Segregation Zone Research Articles

Investigation of the hydrogen induced cracking behaviour of API 5L X65 pipeline steel

The effect of microstructural features on the hydrogen induced cracking (HIC) susceptibility of two API 5L X65 pipeline steels were investigated by cathodic charging, hydrogen permeation and hydrogen microprint experiments. Microstructural evaluation after hydrogen charging revealed cracks at the mid-thickness (segregation zone) of both plates. However, more severe cracks were observed in the plate with higher dislocation density and residual stresses. The plate with lower plastic strain and more {111}-oriented grains had less severe cracks. Inclusions found along the crack path, comprising of Si-enriched oxides and carbides contributed to the initiation and propagation of cracks. The variation of the trapping behaviour and hydrogen diffusion through the plates were examined. The results confirmed that a higher ratio of reversible to irreversible traps contributes to increasing HIC severity in steels. Additionally, hydrogen transport through the steels was most prominent along the grain boundaries, indicating the importance of grain boundary character to HIC.

Clarification of the false liquation crack existed in 9% Cr/CrMoV dissimilar welded joint by comparative etching process and in situ laser scanning confocal microscope

Liquation cracks are generally regarded as thermal defects in various materials, e.g. in steels and nickel-based alloys. However, direct observation of microstructure with crack-like morphology that may be revealed by chemical etching or electrochemical etching methods, might lead to the misinterpretation of liquation cracks. To avoid the possible misinterpretation, comparative etching was employed to reveal the effect of various etching processes on the determination of liquation cracks. As a result, the formation process and mechanism of the false liquation crack in 9% Cr/CrMoV welded joint were clarified. The results obtained by optical microscope (OM) and scanning electron microscope (SEM) in the heat treatment zone of 9% Cr steel (9% Cr-HAZ) after chemical etching clearly demonstrate the existence of crack-like morphology, and the disappearance of element segregation zone after regrinding and re-polishing. Besides, laser scanning confocal microscope (LSCM) was applied to further exhibit the microstructure evolution experiencing high temperature to confirm the possibility of the liquation crack. The results showed that the element segregation and subsequent chemical etching process were the crucial factors to cause the formation of false liquation crack.

Calibrating a ductile damage model for two pipeline steels: method and challenges

This work is part of a project that aims to develop a micromechanics based damage law taking into account hydrogen assisted degradation. A 'vintage' API 5L X56N and a 'modern' API 5L X70M pipeline steel have been selected for this purpose. The paper focuses on an experimental calibration of ductile damage properties of the well known complete Gurson model for the two steels in absence of hydrogen. A basic microstructural characterization is provided, showing a banded ferrite-pearlite microstructure for both steels. Charpy impact tests showed splits at the fracture surface for the X70 steel. Double-notched round bar tensile tests are performed, aiming to provide the appropriate input for damage model calibration. The double-notched nature of the specimens allows to examine the material state at maximum load in the unfailed notch, and the final material state in the failed notch. Different notch radii are used, capturing a broad range of positive stress triaxialities. The notches are optically monitored for transverse necking in two perpendicular directions (transverse to rolling and through thickness) to reveal any anisotropy in plastic deformation and/or damage. It is explained how the occurrence of splits at the segregation zone, and anisotropy complicate the calibration procedure. Calibration is done for each steel and acceptable results are obtained. However, the occurrence of splits did not allow to evaluate the damage model for the highest levels of tested stress triaxiality.

Resistance Increase to Hydrogen-Induced Sheet Cracking for Gas and Oil Pipes Based on the Structure Formation Control in the Central Segregation Zone

In sheets of low-alloy pipe steels with high purity by harmful impurities and non-metallic inclusions, the nucleation and propagation sites of hydrogen-induced cracking (HIC) are extended segregation bands of increased hardness in the axial zone, with sections consisting of high-carbon structures. The use of intensive cooling after controlled rolling from the austenitic region to the bainitic transformation temperatures and a decrease in the concentration of liquor carbon and manganese in steel helps to increase the sheet resistance to HIC due to reducing central segregation heterogeneity.

