Fatigue characteristics of a type 304 austenitic stainless steel in hydrogen gas environment

The effects of a hydrogen gas environment on the fatigue characteristics of a type 304 austenitic stainless steel were investigated and the following results were obtained. The hydrogen effect is not clearly seen by judging fatigue life diagram. However, crack initiation retards and crack propagation accelerates in hydrogen gas environment. The retardation seems to be caused by the absence of oxygen and water vapour. The acceleration seems to be caused by the intrinsic hydrogen effect.     

Effect of pre-charged hydrogen on fatigue crack growth of low alloy steel at 288 °C

Hydrogen effects on mechanical strength and crack growth were studied at high temperatures. The study was motivated by the fact that the environmentally assisted cracking (EAC) of pressure vessel steel SA508 Cl.3 in 288 °C water was suspected to be related to hydrogen embrittlement. Fatigue crack growth rate and tensile tests were performed with hydrogen pre-charged specimens at high temperatures. At 288 °C the fatigue crack growth rate of the hydrogen pre-charged specimen was faster than that of as-received; the fatigue fracture surface of hydrogen pre-charged specimen correspondingly showed EAC like feature. Meanwhile, ductile striation was evident for the case of as-received in both air and argon gas environments. In the dynamic strain aging (DSA) loading condition at 288 °C during tensile tests, the pre-charged hydrogen induced a marked softening (decrease in ultimate tensile strength; UTS) as well as a little ductility loss; this was accompanied by the macrocracks grown from microvoids/microcracks promoted by DSA and hydrogen. These experiments showed that hydrogen embrittlement is an effective mechanism of EAC not only at low temperature but also at the high temperature.     

Evaluation of hydrogen effect on the fatigue crack growth behavior of medium-Mn steels via in-situ hydrogen plasma charging in an environmental scanning electron microscope

Fatigue crack growth(FCG)tests were conducted on a medium-Mn steel annealed at two intercritical annealing temperatures,resulting in different austenite(γ)to ferrite(α)phase fractions and different γ(meta-)stabilities.Novel in-situ hydrogen plasma charging was combined with in-situ cyclic loading in an environmental scanning electron microscope(ESEM).The in-situ hydrogen plasma charging increased the fatigue crack growth rate(FCGR)by up to two times in comparison with the reference tests in vacuum.Fractographic investigations showed a brittle-like crack growth or boundary cracking manner in the hydrogen environment while a ductile transgranular manner in vacuum.For both materials,the plastic deformation zone showed a reduced size along the hydrogen-influenced fracture path in comparison with that in vacuum.The difference in the hydrogen-assisted FCG of the medium-Mn steel with different microstructures was explained in terms of phase fraction,phase stability,yielding strength and hydrogen distribution.This refined study can help to understand the FCG mechanism without or with hydrogen under in-situ hydrogen charging conditions and can provide some insights from the applications point of view.     

Application of the dissipated energy criterion to predict fatigue crack growth of Type 304 stainless steel following a tensile overload

This paper presents the results of numerical simulations of fatigue crack growth performed using three-dimensional elastic–plastic finite element analysis. A simple node release scheme is used to simulate crack advancement. The crack front is assumed to be straight. Crack growth following a tensile overload is simulated. The total energy dissipated per cycle is calculated directly from the finite element analysis and used to predict fatigue crack growth. For comparison, fatigue crack growth rate experiments were performed on Type 304 stainless steel C(T) specimens to determine the effect of a single tensile overload. The dissipated energy per cycle is found to correlate well with the measured fatigue crack growth rate following an overload.     

ASSISTED CRACK GROWTH IN MARAGING STEEL WELDS BY GASEOUS HYDROGEN

Abstract— The effects of environmental hydrogen content on fatigue crack growth rates (FCGRs) in T-250 maraging steel plates and laser welds were investigated. The influence of ageing treatments on fatigue characteristics of the alloy was also studied. Experimental results revealed that the accelerated FCGRs in the presence of hydrogen were always associated with changes in fracture modes that appear in compact-tension specimens. Even for overaged specimens with excellent resistance to gaseous hydrogen embrittlement, such an acceleration of crack growth in hydrogen could not be avoided. The crack path of underaged specimens in hydrogen was found mainly along prior austenite boundaries for steel plates and along coarse columnar boundaries for welds. In gaseous hydrogen, peak-aged welds exhibited intergranular and quasi-cleavage mixed fracture modes, compared to mainly quasi-clevage for similar aged steel plates. Hence, the enhancement of crack growth in hydrogen was more pronounced for the welds. Overaged welds showed higher FCGRs than the same aged steel plates only in hydrogen and for Δ K values greater than 20MPa√m.     

