Acicular Ferrite Research Articles

On the evolution of root weld microstructure and properties during automatic girth welding of high strength pipeline steel

ABSTRACT Properties of root weld are important for service safety of pipelines, while microstructure and properties of root weld would alter due to heat affect of following welding passes. Evolution of root weld microstructure and properties during automatic girth welding of X80 pipeline was studied. Microstructure of the original root weld primarily consists of acicular ferrite (AF) and side-plate ferrite (SPF), and it is mostly affected by hot pass and first filling pass weld, after which SPF and AF coarsened significantly and some interlock ferrite combined with each other. During subsequent welding, no significant microstructural changes was found. However, impact toughness of root weld slightly increased after hot pass and first filling pass in spite of microstructure coarsening. In contrast, hardness of root weld notably dropped due to microstructure coarsening of hot pass and first filling pass, and then it was retrieved in following welding passes due to tempering effect.

Effective strategies for improving quenched microstructure and mechanical properties of hot-rolled seamless steel pipes

The containing Ti-Mg-Ca-O composite particles were innovatively used to achieve microstructure refinement and properties improvement of hot-rolled seamless steel pipes, and compared with conventional steel (C# steel). The results show that the C# steel mainly consisted of parallel lath martensite after on-line quenching. Under the same rolling and heat treatment conditions, dominant Ti-Mg-Ca-O composite particles in modified steel (M# steel) were effective nucleation sites of acicular ferrite, dividing the prior austenite grains and refining microstructures. The strength values of the both steels were at the same level, however, the impact energy of M# steel was significantly higher than C# steel, increasing by 28%. This study provides an effective method for further microstructure refinement of hot-rolled seamless steel tubes.

Revisiting the Concept of Property Optimization of Low Temperature Phase Transformed Microstructures of GIGASTEEL Welds

Although the need for automobile lightening continues to increase in preparation for the era of electrical vehicles, the application of steel, which has strengths in price and recycling in the automotive industry, is expected to continue along with the developments of various driving energy sources. Recently, when lightening and stiffness reinforcement are required for the electric vehicles or medium and large SUVs, newly-designed parts using GIGASTEEL with a tensile strength of 980MPa or higher are being developed. However, it is difficult to secure sufficient weld strength and fatigue performances when applying welding consumables that have been applied normally and ultra-high strength welding materials that have been commercialized so far. It has also limitations that are not easy to apply due to excessive manufacturing costs in the automotive industry. In this study, the application of commercial welding consumables by strength grade to GIGASTEEL was reviewed and the properties of the weldments were compared to address all of the above issues. Here, we consider the effects of C, Mn, Si, Cr, Ni, Mo and microalloying to implement interlocked nature with dense and complex microstructures of lath-like bainitic and acicular ferrite that can effectively improve both strength and toughness of GIGASTEEL welds in a more economical way.

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.

Sintering enhancement process based on fluidity and bonding phase strength

The sintering process is influenced by complex low-grade ores. This study used seven types of imported iron ore, A–F and domestically produced iron ore, G, as raw materials to study their chemical composition, as well as the liquid fluidity and bonding strength of the mixed ore after single and optimised ore blending. The high-temperature characteristic test values of mixed ores are positively correlated with the weighted calculated values of individual ores. Under the sintering cup conditions, the optimal ore mixing includes a 9% increase in liquid fluidity and a 6% increase in bonding strength. The latter significantly improved the sintering yield and drum strength, reaching 75.89% and 65.67%, respectively. In addition, the consumption of solid fuel has slightly decreased by 0.06 kg·t−1, and the utilisation coefficient has slightly increased. In addition, the sintering yield and quality indicators of the scheme have been comprehensively improved. Compared with the benchmark scheme, through liquid fluidity optimisation, an increase in magnetite phase, a decrease in haematite, and a significant decrease in acicular calcium ferrite were observed, interwoven with magnetite. The silicate-bounding iron oxide also showed an increase in this optimisation. In the optimisation of bonding strength, the corrosive structure interwoven with magnetite and needle shaped ferrite significantly increases, while the iron oxide bound with silicate decreases. The improvement effect of liquid phase fluidity and bonding phase strength optimisation on sintering process and mineralisation behaviour was verified through sintering cup test and mineral phase analysis.

