Low Carbon Steel Specimens Research Articles

Model-Assisted Approach for Probability of Detection (POD) in High-Temperature Ultrasonic NDE Using Low-Temperature Signals

Advanced piezoelectric-based ultrasonic transducers offer the potential for in-coolant nondestructive testing (NDT) measurements at high temperatures (HTs), including during hot standby (~260°C) for liquid-sodium–cooled advanced small modular reactors. The reliability of the NDT measurements is typically quantified by the probability of detection (POD) measured at the corresponding temperature. Obtaining such data in liquid sodium is challenging. Using a model-assisted POD approach, a transfer function is reported that enables data obtained on low carbon steel specimens at room temperature to give an estimated POD at an HT. A primary source of the difference in POD between room temperature and HT is due to the transducer material temperature-dependent performance. This paper demonstrates the transfer function approach using data for modified lead zirconium titanate (PZT-5A). A physics-based model was developed using a finite element method and used to quantify reduction in the scattering amplitude for standard reflectors, side drilled holes (SDHs), for a range of sizes, from 15°C to 195°C. Scattering amplitudes for the room-temperature–simulated data are compared with the experimental data measured at 2.25 MHz. A temperature correction and transfer functions were developed to transform the simulated temperature effect in the physics-based model to compare with the experimental data. The model-based approach was validated with experimental data. It was seen and validated for a PZT-5A ultrasonic transducer operating at 2.25 MHz that the 95% POD at 15°C was 0.58 λ, and due to variation in temperature-dependent properties of PZT-5A, the 95% POD was achieved only for a 1.41 λ SDH diameter.

Features of the Hydrogen-Assisted Cracking Mechanism in the Low-Carbon Steel at Ex- and In-situ Hydrogen Charging

Hydrogen embrittlement has been intensively studied in the past. However, its governing mechanism is still under debate. Particularly, the details of the formation of specific cleavage-like or quasi-cleavage fracture surfaces related to hydrogen embrittled steels are unclear yet. Recently it has been found that the fracture surface of the hydrogen charged and tensile tested low-carbon steel exhibits quasi-cleavage facets having specific smoothly curved surface, which is completely different from common flat cleavage facets. In the present contribution we endeavor to shed light on the origin of such facets. For this purpose the notched flat specimens of the commercial low carbon steel were tensile tested using ex- and in-situ hydrogen charging. It is found that in the ex-situ hydrogen charged specimens the cracks originate primarily inside the specimen bulk and expand radially form the origin to the specimen surface. This process results in formation of “fisheyes” – the round-shape areas with the surface composed of curved quasi-cleavage facets. In contrast, during tensile testing with in-situ hydrogen charging, the cracks initiate from the surface and propagate to the bulk. This process results in the formation of the completely brittle fracture surface with the quasi-cleavage morphology - the same as that in fisheyes. The examination of the side surface of the in-situ hydrogen charged specimens revealed the straight and S-shaped sharp cracks which path is visually independent of the microstructure and crystallography but is strongly affected by the local stress fields. Nano-voids are readily found at the tips of these cracks. It is concluded that the growth of such cracks occurs by the nano-void coalescence mechanism and is responsible for the formation of fisheyes and smoothly curved quasi-cleavage facets in hydrogen charged low-carbon steel.

Assessing Fracture Surface Ductility by Confocal Laser Scanning Microscopy

Hitherto there is no generally accepted quantitative parameter which, on the one hand, would reliably characterize the ductility of the whole fracture surface, and, on the other hand, could be relatively quickly measured. This circumstance substantially affects the objectivity of the fractographic analysis which effectiveness is still strongly dependent on the experience and skills of an expert. Recent studies showed that the value of the normalized fracture surface area Rs can serve as the measure of the fracture surface ductility. This parameter can be evaluated by the quantitative confocal laser scanning microscopy (CLSM). In the present study we investigated the Rs value for the fracture surfaces of the low carbon steel specimens tested in the temperature range from 200 to -196 °C where the steel undergoes ductile-to-brittle transition accompanied by the alternation of the fracture mode from ductile to brittle. The temperature dependence of the Rs value is found to have a sigmoidal shape with the sharp drop in the range from 100 to -100 °C. It is demonstrated that the Rs is strongly correlated with the fracture surface appearance: the Rs decreases concurrently with increasing brittleness of the fracture surface.

