Effects of Nb on stress corrosion cracking of high-strength low-alloy steel in simulated seawater

In this study, simulated heat-affected zone (HAZ) of Nb-free and Nb-bearing steel were obtained, and SEM, TEM, and slow strain rate tensile (SSRT) tests were performed to investigate the effect of Nb on the stress corrosion cracking (SCC) behavior of high-strength low-alloy (HLSA) steel in simulated seawater with or without hydrogen charging. The addition of Nb significantly refined the grains and uniformed the microstructure of HLSA. Nb hardly affected the SCC susceptibility of BM and HAZ without hydrogen-charging. However, after charging with 10 mA cm⁻², the SCC resistance of Nb-bearing steel, especially the coarse grain HAZ (CGHAZ) improved drastically, and the process of crack initiation and propagation was inhibited owing to the hydrogen trap function of NbC precipitates.     

Effects of α′ martensite and deformation twin on hydrogen-assisted fatigue crack growth in cold/warm-rolled type 304 stainless steel

The effects of α′ martensite and deformation twin on hydrogen-assisted fatigue crack growth (FCG) were investigated in cold/warm-rolled type 304 stainless steel in 5 MPa hydrogen and argon gas atmosphere. The rate of FCG is reduced in argon gas, while greatly enhanced in hydrogen gas after cold-rolling. The FCG rates of warm-rolled specimens, no matter tested in hydrogen gas or argon gas, are reduced comparing with as-received specimens. After cold-rolling, α′ martensite formed around the grain boundary promotes hydrogen-assisted crack initiation and propagation. The deformation twin plays an important role during FCG besides α′ martensite after warm-rolling, and hydrogen-assisted cracking along the twin boundary and slip band can enhance the FCG rate during cycle loading.     

Strain rate and hydrogen effects on crack growth from a notch in a Fe-high-Mn steel containing 1.1 wt% solute carbon

Effects of strain rate and hydrogen on crack propagation from a notch were investigated using a Fe-33Mn-1.1C steel by tension tests conducted at a cross head displacement speeds of 10⁻² and 10⁻⁴ mm/s. Decreasing cross head displacement speed reduced the elongation by promoting intergranular crack initiation at the notch tip, whereas the crack propagation path was unaffected by the strain rate. Intergranular cracking in the studied steel was mainly caused by plasticity-driven mechanism of dynamic strain aging (DSA) and plasticity-driven damage along grain boundaries. With the introduction of hydrogen, decrease in yield strength due to cracking at the notch tip before yielding as well as reduction in elongation were observed. Coexistence of several hydrogen embrittlement mechanisms, such as hydrogen enhanced decohesion (HEDE) and hydrogen enhanced localized plasticity (HELP) were observed at and further away from the notch tip resulting in hydrogen assisted intergranular fracture and cracking which was the key reason behind the ductility reduction.     

Tensile mechanical properties and fracture behaviors of nickel-based superalloy 718 in the presence of hydrogen

Hydrogen embrittlement of a nickel-based superalloy IN718 was investigated using slow strain rate tensile tests. Post-mortem observation of fractured samples was performed to explore hydrogen-assisted failure mechanisms of the alloy. The results reveal that hydrogen charging reduces yield strength, tensile strength, fracture strain and work hardening rate. With increasing current density, yield strength and tensile strength reduce linearly and fracture strain decreases exponentially. Furthermore, the crack initiation and propagation in hydrogen-charged region depends on the distribution of δ phase in the alloy. For needle-shaped δ phase within the grains, the nucleation of voids takes place at the intersections between dislocation slip bands and δ phase due to the hydrogen-enhanced localized plasticity (HELP)-assisted shear localization and possible hydrogen agglomeration. For δ phase along the grain boundaries, the impingement of slip bands and local hydrogen accumulation at γ-matrix/δ phase interfaces as well as hydrogen-enhanced decohesion (HEDE)-assisted decohesion lead to the void nucleation at the interfaces. Because of the decoration of δ phase at the grain boundaries, hydrogen-assisted cracking preferentially propagates along the grain boundaries. It is hence suggested that the synergistic interplay of HELP mechanism and HEDE mechanism can be used to explain the embrittlement of the alloy.     