Effects of heat input on microstructure evolution and corrosion resistance of underwater laser cladding high-strength low-alloy steel coating

The effects of heat input on the microstructure evolution and corrosion resistance of the high-strength low-alloy (HSLA) steel coating obtained by wire-feed underwater laser cladding were studied. Optical digital microscope, scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), transmission electron microscope (TEM), electron back-scattered diffraction (EBSD), electrochemical tests, laser microscope and other characterization methods were used. As the heat input increased, the smoothness of the underwater cladding coating improved, with little changes in elemental distribution and phase composition. However, the content of acicular ferrite increased, while the amounts of lath bainite, lath martensite, and granular bainite decreased. As the heat input increased from 0.4 to 0.55 kJ/mm, the grain size increased from 25.3 to 55.9 μm, the proportion of low-angle grain boundaries rose from 61.3 % to 69.1 %, and the texture intensity of the (110) crystal plane increased from 16.22 to 23.83. Meanwhile, the dislocation density decreased from 4.9 × 1014 to 4.1 × 1014 m−2. These changes enhanced the corrosion resistance of underwater coating. As the heat input increased from 0.4 to 0.55 kJ/mm, the corrosion current density decreased from 2.13 × 10−5 to 3.35 × 10−6 A/cm2, and the corrosion potential increased from −0.823 to −0.529 V. Additionally, the depth-to-width ratio of the corrosion pits decreased from 0.255 to 0.053. This study laid the foundation for high-quality in-situ underwater repairs of ships and submarines made of high-strength steel.

Mechanical and Metallurgical Characteristics of Wire-Arc Additive Manufactured HSLA Steel Component Using Cold Metal Transfer Technique

Recently, the application of wire-arc additive manufacturing (WAAM) for the production of metallic products is gaining traction. WAAM is associated with the direct energy deposition technique and therefore has a higher deposition rate (approximately 4 kg/h). For this reason, it is of greater interest than powder-based additive manufacturing techniques. Industrial applications such as marine and offshore structures and pressure vessels for space programs commonly utilize high-strength low-alloy (HSLA) steel. HSLA steel components produced by casting methods exhibit defects due to oxidation. Therefore, cold metal transfer (CMT)-WAAM was adopted in this study to fabricate HSLA steel components. The metallurgical properties were analyzed using microscopic and diffraction techniques. The effects of the evolved microstructures on mechanical properties, such as strength, microhardness, and elongation to fracture, were evaluated. To analyze and test the structure, two regions were selected, namely, top and bottom. Microstructural analyses revealed that both regions were primarily composed of acicular ferrite, polygonal ferrite, and bainitic structures. The bottom region exhibited superior mechanical properties compared with the top region. The improved strength at the bottom region can be ascribed to the formation of a high density of dislocations and finer grains.

Correlation between heterogeneous micromechanical properties and fracture toughness in X80 girth weld

The safety accidents caused by the service of pipelines will cause huge economic losses due to the deterioration of the fracture toughness of the girth weld. To clarify the fracture behaviors of girth weld, X80 girth welds with heterogeneous microstructure consisted acicular ferrite (AF) and grain boundary ferrite (GBF) with different micromechanical properties were investigated. The results show that the typical microstructure of X80 girth weld is heterogeneous, which is consist of high strength AF and low strength GBF. With the decrease of the difference in micromechanical properties between AF and GBF, the crack tip opening displacement of three girth welds is 0.13 ± 0.04 mm, 0.18 ± 0.04 mm and 0.29 ± 0.05 mm. Meanwhile, the type of microcracks gradually changed from secondary cracks with GBF aggregation to micro-voids without obvious aggregation, which means the fracture mechanism transforms from cleavage fracture to micro-voids aggregation fracture. The finite element analysis indicates that the change of fracture mechanism is mainly due to the reduction of the gap between the micromechanical properties of AF and GBF, the stress and strain distribution are homogenized, resulting the fracture toughness is significantly improved.

