Processing map for hot working characteristics of a wrought 2205 duplex stainless steel

Processing map on a wrought 2205 duplex stainless steel under hot compression conditions has been developed based on the dynamic material model theories in the range 1223–1473 K and 0.01–10 s⁻¹. The various domains in the map corresponding to different deformation characteristics have been discussed in combination of microstructural observations. The results show that the power dissipation efficiency (η) depends strongly on the dynamic recrystallization (DRX) of austenite which plays a dominant role in microstructural evolution, while the ferrite phase mainly continues to exhibit relatively well-developed dynamic recovery (DRV) at large strain. The optimum hot working domain of wrought 2205 duplex stainless steel is obtained to be in the temperature range 1373–1473 K and at strain rate of 0.01 s⁻¹, with peak efficiency 50% occurring at about 1423 K, in which more uniform microstructure is developed due to the occurrence of complete DRX of austenite. The unstable hot working regimes are predicted by Prasad instability criterion, in good agreement with the macro-and microstructural observations. As predicted, flow instability, which are manifested as twinning, bands of flow localization and the absence of DRX in austenite are observed at lower temperatures and higher strain rates (1223–1273 K and 1–10 s⁻¹); in other cases, wedge cracking is responsible for instability phenomena observed at the temperature range 1373–1423 K and strain rate of 10 s⁻¹.     

Study of hot deformation behaviour of 2205 duplex stainless steel through hot tension tests

The hot working behaviour of duplex stainless steels has been studied in the literature mainly through hot torsion or hot compression tests. The aim of this paper is to investigate the hot deformation behaviour of a duplex stainless steel type 2205 (F51) through hot tension tests, which are easier to carry out and can offer some additional information about the maximum sustainable strain in tension before starting of irreversible damage. In fact, even under the uniaxial compressive action of a forging press, tensile stresses may develop in some zones of the forged product. These zones become most critical and the knowledge of the limiting conditions are important. Three different test temperatures were selected, namely 900, 1000, and 1100 °C. The hot working behaviour was characterised by fitting the mean flow stress, strain rate, and temperature relationship with the hyperbolic sine function defined by Sellars and Tegart. An activation energy Q equal to 430 kJ/mol was obtained for plastic straining at high temperature. It corresponds to values, obtained with hot torsion or hot compression tests, reported in the literature where a large variation in activation energy is also found for duplex stainless steels. It was explained by considering the intrinsic two-phase nature of the investigated steel. The ductility in the different hot working conditions was characterised by the true strain at the onset of damage formation under prevailing tensile stress conditions. The role of the microstructure as well as of damage formation during deformation on the shape of the flow stress curves was analyzed.     

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.     

Comparison between symmetric and asymmetric hot rolling techniques performed on duplex stainless steel 2205

The use of duplex stainless steel represents one possible efficient alternative of austenitic grade and an interesting resources for its high performance against stress corrosion cracking. Unfortunately such material shows some limitations in their use: for instance the sheets or plates of duplex stainless steels present anomalous and poor formability for plastic deformation processes. Such problems are mainly related to an unsuitable normal anisotropy coefficient, which might cause the “necking” and “earing” phenomena, especially during hot rolling. The study deals with the comparison of symmetric and asymmetric rolling technique on stainless steel duplex 2205 specimens. All the experiments were carried out using a laboratory mill, properly equipped with an individual engine for each rolling cylinder. The experimental parameters considered include three different pre-heating temperatures and two asymmetry ratios, while the reduction level is maintained constant for both rolling configurations. Moreover, the study involves also the analysis of the influence of solubilization quenching and the SEM, SEM-EBSD investigation dedicated to establish the microstructure modifications. The specimens were also studied through tensile tests to determine the influence of the rolling techniques on the mechanical properties of the product, focusing on the definition of the average anisotropy coefficient. The results of the experimental trials allow to conclude that the use of asymmetric rolling process induces an improved formability and increases duplex 2205 tensile properties.     

Effect of aging treatments and microstructural evolution on corrosion resistance of tungsten substituted 2205 duplex stainless steel

Abstract

The effect of partial substitution of tungsten for molybdenum on the microstructure and corrosion resistance in 22Cr–5Ni–3Mo duplex stainless steel (DSS) has been investigated following aging heat treatments in a temperature range of 600-1000°C. Electrochemical tests were carried out for the evaluation of corrosion resistance. Aging treatment had hardly influenced the general corrosion resistance. With the increase of aging time, the pitting corrosion resistance of the DSSs had decreased. After aging for 2 min at 700–900°C, the pitting potential of the 3Mo steel decreased remarkably, while that of the W substituted steel hardly changed. During aging, the intermetallic σ and secondary austenite (γ₂) phases were precipitated, and the pitting corrosion and intergranular corrosion resistance were significantly decreased after aging at 700–750°C for 10 h, which could be caused by the γ₂ formation. The γ₂ phase could effect the depletion of molybdenum and chromium in the γ₂ /α and γ₂/σ boundaries.     

