Superaustenitic Steel Research Articles

Effect of solution annealing temperature on the localised corrosion behaviour of a modified super austenitic steel produced in an open-air atmosphere

Two annealing heat treatments were investigated regarding localised corrosion behaviour on Nb and Mn-bearing super austenitic stainless steel. The material was designed based on physical metallurgical principles supported by Thermo-Calc modelling and was produced in an open-air atmosphere based on pre-sorted SS scrap plus ferroalloys elements. The production method aims for metal waste valorisation considering small foundries without an atmosphere-controlled process. Secondary phases at annealing temperatures were experimentally analysed by different microstructural characterisation methods and correlated by the Thermo-Calc modelling. In addition, electrochemical techniques and SKPFM were used to study the relationship between observed phases and localised corrosion performance. As the main results, a good correlation was observed between Thermo-Calc calculations and the microstructural characterisation. In particular, M23C6 carbides, σ phase, and Nb-MX were identified for the sample treated at 1120 °C, while for the sample treated at 1180 °C, σ phase and Nb-MX were identified, no M23C6 were detected. As regards localised corrosion behaviour, the M23C6 carbides and σ phase for sample 1120 °C generated Cr and Mo depletion zones and diminished the corrosion resistance in corrosive aqueous solutions. In the case of sample 1180 °C, the σ-phase with a smaller size and volume fraction than sample 1120 °C was observed, leading to a more uniform Cr and Mo distribution through the microstructure, obtaining high corrosion resistance and showing promising corrosion behaviour, similar to commercial SS 254smo considering its production in open-air.

Improved saline corrosion and hydrogen embrittlement resistances of superaustenitic stainless steel by PIII nitriding

PIII nitriding was applied to superaustenitic steel to improve wear resistance without compromising pitting and hydrogen embrittlement corrosion resistance. Nitriding at temperatures between 300 °C and 400 °C produced modified layers rich in γN phase and with thicknesses ranging from 1.2 ± 0.1 μm to 4.2 ± 0.1 μm. Nanoindentation tests indicated an increase in hardness up to ∼4 times compared to the untreated sample. The potentiodynamic polarization curves indicated an increase in pitting and crevice corrosion resistance after nitriding. In the PIII 350 °C nitrided sample, Ecorr and Epit increased 50% and 14%, respectively, while current densities icorr and ip decreased by 80% and 77% and crevice volume decreased by 93% when compared to the untreated sample. However, in the nitriding condition at 400 °C, the improvement the resistance to saline corrosion is not so marked was observed as a result of the increased passive current density due to formation of CrN The micrographs of surfaces after cathodic hydrogenation also indicated reduction in hydrogen embrittlement in the PIII 300 °C sample. In the PIII 400 °C sample, the hydrogen embrittlement was identified by the delamination and cracking of the modified layer. Thus, the formation of chromium nitrides must be avoided and only the γN phase must be formed to improve the resistance to hydrogen embrittlement and saline corrosion.

Characterization of recycled AISI 904L superaustenitic steel powder and influence on selective laser melted parts

In laser powder-bed fusion processes, excessive metal powder is affected by high laser energy input and contains spatter from the melt pool. Recycled powder therefore can have a negative impact on process quality. In this study, AISI 904L stainless steel powder is recycled up to ten times. The powder is analyzed using scanning electron microscopy and by measuring chemical composition, flowability and particle size distribution in order to examine changes in properties due to recycling. Furthermore, electrical impedance spectroscopy was introduced as a potential method to determine surface oxides on powder particles due to handling and recycling.The recycled powder shows a significant increase in flowability. Furthermore no significant changes of powder properties such as chemical composition and particle size distribution can be found after ten recycling steps, resulting in comparable density and mechanical properties of printed parts. Therefore the present powder material shows an excellent recyclability.

