Fatigue behaviour of duplex stainless steel reinforcing bars subjected to shot peening

The influence of shot peening on the fatigue properties of duplex stainless steel reinforcing bars manufactured using both hot- and cold-rolled processes was studied. The S-N curves of the bars before and after the shot-peening process were determined, showing that shot peening improves the fatigue behaviour of the rebars. This improvement is essentially due to the introduction of a compressive residual stress field in the surface of the reinforcing bars, but also to the smoothing of the surface flaws and cold working generated during the manufacturing process. This improvement is much greater in the case of the hot-rolled bars, mainly as a result of their much higher ability for plastic deformation, whereas cold-rolled bars had a much higher hardness. A more severe peening action capable of promoting greater plastic deformation on the bar surface is judged necessary to improve the fatigue resistance of cold-rolled rebars.     

Environmentally assisted cracking of a paper machine duplex stainless steel suction roll material

Failure analysis of a paper machine duplex stainless steel suction roll material demonstrated that the mechanism of failure was intergranular stress-corrosion cracking followed by fatigue. Crack initiation was related to both the heat treatment of the alloy and the chloride content developed in the system during paper machine shutdowns. Crack propagation occurred during normal operating conditions by a corrosion fatigue mechanism.     

Deformation and microstructure evolution of a duplex stainless steel under out-of-phase thermo-mechanical fatigue

Abstract

Thermo-mechanical fatigue (TMF) of the duplex stainless steel SAF2205 (X2CrNiMoN22-5-3) was studied in the temperature range of 100–350°C. The tests were carried out on the duplex steel and on single-phase ferritic (X6Cr17, AISI 430) and austenitic steels (X2CrNiMo18-14-3, AISI 316L) similar to the two phases of the duplex steel for comparison. The mechanical behaviour of the three steels is analysed and discussed together with microstructural investigations by scanning electron microscopy, including electron backscatter diffraction and electron channelling contrast imaging.     

A fracture mechanics analysis on the fatigue behaviour of cruciform joints of duplex stainless steel

The paper presents the results of an experimental and numerical study on the fatigue behaviour of cruciform load carrying joints made from the duplex stainless steel and failing from the weld root through the weld metal. Fatigue crack growth (FCG) data, obtained in specimens of the weld metal, are presented, as well as threshold data, both obtained for R= 0 and 0.5. The influence of stress ratio is discussed, and the FCGR results are compared with data for low carbon structural steels. S–N data were obtained in the joints, both for R= 0.05 and 0.5, and the fatigue cracking mechanisms were analysed in detail with the SEM. It was found that the cracks propagated very early in the lifetime of the joints, under mixed mode conditions (I + II), but the mode I component was found to be predominant over mode II. The geometries of the cracks were defined in detail from measurements taken in the fracture surfaces. A 2D FE analysis was carried out for the mixed mode inclined cracks obtained at the weld root, and the J‐integral formulations were obtained as a function of crack length and crack propagation angle. The values of the crack propagation angle, θ_i, were obtained for the J_max conditions, and it was found that, in the fatigue tests, the cracks propagated in directions very close to the predicted directions of maximum J. K_I and K_II formulations were obtained, and the K_I data were compared with the formulations given in the PD6493 (BS7910) document, and some differences were found. A more general formulation for K under mixed mode conditions was derived. The derived K solutions were applied to predict the fatigue lives of the joints under crack propagation, and an extremely good agreement was found with the experimental results obtained in the fatigue tests.     

Production of high nitrogen surfaces on 2205 duplex stainless steel substrate using the PTA technique

Abstract

The plasma transferred arc technique has been used for the production of high nitrogen surfaces on 2205 duplex stainless steel substrates. Nitrogen was introduced into the melt using Ar+5%N₂ and Ar+10%N₂ gas mixtures. The nitrided surfaces are austenitic–ferritic and have a thickness of 1140±35 and 1650±31 μm respectively. The change of the austenite crystal lattice, due to the absorption of nitrogen, was determined by X-ray diffraction. Pin on disc tests showed that the wear resistance was increased. The corrosion in 3·5%NaCl and 1 N H₂SO₄ aqueous solutions was also slightly improved. Significant improvement was, however, observed in the pitting corrosion resistance of the nitrided surfaces, with regard to the 2205 duplex stainless steel substrate.     

