fusion zone but had limited effect on the bottom of the fusion zone due to the incomplete mixing of the filler material with the base metal in the bottom root region of the joint. The addition of ER70S-6 filler wire improved the impact toughness of the weld metal at 0°C, due to the formation of an acicular ferrite microstructure. As the temperature decreased to –20°C (–4°F) and –45°C (–49°F), ductile to brittle transition occurred, leading to lower weld metal toughness.

Mechanical, metallurgical, and corrosion investigations on wire arc additively manufactured low carbon steel

Current global 3D metal printing research is focused on powder bed fusion and direct energy deposition. Several materials are presently under research to assess their post-additive manufacturing properties, and to process them in industries using advanced manufacturing techniques. This study investigates the electrochemical, mechanical, and metallurgical properties of wire arc additively manufactured (WAAM) ER70S-6 alloy walls on AISI S235 plates, emphasizing ship component fabrication. The corrosion resistance of both materials is assessed through Electrochemical Impedance Spectroscopy and Potentiodynamic Polarization testing in a 3.5% NaCl solution. Surface and cross-sectional morphologies are analyzed using optical microscopy and scanning electron microscopy, while energy dispersive spectroscopy is employed to study the chemical composition of the corrosion products. Moreover, the hardness of the WAAM-deposited ER70S6 wall is found to be highest in the uppermost layers of deposition due to the presence of acicular ferrite and ferritic-bainite microstructure. Tensile and Charpy impact strength for AISI S235 and WAAM-deposited ER70S-6 exhibit similar performance at room and low temperatures. Interestingly, the WAAM sample demonstrates a 60% higher elongation than the AISI S235 sample. Hence, the findings from this experimental work suggest that WAAM-deposited carbon steel exhibits favourable corrosion resistance and comparable mechanical properties to AISI S235. Consequently, WAAM is emerging as a promising alternative to conventional manufacturing techniques for fabricating ship components, repairing and modifying hull structures, manufacturing nozzles, propeller blades, fin-like structures, structural components, and more.

Limits on Ti Element Transfer in Submerged Arc Welding: Thermochemical Analysis

TiO2 inclusion formation in the weld pool is required to produce potent nucleation sites for acicular ferrite microstructure formation. Weld metal oxygen content must be limited to ensure acceptable materials properties, although sufficient oxygen is required for inclusion formation. Weld pool oxygen is sourced from the decomposition of flux oxides. Weld metal oxygen is controlled with flux chemistry formulation by CaF2 dilution of oxides. In conventional submerged arc welding (SAW), the flux is the source of weld metal Ti. Transfer of Ti from the slag appears to be limited to 400 parts per million (ppm). SAW modification by metal powder additions changes the element transfer reactions. Thermochemical analysis is applied to explain the limitations in Ti transfer from the slag, compared to improved reaction conditions for Ti element transfer in the aluminum‐assisted Ti alloying of weld metal. Low TiO2 activity due to low flux TiO2 content from CaF2 dilution and Ti loss from Ti‐fluoride gas formation limits Ti transfer from slag. Aluminum‐assisted alloying of the weld metal shifts the gas composition from Ti‐fluoride formation to Al‐fluoride formation and lowers the system partial oxygen pressure to increase the weld metal Ti content, with acceptable ppm O remaining in the weld metal.

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.

Crystallographic characteristics of acicular ferrite nucleated on inclusions in a HSLA steel

Acicular ferrite (AF) has been regarded as the optimal microstructure in steel welds and heat affect zones (HAZs), because of its superior combination of strength and toughness. Although various aspects of AF have been investigated, the crystallographic characteristics of AF, including orientation relationship, variant selection and variant pairing, are still not fully studied. In this research, a Ti-deoxidized high strength low alloy steel was prepared and subjected to HAZ thermal cycle simulation tests to attain AF microstructure. Optical micrographs were obtained and EBSD mappings of AF microstructures were conducted. Through crystallographic analysis, the orientation relationship between AF and austenite was found close to that of bainite transformed at a high temperature (580 °C). In the AF microstructure, all 24 variants are formed and variant selection is nearly random. Compared with bainite, the variant pairing of AF is more random despite the slight favour of certain variant pairs. Most of the adjacent AF laths are from different close-packed plane (CP) groups, leading to the classic interlocking morphology. Meanwhile, a large number of neighbouring AF laths belong to different Bain groups, resulting in a high density of high angle grain boundaries (HAGBs). Intragranular nucleation of primary AF laths on inclusions contributes both to the high HAGB density and to the interlocking morphology of AF, while the sympathetic nucleation of secondary AF laths only contributes to the latter. These results suggest that to increase the HAGB density in AF, promoting the intragranular nucleation on inclusions is more important than enhancing the sympathetic nucleation.