The effect of dwell on thermomechanical fatigue in superaustenitic steel Sanicro 25

AbstractSuperaustenitic steel Sanicro 25 has been subjected to out‐of‐phase thermomechanical fatigue cycles with 10‐min dwell at peak temperature in the temperature range from 250°C to 700°C. The effect of the dwells on the cyclic response, internal structure and damage mechanism was studied. Cyclic hardening/softening curves, cyclic stress–strain curves and fatigue life curves were evaluated. The internal structure and nanoparticles representing the obstacles for dislocation motion were analysed and identified by energy dispersive X‐ray spectroscopy in scanning transmission electron microscope. Scanning electron microscopy combined with focused ion beam and electron backscatter diffraction was adopted to reveal the respective mechanisms responsible for fatigue crack nucleation and propagation. Effect of dwells in in‐phase and in out‐of‐phase cycling on the fatigue behaviour, on the modification of internal structure and damage mechanisms, were analysed and discussed.

Cyclic plastic response and damage mechanisms in superaustenitic steel Sanicro 25 in high temperature cycling – Effect of tensile dwells and thermomechanical cycling

The cyclic plastic response and damage evolution in isothermal high temperature constant strain rate cycling and during in-phase thermomechanical fatigue in superaustenitic Sanicro 25 steel have been studied both with and without introduction of dwells at maximum strain. Fatigue hardening/softening curves and fatigue life curves are reported for all types of fatigue tests. Rapid hardening and a tendency to saturation has been found primarily in isothermal cycling with dwells and in thermomechanical cycling. Study of the surface damage evolution using SEM observations and FIB cutting revealed the preferential oxidation of grain boundaries perpendicular to the stress axis. Introduction of dwells in maximum tension leads to the enlargement of the plastic strain amplitude and to additional creep damage in the form of cavities along grain boundaries and internal cracks. Fatigue hardening rate in thermomechanical cycling is higher than in constant strain rate cycling and fatigue life decreases substantially in in-phase thermomechanical cycling. These findings are discussed in the perspective of effectiveness of dominant damage mechanisms relevant to individual types of cyclic straining.

Corrosion Resistance of New Duplex Stainless Steels in Coke Production

The corrosion resistance of SAF 2507 duplex stainless steel and 254 SMO superaustenitic steel is studied in aggressive sulfuric-acid solutions (mother liquor) from sulfate departments; in aggressive media from arsenic–soda desulfurization; and in coke-shop media (from the quenching car and quenching tower). Gravimetric tests are conducted in accordance with State Standard GOST 9.908–85. The research is conducted both at coke plants and in the laboratory. It follows from gravimetric tests and cyclic volt–ampere measurements that these materials are highly resistant to point and pitting corrosion in the aggressive media considered. Such corrosion processes are provoked by the presence of activators (H2S, HCN, HCNS, and Cl–). These findings are confirmed by the considerable life of passive films of molybdenum-bearing steel. Research shows that SAF 2507 (#Kh25H7AM4#) duplex steel and Avesta 254S MO superaustenitic steel are highly resistant to point and slot corrosion and outperform the ordinary #0Kh23N28M3D3T, 10Kh17N13M2T,# and #12Kh18M10T# steels used for heat-exchanger production.

Cold Deformation and Hardness on Superaustenitic Stainless Steel: Evaluation Methods

Superaustenitic stainless steel with high nickel percentage, chromium, molybdenum and nitrogen, has replaced austenitic AISI 304 and 316, mainly in the petrochemical industries, where high toughness, stress corrosion resistance and pitting resistance is suitable. Due to high mechanical stress in the cold deformation manufacturing, residual stresses in the pipes are common. In this case, the possibility of the corrosion process increases. Considering the area reduction after tensile tests, the objective of this study was to evaluate the effects of cold deformation on a superaustenitic steel BS EN 10283 No 14587. The steel was elaborated in an electric induction furnace and the liquid metal was poured out in sand molds. Afterward, samples cut from a specimen submitted to the tensile test were prepared for X-ray diffraction, Vickers hardness measurements, linear polarization and impedance electrochemical tests. The corrosion tests were performed in solution with 3.5% NaCl. The results show that there is a linear increase in hardness and also a decrease in corrosion resistance of the material, with increasing cold deformation. In addition, the cold work reduction was not sufficient to promote strain-induced martensite.

