Intergranular Facets Research Articles

Harmful Effects, Origin, and Control of Banded Structures in Electroplated Fasteners of High‐Carbon Steel

In this study, the harmful effects of a banded structure on the bending fracture of electroplated fasteners are investigated. The origins and ways to control these structures are also explored. The fracture surfaces of the fasteners exhibit stepwise cracks and numerous intergranular facets associated with ductile tearing, which is a characteristic of hydrogen embrittlement. The origin of these stepwise cracks can be traced to the internal banded structures. Therefore, the internal banded structures promote fracturing during the bending test. The internal banded structures in the fasteners can be traced back to the internal cracks in the bloom, and inappropriate soft‐reduction processes lead to internal cracks. Notably, the carbon segregation at the internal cracks is severe. This is attributed to the proximity of the internal cracks to the center of the bloom, causing the enriched liquid in the interdendritic region to be sucked into these internal cracks. The optimal reduction position and amount are determined by employing heat‐transfer and soft‐reduction models. Fluctuations in superheat and casting speed significantly influence this optimal position and reduction amount.

Cause of bending fracture in high-strength fasteners and heredity of surface recarburization in high carbon steel

In this work, the cause of bending fracture in high-strength fasteners after electroplating was investigated and the heredity of surface recarburization from continuously cast high carbon steel blooms to the final fastener products was explored. The fracture surface of the fasteners displayed intergranular cracking and numerous intergranular facets accompanied by ductile tearing. Hydrogen-induced stepwise cracks were identified near fracture zones, indicative of hydrogen embrittlement. The surface decarburized layer on the fasteners featured a significant number of fine partial decarburization grains and a high carbon content, resulting in low electrical conductivity. The observed dense coating, possibly induced by the poor conductivity of the decarburized layer, could elevate the hydrogen content in the substrate, ultimately resulting in hydrogen embrittlement. Additionally, the surface carbon pick-up at the corners of the blooms could be inherited by the rolled billet. The extensive deformation experienced during the hot rolling process resulted in the uniform distribution of recarburization on the surface of both the wire rod and spheroidized wire rod. Consequently, this recarburization contributed to a low degree of decarburization on the surface of the fasteners. Simulation results obtained through DICTRA also indicated that the recarburization on the surfaces of various samples could not be eliminated during the heat treatment process.

Failure analysis of oil refinery heater treater’s fractured fire tube

Overheating, corrosive degradation, hydrogen embrittlement, creep, erosion, and corrosion are the cause of most failure problems in fire tubes. Herein, we have evaluated the macrostructural, microstructural, and mechanical properties at welded U-bend of the fractured fire tube to analyze the failure of the SA 387 grade 22 steel heater treater fire tube that has been under continuous use for 18 years at an oil refinery. We have found that the embrittlement factor at U-bend proved to be the most significant factor in its failure. The embrittlement X factor for weld metal is 45, which is three times more than the recommended value (≤15). It is apprehended that the fractured metal part could be susceptible to temper embrittlement. There is a significant higher hardness value of weld metal as compared to base metal, which is due to metallic phase transition at high temperature. The Charpy impact energy absorbed by the fractured metal is 13 J, which is lower than impact energy absorbed by the base metal 300 J. The macroscopic examination reveals that the crack initiated on the emulsion side and propagated radially in welded U-bend before extending further into the base metal in the axial direction. The scanning electron microscopy demonstrated intergranular crack propagation, discontinuous cracking along grain boundaries, and blunted cracks at grain interiors. The fractography is performed on the fracture surface of the impact test specimens, and it indicated heavily cavitated intergranular facets along the grain boundaries and deep secondary cracks on the fracture surface. Energy disperse spectroscopy identified the carbides formation at the U-bend cracking parts, which counter confirm the microstructural phase transition. These findings suggested that the severe cracking observed in the fire tube at the U-bend is likely due to temper embrittlement (TE) induced by exposure of the fire tube material to elevated temperatures.

