Ferrite Grain Boundaries Research Articles

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.

열가공제어한 저탄소강의 용접부 기계적성질에 미치는 용접기법의 영향

This study investigated the mechanical properties of thermal mechanical control process steel and obtained the following results. In a welded metal (WM) subjected to both single submerged arc welding (SSAW) and tandem SAW (TSAW), grain boundary ferrite (GBF) was generated along with a prior austenite grain boundary and ferrite side plate (FSP). In particular, TSAW with low basicity and a high heat input yielded coarser results than those of SSAW. In the heat affected zone (HAZ), as the heat input increased, the fraction of low-temperature transformation products such as bainite and acicular ferrite decreased, whereas the fraction of primary ferrites such as GBF, FSP, and polygonal ferrite (PF) tended to increase, and the grain size tended to coarsen. The hardness value of the WM increased in the order of electro gas welding, SSAW, and TSAW regardless of the heat input, and the hardness value of the HAZ tended to increase the softening width and decrease the minimum value with an increase in the heat input. Finally, the joint strength of the weld zone tended to decrease as the heat input increased. However, the joint strength of the weld zone under all the applied welding techniques met the value required by the base metal.

Microstructure and corrosive-wear resistance of surfacing layers for underwater wet welding

Underwater wet welding (UWW) is a widely employed method for underwater welding. In this study, Ts306 electrodes were utilized for surfacing on Q235 base material by UWW, and the microstructure and corrosive-wear resistance of the surfacing layers were studied. The results demonstrate that the UWW presents thicker surfacing layer compared with that welded in air, due to the compressive effect of the water on the arc. Meanwhile, the microstructure of the UWW surfacing layer is closely related to the welding current. When the current reached 120 A, the structure of the surfacing layer was mainly pearlite and ferrite. The microstructures of grain boundary ferrite (GBF), acicular ferrite (AF), ferrite with second phase arrangement (FSP), and proeutectoid ferrite (PF) began to exhibit as the current increased to 135 A. When the welding current reached 150 A, the proportion of AF content experienced an increase. The microstructure of the surfacing layer can be refined, leading to an obvious increase in microhardness and corrosive-wear resistance. The substantial increase in the microhardness of the surface layer by 32% and the significant enhancement in corrosive-wear resistance by 60% were observed. Furthermore, the transition of corrosive-wear mechanisms from fatigue wear, corrosion wear, and oxidation wear to abrasive wear, oxidation wear, and adhesive wear was noted.

Effect of N Content on the Microstructure and Impact Properties of Normalized Vanadium Micro-Alloyed P460NL1 Steel

In this work, the effect of nitrogen doping on vanadium micro-alloyed P460NL1 steel is studied in terms of microstructures and impact toughness. As the nitrogen content increased from 0.0036% to 0.0165%, the number of V (C,N) particles increased. The fine precipitates of V (C,N) effectively pin the prior austenite grain boundary, resulting in the refinement of the austenite grain. The intragranular and intergranular V-containing coarse particles enhanced the nucleation of intragranular ferrite and the grain boundaries of polygonal ferrite during air cooling. Accordingly, the proportion of heterogeneously nucleated ferrite increased, and the grain size of ferrite decreased. Notably, the size of the pearlite microstructure decreased, and the bainite microstructure appeared with a high doping of N. With the increase in N content, the impact toughness of vanadium micro-alloyed P460NL1 steel was enhanced. This can be attributed to the refinement of ferrite and the reduction in pearlite, which, in turn, was ascribed to the increase in nitrogen.