Formation of structure and properties of complex alloy cast steel

The patters of the effect of change in the chemical composition and modes of heat treatment on the structure and complex of mechanical properties of cast steel VKL-3 (12Kh2NVML) are studied. The experimental melting of steel with increased content of chromium, silicon, nickel and molybdenum is obtained. The steel of this composition has reduced ductility and toughness. At the same time, experimental steel melting makes it possible to the model segregation zones with increased content of alloying elements in the steel of typical composition that has chemical and structural heterogeneity and heterogeneity of ductility and toughness. It is shown that the use of the fi nal heat treatment with the adjusted modes (in particular, increased temperature and time of tempering) allows increasing the ductility of the steel to the required values, but the impact strength does not reach the required levels. The recommendations for maintaining of the alloying elements content near the lower allowable limit are given.

Hydrogen diffusion and trapping in X70 pipeline steel

In this investigation, the hydrogen permeation experiment was used to determine the parameters for diffusion, and the hydrogen microprint technique was used to visualize the diffusion path in X70 pipeline steels. The samples in mid-layer at the segregation zone and top-layer of the steel were used in this study. The diffusion parameters from the hydrogen permeation experiment enabled us to determine that the calculated density of total, reversible, and irreversible hydrogen trapping sites in the top layer of the steel decreases for larger grains. However, the irreversible and total trapping sites of the mid-layer showed an initial growth and subsequent decay with grain growth due to the inclusions in mid-layer. The observations from the hydrogen microprint experiment allowed us to conclude that the preferential hydrogen diffusion increases in the order of grains, grain boundaries, triple junctions and cementites with cementites being the easiest path for hydrogen diffusion.

Visualization of microstructural factors resisting the crack propagation in mesosegregated high-strength low-alloy steel

While relationship between fracture mechanism and homogeneous microstructures has been fully understood, relationship between fracture mechanism and inhomogeneous microstructures such as the mesosegregation receives less attention as it deserves. Fracture mechanism of the high-strength low-alloy (HSLA) steel considering the mesosegregation was investigated and its corresponding microstructure was characterized in this paper. Mesosegregation refers to the inhomogeneous distribution of alloy elements during casting solidification, and leads to the formation of positive segregation zones (PSZ) and negative segregation zones (NSZ) in ingots. The fracture surface of impact sample exhibits the quasi-cleavage fracture at −21 °C, and is divided into ductile and brittle fracture zone. Meanwhile, the PSZ and NSZ spread across ductile and brittle fracture zone randomly. In ductile fracture zone, micro-voids fracture mechanism covers the PSZ and NSZ, and higher deformation degree is shown in the PSZ. In brittle fracture zone, secondary cleavage cracks are observed in both PSZ and NSZ, but present bigger size and higher quantity in the NSZ. However, some regions of the PSZ still present micro-voids fracture mechanism in brittle fracture zone. It reveals that the microstructures in the PSZ exhibit a higher resistance ability to crack propagation than that in the NSZ. All observations above provide a better visualization of the microstructural factors that resist the crack propagation. It is important to map all information regarding the fracture mechanism and mesosegregation to allow for further acceptance and industrial use.

EBSD study of early fractured phenomena in a 350 grade Maraging steel elbows exposed to hydrofluoric acid

The effect of grain orientations, boundary characteristics, and crystal effects on the crack formation of a 350 high strength Maraging steel elbows exposed to hydrofluoric acid was investigated in the present research. Tempered low tetragonal martensite and fresh martensite were characterized by scanning electron microcopy. Moreover, no segregation zone and precipitates were found near the crack path. Detail analyzing the spatial distribution of crystal orientations by electron backscattered diffraction technique revealed the development of two soft and hard grains. Soft grains oriented approximately along {110} planes provide adequate slip systems to promote dislocation mobility, indicating as low Taylor factor grains distributed well the strain gradient. However, hard grains associated with low compact {001} planes decrease dislocation mobility and increasing the strain accumulation. Micro-cracks initiation and propagation along these high internal energy grains are expected consequently.

Homogenization and Extrudability of 5056 Aluminum Alloy Billet

Through a series of homogenization and extrusion experiments, the extrusion efficacy of 5056 seamless tubes is improved. Although better extrusion practice was apparent with better homogenization parameters for 5056 alloy, a little over-burning structure and phenomena were found on and in the surface and segregation zone of homogenized billet. The extrudability is worse than theoretical level with no aluminum and magnesium oxide particles on the billet surface. Additional, another quality problem in this study is the segregation zone in 5056 larger billets. The over-heat structure was smoothly flowed into the seamless tube by indirect extrusion, and that makes surface bubbles.

Study of the Microstructure and Ring Element Segregation Zone of Spray Deposited SiCp/7055Al.