Determining the Role of Acicular Ferrite Carbides in Cleavage Fracture Crack Initiation for Two Medium Carbon Microalloyed Steels.

Two medium-carbon microalloyed steels with a predominant acicular ferrite microstructure were investigated in this study in order to determine the initial micro-crack formation mechanism and the role of acicular ferrite structure in cleavage fracture. In order to ensure cleavage fracture, samples were investigated at -196 °C for uniaxial tension and four point bending fracture. Previous investigations have shown that cleavage fracture for steels with a predominant acicular ferrite microstructure has not been initiated by the fracture of coarse TiN particles as in ferrite-pearlite, bainite, or martensitic microalloyed steels. The average maximal thickness of cementite plates measured in this work is 0.798 µm and 0.966 µm, for V and TiV steel, respectively. The corresponding stress values required for their fracture according to Griffith's equation are 1970 MPa and 1791 MPa, respectively. Estimated values of the effective surface energy for the V steel with an average cementite volume fraction of 3.8% range from 40 Jm-2 to 86 Jm-2, and for the TiV steel with an average cementite volume fraction of 18.3% range from 55 Jm-2 to 82 Jm-2. The fracture of coarse cementite plates was found to not to be responsible for the cleavage fracture initiation in case of both steels.

Effect of tool rotational speed on microstructure and mechanical properties of friction stir welded DMR249A high strength low alloy steel butt joints for fabrication of light weight ship building structures

The friction stir welding (FSW) was employed for joining 5 mm thick plates of high strength low alloy steel (HSLA) of grade DMR249A in the development of light weight ship building structures. The main objective of this investigation is to study the effect of tool rotational speed (TRS) on microstructure and mechanical properties of friction stir welded (FSWed) DMR249A HSLA steel butt joints. The indigeneously developed Lanthanated Tungsten alloy (W99) tool was used for FSW of DMR249A HSLA steel plates. The TRS of W99 tool was varied from 500 rpm to 700 rpm while maintaining the remaining parameters at constant level. The microstructure of DMR249A steel joints was characterized using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron beam scatter diffraction (EBSD) techniques. The elemental analysis of stir zone (SZ) of DMR249A steel-joint was carried out using energy dispersive spectroscopy (EDS). The tensile properties, hardness and impact toughness of DMR249A steel joints were evaluated and compared to base metal (BM). Results disclosed that the DMR249A steel joints made using the TRS of 600 rpm exhibited superior tensile properties and impact toughness compared to other joints. The joints made using the TRS of 600 rpm revealed the greater tensile strength of 664 MPa, yield strength of 502 MPa, elongation of 19% and impact toughness of 48 J. It showed 8.85% and 14.61% improvement in tensile and yield strength along with 20.83% and 22.58% reduction in ductility and impact toughness compared to BM. It is attributed to the evolution of finer acicular ferritic and upper bainitic microstructure with no debris of W99 tool in SZ.

Investigation of welding of dissimilar high thickness SA106 Gr.C carbon steel and SA335P12 alloy steel to eradicate the failure in boiler components fabricated through SAW and SMAW processes

This study reports the effect of Shielded metal arc welding (SMAW) and Submerged arc welding (SAW) processes used to fabricate high-thickness end cover plates by welding of dissimilar materials SA 335 P12 steel pipes and ASTM A 106 grade C carbon steel. The study emphasizes the structure-property relation of the welded joints made by two different techniques using macrostructure and microstructural examination with optical and electron microscopy and hardness evaluation. The joints were subjected to magnetic particle non-destructive testing and the SAW joint was found to be defect-free. Microstructural examination revealed acicular ferrite microstructure on the SAW joint but bainite and martensite microstructure on the SMAW joint. After heat treatment hardness values disclosed the dissolution of the hard phases. The joints fabricated with SAW demonstrated high ductility with an increase in elongation of 23% and 37.5%, during the bend test and the tensile test respectively. Higher thickness welding of dissimilar joints by SAW is prefereed over SMAW joints, as it requires fewer weld passes and produce superior weld quality without any defects.

