Micromechanical modeling of the behavior of duplex stainless steels

The aim of the present paper is the micromechanical modeling of an aged duplex (austenite/ferrite) stainless steel. While the elastic properties of both phases are almost the same, the plastic mismatch between ferrite and austenite is high due to ferrite aging. Accounting for the complex micro and macrostructure of this material, three scales are investigated. The first one corresponds to each single phase. The second scale consists of a percolated (interwoven) network of both phases. The third scale is an aggregate of percolated bicrystals. For the first scale, each phase is modeled as a single crystal. In the second scale, F.E. simulations are performed on a unit cubic cell representing the percolated networks. A mean field model is then fitted in order to represent that bicrystal. This mean field model is used at the third scale to model the behavior of the aggregate of two-phase grains, using a model for multi-phase materials. At this scale, it becomes possible to represent the distribution of average ferrite stresses in each ferrite-austenite bicrystal. These variations are thought to be responsible for the heterogeneous damage nucleation observed in this material.     

Influence of the microstructure on the plastic behaviour of duplex stainless steels

Microstructure change of α (ferrite) + γ (austenite) two-phase structure in duplex stainless steels deformed by hot torsion tests is briefly analyzed. Two types of stainless steels containing different volume fractions of ferrite and austenite were torsion deformed at temperatures ranging from 900 to 1250 °C. Steel A (25.5Cr-4.9Ni-1.6Mo) contained Cr_eq/Ni_eq = 4.8 and steel B (22.2Cr-5.6Ni-3Mo) contained Cr_eq/Ni_eq = 3.5 bring about different microstructures and flow stress behaviour. The results show that the shape of the flow stress curves depends on the material and on deformation conditions. Four different flow curve shapes were observed. At high temperatures, steel A displayed a plastic behaviour typical of ferritic stainless steels. As the deformation temperature decreased, the flow curves presented peak stresses at low-temperature deformation. When the austenite particles are distributed coarsely in the matrix (steel B), the plastic flow curve displays a stress peak separating extensive regions of hardening and softening. When both phases have the same volume fractions, the microstructure is characterized by percolation of the two phases in the samples, and the plastic flow curve takes on a very distinctive shape in hot torsion tests. The role of the microstructure present during deformation on the shape of the flow stress curves is analyzed.     

Effects of long term thermal aging on high temperature tensile deformation behaviours of duplex stainless steels

Abstract

Tensile deformation behaviours of unaged and thermal aged duplex stainless steels (DSS) were investigated at 350°C in order to understand the effects of long term thermal aging on the high temperature deformation behaviours of DSS. After aging for 20?000?h, the strength of DSS has a slight increase, the plasticity has a considerable decline, and the tensile fracture transfers from ductile to brittle. Nanoindentation tests indicate that ferrite has a considerable increase in hardness, and austenite has only a negligible increase with aging time. Thermal aging embrittlement is primarily concerned with ferrite. After long term thermal aging, spinodal decomposition and G-phase precipitation occur in ferrite and these reactions result in the dramatic decline of the ferrite phases' deformation ability. Cleavage cracks can easily initiate and propagate in ferrite of long term thermal aged DSS.     

Review of temperature indicators and the use of duplex stainless steels for life assessment

This paper presents an overview of the recent development of life assessment techniques for high temperature plants, with special emphasis on a novel temperature-measuring technique, i.e. the ‘Feroplug’.Improved life assessment techniques will enable components to be replaced before failure, so reducing the risk of industrial accidents, as well as minimizing financial loss caused by unscheduled outages. The Feroplug is a new invention and has been patented by the US, UK and European patent bodies, with financial and legal support provided by the British Technology Group. It has been developed to supplement conventional temperature-measuring methods, such as the thermocouple, in extremely hostile environments, to provide information on the operating temperature, which is a major concern for life assessment. The principle of the Feroplug, which consists of piece of duplex (austenite + ferrite) stainless steel with specially designed composition, is based on the transformation of ferrite in the material on heating in the range 300–1000 °C. The extent of the transformation, being an indication of the temperature to which the Feroplug has been exposed, can be easily monitored using magnetic measurement techniques. The characteristics of the ferrite transformation are affected—in addition to the ageing temperature—by many metallurgical parameters, such as the composition of the alloy, thermomechanical pretreatments, etc. The recent progress of research in ferrite transformation is also reviewed in detail to provide the ground for understanding the principles of the Feroplug.     

