Slow Strain Rate Research Articles

Pit-to-crack mechanisms of 316LN stainless steel reinforcement in alkaline solution influenced by strain induced martensite

The pit-to-crack transition of AISI 316LN stainless steel reinforcement exposed to stress corrosion cracking (SCC) in chlorides contaminated alkaline environment, was studied by a combination of slow strain rate testing (SSRT) and electrochemical impedance spectroscopy (EIS). The phase angle shift (Δφ) obtained by EIS at low frequencies was utilized to determine the pit-to-crack transition, differentiating from crack nucleation and propagation as identified by shifts in the frequency range of phase angle (θ) peaks. The pit-to-crack transition was developed once the maximum θ value shifted from the low to high frequencies. EIS analysis was corroborated by assessment of repassivation rates and pit growth, in addition to calculating {Delta G}^{{rm{gamma }}to {rm{alpha }}{rm{mbox{'}}}}. Crack nucleation at lath martensite developed transgranular SCC. Strain-induced martensitic transformation was associated with the brittle failure of AISI 316LN stainless steel, where α’–martensite phase preferentially incubated the pit, and favored crack nucleation, thus promoting pit-to-crack transition.

A study on reducing hydrogen content in steel using ultrasonic cavitation process

In this study, a novel process utilizing ultrasonic cavitation(UC) was proposed to decrease hydrogen content in steel. Electrochemical hydrogen-charging and UC were performed on steels with different elemental species and ratios, and hydrogen embrittlement(HE) susceptibility was evaluated by slow strain rate tensile(SSRT). UC effectively reduced hydrogen-induced plasticity loss by removing hydrogen trapped by traps, particularly in SA508Gr.3. Dehydrogenation effects and recovery mechanisms were explored. Hydrogen trapped by low-energy traps played a critical role in hydrogen-induced hardening. The cleavage platforms of the specimen after UC was reduced, but a large number of microcracks appeared in the fracture due to residual hydrogen.

The Role of Nickel in Low Alloy Steels Exposed to H2S Containing Environments. Part II: Effect of the Electrochemical Potential and Stress Level on Trench Formation

In the first part of this paper, trenches were reported for Ni-containing steels tested using the slow strain rate test (SSRT) method in H2S-containing environments at the open-circuit potential (OCP). Trenches are deep elongated pits, and their appearance is more similar to small blunt cracks. These features can develop a sharp sulfide stress crack at their bottom under certain conditions, which are still not fully understood. In this second and final part, the effect of the electrochemical potential and the stress level in trench nucleation and growth was investigated for the same set of Ni-containing steels with up to 5 wt% Ni. The anodic nature of the trench formation mechanism was verified, and under an anodic polarization a critical stress value for trench formation, σtrench, was estimated from SSRTs and finite element modeling. Applying a cathodic potential suppressed trench formation, but not cracking, because cracks nucleated and propagated by hydrogen stress cracking. The resulting environmental and stress-level dependencies for Ni steels confirmed that trenches could be considered a form of environmental-assisted cracking. It is concluded that the main role of trenches is to provide favorable spots for hydrogen stress crack nucleation at OCP, but their presence is neither necessary nor sufficient for cracking occurrence.

Hydrogen embrittlement behavior of two mining chain steels by slow strain rate test

Two mining chain steels 23MnNiMoCr5-4 and 0.3C0.2Si0.3Mn4.2(Cr+Ni+Mo) with tensile strengths of 1200 MPa and 1250 MPa, respectively, were employed to investigate their hydrogen embrittlement behaviors by slow strain rate tests combined with thermal desorption analyses. It is shown that at initial stage the fracture stress decreases linearly as the hydrogen content increases, and the turning points occur at hydrogen content of 1.2 wppm for 0.3C0.2Si0.3Mn4.2(Cr+Ni+Mo) and 0.26 wppm for 23MnNiMoCr5-4. The fractured surface observation suggests that the ratio of intergranular fracture area to quasi-cleavage area increases dramatically at the turning points. The maximum hydrogen contents resulting from the corrosive HCl solutions with pHs of 2 are approximately 0.6 wppm 0.3C0.2Si0.3Mn4.2(Cr+Ni+Mo) and 0.11 wppm for 23MnNiMoCr5-4, corresponding to the activation energies for hydrogen desorption are 21.57 kJ/mol and 14.53 kJ/mol, respectively.