Microstructure and Impact Toughness of Acicular Ferrite in Low Alloy Steel Weld Joints from Results of Multiple Impact Bending Tests

Microstructure and Impact Toughness of Acicular Ferrite in Low Alloy Steel Weld Joints from Results of Multiple Impact Bending Tests

Evaluation of high cycle fatigue behavior of flux cored arc welded naval grade DMR249 A grade steel joints for ship hull structures

DMR 249A is a micro-alloyed high strength low alloy (HSLA) steel particularly designed for shipbuilding structures due to its excellent strength-to-weight ratio. In this investigation, the rotating-beam fatigue test was performed on the flux-cored arc welded (FCAW) butt joints of DMR 249A steel. And the fatigue test results compared to mechanical properties and microstructural characteristics. The transverse tensile, microhardness, impact, and fatigue properties were evaluated from across the butt weld. The S-N curve was constructed from the experimental data for the stress ratio R = -1. The evaluated fatigue life of the FCAW joint and parent metal was 366 MPa and 394 MPa, respectively. The maximum achieved transverse tensile strength and impact energy absorption of the FCAW joint was 578 MPa &174 J. The weld region contains an acicular ferrite microstructure, which exhibited improved properties of strength, toughness, and fatigue crack resistance. From the results, the fatigue strength of FCAW is 92.8% that of the parent metal and 62.8% equal to the tensile strength of the FCAW joint.

Influence of Filler Material on the Microstructural and Mechanical Properties of 430 Ferritic Stainless Steel Weld Joints

Tungsten Inert Gas (TIG) welding is a commonly used welding technique for ferritic stainless steel, due to its ability to produce high-quality, clean, and precise welds. This welding method provides excellent control over the heat input, making it suitable for thin-walled, high-alloy materials such as ferritic stainless steel. The purpose of this study was to investigate the effect of using two different filler materials, 310 (austenitic) and 410 (ferritic), on the microstructural and mechanical properties of Tungsten Inert Gas (TIG) weld butt joints of 430 ferritic stainless steel (FSS). The results showed that the choice of filler material significantly impacted the dilution percentage, the chromium-nickel equivalent ratio, microstructure, microhardness, and tensile characteristics of the welded joint. The use of 310 filler resulted in a columnar microstructure, whereas the use of 410 filler resulted in a ferritic (acicular ferrite) microstructure with the presence of martensite and austenite. The sample welded with 410 filler demonstrated superior mechanical properties compared to the sample welded with 310 filler. These findings emphasize the importance of selecting the appropriate filler material in order to achieve the desired microstructural and mechanical properties in 430 FSS welded joints.

PENGARUH POSISI SHIELDED METAL ARC WELDING (SMAW) GERAK SPIRAL KAMPUH V TERHADAP KEKUATAN TARIK DAN STRUKTUR MIKRO BAJA KARBON RENDAH

The purpose of this study was to determine the relationship between the position of shielded metal arc welding (SMAW) with spiral motion V on the tensile strength and microstructure of low carbon steel. The research was conducted by welding V-coated low carbon steel with horizontal 2G, vertical 3G and overhead 4G. The test results show that the welding position does not have a significant effect on the tensile strength of low carbon steel welds, where the 3G vertical position provides a higher tensile strength value than the 2G horizontal position and 4G overhead. The microstructure of acicular ferrite (AF) in the vertical position looks dominant uniformly and tightly.