조선용 B Gr. 강의 용접부 미세조직과 기계적 성질에 미치는 용접기법의 영향

The microstructurale and mechanical properties of B Gr. steel were evaluated for shipbuilding according to a welding process. The following conclusions were drawn. In tandem submerged arc welding(TSAW), compared to single submerged arc welding(SAW), the grain boundary ferrite was coarsened, and the fraction was higher owing to the high heat input and low flux basicity ; conversely, the volume fraction of acicular ferrite was lower. Fine intragranular acicular ferrite and polygonal ferrite were observed in the weld metal of the electrogas welding (EGW), which had the highest heat input. In the microstructure near the fusion line, as the amount of heat input increased, the prior austenite grain boundary became coarse. Further, both the intergranular grainboundaryferrite and intragranular bainite became coarse, and the heat-affected zone became wider. The hardness of weld metal was independent of the amount of heat input and was demonstrated to be high in the order of EGW, single SAW, and TSAW owing to the microstructure difference caused by the chemical composition of the filler material and the dilution rate of the base metal. Furthermore, the tensile strength of all three welding processes resulted in stable satisfaction with the class society rules.

Effect of Cr Element in Gas-Shielded Solid Wire for Oil and Gas Long-Distance Pipeline on Microstructure and Low Temperature Toughness of Weld.

In this paper, the influence of Cr element on the mechanical properties of welded joints of gas-shielded solid wire used in oil and gas long-distance pipelines was studied by means of tensile test, impact test, and hardness test, and the microstructure and crack propagation path of weld were characterized by means of an optical microscope, scanning electron microscope, and electron backscattering diffraction. The results show that with the addition of Cr, the strength and toughness of the weld are significantly improved, in which the tensile strength is increased from 607 MPa to 656 MPa, and the impact toughness is increased from 126.37 J to 223.79 J. The proportion of the ferrite side plate in the weld structure is reduced by about 20%, and the effective grain size of acicular ferrite is reduced by about 15%. The reason is that the addition of the Cr element improves the hardenability of the weld structure, inhibits the formation of the ferrite side plate, and promotes the effective refinement of acicular ferrite, which increases the proportion of high-angle grain boundaries in the weld, effectively hindering the crack propagation, improves the crack propagation work, and thus improves the strength and toughness of the weld.

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.

Effect of grain boundary ferrite morphologies on impact toughness of X80 girth weld

The X80 girth weld microstructure formed by high-strength acicular ferrite and low-strength grain boundary ferrite was studied to investigate the effect of grain boundary ferrite morphologies on impact toughness. The results show that due to the difference in direction of heat loss caused by the narrow groove, the grain boundary ferrite morphology of the weld metal center and the weld metal edge are continuous network structure and parallel layered structure, respectively. The continuous network structure grain boundary ferrite causes strain localization, resulting in the formation of a large number of secondary cracks in the crack propagation process, which seriously deteriorates the toughness.

The effect of welding speed on the microstructure, mechanical properties, and fatigue failure of Q690D high-strength steel

This study analyzes the effect of welding speed on the microstructure and mechanical properties of CO2 gas-shielded welded joints of 4 mm thick Q690D high-strength steel. The results show that the weld zone is mainly composed of acicular ferrite. The coarse-grained zone and fine-grained zone contain lath bainite and lath martensite. As the welding speed increases, the degree of recrystallization in the weld zone initially increases and then decreases. At the optimal welding speed of 38 cm/min, the tensile strength of the welded joint reached 692.1 MPa, with an elongation of 9.63 %. The maximum microhardness of the welded joint was 303 HV, and the residual stress was a tensile stress of 167 MPa. After fitting the S-N curve, the ultimate fatigue strength of the welded joint was 210 MPa. Both fatigue and tensile fracture surfaces displayed second-phase precipitate particles, impacting the mechanical properties.

Термомеханическая прокатка при производстве обсадных труб (обзор исследований)