Impact of interlamellar spacing and non-pearlitic features on mechanical properties and cyclic damage initiation in near-eutectoid pearlitic steels

The relationship between microstructural characteristics and mechanical properties was investigated for C72 and C82 pearlitic steel rods patented at temperatures between 500 °C and 600 °C using a lead bath. Results revealed a refinement of interlamellar spacing upon reduction of patenting temperature. At the same time, the fraction of grain-boundary ferrite and Widmanstätten ferrite as common non-pearlitic regions was slightly increased, reaching a maximum of nearly 1.3 vol.-% in the case of C72 steel patented at 500 °C. Reduction of patenting temperature was associated with strengthening, as confirmed by tensile, fatigue and hardness tests. The strengthening was explained in terms of the Hall-Petch effect for ferrite due to increase in the boundary area. The Hall-Petch coefficient, obtained by taking the apparent interlamellar spacing as the mean free path of dislocations, was in the range 70–140 MPa.μm12. To some extent, these low values could be related to the low solute interstitial content of pearlitic ferrite. More importantly, the coefficients are underestimated due to the plate-shaped morphology of pearlitic ferrite, which causes the average mean free path of dislocations be longer than the apparent interlamellar spacing. Scanning electron microscopy examinations after interrupted fatigue tests indicated the preferred occurrence of intrusions and extrusions, as precursors to fatigue crack, in the non-pearlitic ferrite. The enhancement of fatigue limit at lower patenting temperatures—in spite of slight increase in the fraction of ferrite—is justified by the stress shielding effect of pearlite and necessity of crack propagation into pearlitic regions for stable crack propagation. In effect, as long as the fraction of non-pearlitic features remains low, the interlamellar spacing of pearlite serves as the most efficient strength-controlling microstructural parameter. Furthermore, due to the preferred growth of fatigue cracks parallel to lamellae and deflection at colony boundaries, the refinement of colonies at reduced patenting temperatures is an additional contribution to the fatigue strength.

Corrosion fatigue behavior of X65 pipeline steel welded joints prepared by CMT/GMAW backing process

The corrosion-fatigue behavior of X65 welded joints, processed by cold metal transfer (CMT) and gas metal arc welding (GMAW) backing, was examined. The application of the CMT backing process led to a 22% increase in the corrosion-fatigue performance, which was attributed to the optimization of the microstructure (synergistic cracking-arrest ability of acicular ferrite (AF) and grain boundary ferrite (GBF)) and the reduction in the stress concentration at the weld root. Nonetheless, a lower heat input contributed to a consequent decrease in corrosion-fatigue performance. In addition, H2S corrosion fatigue was governed by the combined effect of anodic dissolution and hydrogen embrittlement.

The effect of variation of the electric current on physical and mechanical properties in A36 steel welding

The research objective was to determine the effect of current on microstructure, hardness values, and tensile strength of A36 steel before (raw material) and after welding using SMAW using electrodes E7016 diameter 3.2 mm. The research adopted an experimental method and utilized Olympus Metallurgical Microscope, Micro Vickers Hardness Tester, and Universal Testing Machine for data analysis. The results revealed that welding caused a change in the microstructure of the weld area into Grain boundary Ferrite (GF), Widmanstatten Ferrite (WF), and Acicular Ferrite (AF). The hardness test showed that higher current leads to greater hardness values. The specimens with a current of 130 Ampere had a hardness level of 181.39 VHN, which was higher than specimens with 120 Ampere (176.21 VHN) and 110 Ampere (159.56 VHN). The raw material had a hardness value of 125.4 VHN. The welding resulted in a difference in tensile stress, with specimens using 130 Ampere having the highest tensile strength level of 520.20 MPa, followed by 120 Ampere with 504.61 MPa, and 110 Ampere with 483.03 MPa. The research concludes that varying the electric current during welding affects the microstructure, tensile strength, and hardness value of Steel ASTM A36.

