Forged duplex steels possess a highly anisotropic two-phase microstructure. Due to the mismatch of the coefficients of thermal expansion thermal stresses evolve during cyclic thermal loading which ...can lead to plastification of one or both phases. While the macroscopic deformation of specimens is well understood, open questions related to the effect of traction free surfaces still remain. Experimental evidence is found for a characteristic evolution of the surface roughness depending on the microstructure of the material. In this work micromechanical models based on a continuum mechanical description of the phase domains are evaluated with respect to surface roughness. Parameters for the description of the amplitude and the anisotropy of surface deformations are proposed and the model predictions are compared with experimental results.
This paper presents the results of a study which has been carried out to investigate the crevice corrosion behavior of high-alloy stainless steel in a SWRO pilot plant. The study evaluated the ...corrosion performance of some austenitic and duplex steels in a crevice-forming environment created in a RO plant, especially in high-pressure feed and brine lines. The study of the effect of chemical dosing on crevice corrosion in the RO plant and electrochemical testing of crevice corrosion in the laboratory were the main objectives of this test program. High-alloy stainless steels, namely AL 6XN® and 254 SMO (superaustenitic), 2205, 2507 and DP3W (Duplex) were used in the test program. The tests were carried out in natural seawater and RO concentrate (conductivity: 75,000 to 80,000 μS/cm) at ambient temperature by operating the test plant at normal SWRO operating feed pressure of 54 bar. FeCl
3 was added as coagulant to maintain a silt density index of ∼3, and H
2SO
4 was added to feed in order to maintain the feed pH of ∼ 6.5. Chlorination and subsequent dechlorination agents were not added to the feed. For crevice corrosion tests in RO unit, the exposure periods were 6 and 12 months, respectively. The results of the tests showed that alloy DP3W has the best pitting resistance in crevice forming environment of seawater. In RO concentrate, alloys 2507 and 254 SMO showed lowest “maximum pit depth”. The results of potentiodynamic cyclic polarization (PCP) indicate that all the alloys have high pitting potential and small hysteresis loop. The results of critical crevice solution pH (CCSpH) indicated excellent resistance of alloys 254 SMO and DP3W against crevice corrosion attack and 2205 had least resistance in most aggressive sodium chloride solution.
For the prediction of the yield strength of nitrogen alloyed ferritic-austenitic duplex steels, accurate knowledge on the single phases' yield strength and their geometrical arrangement within the ...duplex microstructure is required. Since the matrix-inclusion character of the phases markedly influences the duplex yield strength
σ
y
d
, linear models for
σ
y
d
(Voigt-model) can not serve for an accurate prediction of
σ
y
d
. A non-linear rule of mixture, however, is a more sophisticated approach to calculate
σ
y
d
. Micromechanical models combined with finite element computations are efficient tools to accurately predict the influence of the topology of the microstructure on
σ
y
d
only if the yield strengths of the single phases are distinctly different, i.e. the yield strength ratio of ferrite and austenite, ψ, is larger than 2. Experiments on duplex steels show, however, a marked influence of the phase arrangement on
σ
y
d
even for ψ ~ 1. To explain this behavior a modified non-linear rule of mixture is proposed, which incorporates the in situ yield strengths of the phases as upper and lower bounds for
σ
y
d
. A Hall-Petch analysis, which considers the interactions between adjacent grains gives excellent agreement between experiments and computational predictions.
Ultra-high-strength (over 1 GPa) hot-rolled steel sheets have been actively developed to protect passengers in cases of vehicle crashes, and their applications have been expanded to cold-rolled steel ...sheets. A major alloying element for forming meta-stable austenite is Mn in (austenite + martensite) duplex microstructures, which is readily obtained at medium-Mn level ((3–10) wt.%). However, these medium-Mn hot-rolled duplex microstructures inevitably include Mn-segregated bands, which often lead to anisotropic mechanical properties and deteriorate the strength or uniform elongation. However, in this study, we show favorable effects of the Mn-segregated band, by carefully controlling the composition, size, and shape of austenite in Mn-rich and Mn-lean bands in medium-Mn duplex steels (composition; Fe-0.1C-10Mn-1Si-0.3Mo-0.5 V (wt.%)). The austenite grown coarsely in the Mn-rich band provoked transformation-induced plasticity (TRIP) more efficiently than the austenite finely transformed from the martensite in the Mn-lean band. The Mn composition acted more dominantly on the austenite stability than the austenite size, resulting in continuous TRIP in the austenite of the Mn-rich band. This austenite enables continuous strain hardening, thereby leading to high yield and tensile strengths of (1.0–1.6) GPa together with large ductility of 20%, which offers promise for new applications to ultra-high-strength automotive hot-rolled steel sheets.
