Cracking Of Steel Research Articles

SEM Study of the Influence of Microstructure on Low Cycle Fatigue Crack Growth in Martensitic Steel I

Distribution of fatigue cracks in chromium martensitic steel after low cycle fatigue (LCF) tests at room temperature has been studied using SEM, and the experimental evidences of localized plastic flow (LPF) are presented. The influence of the location of LPF and the microstructure elements on the trajectory and growth of microcracks is also considered. The dimensions of plastic zones ahead of macrocrack tip as well as at its edges were measured in the process of crack propagation inside of the sample. The processes occurring in plastic zone, particularly ahead of macrocrack tip, were analyzed. Distribution, orientation and the reasons of slip bands’ formation as well as the microstructure elements at which they were nucleated have been studied. The impact of the slip bands’ orientation on the process of macrocrack growth was also analyzed. In addition the interactions of a crack with the boundaries of former austenite grains, martensitic packets, martensitic laths, slip bands and precipitates have been discussed.

3D EBSD Analysis of Hydrogen Induced Cracking in Pipeline Steel Using Serial Section Broad Ion Beam Tomography

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Fatigue life improvement of steel structures using self-prestressing CFRP/SMA hybrid composite patches

While prestressed CFRP patches are considered as attractive alternatives to traditional welding or bolting methods for enhancing the fatigue life of cracked steel structures, they commonly require hydraulic actuators and pumps, and complex setups and installations for prestressing, limiting the applicability of this method. The primary objective of this research is to investigate the use of shape memory alloys (SMAs) in combination with CFRP composites to develop self-prestressing CFRP/SMA hybrid patches. The concept of the self-prestressing is to embed prestrained SMA wires into the CFRP patches and activate them through direct heat or electric current, thereby, generating prestressing forces in the cracked member. Two types of NiTi SMA wires, namely, the body-temperature and as-drawn heat-treated NiTi SMA wires are used in the fabrication process of the patches. The results demonstrate that the developed self-prestressing CFRP/SMA hybrid patches can be used as simple and effective solutions to significantly enhance the fatigue life of cracked steel structures.

Formation Analysis of Edge Cracks of 33MnCrTiB Fork Steel

A 33MnCrTiB is a high-strength low alloy steel used for a forklift truck. This study investigates the root causes of the edge cracks in 33MnCrTiB steel produced by a steel manufacturer. After sampling, inspecting and analyzing the 33MnCrTiB flat steel with edge cracks taken from the plant, it was found that the crack and porosity contain similar residues to the deoxidation products and continuous casting mold flux. It can be concluded that the edge surface cracking in the flat steel are mainly caused by the non-deformable inclusions in steel, slag sticking and surface cracks on the original billet. At the same time, the further expansion of cracks is closely related to the serious decarburization during the heating treatment in the billet heating process, and the improper rolling deformation system during the rolling process.

Intergranular stress corrosion cracking of steel gas pipelines in weak alkaline soil electrolytes

Abstract The paper presents an overview of the main types of stress corrosion cracking (SCC) of pipeline steels under operating conditions of high-pressure buried gas pipelines. Up to the present time, there have been cases of transgranular cracking in gas pipelines in near-neutral pH soil electrolytes in Russian territory. The present work studies fragments of pipes containing cracks as well as soil samples at the excavation sites of gas pipelines running through the territory of the Astrakhan region. It was found that the cracks formed as a result of SCC of pipe metal and are of intergranular character of propagation. It is also established that the chemical composition of the soil along the gas pipeline differs from the previously studied compositions of corrosive environments that encourage the development of intergranular SCC of pipeline steels.

Why Do Secondary Cracks Preferentially Form in Hot-Rolled ODS Steels in Comparison with Hot-Extruded ODS Steels?

Secondary cracks are known to absorb energy, retard primary crack propagation and initiate at lower loads than primary cracks. They are observed more often in hot-rolled than in hot-extruded ODS steels. In this work, the microstructural factors responsible for this observation are investigated. Better understanding of these factors can lead to tailoring of improved ODS steels. Fracture toughness testing of two batches of 13Cr ODS steel, one hot-rolled and the other hot-extruded, was carried out. The fracture behaviour of secondary cracks was investigated using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Crystallographic texture and grain morphology play a predominant role in preventing secondary cracks in hot-extruded ODS steels. At lower temperatures, secondary cracks occur predominantly via transgranular cleavage. The fracture mode changes to ductile and intergranular at higher temperatures.