Multi-objective Optimization of Welding Parameters of Pulsed Current Micro Plasma Arc Welded AISI 904L Super Austenitic Steel

In the present work, AISI 904L super austentic steel sheets of 0.4mm thick is butt welded using Micro Plasma Arc Welding. Welding input parameters like peak current, base current, pulse rate and pulse width are considered and output responses like fusion zone grain size, hardness and ultimate tensile strength of the welded joint are considered. 31 experiments are performed as per Central Composite Design (CCD) design matrix of Response Surface Method (RSM) by considering four factors and five levels of weld input parameters. Grey Relational Analysis (GRA) is carried out by minimizing fusion zone grain size and maximizing fusion zone hardness and ultimate tensile strength to find the optimal combination of weld input parameters. The order of importance of weld input parameters are also identified and improvement in Grey Relational Grade was found.

Cyclic plastic response and damage in superaustenitic steel in high temperature cycling with dwells and in thermomechanical cycling

The cyclic plastic response and damage evolution in high temperature cycling with dwells and during thermomechanical fatigue of superaustenitic steel Sanicro 25 has been studied. Fatigue hardening/softening curves in cycling with dwells have been compared with those in constant strain rate cycling. Rapid hardening and early saturation has been found in cycling with dwells. Study of the surface evolution using SEM observations and FIB cutting revealed the preferential oxidation of grain boundaries which were perpendicular to the stress axis. Introduction of dwells in maximum tension lead to the opening of the early cracks, their sliding and formation wedge cracks. Fatigue hardening/softening curves in thermomechanical in-phase cycling do not exhibit saturation and the hardening is higher than in constant strain rate cycling. Damage mechanism was similar to that in isothermal cycling and even more damaging as found by plotting Manson-Coffin curves.

Effect of an environment in stainless steel on the damage mechanisms under LCF conditions

The effect of an environment on the damage mechanism was addressed in this study. Super austenitic steel Sanicro 25 grade was subjected to isothermal elasto-plastic cyclic loading at 700°C. The material was exposed to different environmental conditions to assess the effect of laboratory air comparing to high vacuum. The mechanism of the crack nucleation was the matter of the inquiry by means of scanning electron microscope equipped with focused ion beam technique. Distinctive mechanism of the crack nucleation was observed for different loading conditions. The isothermal fatigue loading in the air resulted in the early initiation of the cracks from the oxidized grain boundaries arranged perpendicularly to the loading axis. The significant change was revealed for the specimens strained under high vacuum conditions. The crack was found to be nucleated due to cavity formation and their subsequent mutual coalescence during further loading. Moreover, the crack formation was observed in the interior of the material. The effect of the environmentally related damage mechanism is documented.

MATERIALS EVALUATION FOR GEOTHERMAL APPLICATIONS

In order to provide basic information on corrosion resistance to the designers and users of geothermal plants different metallic materials including duplex and austenitic stainless steels as well as a nickel alloy have been evaluated in artificial geothermal fluids simulating the conditions in some locations with geothermal potential in Germany as well as two sites in Indonesia. By electrochemical and long-term exposure tests at 100 °C and 150 °C the suitability of low alloyed steel UNS G41300, stainless steels UNS S31603 UNS S31803, UNS S32760, super austenitic steel UNS N08031 and nickel based alloy UNS N06059 was investigated in these geothermal fluids, using critical potentials and corrosion rates. In high-saline environments the crevice corrosion turned out to be the determining mechanism. The nickel based alloy shows excellent corrosion resistance against pitting corrosion. Excluding its high cost, it is very good to be used in the construction of geothermal facilities having highly saline brines. Stainless and duplex steels exhibit a limited corrosion resistance concerning pitting and crevice corrosion. Therefore they are not suitable for highly saline brines. The super austenite UNS N08031 showed a temperature depending behavior. In non-saline environments the low-alloyed steel UNS G41300 (beside of the higher alloyed materials) could be employed as a constructional material for the geothermal power plant, as long as a sufficient wall thickness of the material is considered.