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The mechanical and microstructural evolution of Alloy 2205 during severe plastic deformation is examined in this study. A combination of accumulative roll bonding (ARB) and cold rolling results in ...the successful formation of a nanograined dual-phase microstructure of austenite and ferrite with some transformed martensite. Severe deformation to cumulative reductions of 80.5, 92.5, 95, and 97 pct were performed. Microscopy indicates that grain dimensions in the sheet normal direction is less than 100 nm for reductions ≥ 92.5 pct. Shear banding is observed at reductions ≥ 95 pct while twinning is only observed at reductions < 92.5 pct. Neutron diffraction measurements indicated the presence of martensite for reductions ≥ 95 pct at ~ 8 pct volume fraction. Taken in conjunction, it appears that during initial ARB processing, both slip and twinning are active plastic mechanisms. As twinning becomes exhausted, martensitic transformation, slip, and intermittent shear banding account for the active plasticity mechanisms. Material hardness saturates at 92.5 pct reduction, with a maximum hardness of 45 HRC. Sub-sized tensile testing confirms this approximate hardness with measurements indicating a UTS of ~ 1440 MPa. Texture analysis of crystal orientation distributions in the plate normal direction suggest an approximate Kurdjumov–Sachs orientation relationship at all reductions above 80 pct indicating stability of the orientation relationship at high strains. The intragranular structure develops a fine scale sub-grain content with increasing deformation, resulting in a continual evolution of texture up to and including 97 pct reduction. The final structure presents strong components of Goss and rotated cube texture in both the austenite and ferrite. In this body of work we aim to compare ARB of an industrially relevant FCC/BCC system (Alloy 2205) to historical model FCC/BCC systems such as Cu/Nb.
This work explains the occurrence of transformation-induced plasticity via stress-assisted martensite, when designing ultrafine-grained duplex steels. It is found that, when the austenite is reduced ...to a fine scale of about 300nm, the initial deformation-induced microstructure can be dominated by parallel lamellae of ε martensite or mechanical twinning, which cannot efficiently provide nucleation sites for strain-induced martensite. Hence, α′ martensite nucleation occurs independently by a stress-assisted process that enhances transformation-induced plasticity in ultrafine-grained austenite. This metallurgical principle was validated experimentally by using a combination of transmission Kikuchi diffraction mapping, transmission electron microscopy and atom probe microscopy, and demonstrated theoretically by the thermodynamics model of stress-assisted martensite.
Despite their good corrosion resistance and optimal mechanical properties, duplex stainless steels are affected by hydrogen embrittlement. Therefore, understanding the hydrogen-defect interactions in ...these steels is crucial. This study uses internal friction to evaluate the interactions of hydrogen with microstructural defects. Analysis of the internal friction spectra of the steels subjected to straining and hydrogen charging, together with thorough microstructural characterization, gives new insights in hydrogen interactions with defects present in the different phases, i.e. ferrite (body centered cubic) and austenite (face centered cubic).
While no significant effect of tensile deformation can be observed by thermal desorption spectroscopy, the internal friction spectra show a clear influence of the presence of defects. Detailed analysis of these spectra reveals the interactions in the austenite as dominant, while no indications for hydrogen-dislocation interactions in ferrite are observed. This can be related to limited trapping in ferrite due to the austenite sink action or to limited dislocation formation in ferrite. Indications for hydrogen interactions with dislocations in the austenite are found, possibly suggesting enhanced dislocation mobility when surrounded by hydrogen. Moreover, a pronounced influence of hydrogen charging on the vacancy cluster related peaks is observed, indicating strong interactions between hydrogen and vacancy clusters in austenite. This can be put in contrast to behavior of pure ferritic steels, where dislocations provided the strongest hydrogen interactions. As these specific defects are of primary interest in the hydrogen embrittlement mechanisms, internal friction is concluded to provide important unique insights in hydrogen-defect interactions, even for complicated multiphase microstructures.
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The experimental quantification of retention factors related to the post-fire strength as well as the post-fire ductility of intentionally selected stainless steel grades applied in construction is ...the objective of the research presented here. These steel grades are characterized by a two-phase austenitic-ferritic microstructure of the duplex type. In this context, two mutually corresponding chromium-nickel-molybdenum steel grades are subjected to analysis, namely
steel belonging to the standard duplex group (DSS 22% Cr) and
steel belonging to the lean duplex group (LDSS). The similarities and differences in the mechanical properties exhibited by these steel grades after effective cooling, following more or less prolonged simulated fire action conforming to several development scenarios, are identified and indicated. The resistance of a given steel grade to permanent structural changes induced by the heating program proved to be the critical factor determining these properties and resulting in many cases in increased susceptibility to brittle fracture. The results obtained experimentally seem to confirm the quantitative estimates of post-fire retention factors forecast by Molkens and his team, specified for the steels exhibiting a duplex-type structure and tested by us. However, several of these estimates might be considered somewhat risky. Nevertheless, our results do not confirm the significant post-fire strengthening of steel grades belonging to the LDSS group following prior heating at a sufficiently high temperature, as reported earlier by Huang Yuner and B. Young.
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