A. Mateo

Hot deformation of duplex stainless steels

Abstract Duplex stainless steels (DSSs) have become established materials, successfully employed in many industrial applications. Their combination of mechanical properties and corrosion resistance is particularly appreciated in the petrochemical field. Hot deformation of these two-phase materials is still a critical point because the different mechanical response of austenite and ferrite often leads to the formation of edge cracks. In the present research, two DSSs with different nitrogen contents, i.e. EN 1.4462 and EN 1.4410, have been subjected to uniaxial hot compression tests in a wide range of temperatures and strain rates. The microstructural changes produced as a consequence of the distinct test conditions have been analyzed by means of optical and electron microscopy. The characteristics of high temperature plastic flow of both DSSs are interpreted in terms of the classical hyperbolic sine equation. The results are finally discussed considering the intrinsic two-phase nature of the materials studied.

Characterization of the intermetallic G-phase in an AISI 329 duplex stainless steel

Duplex austenite–ferrite stainless steels are susceptible to a variety of decomposition processes when aged within the intermediate range of temperatures (250–500 °C). One of these phenomena is the precipitation of the intermetallic G-phase. In the present investigation, the crystal structure and the chemical composition of the G-phase, precipitated in the ferritic phase of an AISI329 duplex stainless steel, is studied by electron microdiffraction and energy dispersive X-ray spectroscopy. It is determined that the space group of the G-phase is F 3 with a lattice parameter four times that of the ferritic matrix. The precipitation mechanism of the G-phase showed a synergetic relation with the ferrite decomposition in Cr-rich and Fe-rich domains. Based on the obtained results, the structural proximity of ferrite matrix and G-phase has been studied. Further analysis allows to suggest that the spinodal decomposition leads to an interdomain of a ferritic structure which is thermodynamically unstable and serves as a precursory site to the development of the G-phase by atomic position readjustments inferior to the atomic distances.

Aging effects on the cyclic deformation mechanisms of a duplex stainless steel

Abstract Aging effects on the cyclic deformation mechanisms of an AISI-329 duplex stainless steel have been studied on the basis of the cyclic hardening-softening response, cyclic stress-strain curve and substructure evolution within the individual phases. The cyclic behavior of an unaged and two aged materials shows, in terms of plastic strain amplitude (e pl ), three well-defined stages. In the first regime, at low e pl , no differences are observed among the response of the three materials as a consequence of the dominance of “austenitic-like” deformation mechanisms for all the materials. In the second regime, at intermediate e pl , the cyclic behavior of unaged material is associated with a mixed “austenitic/ferritic-like” character, mainly due to plastic activity of both phases. On the other hand, the cyclic response of aged material within this intermediate e pl range is rather correlated to “austenitic-like” cyclic deformation mechanisms because of the intrinsic brittleness of the ferritic matrix. A third regime, at relatively large e pl , suggests a synergetic phenomenon of dislocation activity, deformation twinning and demodulation of spinodal microstructure in ferrite that enables this phase to sustain plastic deformation. Thus, in this e pl interval, the observed mechanical and substructural behavior within ferrite may be considered as relatively similar to that observed in unaged material at much lower stress levels; and therefore is amenable to be associated with “ferritic-like” cyclic deformation mechanisms. Finally, based on the results presented, the prevalence of “austenitic-like” or “ferritic-like” cyclic deformation mechanisms, for a given plastic strain range, is discussed in terms of the different role played by the ferritic matrix in each material investigated, depending upon its embrittlement degree.

Cyclic stress-strain response and dislocation substructure evolution of a ferrite-austenite stainless steel

The hardening-softening response, the cyclic stress-strain behavior and the evolution of dislocation structures of an AISI-329 ferrite-austenite stainless steel have been studied. Fatigue testing has been conducted under fully reversed total strain control and constant total strain rate. Detailed transmission electron microscopy studies have been carried out in order to determine the individual substructure evolution, as a function of increasing imposed strain amplitude, in each constitutive phase. In general, the cyclic response of the studied material may be described in terms of three different regimes within the plastic strain amplitude (ϵpl) range investigated, i.e. from 2 × 10−5 to 6 × 10−3:at ϵpl below 10−4 the dominant cyclic deformation mechanisms are those correlated to planar glide of dislocations within the austenite which is the phase which carries a large part of the macroscopic strain in this first regime. On the other hand, at ϵpl higher than 6 × 10−4 the dominant substructure evolution is observed inside the ferritic matrix. In this case, strain localization is enhanced, within the ferritic grains, through the development of veins into the wall structure. Such evolution induces a pronounced decrease of the cyclic strain hardening rate in the cyclic stress-strain curve. At ϵpl in-between these values, the cyclic behavior is characterized by a relatively high strain hardening rate and may be classified as a mixed “ferritic/austenitic-like” behavior. In this intermediate regime substructural changes are observed in both phases and the dislocation activity in each of them seems to be strongly influenced by their particular cyclic strain hardening behaviors. Finally, the results are analyzed and compared with data from the literature in terms of volume fraction and chemical composition of the constitutive phases.