Electrochemical potential dependence of SCC initiation in X60 pipeline steel in near-neutral pH environment

Stress corrosion crack (SCC) in pipelines directly affects the long-term safety service and has a major impact on the continuity of oil and gas delivery. The near-neutral pH SCC propagation process has been widely reported in the past decades, while the crack initiation mechanism is rarely studied in-depth. Here, the crack initiation behavior of ×60 pipeline steel under different electrochemical potential levels was investigated by interrupted slow strain rate tensile testing (SSRT) and microstructural characterization. The experimental results show that the SCC initiation is dominated by hydrogen-facilitated anodic dissolution (AD) under the open circuit potential (OCP) environment, while it is by the hydrogen embrittlement (HE) under −1.1 and −1.2 VSCE cathodic potentials. All the SCC cracks in ×60 pipeline steel show transgranular features, pathing through the grains in different crystallographic orientations, which showed dependence on the electrochemical potential of the steel: initiation via slip-enhanced dissolution of {110}//ND or {112}//ND texture under OCP environment, but mostly by quasi-cleavage cracking of {100}//ND texture under −1.1 and −1.2 VSCE cathodic potentials.

Enhancement of the resistance to hydrogen embrittlement by tailoring grain boundary characteristics in a low carbon high strength steel

In this work, the feasibility of using “grain boundary engineering” to reduce the susceptibility to hydrogen embrittlement in a low carbon low alloy steel was examined. By adding Cu element, the fraction of random grain boundary (RGB) exhibited the highest value in the steel with 1%Cu, while the fraction of special grain boundary (SGB) showed a monotonical decline. The slow strain rate tensile (SSRT) test revealed that the elongation loss presented an increase with Cu addition increasing from 1 % to 3 %. This can be explained in terms of hydrogen diffusion, hydrogen trap and crack propagation. The steel with 1%Cu had a higher fraction of high angle grain boundary (HAGB), which contributed to a higher density of hydrogen traps and a lower hydrogen diffusion rate. Moreover, the steel with 1%Cu had the highest fraction of SGB (∑3, ∑5, ∑7 and ∑≥9), which was beneficial to the resistance to crack propagation. Under the combined effect of RGB and SGB, a higher resistance to hydrogen embrittlement was achieved in the steel with 1%Cu.

Insights into the role of CO in inhibiting hydrogen embrittlement of X80 steel weld at different hydrogen blending ratios

In this study, slow strain rate tension tests were conducted on an X80 steel weld in 12 MPa mixed gases with 2 %–75 % H2 ([H2]) and 0 %–0.75 % CO ([CO]). The influence mechanism of CO on inhibiting hydrogen embrittlement (HE) was explored with the aid of the first-principles method and hydrogen permeation tests. The results show that the HE index first decreases rapidly with the increase of [CO] or [CO]/[H2], and then tends to stabilize. CO can restore the hydrogen induced plastic loss to a certain value, and the recovery degree decreases with the increase of [H2]. CO preferentially occupies the hydrogen adsorption site when co-existing with H2, and can reduce but not completely inhibit hydrogen permeation, this is the essential reason why the change of [CO] affects HE susceptibility.

Stress corrosion cracking behavior on carbon steel under the synergistic effect of chloride and bicarbonate ions in alternating wet-dry environment

The stress corrosion cracking mechanism of X80 carbon steel under the combined actions of chloride and bicarbonate ions in alternating wet-dry environment was investigated by performing slow strain rate tensile test and in-situ electrochemical detection during the SCC process under constant load. The results showed that the oxidation and reduction reaction cycle of iron oxide during wet-dry cycle would accelerate the corrosion process. The pitting under the corrosion product layer at high concentrations of chloride and bicarbonate was the origin of SCC under the action of stress concentration, which made steel display high SCC susceptibility.