Study of Heat Affected Zone for SMAW Process for Low Carbon Steel Specimen with controlled parameters

Study of Heat Affected Zone for SMAW Process for Low Carbon Steel Specimen with controlled parameters

On DIC measurements of DeltaKeff to verify if it is the FCG driving force

Redundant data obtained under quasi-constant {DeltaK, Kmax} loading conditions is used to verify if the effective stress intensity factor (SIF) range DeltaKeff DeltaKmax DeltaKop is indeed the fatigue crack driving force. The crack opening SIF Kop is measured along the entire crack path on DC(T) low carbon steel specimens by a series of strain gages bonded along the crack paths, by a strain gage bonded on their back faces, and by a digital image correlation technique. All such measurements showed a significant Kop decrease as the crack sizes increased, while the fatigue crack growth rates remained essentially constant both in the thin and thick specimens, a behavior that cannot be explained by DeltaKeff arguments.

Confocal laser scanning microscopy: The technique for quantitative fractographic analysis

Brittle and ductile fracture surfaces of low-carbon steel specimens having different grain sizes were investigated by means of confocal laser scanning microscopy in conjunction with conventional scanning electron microscopy and electron backscattered diffraction. The fracture surfaces were quantitatively characterized in terms of the areal surface roughness and normalized surface area. It was demonstrated that the normalized fracture surface area rather than areal roughness can be used as a measure of fracture surface ductility. The misorientation between cleavage facets and their diameters were evaluated statistically. Good agreement was found between characteristics of fracture surface elements and the underlying microstructure.

Comparison of magnetic nondestructive methods applied for inspection of steel degradation

A series of low carbon steel specimens is investigated in the frame of a chain of magnetic non-destructive measurements on round robin samples, organized by the Universal Network for Magnetic Non-Destructive Evaluation. The samples have been plastically deformed by cold rolling to five consecutive stages of deformation. They were examined by several different nondestructive magnetic methods and the results were compared with each other and with the destructive mechanical measurements of Vickers hardness and ductile-brittle transition temperature. Very well correlated and qualitatively very similar relations between the magnetic and mechanical parameters were found in all cases, and for all the methods. Difference between the results of different measurements was found only in the sensitivity of the measurement with respect to the independent parameter, which is a promising message for the potential practical applications and possible future standardization of magnetic nondestructive methods.

Hydrogen trapping and fatigue crack growth property of low-carbon steel in hydrogen-gas environment

Fatigue crack growth (FCG) tests for compact-tension specimens of low-carbon steel were performed under various combinations of hydrogen pressures (0.1–90MPa), test frequencies (0.001–10Hz), and test temperatures (room temperature, 363K, and 423K). For quantifying the FCG acceleration, the hydrogen trapping and diffusivity of prestrained specimens were determined. Depending on the test conditions, the FCG was accelerated. The hydrogen-assisted FCG acceleration always accompanied localized plastic deformations near the crack tip. The onset of the FCG acceleration was roughly quantified by using a simplified parameter representing the gradient of the hydrogen concentration at the crack tip.

Effect of Micro-Alloying Elements and Transformation Temperature on the Correlation of Microstructure and Tensile Properties of Low-Carbon Steels with Ferrite-Pearlite Microstructure

This present study deals with the effect of micro-alloying elements and transformation temperature on the correlation of microstructure and tensile properties of low-carbon steels with ferrite-pearlite microstructure. Six kinds of lowcarbon steel specimens were fabricated by adding micro-alloying elements of Nb, Ti and V, and by varying isothermal transformation temperature. Ferrite grain size of the specimens containing mirco-alloying elements was smaller than that of the Base specimens because of pinning effect by the precipitates of carbonitrides at austenite grain boundaries. The pearlite interlamellar spacing and cementite thickness decreased with decreasing transformation temperature, while the pearlite volume fraction was hardly affected by micro-alloying elements and transformation temperature. The room-temperature tensile test results showed that the yield strength increased mostly with decreasing ferrite grain size and elongation was slightly improved as the ferrite grain size and pearlite interlamellar spacing decreased. All the specimens exhibited a discontinuous yielding behavior and the yield point elongation of the Nb4 and TiNbV specimens containing micro-alloying elements was larger than that of the Base specimens, presumably due to repetitive pinning and release of dislocation by the fine precipitates of carbonitrides.