Composites of 7055 aluminum (Al) matrix reinforced with SiC particles were prepared using the spray deposition method. The volume fraction of the phase reinforced with SiC particles was 17%. The effect of the introduction of SiC particles on the deposited microstructure and properties of the composites was studied in order to facilitate the follow-up study. The structure and element enrichment zone of spray-deposited SiCp/7055 Al matrix composites were studied by Optical Microscope (OM), X-ray diffraction (XRD), Scanning Electronic Microscopy (SEM) and Transmission electron microscopy (TEM). The results show that the reinforcement phases of the SiC particles were uniformly distributed on the macro and micro levels, and a few SiC particles were segregated into annular closed regions. C and Si on the surface of SiC particles diffused to the Al matrix. The distribution of the two elements was gradient weakening with SiC particles as the center, and the enrichment zones of Si, Mg and Cu formed in the middle of the closed annular area of a few SiC particles. The enrichment zones were mainly composed of alpha-Al, SiC, Al2CuMg, Al2Cu and MgZn2. AlCu and AlMgCu phase precipitate on the surface of the SiC particles, beside the particle boundary, and had the characteristics of preferred nucleation. They tended to grow at the edges and corners of SiC particles. It was observed that the formation of nanoparticles in the alloy had a pinning effect on dislocations. The different cooling rates of the SiC particles and the Al matrix led to different aluminum liquid particle sizes, ranging from 20 to 150 μm. In the region surrounded by SiC particles, the phenomenon of large particles extruding small particles was widespread. Tearing edges and cracks continued to propagate around the SiC particles, increasing their propagation journey and delaying the fracture of the materials.

Synthesis and Study of Gas Transport Properties of Polymers Based on Macroinitiators and 2,4-Toluene Diisocyanate.

Nowadays, block copolymers hold great promise for the design of novel membranes to be applied for the membrane gas separation. In this regard, microporous block copolymers based on a macroinitiator with an anionic nature, such as potassium-substituted block copolymers of propylene oxide and ethylene oxide (PPEG) and 2,4-toluene diisocyanate (TDI), were obtained and investigated as effective gas separation membranes. The key element of the macromolecular structure that determines the supramolecular organization of the studied polymers is the coplanar blocks of polyisocyanates with an acetal nature (O-polyisocyanate). In the present research, the influence of the content of peripheral polyoxyethylene (POE) blocks in PPEG on the supramolecular structure processes and gas transport characteristics of the obtained polymers based on PPEG and TDI was investigated. According to the study of polymers if the POE block content is 15 wt %, the polyoxypropylene segments are located in the internal cavity of voids formed by O-polyisocyanate blocks. When the POE block content is 30 wt %, the flexible chain component forms its own microphase outside the segregation zone of the rigid O-polyisocyanate blocks. The permeability for polar molecules, such as ammonia or hydrogen sulfide, significantly exceeds the permeability values obtained for non-polar molecules He, N2 and CH4. A relatively high permeability is also observed for carbon dioxide. At the same time, the content of POE blocks has a small effect on the permeability for all studied gases. The diffusion coefficient increases with an increase in the POE block content in PPEG for all studied gases.

Industrial development of wide and thick plate X65 pipeline steel

The 20.62mm thick X65 trial was carried out on the production line of wide and thick plate. The microstructures, inclusions and mechanical properties were studied by scanning electron microscopy and optical microscopy, low magnification and impact were shown that: (1) The wide plate X65 slab center area is severe and the degree to C level. (2) Inclusions in the finished product is mainly alumina oxide, silicate and composite inclusions, large-scale abnormal non-metallic inclusions were not found. (3) After the first rolling with the microstructure is relatively uniform and small acicular ferrite, grain size acts as 12 grades while the thickness has the center of a slight negative segregation zone, the mechanical properties are to meet the standard requirements but less margin. (4) After adjustment about the second time on the Mn, V composition and rolling process, Compared with the first batch of steel, the yield strength is more than 20 MPa, which fully meets the requirements of X65.