电解充氢对堆焊熔合区疲劳裂纹扩展行为的影响

研究了阴极充氢前后堆焊熔合区疲劳裂纹扩展行为,发现堆焊熔合区对疲劳裂纹的扩展有阻碍作用,充氢对裂纹扩展速率无明显影响,但使熔合区出现大量二次裂纹,随着充氢时间的延长,二次裂纹将更加严重。     

Effect of internal hydrogen on very high cycle fatigue of precipitation-strengthened steel SUH660

The fatigue life of SUH660 steel is dominated by crack initiation in the region of very high cycle fatigue owing to the new crack initiation behavior near the tip of temporarily arrested crack. The effect of internal hydrogen on very high cycle fatigue life is investigated focused on crack initiation life via fatigue and Vickers hardness tests. Hydrogen inhibits cracks initiation, and accelerates the increase in crack initiation lives with decreasing stress in low and medium hardness zones. Hydrogen increases the hardness in low and medium hardness zones. Hydrogen extends new crack initiation lives and causes longer very high cycle fatigue life.     

Effects of microstructure and residual stress on fatigue crack growth of stainless steel narrow gap welds

The effects of weld microstructure and residual stress distribution on the fatigue crack growth rate of stainless steel narrow gap welds were investigated. Stainless steel pipes were joined by the automated narrow gap welding process typical to nuclear piping systems. The weld fusion zone showed cellular–dendritic structures with ferrite islands in an austenitic matrix. Residual stress analysis showed large tensile stress in the inner-weld region and compressive stress in the middle of the weld. Tensile properties and the fatigue crack growth rate were measured along and across the weld thickness direction. Tensile tests showed higher strength in the weld fusion zone and the heat affected zone compared to the base metal. Within the weld fusion zone, strength was greater in the inner weld than outer weld region. Fatigue crack growth rates were several times greater in the inner weld than the outer weld region. The spatial variation of the mechanical properties is discussed in view of weld microstructure, especially dendrite orientation, and in view of the residual stress variation within the weld fusion zone. It is thought that the higher crack growth rate in the inner-weld region could be related to the large tensile residual stress despite the tortuous fatigue crack growth path.     

Fatigue crack propagation behavior in AISI 304 steel welded joints for cold‐stretched liquefied natural gas (LNG) storage tank at cryogenic temperatures

AISI304 steel welded joints are used in cold‐stretched liquefied natural gas (LNG) storage tanks used for storing and transporting of liquefied gases. Compared with a conventional liquefied natural gas storage tank, a cold‐stretched liquefied natural gas storage tank has many advantages such as reduced thickness, light weight, low cost and low energy consumption. However, liquefied natural gas storage tanks can be subjected to alternative loads at cryogenic temperatures; thus, it is important to investigate the fatigue crack propagation behavior in AISI 304 steel welded joints at cryogenic temperatures. Specimens were machined from a cold‐stretched liquefied natural gas storage tank with a welding structure. The crack length was determined using compliance method and confirmed by examination with traveling microscope. Fatigue crack propagation rates were evaluated at various stress ratios and temperatures. The fatigue crack growth rate of all specimens a little appears the effect of stress ratio, but it has a great influence at a cryogenic temperature. The fatigue crack growth rate of longitudinal welded joint is the fastest at room and cryogenic temperature. Fracture mechanism in the specimen is examined using a scanning electron microscope.     

The effect of overload on the fatigue crack growth behaviour of 304 stainless steel in hydrogen

Fatigue crack growth (FCG) behaviour and its characteristics following tensile overloads were investigated for AISI 304 stainless steel in three different atmospheres; namely dry argon, moist air and hydrogen. The FCG tests were performed by MTS 810 servohydraulic machine. CT specimens were used for the tests and crack closure measurements were made using an extensometer. FCG rates of 304 stainless steel at both dry argon and moist air atmospheres have shown almost the same behaviour. In other words, the effect of moisture on FCG of this material is very small. However, in a hydrogen atmosphere, the material showed considerably higher crack growth rate in all regimes. In general, for all environments, the initial effect of overloads was to accelerate the FCG rate for a short distance (less than a mm) after which retardation occurred for a considerable amount of time. The main causes for retardation were found as crack blunting and a long reinitiation period for the fatigue crack. Regarding the environmental effect, the overload retardation was lowest in a hydrogen atmosphere. This low degree of retardation was explained by a hydrogen embrittlement mechanism. In a general sense, hydrogen may cause a different crack closure mechanism and hydrogen induced crack closure has come in to the picture. Scanning electron microscope and light microscope examinations agreed well with the above results.     