Short crack growth and fatigue life in austenitic-ferritic duplex stainless steel

The kinetics of short crack growth has been studied in austenitic‐ferritic 2205 duplex stainless steel. Smooth cylindrical specimens and specimens with shallow notch were subjected to constant plastic strain amplitude loading. The crack growth was studied in notched specimens. The notch area has been mechanically and electrolytically polished to facilitate the observation of crack initiation and growth. The initiated cracks were observed in an SEM (scanning electron microscope). The crack growth was studied using long distance QUESTAR optical microscope equipped with high‐resolution camera. In constant plastic strain amplitude loading the microcracks were initiated and their growth kinetics has been studied. The characteristic features of the crack growth at different plastic strain amplitudes were recorded. Two approaches to analyse the crack growth rates were adopted. The comparison of the prediction of the fatigue life using the plastic‐strain‐dependent crack growth rate was compared with Manson–Coffin law and the relation between parameters of this law and parameters of the short crack growth law were established.     

Microcrack growth and fatigue behavior of a duplex stainless steel

The kinetics of microcrack growth during cycling has been studied in a S32205 duplex stainless steel in the as-received and aged (100 h at 475 °C) conditions. Cylindrical specimens with a shallow notch were subjected to a constant plastic strain range of 0.3% in both thermal conditions. The characteristic features of surface damage and crack growth showed striking differences in microcrack density, nucleation location and propagation rate between the two thermal conditions even though the fatigue lives are comparable. In the as-received material, microcrack density is low and they nucleate mainly at grain and phase boundaries or second-phase particles. In the aged condition, slip markings first appear in the ferritic phase and they are the preferred site for microcrack nucleation. Crack propagation takes place along slip markings in adjacent grains for crack lengths less than 100 μm. A comparison between fatigue life and the relevant parameters of a microcrack growth law was made.     

Multi-scale study on the heterogeneous deformation behavior in duplex stainless steel

The heterogeneous deformation behavior of austenite and ferrite in the 2205 duplex stainless steel was subjected to multiscale analysis based on the in situ synchrotron-based high energy X-ray diffraction,microscopic digital image correlation,electron backscatter diffraction,and transmission electron microscopy.It is found that the heterogeneous deformation triggers from the yielding of austenite.During this deformation stage,austenite experiences greater strain in the area near the phase boundaries because of the impeded function of the phase boundaries to dislocations.Owing to the relatively small difference in hardness between the constituent phases,the strain in austenite grains extends into the adjacent ferrite grains when entering into the ferrite yielding stage.In addition,the strain distribution of the austenite grains is more homogeneous than that of the ferrite grains because of the lower stacking fault energy of austenite,which results in a planar slip,and higher stacking fault energy in case of ferrite,causing cross slip.The interaction between austenite and ferrite becomes considerably obvious when the strain further increases after both constituent phases yielding because of the back stress and forward stress in austenite and ferrite,respectively,which are generated by the pile-up of the geometrically necessary dislocations.     

The transformation characteristics of ferrite in a cast of duplex stainless steel and its applications in temperature measurement

The transformation characteristics of ferrite transformation in a cast of specially designed duplex stainless steel have been studied. The ferrite content in the duplex stainless steel was found to settle to steady values after long-term exposure to high temperatures. This feature can be used for developing an average temperature indicator that is very easy to use and requires only very simple equipment.     

Fatigue crack propagation in cast duplex stainless steels: thermal ageing and microstructural effects

The ferrite phase of cast duplex stainless steels becomes embrittled after thermal ageing, leading to a significant decrease in fracture properties. In the present paper, the influence of ageing and solidification structure on the fatigue crack growth rates (FCGRs) and on the fatigue crack growth mechanisms in a cast duplex stainless steel is studied. FCGRs measured at room temperature increase slightly after ageing at 400 °C, due to ferrite cleavage and to the resulting irregular shape of the crack front. The crack propagates without any preferential path by successive ruptures of ferrite and austenite phases. The macroscopic crack propagation plane depends on the crystallographic orientation of the ferrite grain. Secondary cracks can appear due to the complex solidification structure. This in turn influences the FCGR. The fatigue crack closure level decreases with increasing ageing. This can be explained by a decrease in the kinematic cyclic hardening of these materials.     