SVET Study of Amines Attack of MnS Inclusions and the Consequences for Pitting and SCC Nucleation

Sulfide inclusions are preferred nucleation sites for the localized corrosion of carbon and stainless steels. Thus, inclusions distribution and composition are important for the occurrence of pitting, crevice and the different forms of stress corrosion cracking. Amines are usually used for the removal of sulfides, with the precipitation of carbamates by the production of environmental friendly fuels with low S contents. On the other hand, amines are also used associated to formation/injection waters simultaneously to CO2 reinjection by the offshore oil production at high pressures, usually under the assumption that it will act as a corrosion inhibitor, as well. In the present work, we show that amines, usually seen as corrosion inhibitors may enhance the attack to sulfide inclusions in unalloyed carbon steels. For this, artificial galvanic elements with carbon steel (AISI 1006) and MnS tablets (Figure 1.a and 1.c), simulating an inclusion, were studied by SVET (scanning vibration electrode technique) (Figure 1.b and1.d) and other techniques to evaluate the influence of amine (1% tetraethylenepentamine-TEPA) on the corrosion in Na2SO4solutions. Carbon steels (e.g. SAE1006) exposed to dilute sulfate solutions show the typical passive behavior with pitting at higher potentials. The amine “TEPA” shifts the pitting potential to higher values, as expected (Fig. 3a). MnS inclusions show the opposite behavior and its dissolution is promoted when TEPA is added in significant concentrations (Fig. 3b). Accordingly, SVET measurements show that the addition of TEPA can invert the galvanic pair formed by Fe-MnS (Fig. 1). At OCP, a synthetic MnS inclusion is attacked progressively by TEPA as Fig.2 shows for different exposure times. This has significant consequences for the initiation of pitting and stress corrosion cracking of steels, as verified by SVET measurements under simultaneous mechanical strain. Figure 1

Theoretical and Numerical Study on Stress Intensity Factors for FRP-Strengthened Steel Plates with Double-Edged Cracks.

This paper presents a theoretical and numerical study on the stress intensity factors for double-edged cracked steel plates strengthened with fiber reinforced polymer (FRP) plates. Based on the stress intensity factor solution for infinite center-cracked steel plates strengthened with FRP plates, expressions of the stress intensity factors were proposed for double-edged cracked steel plates strengthened with FRP plates by introducing two correction factors: β and f. A finite element (FE) simulation was carried out to calculate the stress intensity factors of the steel plate specimens. Numerous combinations of the specimen width, crack length, FRP thickness and Young’s modulus, adhesive thickness, and shear modulus were considered to conduct the parametric investigation. The FE results were used to investigate the main influencing factors of the stress intensity factors and the correction factor, β. The expression of the correction factor, β, was formulated and calibrated based on the FE results. The proposed expressions of the stress intensity factors were a function of the applied stress, the crack length, the ratio between the crack length and the width of the steel plate, the stiffness ratio between the FRP plate and steel plate, the adhesive thickness, and the shear modulus. Finally, the theoretical results and numerical results were compared to validate the proposed expressions.

FIBER SEGMENTATION IN CRACK REGIONS OF STEEL FIBER REINFORCED CONCRETE USING PRINCIPAL CURVATURE

This paper tackles the non-trivial image-processing task to segment hook-ended fibers in three-dimensional images. For this purpose, a novel segmentation method is presented that relies on the following observation: For a single fiber the configurations of principal curvatures that can occur on its surface are limited. Deviations from these configurations indicate potential overlaps of fibers. The method that was developed based on this observation is used to separate several simulated clusters of touching fibers as a proof-of-concept. Further, it is applied to two images of cracked steel fiber reinforced concrete specimens arising from a 4-point bending test. The method's performance is compared to manual separation. Overall, we can state that the proposed method yields satisfying results when data meets the following criteria: Low fiber volume density, circular fiber cross section and sufficient spatial resolution of fiber-fiber contacts.

3D characterization of the propagation of small fatigue cracks in steels with different forging conditions

The influence of forging conditions on the propagation of physically small fatigue cracks has been studied for two high strength steels. Two surface conditions were produced after the forging process. The subsurface microstructure of the materials has been characterized by EBSD. Small samples extracted from the original specimens were used to perform in situ fatigue tests monitored by high resolution synchrotron X-ray tomography. Fatigue cracks were initiated from an artificial defect (100 μm × 50 μm deep) introduced in the forging skin by laser machining. 3D images of the initiation and growth of those physically small fatigue cracks have been obtained. It was found that the presence of a shot-blasted skin containing a hardness and microstructure gradient influences the 3D crack shape during propagation in comparison with the materials without material properties gradient. The 3D crack shapes are rationalized in terms of crack closure effects induced by the forging processes, close to the surface.