Monitoring of stress corrosion cracking of sensitised 304H stainless steel in nuclear applications by electrochemical methods and acoustic emission

Abstract

A hybrid monitoring technique for stress corrosion cracking (SCC) has been developed that employs simultaneously localised corrosion monitoring, electrochemical noise and acoustic emission (AE) techniques. The application of the hybrid technique for detection of SCC initiation and propagation in sensitised 304H stainless steel in dilute tetrathionate solutions at ambient temperature is reported. Initial result shows that SCC initiation and its early stage propagation can be detected by the localised corrosion monitoring and electrochemical noise methods. The dimensions of the crack can be estimated from the charge values derived from the detected transients. The locations of AE events determined using two sensors are in good agreement with the locations of cracks observed in the specimen. The AE technique is sensitive to rapid crack propagation, but does not appear to be sensitive to SCC initiation and early stage propagation for the present material environment load combination. It is postulated that AE is sensitive to SCC propagation involving a relatively large volume of plastic deformation. On the basis of test results and on information from the literature, it is suggested that in this material environment system SCC cracks initiate via slow anodic dissolution at the chromium depleted grain boundaries. Subsequently, elemental sulphur adsorbed on the surface around the crack tip catalyses the entry of hydrogen atoms produced by the hydrogen reduction reaction into the steel matrix ahead of the crack tip; this hydrogen accumulates gradually over a relatively long period of time and preferentially at carbide/matrix interfaces, eventually causing hydrogen induced brittle fracture along grain boundaries.     

Gaseous hydrogen embrittlement of a Ni-free austenitic stainless steel containing 1 mass% nitrogen: Effects of nitrogen-enhanced dislocation planarity

We investigated the effect of hydrogen on degradation of tensile properties in a Fe–25Cr–1N austenitic stainless steel. Hydrogen was introduced by exposure to a hydrogen gas atmosphere at 100 MPa and 270 °C. Hydrogen charging caused significant ductility loss associated with nitrogen-enhanced dislocation planarity. Specifically, even without hydrogen, the nitrogen-enhanced planar dislocation glide induced micro-stress concentration, which assisted the occurrence of hydrogen-induced intergranular and quasi-cleavage fractures. The hydrogen-assisted intergranular cracking occurred along boundaries of grains where primary slip was predominantly activated. On the other hand, the hydrogen-assisted quasi-cleavage fracture took place when multiple slip systems were activated. The hydrogen-related cracks emerged, but their growth was arrested via crack blunting associated with a significant plastic deformation. Instead, new cracks formed near the crack tips. Therefore, hydrogen-assisted crack propagation occurred through repetition of crack blunting, initiation, and coalescence.     

Hydrogen embrittlement associated with strain localization in a precipitation-hardened Fe–Mn–Al–C light weight austenitic steel

Hydrogen embrittlement of a precipitation-hardened Fe–26Mn–11Al-1.2C (wt.%) austenitic steel was examined by tensile testing under hydrogen charging and thermal desorption analysis. While the high strength of the alloy (>1 GPa) was not affected, hydrogen charging reduced the engineering tensile elongation from 44 to only 5%. Hydrogen-assisted cracking mechanisms were studied via the joint use of electron backscatter diffraction analysis and orientation-optimized electron channeling contrast imaging. The observed embrittlement was mainly due to two mechanisms, namely, grain boundary triple junction cracking and slip-localization-induced intergranular cracking along micro-voids formed on grain boundaries. Grain boundary triple junction cracking occurs preferentially, while the microscopically ductile slip-localization-induced intergranular cracking assists crack growth during plastic deformation resulting in macroscopic brittle fracture appearance.     

Evaluation of the cementite morphology influence on the hydrogen induced crack nucleation and propagation path in carbon steels

The effect of cementite morphology on the crack initiation and growth path was studied using in situ electrochem-ical micro-cantilever bending (ECCB) technique under hydrogen (H) charging. Two carbon steels with lamellar cementite morphology (pearlitic microstructure) and spherical or broken lamellas cementite morphology (spheroidite microstructure), both with approximately the same carbon equivalent, were used in this study. The ECCB tests were performed in H-free and two H charging steps with ?1050 mV and ?1550 mV charging potential versus Ag/AgCl reference electrode. The results show that both materials are resistant to crack initiation in the H-free condition while under ?1050 mV charging, crack propagates through the grain boundaries in a tortuous path in spheroidite mi-crostructure and the lamellar microstructure displayed a higher strength with small cracks propagating through both the grain boundaries and the lamellas. A drastic load decrease in the load-displacement (L-D) curve happened under ?1550 mV charging for both microstructures accompanied by a straight crack growth path in spheroidite microstruc-ture, independent of grain boundaries or ferrite-cementite interfaces while a competition between the shear crack growth mechanism and the interfacial cracking determines the crack growth path in the lamellar microstructure.     

