Investigation of stress corrosion cracks in a UNS S32750 superduplex stainless steel

Duplex and superduplex stainless steels are corrosion resistant alloys with many uses in chemical and petrochemical industries. It is generally accepted that these alloys present stress corrosion resistance superior to austenitic grades, but it does not mean that they are immune to this type of failure. Under severe conditions of temperature, stress, low pH, high chloride and H₂S contents superduplex steels may fail environmentally assisted cracking (EAC). In this work, superduplex UNS S32750 steel specimens were subjected to critical environmental conditions which produced stress corrosion cracks. In the first experiment the material was tested by slow strain rate tensile tests at 80 °C in a solution with 115,000 ppm of chloride, H₂S partial pressure of 6.75 psia, and pH = 3.0. In a second experiment the material was subjected to four bend beam test in a solution similar to experiment 1, but with a H₂S partial pressure of 30.0 psia. Finally, a third test was conducted in a bend plate of superduplex steel subjected to MgCl₂ saturated solution at 154 °C. The cracks produced in the three experiments showed quite different features, which were investigated by optical and scanning electron microscopy.     

Modeling of deleterious phase precipitation during isothermal treatments in superduplex stainless steel

The deleterious constituents σ, χ, and eutectoid γ₂ can precipitate during hot forming and welding operations in superduplex stainless steels. In this study the precipitation of these phases in superduplex UNS S32750 was investigated. Two materials with similar composition and quite different grain sizes were used in the study. Specimens were subjected to isothermal treatments in the 800–950 °C temperature range for time periods up to 240 min. Under these conditions, quantifications of deleterious phases by light optical microscopy were carried out in order to model the precipitation kinetics of these deleterious constituents. A sigmoidal equation, similar to the one developed by Johnson–Mehl–Avrami was fitted to all the analyzed temperatures. Precipitation velocity was evaluated so as to compare the kinetics of deleterious phases in the two materials. The activation energies for precipitation were calculated based on Arrhenius equation.     

Annealing temperature effects on superduplex stainless steel UNS S32750 welded joints. II: pitting corrosion resistance evaluation

This study deals with the effect of the annealing temperature on the pitting corrosion resistance of UNS S32750 submerged-arc welded joints. In a companion article (Part I), the influence of post-weld annealing temperature on microstructure evolution and chemical composition of austenite and ferrite was analyzed; this study can thus be considered directly connected with the previous one. The pitting corrosion resistance of the heat-treated welded joints was evaluated by using both electrochemical measurements and ASTM G48 standard gravimetric tests; examinations of initiation sites of pitting attack were carried out in order to correlate the experimental data obtained in this study with the predicted pitting corrosion behavior obtained by using the results described in Part I. Generally, the post-weld annealing treatment enhances the pitting corrosion resistance of UNS S32750 welded joints. By using PREN analysis of single phases, a correlation between the chemical composition evolution of ferrite and austenite and the experimental pitting behavior of the welded joints was found, in relation to welding and post-welding heat treatment temperature. In particular, an exponential relationship between PREN of weaker phase and pitting potential in 3.5% NaCl solution at 80 °C for the weld metal was obtained. The most favorable annealing temperature for the analyzed welded joints was found to be 1100 °C.     

Characterization of microstructure,chemical composition,corrosion resistance and toughness of a multipass weld joint of superduplex stainless steel UNS S32750

The superduplex stainless steels have an austeno-ferritic microstructure with an average fraction of each phase of approximately 50%. This duplex microstructure improves simultaneously the mechanical properties and corrosion resistance. Welding of these steels is often a critical operation. In this paper we focus on characterization and analysis of a multipass weld joint of UNS S32750 steel prepared using welding conditions equal to industrial standards. The toughness and corrosion resistance properties of the base metal, root pass welded with gas tungsten arc welding, as well as the filler passes, welded with shielded metal arc welding, were evaluated. The microstructure and chemical composition of the selected areas were also determined and correlated to the corrosion and mechanical properties. The root pass was welded with low nickel filler metal and, as a consequence, presented low austenite content and significant precipitation. This precipitation is reflected in the corrosion and mechanical properties. The filler passes presented an adequate ferrite:austenite proportion but, due to their high oxygen content, the toughness was lower than that of the root pass. Corrosion properties were evaluated by cyclic polarization tests in 3.5% NaCl and H₂SO₄ media.     

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.     