Fatigue performance test on inclined central cracked steel plates repaired with CFRP strand sheets

An experimental study was conducted on central-cracked steel plates with different crack inclination angles that had been repaired with carbon fiber reinforced polymer (CFRP) strand sheets. Initial cracks with five different inclined angles (0–60°) were artificially induced from a central hole of steel plates. All the cracks had the same projection length, running perpendicular to the loading axis. Crack lengths, crack trajectories, failure modes and the effects of different repair configurations were investigated, and fatigue crack propagation lives were compared. A typical adhesive failure mode, oblique compression failure in the adhesive layer, was observed. A conclusion was drawn that fatigue life was directly determined by the projection length, while the projection length was calculated from the initial crack length and inclination angle. The average fatigue life extension ratio of specimens with single-sided repairs was 1.44; that of specimens with double-sided repairs was 3.03.

Effect of Crack Length on Mechanical Field of Stress Corrosion Crack Tip of Cold Working 316L Stainless Steel

The effect of crack length at different cold working level on stress and strain state and fracture parameters of stress corrosion cracking tip of 316L stainless steel in high temperature water environment of the nuclear pressure vessel was studied by sub-model technology of elastic plastic finite element method. It combined with the mechanical properties of 316L stainless steel under different cold working level, and the effect of crack length on the Mises stress, equivalent plastic strain, tensile stress, tensile strain and J integral of 316L stainless steel under different cold working level was compared. The results indicate that crack length of stress corrosion cracking has certain effect on stress and strain state at crack tip. With the increase of crack length, Mises stress and equivalent plastic strain, tensile stress and tensile strain increase at crack tip, at the same time, J integral increases with increase of crack length. Cold working level has changed the mechanical properties of 316L stainless steel. When the stress intensity factor K is constant, with the increase of cold working level, Mises stress increases and equivalent plastic strain decreases at crack tip, while tensile stress increases and tensile strain decreases at crack tip. The rate of change of stress strain curve from 0° to 90° is different because of cold working level. The higher the level of cold working, the faster the Mises stress and the tensile stress increase at 90° in front of the crack tip, and equivalent plastic strain and tensile strain decrease most slowly at 90° in front of the crack tip. As the degree of cold working increases, the J integral of crack tip also increases. The increase of crack length and the level of cold working aggravate the stress corrosion cracking of 316L stainless steel under high temperature and high pressure in a certain range.

Factors Affecting the Life of Steel Grating Bars in Roasting Cars

To understand the factors affecting the working life of 40Kh24N12SL steel grating bars in roasting cars, we compare data regarding the grating bars after operation with the results of simulating their operating conditions. The results obtained by finite-element modeling are highly reliable. In modeling, the maximum possible number of boundary conditions is specified on the basis of the data regarding the grating bars already obtained and literature data. Modeling shows that a considerable temperature is formed over the part’s cross section, with local heating zones. The heating is predictable and is associated with the supply of coolant to the working zone in the course of operation. Over time, zones with large internal stress and strain are formed locally in the part. The appearance of these zones depends strongly on the presence of structural inhomogenities of the part and is thought to be associated with the geometric complexity of the casting. In the presence of shrinkage cavities, all of the stress and strain values increase sharply, especially at local maxima. The distribution of the local zones with high stress and strain remains practically unchanged in the presence of shrinkage cavities. Analysis of the position of the zones with high stress and strain sheds light on the grating bar failure by cracking in the course of operation. The presence of shrinkage cavities in the metal is found to be a major cause of the buckling, cracking, and fracture of 40Kh24N12SL steel grating bars in roasting cars. Simulation of the operating conditions permits explanation of the defect formation in the grating bars in accordance with observational data.

Advanced Non-Destructive Testing in Steels

The quality of steel and its corresponding products depends on the distribution and level of stress on the steel’s volume and surface, since the stress gradient is responsible for steel cracking generation and failure[...]

Effect of pearlite on stress corrosion cracking of carbon steel in fuel-grade ethanol

AbstractThe selective dissolution of ferrite phase from the pearlite was studied in fuel-grade ethanol (FGE) to understand how it affects the stress corrosion cracking (SCC) mechanism of carbon steel in FGE. It was shown that microgalvanic coupling occurs between ferrite and cementite phases of the pearlite, leading to localized corrosion, which affects the SCC mechanism. The intergranular SCC mechanism stops at the pearlite, and the selective dissolution promotes the transgranular SCC mechanism. Cathodic polarization curves were measured for pure iron and cementite exposed to various FGE conditions. According to the results, cementite phase is, in most cases, a more favorable cathode in FGE.

