Lean Duplex Stainless Steel UNS Research Articles

Microstructure and Properties of Lean Duplex Stainless Steel UNS S32101 Welded Joint by Hot Wire TIG Welding

Lean duplex stainless steel UNS S32101 was welded by hot wire TIG welding and traditional TIG welding, and nice formed welds with no visible defects were obtained. Metallographic microstructure, phase ratio, mechanical properties and pitting corrosion resistance property of the welded joints were tested. Microstructure analysis showed that the hot wire TIG and traditional TIG welded joints had similar microstructures. The welded metal was composed of ferrite, grain boundary austenite (GBA), Widmanstatten austenite (WA), intragranular austenite (IGA). The high temperature heat affected zone (HTHAZ) consisted of ferrite, GBA and IGA. The low temperature heat affected zone (LTHAZ) had semblable microstructures with base metal. The phase ratio of welded joints was measured by manual point count method. The ferrite/austenite ratio of hot wire TIG welded metal was close to 1:1. The welded joints of hot wire TIG and traditional TIG had same hardness distribution. The hardness of hot wire TIG with an average value of 291 HV10 was a little higher than that of traditional TIG with an average value of 280 HV10. Charpy impact test at -40°C showed that the impact values of hot wire TIG and traditional TIG welded joints meet the standard requirements. The results of chemical weight loss method showed that the corrosion rate of hot wire TIG welded joint was less than 10 mdd. Potentiodynamic polarization method results showed that the pitting corrosion resistance of hot wire TIG welded joints was slightly lower than that of base metal. Solid solution treatment significantly increased the pitting corrosion resistance of welded joints and base metal. The hot wire TIG and traditional TIG had similar microstructure and properties under the same arc power, however the welding speed of hot wire TIG was 1.5 times higher than that of traditional TIG and the welding efficiency was greatly improved.

Microstructural and Corrosion Resistance of Lean Duplex Stainless Steel UNS S32304 Welded by SAW with Cold Wire Addition

The results of the corrosion tests, in chloride environment, of lean duplex stainless steel UNS S32304 welded by submerged arc welding (SAW) process variant with cold wire, indicated that the sample with higher heat input heat input (HI) (2.7 kJ/mm) had higher corrosion resistance, which can be explained by higher amount of austenite and lower amount of precipitates. Therefore, the results indicate that the use of HI above the limit recommended by literature (2.5 kJ/mm) may produce welds with adequate corrosion resistance with SAW with cold wire addition, because this technique has the potential to deliver higher productivity still controlling the process HI.

Effects of Isothermal Aging on the Microstructure Evolution and Pitting Corrosion Resistance of Lean Duplex Stainless Steel UNS S32003

UNS S32003 materials were aged at temperatures in the range of 873 K to 1173 K (600 °C to 900 °C) for 30 minutes and 2 hours. Nitrides were found to precipitate along the ferrite/austenite phase boundaries and within the ferrite phase. The precipitation kinetics was the fastest at 1073 K (800 °C), at which temperature the Cr-depleted zones were observed to be in the vicinity of the precipitates. Precipitation and resultant Cr-depleted zone led to the highest pitting susceptibility of the UNS S32003, aged at 1073 K (800 °C), when tested in 0.6 mol/L NaCl solution. To observe the precipitation evolution, the UNS S32003 specimens were aged at 973 K (700 °C) for different time periods for up to 120 hours. Within aging time up to 24 hours at 973 K (700 °C), only nitrides precipitates were observed and an intermetallic phase was observed after 120 hours of aging along with an increase in the percentage of precipitates. Pitting potential dropped with increasing aging time and metastable pitting rate increased with longer aging time period, both indicating a negative effect of aging on pitting corrosion resistance of the lean duplex stainless steel UNS S32003.

Multicomponent Phase Diagram of Lean Duplex Stainless Steel UNS S82441 and Its Application to Evaluate the Microstructure in the Heat Affected Zone

Duplex stainless steels are Fe-Cr-Ni-Mo-N alloys with nitrogen replacing some nickel to improve its properties. These steels have a balanced microstructure, which means that ferrite (α) and austenite (γ) are in equal proportions of approximately 50% by volume. These alloys have a tendency to form intermetallic precipitates such as chromium nitrides and sigma phase (σ), depending on their chemical composition and welding thermal history. Precipitation of these intermetallic phases can impair mechanical and corrosion properties. In addition, material processes such as welding and heat treatment can change the balanced microstructure. The objective of this work was to compare phases formed during heat treatment and welding of duplex stainless steel UNS S82441 with the equilibrium phases predicted by Thermo-calc® software. In general, the results showed good agreement between predicted and measured phases, as well as, agreement between the measured volume fraction of austenite in the heat-affected zone and austenite predicted by Thermo-calc® software.