Twin nucleation in cold rolled low carbon steel subjected to plate impacts

Shock loading tests have been carried out on low carbon steel specimens using a single stage gas gun with projectile velocities ranging from 200 to 800m/sec. In addition to dislocation slip, twinning was observed to be a contributing deformation mode. Similar impact tests were performed on specimens that have been previously subjected to cold rolling and it was shown that pre-straining increases the threshold stress for twin formation under shock loading. Efforts are made to determine the critical stress and strain rate required for twinning in cold rolled specimens. For this purpose a set of compression tests were performed on as-received and pre-strained specimens using a split Hopkinson pressure bar at various strain rates. These tests were carried out at liquid nitrogen temperatures where the thermal activation of dislocations is assumed to be higher than that for twin nucleation. Results of these tests are combined with FE analysis to provide knowledge of the twin stress as a function of pre-strain and strain rate. In addition, post-impact specimens were examined to identify characteristics of the deformation induced twins. It is shown that more than one {112} <111> type twin system is active at high stress levels as evident by the non-parallel nature of twins formed at the higher impact pressures. This observation was incorporated into a constitutive model based on one dimensional wave and conservation equations for predicting twin volume fraction by resolving the shear stress in multiple directions and applying twin growth equations for each plane. Results of this model show similar trends when compared with experimental outcomes for as-received and pre-strained steel.

Study on High Temperature Corrosion Cast and Wrought Steel

This paper represents the morphology of the iron oxide of medium carbon, low carbon steel and high carbon steel with the help of SEM micrographs and EDX analysis. This paper includes the study on high temperature corrosion of medium carbon steel specimens, study the highest oxidation layer thickness. Cross-section of the medium carbon steel or low carbon steel specimen are studied under this paper.

Acoustic investigations of the steel samples deformation during the tensile

The paper presents an experimental study of ultrasonic surface waves propagation in the low carbon steel specimens with different degree of degradation of microstructure and mechanical properties, subjected to tensile deformation. The purpose of this paper is to investigate and analyze the dependence between microstructure, mechanical and acoustic properties and to find appropriate parameters for stress state evaluation. For identification of degradation and stress state we use ultrasonic Rayleigh waves (URW) velocity variation obtained by in-line and off-line measurements. The degradation is imitated by structural changes caused by heat treatment.

Influence Nb Addition and Transformation Temperature on Impact Properties of Low-Carbon Steels

In this study, six kinds of low-carbon steel specimens with different ferrite-pearlite microstructures were fabricated by varying the Nb content and the transformation temperature. The microstructural factors of ferrite grain size, pearlite fraction, interlamellar spacing, and cementite thickness were quantitatively measured based on optical and scanning electron micrographs; then, Charpy impact tests were conducted in order to investigate the correlation of the microstructural factors with the impact toughness and the ductile-brittle transition temperature (DBTT). The microstructural analysis results showed that the Nb4 specimens had ferrite grain size smaller than that of the Nb0 specimens due to the pinning effect resulting from the formation of carbonitrides. The pearlite interlamellar spacing and the cementite thickness also decreased as the transformation temperature decreased. The Charpy impact test results indicated that the impact-absorbed energy increased and the ductile-brittle transition temperature decreased with addition of Nb content and decreasing transformation temperature, although all specimens showed ductile-brittle transition behaviour.

Generalized evaluation method for determining transition crack length for microstructurally small to microstructurally large fatigue crack growth: Experimental definition, facilitation, and validation

We proposed a generalized method to determine the transition crack length from a microstructurally small to a microstructurally large crack growth, l0. Artificial errors are minimized in the data analysis process of the proposed method. In this study, we used low carbon steel specimens, each with eight micro notches (the ferrite grain size was 25μm). The micro notches were introduced by focused ion beam technique, which is regarded as pre-cracks in steel. The obtained l0 (188μm) agrees with previous studies. With the present specimen geometry, l0 can be determined using two specimens—an approach that is much easier than conventional methods.