Temperature dependency of hydrogen embrittlement resistance of austenitic Fe–24Mn–3Cr-0.5Cu-0.47C steel

Currently, austenitic stainless steel has been used as a tank material for liquid hydrogen storage. However, in this study, the resistance to hydrogen embrittlement (HE) of austenitic Fe–24Mn–3Cr-0.5Cu-0.47C steel was investigated at a wide temperature range from 300 K to 20 K through slow strain rate tensile tests, aiming to explore the possibility of Fe-high Mn steel as an alternative material. To date, there are few papers on the HE resistance of austenitic high Mn steel, especially at cryogenic temperatures. The HE resistance of the high Mn steel was compared to that of austenitic 304 stainless steel (STS 304). The HE resistance of the high Mn steel was continuously enhanced with decreasing tensile temperature from 300 K to 77 K, despite the occurrence of mechanical twinning due to the significant reduction in H diffusion. However, the HE resistance slightly decreased again at 20 K, primarily due to cracking induced by dislocations with an edge component. Although both the high Mn steel and STS 304 had similar resistance to HE in the temperature range of 123 K–20 K, the high Mn steel demonstrated superior resistance to HE at the temperature range of 123 K–300 K. This indicates that the strain-induced α′-martensitic transformation occurring in STS 304 during tensile deformation in that temperature range is more detrimental to the HE resistance compared to the mechanical twinning occurring in the high Mn steel.

Stress corrosion cracking behavior of buried oil and gas pipeline steel under the coexistence of magnetic field and sulfate-reducing bacteria

Magnetic field and microorganisms are important factors influencing the stress corrosion cracking (SCC) of buried oil and gas pipelines. Once SCC occurs in buried pipelines, it will cause serious hazards to the soil environment. The SCC behavior of X80 pipeline steel under the magnetic field and sulfate-reducing bacteria (SRB) environment was investigated by immersion tests, electrochemical tests, and slow strain rate tensile (SSRT) tests. The results showed that the corrosion and SCC sensitivity of X80 steel decreased with increasing the magnetic field strength in the sterile environment. The SCC sensitivity was higher in the biotic environment inoculated with SRB, but it also decreased with increasing magnetic field strength, which was due to the magnetic field reduces microbial activity and promotes the formation of dense film layer. This work provided theoretical guidance on the prevention of SCC in pipeline steel under magnetic field and SRB coexistence.

Determining SCC resistance of stainless steel claddings in high- temperature water by constant load crack growth tests and slow strain rate tests

Stress corrosion cracking (SCC) behaviors of 309 L and 308 L weld overlays are characterized in oxygen-free and oxygen-bearing high-temperature water using slow strain rate tests (SSRTs) and crack growth rate (CGR) tests with compact tension specimens. There is no apparent SCC indication on 308 L, while local SCC is observed on 309 L after SSRT in oxygen-free water. The SCC sensitivities of 308 L and 309 L increase with the increase of dissolved oxygen (DO) content, while 308 L shows relatively higher SCC resistance than 309 L in water with 200 ppb and 8000 ppb DO. SSRT and CGR test results show a similar effect of DO concentration on SCC of 309 L and 308 L. The fracture surface of 309 L exhibits extensive intergranular SCC, while 308 L shows only local interdendritic SCC after CGR tests. The crack tip blunting decreased the CGR in the low alloy steel. The highly island-shaped δ-ferrites and moderate strain hardening in 308 L contribute to its SCC resistance over 309 L.

Enhanced hydrogen induced stress corrosion cracking resistance of Ni-advanced weathering steel by Ni and Mn modification

The stress corrosion cracking (SCC) behavior of three kinds of Ni-advanced weathering steel and a common weathering steel are examined in a simulated harsh marine environment by X-ray diffraction, scanning electron microscopy, electron backscattered diffraction, transmission electron microscopy, electrochemical tests, and slow strain rate tensile tests under different applied potentials. The results show that the SCC susceptibility of all test steel is low at open circuit potential. However, the SCC sensitivity of the conventional weathering steel increases significantly at − 1200 mV, while the Ni-advanced weathering steel exhibits good resistance to hydrogen embrittlement, which is attributed to the improved stacking fault energy, decrease of martensite-austenite constituents and refined grain size caused by increase in Ni and Mn content.

Effect of Microstructure Modifications on Stress Corrosion Endurance of 15-5 PH Stainless Steel Formed by Wire Laser Additive Manufacturing (WLAM)

Additive manufacturing (AM) technology using the direct energy deposition (DED) process and wires as feedstock material is commonly used to produce large components at an affordable cost. The wire laser AM (WLAM) process is one type of DED technology that uses welding wire as the raw material and a laser beam as the energy source. The goal of this study was to understand and evaluate the effect of microstructure modifications on the stress corrosion endurance of 15-5 PH stainless steels produced through WLAM, compared to their counterpart wrought alloy AISI 15-5 PH. All the tested alloys were heat treated using a standard age hardening treatment (H-1150M) prior to their examination. The microstructure analysis was performed using optical and electron microscopy (SEM and TEM) and X-ray diffraction analysis. The environmental behavior was characterized through electrochemical examination using potentiodynamic polarization and impedance spectroscopy analysis, while stress corrosion behavior was evaluated by means of slow strain rate testing (SSRT). The corrosion experiments were conducted in a simulated corrosive environment in the form of a 3.5% NaCl solution. The results showed that the microstructure modifications in the WLAM alloy (mainly in terms of austenite content, passivation capability and inherent printing defects) have a significant detrimental effect on stress corrosion resistance.

