Effect of cold rolling on microstructure and magnetic properties in a metastable lean duplex stainless steel

Microstructural and magnetic properties changes of a metastable ferritic–austenitic stainless steel due to cold rolling were studied together with the possibility to develop a new ferritic–martensitic stainless steel. In order to reduce costs low-Ni content was maintained in the lean duplex stainless steel considered, making it more susceptible to strain-induced martensitic transformation. In this study a practically complete γ → α′ transformation was found for 80% of thickness reduction, resulting a new two-phase ferritic–α′ martensitic stainless steel. To investigate the structural evolution different values of thickness reduction were applied. Light optical and scanning electron microscopy were performed to characterize the morphology and grain refining of the structure after each rolling step. Martensitic transformation and work hardening were detected and analyzed by studying of magnetic properties (saturation magnetic polarization, relative magnetic permeability, coercivity). Additionally, hardness tests were performed. The results highlighted a strong grain refining and increase in martensitic phase and hardness with increasing cold deformation. A direct relationship between microstructure and magnetic properties was revealed. In particular the reciprocal of relative magnetic permeability and the coercivity increased with martensite content and the amount of cold deformation. Therefore, the possible application of magnetic measurements as non-destructive tests to study microstructural evolution during cold rolling was shown for the steel considered.     

Influence of thermal cycling on the microstructure of a ferritic-austenitic duplex stainless steel

The influence of thermal cycling without any applied load on the microstructural features of a forged stainless ferritic-austenitic duplex steel was investigated by transmission electron microscopy. The steel was examined in uncycled and cycled conditions. Thermal cycling between 20 °C and 900 °C did not alter the volume fractions and the anisotropic grain structure of ferrite and austenite. On the other hand, thermal cycling had a strong influence on the dislocation arrangement in both phases. Three different types of precipitate were identified in the uncycled material. The amount of these particles was increased by thermal cycling. There was a strong evidence for the precipitation of ′ particles in the cycled material.     

Influence of the microstructure on the toughness of a duplex stainless steel UNS S31803

The microstructure of a duplex stainless steel UNS S31803 was varied by high temperature treatments (1300°C) followed by different cooling rates. A wide range of microstructures, with differents morphologies and phase proportions, were obtained by this way. Some samples were solution treated at 1000°C and fast cooled after the high temperature treatment. The impact toughness in all conditions were evaluated by reduced size (2.5 mm) Charpy impact tests. The highest toughness was obtained in the samples cooled in furnace from 1300 to 1000°C and then air cooled to room temperature. The microstructure at this condition was very fine with 55.4% of austenite. The lower toughness value was obtained in the water cooled sample, which presented only 17.1% of austenite and large grains of ferrite. The toughness of these and other microstructures was improved by the solution treatment.     

Effect of aging on the corrosion resistance of 2101 lean duplex stainless steel

The microstructure and localized corrosion behavior of a 2101 lean duplex stainless steel aged at 700 °C were investigated. The results showed that changes in the microstructure of the duplex stainless steel, due to the formation of precipitates, affected its pitting corrosion resistance. The values of the pitting potential and the critical pitting temperature dropped drastically before aging time up to 30 min. The potentiostatic pitting corrosion measurement indicated more sensitive to the small amount of precipitates compared to the potentiodynamic test. Pitting nucleated mainly in the ferrite phase for the solution-annealed specimen, while the initiation of pitting corrosion for the aged specimen took place at Cr-depletion area around the precipitates, i.e. in the newly formed secondary austenite phase.     

Effects of deformation on hydrogen degradation in a duplex stainless steel

Hydrogen embrittlement of annealed, 20 and 40% cold worked 2205 duplex stainless steels has been evaluated using electrochemical permeation measurement, hydrogen microprint technique and tensile test in this study. Due to hydrogen transport in 2205 duplex stainless steel is mainly lattice diffusion in ferritic phase, more hydrogen distribution, higher permeation rate and effective diffusion in ferritic phase were detected. Hydrogen trapping and mechanical property effects were studied for cold worked specimens. Fractographic investigation revealed that hydrogen absorption promoted transgranular fracture in cold worked specimens. These results exhibits that the cold worked duplex stainless steels are more susceptible to hydrogen embrittlement.     

