Quasi-cleavage Fracture Surface Research Articles

Rapid Evaluation of Hydrogen Embrittlement Resistance for Spot-Welds of High Tensile Strength Steel Sheet by Slow Rate Tensile Shear Test under Hydrogen Charging Conditions

Automobile manufacturers are accelerating adoption of spot welding of Advanced High-Strength-Steels (AHSS) sheets to reduce weight of automobile bodies. Rapid evaluation of the hydrogen embrittlement (HE) resistance for the spot-welds of AHSS sheets is required, since it is worried that the HE resistance of the nugget will deteriorate compared to the base metal due to the difference in microstructure caused by rapid cooling and solidification during spot welding. However, evaluation of the HE resistance for the spot-welds has not been established. In this study, we prepared spot-welded specimens using AHSS sheets and performed tensile shear tests with varying tensile rates under hydrogen charging to evaluate the relationship between diffusible hydrogen content and tensile shear strength. As a result, the tensile shear strength of spot welds decreased as the amount of diffusible hydrogen increased. The quasi-cleavage fracture surface and intergranular fracture surface were observed at the nugget and inside the crack generated at the nugget-heat affected zone interface. Furthermore, as the results of crack growth behavior and hydrogen thermal desorption spectroscopy analysis, hydrogen embrittlement in spot welds can be attributed to the stress-induced diffusion of hydrogen and the hydrogen trapped in dislocation and vacancy clusters at crack tips. This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 85 (2021) 75–83.

Количественное сравнение поверхностей разрушения с морфологией скола и квазискола в охрупченной водородом низкоуглеродистой стали

Количественное сравнение поверхностей разрушения с морфологией скола и квазискола в охрупченной водородом низкоуглеродистой стали

Characterization of crystallographic fracture surfaces in Fe–33Mn–6Si alloy

The fatigue fracture surfaces of Fe-33Mn-6Si alloy were observed for detection of the microscopic features of quasi-cleavage fracture, which are reported for γ/ε dual-phase steels. Facet-like morphologies were found in both the fatigue crack origin and growth regions; however, their microscopic features were different. Smooth features implying single-slip deformation were observed at the origin, while characteristic regular triangular and tetrahedral configurations, reflecting {1 1 1} planes and indicating multiple-slip deformation, were observed at the growth region. Some unique morphological features were found in the tensile fracture region following the fatigue fracture region. These distinct microscopic features might indicate the fracture of annealing twin boundaries. Other features on the surface of the primary ε-martensite variant from collision with the secondary ε-martensite variant were also recognized. The variety of microscopic features on the quasi-cleavage fracture surface detected in this study may guide understanding of the deformation and fracture mechanism of γ/ε dual-phase steels.

Hydrogen Desorption Spectra from Excess Vacancy-Type Defects Enhanced by Hydrogen in Tempered Martensitic Steel Showing Quasi-cleavage Fracture

An attempt was made to separate and identify hydrogen peaks desorbed from plastic-strained, hydrogen-enhanced lattice defects from among various trapping sites in tempered martensitic steel showing quasi-cleavage fracture using thermal desorption spectroscopy from a low temperature (L-TDS) and positron annihilation spectroscopy (PAS). The amount of the lattice defects beneath the quasi-cleavage fracture surface was measured by L-TDS. The L-TDS results made it possible to separate two peaks, namely that of the original desorption and also that of new desorption from the steel specimens due to the application of plastic strain in the presence of hydrogen. The PAS results revealed that the new desorption obtained by L-TDS corresponded to vacancy-type defects. Hydrogen and plastic strain noticeably enhanced lattice defects formed within 1.5 mm from the fracture surface, where the average concentration of vacancy-type defects reached approximately 10−5 order in terms of atomic ratio. These results indicate that the accumulation of excess vacancy-type defects enhanced by hydrogen in the local region can lead to nanovoid nucleation and coalescence in plastic deformation, resulting in quasi-cleavage fracture of tempered martensitic steel.