Effect of hydrogen on very high cycle fatigue properties of 17-4 PH martensite stainless steel

Very high cycle fatigue properties of 17-4 PH stainless steel aged for 1 h and 4 h after hydrogen charging were studied. Results show that hydrogen reduces fatigue strength and changes crack initiation mode to mainly inducing by inclusions. The existence of intergranular facets on fracture surface indicates that hydrogen accelerates crack initiation and early crack growth. Hydrogen-granular bright facet (GBF) mechanism may be interpreted as lath martensite fracture and dislocation cells formation, where hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE) mechanisms occur. The degradation of hydrogen on fatigue properties is suppressed by hydrogen trapping effect of ε-copper precipitates.

On improved fatigue properties of aluminum alloy 5086 weld joints

In this work, we aimed to investigate the fatigue properties of AA5086-H321 and its weld joints. The AA5086-H321 plates were welded in a butt configuration using the friction stir welding. Load-controlled axial sinusoidal cyclic tests were conducted in the ambient conditions, at a frequency 20 Hz, and at different load ratios i.e., Rσ=-1, 0.1, and 0.5. The weld joints, in general, are known to have reduced properties. However, in this study, the base alloy and its weld joint had similar hardness and ultimate tensile strength, but with marginally reduced elongation (∼4.4%). The endurance limits of the weld joints were either higher (∼28% and ∼11% at R=-1 and 0.1, respectively) or same (at R=0.5) as compared to that of the base alloy. The difference in fatigue lives of base alloy and its weld joint kept diminishing with increasing R-ratio. Microscopic examination of fracture surfaces in weld joints revealed three types of crack initiations: voids, inclusions, and tool-marks, and two types of crack propagation mechanisms: intergranular and transgranular. However, in base alloys, only defect-free surface initiation with transgranular crack growth was observed. Mean stress, as expected, had a detrimental effect on fatigue properties, with no definitive control on the type of crack initiation or its propagation mechanism. An increase in the fatigue lives of weld joints was attributed to the crack growth retardation during the intergranular fatigue crack propagation due to frequent defection of crack path, which was absent in the base alloys. Most of the intergranular facets were observed near the surfaces (top or bottom) of the weld nugget zone and disappeared when the crack front reaches its interface boundary with the heat affected zone. All these aspects of fatigue damage in microstructurally heterogeneous weld joints and the plausible explanations for the observed mechanical fatigue properties are presented in this study.

Stress corrosion cracking in fuel-grade ethanol: The role of the testing methodology

The increase in fuel-grade ethanol (FGE) consumption demands efficient and safe storage and transportation. However, depending on the raw material used for ethanol production, stress corrosion cracking (SCC) might be a major concern. This work evaluated the API 5L X70 steel susceptibility to SCC in simulated fuel-grade ethanol (SFGE), corn, and sugar cane FGE. The maximum stress intensity factor (Kmax.) was determined from KR curves, and the stress intensity factor threshold for environment-assisted cracking (KEAC) was obtained from the direct current electric potential difference (DCEPD) versus load-line displacement (LLD) curves. The results showed that Kmax. was weakly affected by the environment (different variants of ethanol), although brittle fracture comprising cleavage-like features, intergranular facets, and secondary cracks was observed for SFGE and corn FGE. Under the specific set of testing conditions adopted here, the parameter KEAC was effective in showing that the API 5L X70 steel is quite sensitive to SCC in SFGE and corn FGE, and not susceptible to this progressive cracking process in sugar cane FGE.

The influence of rotation angle between layers in laser-based powder bed fusion on the hydrogen embrittlement of Ni

This work investigated the relation between hydrogen embrittlement and the scanning rotation angle between adjacent layers in laser-based powder bed fusion (L-PBF) fabricated Ni. The microstructure analysis indicated that all samples failed with an intergranular mode, and all intergranular facets were propagated along the fine-grain region. However, the rotation angle during manufacturing changed the grain shape and size distribution in Ni, which in turn modified the pathway for hydrogen-induced cracking. The sensitivity of LPBFed Ni to hydrogen embrittlement is influenced by the scanning strategy, and the samples with a rotation angle of 0° present the highest resistance to hydrogen embrittlement.