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.     

Effect of heat input on microstructure and mechanical properties of dissimilar joints between super duplex stainless steel and high strength low alloy steel

In the present study, microstructure and mechanical properties of UNS S32750 super duplex stainless steel (SDSS)/API X-65 high strength low alloy steel (HSLA) dissimilar joint were investigated. For this purpose, gas tungsten arc welding (GTAW) was used in two different heat inputs: 0.506 and 0.86 kJ/mm. The microstructures investigation with optical microscope, scanning electron microscope and X-ray diffraction showed that an increase in heat input led to a decrease in ferrite percentage, and that detrimental phases were not present. It also indicated that in heat affected zone of HSLA base metal in low heat input, bainite and ferrite phases were created; but in high heat input, perlite and ferrite phases were created. The results of impact tests revealed that the specimen with low heat input exhibited brittle fracture and that with high heat input had a higher strength than the base metals.     

The effect of duplex stainless steel microstructure on its cavitation morphology in seawater

An ultrasonically induced cavitation facility was used to study the effect of a cast duplex stainless steel (DSS) microstructure on its corrosion behavior in seawater. Under cavitation conditions, small cavities initiated in the ferritic matrix and at the ferrite–austenite boundaries. With the progress of cavitation, the attack concentrated in the austenitic phase and then spread to the ferritic phase and was associated with cleavage-like facets, ductile tearing, river patterns and crystallographic steps at later stages. Cross-sections of specimens revealed microcracks initiating from the ferritic matrix at the bottom of cavities. Crack propagation into the matrix was impeded by the austenitic islands.     

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.     

Influence of inclusions and heat treated microstructure on hydrogen assisted fracture properties of aisi 316 stainless steel

An attempt has been made to assess the influence of nonmetallic inclusions and heat treated microstructure on hydrogen assisted cracking of AISI 316 austenitic steel. The steel obtained in two levels of cleanliness was given solution annealing treatment in the temperature range of 1173–1473 K, and additional sensitization treatment of 973 K for 26 hours. Hydrogen embrittlement of this steel has been studied by charging Charpy and Compact Tension specimens of ASTM specification, with hydrogen through cathodic polarization. It is found that hydrogen embrittlement susceptibility increases with the presence of large size inclusions, larger grain and sensitized microstructure.     

Influence of carbon diffusion on microstructure and wear behaviour of duplex stainless steel surface layers on lamellar grey cast iron

Surface welding with duplex stainless steel was performed to enhance the wear and corrosion properties of grey cast iron, which is used as material for applications as pump components in maritime and chemical environments. The method used for surface welding and the corresponding process parameters determine the chemical composition and microstructure, which both determine the corrosion and wear properties of the surface layer. High heat input leads to high chemical dilution and thus, reduced corrosion resistance. Slow cooling rates, which are recommended for welding of grey cast iron components, facilitate the formation of carbides in the fusion zone of the chromium‐rich duplex stainless steel surface layer. On the one hand, carbides lead to increased hardness and thus, improved wear resistance of the surface layers. On the other hand, carbides and high chemical dilution rates reduce the corrosion resistance and therefore should be avoided. Under high cooling rates, the risk of cracking in the heat affected zone of the grey cast iron increases due to martensitic phase transformations. The paper describes the correlation of process parameters, microstructure and chemical composition with a focus on carbon diffusion and carbide formation, ever considering the effect on the wear behaviour in an oscillation tribometer and under erosion‐corrosion conditions.     