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

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

Effects of finish rolling deformation on hydrogen-induced cracking and hydrogen-induced ductility loss of high-vanadium TMCP X80 pipeline steel

An optimum finish rolling deformation (FRD) of thermomechanical controlled processing (TMCP) is suggested to improve the hydrogen-induced ductility loss of high-vanadium X80 pipeline steel in this study. The results demonstrate that with increasing FRD the microstructure refines, the grain size of the steel decreases and the recrystallization degree deepens. The increase of FRD leads to the reduction of low angle grain boundaries (LAGBs) and the grains oriented with plane {100} parallel to normal direction ({100}//ND) fibres, which plays a significant role in improving the resistance of crack propagation. Besides, the differences of effective hydrogen diffusion coefficient and diffusible hydrogen concentration are negligible among four experimental steels with various FRD. However, the best hydrogen-induced ductility loss resistance is obtained in the steel with 40% FRD containing the most nano-scale precipitates acting as effective hydrogen traps.     

Investigation on hydrogen induced cracking behaviors of Ni-base alloy

Hydrogen embrittlement of a Ni-base alloy at room temperature was investigated by slow strain rate tensile test (SSRT) under precharging or dynamic charging conditions. It was found that hydrogen embrittlement susceptibility of this alloy increased with increasing charging current density in both charging conditions. In-situ observation of hydrogen induced cracking revealed that surface crack initiation at both grain boundaries and slip bands, which should be attributed to decomposition of hydride phase during aging at room temperature. SSRT result exhibited that hydrogen diffusion in the alloy could be facilitated by deformation and as a result induced transgranular fracture of the sample. Both hydrogen induced cracking and the interaction between hydrogen and deformation played combined roles on hydrogen embrittlement of this alloy.     

Hydrogen embrittlement of the coarse grain heat affected zone of a quenched and tempered 42CrMo4 steel

The coarse grain heat affected zone (CG-HAZ) of welds produced in a quenched and tempered 42CrMo4 steel was simulated by means of a laboratory heat treatment consisting in austenitizing at 1200 °C for 20 min, oil quenching and finally applying a post weld heat treatment at 700 °C for 2 h (similar to the tempering treatment previously applied to the base steel). A tempered martensite microstructure with a prior austenite grain size of 150 μm and a hardness of 230 HV, similar to the aforementioned CG-HAZ weld region, was produced. The effect of the prior austenite grain size on the hydrogen embrittlement (HE) behaviour of the steel was studied comparing this coarse-grained microstructure with that of the fine-grained base steel, with a prior austenite grain size of 20 μm.The specimens used in this study were charged with hydrogen gas in a reactor at 19.5 MPa and 450 °C for 21 h. Cylindrical specimens were used to determine hydrogen uptake and hydrogen desorption behaviour. Smooth and notched tensile specimens tested under different displacement rates were also used to evaluate HE.Embrittlement indexes, EI, were generally quite low in the case of hydrogen pre-charged tensile tests performed on smooth tensile specimens. However, very significant embrittlement indexes were obtained with notched tensile specimens. It was observed that these indexes always increase as the applied displacement rate decreases. Moreover, hydrogen embrittlement indexes also increase with increasing prior austenite grain size. In fact, the embrittlement index related to the reduction in area, EI_(RA), reached values of over 20% and 50% for the fine and coarse grain size steels, respectively, when tested under the lowest displacement rates (0.002 mm/min).A comprehensive fractographic analysis was performed and the main operative failure micromechanisms due to the presence of internal hydrogen were determined at different test displacement rates. While microvoids coalescence (MVC) was found to be the typical ductile failure micromechanism in the absence of hydrogen in the two steels, brittle decohesion mechanisms (carbide-matrix interface decohesion, CMD, and martensitic lath interface decohesion, MLD) were observed under internal hydrogen. Intergranular fracture (IG) was also found to be operative in the case of the coarse-grained steel tested under the lowest displacement rate, in which hydrogen accumulation in the process zone ahead of the notch tip is maximal.     