On the Stacking Fault Energy Evaluation and Deformation Mechanism of Sanicro-28 Super-Austenitic Stainless Steel

In the present study, the reliance of deformation mechanism of a super-austenitic steel (Sanicro-28) on the external stress and stacking fault energy (SFE) was quantitatively investigated by analyzing the stacking fault separation. The stacking fault energy of the experimental alloy was determined using line profile analysis through x-ray diffraction measurements. Considering the calculated SFE and external stress, the predominant deformation mechanism was predicted by a quantitative model. A set of experimental examinations were carried out validating the applied model. The experimental findings reveal that the plasticity mechanism of steel can be divided into two stages. In the first stage (at lower strain), no mechanical twin was observed in the microstructure and the dislocation glide would control the plasticity. In the second stage (at higher strain), the mechanical twinning was considered as the predominant plasticity mechanism. Furthermore, regarding the stress threshold of mechanical twin formation, as the SFE increases, the critical stress for mechanical twinning initiation intensifies. However, in the present super-austenitic stainless steel, the mechanical twinning was observed at the stresses lower than those predicted by the applied model. This was related to the various interactions between dislocation and different barriers in the solid solution matrix.

URANUS 926

Abstract Uranus 926 is a super-austenitic steel with very high nickel, molybdenum, and chromium contents. This datasheet provides information on composition, physical properties, hardness, elasticity, tensile properties, and shear strength as well as fracture toughness. It also includes information on corrosion resistance as well as forming, heat treating, machining, and joining. Filing Code: SS-1194. Producer or source: Industeel USA, LLC.

URANUS 31

Abstract Uranus 31 is a super-austenitic steel with very high nickel, chromium, and molybdenum content. This datasheet provides information on composition, physical properties, hardness, elasticity, and tensile properties as well as fracture toughness. It also includes information on corrosion resistance as well as forming, heat treating, and joining. Filing Code: SS-1191. Producer or source: Industeel USA, LLC.

URANUS 28

Abstract Uranus 28 is a super-austenitic steel with very high nickel and chromium contents. This datasheet provides information on composition, physical properties, hardness, elasticity, and shear strength as well as fracture toughness. It also includes information on corrosion resistance as well as forming, heat treating, machining, and joining. Filing Code: SS-1187. Producer or source: Industeel USA, LLC.

New generation super alloy candidates for medical applications: Corrosion behavior, cation release and biological evaluation

Three super alloy candidates (X1 CrNiMoMnW 24-22-6-3-2 N, NiCr21 MoNbFe 8-3-5 AlTi, CoNiCr 35-20 Mo 10 BTi) for a prolonged contact with skin are evaluated in comparison with two reference austenitic stainless steels 316L and 904L. Several electrochemical parameters were measured and determined (Eoc, Ecorr, icorr, ba, bc, Eb, Rp, Ecrev and coulometric analysis) in order to compare the corrosion behavior. The cation release evaluation and in vitro biological characterization also were performed.In terms of corrosion, the results reveal that the 904L steels presented the best behavior followed by the super austenitic steel X1 CrNiMoMnW 24-22-6-3-2 N. For the other two super alloys (NiCr and CoNiCr types alloys) tested in different conditions (annealed, work hardened and work hardened+age hardened) it was found that their behavior to corrosion was weak and close to the other reference stainless steel, 316L. Regarding the extraction a mixture of cations in relatively high concentrations was noted and therefore a cocktail effect was not excluded. The results obtained in the biological assays WST-1 and TNF-alpha were in correlation with the corrosion and extraction evaluation.

Influence of sinter-cooling rate on intergranular corrosion of powder metallurgy superaustenitic stainless steel

Powder metallurgy superaustenitic stainless steel sintered in N2–H2 atmosphere (95%–5%) was obtained. Three different processes were applied by controlling the sinter-cooling rate (furnace, gas and water). Intergranular corrosion resistance was evaluated. Furthermore, microstructure, density, porosity, macro- and microhardness were investigated. Microstructures were characterised by optical microscopy and scanning electron microscopy with energy dispersive analysis of X-rays. Intergranular corrosion behaviour was evaluated by immersion and by single and double loop electrochemical potentio-kinetic reactivation tests. The superaustenitic steel after fast cooling by water quenching showed the best intergranular corrosion behaviour, the highest microhardness values and the most homogeneous microstructure.