Cyclic deformation behaviour of superduplex stainless steels

Abstract The cyclic hardening–softening response, the cyclic stress–strain curve (CSSC) and the substructure evolution within the constitutive phases of a high nitrogen superduplex stainless steel SAF 2507 have been studied and the results compared with reference to low and medium nitrogen duplex stainless steels (DSSs), particularly AISI 329 and SAF 2205 grades, respectively. All three materials were manufactured in the form of hot rolled and solution annealed cylindrical bars with the same diameter. It is shown that the previously reported description of the CSSC of a DSS in terms of three regimes, each associated to distinct controlling cyclic deformation mechanisms, applies well in the case of the superduplex steel too. The main differences between the cyclic deformation behaviour of the DSSs studied are: (i) the CSSC of the SAF 2507 steel is placed at higher stress levels, mainly as a consequence of nitrogen alloying; (ii) the initial hardening period is shorter whereas the softening stage is more prolonged; and (iii) increasing nitrogen content promotes a more planar glide deformation character in austenite.

Anisotropy effects on the fatigue behaviour of rolled duplex stainless steels

Duplex stainless steels (DSSs) produced by rolling have a significant anisotropy in mechanical properties. The present research deals with this anisotropy, paying particular attention to fatigue behaviour in the high-cycle fatigue regime. In doing so, the mechanical response of a 5 mm thick rolled plate of EN 1.4462 type duplex steel was characterised. Marked anisotropy effects are observed, with the transverse orientation exhibiting a mechanical response higher than that of the longitudinal one. These experimental findings are rationalised considering the correlation between the crystallographic texture of each phase and their behaviour with respect to crack nucleation and early growth of short cracks.

High cycle fatigue behaviour of a standard duplex stainless steel plate and bar

The fatigue-life behaviour of an EN 1.4462 duplex stainless steel was characterised for two different products, namely a rolled plate and a hot rolled bar. The results indicate that strength characteristics, under both monotonic tensile and high cycle fatigue conditions, are lower for the bar than for the plate. Further, marked anisotropy effects are observed for the plate material with the transverse orientation exhibiting a higher mechanical response than the longitudinal one, independent of the testing conditions under comparison. The experimental findings may be rationalised, considering a direct correlation between crystallographic texture, quantified in terms of the Taylor factors for the individual phases, and crack nucleation resistance. In doing so, the latter parameter is assumed to be intimately associated with resistance to plastic deformation, i.e. flow stress, in agreement with the dominant cyclic deformation and crack nucleation mechanisms observed.

Effect of testing atmosphere (air/in vacuo) on low cycle fatigue characteristics of a duplex stainless steel

The effect of testing atmosphere on low cycle fatigue characteristics of a duplex stainless steel has been studied at room temperature. Fatigue tests have been conducted under fully reversed plastic strain control and constant plastic strain rate in two different environments: air and in vacuo. The material has been investigated under two distinct conditions: as annealed or unaged and as thermally aged, corresponding to different dominant cyclic deformation mechanisms at the plastic strain amplitudes chosen for the study. In vacuo testing resulted in longer fatigue lives, and consequently, higher cumulative plastic strain than in air experiences for both material conditions. Although prominent fatigue micromechanisms for a given plastic strain amplitude did not seem to be affected by testing atmosphere, for both unaged and aged conditions, strain localization and cracking phenomena were enhanced in air as compared to vacuum. The experimental results were finally discussed in terms of fatigue micromechanisms-environment interactions.

On the high cycle fatigue behavior of duplex stainless steels: Influence of thermal aging

Abstract In this investigation the high cycle fatigue (HCF) behavior of an austenite-ferrite duplex stainless steel (DSS) is studied as a function of aging at 475 °C. It is found that the HCF strength and the fatigue sensitivity of DSS increase with aging. The fatigue strength improvement results from the higher cyclic yield stress of the spinodally-hardened ferrite which induces an increasing difficulty for early propagation, into and through the ferritic matrix, of microcracks nucleated within austenite. The fatigue sensitivity impairment is suggested to be correlated to an easier microcrack nucleation stage in the aged DSS as compared to the annealed or unaged one.

Evaluation of fatigue damage for duplex stainless steels in aggressive environments by means of an electrochemical fatigue sensor (EFS)

Abstract The EFS (Electrochemical Fatigue Sensor) is a device that is based on a study of electrochemical–mechanical interactions, providing information about the status of fatigue damage in a material, and promises to become a useful non-destructive, niche testing tool. The EFS has been shown to function for several commercial alloys, as well as in different environments. The present work is to confirm the feasibility of applying the EFS for assessing fatigue damage in duplex stainless steels. A second goal is to investigate the possibility of using an aggressive environment as an electrolyte within the EFS device. In doing so, corrosion-fatigue testing was conducted in EFS solutions at three different pH values: 8.4, 6 and 2. The results show that the fatigue life is longer at a pH value of 6. Such an unexpected environmental effect is discussed and related to the dissolution of the surface roughness generated by cyclic deformation, together with a higher repassivation rate.