Quasi-cleavage hydrogen-assisted cracking path investigation by fractographic and side surface observations

Despite recent substantial advances in understanding of hydrogen embrittlement (HE), many important aspects of this widespread phenomenon remain a subject of debates. Particularly, remarkably different opinions have been expressed on the nature of hydrogen-assisted cracking (HAC), which produces quasi-cleavage (QC) fracture surfaces in iron and low-carbon steels. Basically, two conflicting groups of theories are distinguished concerning causes the QC phenomenon: brittle cleavage-like models and ductile models exploiting localized ductile micro- or nanovoid concepts at the core. The present study was aimed at gaining a new insight into the QC mechanism through the detailed investigation of the HAC path by the microscopic observations of the side and fracture surfaces of the annealed low-carbon steel, which was tensile tested in the ex- and in-situ cathodic hydrogen charged conditions. Using a combination of the scanning electron microscopy, confocal laser scanning microscopy and electron-backscattering diffraction, it was found that the in-situ charging is more suitable for the investigation of HAC by side surface observations because it provokes QC cracking on the side surface and suppresses normal ductile fracture. As opposes to this, HAC after ex-situ charging occurs mainly internally. In this condition, HAC interferes with the normal ductile fracture on the side surface. It is established that the path of secondary QC cracks in the in-situ charged specimen is determined predominantly by the local stress distributions. This holds even on the scale of individual grain. Large deviations of the quasi-cleavage cracks paths from the specific crystallographic planes are found and explained. Nano-voids are regularly observed at the crack tips. Thus, the nano-voids coalescence is supposed to be an integral part of QC HAC.

Influence of phase composition and microstructure on mechanical properties of hot-rolled Ti-χZr-4Al-0.005B alloys

The effects of phase composition and microstructure evolution on mechanical properties of hot-rolled Ti-χZr-4Al-0.005B alloys (abbreviated as, TχZAB with χ = 0, 10, 20, 30, and 40 wt%) were investigated by means of theoretical calculations, optical microscopy, scanning electron microscopy, and transmission electron microscopy. The as-prepared TχZAB alloys showed an excellent combination of the tensile strength and ductility features. For example, the hot-rolled T40ZAB alloy displayed the ultrahigh tensile strength of 1535 MPa at 6.06% elongation. As Zr content increased, the fracture morphologies revealed typical transitional processes from dimple to quasi-cleavage fracture surfaces. The theoretical calculations agreed well with results from X-ray diffraction, where examined alloys showed only α/α′ crystal phases. Under the same hot-rolled condition, the microstructure evolved from distorted (or broken) lamellar α phase to crisscross acicular α′ martensite phase as the Zr content increased. Finally, in search for ideal structural titanium alloys, phase composition and microstructure and relationship with mechanical properties were examined and discussed.

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.

Hydrogen effects on mechanical properties of 18%Cr ferritic stainless steel

Role of microstructure in susceptibility of 18Cr ferritic stainless steel to hydrogen embrittlement was studied. Specimens of the studied steel were charged with hydrogen electrochemically from 0.1N H2SO4 solution under controlled cathodic potential providing a homogeneous hydrogen distribution over the specimen cross-sections. Thermal desorption spectroscopy analyses were carried out investigating the uptake, trapping and diffusion of hydrogen in the ferritic stainless steel.Microstructural change caused by heat-treatment at 1050°C and 1200°C associated preferably with grain size growth from 18µm to 65µm and 349µm, respectively, resulting in significant degradation of the mechanical properties of the studied steel. The effect of the grain size growth on hydrogen susceptibility was studied with constant extension rate test (CERT) performed under continuous hydrogen charging. It is found that hydrogen has a remarkable effect on the elongation to fracture of the Fe-Cr ferrite: in the presence of H elongation to fracture of the steel reduces up to 75% compared to the H-free counterpart. In general, the hydrogen sensitivity of the mechanical properties increases with increase of the mean grain size of the studied ferritic stainless steel. However, the detailed analysis reveals a complicated, non-linear behavior of the hydrogen sensitivity. Scanning electron microscopy (SEM) of the fracture surfaces of the tensile specimens tested during continuous hydrogen charging reveals a quasi-cleavage fracture surface morphology. Hydrogen-induced cracking in the studied 18Cr ferritic steel was studied using electron backscatter diffraction (EBSD) analysis from the side surfaces of the tensile tested specimens.