Biofilm inhibition and corrosion resistance of 2205-Cu duplex stainless steel against acid producing bacterium Acetobacter aceti

Acid producing bacterium Acetobacter aceti causes pitting corrosion of stainless steel(SS).This work investigated the enhanced resistance of 2205-Cu duplex stainless steel(DSS) against biocorrosion by A.aceti in comparison with 2205 DSS using electrochemical and surface analysis techniques.With the addition of Cu to 2205 DSS,biofilms on the 2205-Cu DSS surface were inhibited effectively.The largest pit depth on 2205-Cu DSS surface in the presence of A aceti was 2.6 μm,smaller than 5.5 μm for 2205 DSS surface.The i_(corr) was 0.42±0.03 μA cm~(-2) for 2205-Cu DSS in the biotic medium,which was much lower than that for 2205 DSS(3.69±0.65 μA cm~(-2)).All the results indicated that the A aceti biofilm was considerably inhibited by the release of Cu~(2+) ions from the 2205-Cu DSS matrix,resulting in the mitigation of biocorrosion by A aceti.     

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.     

Processing of duplex stainless steel by WEDM

This study investigates the manufacturing process of 2205 duplex stainless steel by wire electrical discharge machining where the effects of pulse-on time (PONT), wire tension and pulse-off time (POFT) on surface finish, kerf width, and material removal rate (MRR). It was found that the kerf width was unchanged with the change of PONT at long pulse-of time and higher wire tension. However, it decreased initially and then increased due to the rise of PONT at low values of wire tension and POFT. Low wire tension and PONT, POFT and contributed towards widest kerf. Longer PONT increased MRR due to higher machining/processing speed. Lower wire tension and shorter POFT increased MRR more than that of higher wire tension and POFT. Craters and recast layer were on the machined surfaces at all machining conditions. Increased PONT raised surface roughness at the lower POFT and tension in the wire. The surface finish at high wire tension and longer PONT is always better than that at smaller PONT and lower tension in the wire. The microstructure underneath the recast layer remains unchanged and the failure of wire electrode occurred at higher wire tension, longer PONT and shorter POFT.     

Evaluation of Pitting Behavior on Solution Treated Duplex Stainless Steel UNS S31803

The pitting corrosion resistance of duplex stainless steels UNS S31803 annealed at different temperatures ranging from 1050 ℃ to 1200 ℃ for 24 h has been investigated by means of potentiostatic critical pitting temperature(CPT).The microstructural evolution and pit morphologies of the specimens were studied through optical microscopy and scanning electron microscopy.The potentiostatic CPT measurements show that the CPT was elevated with the annealing temperature increased from 1050 ℃ to 1150 ℃ and decreased as the temperature further increased to 1200 ℃.The specimens annealed at 1150 ℃ exhibited the highest CPTand the best pitting corrosion resistance.The pit morphologies show that the pit initiation sites transfer from austenite phase to ferrite phase as the annealing temperature increases.The results were explained by the variation of pitting resistance equivalent number(PREN) of ferrite and austenite phases as the annealing temperature was varied.     

Effect of partial replacement of carbon by nitrogen on intergranular corrosion behavior of high nitrogen martensitic stainless steels

The microstructure evolution and intergranular corrosion (IGC) behavior of high nitrogen martensitic stainless steels (MSSs) by partial replacing C by N were investigated by using microscopy, X-ray diffraction, nitric acid tests and double-loop electrochemical potentiokinetic reactivation (DL-EPR) tests. The results show that the partial replacement of C by N first reduces and then increases the size and content of precipitates in high nitrogen MSSs, and converts the dominant precipitates from M₂₃C₆ to M₂N, furthermore first improves and then deteriorates the IGC resistance. The high nitrogen MSS containing medium C and N contents provides good combination of mechanical properties and IGC resistance.     