The Role of Nickel in Low Alloy Steels Exposed to H2S-Containing Environments. Part I: Trench Formation at the Open-Circuit Potential

The nickel content in low alloy steels (LAS) for oil and gas exploration and production is limited to a maximum of 1 wt% according to ANSI/NACE MR 0175/ ISO 15156. This restriction is imposed to avoid sulfide stress cracking (SSC) in sour (H2S-containing) environments. In this work, the effect of Ni on the SSC of LAS was studied independently of other alloying elements. For this purpose, quenched and tempered steels heat treated to a yield strength of 610 MPa with a Ni content below and above the 1 wt% threshold were evaluated at the open-circuit potential (OCP) in unstressed specimens, and in slow strain rate tests at room temperature. Thiosulfate was used as a surrogate of H2S, according to the Tsujikawa method. It is concluded that Ni contributes to the stabilization of the sulfide films that form on the steel’s surface at OCP. The rupture of this film due to tensile stress promotes the nucleation of deep planar features, referred to as trenches, which can act as sulfide stress crack initiators. Trenches were observed exclusively in stressed, Ni-containing specimens. Moreover, trenches’ morphology, dimensions, and distribution varied with the Ni content in the steels. For the steels studied in this work, the Ni effect on trenching persisted below the 1 wt% threshold.

Study on the wear resistance and corrosion behaviour of self-sealed MAO/ZrO2 coatings prepared on 7075 aluminium alloy

Self-sealed microarc oxidation (MAO) coatings were prepared on 7075 aluminium alloy by incorporating different concentrations of ZrO2 particles. The wear resistance and corrosion behaviour of MAO- and MAO/ZrO2-coated alloys were investigated. The results show that the tribological behaviour of the coatings was controlled by an abrasive mechanism. The coefficient of friction (COF) for the MAO/ZrO2-coated samples decreased to 0.82, indicating that the wear resistance was significantly improved due to the incorporation of hard ZrO2 particles and the enhanced compactness and surface roughness of the coatings. The sealed pores effectively prevented the permeation of the corrosion solution, thus increasing the electrochemical corrosion resistance. Pores and microcracks would be enlarged and preferentially fractured during slow strain rate testing (SSRT) and thus were the initial sites for corrosion pitting and SCC crack propagation. The interlocking mechanism between the pores (microcracks) and ZrO2 particles enhanced the strength of the pores (microcracks); therefore, the stress corrosion cracking (SCC) susceptibility of the MAO/ZrO2-coated samples was also reduced. Moreover, the MAO/ZrO2 coatings formed with the addition of 4 g/L ZrO2 particles had the best corrosion resistance, and the SCC susceptibility was reduced by 8.1% compared with that of the Al matrix.

In vitro corrosion-assisted cracking of AZ31B Mg alloy with a hybrid PEO+MWCNTs/PCL coating

The effects of multi-walled carbon nanotubes (MWCNTs) incorporation and polycaprolactone (PCL) post-treatment on the environmental-assisted cracking behaviour of a plasma electrolytic oxidation (PEO) coated AZ31B Mg alloy were elucidated in this study. Slow strain rate tensile (SSRT) experiments were carried out in simulated body fluid (SBF) for the bare material and different coating systems with and without MWCNTs and PCL overlay. Electrochemical impedance spectroscopy (EIS) and microscopic examinations (SEM and optical) were also conducted to reveal the role of corrosion on the mechanical response. In spite of the significant positive influence of the PEO coatings (with and without MWCNTs) on the bio-electrochemical behaviour of the AZ31B alloy, the environmental-assisted cracking performance was only marginally improved. Furthermore, PEO+MWCNTs/PCL coating system increased the fracture strain of the specimens by 7% compared to the uncoated specimens. Based on the SEM and optical micrographs, hydrogen embrittlement was suggested as the main cause of failure of the coated specimens under the in vitro slow strain rate test conditions.