Microstructural characteristics and impact toughness in YS690MPa steel weld metal for offshore structures

The fine-grained mixed microstructure of acicular ferrite (AF) and bainite in YS690MPa steel weld metal contributes to attain high-impact toughness. The morphology and evolutionary mechanism of fine-grained mixed microstructure in this weld metal were investigated. Single or multiple AF grains were nucleated on complex inclusions by forming Mn-depleted zones, where Mn spontaneously diffused into Ti oxide inclusions due to the cation vacancies. It is in good agreement with the theoretical calculation by first principle. The bainite nucleated on austenite grain boundary and then assisted the pre-formed AF to partition the austenite grain into small and separate regions. Furthermore, the later formed ferrite nucleated on the broad surface of pre-formed ferrite plates and grew in those small regions with limited grain size. All of them resulted in the formation of fine-grained mixed microstructure, which provided excellent impact toughness in this weld metal with dimples and quasi-cleavage fracture surface combination.

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.

Texture evolution and mechanical anisotropy of biomedical hot-rolled Co–Cr–Mo alloy

Crystallographic textures and their effect on the mechanical anisotropy of a hot-rolled biomedical Co–Cr–Mo alloy were investigated. The hot-rolled Co–28Cr–6Mo–0.13N (mass%) alloy examined here exhibited a monotonic strength increment following hot-rolling reduction, eventually reaching a 0.2% proof stress of 1400MPa while maintaining acceptable ductility (>10%). The dominant hot-rolling texture was a brass-type component, which is characterized by the alloy’s peculiarly low stacking fault energy (SFE) even at hot rolling temperatures, although the minor peaks of the near copper component were also identified. However, because of the onset of dynamic recrystallization (DRX) during the hot rolling process, the texture intensity was relatively weak even after 90% hot rolling, although the grain refinement originating from the DRX was not significant (the “less active DRX” condition increased the strain accumulation during the process, resulting in high-strength samples). The weakened texture development resulted in negligible in-plane anisotropy for the hot-rolled specimen strength, when the specimens were tensile strained in the rolling direction (RD) and transverse direction (TD). The elongation-to-failure, however, exhibited a difference with respect to the tensile loading axis. It is suggested that the ductility anisotropy is closely related to a strain-induced γ (fcc)→ε (hcp) martensitic transformation during tensile loading, resulting in a difference in the proportion of quasi-cleavage fracture surfaces. The obtained results will be helpful in the development of high-strength Co–Cr–Mo alloy plates and sheets, and have implications regarding plastic deformation and texture evolution during the hot rolling of non-conventional metallic materials with low SFE at elevated temperatures, where planar dislocation slips of Shockley partial dislocations and thermally activated process interplay.

Small fatigue crack growth characteristics and fracture surface morphology of low carbon steel in hydrogen gas

Owing to energy conservation and environmental concerns, hydrogen has been suggested as a next-generation energy source. However, hydrogen known to seep into a metal, degrade its strength, and accelerate fatigue crack growth rates. We have investigated the effects of hydrogen gas on the small fatigue crack growth characteristics of low carbon steel JIS S10C by conducting bending fatigue tests on a specimen with a small blind hole and placed in a low-pressure hydrogen environment. The fatigue crack growth rate in hydrogen was higher than that in nitrogen. The fracture surface of the specimen in hydrogen showed intergranular facets in the low- growth-rate range and a quasi-cleavage fracture surface with brittle striations in the high-growth-rate range. The specimen only showed a ductile fracture surface for nitrogen. The small-fatigue-crack growth rate for nitrogen is given by \({dl/dN\propto \Delta \varepsilon_{p}^{n}l}\), where l, N, and \({\Delta \varepsilon_{p}}\) represent the crack length, number of repetitions, and plastic strain range, respectively. This equation was also satisfied for hydrogen, but only over a short strain range from \({\Delta \varepsilon_t = 0.25}\) to 0.37 % in which the fracture surface exhibited intergranular facets and a ductile morphology, but no quasi-cleavage fracture. The exponent n of the equation was 1.22 in nitrogen and 0.66 in hydrogen environment. The small-fatigue-crack growth law can be used for safe material designs in hydrogen environments.

