Formation Of Acicular Ferrite Research Articles

Effect of Filler Wire Composition on Weld Metal Microstructure and Mechanical Properties in X80 Steel Laser Welds.

Laser welding was performed using different filler wires, ER70S steel, commercially pure iron, and pure nickel filler, in the context of welding X80 pipeline steel to assess the microstructure and mechanical properties of the weld metal. Introducing an ER70S wire promoted acicular ferrite formation in the fusion zone, compared to a bainitic microstructure in an autogenous laser weld. The use of pure iron wire was considered as a potential strategy for reducing hardenability, as it led to the dilution of alloying elements in the fusion zone, increasing ferrite content and reducing weld metal hardness to a level compliant with API pipeline standards. The addition of pure nickel wire was used to reveal the degree of weld metal mixing imposed by the laser (thus providing an unambiguous tracer element) when it is combined with filler material dilution in the fusion zone, revealing that the upper region contained 38% wire material and the lower region only 12%. This accounts for the differences observed between the upper versus lower portions of the weld metal when other wires are used, and the use of hardness mapping and micro-indentation demonstrates the correlation between the variations in mechanical properties and microstructural differences introduced by incomplete mixing of the filler wire elements.

Relationship between microstructure and mechanical properties of V-N microalloyed high strength ship plate steel

V-N microalloying treatment is an important way to improve the service performance of non-quenched and tempered ship plate steel. Herein, the influence of V(C, N) on the evolution of microstructure and improvement of mechanical properties was studied. In addition, the relationship between microstructure and mechanical properties of V-N microalloyed high strength ship plate steel was revealed. The results showed that the composite addition of V and N not only formed a fine dispersed precipitated phase, but more importantly, significantly refined the ferrite/pearlite microstructure, promoted the formation of intragranular acicular ferrite, increased the proportion of high angle grain boundaries, and decreased the kernel average misorientation value. The optimization of microstructure brought about by V-N microalloying achieved synchronous improvement of strength and cryogenic toughness. The impact energy of V-N microalloying ship plate steel increased from 97 J of V-N-free ship plate steel to 239 J at −40 °C, and the impact fracture mode changed from brittle quasi-cleavage fracture to microvoid coalescence fracture with a large number of equiaxial dimples.

Impact of Permanent Magnet Stirring on Inclusion Evolution, Solidification Structure and Mechanical Properties of AH32 Shipbuilding Steel

An experimental investigation has been conducted with respect to the effect of permanent magnet stirring (PMS) under different rotation speeds (0, 60, 180 rpm) on the solidification of AH32 shipbuilding steel by analyzing the inclusion evolution, solidification structure, and mechanical properties. As the rotation speed increased from 0 to 180 rpm, the mean size of inclusions (Al2O3, MnS, and Al2O3–MnS) reduced significantly from 6.23 to 2.92 μm, their number increased from 49 to 82 n mm−2 and the ratio of complex inclusions increased from 30.6% to 75.6% in the AH32 steel ingot. Besides, the increasing number of small complex inclusion promotes the formation of acicular ferrite in steel, resulting in an elevation of the area ratio from 10.5% to 27.8%. Furthermore, under a rotation speed of 180 rpm, the mean grain size decreased to 99.4 μm and the frequency of high‐angle grain boundaries (HAGBs) increased to 86.1%. In addition, the as‐cast AH32 steel exhibited an optimized ultimate tensile strength of 532.0 MPa and an improved elongation of 24.2% after an enhanced PMS, which was attributed to more acicular ferrite, higher HAGBs ratio, and refined grain size.

Nichrome Dependency in Welding Layer Using In situ Fabrication on Hardness and Corrosion Properties

The use of shielded metal arc welding (SMAW) equipment to synthesize alloys is one of the most promising methods as it can be used for in-situ fabrication. The hardness and corrosion properties of the deposited layer during the welding process can be enhanced by dilution of nichrome (NiCr) during the process. Therefore, the alloy was synthesized on the surface of the mild steel using SMAW equipment with various NiCr additions. The properties of the alloy were investigated using an X-ray diffraction (XRD), an Optical microscope, a Potentiostat, and a Vickers hardness apparatus. The appearance of martensite overlaps with the ferrite phase could be affected by the addition of NiCr to the alloy. An increase in NiCr content is followed by an increase in the lattice parameter value of the ferrite phase. The presence of a certain amount of nickel (Ni) contributes to the formation of lower transformation phases, including martensite. At the same time, the incorporation of chromium (Cr) also plays a significant role in promoting the formation of acicular ferrite. An increase in NiCr content in NiCr2 and NiCr3 samples results in a quantitative increase in martensite formation. Adding 0.434 g of NiCr to the weld deposited layer could result in the corrosion rate of the alloy being 0.1118 mm/y. Higher hardness is found in the NiCr3 sample, around 742.06 HV.