Hydrogen induced blister cracking and mechanical failure in X65 pipeline steels

The present work aims to investigate the role of hydrogen induced blisters cracking on degradation of tensile and fatigue properties of X65 pipeline steel. Both tensile and fatigue specimens were electrochemically charged with hydrogen at 20 mA/cm² for a period of 4 h. Hydrogen charging resulted in hydrogen induced cracking (HIC) and blister formation throughout the specimen surface. Nearly all the blisters formed during hydrogen charging showed blister wall cracking (BWC). Inclusions mixed in Al-Si-O were found to be the potential sites for HIC and BWC. Slow strain rate tensile (SSRT) test followed by fractographic analysis confirmed significant hydrogen embrittlement (HE) susceptibility of X65 steel. Short fatigue crack growth framework, on the other hand, specifically highlighted the role of BWC on accelerated crack growth in the investigated material. Coalescence of propagating short fatigue crack with BWC resulted in rapid increase in the crack length and reduced the number of cycles for crack propagation to the equivalent crack length.     

Effect of grain size on hydrogen embrittlement in stable austenitic high-Mn TWIP and high-N stainless steels

Two stable austenitic steels, 20Cr-11Ni-5Mn-0.3N (wt%) stainless steel (STS) and 18Mn-1.5Al-0.6C (wt%) twinning-induced plasticity steel (TWIP), were investigated to understand the effect of grain size on hydrogen embrittlement (HE). Grain refinement promoted HE in the STS but suppressed HE in the TWIP. These opposite effects occurred because the steel composition affected deformation mechanism. Cr-N pair enhanced short-range ordering (SRO) in STS, which promoted planar slip and delayed mechanical twinning. In contrast, TWIP exhibited mechanical twinning which was more active in coarser grains. Final dislocation density after tensile deformation was increased by grain refinement in STS, but was decreased in TWIP. The damaging effects of hydrogen on strain energy at interfaces and on interfacial bonding strength were controlled by dislocation density; therefore, increase in dislocation density led to increase in susceptibility to HE.     

Hydrogen environment embrittlement of an ODS RAF steel – Role of irreversible hydrogen trap sites

The microstructure and the effects of 10 MPa hydrogen atmosphere on the tensile properties of a oxide dispersion strengthened (ODS) reduced activation ferritic (RAF) steel were investigated. The microstructure consists of a fine grained ferritic matrix with Me₃O₄ (Me = Cr, Fe or Mn), VN and Cr₂₃C₆ grain boundary precipitates as well as dispersed yttrium oxide nano precipitates in the ferritic matrix. The yield and ultimate tensile strength were unaffected by the H₂ atmosphere whereas elongation at fracture and reduction in area were markedly reduced. In H₂ atmosphere, the fracture morphology was found to be a mixture of intergranular H-assisted fracture and a smaller amount of transgranular hydrogen enhanced localized plasticity (HELP) fracture. The sensitivity of the ODS RAF steel to hydrogen embrittlement is attributed to the large number grain boundary precipitates which enhance the tendency for intergranular fracture.     

Grain boundary control for improved intergranular stress corrosion cracking resistance in austenitic stainless steels: new approach

Abstract

The present paper provides an overview of a new approach which has focused on the behaviour of special grain boundaries in sensitised austenitic stainless steel. The aim of the work was to develop a general model for stress corrosion cracking, which would ultimately be capable of predicting the effects of the degree of sensitisation, the connectivity of special boundaries and the influence of stress gradients, such as those developed from surface preparation (machining or peening) or due to the stress concentration effect of pit formation. Experimental work using electron backscatter diffraction analysis and in situ high resolution computed X-ray tomography has correlated cracking with the microstructure in a type 304 austenitic stainless steel. In situ three-dimensional observations demonstrated that annealing twins cause local crack arrest and diversion, leaving non-fractured ligaments in the wake of the cracking path. The mechanical effects of the deformation and failure of these bridges have been modelled, demonstrating that special grain boundaries cause crack tip shielding. Increasing the fraction of special boundaries and decreasing grain size are both predicted to increase stress corrosion cracking resistance. Experimental observations using room temperature intergranular stress corrosion tests and high temperature autoclave tests confirm these predictions for thermomechanically processed microstructures. The effects of applied stress and stress gradients are also predicted by the model, which may be extended to include the kinetics of crack growth, clustering of grain boundary types and variation of the degree of sensitisation.     