SSAW 용접기법이 강재의 용접부 미세조직에 미치는 영향

This study investigated the effects of self-shielded arc welding on the microstructures of welded parts in laminar-cooled-thermo-mechanical-controlled-process (LC-TMCP), titanium nitride (TiN), and B grade steels. The microstructure of the fusion zone was the same in all three samples, consisting of ferrite (grain boundary ferrite (GBF)+acicular ferrite+ferrite side plate) and pearlite phases, and the LC-TMCP and TiN steels had a higher volume fraction of ferrite and smaller grain sizes than those of the B grade steel. In all three samples, the microstructure near the fusion line consisted of ferrite and pearlite phases with a small amount of bainite, and the grain sizes of the LC-TMCP and TiN steels were smaller than those of the B grade steel. In addition, the TiN steel was smaller than the TMCP steel, and the volume fraction of ferrite (GBF, etc.) was the highest in the B grade steel and lowest in the TiN steel. The microstructure of the heat-affected zone included a phase in which a small amount of bainite was present in the ferrite and pearlite phases near the fusion line (1 mm) in all three samples. However, the shape was different from that of the base material, and the grain size was the largest in the B grade steel and smallest in the TiN steel. The volume fraction of ferrite (GBF, etc.) was the highest in the B grade steel and lowest in the TiN steel. All three samples had the same shape as the base metal phase at 5 mm from the fusion line. However, grain was growth.

Phosphorus and transition metal co-segregation in ferritic iron grain boundaries and its effects on cohesion

The phenomenological interplay in the segregation of phosphorus (P) and transition metal (TM) elements at grain boundaries (GBs) in steels has long been suspected to be the main contributor to temper embrittlement. However, many of the details remain unclear. Here, we investigate the segregation, co-segregation and cohesion effects of TMs (Co, Cr, Cu, Mn, Mo, Ni, Nb, Ti, V, W) along with P in various ferritic iron (α-Fe) GBs utilising density functional theory and simulations of kinetics. Our findings demonstrate that P is unlikely to cause intergranular fracture via weakened interfacial bonding when segregated by itself. Nevertheless, the stronger segregation binding of P compared to TMs can explain the ubiquitous presence of P segregated at GBs. We find that most P-TM interactions at ferritic GBs are repulsive and differ significantly from the corresponding interactions in the bulk. Due to the repulsive interactions and strong segregation binding of P, the enrichment of P over time at GBs leads to the depletion of prior-segregated cohesion-enhancing solutes at general GBs. Additionally, certain P-TM co-segregation combinations that are cohesion-lowering are energetically favoured at such GBs. We posit these mechanisms act in tandem as critical causalities of P-induced temper embrittlement in alloyed steels. Finally, we reveal a contradiction in the predicted cohesion effects of segregated P calculated in the Rice–Thomson–Wang theory of interfacial embrittlement compared to that as assessed by chemical bonding strength, calculated in the DDEC6 bond-order framework. These findings have important implications for GB engineering for interfacial cohesion.

Mechanisms of hardening of 12 % chromium ferritic-martensitic steel EP-823

Based on experimental data on microstructure parameters of the reactor high-strength high-chromium (12 % Cr) ferritic-martensitic steel EP-823, the authors identified the main factors responsible for its strength properties. The hardening mechanisms of this steel were analyzed after processing according to the modes that provide different level of steel strength properties. Traditional heat treatment (THT) and promising modifying high-temperature thermomechanical treatment (HTMT) are considered. The main mechanisms of steel hardening, regardless of the processing mode, are: dispersed hardening by nanoscale particles of the MeX type (Me = V, Nb, Mo; X = C, N) by the Orovana mechanism; grain-boundary hardening by high-angle boundaries of martensitic blocks and ferrite grains; substructural hardening by small-angle boundaries of martensitic lamellae; dislocation hardening by increased dislocation density. HTMT mode, which includes hot deformation in the austenitic area, leads to a significant modification of the structural-phase state of steel relative to THT: a decrease in the average size of blocks and lamellae of martensite, as well as ferrite grains, an increase in the density of dislocations and the volume fraction of nanoscale particles of the MeX type. At the same time, the corresponding contributions to value of the steel yield strength from grain boundary, substructural and dispersed hardening increase by 1.2, 1.3 and 1.8 times in comparison with THT. The relative contributions of the considered hardening mechanisms to the yield strength of ferritic-martensitic steel EP-823 were discussed. The values closest to the experimental yield strength after two treatment modes studied are obtained when the Langford-Cohen model is used to estimate the magnitude of substructural hardening.