Inclusion Size Dependency of Acicular Ferrite Formation in EH36 Shipbuilding Steel Weld Metal Treated by CaF2–TiO2 Flux

Inclusion Size Dependency of Acicular Ferrite Formation in EH36 Shipbuilding Steel Weld Metal Treated by CaF2–TiO2 Flux

Investigation on the formation mechanism of collapse defects, microstructural evolution and mechanical properties in full penetration laser welding of thick-section steel

In this paper, we described full penetration laser welding (FPLW), a process capable of achieving single-pass welding of thick-section steel. This work summarized a welding process window encompassing various typical formations, including those well-formed and collapse defects. Furthermore, collapse defects were categorized as “collapse formation with backside humping” and “collapse formation with underfill”. High-speed camera (HSC) images reveal that the formation mechanisms of these two defects are attributed to the accumulation of liquid metal at the rear edge of the bottom molten pool and the ejection of a large amount of liquid metal from the weld in a spatter form, respectively. A computational fluid dynamics (CFD) numerical simulation model well-aligned with the experimental results has been established. The analysis of the molten pool flow behavior and keyhole evolution revealed that during well-formed welding, the keyhole status in the welding process exhibits a periodic closure-collapse mode. This periodic mode can instantaneously release the pressure at the bottom of the keyhole, inhibiting the excessive accumulation of liquid metal at the bottom of the weld and thus inhibiting the formation of collapse defects. The cooling rate in the upper part of the weld exceeds that in the middle and lower parts, primarily resulting in the formation of fine acicular ferrite and martensite. The increased tensile strength exhibited by the welded joint relative to the base material predominantly emanates from the mechanisms of grain boundary reinforcement and dislocation strengthening, precisely caused by the refined microstructure.

Role of Ti and Cr on microstructure and hydrogen embrittlement of welded joint of low-alloy steel used for armor layer

Welded joints suffer from hydrogen embrittlement (HE) owing to the internal stress generated during welding. Alloying elements play a vital role in the metallurgical behavior of welded joints. In this study, the influence of chromium (Cr) and titanium (Ti) on the HE behavior of welded joints in low-alloy steel was investigated by examining the microstructural morphology, grain characteristics, precipitate particles, engineering stress–strain curves, and fracture modes. The experimental results indicated that, compared with the addition of Cr, the addition of Ti promoted the formation of acicular ferrite in the lower fusion zone (FZ) via Ti-bearing oxides, and refined the grains of the FZ. The strength and elongation of the Ti joint were superior to those of the Cr joint in both air- and hydrogen-containing environments. Furthermore, compared with the addition of Cr, the addition of Ti improved the HE sensitivity. The fracture modes of both the Cr and Ti welded joints changed from ductile to brittle when the prepared joints were exposed to an H-containing environment.

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.

Effect of heat input on the mechanical properties of welded Q960E ultra-high strength steel butt joints

The influence of welding heat input on the tensile properties and low-temperature impact toughness of Q960E weld metals (WMs) and butt welded joints (BWJs) was investigated. Prediction models were established to correlate the heat input range (4–17.5 kJ/cm) with the yield strength, tensile strength, and elongation of both WMs and BWJs. Impact toughness was examined at different temperatures for the WMs, heat-affected zones (HAZs), and BM. Microstructure transformations were revealed in different regions of the BWJs, and their impact on the tensile properties and toughness. The results demonstrated that the yield strength, tensile strength, and elongation of the BWJs were 0.80–0.99, 0.9–1.0, and 0.44–0.82 of the BM, respectively; this was attributed to variations in the average effective grain size and Taylor factor in different regions of the BWJs, which led to distinct strength and plastic deformation capacities. The impact toughness of both the WMs and HAZs was notably diminished when compared to the BM. For the WMs, the Charpy impact absorption energy values were only 0.24–0.34, 0.50–0.59, and 0.40–0.50 of that of the BM for heat inputs of 4, 7.4, and 11.2 kJ/cm, respectively. For the same heat inputs, in the case of the HAZs, the Charpy impact absorption energy values were only 0.71–0.90, 0.63–0.70, and 0.63–0.80 of that of the BM, respectively. The presence of martensite–austenite (M–A) islands in the HAZ contributed to the decrease in the impact toughness. Increasing the heat input resulted in larger M–A components, thus further reducing the impact toughness of the HAZ. Conversely, a higher heat input for the WMs promoted the formation of acicular ferrite, thereby enhancing its impact toughness. However, owing to the abundant lath martensite present in the WM, its impact toughness was significantly inferior to that of the BM.