Fracture process of C–Mn steel embrittled by hydrogen

Fracture morphologies of tested specimens, by means of the slow strain rate tensile testing, were examined to study fracture modes by using scanning electron microscopy.This study has led to the determination of three categories of specimens: unembrittled specimens, slightly embrittled specimens and severely embrittled specimens. The first category of unembrittled specimens includes specimens tested in air, at potentials of −0.80 and −0.85V (SCE) with embrittlement ratio (ER)⩾0.97. In the category of specimens slightly embrittled, only the specimen tested at −0.9V with an embrittlement ratio (ER) of 0.69 is included. The last category of severely embrittled specimens includes the specimens with embrittlement ratio (ER)<0.69.The fractographic studies are consistent with the embrittlement of steel being due to hydrogen embrittlement. The intergranular and transgranular quasicleavage fracture surface morphologies observed with embrittled specimens are typical and characteristic of hydrogen embrittlement.

GS16 SUS316ステンレス鋼の引張強度特性に及ぼす水素の影響

The effect of hydrogen on the tensile properties of SUS316 stainless steel are investigated. 0, 5 and 10% pre-strained specimens are exposed to hydrogen atmosphere of 100MPa in pressure at 393K for 400h. It is observed that the tensile strength does not vary but the reduction of area reduces by performing hydrogen exposure. Cup-and-cone fracture occurs in the unexposed specimens. Fracture surface of hydrogen exposed specimens is quasi-cleavage fracture surface in the region from the specimen surface to 200μm. In the inside of the quasi-cleavage fracture area, the fracture surface is covered by dimples. Slip bands are seen in the side surface of unexposed specimens, and many cracks of 300-500μm in length are found in the side surface of hydrogen exposed specimens.

Accelerated corrosion exposure in ultra thin sheets of 2024 aircraft aluminium alloy for GLARE applications

The effect of corrosion exposure on ultra thin (t<0.4mm) 2024-T3 aluminium sheet thickness is investigated. Microstructural analysis showed that for low accelerated corrosion exposure times, no surface deterioration existed and hence corresponding mechanical properties degradation was assumed to be incited by hydrogen diffusion and subsequent embrittlement. Medium exposure times resulted in cross-section reduction due to formation of sub-surface corrosion products, while crevice corrosion was evident for higher exposure times. Tensile specimens were pre-corroded for single or double sides and then mechanically tested. Decrease in mechanical properties was noticed for both L and T sheet rolling directions, even after a few minutes exposure, with transverse rolling direction having higher degradation values for same exposure times. It was noticed that a single sided corroded specimen needs approximately double exposure time to reach the same ductility decrease of a double sided specimen. Quantitative tensile properties degradation results are discussed; the results were correlated to observed corrosion degradation mechanisms. The stepwise ductile-to-brittle transition of tensile fracture mechanism with increasing corrosion exposure time was denoted via analysis of the ‘un-corroded’ region of tensile fracture surface. Single sided specimens exhibited quasi-cleavage fracture surfaces at the region below protected surface to corrosion.

Effect of Loading Rate and Tempering Temperature on Fracture Toughness of Hydrogen-Charged Low Alloy Steel SCM440

J - R curves under continuous hydrogen charge were obtained to evaluate the effect of absorbed hydrogen on fracture toughness of SCM440 low alloy steel. The effect of absorbed hydrogen, loading rate and tempering temperature were investigated. Five materials which were tempered at different temperature to change Vickers hardness were used. Continuous hydrogen charge, which isolated the crack tip from the electrolyte and kept the crack in dry condition, was conducted during the test. Material hardness and loading rate significantly affected J - R curves and JIC. Low loading rate and high hardness resulted in drastically decreased JIC. The materials which were tempered at 803K showed 90% reduction of JIC compared with that of uncharged specimen. The significant reduction was experienced in materials with Vickers hardness higher than 310. Quasi-cleavage fracture surface was predominantly observed in the tests which resulted in low JIC. It is indicated that the hardness of low alloy steel used as a structural material should be limited below 300 on the viewpoint of fracture toughness. This study showed that low loading test that lasts for eighty hours at least is necessary to assess the JIC of hydrogen charged material.