Low cycle fatigue behaviour of duplex stainless steel: influence of isothermal aging treatment

The low‐cycle fatigue behaviour of a duplex stainless steel was comparatively studied in standard heat‐treated and isothermally aged (at 475 °C for 100 h) conditions by mechanical testing, scanning and transmission electron microscopy. It was established that fatigue life is longer in the aged condition as compared to the annealed condition at lower values of total strain amplitude Δ?/2 (Δ?/2 = 4.0 × 10^?3 and 6.0 × 10^?3) and becomes similar in annealed and aged conditions at Δ?/2 = 8.0 × 10^?3. Fatigue resistance of the material converges with increase in Δ?/2 values as a result of rapid cyclic softening of the ferritic phase in the aged condition.     

Enhanced resistance of 2205 Cu-bearing duplex stainless steel towards microbiologically influenced corrosion by marine aerobic Pseudomonas aeruginosa biofilms

An antibacterial 2205-Cu duplex stainless steel(DSS)was shown to inhibit the formation and growth of corrosive marine biofilms by direct contact with copper-rich phases and the release of Cu~(2+)ions from the2205-Cu DSS surface.In this work,the microbiologically influenced corrosion(MIC)resistance of 2205-Cu DSS in the presence of the corrosive marine bacterium Pseudomonas aeruginosa was investigated.The addition of copper improved the mechanical properties such as the yield strength,the tensile strength and the hardness of 2205 DSS.Electrochemical test results from linear polarization resistance(LPR),electrochemical impedance spectroscopy(EIS)and critical pitting temperature(CPT)measurements showed that 2205-Cu DSS possessed a larger polarization resistance(R_p),charge transfer resistance(R_(ct))and CPT values,indicating the excellent MIC resistance of 2205-Cu DSS against the corrosive P.aeruginosa biofilm.The live/dead staining results and the SEM images of biofilm confirmed the strong antibacterial ability of 2205-Cu DSS.The largest pit depth of 2205-Cu DSS was considerably smaller than that of 2205 DSS after 14 d in the presence of P.aeruginosa(2.2μm vs 12.5μm).2205-Cu DSS possessed a superior MIC resistance to regular 2205 DSS in the presence of aerobic P.aeruginosa.     

Micromechanical interactions in a superduplex stainless steel subjected to low cycle fatigue loading

Micromechanical interactions in an austenitic‐ferritic SAF 2507 steel under lowcycle fatigue loading was studied by experiment and simulation. Neutron diffraction measurements of residual lattice strains were made on specimens unloaded from different cyclic deformation stages, namely cyclic hardening, softening and saturation. With self‐consistent modelling, the micromechanical behaviour of the constituent phases was studied for the first loading cycle. The evolution of the residual lattice strain distributions with cyclic loading and the development of phase stresses have been analysed with respect to the initial residual stress field and the different mechanical properties of the constituent phases.     

Fatigue failures of austenitic stainless steel orthopedic fixation devices

Failures of four different 300-series austenitic stainless steel biomedical fixation implants were examined. The device fractures were observed optically, and their surfaces were examined by scanning electron microscopy. Fractography identified fatigue to be the failure mode for all four of the implants. In every instance, the fatigue cracks initiated from the attachment screw holes at the reduced cross sections of the implants. Two fixation implant designs were analyzed using finite-element modeling. This analysis confirmed the presence of severe stress concentrations adjacent to the attachment screw holes, the fatigue crack initiation sites. Conclusions were reached regarding the design of these types of implant fixation devices, particularly the location of the attachment screw holes. The use of austenitic stainless steel for these biomedical implant devices is also addressed. Recommendations to improve the fixation implant design are suggested, and the potential benefits of the substitution of titanium or a titanium alloy for the stainless steel are discussed.     

Photoelectrochemical investigations on individual ferritic and austenitic grains of a duplex stainless steel oxidized in water vapour

Abstract

This work presents a multiscale photoelectrochemical characterization of oxide layers, and aims mainly at introducing the mesoscopic scale (ca 30 µm). For the first time, photocurrent energy spectra could be recorded on individual ferritic and austenitic grains of a unique 2205 duplex stainless steel sample oxidized for 5 minutes at 650°C under 20% water vapour in nitrogen. These results allowed us to explain contrasting issues in the photoelectrochemical images obtained from the oxidized sample. Moreover, the bandgap energies obtained by fitting these individual mesoscopic photocurrent energy spectra with a novel approach developed in our laboratory, showed that all oxide scales were constituted of Fe₂O₃ and Cr₂O₃ and of an Fe_2-xCr_xO₃ solid solutions, but that the x-value of the latter was different depending on the metallurgic phase of the oxidized substrate. The latter results were shown to be in agreement with those of additional Raman analyses of the oxidized austenitic and ferritic grains.     