Special Features of Metal Lamination in Hydrogen Sulfide Absorber of Catalytic Reforming Unit and Hydraulic Cleaning

The paper analyzes the technical state of the hydrogen sulfide absorber of the catalytic reforming unit and hydraulic cleaning containing hydrogen and hydrogen sulfide with its operation time of 168,200 h (21 yr). 17G1S manganese-silicon steel is used in the absorber structure. In operation, in the vessel shell, a cone-shaped region of subsurface metal lamination is developed across the entire periphery. The region is under constant observation using the methods of nondestructive testing. In case the region of the damaged metal intersects the longitudinal welded joint, the vessel shell is being repaired. A low-temperature (to the boiling point) hydrogen-related fracture of metal in the petroleum refining due to hydrogen saturation and its accompanying cracking are considered to be the most critical types of corrosion of oilfield equipment. In view of this, a set of laboratory tests using specimens of metal templates cut out of the damaged shell region has been performed to evaluate the serviceability of the absorber, as well as to determine the effect of hydrogen sulfide on the structure and mechanical characteristics of the metal. It is found that the process of lamination is detected inside the metal intersection and is caused by the macroinhomogeneity of rolled metal, i.e., the axial segregation zone, which is the most critical region of the deformed metal. X-ray phase analysis lends credence to the metallurgical nature of lamination of the shell plate. The results of metallographic examination imply that the subsurface hydrogen-induced cracking is caused by aggregation of brittle refined nonmetallic inclusions, mainly of MnS, and it propagates chiefly over sulfide films along the rolled metal direction. The results of experimental investigations on the specimens are provided. From the fractographic investigations on the fractured surface of specimens after mechanical testing it is established that the process of fracture is ductile occurring via microvoids coalescence in the fracture of bridges between them. The key mechanism of microvoids formation is the inhomogeneity of the plastic strain in the micro-volumes around the nonmetallic inclusions.

Influence of die-forging deformation on microstructure of 1538MV non-quenched and tempered steel for crankshaft

Non-quenched and tempered steels offer many advantages such as energy saving, emission reduction, simple processing, short production time, and low cost. Demand for energy saving and emission reduction is increasing with the rapid increase of car production and ownership. As a result, the usage of non-quenched and tempered steel in automotive parts has attracted increasing attention. The main problem with non-quenched and tempered steel in actual production is the lack of a hot deformation process and a cooling process that can be used to precisely control the microstructure and properties of the material. The material 1538MV is a type of pearlite+ferrite non-quenched and tempered steel, and its use for building crankshafts has not been studied sufficiently thus far. The main factors affecting the structure of a crankshaft are deformation, final forging temperature, and metal flow and cooling after forging. In this paper, the forging process of 1538MV non-quenched and tempered steel for building a crankshaft was studied by means of numerical simulation, microstructures of the rolled material and finished crankshaft were analyzed, and influence of deformation on the microstructure of the crankshaft was discussed. The results show that the microstructure of a crankshaft forged at a higher temperature and with smaller deformation is coarser than that of a crankshaft made of rolled material. The inhomogeneity of the microstructure is caused by unevenness of temperature and strain distribution during crankshaft deformation. The ferrite content of the crankshaft and the pitch of the pearlite were lower than those of the rolled material, and the bainite structure appeared in a few parts, which indicated that the cooling rate was too fast during phase transformation. Therefore, the cooling process should be optimized further. In addition, the metal flow in the segregation zone during the crankshaft forging process significantly influences the forged microstructure of the crankshaft, which is another cause of the formation of bainite. Therefore, the quality of the rolled material should be strictly controlled. The above results provide directions for improving the quality of the rolled material, optimization of the crankshaft forging process, and optimization of the cooling process after forging.

Modeling Bimolecular Reactive Transport With Mixing‐Limitation: Theory and Application to Column Experiments

AbstractThe challenge of determining mixing extent of solutions undergoing advective‐dispersive‐diffusive transport is well known. In particular, reaction extent between displacing and displaced solutes depends on mixing at the pore scale, that is, generally smaller than continuum scale quantification that relies on dispersive fluxes. Here a novel mobile‐mobile mass transfer approach is developed to distinguish diffusive mixing from dispersive spreading in one‐dimensional transport involving small‐scale velocity variations with some correlation, such as occurs in hydrodynamic dispersion, in which short‐range ballistic transports give rise to dispersed but not mixed segregation zones, termed here ballisticules. When considering transport of a single solution, this approach distinguishes self‐diffusive mixing from spreading, and in the case of displacement of one solution by another, each containing a participant reactant of an irreversible bimolecular reaction, this results in time‐delayed diffusive mixing of reactants. The approach generates models for both kinetically controlled and equilibrium irreversible reaction cases, while honoring independently measured reaction rates and dispersivities. The mathematical solution for the equilibrium case is a simple analytical expression. The approach is applied to published experimental data on bimolecular reactions for homogeneous porous media under postasymptotic dispersive conditions with good results.