STRUCTURAL-PHASE STATE AND METAL FRACTURE SURFACE OF WELDS MADE WITH THE USE OF NEW MATERIALS BASED ON TECHNOGENIC RAW MATERIALS

The structural-phase state and the metal fracture surface of 09G2S steel welds made using new welding materials based on technogenic raw materials of metallurgical production (silica manganese slag and aluminum electrofilter dust) were studied by scanning and transmission electron microscopy methods. The production of welded joints was carried out in laboratory conditions using welding wire of the Sv-08GA brand on the equipment of the scientific and production center "Welding Processes and Technologies" of SibSIU. Quantitative analysis of the parameters of the structure and dislocation substructure of the weld metal was performed, using the equipment of the Institute of High-current Electronics SB RAS (Tomsk). The contributions of scalar and excessive dislocation density to the strength of the weld metal are estimated. It is established that the main metal phase of the studied welds (regardless of the introduction of the additive) is a solid solution based on α-iron (ferrite, BCC crystal lattice, α-phase). In addition to α-iron, particles of iron carbide (cementite) and (mainly in the sample without additives) iron silicide of the composition Fe3S are present in the weld metal. Analyzing electron microscopic images of the structure of the weld metal, it was noticed that the narrowest contours are formed at the interfacial boundaries of the inclusion – matrix. Such inclusions detected by transmission electron microscopy methods may be cementite particles located in the volume and at the boundaries of ferrite grains, and iron silicide particles of rounded (spherical) shape located in the volume of ferrite grains. It is revealed that the inclusions of the second phase present in the weld metal are stress concentrators and can be centers of microcrack nucleation under mechanical action on the material.

Effect of Ca Deoxidation on Toughening of Heat-Affected Zone in High-Strength Low-Alloy Steels after Large-Heat-Input Welding

Large-heat-input welding can effectively increase the efficiency and reduce the cost of manufacturing a super-large container ship for marine trade worldwide with thick, high-strength low-alloy (HSLA) steel plates; however, it significantly degrades the toughness of the welding heat-affected zone (HAZ). This paper describes the effect of Ca deoxidation on the impact toughness of simulated coarse-grained HAZs (CGHAZs) in HSLA steels after large-heat-input welding at 400 kJ cm−1. The average impact energy of the CGHAZ increases with an increase in Ca content; in particular, the energy of the steel with 25 ppm Ca content is satisfactorily high, owing to the uniform and fine prior austenite grains. In contrast, the grains in the CGHAZs of the steels with relatively low Ca contents are not uniform, leading to large test variabilities at −20 °C. Failure analyses reveal that the major and secondary cracks preferentially propagate along the coarse and brittle grain boundary ferrite (GBF), but their propagation is retarded by the fine and interlocking intragranular acicular ferrite (IAF) as the grain size decreases. It is therefore recommended to increase the Ca content to about 25 ppm during the deoxidation of steelmaking to improve HAZ toughness by forming fine and uniform prior austenite grains and IAF within grains.

Structure, defect substructure and fracture surface of low-carbon alloy steel welds

By the methods of scanning and transmission electron microscopy a structural phase state, defect substructure and fracture surface of low carbon alloy steel welds produced with use of carbon-containing addition and without it are investigated. The study of etched metallographic section structure of welds revealed a presence of a large amount of second phase particles (carbides, sulfides, oxides, etc.). It is shown that in weld with a carbon-containing addition the particles locate chaotically, and in weld without addition they decorate boundaries of ferrite grains. Dimensions of second phase particles vary in the limits from 0.8 μm to 5 μm and are identical for both types of welds. Analysis of fracture surface structure of welds is carried out. A presence of nonmetallic inclusions is detected. They are typical of mainly weld produced without a carbon-containing addition which may indicate its increased brittleness. Amount of micropores in fracture of weld with carbon-containing addition is many times greater than that in weld without addition, which is indicative of gas removal in use of carbon-containing addition. The quantitative analysis of structure and dislocation substructure parameters of weld metal is carried out, the estimates of contributions of scalar and excess dislocation density to strength of welds are performed. It is shown that higher values of scalar and excess dislocation density in the weld formed without carbon-containing addition in flux may be responsible for material embrittlement. A large quantity of stress concentrators in the weld without carbon-containing addition may result in material embrittlement.