S32750双相不锈钢相界与晶界特征对其力学性能和耐蚀性能的影响EI北大核心CSCD

通过固溶处理获得不同初始组织状态的S32750双相不锈钢样品,然后进行厚度压下量80%的冷轧变形和1050℃的退火处理,采用SEM-EBSD和XRD技术研究合金相界与晶界特征以及相组成分布情况,并利用拉伸实验、纳米压痕和双环电化学动电位再活化法(DL-EPR)分析不同初始状态样品的组织对力学性能与耐晶间腐蚀性能的影响规律。结果表明:高温固溶处理的合金样品经冷轧退火后晶粒细小均匀,两相分布接近1∶1,且相界占内界面(晶界+相界)比例较高,同相晶粒团簇程度最低,表现出优异的综合力学性能。合金样品经敏化处理后,σ相易沿α相晶界析出,高温固溶并经轧制退火后的样品中,由于α晶界比例较少且满足K-S取向关系的相界比例较高则又表现出良好的晶间腐蚀抗力。因此,通过适当的工艺来调控合金的相界与晶界分布可以实现材料强度和晶间腐蚀抗力的同步改善。     

Effects of cooling regimes on the microstructural and mechanical properties of the transient liquid phase joints of UNS S32750 super duplex stainless steel/BNi-2/AISI 304 stainless steel

Journal of Materials Science - UNS S32750 super duplex stainless steel and AISI 304 stainless steel were transient liquid phase (TLP) bonded using BNi-2 at a bonding temperature of...     

Characterization of weld strength and impact toughness in the multi-pass welding of super-duplex stainless steel UNS 32750

This paper investigates the weldability, metallurgical and mechanical properties of the UNS 32750 super-duplex stainless steels joints by Gas Tungsten Arc Welding (GTAW) employing ER2553 and ERNiCrMo-4 filler metals. Impact and tensile studies envisaged that the weldments employing ER2553 exhibited superior mechanical properties compared to ERNiCrMo-4 weldments. Microstructure studies performed using optical and SEM analysis clearly exhibited the different forms of austenite including widmanstatten austenite on the weld zone employing ER2553 filler. Also the presented results clearly reported the effect of filler metals on strength and toughness during the multi-pass welding. This research article addressed the improvement of tensile and impact strength using appropriate filler wire without obtaining any deleterious phases.     

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.     

Annealing temperature effects on super duplex stainless steel UNS s32750 welded joints. I: microstructure and partitioning of elements

Welding of austenitic-ferritic stainless steels is a crucial operation and all the materials and parameters used in this process must be optimized in order to obtain the suitable corrosion and mechanical properties. Since a great part of super duplex stainless steels is used in very aggressive environment, their corrosion resistance, referred in particular to pitting and crevice corrosion, is an all-important facet in production and processing of this type of steels. Pitting corrosion resistance of super duplex stainless steels welded joints depends on several aspects: microstructure of the bead, elemental partitioning between ferrite and austenite, and the possible presence of secondary phases. For these reasons, a post-weld annealing is generally performed to homogenize the microstructure. The annealing temperature is the most important parameter to be optimized in this heat treatment. In the present work, a comparison between the as-welded and solution-treated joints is carried out. An effort has been made to correlate the main factors that affect pitting corrosion of the welded joints (microstructure, secondary phases, chemical composition of single phases) with the experimental data obtained from corrosion tests. In this first part of the work the results regarding microstructure and partitioning of elements are presented. The phase balance and the austenite morphology are locally upset during submerged-arc welding of UNS S32750. In the fusion zone, the two phases (ferrite and austenite) result to have approximately the same composition regarding Cr, Mo, and Ni content, while nitrogen is heavily concentrated in austenite. After annealing treatment, the austenite volume fraction increases and the partitioning ratios of elements reach the equilibrium values. The base material results to be less sensitive to annealing treatment than the fusion zone, and the partitioning of elements in the base material is in agreement with previous works reported in the literature.     

Quantitative aspects of precipitation at grain boundaries in an austenitic stainless steel

This paper shows, in a quantitative manner, how the precipitation of niobium carbide in an austenitic stainless steel is affected by varying the amount of deformation prior to ageing. In particular, the extrinsic dislocation content of grain boundaries is shown to govern the overall size distribution of grain-boundary precipitates developed during ageing.     

Spinodal decomposition mechanism study on the duplex stainless steel UNS S31803 using ultrasonic speed measurements

This work, focuses on the spinodal decomposition mechanism study on the duplex stainless steel duplex UNS S31803, composed by austenite (γ) and ferrite (α) phases, at 425 °C and 475 °C temperatures by ultrasonic speed measurements. This temperature range is responsible for the transformation mechanism of α_initial phase to α phases (poor in chromium) and α′ (rich in chromium) by spinodal decomposition. The techniques to accomplish this analysis are based mainly on X-ray diffraction measures and ultrasonic speed. The obtained results show that it is possible to conclude that the use of ultrasonic speed measurements indicates a promising technique for following-up the phase transformation and spinodal decomposition on the steel studied.     

Effect of pre-strain on grain size distributions in 316H austenitic stainless steel

An experimental study addressing the effect of tensile deformation on recrystallized grain size has been undertaken to explore the conditions leading to abnormal grain growth in Type 316H austenitic stainless steel. Following a solution heat treatment, a Type 316H stainless steel has been subjected to various tensile deformations up to a maximum of approximately 50% strain and then heated at a temperature of 1150 °C for 0.5 h followed by furnace cooling. A fraction of abnormally large grains is observed following a prior strain of approximately 20%. The results are presented, in terms of standard statistical analysis, and also graphically. The graphical presentation provides a clear, visual appreciation of uni- and bi-modal distributions, which may be of general help in other analyses of this nature.     