A note on fatigue damage assessment by x-ray diffraction techniques for a 304L stainless steel specimen

The progress of fatigue damage in type 304L solution treated stainless steel was studied by surface residual stress measurements and measurements of the formation of martensite, as determined from x-ray diffraction techniques. The specimen was subjected to tension-tension (R=0.1) cycling in air at a maximum stress _max=422 MPa, which was approximately 160% off the yield stress. The initial longitudinal residual stress distribution was determined for 13 locations along the gage length and found to vary around –80±17 MPa (compression). After several fatigue cycles, the residual stress changes to approximately +115±17 MPa (tension). At about 7% of the life (N/N _f =0.07), the variation of the residual stress along the gage length reaches a minimum. With additional cycling, the residual stress first drops, reaching a minimum at approximately 10% of the life (100±8 MPa), then increases to a maximum at between 20 and 40% of the life (125±18MPa), followed by a marked relaxation to a minimum of 90±18 MPa at approximately 65% of the life. The surface martensite content of the sample continuously increases from its initial value of around 0.6%, first slowly up to 10% of the total fatigue life, and more rapidly thereafter to final failure. On the basis of additional measurements of the residual stress and martensite content as a function of depth below the surface, surface layer hardening of austenite and martensite is suggested as the principal reason for the residual stress development.     

Effect of 475°C embrittlement on the low cycle fatigue behaviour of lean duplex stainless steels

Lean duplex stainless steels (LDSSs) with lower nickel and molybdenum are less susceptible to suffer spinodal decomposition than standard duplex stainless steels. It is the purpose of this work to study the effect of thermal embrittlement on the low cycle fatigue behaviour of 2 LDSSs with different Cr_eq and Ni_eq. The correlation between the fatigue behaviour and the dislocation structure is attempted. Transmission electron microscopy was used to observe the dislocation microstructure. Additionally, STEM‐EDS technique in conjunction with Vickers microhardness measurements was used to characterize the amplitude of the spinodal decomposition. The results show that the LDSS with lower Cr_eq and Ni_eq values exhibits improved fatigue properties in the as received and aged conditions. Furthermore, it is important to emphasize that with an adequate volume fraction of phases in LDSSs, the ageing treatment leads to an increase in strength without causing a great detriment in low cycle fatigue life.     

Fatigue crack propagation in tensile shear stainless steel spot welded specimens

Fatigue tests were carried out on 4 mm thick spot welded joints; the material was stainless steel AISI 301, quarter hard. Some specimens were instrumented with a strain gauge bonded in correspondence with one of the edges of the spot weld. Strain gauge output was demonstrated to be a reliable instrument to monitor the nucleation and propagation of fatigue cracks. A good correlation was found between strain gauge output and spent fatigue life. Some fatigue tests were suspended when the strain gauge output was equal to pre-fixed values, corresponding to fatigue life in the range from 15 to 85%. Subsequently, the specimens were dissected to observe fatigue cracks. The same correlation existed between crack depth and fatigue life. Small cracks were observed in specimens fatigue tested up to 15% of the mean fatigue life; fatigue cracks in the joints under examination would be nucleated between 5 and 10% of fatigue life.
Finite Element calculations were carried out, introducing in the models cracks similar to those observed in the fatigue tests. Calculated strain at the external surface compared well with the measured strain as a function of crack depth. Calculations demonstrated that small errors in strain gauge position can be tolerated without appreciable deterioration in crack dimension prediction.     

An approach to calculate the J‐integral by digital image correlation displacement field measurement

This paper presents a combined experimental‐numerical technique for the calculation of the J‐integral as an area integral in cracked specimens. The proposed technique is based on full‐field measurement using digital image correlation (DIC) and the finite element method. The J‐integral is probably the most generalised and widely used parameter to quantify the fracture behaviour of both elastic and elastoplastic materials. The proposed technique has the advantage that it does not require crack length measurements nor is it limited to elastic fracture mechanics, provided that only small scale yielding is present. Evaluated are three test geometries; compact tension, three‐point bend and the double torsion beam. Possible errors and their magnitude and the limitations of the method are considered.