02.06: Weldability of old mild steels within the rehabilitation of historical steel structures

ABSTRACTWelding process causes notch effects and residual stresses. Accordingly, the risk of brittle fracture grows. In comparison with currently used steels, old mild steels have a lower toughness. The weldability of mild steel is limited and only feasible under specific conditions. If for structural reasons welding is preferred instead of the use of bolts, the metallurgical characteristics of the steels have to be considered. These include the concentrations of impurities in the zones of segregation and the tendency to embrittlement by nitrogen‐induced ageing.In the corresponding paper, experimental and analytical studies of the weldability of old mild steels are presented. Extensive material analyses to determine the mechanical and the technological properties of the material will be an essential part of the investigations. Particularly, the increased impurities of phosphorus, sulfur, nitrogen and also oxygen in the segregation zones as well as the distinctive non‐metallic inclusions complicate to produce load‐bearing butt welds. Welding materials currently available are not designed for welding such materials. Nevertheless, in the context of a research project welding tests on old mild steels has been carried out using current welding electrodes. Supported by involved industrial partners, a stick electrode adapted to the characteristics of old steels will be developed.

Microstructure characteristics of segregation zone in 17-4PH stainless steel piston rod

The segregation of Cu and Ni in a 17-4PH stainless steel piston rod has been confirmed to be responsible for the cracking after heat treatment. Further investigation showed that the segregation zone was composed of three layers, namely the fine grain martensitic layer, the coarse grain martensitic layer and the coarse grain austenitic layer from the matrix to the crack surface. Three button ingots with the same chemical compositions as those three layers have been prepared to evaluate the grain size distribution, microstructure and mechanical properties. The effects of Cu and Ni segregation on the microstructures of those three layers have been explored by thermodynamic calculation. Based on the microstructure and mechanical properties results, an intensive understanding of the cracking in the segregation zone was therefore reached.

Effect of Macrosegregation on the Microstructure and Mechanical Properties of a Pressure-Vessel Steel

Macrosegregation refers to the chemical segregation, which occurs quite commonly in the large forgings such as nuclear reactor pressure vessel. This work assesses the effect of macrosegregation and homogenization treatment on the mechanical properties of a pressure-vessel steel (SA508 Gr.3). It was found that the primary reason for the inhomogeneity of the microstructure was the segregation of Mn, Mo, and Ni. Martensite, and coarse upper bainite with M-A (martensite-austenite) islands have been obtained, respectively, in the positive and negative segregation zone during a simulated quenching process. During tempering, the carbon-rich M-A islands decomposed into a mixture of ferrite and numerous carbides which deteriorated the toughness of the material. The segregation has been substantially minimized by a homogenizing treatment. The results indicate that the material homogenized has a higher impact toughness than the material with segregation, due to the reduction in M-A island in the negative segregation zone. It can be concluded that the microstructure and mechanical properties have been improved remarkably by means of homogenization treatment.

Población rural y consumo de lo rural de la región Este de Antioquia - Colombia

How is the eastern Antioquia's Rural population distributed throughout the region? What effect does this distribution have for the region local development? Those are the questions for this article. A comparative and interpretative case study was used for qualitative methodology to answer the question above. Documental revision and semistructured interviews were techniques used during research. This article is composed in three parts: the first part is a basic presentation of some theories that contribute to the issues. The second part is comprised of result and third part includes conclusions. The result indicates that there is a significant rural population (47%) with an uneven of property and rural rent (Gini of 0.50) and different lifestyle and consuming habits. This leads to the conclusion that the infrastructure works and rural rent are creating region of Antioquia zones of spatial segregation inside the Eastern region of Antioquia.

Static recrystallization behaviour of cold rolled Mg-Zn-Y alloy and role of solute segregation in microstructure evolution

We examined the microstructure and texture evolution during the recrystallization annealing of cold rolled Mg-1Y and Mg-0.2Zn-1Y alloys. At the early annealing stage, new grains are nucleated on sub-units with 200–300nm surrounded by dislocation walls within shear bands. The Zn and Y addition reduced the stacking fault energy which results in the high activity of non-basal deformation mechanisms and formation of segregation zones along the stacking faults. This causes the orientation of recrystallization nuclei to be effectively retained so that the texture weakens during the recrystallization without reducing the basal pole split into the sheet transverse direction.