Effects of Thermal Treatment Processes (TTP) on the Tensile Properties of 0.165% Carbon Steel

In practice, welded low carbon steels do fail at the welded joints in use, thus leading to structural defects, material wastages, structural failure, and at times loss of lives, among others. This has been a great concern to practicing Engineers and Researchers. This study tends to proffer solution to this problem of concern through application of post welded thermal treatments. The welded samples were subjected to some post-weld thermal-treatment (TTP) operations such as normalizing, annealing and quench-hardening using different quenching media (Water, Palm oil, Quartz 5000 Total Engine oil, and Ground nut oil). The Tensile properties of the steel (such as tensile stress, tensile strain, and toughness) were determined before and after welding operations. At yield points, the thermal treatment processes adversely affected the strength of the welded steel. Meanwhile, normalizing and annealing processes enhanced the steel’s ductility and toughness, while quench-hardening process, irrespective of medium of quenching used reduced the steel toughness value. The toughness of the welded steel at the fracture point was also reduced through all the adopted thermal processes, except for normalizing process. The steel ultimate tensile stress and strain and its toughness values were equally reduced after TTP. Improvement of the properties of welded low carbon steel and the reduction of mechanical hazard were achieved through effective TTP. Thus, a better tensile property of welded low carbon steel was elicited by post-weld normalizing and annealing operations. Hence, butt-welded annealed and normalized low carbon steel specimens tend to be more resilient to failures at welded joints.

Anti-corrosive properties of Poly(aniline-co-2,3-xylidine)/ZnO nanocomposite coating on low-carbon steel

A copolymer nanocomposite Poly(aniline-co-2,3-xylidine)/ZnO [Poly(AN-co-XY)/ZnO], pure copolymer and its homopolymers namely, Poly(aniline-co-2,3-xylidine) [Poly(AN-co-XY)], Polyaniline (PANi) and Poly (2,3-xylidine) were synthesized by chemical oxidative polymerization using ammonium persulfate as an oxidant in hydrochloric acid medium. The synthesized compounds were characterized by FTIR, XRD, SEM, and TEM techniques. Saturated solutions of the synthesized compounds were made in N-methyl-2-pyrrolidone and casted on low-carbon steel specimens using 10% epoxy resin as a binder. The anticorrosion behavior of polymeric coatings was studied in 3.5 wt% NaCl solution at a temperature of 30 °C by electrochemical techniques, which include: open-circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy. Protective properties of nanocomposite coating were also evaluated at different immersion times for an extended period of 60 days. Anticorrosion properties of nanocomposite coating were compared with parent copolymer and individual homoplymers. SEM photomicrographs of the coated surface showed that Poly(AN-co-XY)/ZnO nanocomposite coating is crack free, uniform, and compact, whereas, copolymer and homopolymer coatings have surface defects. The performance of the polymer coatings followed the order: Poly(AN-co-XY)/ZnO > Poly(AN-co-XY) > PANi > Poly(2,3-xylidine). The presence of ZnO nanoparticles in copolymer resulted in significant improvement in corrosion resistance and provided better barrier properties.

Polypyrrole/graphene nanosheets/rare earth ions/dodecyl benzene sulfonic acid nanocomposite as a highly effective anticorrosive coating

Abstract Organic-inorganic nanocomposites (PPy/GNS/RE3 +/DBSA) involving pyrrole (Py), graphene nano sheets (GNS), rare earth elements (RE3 + = La3 +, Sm3 +, Nd3 +) and dodecyl benzene sulfonic acid (DBSA) were synthesized via in situ chemical oxidative polymerization using FeCl3 as an oxidant. PPy/GNS/DBSA and PPy/DBSA was also synthesized by following the identical synthesis route. The resultant nanocomposites were characterized by FTIR, XRD, SEM and TEM. The synthesized nanocomposites were chemically deposited on low carbon steel specimens by solvent evaporation method using N-Methyl-2-pyrrolidone (NMP) as solvent and 10% epoxy resin (by weight) as binder. The anticorrosive nature of polymer coatings were studied in 0.1 M HCl solution by subjecting them to various corrosion tests, which includes: EIS, potentiodynamic polarization, free corrosion potential (OCP) measurement and immersion test. The surface morphology of coated samples before and after immersion was evaluated using SEM.