Introduction. The modern oil and gas industry requires the development of high strength materials for well casing. Changes in rolled steel production technologies are one of the urgent tasks. Reducing the cost of high quality steel well casing is becoming a major challenge for the oil and gas industry. Multiphase microstructures containing acicular ferrite or an acicular ferrite-dominated phase exhibit good complex properties in HSLA steels. This paper focuses on the results obtained using modern methods of thermomechanical rolling. Results and discussion. This work analyzes the characteristics of thermomechanical rolling technologies and its impact on the microstructure of rolled steel for well casing. It is shown that predicting mechanical properties based on the microstructural characteristics of steel is complicated due to the large number of parameters involved. This requires an optimal microstructure of the steel. A satisfactory microstructure depends on several factors, such as chemical composition, hot work processing, and accelerated cooling. Alloying elements have a complex effect on the properties of steel, and alloying additives are usually introduced into the steel composition. From a metallurgical point of view, the choice of alloying elements and the metallurgical process can greatly influence the resulting microstructure. Conclusion. This review reports the most representative study regarding thermomechanical rolling technologies and microstructural factors in well casing steels. It includes a summary of the most important process variables, material properties, regulatory guidelines, and microstructural and mechanical properties of the metal for well casing production. This review is intended to benefit readers from a variety of backgrounds, from non-metal forming or materials scientists to various industrial application specialists and researchers.

Study on the evolution of multistage and multiscale Ti-bearing precipitation and microstructure in ultrahigh-strength titanium microalloyed weathering steels

In this study, 900 MPa ultrahigh-strength weathering steels were successfully developed through thermomechanical controlled processing (TMCP). Advanced microstructure characterization, combined with precipitation thermodynamics and kinetics models, elucidated the evolution of Ti-bearing precipitation and microstructure. The results showed that coiling temperature (CT) significantly impacts phase fractions, grain boundary density, and misorientation angles, while both CT and finishing rolling temperature (FRT) influence grain sizes in acicular ferrite (AF) and granular bainite. The lower coiling temperature resulted in a higher dislocation density of the test steel, which provided more nucleation sites for AF and TiC, favoring a higher number of TiC particles and a higher proportion of AF. Above 1050 °C, the addition of nitrogen changed the shape of the precipitation kinetics curve, reduced the nucleation energy barrier of TiC, decreased the critical nucleation size, and improved the nucleation rate. Meanwhile, the addition of nitrogen accelerated the precipitation transformation of TiC, which promoted the formation of Ti(C, N). Furthermore, increasing the deformation stored energy (DSE) further accelerated the precipitation of Ti(C, N) and significantly increased the nucleation rate. The formation mechanism of large-size Ti(C, N) and the transformation mechanism of Ti(C, N) to TiC are revealed by precipitation thermodynamics and kinetics. The completion time of TiC precipitation on dislocations is shorter than that on grain boundaries, which results in the TiC on dislocations being prone to coarsening during prolonged coiling. These findings provide crucial insights for optimizing the industrial production of ultrahigh-strength titanium microalloyed weathering steels.

Continuous cooling transformation behavior of V microalloyed hot-bent X80 pipeline steel

Abstract Continuous cooling transformation (CCT) behavior of a vanadium-microalloyed hot bent pipeline steel was studied by using Gleeble 3800 thermal simulation machine, and the hot-rolling technique of the tested steel was designed. Acicular ferrite and granular bainite can be observed with the cooling speeds ranging from 1~ 40 °C/s. Higher cooling speed leads to grain refinement. The yield strength is 570 MPa. The ultimate tensile strength (UTS) is 757 MPa. The fracture elongation is 22.50 %. The low temperature toughness at -40 °C is 325 J. The tested steel has both high strength and good low temperature toughness, and meets the mechanical performance requirement of API Spec 5L specification for X80 pipeline steel.

Micro-structure evolution, precipitation behavior and strengthening mechanism of Ni-Cr-Mo system steel fabricated via laser powder bed fusion and subsequent quenching control