Improving Corrosion and Stress Corrosion Cracking Performance of Machined Biodegradable Alloy ZX20 by HF-Treatment

The treatment with hydrofluoric acid (HF-treatment) was suggested to be an effective way of improving the corrosion resistance of Mg alloys, including Mg-Zn-Ca (ZX) ones used for biodegradable implants. However, the effect of the HF-treatment on the stress corrosion cracking (SCC) susceptibility of ZX alloys has not been reported yet, although this phenomenon can induce premature brittle failures of the metallic medical devices, and thus, it is critical for their in-service structural integrity. In the present study, the effect of the HF-treatment on the microstructure, cytotoxicity, corrosion rate, mechanical properties, and fracture and side surface characteristics of the as-cast ZX20 alloy were investigated with the use of scanning electron microscopy, immersion, and slow-strain rate tensile testing in Hanks’ solution and indirect cell viability tests. It is found that the HF-treatment exerts no cytotoxic effect and results in a significant reduction in corrosion rate (up to 6 times of magnitude) and SCC susceptibility indexes (up to 1.5 times of magnitude). The observed improvement of corrosion and SCC performance of the alloy by the HF-treatment is found to be attributed to three effects, including (i) formation of the protective surface film of MgF2, (ii) removal of surficial contaminations originating from sample preparation procedures, and (iii) dissolution of surficial secondary phase particles. The mechanism of corrosion and SCC in the specimens before and after the HF-treatment are discussed.

Elucidating the dynamics of hydrogen embrittlement in duplex stainless steel

We used slow-strain rate testing with in-situ microstructure imaging during electrochemical hydrogen charging to understand hydrogen embrittlement of super duplex stainless steel. Tensile deformation during hydrogen absorption softens the austenite and ferrite phases, lowering the macroscopic yield point and fracture strain. In contrast, when hydrogen absorption precedes micro-tensile testing, it strengthens the microstructure, highlighting a complex dual response. Computational analyses showed hydrogen atoms are trapped at phase boundaries, increasing the ferrite phase's hardness but reducing the austenite phase's hardness. However, once the boundaries are passivated, further entering hydrogen can diffuse rapidly without energy barriers, resulting in softening of both phases.

Hydrogen embrittlement in dissimilar welded joints interfaces – Influence of manufacturing parameters and evaluation of methodologies

The resistance of dissimilar joints to hydrogen was assessed using fracture toughness in an aggressive environment and slow strain rate tests. Six weld joint conditions were evaluated. To determine the distribution of microconstituents and elements along the interfaces, techniques such as optical and scanning electron microscopy, tomography, and nano hardness were employed. The results demonstrated that the susceptibility to hydrogen embrittlement is directly linked to the fraction of discontinuous regions present along the dissimilar interface. Additionally, a critical evaluation of the methodology concerning the positioning of stress concentration at the dissimilar interface was carried out, and alternative approaches were presented.

Effect of pre-strain on hydrogen induced cracking of PAW welded 304 austenitic stainless steel

Strain-hardened austenitic stainless steel (γ-SS) is extensively used in high-pressure hydrogen systems after welding. The weld may experience varying degrees of hydrogen embrittlement (HE) with different levels of pre-strain, which seriously threatens the reliability of hydrogen systems. Therefore, it is vital to research the effect of pre-strain on hydrogen-induced cracking of γ-SS welds. In this research, the impact of hydrogen on the mechanical properties of type 304 γ-SS plasma arc welding (PAW) welds with different levels of pre-strain are studied through slow strain rate tensile (SSRT) test. The pre-strain of the weld after hydrogen charging is optimized, then the causes of hydrogen-induced cracking are illustrated by microstructure analysis. The weld metal zone (WMZ) has a greater HE susceptibility than the base metal zone (BMZ). The HE susceptibility of the weld can decrease after solution-treated. The fracture of the weld with pre-strain of 5% is mainly due to dislocation and ferrite increasing the HE susceptibility. When the pre-strain is above 10%, the weld metal and the base metal have undergone a severe transformation from austenite to α′ martensite. The content of α′ martensite rises with the rising pre-strain. The HE susceptibility of the weld is greater than the base metal due to the influence of dislocations and ferrite. The brittle fracture is initiated primarily at the phase boundary. The strength of welds can be improved while keeping good HE resistance with pre-strain of 5%.