Effect of different feedstocks on the microstructure and mechanical properties of HSLA steel repaired by underwater laser direct metal deposition

Underwater laser direct metal deposition (UDMD) technique shows great potential for the on-site repair of damaged structures in the ocean. In this study, aiming at the possibility of flexible feedstock substitution, two powders with different properties are employed to repair the preprepared HSLA steel plates using UDMD. The combined effects of alloy compositions and UDMD processing on the repair performance including microstructure, mechanical properties and corrosion behavior, are systematically studied. The results show that the localized laser melting/solidification and rapid underwater cooling effect significantly increase the cooling rates, and thus lead to the formation of fine-grained lath martensite and acicular ferrite. The high Ni content in the sample repaired with ultra-high strength (UHS) powder contributes to the interlath austenite. Compared with that of the sample repaired with high strength (HS) powder, the microhardness of the sample repaired with UHS powder is high due to the solid solution strengthening and grain boundary strengthening. In addition, the tensile properties of the UDMD samples exceed the marine steel requirement (yield strength ≥690 MPa). The acceptable impact toughness of the UDMD samples is due to the existence of acicular structures and high fraction HAGBs. Furthermore, the UDMD samples present a better corrosion resistance in comparison with the substrate. The experimental findings demonstrate the employment feasibility of powder substitution on the UDMD repair of complex steel structures in the ocean.

Influence of austenitizing temperature on acicular ferrite formation on Ti-Rare Earth Metal (REM)-Zr oxide in low carbon steel

Influence of austenitizing temperature on acicular ferrite formation on Ti-Rare Earth Metal (REM)-Zr oxide in low carbon steel

Microstructure and mechanical properties of laser beam welded 10 mm-thick Q345 steel joints

The present study systematically investigated the effects of a laser welding speed and a defocusing amount on the microstructure and mechanical properties of low alloy high strength Q345 steel joints. The macro-morphology, microstructure, Vickers microhardness, and tensile properties of as-welded joints were analyzed. The results indicated a significant correlation between the laser welding speed and the size of individual zones within the welded joint. As the laser welding speed increased, the size of each zone decreased noticeably. The microstructure of the fusion zone was composed of proeutectoid ferrite, ferrite side-plate, and acicular ferrite. An increase in welding speed and defocusing amount facilitated microstructural changes towards formation of ferrite side-plate and acicular ferrite. Maximum tensile strength and maximum elongation of as welded joints were measured to be 669 MPa and 21.8%, respectively. The joints with maximum tensile strength tended to fracture within the fusion zone. The increase in welding speed and defocusing amount caused a significant decrease in normalized enthalpy and volumetric energy density.

Acicular ferrite formation and Mn depletion behavior around the inclusions in Ti-Mg oxide metallurgy steel with different holding times

In this study, the effect of different holding times at 1250 °C on acicular ferrite formation, the inclusion characteristics, and Mn depletion behavior at the inclusion-matrix interface in Ti-Mg oxide metallurgy steel was systematically investigated. The results suggested that the volume fraction of grain boundary products decreased with increased holding time from 60 s to 3600 s, with the volume fraction of acicular ferrite initially increasing and then decreasing. Regarding the inclusion characteristics, different holding times showed no obvious effect on the inclusion type, but they affected the size and area number density of the inclusions. The domain inclusions at different holding times all consisted of MgTi2O4 +MnS, and these inclusions were found to be effective for the nucleation of acicular ferrite. The average diameter of the inclusions increased, and the area number density of the effective inclusions significantly decreased when the holding time was increased from 900 s to 1800 s and 3600 s. Mn-depleted zones (MDZs) with larger widths and higher Mn concentration gradients formed at the inclusion-matrix interfaces at holding times of 60 s and 900 s, and MDZ formation was weakened by the continuous diffusion of Mn when the holding time was increased to 1800 s and 3600 s. At a holding time of 900 s for the Ti-Mg oxide metallurgy steel, the volume fraction and relative nucleation potential of acicular ferrite reached their maximum values, and MDZ formation with the lowest Mn concentration (∼0.8 wt%) was observed around the MgTi2O4 +MnS inclusions. Therefore, 900 s was the optimal holding time for acicular ferrite formation.