Microstructural evolution and mechanical properties of a low-carbon quenching and partitioning steel after partial and full austenitization

In this work, low-carbon steel specimens were subjected to the quenching and partitioning process after being partially or fully austenitized to investigate their microstructural evolution and mechanical properties. According to the results of scanning electron microscopy and transmission electron microscopy observations, X-ray diffraction analysis, and tensile tests, upper bainite or tempered martensite appears successively in the microstructure with increasing austenitization temperature or increasing partitioning time. In the partially austenitized specimens, the retained austenite grains are carbon-enriched twice during the heat treatment, which can significantly stabilize the phases at room temperature. Furthermore, after partial austenitization, the specimen exhibits excellent elongation, with a maximum elongation of 37.1%. By contrast, after full austenitization, the specimens exhibit good ultimate tensile strength and high yield strength. In the case of a specimen with a yield strength of 969 MPa, the maximum value of the ultimate tensile strength reaches 1222 MPa. During the partitioning process, carbon partitioning and carbon homogenization within austenite affect interface migration. In addition, the volume fraction and grain size of retained austenite observed in the final microstructure will also be affected.

Investigation of the Corrosion Behavior of Poly(Aniline-co-o-Anisidine)/ZnO Nanocomposite Coating on Low-Carbon Steel

A copolymer of aniline (AN) and o-anisidine (OA), Poly(AN-co-OA) and its nanocomposite with ZnO nanoparticles, Poly(AN-co-OA)/ZnO were synthesized by chemical oxidative polymerization using ammonium persulfate as an oxidant in hydrochloric acid medium. The synthesized compounds were characterized using FTIR, XRD, SEM-EDS, TEM, and electrical conductivity techniques. The copolymer and nanocomposite were separately dissolved in N-methyl-2-pyrrolidone and were casted on low-carbon steel specimens using 10% epoxy resin as a binder. The anticorrosive properties of the coatings were studied in different corrosive environments such as 0.1 M HCl, 5% NaCl solution, and distilled water at a temperature of 30 °C by conducting corrosion tests which include immersion test, open circuit potential measurements, potentiodynamic polarization measurements, and atmospheric exposure test. The surface morphology of the coatings prior to and after one-month immersion in corrosive solution was evaluated using SEM. It was observed that the nanocomposite coating exhibited higher corrosion resistance and provided better barrier properties in comparison with copolymer coating. The presence of ZnO nanoparticles improved the anticorrosion properties of copolymer coating in all corrosive media subjected to investigation.

Quantitative characterization of cleavage and hydrogen-assisted quasi-cleavage fracture surfaces with the use of confocal laser scanning microscopy

“True” cleavage (TC) and quasi-cleavage (QC) fracture surfaces of low-carbon steel specimens tested in liquid nitrogen and after hydrogen charging respectively were investigated by quantitative confocal laser scanning microscopy (CLSM) and conventional scanning electron microscopy (SEM) with electron-backscattered diffraction (EBSD). Topological and crystallographic features of the TC fracture surface are found in good agreement with the generally accepted cleavage mechanism: TC facets diameters correspond to those of grains; the crack path strictly follows the crystallographic orientation of grains and the most of the cleavage cracks are parallel to {100} planes. On the 2D SEM images, the QC facets appeared resembling the TC ones in terms of river line patterns, shapes and sizes. However, the substantial differences between the topography of these two kinds of fracture surfaces were revealed by 3D CLSM: the average misorientation angle between QC facets and the roughness of the QC fracture surface were much lower than those measured for TC. It is demonstrated that all these features are attributed to the specific fracture mechanism operating during hydrogen-assisted cracking.