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.     

Superplastic boronizing of duplex stainless steel under dual compression method

In this work, SPB of duplex stainless steel (DSS) under compression method is studied with the objective to produce ultra hard and thick boronized layer using minimal amount of boron powder and at a much faster boronizing time as compared to the conventional process. SPB is conducted under dual compression methods. In the first method DSS is boronized using a minimal amount of boron powder under a fix pre-strained compression condition throughout the process. The compression strain is controlled in such a way that plastic deformation is restricted at the surface asperities of the substrate in contact with the boron powder. In the second method, the boronized specimen taken from the first mode is compressed superplastically up to a certain compressive strain under a certain strain rate condition. The process in the second method is conducted without the present of boron powder. As compared with the conventional boronizing process, through this SPB under dual compression methods, a much harder and thicker boronized layer thickness is able to be produced using a minimal amount of boron powder.     

Interface structure of diffusion bonded duplex stainless steel and medium carbon steel couple

Diffusion bonding of duplex stainless steel to medium carbon steel was carried out with different temperatures for sound bonds. In the bonding process, relatively intermediate temperatures such as 750, 800, 850 and 900 °C were used with a bonding time of 30 min. In this study, microstructural changes and mechanical properties in the interface region of duplex stainless steel and medium carbon steel couples were determined. The results showed that, in interface region, Cr₂₃C₆ was formed on the stainless steel side, while ferrite formation was observed on the carbon steel side as a result of mutual diffusion of C and Cr.     

Interaction between austein-ferrite phases on passive performance of 2205 duplex stainless steel

The passive behavior of 2205 duplex stainless steel (DSS) and its individual phases (α-phase, γ-phase) in neutral 3.5% NaCl solution was investigated by various electrochemical methods. The results indicated that galvanic effect between α and γ phases cannot deteriorate local corrosion, but favors the enhancement of the passive film. Under the galvanic effect, the diffusion of the dissolved passive cations would be promoted in a short distance between α and γ zones, leading to modifications of the chemical composition and semiconductive property of the passive film and therefore the enhancement of the corrosion resistance of DSS 2205.     

The effect of niobium on the microstructure of ferritic stainless steel

The effect of different amounts of Nb and of homogenization on the ferritic stainless steels containing 17–18 wt.% Cr was investigated with scanning electron microscopy (SEM), optical microscopy, energy-dispersive spectrometry (EDS), X-ray diffraction (XRD) and differential thermal analysis (DTA). It was observed that M₂₃C₆, NbC and sigma phase formed in these steels. In addition, the formation of Nb₂C was observed in the sample containing 3.0 wt.% Nb. While the amount of Nb increased from 0.5 to 3.0 wt.% Nb, the microhardness of the matrix and the amount of M₂₃C₆ decreased and the toughness of the samples increased. After homogenization, the increase in the toughness of the samples containing 1.5–3.0 wt.%Nb was considerable and impressive.     

Experimental characterization of short crack nucleation and growth during cycling in lean duplex stainless steels

Considering that many applications of Lean Duplex Stainless Steels (LDSSs) involve cyclic loading, the aim of this paper is to study short crack initiation and growth during low (LCF) and high cycle fatigue (HCF) in AL 2003 (UNS S32003). Electron Backscattered Diffraction (EBSD) analysis of plastically active grains allows to determinate the slip systems and their associated Schmid factor (SF). Additionally, the dislocation structure developed during cycling is observed by transmission electron microscopy (TEM). Whereas in HCF cracks nucleate at grains boundaries, during LCF cracks nucleate along intrusion/extrusions in ferritic grains and as they reach austenitic grains grow along active slip systems or by double slip system. Moreover, phase boundaries and grain boundaries act as effective barrier against crack propagation.     