On the formation and nature of quasi-cleavage fracture surfaces in hydrogen embrittled steels

Quasi-cleavage, a common feature of hydrogen-induced fracture surfaces, is generally taken as being cleavage-like but not along a known cleavage plane. Despite the frequency with which this surface is observed, the relationship to the underlying microstructure remains unknown. Through a combination of topographical reconstruction of secondary electron microscope fractographs and a transmission electron microscopy study of the microstructure from site-specific locations, it will be shown that the features on quasi-cleavage surfaces are ridges that can be correlated with sub-surface intense and highly localized deformation bands. It will be demonstrated that the fracture surface arises from the growth and coalescence of voids that initiate at and extend along slip band intersections. This mechanism and process is fully consistent with hydrogen enhancing and localizing plastic processes.

Low-temperature fracture mechanisms in a spheroidised reactor pressure vessel steel

The micromechanisms of fracture of a spheroidised A533B reactor pressure vessel steel over the temperature range of −190°C to + 60°C were investigated by performing uniaxial tensile tests on double-notched cylindrical specimens. Failure was by quasi-cleavage at temperatures between −190°C and −145°C. Quasi-cleavage fracture surfaces are characterised by clusters of planar facets that are separated from other facets either by large voids or by clusters of microvoids. At temperatures between −145°C and −25°C failure was by mixed microvoid coalescence and cleavage while complete microvoid coalescence was observed at temperatures higher than −25°C. Over the whole temperature range studied, fracture nucleation was either from large single voids or localised regions of microvoids.

Quantitative Evaluation of the Impact Fracture Surfaces of SS400 Steel by the Three-dimensional Geometrical Analysis

The fractal dimension and the surface roughness of the fracture surface was estimated using the height data generated by the stereo matching method on Charpy V-notch specimens of the SS400 steel impact-fractured in the temperature range from 194 K (−79°C) to 273 K (0°C). The representative fracture surfaces were quasi-cleavage fracture surface with river patterns and steps between facets at 194 K and ductile fracture surface with dimples and shear steps at 273 K. On the region close to the notch root (fracture initiation site), a definite correlation was not found between the absorbed energy and the fractal dimension of the fracture surface, but both absorbed energy and surface roughness increased with temperature because of increased proportion of ductile fracture surface with the larger surface roughness. There was essentially no difference in the fractal dimension between quasi-cleavage fracture surface and ductile fracture surface. Fracture process including fracture initiation, local crack growth or finally fractured part could be examined by the detection of characteristic fracture patterns using the fractal dimension map (FDM) and the surface roughness map (SRM). Detected steps of various sizes were associated with river patterns or fracture at ferrite grain boundaries on the quasi-cleavage fracture surface, and were associated with dimple walls or shear steps on the ductile fracture surface. Steps on both fracture surfaces were formed by common mechanism, namely, by plastic deformation (shear deformation). The density of steps was larger on the quasi-cleavage fracture surface than on the ductile fracture surface. These steps led to a large fractal dimension of the quasi-cleavage fracture surface comparable with the ductile fracture surface with dimples.

Nitride Precipitation in Duplex Stainless Steel Weld Metal.

Duplex stainless steel base material was welded using gas tungsten arc welding with an Ar-10%H2 shielding gas and laboratory-made filler wires were employed to deposit duplex and fully ferritic weld metals having different nitrogen contents. Weld metal slow extension rate tensile (WM-SERT) testing was used to examine the hydrogen-induced cracking susceptibility and fractography of the weld metals. An increase in nitrogen content in fully ferritic stainless steel weld metal increased the density of precipitates and the hydrogen-induced cracking susceptibility. The facets on the quasi-cleavage fracture surfaces of broken WM-SERT test specimens were parallel to the {100} plane in ferrite. Scanning and transmission electron microscope observations revealed the crystallographic features and morphology of the precipitates. The precipitates were rod-like Cr2N nitrides. Many of them had directions and were parallel to the cleavage {100} plane in ferrite. An orientation relationship shown between Cr2N precipitates and ferrite suggested that the axes of the Cr2N precipitates were parallel to direction in ferrite and that they were more coherent along their long faces than at their tips. As a result, the tip of these Cr2N precipitates could act as sinks for hydrogen and may be preferential sites for initiation of hydrogen cracking; this could promote crack propagation on {001} cleavage planes in ferrite on which Cr2N precipitates are located. Higher densities of Cr2N precipitates were nucleated at solidification boundaries and at oxide inclusions in ferrite.