Effect of weld microstructure on the tensile properties and impact toughness of the naval, marine-grade steel weld joints

Pulsed direct current (PDC) gas tungsten arc welding (GTAW) of 12 mm thick plates of naval, marine-grade high strength low alloy steel (HSLA) using ER80S–Ni3 filler metal was investigated. The microstructural characteristics were examined by both optical microscopy and field-emission scanning electron microscopy (FE-SEM) techniques. The fusion zone is comprised of mixed microstructures of acicular ferrite (AF), polygonal ferrite (PF), grain boundary ferrite (GBF), and bainite ferrite (BF) laths in the various welding passes. The yield and tensile strengths of the PDCGTA weld joints were found to be superior to that of the base metal. A joint efficiency of 122% was observed for the weld seams while conducting the notch tensile studies (NTS). The impact toughness of the weld joints was higher than that of the base metal. The tensile and impact properties in the room temperature (RT) conditions were superior due to the formation of acicular ferrite (AF) and bainite ferrite (BF) laths in the weld seam microstructure. A higher toughness value of 173 J and ductile fracture mode was observed when the joints were impact loaded at −40 °C. On the other hand, the brittle cleavage fracture was observed for the joints subjected to impact loading at −196 °C. The impact toughness data indicated that the joints experienced a transition from a ductile to a brittle mode of fracture on lowering the temperature.

Effect of H2S Corrosion on the Fracture Toughness of the X80 Pipeline Steel Welded Joint.

To analyze the causes and mechanisms affecting the fracture toughness of X80 pipeline steel welded joints against H2S, the fracture toughness of different zones of X80 pipeline steel welded joints in both air and saturated H2S solution was investigated. The fracture toughness of welded joints degraded significantly in the saturated H2S solution, where the crack tip opening displacement (CTOD) characteristic value in the coarse grain heat-affected zone (CGHAZ) and weld metal (WM) was only 8% and 12% of that in air, respectively. However, the sub-critical grain heat-affected zone (SCHAZ) showed better resistance to H2S corrosion, with the CTOD characteristic value reaching 42% of that in air. The resistance of the welded joint to H2S corrosion was sensitive to microstructures. The grain boundary ferrite (GBF) presented in WM, and the angle of grain boundary orientation in CGHAZ was not conducive to hindering crack propagation. Moreover, the formation of the resultant hydrogen cracks owing to the H2S corrosion also reduced the fracture toughness of the welded joint.

Three-Dimensional Morphology and Analysis of Widmanstätten Sideplates Ferrite

The three-dimensional (3D) morphology and crystal structure of Widmanstätten sideplate ferrite were simulated using a focused ion beam (FIB) scanning electron microscope equipped with electron backscatter diffraction (EBSD). The primary Widmanstätten sideplates nucleated and grew directly at the austenite grain boundary (GB). A certain included angle between the sideplates and the austenite GB was observed. The sideplates grew approximately parallel to the grain, and were separated by a small-angle GB. The primary Widmanstätten sideplates are best described as “∃” shaped, with a long intermediate ferrite strip. The interface with the austenite GB was smooth and flat, and the sideplate surface contained pits and holes. The secondary Widmanstätten sideplates nucleated and grew on the surface of the proeutectoid GB ferrite, with the sideplates and GB ferrite perpendicular to each other. Sideplates parallel to one another grew into the grain, and were separated by small-angle GB. The 3D morphology was distinguished by its “comb” shape. The sideplates’ tail was clustered and its front end remained sharp. The contact side of the GB ferrite was smooth and flat. The surface contained several uneven pits and defects.