Laser dissimilar welding of CoCrFeMnNi-high entropy alloy and duplex stainless steel

High entropy alloys(HEAs)have superior mechanical properties that have enabled them to be used as structural materials in nuclear and aerospace applications.As a dissimilar joint design is required for these applications,we created a dissimilar joint between CoCrFeMnNi-HEA and duplex stainless steel(DSS)through laser beam welding;a technique capable of producing a sound joint between the two materials.Microstructure examination using SEM/EBSD/XRD analysis revealed that the weld metal(WM)exhibits an FCC phase regardless of the postweld heat treatment(PWHT)temperature(800 and 1000℃)without forming detrimental intermetallic compounds or microsegregation.The heat-affected zone of the CoCrFeMnNi-HEA showed CrMn oxide inclusions while that of the DSS showed no inclusions.Moreover,a lower hardness was recorded by the WM compared to the base metal after welding.After PWHT,the hardness of the WM,CoCrFeMnNi-HEA,and DSS decreased with an increase in the PWHT temperature.However,the decrease in the hardness of the HEA was more significant than in the WM and DSS.The cause for this reduction in hardness was attributed to recrystallization and grain growth.In addition,a strength of 584 MPa with low ductility was recorded after welding.The obtained strength was lower than that of the BMs,but comparable to that of the welded CoCrFeMnNi-HEA.The application of PWHT resulted in over a 20％increment in ductility,with only a marginal reduction in strength.The deformation mechanism in the as-weld joint was mainly dominated by dislocation while that for the PWHT joint was twinning.We propose laser beam offset welding as a technique to improve the mechanical properties of the dissimilar joint,which will be the subject of future studies.     

The use of X-ray diffraction, microscopy, and magnetic measurements for analysing microstructural features of a duplex stainless steel

X-ray diffraction, light optical microscopy, and magnetization saturation measurements were employed to analyse the microstructural features of a UNS S31803 duplex stainless steel modified by high-temperature treatments. The samples were heated to 1300 °C and cooled by different ways to produce five different microstructures. Solution treatments at 1000 °C were also employed to produce another five conditions. Three methods were employed to determine the austenite/ferrite proportions. X-ray diffraction gave higher austenite values than the other methods, due to the influence of texture, but can be successfully used to determine the microstrain level in each phase. Magnetic saturation measurement is a very simple and precise method for quantification of austenite and ferrite volume fractions in samples that were fast-cooled and slow-cooled. Light microscopy can give a fast and precise measurement of the phase proportions and reveals important features related to the morphology of the phases, but in the samples where the austenite content is low, quantification becomes difficult and imprecise.     

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 effect of short time post-weld heat treatment on the fatigue crack growth of 2205 duplex stainless steel welds

The influence of γ content and its morphology on the impact and fatigue crack growth behavior of 2205 duplex stainless steel (DSS) welds were studied in this work. Short time post-heating was able to effectively raise the γ content and the impact toughness of the weld. The variation in microstructures showed less influence on the fatigue crack growth rate (FCGR) of the steel plate and weld except in the low ΔK regime. In contrast, residual welding stresses played a more significant affection on the FCGR of the DSS weld than microstructural factors did. Plastic deformation induced martensitic transformation within a definitely thin layer was responsible for the difference in crack growth behavior between specimens in the low ΔK range. Coarse columnar structure was more likely to have tortuous crack path in comparison with the steel plate.     

Mechanical,electrochemical and magnetic behaviour of duplex stainless steel for biomedical applications

Mechanical, electrochemical and magnetic properties of duplex stainless steel were analysed to evaluate its use as biomaterial, comparing the results with those obtained for austenitic stainless steel. Yield and ultimate tensile strengths are almost twice in duplex stainless steel, being the values 870?MPa and 564?MPa, respectively. The electrochemical test revealed that this material has lower susceptibility to localised corrosion because of its greater passive range, 1?V from the open circuit potential, while the austenitic stainless steel exhibited a passive region of 0.370?V. Both steels behave as soft magnetic materials, however, duplex stainless steel has higher magnetic saturation and remanence, while austenitic stainless steel is more prone to heating when exposed to a magnetic field.     