Deformation and microstructure evolution of a duplex stainless steel under out-of-phase thermo-mechanical fatigue

Abstract

Thermo-mechanical fatigue (TMF) of the duplex stainless steel SAF2205 (X2CrNiMoN22-5-3) was studied in the temperature range of 100–350°C. The tests were carried out on the duplex steel and on single-phase ferritic (X6Cr17, AISI 430) and austenitic steels (X2CrNiMo18-14-3, AISI 316L) similar to the two phases of the duplex steel for comparison. The mechanical behaviour of the three steels is analysed and discussed together with microstructural investigations by scanning electron microscopy, including electron backscatter diffraction and electron channelling contrast imaging.     

Effect of heat treatment on susceptibility of duplex stainless steel to embrittlement by hydrogen

Abstract

Slow straining of smooth tensile specimens of heat treated duplex stainless steel from a fabricated pipe has been used to assess susceptibility to embrittlement by hydrogen. The effect of the proportions of ferrite and austenite in the microstructure, produced by quenching after solution treatment at temperatures between 1000 and 1300°C, on the ductility was measured. Tests were carried out by either straining in a hydrogen atmosphere or in air after thermal charging in high pressure hydrogen. The measured susceptibility increases proportionately with increase in the amount of ferrite in the structure and reflects the role of austenite in arresting propagating cracks. However, there is little doubt that the amount of austenite presents greater dominance than its orientation in this respect.     

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.     

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.     

Effects of dynamic impact on mechanical properties and microstructure of special stainless steel weldments

Variations in weld metal microstructure after impact at different strain rates for three weld metals, SAF 2507 DSS, SAF 2205 DSS and 254 SMO steel, are addressed. The twin grain forms that appear inside the γ grain of duplex stainless steel at low impact strain rates may be caused by adiabatic heat. The needle-like or thick streak-like twins occur in the γ grain of duplex stainless steel, caused by shear stress at a high strain rate. The impact induces dislocation loop formation in the γ phase of 254 SMO steel at low strain rate, but martensite is produced at a high strain rate. The chemical composition affects the stacking fault energy coupled with adiabatic heat to determine the final structure of the impact.     

Effect of rolling deformation and solution treatment on microstructure and mechanical properties of a cast duplex stainless steel

The present study deals with the effect of rolling deformation and solution treatment on the microstructure and mechanical properties of a cast duplex stainless steel. Cast steel reveals acicular/Widmanst?tten morphology as well as island of austenite within the $\boldsymbol\delta $ -ferrite matrix. Hot rolled samples exhibit the presence of lower volume percent of elongated band of $\boldsymbol\delta $ -ferrite ( $\boldsymbol\sim $ 40%) and austenite phase which convert into finer and fragmented microstructural constituents after 30% cold deformation. By the solution treatment, the elongated and broken crystalline grains recrystallize which leads to the formation of finer grains (<10? $\boldsymbol\mu $ m) of austenite. X-ray diffraction analysis has corroborated well with the above-mentioned microstructural investigation. Enhancement in hardness, yield strength and tensile strength values as well as drop in percent elongation with cold deformation increases its suitability for use in thinner sections. 30% cold rolled and solution treated sample reveals attractive combination of strength and ductility (25·22?GPa%). The examination of fracture surface also substantiates the tensile results. The sub-surface micrographs provide the potential sites for initiation of microvoids.     

Effect of brazing temperature on tensile strength and microstructure for a stainless steel plate-fin structure

This paper presented a vacuum brazing technology for 304 stainless steel plate-fin structures with BNi2 filler metal. The effect of brazing temperature on tensile strength and microstructure has been investigated. The tensile strength is increased along with the increasing of brazing temperature. The microstructure is very complex and some Boride compounds are generated in the brazed joint. Full solid solution can be generated in the middle zone of joint when the brazing temperature is increased to 1100 °C. The brittle phases always exist in the fillet no matter how the brazing temperature changes, but the microstructure in fillet becomes more uniform and the tensile strength is increased with the brazing temperature increasing. In total, the brittle Boride compounds are decreased with the brazing temperature increase. Brazing with a filler metal thickness 105 μm and 25 min holding time, 1100 °C is the best suitable brazing temperature and a tensile strength of 82.1 MPa has been achieved for 304 stainless steel plate-fin structure.