VIABILITY ANALISYS FOR LASER DIRECTED ENERGY DEPOSITION (L-DED) OF POWDER MATERIAL15CDV6

The aim of the present research work is the characterization of the 15CDV6 powder material for Laser Directed Energy Deposition (L DED) processes. The novelty of the work lays on the fact that this aeronautical steel has never been employed before in powder L DED process. Therefore, the mentioned steel powder was atomized specifically for the present work and the chemical composition and shape of the particles was measured to ensure the quality of the material. Afterwards, the suitability of the 15CDV6 steel for the L-DED process was studied and the employed methodology consisted of 3 sequential steps: single bead tests, overlapping beads deposition and thin wall construction. The results of each sequence are used as the basis for the following ones. In addition to the determination of the main process parameters, the influence of the deposition strategy on the process efficiency is analyzed and a correlation between the microstructure resulting from the thermal process and the hardness HV0.3 values was obtained. To finish the characterization of the process, a demonstrator part was fabricated using the optimum parameters defined during the tests. Based on the obtained results, the viability of employing 15CDV6 steel in the L-DED process is ensured. Also, it is concluded that if a sufficient cooling rate is ensured, Acicular Ferrite microstructure is obtained, which provides good mechanical properties to the L-DED manufactured parts. Nevertheless, the thermal evolution of the process needs to be controlled in order to avoid heat accumulation and cooling stops are to be applied when required. Keywords: 15CDV6, powder, L-DED, characterization, atomization.

Evaluation of the corrosion behavior of different regions of the deposited weld metal from the Ni-modified E8018-G electrode

In this study, the corrosion behavior of different regions of the weld metal produced from the E8018-G electrode containing 7.0 wt% Ni in a corrosive solution has been investigated. X-ray diffraction (XRD) method has been used to compare the phase intensity in different welding regions. Electrochemical evaluations show that in the weld joint, by moving from the root of the weld metal to the upper layers of the center and cap, the corrosion resistance of the weld metal increases. In addition, the results demonstrates that the corrosion of the St-52 welded joint mainly occurs in the heat affected zone (HAZ). According to the microstructures observed from the spectra obtained by the field emission scanning electron microscopy (FE-SEM), the corrosion behavior of the weld metal has been evaluated. The examination of the local weld metal layers indicates that not only the corrosion resistance of the cap layer has improved compared to the central and root layers, but also the corrosion resistance has increased compared to the base metal. This improvement is attributed to the existence of a homogeneous acicular ferrite microstructure in the cap layer and higher nickel content.

Performance and Characteristics of Stationary Arc and Rotating Arc-Gas Metal Arc Welded DMR 249 Naval Grade Steel Joints

Abstract In this investigation, 12-mm thickness naval grade high-strength low-alloy steel plates were welded by rotating arc-gas metal arc welding (RA-GMAW) and stationary arc-gas metal arc welding (SA-GMAW) processes. The main objective of this work was to carry out the comparative analysis of mechanical properties and microstructural characteristics of the joints fabricated by the outlined welding processes. From this experimental work, it is found that the ultimate tensile strength, yield strength, and impact toughness of the RA-GMAW joint are 15 % higher than those of the SA-GMAW joint. This is because of the formation of a higher volume percentage of acicular ferrite microstructure and island martensite/austenite constituent in the weld metal region of the RA-GMAW joint. Moreover, from the productivity point of view, the number of passes in the RA-GMAW process was minimized to three (from six passes), and the width of the heat-affected zone was also reduced by 45 % to SA-GMAW processes. Overall, the RA-GMAW joints are found to have superior mechanical properties and microstructural characteristics compared to SA-GMAW joints.

Welding heat input for synergistic improvement in toughness and stress corrosion resistance of X65 pipeline steel with pre-strain

The microstructure, hydrogen embrittlement sensitivity, fracture toughness, and influence of pre-strain on the sulfide stress corrosion cracking (SSCC) sensitivity of the coarse-grained heat-affected zone of X65 pipeline steel with large deformation were studied to determine the optimal welding heat input in an acidic H2S environment. After pre-straining, the microstructures of the specimens changed insignificantly, but their ductility decreased. The acicular ferrite microstructure obtained under a low heat input demonstrated the synergistic improvement in the SSCC resistance and fracture toughness under pre-strain, especially at t8/5 = 20 s. The M/A constituents interface potentially causes microcrack initiation and interconnection under pre-strain.

Critical assessment 42: acicular ferrite formation and its influence on weld metal and heat-affected zone properties of steels

Toughness and fatigue resistance of welds and associated heat-affected zones (HAZs) in high-strength steels are important design parameters in structural applications. Acicular ferrite microstructures, which can be achieved through control of weld process and nucleant (inclusions or precipitates) characteristics, are often regarded as key to enhancing weld metal and HAZ properties of welded steels. This paper addresses the transformation characteristics of acicular ferrite related to alloying and weld processing as well as the specific role of acicular ferrite in weld performance, along with future research needs to identify weld processes suitable to specific alloying strategies.