Effect of grain size on the hydrogen-assisted cracking in duplex stainless steels

The embrittlement behavior of 2205 duplex stainless steels with two different grain sizes in 26 wt% NaCl (pH 2) under cathodic potential were investigated by slow strain rate testing. The electrochemical permeation technique was used to characterize the permeation rate and effective diffusivity of hydrogen. The results indicated that both the effective diffusivity and the susceptibility of hydrogen embrittlement were lower for the finer grain size specimen. Ultimate tensile strength (UTS) and uniform elongation (UEL) decrease linearly with decreasing logarithm of strain rate. The dependence of UTS and UEL on the logarithm of strain rate was higher for the finer grain specimen. The microstructural examination revealed that internal cracks resulted from hydrogen embrittlement of the ferrite phase under cathodic charging conditions were arrested by austenite in duplex stainless steels.     

Evolution of interface character distribution in duplex stainless steel processed by cross-rolling and annealing

Duplex stainless steel UNS S31803 samples were cross-rolled with a true strain of ε = 2 followed by annealing at 1323 K for 2 min and 240 min, respectively. The distributions of intervariant boundary planes in the precipitated austenite (A) from ferrite (F) and phase boundary planes conforming to Kurdjumov-Sache (K-S) orientation relationship (OR) were characterized by electron backscatter diffraction (EBSD) and the five-parameter analysis (FPA) method, respectively. The intervariant boundary planes with misorientation angle of 60° around <111> and <011> occur frequently and tend to terminate on the {111} plane. At the grain size level of 4 μm, the phase boundary appears to be connected with the K-S OR terminating on {110}_F6{111}_A at the early stage of annealing. When the grain size reaches approximately 20 μm, phase boundary was modified into {541}_F6{533}_A due to twinning in austenite during annealing.     

Hot deformation characteristics of 2205 duplex stainless steel based on the behavior of constituent phases

High temperature behavior of 2205 duplex stainless steel was studied by considering behavior of each constituent phase. The specimens were subjected to hot compression tests at temperatures of 800–1100 °C and strain rates ranging from 0.001 to 1 s⁻¹ at intervals of an order of magnitude. The flow stress analysis showed that hot working empirical constants are different at low and high temperatures. The strain rate sensitivity m was determined and found to change from 0.12 to 0.21 for a temperature rise from 800 °C to 1100 °C. The apparent activation energy Q was calculated as 554 and 310 kJ/mol for low and high temperature, respectively. The validity of constitutive equation of hyperbolic sine function was studied and stress exponent, n, was assessed to be 4.2. Assuming the hyperbolic sine function for determination of strain rate and application of the rule of mixture, the interaction coefficients of δ-ferrite, P, and austenite, R, were estimated at different hot working regimes. It was found that the interaction coefficients are functions of Zener–Hollomon parameter Z and obey the formulas P = 1.4Z^−0.08 and R = 0.76Z^0.005. Therefore, it was concluded that at low Z values δ-ferrite almost accommodates strain and dynamic recovery is the prominent restoration process which may even inhibit dynamic recrystallization in austenite. Otherwise, at high Z, austenite controls the deformation mechanism of material and dynamic recrystallization leads in finer microstructure.     

Chi-phase precipitation in a duplex stainless steel

The aim of this study is to investigate the precipitation of intermetallic phases, especially the chi-phase, in a 45N (type UNS S31803) duplex stainless steel through aging heat-treatments carried out at 700 and 750 °C. Two intermetallic phases are detected: chi (χ) and sigma (σ). The χ-phase precipitates at ferrite/ferrite grain boundaries prior to the σ-phase precipitation, which occurs preferentially at ferrite/austenite interfaces and at ferrite/ferrite grain boundaries. The σ-phase precipitation is a eutectoid type reaction of ferrite leading to σ-phase phase and austenite. The χ-phase is consumed in the σ-phase precipitation after becoming completely surrounded by both the σ-phase and the newly formed austenite.     

Precipitation of secondary phases in super duplex stainless steel ZERON100 isothermally aged

Abstract

In the present paper the results of secondary phases determination and quantification in ZERON100 duplex steel, heat treated at 850–1000°C for 180–2400 s, are presented. During the isothermal heat treatments, at 850°C, the χ phase is the only phase to precipitate at α/γ boundaries and triple points, while at 900 and 950°C, χ phase is the first phase to precipitate after 180 s, followed by σ phase 300 s later. At all the temperatures the total amount of secondary phases is ～3% after <600 s aging.     