Effect of dissolved oxygen on electrochemical corrosion behavior of 2205 duplex stainless steel in hot concentrated seawater

The corrosion behavior of 2205 duplex stainless steel was investigated in hot concentrated seawater with different dissolved oxygen(DO) concentration by electrochemical measurement techniques and surface analysis methods. DO obviously enhances the cathodic reaction process, the formation of passive film and polarization resistance. With increasing the DO concentration from 0.34 to 3.06 mg L^-1, the relative contents of Fe₂O₃ and Cr₂O₃ and the Cr-enrichment gradually enlarge in the passive film. The higher DO concentrations result in lower defect densities and thicker of space charge layers in the passive films,whichmayeffectively inhibit the intrusion of aggressive chloride ions. The increment inDOconcentration clearly increases the pitting potential, but decreases the repassivation potential. It may weaken both the occurrence and repassviation tendencies of stable pitting corrosion.     

Investigation of stress corrosion cracking behavior of super 13Cr tubing by full-scale tubular goods corrosion test system

In this paper, a self-built device called “full-scale tubular goods corrosion test system” was used to test a 6 m length super 13Cr tubing (with coupling) to study its corrosion performance in spent acid. The specimen fractured at the tubing and was investigated by visual inspection, optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), and mechanical test. It was the joint function of tensile force (78.6% actual yield strength), inner pressure (70 Mpa) and spent acid that induced stress corrosion cracking (SCC) of the tubing at 120 °C. Three different areas were found on the fracture surface, including crack initiation area, crack expansion area, and final fracture area. The fracture initiated from the “X” shape corrosion cracks which were evolved from small corrosion pits. The reduction of ductility and toughness may also facilitate SCC of the tubing.     

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.     

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.     

Effect of annealing temperature on the pitting corrosion resistance of super duplex stainless steel UNS S32750

The pitting corrosion resistance of commercial super duplex stainless steels SAF2507 (UNS S32750) annealed at seven different temperatures ranging from 1030 °C to 1200 °C for 2 h has been investigated by means of potentiostatic critical pitting temperature. The microstructural evolution and pit morphologies of the specimens were studied through optical/scanning electron microscope.Increasing annealing temperature from 1030 °C to 1080 °C elevates the critical pitting temperature, whereas continuing to increase the annealing temperature to 1200 °C decreases the critical pitting temperature. The specimens annealed at 1080 °C for 2 h exhibit the best pitting corrosion resistance with the highest critical pitting temperature. The pit morphologies show that the pit initiation sites transfer from austenite phase to ferrite phase as the annealing temperature increases. The aforementioned results can be explained by the variation of pitting resistance equivalent number of ferrite and austenite phase as the annealing temperature changes.     

含Cu抗菌不锈钢的工艺与耐蚀性能

与普通0Cr17铁素体不锈钢和0Cr18Ni9奥氏体不锈钢相比，含铜铁素体和奥氏体抗菌不锈钢均具有良好的冷热加工性能和焊接性能．通过提高浇铸温度，抗菌不锈钢能保持良好的铸造性能．奥氏体抗菌不锈钢的抗应力腐蚀性能比0Cr18Ni9不锈钢有很大的提高，而铁素体抗菌不锈钢比0Cr17有明显的下降．与相应的普通不锈钢相比，两种类型抗菌不锈钢的耐点蚀性能均略有下降．     

A comparative analysis of metallurgical and mechanical properties of friction welded and post weld heat treated (oil quenched) duplex stainless steel joints

The effects of post weld heat treatment (PWHT) and oil quenching on the metallurgical and mechanical properties of the duplex (UNS S31803) welded joints were evaluated at three different temperatures namely 1080, 1150 and 1200 °C. The microstructural variation, austenite/ferrite phase changes, grain size measurements and microhardness aspects of the welded joint were observed. The fraction of ferrite and austenite phases was equivalent at 1150 °C. Nickel element was more efficient in controlling the twin phase balance. Finer grain structure was achieved at 1150 °C due to recrystallization effect. Twin phase presence and absence of precipitates were confirmed through XRD and TEM which followed Kurdjumov–Sachs relationship. At a heating pressure of 40 MPa, heating time of 4 s, an upsetting pressure of 80 MPa, and an upsetting time of 2 s during a PWHT at 1150 °C, a 50/50 balance between the duplex phases, fine grains, and increased microhardness were obtained.     