Effects of Mn Segregations on Intergranular Fracture in a Medium‐Mn Low‐Density Steel

Al‐bearing medium‐Mn low‐density steels possess great potential in the automotive industry because of their excellent mechanical properties based on transformation‐induced plasticity and low specific weight. Reducing the austenite stability against deformation‐induced martensitic transformation enables a high strain‐hardening capacity to be obtained; however, undesirably low stability often results in considerably reduced tensile ductility and brittle fracture. Herein, the brittle fracture that occurs with increasing annealing temperature for a Fe−0.3C–9Mn−5Al (wt%) steel is investigated in relation to Mn segregation at the phase boundaries between ferrite and austenite. The results demonstrate that annealing at 850 and 900 °C leads to ductile fractures with 72% and 95% tensile elongation, respectively, whereas only 25% elongation is achieved for the specimen annealed at 950 °C, exhibiting predominant intergranular facets. 3D atom probe tomography reveals that annealing at 950 °C promotes considerable Mn segregation at the ferrite/austenite phase boundaries with a peak composition of ≈19 at%, which is sufficient to reduce the boundary cohesion for intergranular fracture. Thermodynamic moving boundary simulation reveals that intercritical annealing is not a prerequisite for segregations; however, low‐temperature and prolonged holding should be accompanied, such as the coiling procedures.

ВІЗУАЛІЗАЦІЯ ФРАКТОГРАФІЧНИХ ОЗНАК ЕКСПЛУАТАЦІЙНОЇ ДЕГРАДАЦІЇ ТЕПЛОТРИВКОЇ СТАЛІ

Проаналізовано злами зразків після випробувань на ударну в’язкість, статичну тациклічну тріщиностійкість тривало експлуатованої (a2·105год ) теплостійкої сталі15Х1М1Ф. Фрактографічним аналізом зламів виявлено відмінності між станом металу, щопід час експлуатації переніс різну кількість зупинок блоків (501 та 576). А саме в металі збільшою кількістю зупинок частка міжзеренних фрагментів на фоні типового крізьзеренногорельєфу в зоні спонтанного руйнування зразків випробуваних на ударну в’язкість та статичнутріщиностійкість була високою, що пов’язано з його інтенсивнішою деградацією. Крім того, після випробувань на удар частка міжзеренних фасеток була більшою ніж за випроб настатичну тріщиностійкість, що пов’язано з більшим відхиленням тріщини (навіть надекілька зерен) від магістрального напряму її поширення. Тоді як при статичнихвипробуваннях руйнування відбувалося лише по тих ослаблених деградацією меж зерен, якітраплялися на шляху фронту магістральної тріщини. При циклічних випробуваннях наприпороговій ділянці росту тріщини на зламах експлуатованої сталі на фоні класичногокрізьзеренного втомного руйнування, виявили фрагменти міжзеренного руйнування, якіпов’язали з межами зерен пошкодженими внаслідок повзучості, які утворилися під частривалої експлуатації. Отримані результати дають підстави рекомендувати площуміжзеренного рельєфу, що припадає на одиницю площі зламів зразків з експлуатованоїтеплотривкої сталі, для кількісного оцінювання її структурно-механічного стану

Hot ductility behavior of a Fe-0.3C-9Mn-2Al medium Mn steel

The hot ductility of a Fe-0.3C-9Mn-2Al medium Mn steel was investigated using a Gleeble3800 thermo-mechanical simulator. Hot tensile tests were conducted at different temperatures (600–1300°C) under a constant strain rate of 4 × 10−3 s−1. The fracture behavior and mechanism of hot ductility evolution were discussed. Results showed that the hot ductility decreased as the temperature was decreased from 1000°C. The reduction of area (RA) decreased rapidly in the specimens tested below 700°C, whereas that in the specimen tested at 650°C was lower than 65%. Mixed brittle-ductile fracture feature is reflected by the coexistence of cleavage step, intergranular facet, and dimple at the surface. The fracture belonged to ductile failure in the specimens tested between 720–1000°C. Large and deep dimples could delay crack propagation. The change in average width of the dimples was in positive proportion with the change in RA. The wide austenite-ferrite inter-critical temperature range was crucial for the hot ductility of medium Mn steel. The formation of ferrite film on austenite grain boundaries led to strain concentration. Yield point elongation occurred at the austenite-ferrite intercritical temperature range during the hot tensile test.