316L不锈钢管板试压时的开裂失效

对开裂的316L不锈钢管板进行了化学成分分析、超声波测厚、渗透探伤、母材金相分析、开裂部位宏观分析及断口扫描电镜分析、微区能谱分析。认为试压开裂的原因为管板金相组织粗大，晶界析出较多碳化物，使材料变脆，同时材料中有大量的夹杂物和夹渣，材料性能变差，因此在试压过程中受力开裂。最后并就如何对受压元件进厂验收提出了一些可行的建议。     

Mechanical and microstructural analysis on hydrogen-related fracture in a martensitic steel

The present study quantitatively evaluated mechanical response of hydrogen-related fracture in the as-quenched martensitic steel and correlated it to crack propagation behavior analyzed by microstructure observations. The crack-growth resistance curves revealed that the hydrogen-related intergranular cracks propagated in a stable manner even when the diffusible hydrogen content was large. Fracture initiation toughness was decreased significantly by small amounts of diffusible hydrogen. With further increasing diffusible hydrogen content, however, the fracture initiation toughness did not change and remained almost constant. On the other hand, tearing modulus, corresponding to crack-growth resistance, decreased rather gradually with increasing diffusible hydrogen content. The microstructure observations confirmed that the hydrogen-related crack propagated discontinuously in a stepwise manner on a microscopic scale. Accordingly, it was proposed that the microscopic discontinuous crack propagation could be the possible reason for the stable crack propagation.     

On the physical differences between tensile testing of type 304 and 316 austenitic stainless steels with internal hydrogen and in external hydrogen

Seventeen metastable austenitic stainless steels (type 304 and 316 alloys) were tested in tension both with internal hydrogen and in external hydrogen. Hydrogen-assisted fracture in both environments is a competition between hydrogen-affected ductile overload and hydrogen-assisted crack propagation. In general, hydrogen localizes the fracture process, which results in crack propagation of particularly susceptible materials at an apparent engineering stress that is less than the tensile strength of the material. Hydrogen-assisted crack propagation in this class of alloys becomes more prevalent at lower nickel content and lower temperature. In addition, for the tests in this study, external hydrogen reduces tensile ductility more than internal hydrogen. External hydrogen promotes crack initiation and propagation at the surface, while with internal hydrogen surface cracking is largely absent, thus preempting hydrogen-assisted crack propagation from the surface. This is not a general result, however, because the reduction of ductility with internal and external hydrogen depends on the specifics of the testing conditions that are compared (e.g., hydrogen gas pressure); in addition, internal hydrogen can promote the formation of internal cracks, which can propagate similar to surface cracks.     

Correlation between grain size variation and hydrogen embrittlement in a cost-effective Fe40Mn40Ni10Cr10 austenitic medium entropy alloy

The effect of grain size variation (11 μm, 34 μm) on the hydrogen-induced tensile properties degradation of a Co-free cost-effective Fe₄₀Mn₄₀Ni₁₀Cr₁₀ austenitic medium entropy alloy was investigated using a slow strain rate test. Despite improving both strength and ductility with decreasing the grain size in non-charged conditions, the fine-grain alloy showed a higher relative elongation loss after electro-chemical hydrogen charging. The larger ductility loss in the fine-grain alloy was ascribed to the fast propagation rate of major intergranular cracks and the drastic strain hardening rate drop of the alloy under hydrogen charging. The Schmid factor analysis showed that the enhanced dislocation activity in the fine-grain alloy compared with coarse-grain one was responsible for rapid hydrogen transfer to the grain boundaries, fast dislocation pile-up behind the grain boundaries, and, consequently, more severe hydrogen embrittlement. The significant stress concentration near the grain boundaries and fast intergranular crack propagation were recognized to be the main reason for premature fracture in fine-grain alloy.