Observation of hydrogen effects on fatigue crack growth behaviour in an 18Cr-8Ni austenitic stainless steel

The hydrogen effect on crack growth behaviour in a type 304 austenitic stainless steel was investigated and the following results were obtained. The crack growth rate in hydrogen gas is accelerated compared with that in air. In order to clarify the mechanism of the acceleration, the growth behaviours of a crack propagating in a grain and propagating along the boundary to be a fracture facet were investigated. Slip behaviour, opening displacement and fractography showed that the slip-off mechanism in fatigue crack growth is valid even in hydrogen gas. Hydrogen mainly affects slip behaviour such that slip bands concentrate at a crack tip and result in acceleration of the growth rate. The facets are not significantly responsible for the acceleration. The ratio of facets to the entire area is low, and a crack nearly compensates for the temporary acceleration by the facets with subsequent deceleration.     

Comparison of the simulation and experimental fatigue crack behaviors in the nanoseconds laser shocked aluminum alloy

This investigation was performed to compare the simulation and experimental results of the fatigue crack growth rates and behaviors of the 7050-T7451 aluminum alloy by nanoseconds laser shock processing (LSP). Forman–Newman–deKoning (FNK) model embedded in the Franc2D/L software was utilized to predict fatigue crack growth rate, which was conducted to weigh the stress intensity factor (SIF) changing on the surface cracks. LSP induced high compressive residual stresses that served to enhance fatigue properties by improving the resistance against fatigue crack initiation and propagation. The circulating times of crack growth obtained from the simulation and experimental values indicated a slower fatigue crack growth rates after LSP. The relationships between the elastic–plastic materials crack growth rates and the SIF changing after LSP are resolved.     

轴承钢疲劳裂纹扩展速率的研究

研究了轴承钢碳化物及晶粒细化对轴承钢疲劳寿命的影响。结果表明;细化轴承钢中的碳化物可以使其疲劳裂纹扩展速率下降,而同时细化轴承钢中的碳化物和晶粒,会使其疲劳裂纹扩展速率下降更明显。     

Hydrogen-enhanced cracking of 2205 duplex stainless steel

Tensile and fatigue crack growth tests of 2205 duplex stainless steel (DSS) were performed in laboratory air, gaseous hydrogen at 0.2 MPa and saturated H₂S solution. The longitudinal specimen showed a lesser degradation of tensile properties than the transverse ones in saturated H₂S solution. The orientation of specimens with respect to rolling direction had little influence on the fatigue crack growth rate (FCGR) of the alloy in air. Furthermore, 2205 duplex stainless steel was susceptible to hydrogen‐enhanced fatigue crack growth. Transmission electron micrographs, in addition to X‐ray diffraction, revealed that the strain‐induced austenite to martensite transformation occurred near the crack surface within a rather narrow depth. Fatigue fractography of the specimens tested in air showed mainly transgranular fatigue fracture with a small amount of flat facet fracture. Furthermore, extensive quasi‐cleavage fracture of 2205 duplex stainless steel was associated with the hydrogen‐enhanced crack growth.     

Environmental effect on fatigue strength of stainless steel in PWR primary water – Role of crack growth acceleration in fatigue life reduction

The influence of the pressurized water reactor (PWR) water environment on fatigue life and fatigue crack growth rate was discussed. The fatigue lives of Type 316 stainless steel in the PWR water environment were investigated using cylindrical hollow specimens. The acceleration in the crack growth due to the environment was quantified by investigating spacing of striations and crack growth tests using compact tension specimens. The growth rates obtained could be represented by the strain intensity factor. It was shown that the fatigue lives estimated by crack growth prediction agreed with those obtained by the tests. Then, it was concluded that the reduction in the fatigue life due to the PWR water environment was brought about not by enhancement of crack initiation but by the acceleration of the crack growth.     