Grain-boundary phosphorus segregation in highly neutron-irradiated reactor pressure vessel steels and its effect on irradiation embrittlement

Reactor pressure vessel (RPV) steels for pressurized water reactors (PWRs) with bulk P contents ranging from 0.007 to 0.012wt.% were subjected to neutron irradiation at fluences ranging from 0.3 to 1.2 × 1020 n/cm2 (E > 1 MeV) in PWRs or a materials testing reactor (MTR). Grain-boundary P segregation, which was analyzed using Auger electron spectroscopy (AES) on intergranular facets, increased with increasing neutron fluence. A rate theory model based on four diffusion-reaction equations for substitutional P atoms, octahedral interstitial P atoms, vacancies, and self-interstitial atoms was also used to simulate the increase in grain-boundary P segregation for RPV steels with a bulk P content up to 0.020wt.%, using parameters optimized by the present AES data. The increase in grain-boundary P segregation in RPV steel with a bulk P content of 0.015wt.%, which is the maximum P concentration in RPV steels used in Japanese nuclear power plants intended for restart, was estimated to be less than 0.1 in monolayer coverage at 1 × 1020 n/cm2 (E > 1 MeV). A comparison of the PWR data with the MTR data, including that from the literature, showed that neutron flux had no effect upon grain-boundary P segregation for A533B steels. The relationships of the ductile-brittle transition temperature (DBTT) shifts to grain-boundary P segregation and to yield strength were also discussed. A linear relationship between the yield strength and the DBTT shift with a slope of 0.63 was obtained for RPV steels with a bulk P content up to 0.026wt.%, which is higher than that of most U.S. A533B steels. It is concluded that the intergranular embrittlement is unlikely to occur for RPV steels irradiated in PWRs.

Fatigue crack growth in operated gas pipeline steels

Regularities of fatigue crack growth for pipeline steels of different strength are presented, and the changes in fatigue behavior of these steels after long term operation are analyzed. Threshold values of stress intensity factor range are lower for operated steels comparing to the corresponding values for as received ones. During the testing in the simulated soil solution NS4, a barely noticeable tendency to increase the threshold values of SIF was traced. It was explained by the appearance of intergranular fracture elements on the backgrownd of the typical flat fatigue relief already in the near-threshold region of fatigue crack growth curves in the soil solution. A higher relief of intergranular facets provided favorable conditions for occurrence of crack closure effect.Fatigue testing was performed using steel specimens after in-laboratory and in-service degradation, and it was shown that results for both degraded steels are very close to each other proving the validity of the method of in-laboratory degradation. A new methodic approach to fatigue testing of pipe steels is presented which allows simulating working conditions of gas pipelines, namely the hydrogen diffusion through the pipe wall to its external surface, and estimating its possible effect on SCC. It consists in evaluation of the influence of hydrogen reached the crack tip only due to its diffusion, on the crack growth. It is found that hydrogen absorbed by metal during the test providing such conditions, causes a leap of crack growth rate in the Paris region of the fatigue crack growth curve of the tested 17H1S steel. Intergranular mechanism of fracture detected on the specimen fracture surface is suggested as a clear evidence of embrittlement of grain boundaries as a result of its hydrogenation.

Correlative analysis of digital imaging, acoustic emission, and fracture surface topography on hydrogen assisted cracking in Ni-alloy 625+

Failures of Ni-based alloy components, used in oil and gas environments have been attributed to hydrogen embrittlement (HE). This has motivated the development of HE resistant Ni alloys without compromising their superior mechanical and corrosion properties. An understanding of crack initiation and propagation mechanisms is essential for improving the Ni alloy design and its resistance to HE. The objectives of this paper are: to elucidate the fracture events involved during a slow strain rate test (SSRT) of a Ni-based alloy, UNS N07716, under an electrochemical hydrogen (H) charging environment, and; to correlate these events with in-situ and offline failure analysis techniques. The in-situ techniques involved continuous monitoring of fracture events during a SSRT using a high-resolution digital camera and an acoustic emission (AE) sensor. The offline techniques include Fracture Surface Topography Analysis (FRASTA) and Finite Element Analysis (FEA). The results obtained from these analyses were correlated with the stress-strain behaviour to determine the crack initiation stress. It is observed that the ductility (based on elongation to failure) of UNS N07716 was reduced significantly under H environment when compared with its ductility in an inert environment. Furthermore, the fracture morphology of a sample tested in H was characterised by intergranular facets in contrast to dimples describing void coalescence in the inert environment. The failure analysis techniques used suggest that the time taken for crack initiation event is at variance between the individual techniques; however, it is clear that the fracture processes in an H environment include early brittle crack initiation with limited plastic deformation, in comparison with the uniform elongation leading to cup-and-cone ductile fracture in air. Early crack initiation is primarily attributed to the loss of fracture toughness in the H affected region, and the local fracture toughness is evaluated using FRASTA. The average value of local fracture toughness in the H induced fracture region was 55 MPa √m as compared to 270 MPa √m in air.