Hot working behavior of 2205 austenite-ferrite duplex stainless steel characterized by constitutive equations and processing maps

The hot deformation characteristics of the 2205 duplex stainless steel were analyzed using constitutive equations and processing maps. The hot compression tests were performed at temperature range of 950-1200 °C and strain rate of 0.001-1 s⁻¹. Flow stress was modeled by the constitutive equation of hyperbolic sine function. However, the stress exponent and strain rate sensitivity were different at low and high deformation temperatures where austenite and ferrite are dominant, respectively. It was recognized that strain at the peak point of flow curve increases with the Zener-Hollomon parameter, Z, at low temperature deformation while at high temperature deformation it actually decreases with Z. The power dissipation map, instability map and processing map were developed for the typical strain of 0.3. It was realized that dynamic restoration mechanisms could efficiently hinder the occurrence of flow instability at low and medium strain rates. Otherwise, the increase in strain rate at low and high temperatures could increase the risk of flow instability.     

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.     

Fatigue performance and microanalysis of heat treated 2205 duplex stainless steel

Abstract

The reduced corrosion fatigue performance of a heat treated grade of duplex SAF 2205 stainless steel has been quantified using Wöhler rotating bending fatigue machines. The reduction in fatigue performance has been related to elemental profiles around intergranular and intragranular precipitates and grain boundary segregation measured using EDAX analysis and scanning transmission electron microscopy.     

Formability studies on plasma arc welded duplex stainless steel 2205 sheet

Sheet metal forming is cost-effective manufacturing process and hold a significant key position in fabrication works. Welding being a popular technique the industries primarily prefer for joining of sheet metal formed parts. The inherent material properties changes at the weld metal and heat affected zone post welding process, hence the impact and changes on mechanical strength aspects need to be studied. The current study focuses on influence of plasma arc welding on formability of 1.6 mm thick duplex stainless steel 2205 sheet using Erichsen cupping test by gauging the height of the cup formed. The performed tests such as uniaxial tensile, microhardness showed better mechanical properties and decrease in formability noted from Erichsen cupping test for weld metal compared to base metal. Finite element analysis of Erichsen cupping test is conducted using ABAQUS software and results are matched with experimental outcomes for validation. The comparison shows that a deviation of less than 5 % is noticed between actual and predicted formability index values. Microstructure examination reveals that, equiaxed grains at the weld center and columnar grains at sides typically formed in the weld metal region. The decrease in formability of weld blank compared to base blank is attributed to an increase in ferrite content. This is supported by amount of ferrite content measured in weld metal is 55 % and base metal is 52 %. The scanning electron micrographs (base and weld blank) reveal the mode of failure is ductile.     

Effects of microstructure changes on the superplasticity of 2205 duplex stainless steel

The high temperature deformation behavior of 2205 duplex stainless steel under different conditions had been studied by tensile tests. The whole tensile test was conducted at a constant temperature 950 °C with an initial strain rate 1.5 × 10⁻³/s. Some tests were interrupted purposely and then the samples were quenched using water. Elongations of the fractured specimens were calculated. Microstructure changes just before and during the deformation were observed. Phase ratio of σ precipitate was analyzed. The results showed that the superplasticity of 2205 duplex stainless steel was directly affected by the microstructure before the deformation. The recrystallization phenomenon was distinct along with the homogenizing time and the grains became equiaxed and stable. Meanwhile, the quantity of σ phase increased when prolonged the homogenizing time. After homogenized for 7 min before the tensile test, the σ phase ratio was about 4.8% and the grain size was about 998 nm, the maximum elongation value 1260% was obtained. During the deformation progress, dynamic recrystallization was observed and quantity of σ phase increased with the increasing of deformation strain. The σ phase restricted the grain growth and kept the equiaxed duplex structure stable with a grain size of about 1 μm.     

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.     