Influence Of External Magnetic Field On Mechanical And Metallurgical Properties Of Pressure Vessel Steel (Sa 516 Grade 70) Welds Using Gas Tungsten Arc Welding

Abstract The mechanical and metallurgical properties of weld play the most important role in the performance of welds which needs to be enhanced. The superimposing external magnetic field to the welding arc provided wonderful and favourable results for improving the shape and properties of weld. In this research, different configurations of specially designed electromagnets were used to investigate their effects on weld characteristics of SA 516 grade 70 welds. It is observed that 0-0-S-N configuration yielded maximum penetration (3.92 mm) compared with other configurations as well as the conventional gas tungsten arc welding (GTAW) process. Tensile test, Charpy impact test and microhardness were performed to investigate the mechanical properties and microstructure analysis was used to determine the metallurgical properties of the weld joints. The result shows that the tensile strength, impact strength, and microhardness of magnetically controlled GTAW (MCGTAW) weld was 620MPa, 275J and 198HV respectively which is 3.16%, 22.76%, and 1.51% higher than weld produced by GTAW process. It has also been observed that electromagnetic stirring refined the microstructure of the weld pool. The average grain size of MCGTAW weld was 31.035µm whereas 42.558µm average grain was obtained in conventional GTAW weld. The electromagnetic stirring enhanced the weld cooling rate resulting formation of more acicular ferrite which is desirable.

Effect of Ti addition on mechanical properties and corrosion resistance of X80 pipeline steel

In this study, the effects of Ti addition on the mechanical properties and corrosion resistance of X80 pipeline steel were investigated using tensile impact, electrochemical and neutral salt spray tests, scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). The results show that the addition of Ti can promote the formation of acicular ferrite (AF), resulting in significant grain refinement. It can improve the tensile strength, yield strength and impact toughness of X80 pipeline steel, with the highest value of tensile strength at 0.04 wt % Ti content, which is 8% higher than the original; yield strength is 18% higher; and impact work is 9% higher. The best corrosion resistance of the pipeline steel is achieved at a Ti addition of 0.04 wt %, with a reduced corrosion weight loss rate, 28% less than the original. The Tafel curve is shifted forward, the self-corrosion potential Ecorr increases and the corrosion current density Icorr decreases, improving by 10% from the original. The capacitive arc radius and Rct increase, the resistance to corrosion reaction increases. The corrosion products are mainly loose and porous γ-FeOOH and β-FeOOH in the early stage of corrosion, and more α-FeOOH and Fe3O4 are generated in the later stage.

Effect of Al or Ti addition on the ferrite formation in ultra-low oxygen weld metal of low carbon steel

Effect of Al or Ti addition on the ferrite formation in ultra-low oxygen (about a few tens ppm) weld metal of low carbon steels were investigated. A bead-on-plate welding experiments were performed with a Tungsten Inert Gas (TIG) welding system. The double nozzle was attached to the welding torch used, Ar was passed along the outer nozzle and He is added along the inner nozzle. Optical micrographs showed that primary ferrite appeared along prior austenite grain boundaries and that as the temperature is decreased ferrite side plates were grew into grain interior. Their formation positions were approached to the welding end as the decreasing Al and increasing Ti content. This change in ferrite formation position attributed on the ferrite formation temperature, which was assumed to increase as the decreasing Al and increasing Ti content. Energy dispersive X-ray spectroscopy analysis of inclusions showed that Al and Ti were mainly associated with 0, it was obvious that the inclusions in the steels were Al- and Ti- oxide inclusions. Al-oxide inclusions were not favourable acicular ferrite nucleation sites and the inclusions contributing to acicular ferrite formation were Ti-oxide ones. The number density of acicular ferrite was small. This indicated that acicular ferrite formation was suppressed because the number density of inclusions was low under the ultra-low oxygen welding condition.