A frequency dependent embrittling effect of high pressure hydrogen in a 17-4 PH martensitic stainless steel

The effects of hydrogen on tensile and fatigue-life properties of 17-4PH H1150 steel have been investigated by using a smooth, round-bar specimen for tensile tests and circumferentially-notched specimen for fatigue-life tests. The specimens were precharged by an exposure to 35-100 MPa hydrogen gas at 270°C for 200 h. For the 100 MPa hydrogen exposure, the steel showed a significant degradation in ductility loss, translated by a relative reduction in area, RRA, of 0.31. The fatigue-life test of the present notch specimen (stress concentration factor of 6.6) reflects the fatigue crack growth (FCG) for long cracks. The fatigue limit of the non-charged and H-charged notched specimens, defined by the threshold of non-propagation for long cracks, was not affected by hydrogen. At a higher stress amplitude, the H-charged specimen showed a significant FCG acceleration ratio compared to the non-charged specimen. Although, an upper bound of the FCG acceleration seemed to exist, this ratio was approximately 100. The fracture surface of the H-charged specimen was covered with quasi-cleavage (QC) at a lower stress amplitude and with a mixture of QC and intergranular (IG) facets at higher stress amplitudes. It has been suggested that a cycle-dependent crack growth accompanied by QC occurs at a lower stress amplitude, whereas a mixture of cycle-dependent crack growth (accompanied by QC) and time-dependent crack growth (accompanied by IG) occurs otherwise. This mixture justifies the 100 times FCG acceleration ratio in spite of the existence of the upper bound.

Thermomechanical fatigue behavior of annealed Cu-Cr-Zr-Ti alloy in argon atmosphere

Strain controlled thermomechanical fatigue (TMF) behavior of Cu-Cr-Zr-Ti alloy was studied in the temperature range of 375–525°C in high purity argon atmosphere under both in-phase (IP) and out-of-phase (OP) loading. In the case of OP TMF tests, the material exhibited continuous hardening without any saturation of stress. On the other hand, it showed cyclic hardening for the first few cycles followed by saturation of stress in IP TMF tests. The material exhibited superior lives under OP TMF loading compared to that under IP TMF loading especially at lower strain amplitudes. This may be attributed to the fact that the material experienced peak tensile stress at lower temperatures in OP TMF tests, where the resistance to crack initiation is expected to be higher. Microstructural observations and fractographic studies indicated mixed mode of fracture involving Intergranular facets and transgranular cracking in both IP and OP TMF tests.

Evolution of damage under combined low and high cycle fatigue loading in a type 316LN stainless steel at different temperatures

The present study investigates the effect of different damage modes like low cycle fatigue (LCF), high cycle fatigue (HCF), creep and ratcheting during combined cycling at various temperatures ranging from ambient to 923K, in a type 316LN austenitic stainless steel. The experiments were designed with multi-step load sequences where specific number of small amplitude HCF cycles (referred to as blocks) were introduced at the stabilized cyclic load under LCF for a given strain amplitude and repeated until failure. Cyclic life was found to decrease with increase in temperature as well as block-size. The decrease in cyclic life with block-size is more significant at 923K where multiple damage modes like creep and ratcheting are activated. Dynamic strain aging (DSA) was found to operate in the temperature range, 823–873K where the decrease in cyclic life with block-size gets saturated. Typically, transgranular fatigue fracture, intergranular creep fracture or dimpled rupture was identified when failure was dictated by LCF, creep and ratcheting respectively. However, synergistic interaction between the above damage modes leading to a mixed mode fracture carrying signatures of fatigue striations, intergranular facets and dimples occurred at specific combinations of block size and temperature. HCF damage played an important role for some specific loading conditions by acting as a link between intergranular (creep) cracks, thus facilitating the crack propagation and final failure. The regimes of dominant failure modes and interactions among them were suitably mapped as a function of temperature and block size.