The influence of the surface finishing on the electrochemical behaviour of austenitic and superaustenitic steels

AbstractThe influence of surface roughness on the pitting potential and the corrosion potential of the DIN W.Nr. 1.4541 and DIN W.Nr 1.4465 stainless steels were examined using electrochemical techniques. Roughness and electrochemical tests were performed on samples with various surface finishing. Electrochemical studies were conducted in an electrochemical cell where steel electrodes were immersed in 0.1 M NaCl and 0.1 M NaCl + 0.1 M Na2SO4 aqueous solutions. We found for both materials when immersed in 0.1 M NaCl aqueous solution lower pitting potential than in 0.1 M NaCl + 0.1 M Na2SO4. The results showed that the electrochemical behaviour of the DIN W.Nr. 1.4465 is better than of DIN W.Nr. 1.4541.

Application of Ab Initio Results in Modeling Phase Diagrams Containing Complex Phases

Ab initio electronic structure theory has achieved considerable reliability concerning predictions of physical and chemical properties and phenomena. It provides understanding of matter at the atomic and electronic scale with an unprecedented level of details and accuracy. In the present contribution, the electronic structure theory and state-of-the-art ab initio calculation methods in solids are briefly reviewed and the application of the calculated total energy differences between various phases (lattice stabilities) is illustrated on construction of phase diagrams by the CALPHAD (CALculation of PHAse Diagrams) method in systems containing phases with complex structures, as e.g. sigma phase. Particular examples include description of the sigma phase in the Fe-Cr system and prediction of the phase composition of super-austenitic steels. It is shown that the utilization of ab initio results introduces a solid basis of the energetics of systems with complex phases, allows to avoid unreliable estimates and extrapolations of Gibbs energies and brings more physics into the CALPHAD method.

Use of Alloy 59 for the Transport of Highly Corrosive Dangerous Goods

Alloy 59 (NiCr23Mo16Al) with a lot of chromium, molybdenum and nickel possesses excellent resistance not only to reducing but also oxidizing chemicals. Both the Nickel alloy 59 and the superaustenitic steel alloy 31 have already been used as shell materials for tank vehicles or tank containers. Use of these alloys allows the transport of a signifi-cantly more wider variety of chemicals and, especially, waste mixtures than the use of common aus-tenitic steels. Another advantage is the extension of test intervals of for transport tanks. In Germany the “BAM-List – Requirements for Tanks for the Transport of Dangerous Goods” is the basis for substance-related prototype approvals for tank containers designed for the car¬ri¬age of dangerous goods issued by the Federal Institute for Materials Re¬search and Testing (BAM). Com-patibility evaluations of selected metallic material groups as well as polymeric gasket and lining materials under the influence of approximately 7000 dangerous goods and water-polluting sub-stances are published in the BAM-List. Alloy 59 belongs to the group of metallic materials in the BAM-List. Due to the large number of dangerous goods in the BAM-List BAM, IKS Dresden and ThyssenKrupp VDM performed a comprehensive corrosion test programme with welded specimens of the nickel alloy 59 and the superaustenitic steels alloy 926 and alloy 31 in the period 2002 - 2010. Especially In particular alloy 59 and alloy 31 were exposed to a large number of corrosive sub-stances such as various mixtures of both nitric acid/sulphuric acid and nitric acid/phosphoric acid at 55 °C. Other corrosive test substances were different organic and inorganic halogenides, peroxyace-tic acid and molten substances. In the case of molten chemicals such as monochloroacetic acid the test temperature was increased to more than 100 °C. The test results presented in this paper are al-ready included in the 10th edition of the BAM-List and, therefore, available to the customer.