Corrosion studies using potentiodynamic and EIS electrochemical techniques of welded lean duplex stainless steel UNS S82441

The corrosion characterisation of lean duplex stainless steel (1.4662) UNS S82441 welded joints using the potentiodynamic test and electrochemical impedance spectroscopy in 1M NaCl solution are discussed. The influence of autogenous TIG welding parameters (amount of heat input and composition of shielding gases like Ar and Ar–N2 and an Ar–He mixture), as well as A-TIG welding was studied. The influence of welding parameters on phase balance, microstructural changes and the protective properties of passive oxide films formed at the open circuit potential or during the anodic polarisation were studied.From the results of the potentiodynamic test and electrochemical impedance spectroscopy of TIG and A-TiG, welded joints show a lower corrosion resistance compared to non-welded parent metal, but introducing heat input properly during welding and applying shielding gases rich in nitrogen or helium can increase austenitic phase content, which is beneficial for corrosion resistance, and improves surface oxide layer resistance in 1M NaCl solution.

Repassivation Behaviour of UNS S32101 and UNS S30403 Stainless Steels after Cathodic Stripping of the Native Passive Film in a CO<sub>2</sub>-Saturated Oilfield Brine

Repassivation behaviour of the passive film formed on lean duplex stainless steel UNS S32101 and austenitic stainless steel UNS S30403 in a CO2-saturated oilfield environment has been studied. The native passive film on the alloys was thinned/removed by stepping the potential of the alloy to ﹣850 mV/Ag/AgCl for 30 minutes. Potentiostatic measurements were then taken at potentials of ﹣200, ﹣100, 0, 100 and 200 mV versus Ag/AgCl. Results show that the passive film repassivates at potentials of ﹣200 and ﹣100 mV and 0 mV for both alloys at 50°C. The current density however continues to rise for potentials of 100 and 200 mV. This shows that both alloys are susceptible to pitting at potentials above 100 mV at the test temperature of 50°C.

Brasagem dos aços inoxidáveis duplex 'baixa liga' UNS S32101, UNS S32304 e superduplex UNS S32750 com metal de adição AWS A5.3 tipo BNi-7 (Ni-Cr-P)

Os aços inoxidáveis duplex, devido à sua resistência à corrosão, têm uma grande aplicação em diversos segmentos industriais, onde a resistência à corrosão é uma propriedade requerida. Por terem resistência mecânica superior aos aços inoxidáveis ferríticos e austeníticos convencionais, podem produzir equipamentos mais leves. Quando ciclados termicamente, estes aços podem ter suas propriedades alteradas, principalmente quando se emprega processos de junção, como a brasagem, e quando ocorre o desbalanceamento da fração volumétrica ferrita/austenita. O objetivo deste trabalho é caracterizar a brasagem de aços inoxidáveis duplex 'baixa liga' e superduplex, verificando o efeito da folga e do tempo de brasagem na formação de intermetálicos contínuos no interior da junta brasada. Neste trabalho foram brasados, em forno continuo com atmosfera de hidrogênio, corpos-de-prova de aço inoxidável duplex UNS S32101, UNS S32304 e UNS S32750 com a adição para brasagem BNi-7 (Ni-Cr-P), na temperatura de 1100ºC, por tempos de 12 min e 32 min, utilizando-se folgas das juntas de 0,0 e 0,3 mm, seguidos de resfriamento em forno. Os corpos de prova foram caracterizados utilizando-se microscópio óptico, microscópio eletrônico de varredura com EDS e difração de raios X. Os resultados mostraram que a melhor condição para a brasagem em forno empregado foi a brasagem com a folga de 0,0 mm. Ambas as folgas apresentaram microestruturas variadas, porém a folga de 0,0 mm mostrou a presença de uma fase contínua CFC (cúbica de face centrada), rica em níquel, que é denominada, frequentemente, de γ-Ni. Para folgas de 0,3 mm, forma-se uma região eutética contínua no centro da junta, que pode prejudicar as propriedades mecânicas da mesma. Esta região é composta de fases intermetálicas ricas em fósforo. Foram observadas precipitações de fase sigma no metal de base no AID UNS S35750, nas condições de brasagem estudadas.