Fatigue crack growth behavior of JIS SCM440 steel near fatigue threshold in 9-MPa hydrogen gas environment

In this study, stress intensity factor range (ΔK) decreasing tests were conducted and the in-situ observations were used to investigate the fatigue crack growth behavior of JIS SCM440 steel near the fatigue threshold in a 9-MPa hydrogen gas environment. The fatigue crack growth rate reflected the threshold behavior of the material, although the crack propagation knee point immediately before the threshold stress intensity factor range (ΔKth) could not be distinctly identified. The fatigue crack was also observed to exhibit uneven propagation immediately before ΔKth. In contrast, the knee points in a helium gas environment and air were very distinct. Fractographic analysis further revealed the existence of intergranular facets, which were observed immediately before ΔKth in the hydrogen gas environment. Conversely, no facet was observed immediately before ΔKth in the helium gas environment and air. The formation of the facets was considered to be one of the causes of the uneven crack propagation immediately before ΔKth in the hydrogen gas environment.

Microstructure characterization and mechanical behavior for Ag3Sn joint produced by foil-based TLP bonding in air atmosphere

Low-temperature transient liquid phase (TLP) bonding for Ag-plated substrates was systematically investigated by using foil-based interlayer of pure Sn foil or preformed Sn/Cu/Sn sandwich structure in air atmosphere. The influences of bonding process, such as bonding temperature, bonding time and foil thickness, on the microstructure characterization and mechanical behavior of TLP joint were discussed. Experimental results show that Ag-plated substrates can be successfully TLP bonded in air atmosphere by the protection of flux. The formation of pores in intermetallic compounds (IMCs) is a serious problem for Ag/Sn/Ag TLP bonding, which is attributed to the volume shrinkage of isolated Sn areas during isothermal solidification. Prolonging homogenization time, properly increasing bonding pressure, decreasing temperature, or reducing interlayer thickness can effectively reduce the shrinkage porosity, but is still incapable of eliminating pores thoroughly. Both shear bands and intergranular facets are simultaneously observed on the fracture surface of Ag3Sn joint. Since the micro voids distributing along Ag3Sn grain boundaries weaken the cohesion strength between two neighboring Ag3Sn grains in some areas. Using preformed Sn/Cu/Sn interlayer is available to enhance the mechanical integrity, which is strongly depended on the Cu thickness and Sn thickness. The joint shear strength can be increased by even more than 100% by the introduction of Cu foil. Moreover, the remained Cu layer in the IMCs can act as a buffer layer during fracture process, leading to the improvement of the ductility of TLP joint.

Effect of biaxial cyclic tension on the fatigue life and damage mechanisms of Cr–Mo steel

Combined cyclic tension and internal pressure tests with various proportions of each loading were run on a 2.5%Cr–1%Mo steel to investigate the effect of positive stress biaxiality on fatigue lives and damage mechanisms. While moderate stress biaxiality had a beneficial effect on fatigue lives, attributed mainly to a retardation of crack initiation, equibiaxial tension had a slightly detrimental effect, attributed to a “pseudo size effect”. Intergranular facets associated with temper and hydrogen embrittlement were observed on the fracture surfaces. The evolutions of their surface fraction with ΔK and load biaxiality suggested a possible reduction in crack growth rate at moderate biaxialities. Several popular multiaxial fatigue criteria failed to describe all fatigue data. Thus, a new fatigue criterion based on Gerber’s parabola has been proposed. It captures the evolution of the endurance limit under the combined effects of a positive mean stress and positive biaxiality.

Hot Ductility Loss in a Fe-Ni-Based Superalloy

High temperature tensile tests have been conducted on samples of a Fe-Ni based superalloy, Incoloy A-286, and significant ductility loss has been observed at 1220 °C. Titanium-rich, thin-film-like phase has been found on the inter-granular facets of fracture surfaces. It appears that sulfur content of Ti-rich phase was higher than that of the matrix. At 1220 °C, liquation of Ti-rich phases has resulted in thin-film-like morphology along the grain boundary and caused the ductility loss during tensile deformation.