Fatigue crack growth of a metastable austenitic stainless steel

The fatigue crack growth behavior of an austenitic metastable stainless steel AISI 301LN in the Paris region is investigated in this work. The fatigue crack growth rate curves are evaluated in terms of different parameters such as the range of stress intensity factor ΔK, the effective stress intensity factor ΔK_eff, and the two driving force parameter proposed by Kujawski K¹.The finite element method is used to calculate the stress intensity factor of the specimens used in this investigation. The new stress intensity factor solution has been proved to be an alternative to explain contradictory results found in the literature.Fatigue crack propagation tests have been carried out on thin sheets with two different microstructural conditions and different load ratios. The influence of microstructural and mechanical variables has been analyzed using different mechanisms proposed in the literature. The influence of the compressive residual stress induced by the martensitic transformation is determined by using a model based on the proposal of McMeeking et al. The analyses demonstrate the necessity of including K_max as a true driving force for the fatigue crack growth. A combined parameter is proposed to explain the effects of different variables on the fatigue crack growth rate curves. It is found that along with residual stresses, the microcracks and microvoids are other factor affecting the fatigue crack growth rate in the steel studied.     

Corrosion fatigue short crack growth behaviour in a high strength steel

The influence of an aggressive environment (0.6 M, aerated NaCl solution) on short fatigue crack initiation and growth behaviour has been studied. The study involved three major test series, namely: air fatigue, corrosion fatigue, and intermittent air fatigue/corrosion fatigue. The above tests carried out under fully reversed torsional loading conditions at a frequency of 5 Hz, showed that it was the non-metallic inclusions which took part in crack initiation resulting from debonding at metal matrix/inclusion interface and pitting of inclusions in both air and corrosove environments, respectively. Short fatigue crack growth results in these two environments obtained by using plastic replication technique, indicated a large effect of microstructure i.e. prior austenite grain boundaries. The stage/stages at which the environmental contribution was dominant has been discussed by considering the results achieved from intermittent tests. However, the mechanisms involved in corrosion fatigue short crack growth have also been described in the light of results obtained from futher investigations carried out by conducting corrosion fatigue tests under applied cathodic potential conditions and tests on hydrogen pre-charged specimens under air fatigue and uniaxial tension conditions.     

Effect of hydrogen on fatigue crack growth of metals

The present paper shows several important phenomena obtained by investigations of the effect of hydrogen on fatigue crack growth behaviour, including the measurement of the hydrogen content in various materials such as low-carbon, Cr-Mo and stainless steels. Particularly important phenomena are the localization of fatigue slip bands, strain-induced martensite in Types 304, 316 and even 316L, and also strong frequency effects on fatigue crack growth rates. For example, with a decrease in frequency of fatigue loading down to the level of 0.2 Hz, the fatigue crack growth rate of a Cr-Mo steel is accelerated by 10-30 times. The same phenomenon also occurs even in austenitic stainless steels at the frequency of the level of 0.001 Hz. Striation morphology is also influenced by hydrogen. It has been revealed by re-analysing the results of the authors’ separately published reports that this basic hydrogen embrittlement mechanism is essentially the same throughout all the materials, i.e. low-carbon, Cr-Mo and stainless steels. Thus, the coupled effects of hydrogen content, hydrogen diffusion coefficient (for BCC or FCC), load frequency, localization of fatigue slip bands and strain-induced martensite must be always considered in fatigue test and analysis of hydrogen embrittlement.     

Fatigue crack growth through particulate clusters in polycarbonate material

The interaction of a crack with a perfectly bonded inclusion or a cluster of inclusions in polycarbonate matrix was investigated through both numerical simulations and fatigue tests. Stress intensity factors (K_I) were evaluated by boundary element method for several particle sizes, position and finally for inclusion cluster as a precursor study for the experiments. The numerical simulation has shown the crack tendency to circumvent the inclusions with consequential reduction of the growth rate. Fatigue crack growth tests were carried out on several particle-filled specimens at constant value of the applied stress intensity factor range (ΔK_Iapp) highlighting the crack delay due to the presence of the stiff second phase. The experiments demonstrated that the inclusion effect on the crack growth rate can be explained with a model based on the crack shielding effect in which the particle would act to reduce the effective stress intensity factor at crack tip (K_Ieff). Finally, the crack growth rate was predicted with an analytical model, and then compared to that obtained by the fatigue testing. Possible explanations for differences are discussed.     

Fatigue life prediction of a structural steel under service loading

Fatigue life prediction techniques for variable amplitude load histories are reviewed. The fatigue crack growth rate and crack closure responses of BS4360 50B steel are determined for a service load history experienced by a gas storage vessel. Crack propagation rates are found to be independent of specimen thickness. Crack growth is successfully predicted by linear summation using the Paris law; no significant improvement is achieved by incorporating crack closure into the analysis. The particular choice of cycle counting technique is also found to have an insignificant effect on the predicted fatigue life. The load-interaction model proposed by Willenborg et al correctly indicates the absence of retarded growth, whilst the Wheeler and Führing models erroneously predict retarded crack growth.