Quantitative fractography of 2205 duplex stainless steel after a sulfide stress cracking test

This paper presents the results of SSC (Sulfide Stress Cracking) investigations of duplex stainless steel 2205 resistance to cracking failure under the combined action of tensile stress and aqueous environments containing hydrogen sulfide according to the NACE Standard TM0177-96. The investigations were carried out on 9 test pieces that were loaded with a tensile stress ranging from 1.02 × the yield stress (YS) to 0.72 × YS. The tests were terminated either when the test specimen failed or after 720 h (30 days) – whichever occurred first – in accordance with the standard requirements. Only two of the specimens examined, one loaded with 1.02 × YS and the second one with 0.72 × YS, failed during the test. The resulting fracture surfaces were subjected to qualitative and quantitative fractographic examinations. Quantitative fracture analysis included an estimation of such parameters as linear roughness index R_L, fractal dimension D_F and overlap index O_L.     

The effect of thermal cycling in superplastic diffusion bonding of 2205 duplex stainless steel

In view of the requirement of large cold rolling deformation and bonding pressure in the conventional superplastic diffusion bonding of 2205 duplex stainless steel, a novel method of introducing thermal cycling into the process was proposed. During the thermal cycling process, due to the change of temperature, surface chemical activity of 2205 duplex stainless steel was improved, activity of atoms and grain boundaries were improved, and the recrystallized grains were refined. The shear bond strength of joint prepared in the mode of thermal cycling using specimens with the cold roll reduction of 60% was 15 MPa higher than that of conventional bonding using specimens with the cold roll reduction of 85%. Compared to the shear bond strength of 430 MPa under the specific pressure of 10 MPa after conventional bonding, shear bond strength of 623 MPa was obtained under the condition of T_max = 1000 °C, T_min = 900 °C, cycle number of heating and cooling N = 3, and specific pressure P = 5 MPa.     

Effect of aging treatments on microstructure and impact properties of tungsten substituted 2205 duplex stainless steel

Abstract

The effects of partial substitution of tungsten for molybdenum on the microstructure and impact properties in 22Cr–5Ni–3Mo (wt-%) duplex stainless steel (DSS) have been investigated following aging heat treatments in the temperature range 600–1000°C. During aging the intermetallic σ and χ phases were precipitated, and the impact toughness was significantly decreased with an increase in the σ phase content. The χ phase had been precipitated on the α/γ boundary in the early stages of aging. Ferrite and χ phases in tungsten substituted duplex stainless steel contain a large amount of tungsten, and their decomposition rates are much lower compared with those in steel containing only molybdenum. Consequently, the precipitation of the σ phase is retarded in tungsten substituted DSSs, which results in high impact toughness. However, after aging for a long time, the α and χ phases transformed to the σ and austenite phases in the tungsten substituted steels, and the steels showed low impact toughness.     

Synergistic effect of erosion and hydrogen on properties of passive film on 2205 duplex stainless steel

Stainless steels have shown great potential in the application of offshore oil and gas industry.However,the internal surface of stainless steel pipeline may simultaneously suffer erosion from the fluid media inside the pipeline and the damage of hydrogen that is generated from the external activities such as cathodic protection.The synergistic effect of erosion and hydrogen on the properties of passive film on 2205 duplex stainless steel was studied for the first time in a loop system coupled with a hydrogen-charging cell.The components,protective performance and semiconductive structure as well as properties of the passive film under different conditions were investigated using in-situ electrochemical techniques,surface characterization and computational fluid dynamics simulation.The results show that the combination of erosion and hydrogen could greatly thin the passive film,furthermore,the Fe³⁺/Fe²⁺ratio and O₂^-/OH^-ratio in the passive film also decrease dramatically under such a condition.Therefore,the hydration degree of the passive film greatly increases,resulting in an increase in active sites and a decrease in the stability of the passive film.Erosion could destroy the passive film through the impact of sand particles and accelerate the mass transfer process of electrochemical reaction.While hydrogen can not only enhance the charge transfer process,but also make the passive film highly defective.Under the combination of erosion and hydrogen condition,erosion could enhance the hydrogen damage and simultaneously hydrogen could also enhance erosion.Therefore,the synergistic effect of erosion and hydrogen could dramatically change the passive film component,decrease the protective performance,and increase the susceptibility of pitting corrosion of 2205 stainless steel in Cl-containing environment.     

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.     

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.     

Influence of isothermal aging on secondary phases precipitation and toughness of a duplex stainless steel SAF 2205

In this paper the results of secondary phases determination in a SAF 2205 duplex steel, heat treated at 780, 850 and 900 °C for 10–40 min, are presented. χ-phase is the first phase to precipitate at all the temperatures, followed by σ-phase. The main drop of toughness is related to very low contents of χ-phase and σ-phase. Hardness is not a sensitive parameter to low amounts of secondary phases.