Role of minor addition of B in submerged arc weld metal for 500 MPa grade seismic steel plates

In this study, we investigated the effects of boron content on the toughness of a 500 MPa-class structural steel weld. Submerged arc welding was performed using welding consumables with boron contents of 0, 0.001, and 0.003 wt% for heat input conditions of 34, 42, and 56 kJ/cm, respectively. The addition of boron reduced the grain boundary ferrite (GBF) and ductile–brittle transition temperature, resulting in improved impact toughness. Moreover, the addition of 0.001 wt% boron further increased the toughness of the weld metal. This was because the presence of boron at the austenite grain boundaries reduced its surface energy, thus delaying ferrite nucleation. In addition, the minor boron element prevented the formation of grain boundary ferrite (GBF) structures during solidification, and an acicular ferrite structure was created in the alloy during sufficiently slow cooling rates (higher heat input condition).

Research of the occurrence and development of metal defects in sulfide stress cracking inducing environment

Comparative studies of the metal of equipment after long-term operation in high hydrogen sulfide environment are carried out. To compare their resistance to sulfide stress cracking, the samples were hydrogenated under a stress of 0.6-0.9 of yield stress in a hydrogen sulfide environment. Characteristic of surface defects (blisters and bubbles) were identified, their size and time of occurrence were determined. To study the further development of the process, the samples were subjected to intense hydrogenation for 6 hours at maximum tensile stresses of 0.95 of the yield stress. Then they were cut across for conducting a metallographic study in order to identify the initiation and development of cracks in the metal. It is shown that steel containing cerium, which promoted the formation of relatively small globular sulfides, has the highest resistance to sulfide stress cracking. Such inclusions, in contrast to sulfide stringers of the FeS and MnS type, were not sources of crack initiation. Another source of crack initiation was the boundaries of pearlite and ferrite grains contaminated with microimpurities.

Effect of soft reduction technique on microstructure and toughness of medium carbon steel

This paper investigated the effect of soft reduction technique on microstructure evaluation and toughness of medium carbon steel. The scanning electron microscope (SEM) was used in conjunction with fracture test to identify the comparative microstructure-toughness relationship, while the morphology of grain boundary ferrite (GBF) and widsmanstatten ferrite (WF) was analyzed by electron backscatter diffraction (EBSD) at different soft reduction amounts. It was observed that the area ratio of grain boundary ferrite phases (GBF + WF) decreases with increasing the reduction amount. Both ferrite phases nucleated at the austenite grain boundary and grow into their neighbouring grain due to the different orientation relationship. At high reduction amounts the GBF holds non K-S orientation with prior austenite grain into which it grows, caused by the presence of high energy interface between the GBF and martensite matrix. This criterion restricts the formation of GBF along the prior austenite grain boundary (PAGB) and resulting in its discontinuous growth into the adjacent austenite grain. The continuous grain boundary ferrite precipitation is more detrimental as compared to the discontinuous precipitation. Furthermore, film-like retained austenite was aligned along the growth direction of widsmanstatten ferrite plates with its increased volume fraction at high reduction amounts, which manifested the improved fracture toughness.

Investigation on microstructure and mechanical properties of double-sided synchronous TIP TIG arc butt welded duplex stainless steel

Double-sided synchronous TIP TIG arc butt welding (DSSTTABW) is firstly carried out to join 5-mm SAF 2205 duplex stainless steel (DSS) plates without grooving. The microstructure and mechanical properties of DSSTTABW joints, including microhardness, transverse tension, and bend properties, are evaluated systematically. Results show that sound DSS joint, with excellent appearance and properties, can be obtained through the DSSTTABW process. In DSSTTABW joints, a deal of second austenite (γ2) appears in the weld metal, resulting in a slight increment of austenite volume fraction of the weld zone. Meanwhile sigma (σ) phase precipitates at the grain boundaries of delta ferrite (δ-Fe) and austenite of the weld zone. Mechanical property tests show that the DSSTTABW weldment exhibits higher tensile strength and microhardness but lower ductility compared to 2205 DSS base metal due to the existing Widmanstten-type austenite (WA) and σ phase. All weldments fracture at the weld zones in the mode of ductile. After bending tests, no surface cracks are observed on the DSSTTABW joint.