Prediction of Bending Fatigue Life of Cracked Out-of-Plane Gusset Joint Repaired by CFRP Plates

Carbon fiber reinforced polymer (CFRP), plates bonding repair method is one of the simple repair methods for cracked steel structures. In this study, the influence of width of CFRP plates on bending fatigue life of out-of-plane gusset joint strengthened with CFRP plates was investigated from the experimental and numerical point of view. In the bending fatigue test of cracked out-of-plane gusset joint strengthened with CFRP plates, the effect of width of CFRP plates on crack growth life was clarified experimentally. Namely, it was revealed that the crack growth life becomes larger with increasing the width of CFRP plates. In the numerical approach, the stress intensity factor (SIF) at the surface point of a semi-elliptical surface crack was estimated based on the linear fracture mechanics. Furthermore, the extended fatigue life of cracked out-of-plane gusset joint strengthened with CFRP plates was evaluated by using the estimated SIF at the surface point and the empirical formula of the aspect ratio of semi-elliptical crack. As the results of numerical analysis, the estimated fatigue life of the specimen strengthened with CFRP plates showed the good agreement with the test results.

Fatigue crack growth rate in CFRP reinforced constructional old steel

PurposeThe purpose of this paper is twofold: first, to observe an influence of different Composite Fibre-Reinforced Polymer (CFRP) patches, whose application to metals is very easy, in suppling and significantly elongating the service time; and second, the numerical calculation of the reduced stress intensity factor (SIF) range for strengthened cracked steel specimens.Design/methodology/approachOne of the successful strengthening methods is the CFRP patching along the fatigue crack paths. The presented approach has been studied and discussed in this paper on the background of the numerical and experimental data. As it was expected, the proposed strengthening method is efficient and promising in case of the “immediate” repairs of critical members with cracks. The manufacturing process of specimens and test methodology as well as numerical approach to calculate SIFs for various reinforcements of steel specimens are presented. For this purpose, the Extended Finite Element Method was involved and described.FindingsThe main mechanism of fatigue crack growth retardation is associated with local ΔK reduction due to CFRP patches; any type of reinforcement results in an increase in af and a significant decrease in SIF values. The beach-marking method is described as a good, reliable and comprehensive method to capture the crack propagation in structures consisting of various materials and could be applied successfully for mixed mode testing.Originality/valueA detailed experimental-numerical approach for fatigue crack growth in long-term operated structures made of steel is presented. The strengthening methodology is presented with consideration of the various CFRP patches configurations.

Empirical Stress Intensity Factor Equations for Cracked Steel Plates Repaired with Double-Sided FRP Patches

This paper presents an approach that combines the finite element (FE) modeling and genetic programming (GP) to provide accurate empirical stress intensity factor (SIF) equations for center-cracked steel plates repaired with adhesive-bonded double-sided fiber-reinforced polymer (FRP) patches. Several past studies in recent years independently showed that the reduction on the SIF of cracked structures after the patch repair is dependent on many factors such as bonding techniques, material parameters, geometric parameters, environmental factors. In this study, the SIF of the repaired cracked steel plate was considered to be a function of seven parameters including the crack length, elastic modulus of FRP material, shear modulus of adhesive material, dimensions (width, length, and thickness) of rectangular FRP patches, and thickness of adhesive layers. Empirical SIF equations were created by the data mining process of genetic programming analyses performed on a database created from the FE results. The SIF values obtained from these equations were also compared with an analytical equation to assess whether their ability to perform well on a certain design or not. It was found that the proposed SIF equations fitted well with the FE results as the squared Pearson correlation coefficients R2 are higher than 0.9. In addition, the correlations between proposed equations and the analytical equation are approximately 0.8.

The occurrence of quenching cracks in high-carbon tool steel depending on the austenitizing temperature

Abstract Heat treatment has the greatest impact on the microstructure and properties, as well as the residual stresses and dimensional control of steel. Most of the problems that occur in heat-treated parts are attributed to improper heat-treatment practices, deficiency in the used grade of steel, poor part design or part defects. Inappropriate heat-treatment practices include overheating, burning, non-uniform heating, incorrectly selected austenitizing temperature and improper quenching. The formation of cracks during and after the quenching of austenitized steels is a major problem. As the carbon content of steel increases, the tendency to crack increases too. The tendency for quench cracking is also increased with higher austenitizing temperatures. The aims of the research were to determine the cause of the cracks in high‑carbon tool steel and to analyze the influence of the austenitizing temperature on the occurrence and propagation of quenching cracks. The results show that the main cause of quenching cracks is a too high austenitizing temperature caused by poor temperature control. A temperature that is too high leads to excessive austenite grain growth, distinctive grain boundaries with coarser martensite and the presence of retained austenite. On the other hand, the best properties and the absence of quenching cracks occur when C85S steel is quenched from a temperature below the Acm temperature.

Physio-mechanical Materials Testing Using Scanning Contact Potentiometry Method

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