Effect of Ce content on the hydrogen induced cracking of X80 pipeline steel

In this study, the effect of Ce content on hydrogen induced cracking (HIC) of X80 pipeline steel has been investigated. The results show that as the Ce content increased from 0 wt% to 0.0042 wt%, 0.016 wt% and 0.024 wt%, the HIC susceptibility of tested steels decreased first and then increased. The steel containing 0.016 wt% Ce possessed the lowest HIC susceptibility because Ce modified inclusions, promoted the formation of acicular ferrite, and decreased the number of hydrogen traps and intergranular cracks.

Effect of Boron Content on Microstructure and Impact Toughness in CGHAZ of Shipbuilding Steel Plates with Ca Deoxidation

Herein, the effect of boron (B) content on the microstructure and impact toughness in the coarse‐grained heat‐affected zone (CGHAZ) after large heat input welding of 400 kJ cm−1 for the EH40 shipbuilding steel plate with Ca deoxidation is studied. As the B content is increased from 0.0002 to 0.0024 wt%, the average width of grain boundary ferrites (GBFs) is decreased from 14.6 to 11.3 μm, the formation of acicular ferrite (AF) is promoted, and the formations of GBF and Widmanstatten ferrite (WF) are hindered in CGHAZ. The heat‐affected zone (HAZ) impact toughness is increased from 39 to 97 J at −20 °C. The fraction of low‐angle grain boundaries (LAGBs) is decreased from 40 to 29%, and the high‐angle grain boundaries (HAGBs) is increased from 60 to 71%. When the B content is 0.0024 wt%, the B atom segregates on the grain boundaries, and the widths of the segregation lines range from 2.3 to 3.6 μm. GBF is found to be the weakest zone in the CGHAZ. B addition can effectively reduce the formation of GBF, enhancing the critical crack stress and improving the HAZ toughness.

The microstructure evolution and influence factors of acicular ferrite in low alloy steels

The excellent combination of strength and toughness renders acicular ferrite to be a desirable microstructural contexture in C-Mn and high-strength low-alloy steels. The formation of acicular ferrite in steels is generally affected by factors such as composition, cooling rate, prior austenite grain size (PAGS), and impurity inclusions. Due to the complexity of multi-factor coupling, it is difficult to explore the influence of a single factor through actual experiments. In the current work, the influences of cooling rates, PAGS, and impurity inclusion density on the microstructure evolution of austenite-acicular ferrite transformation in low alloy steels are quantitatively evaluated by multi-phase-field simulation. The numerical results demonstrate that reducing PAGS during austenite-ferrite transformation, increasing cooling rate and increasing inclusion nucleation density can effectively refine acicular ferrite grain size. Moreover, the C element diffuses from acicular ferrite to austenite and accumulates at the phase interface during the phase transformation from austenite to ferrite. The findings in the current work contribute to better regulating and designing acicular ferrite in steels.

Effects of substituting municipal waste incineration fly ash for lime on generating a bonding phase during the sintering process

As the main solid by-product of municipal waste incineration, the safe treatment and resource utilization of municipal waste incineration fly ash (hereafter referred to as fly ash), comprising a large amount of CaO, SiO2, Al2O3, and related compounds, has become an exceedingly important environmental concern. Owing to its alkaline composition, herein we propose that fly ash replaces lime as a part of the flux used in the sinter process. Following a combined approach using thermodynamic calculations and experimental verification, we found that CaO from fly ash can combine with Fe2O3 to form complex calcium ferrite, which can effectively replace lime to form the binder phase for the sintering process. In addition, a small amount of Al2O3 impurities present in fly ash promotes the formation of acicular calcium ferrite with an interwoven structure, which improves the reducibility and mechanical strength of the sintered ore. However, when the proportion of fly ash replacing lime is too high and results in an Al2O3 content greater than 2%, the acicular calcium ferrite is transformed to the plate state, which deteriorates the metallurgical properties of the sintered ore. Furthermore, alkaline metal chlorides (e.g., KCl and NaCl) present in fly ash can improve the dissolution of Al2O3 and SiO2 in silicate, which impedes the performance of the binder phase. However, most of the alkali metal chlorides are volatilized, and only a small amount remains in the form of oxides, which have little effect on the performance of the sintered binder phase. Overall, the best metallurgical properties were obtained when fly ash replaced 25% lime as the sintering alkaline flux. Therefore, replacing lime with fly ash to form the binder phase during sintering holds excellent application prospects.