The precipitation of niobium carbide at grain boundaries in an austenitic stainless steel

Electron microscopy has been used to show that the precipitation of niobium carbide at the grain boundaries of an austenitic stainless steel can occur in a spatially non-random fashion. Preferential nucleation and growth of carbides occurs on grain-boundary defect structures. Where present, these defect structures include extrinsic grain-boundary dislocations and topographical discontinuities. An example of the precipitation of niobium carbide on an intrinsic dislocation array is also shown.     

The influence of grain size on the ductility of micro-scale stainless steel stent struts

Vascular stents are used to restore blood flow in stenotic arteries, and at present the implantation of a stent is the preferred revascularisation method for treating coronary artery disease, as the introduction of drug eluting stents (DESs) has lead to a significant improvement in the clinical outcome of coronary stenting. However the mechanical limits of stents are being tested when they are deployed in severe cases. In this study we aimed to show (by a combination of experimental tests and crystal plasticity finite element models) that the ductility of stainless steel stent struts can be increased by optimising the grain structure within micro-scale stainless steel stent struts. The results of the study show that within the specimen size range 55 to 190 μ m ductility was not dependent on the size of the stent strut when the grain size maximised. For values of the ratio of cross sectional area to characteristic grain length less than 1000, ductility was at a minimum irrespective of specimen size. However, when the ratio of cross sectional area to characteristic grain length becomes greater than 1000 an improvement in ductility occurs, reaching a plateau when the ratio approaches a value characteristic of bulk material properties. In conclusion the ductility of micro-scale stainless steel stent struts is sensitive to microstructure and can be improved by reducing the grain size.     

Effects of grain size on the properties of a low nickel austenitic stainless steel

The effect of the grain size (varying in the range of 2.5–50 m) on the mechanical properties and on the wear and corrosion resistance of a low nickel austenitic stainless steel is reviewed. In particular, the austenite-martensite transformation followed by annealing for martensite reversion in high nitrogen stainless steel is investigated. In order to study the effect of this thermo-mechanical process on grain refinement, the effect of cold reduction, annealing temperature and annealing times were analysed. After obtaining ultrafine grains, the effect of the grain size on the hardness and the tensile properties was evaluated and showed a Petch-Hall dependency in the fully analysed range (down to a 2.5 m grain size).The fatigue behaviour of the steel is studied as a function of the grain size showing a poor influence of grain refining on the fatigue resistance. An increase of both the wear resistance and of the localized corrosion resistance with grain refining is also detected. Results are compared to those of similar measurements on a standard AISI 304 steel.     

Influence of chloride ion concentration and temperature on the electrochemical properties of passive films formed on a superduplex stainless steel

Polarization measurements were conducted to monitor the corrosion behavior of superduplex stainless steel ASTM A995M-Gr.5A/EN 10283-Mat#1.4469(GX2CrNiMo26-7-4) when exposed to a) an electrolyte containing 22,700 parts per million (ppm) of chloride ions at seven different temperatures and b) an electrolyte at 25 °C and different chloride ion concentrations (5800, 22,700, 58,000 and 80,000 ppm of Cl^?). The polarization curves indicate that the passive films formed are only slightly affected by NaCl concentration, but the pitting potential decreases drastically increasing the temperature, in particular > 60 °C. The image analysis of the microstructure after potentiodynamic polarization showed that the pitting number and size vary in function of the temperature of the tested medium. Nyquist diagrams were determined by electrochemical impedance spectroscopy to characterize the resistance of the passive layer. According to Nyquist plots, the arc polarization resistance decreases increasing the temperature due to a catalytic degradation of the oxide passive films.     

Predicting recrystallized grain size in friction stir processed 304L stainless steel

A major dilemma faced in the nuclear industry is repair of stainless steel reactor components that have been exposed to neutron irradiation. When conventional fusion welding is used for repair, intergranular cracks develop in the heat-affected zone(HAZ). Friction stir processing(FSP), which operates at much lower peak temperatures than fusion welding, was studied as a crack repair method for irradiated 304 L stainless steel. A numerical simulation of the FSP process in 304 L was developed to predict temperatures and recrystallized grain size in the stir zone. The model employed an Eulerian finite element approach,where flow stresses for a large range of strain rates and temperatures inherent in FSP were used as input. Temperature predictions in three locations near the stir zone were accurate to within 4%, while prediction of welding power was accurate to within 5% of experimental measurements. The predicted recrystallized grain sizes ranged from 7.6 to 10.6 μm, while the experimentally measured grains sizes in the same locations ranged from 6.0 to 7.6 μm. The maximum error in predicted recrystallized grain size was about 39%, but the associated stir zone hardness from the predicted grain sizes was only different from the experiment by about 10%.