Effect of aluminum on microstructure,mechanical properties and pitting corrosion resistance of ultra-pure 429 ferritic stainless steels

Effect of aluminum on microstructure, mechanical properties and pitting corrosion resistance of ultra-pure 429 ferritic stainless steels has been investigated. Aluminum can significantly increase the ratio of equiaxed crystal grains, but the promotion effect has great relation with aluminum content. Aluminum can stabilize ferrite phase and significantly reduce recrystallization temperature. Increased aluminum content can also lead to the precipitate of AlN and Al₂O₃ at higher temperature. The increased amount of AlN may partly contribute to the reduced nitrogen element to form austenite at high temperature, hence the high temperature phase transformation of α + γ → α occurs. The fine and large number of Al₂O₃ particles can refine grain size and then promote recrystallization. The highest intensity of γ-fiber texture {1 1 1}〈1 1 2〉 is observed in the steel with 0.19 wt.% aluminum, which can improve the formability of steels. With the increase of aluminum content, the tensile strength increases linearly but the elongation and plastic strain ratio first increase then decrease, the working hardening index varies slightly among the steels. Appearance of Al₂O₃ inclusions with small size and decreased content of MnS benefit pitting corrosion resistance. However, the large dimension Al₂O₃ inclusions have significantly negative influence on pitting corrosion resistance.     

Microscopic observations of both sides of scale-substrate interfaces after adhesion measurements of chromia-rich scales on ferritic stainless steels using the inverted blister test

Abstract

Ferritic stainless steels were oxidized cyclically at 850°C in air. Four grades were studied : one reference grade without stabilizers, one stabilized with Ti, one with Nb and one with both Ti and Nb. The mass gain evolution of these grades versus number of cycles was recorded and gave relevant information about their oxidation kinetics. Indeed, very few oxide spallation was noted (<1%). Ti appeared to accelerate kinetics significantly, mainly because of its internal oxidation. Nb seemed to have no effect on kinetics in air at this temperature. The adhesion of oxide layers was quantified by the inverted blister-test. A beneficial effect of titanium on the adhesion was noted. For Ti-containing grades, adhesion energies were about 40 J/m² whereas for other grades, they were lower, about 10 J/m². This effect is thought to be due to internal oxidation of titanium, playing a role of mechanical keying of the oxide scale. On the contrary, niobium is not oxidized at the interface and remains as an intergranular intermetallic ironniobium phase. Its influence on oxide adhesion is therefore negligible.     

Direct observation and analysis of the oxide scale formed on Y-treated austenitic stainless steels at high temperature

Abstract

We have studied the oxidation behavior of conventional austenitic stainless steels using same small amounts of Y as is added for deoxygenation and desulphurisaton in steel making.

The direct observation and analysis of the oxide scale formed on 19Cr–10Ni–l .5Si steels with and without small amounts of Y at high temperature have been carried out using several types of equipment. The following results were found: (1) Steel with 0.03Y showed good resistance to oxidation at l,000°C.

(2) Oxide scale was composed mainly of Cr oxide, and Si oxide was also detected at the oxide scale–metal interface and in the internal oxides. The Si oxide formed a network cell structure in the inner oxide scale with deeper internal penetrations. The steel with Y formed a uniform oxide scale in every oxide layer.

(3) Small amounts of Y and Si were detected at the grain boundaries of the inner oxide scale, but no Y was detected in the oxide grains.

The beneficial effect of Y addition was more notable in the Si containing austenitic stainless steels, as the existence of Y or Si prevents the diffusion of cations and anions through the oxide grain boundaries. As consequence, the steel treated with Y showed good resistance to oxidation.     

Comparative tribological studies of duplex surface treated AISI 1045 steels fabricated by combinations of plasma nitriding and aluminizing

Duplex surface treatments via aluminizing and plasma nitriding were carried out on AISI 1045 steel. A number of work pieces were aluminized and subsequently plasma nitrided (Al–PN) and other work pieces were plasma nitrided and then aluminized (PN–Al). Aluminizing was carried out via pack process at 1123 K for 5 h and plasma nitriding was performed at 823 K for 5 h. The fabricated steels were characterized using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and microhardness testing. Tribological behaviors of the duplex treated AISI 1045 steels were examined against tungsten carbide pin using a pin-on-disc apparatus at room temperature. The PN–Al specimen showed higher surface hardness, lower wear rate and coefficient of friction than the Al–PN one. It was noticed from the worn surfaces that tribo-oxidation plays an important role in wear behavior of both specimens.