Strengthening of AISI 2205 duplex stainless steel by strain ageing

In this study, static strain ageing behavior of commercially available and solution heat treated duplex stainless steel was investigated and the effect of static strain ageing on the mechanical properties was also determined in detail. Some of as-received duplex stainless steel test specimens were pre-strained in tension by 5% and then aged at 100 °C, 200 °C, 300 °C, 400 °C, 500 °C and 600 °C for 30 min in furnace. Some of duplex stainless steel test specimens were solution heat treated at 1050 °C for 30 min, water quenched and then pre-strained for 5% in tension shortly after the solution heat treatment.In order to identify the effect of static strain ageing on the mechanical properties, the tensile strength, the change in the strength due to ageing (ΔY), elongation fracture and hardness were determined. The test results showed that the mechanical properties were affected by static strain ageing mechanism which was applied at different temperatures for same time interval.     

A mechanism of ferrite softening in a duplex stainless steel deformed in hot torsion

A mechanism of dynamic softening of ferrite was studied in a 21Cr-10Ni-3Mo austenite/ferrite duplex stainless steel subjected to torsion at a strain rate of 0.7 s⁻¹ at 1200°C. Transmission electron microscopy together with convergent beam electron diffraction were used with major emphasis on the study of misorientations across ferrite/ferrite boundaries. No evidence of discontinuous dynamic recrystallisation involving nucleation and growth of new grains was found within ferrite contrary to some suggestions made in the literature for similar experimental conditions. The softening mechanism has been classified as extended dynamic recovery characterised by a gradual increase in misorientations between neighbouring subgrains that were created by dynamic recovery processes at the earlier stages of deformation. The resulting dislocation substructure was a complex network of subgrain boundaries composed of a mix of higher- and lower-angle walls characterised by misorientation angles not exceeding 20° at a maximum obtained strain of 1.3.     

Chemical-state information obtained by AES and XPS from thin oxide layers on duplex stainless steel surfaces

Thin oxide films were produced by the exposure of polished, sputter-cleaned metallic surfaces kept in UHV to several thousands of Langmuirs at 10⁻⁵ mbar oxygen yielding oxide layers of several nanometers. Metallic substrates used were iron, chromium and duplex stainless steel (DSS 2205). AES and XPS profiling analyses were performed. An attempt was made to use certain features observed in the AES spectra, i.e. to correlate Fe and Cr MNN peak shapes with the chemical state of the corresponding element. A similar approach has been tried before and may, when combined with high-lateral-resolution AES, provide small-area chemical-state information. Localized chemical-state information derived from the MNN peak shapes by a Linear Least Squares Fit (LLSF) procedure appeared to match reasonably well with that provided by XPS, which is averaged over approximately 2 mm². This is a plausible result for a thin homogeneous layer on a polished substrate.     

Laser surface modification of UNS S31603 stainless steel. Part II: cavitation erosion characteristics

Austenitic stainless steel UNS S31603 was laser surface alloyed with various elements (Co, Ni, Mn, C, Cr, Mo, Si) and alloys/compounds (AlSiFe, Si₃N₄ and NiCrSiB) as presented in Part I together with the microstructures and the corrosion characteristics of the alloyed specimens. In Part II, the cavitation erosion characteristics of the alloyed specimens in 3.5% NaCl solution at 23°C were studied by means of a 20 kHz ultrasonic vibrator at a peak-to-peak amplitude of 30 μm. The hardness profile and the compositional profile of the alloyed layers were investigated by a Vickers hardness tester and by EDX respectively. The cavitation erosion resistance of specimens alloyed with AlSiFe, C and NiCrSiB were highest, reaching 11.1, 10.5 and 7.9 times that of the substrate respectively. The damage mode was identified to be ductile fracture for specimens containing austenite as the major phase, and brittle fracture when the major phase was ferrite or intermetallic. Cavitation erosion was initiated at the phase boundaries where there was an abrupt change in mechanical properties (e.g. hardness) and then propagated into the weaker phase. It was also noted that large improvement in cavitation erosion resistance and corrosion resistance could not be simultaneously achieved in the present study.