High-performance 24CrNiMo steel was fabricated using Laser Powder Bed Fusion (LPBF). Subsequent quenching treatment was applied and the influence of quenching temperatures on micro-structure evolution and properties was systematically characterised and analysed. The micro-structure of the as-built steel consisted of two parts. The first part comprised martensite with twins combined with ω-Fe nano-particles, and the second part consisted of lower bainite in the molten pool, as well as upper bainite, granular bainite and tempered martensite in the heat-affected zone. With the quenching temperatures varying from 800 °C to 950 °C, the micro-structure gradually transformed from acicular ferrite + martensite to tempered martensite +θ-Fe3C carbides, and the grain size exhibited noticeable growth. Moreover, quenching treatments could eliminate the anisotropy and inhomogeneity of the micro-structure. The rod-shaped nanosized η-Fe2C and θ-Fe3C precipitates were clearly observed, which were converted from ω-Fe and distributed at multiple angles in the lath. The size and number of nano-precipitates, triggered by the high self-tempering degree of martensite, gradually increased. The relationships among grain size, the twins, dislocation density and nano-precipitation and the dramatically improved performance of quenched samples were analysed using strengthening mechanisms. After quenching at 850 °C, the as-built 24CrNiMo steel attained ultra-high mechanical properties including hardness, Ultimate Tensile Strength (UTS), Elongation (El) and impact energy with values of 480.9 HV1, 1611.4 MPa, 9.8% and 42.8 J, respectively. Meanwhile, both the wear and thermal fatigue resistance increased by approximately 40%. This study demonstrated that LPBF-fabricated 24CrNiMo steel, with matching good performances, can be achieved using a subsequent one-step quenching process.

Microstructural and mechanical characterizations of weld metal of S960QL ultra high strength steel joints obtained with different multi-pass laying techniques using GMAW

Abstract 15 mm thick ultra-high strength steel plates with 960MPa yield strength were welded using different multi-pass laying techniques (i.e., stringer and weaving beads) with torch manipulation. Weld metals obtained were compared using different mechanical (i.e., micro tensile tests and Vickers hardness maps) and microstructural (i.e., optical microscope, scanning electron microscope, X-ray diffraction, electron backscatter diffraction) characterization techniques. Coarser grains and acicular ferrite were observed in weld metal obtained with weaving pass procedure. There were hardness differences in face and root passes of both weld metals. Yet, hardness values were 19% and 11% higher for face and root regions of the joint obtained by stringer pass procedure, respectively. Fractographs of micro tensile test specimens revealed dimples depicting ductile network structure for both joints.

Transverse crack micromechanisms in high-strength steel weld metal with microstructure heterogeneity under hydrogen-containing environment

The transverse crack susceptibility for high-strength steel weld metal with microstructure heterogeneity was investigated under various environment. Transverse cracking probability (C%) increases with increasing microstructure heterogeneity and diffusible hydrogen content. The dominant transverse crack initiation are soft massive pre-eutectic ferrite (PF), the deformation-uncoordinated acicular ferrite (AF)-PF interface and inclusions aggregation. PF, AF-PF interface and inclusions acts as the principal crack extension channel during crack propagation. Transverse cracks pass AF mainly extend along {100} plane while only along {110} when passing PF. MAGBs and HAGBs impede crack propagation. The Ʃ3 boundaries promote the crack propagation, while Ʃ11, Ʃ13b and Ʃ17b hinder the crack propagation. The transverse crack of weld metal under hydrogen-containing environment is attributed to the combination of HEDE and HELP mechanisms in the presence of microstructure heterogeneity and inclusions.

A novel method for fully penetrated U-rib-to-deck joints: Single-sided oscillating laser arc hybrid welding

U-rib-to-deck (RTD) joints represent one of the most critical and vulnerable welding details of orthotropic steel decks. Aiming for full penetration and high-level service performance, a novel method with single-sided oscillating laser-arc hybrid welding (OLAHW) technique was proposed for the RTD joint. The weld formation, microstructure and mechanical properties were extensively investigated. The results indicated that the porosity and lack of fusion (LoF) defects effectively suppressed with the beam oscillation during LAHW process. The OLAHW technique has good spatial accessibility with larger laser beam incidence angle of 20° Beam oscillation led to an increase in acicular ferrite in the fusion zone (FZ) while simultaneously a reduction of martensite in the heat effected zone (HAZ). This was related to the cooling time, and the thermal simulation results revealed that the OLAHW sample demonstrated comparable cooling times in both the Laser-FZ and arc-FZ. Consequently, beam oscillation can generated more homogeneous microstructures and microhardness. Although the microhardness of the OLAHW sample in the FZ was increased compared to the conventional arc welding (CAW) and LAHW sample, the highest microhardness value remained within acceptable limits (380 HV). Moreover, the ultimate tensile strength of the OLAHW weld sample increased by 7 % compared to the CAW weld sample. The application of the OLAHW technique demonstrates its feasibility for engineering applications in orthotropic steel decks manufacturing.