Martensitic transformation induced by cold deformation of lean duplex stainless steel UNS S32304

Lean duplex UNS S32304 is an austenitic ferritic stainless steel with pitting resistance equivalent number around 24-25. Its corrosion resistance is similar to AISI 316L steel, but with the advantage of a yield limit two times superior to conventional austenitic grades. Deformation induced martensitic transformations (γ→e and γ→ α') have been extensively studied in austenitic steels, but few works were published about these reactions in austenitic ferritic grades. In the present work, deformation induced martensitic transformation in lean duplex UNS S32304 was investigated by means of magnetic measurements and X-ray diffraction. The curves of α' martensite volume fraction against true deformation by cold rolling were constructed and compared to duplex and austenitic grades. It was found that lean duplex UNS S32304 is much more susceptible to martensitic transformation than duplex UNS S31803, which can be explained by its lower Mo and Ni contents.

Microstructural evolution and pitting resistance of annealed lean duplex stainless steel UNS S32304

Abstract The effect of annealing temperature in the range from 1000 to 1200 °C on the pitting corrosion behavior of duplex stainless steel UNS S32304 was investigated by the potentiodynamic polarization and potentiostatic critical pitting temperature techniques. The microstructural evolution and pit morphologies were studied using a scanning electron microscopy with energy dispersive X-ray spectroscopy. The results demonstrated that the nucleation of metastable pits transformed from austenite phase to ferrite phase with the increasing annealing temperature. As the annealing temperature increased, the pitting corrosion resistance firstly increased and then decreased. The highest pitting corrosion resistance was obtained at 1080 °C with the highest critical pitting temperature value and pitting nucleation resistance. The results could be well explained by the microstructural evolution of ferrite and austenite phases induced by annealing treatment.

New Trends Emerge in Thermal Desalination Evaporator Materials

Lessons Learned In the last half of the 20th century, the main evaporator body for an MSF was made of carbon steel of varying thickness, depending on the evaporator component. The general trend was to first establish the evaporator structure’s required thickness and add a corrosion allowance based on an assumption of general uniform corrosion to meet the evaporator’s expected life span. The highest corrosion allowance for an evaporator component was for areas subject to severe corrosion, such as the stage division wall and flashing brine gates deflector, which can reach 25 mm. Because carbon steel corrosion was never uniform, an evaporator was plagued by frequent shutdowns to patch leaks in the evaporator body or water boxes. The immediate impact was higher maintenance costs and production loss, eventually lowering evaporator availability. Bare carbon steel can’t withstand aerated seawater or the high salinity of flashing brine. Operating experience proved that bare carbon steel wasn’t the correct material for an evaporator shell and internal components. The selected material must exhibit high corrosion resistance to aerated seawater, hot brine recycle, and concentrated brine. Water boxes were protected by 3 mm 90/10 copper–nickel (Cu–Ni). The evaporator floor was lined with 3 mm 317L (3 percent molybdenum), which exhibits excellent resistance to pitting corrosion. The evaporator’s side walls, from stage floor to ceiling, were lined with 316L. The ceiling was protected by 304L, and the distillate trough, division walls, flashing brine gates, and deflectors were replaced by solid 316L. After using austenitic stainless steel to control corrosion of an evaporator’s internal components, it was extremely important to reduce dissolved oxygen in the evaporator, reduce dissolved oxygen in the feedwater to less than 20 ppb, and inject sodium sulfite in the brine recycle to scavenge the remaining dissolved oxygen to nil ppm. This important step controls corrosion of the evaporator’s internal components and condenser tubes. By selecting the proper materials, it was possible to reduce evaporator operating costs, increase availability to 93 percent, and achieve sustainable production. Because the price of nickel increased dramatically, reverting to lean duplex stainless steel made evaporator capital costs more competitive. A new generation of MSF and MED evaporators replaced austenitic stainless steel 304L, 316L, and 317L with lean duplex stainless steel UNS 32202, UNS 32304, and UNS 32205, respectively. Lean duplex stainless steel is characterized by low-nickel, highchromium content, which improves stress corrosion properties and provides greater pitting resistance than austenitic stainless steel. To better understand the importance of proper material selection, consider an MSF evaporator’s primary components.

Role of Microstructure on the Corrosion Susceptibility of UNS S32101 Duplex Stainless Steel

Abstract Effect of heat treatments and resulting changes in microstructure on general and localized corrosion susceptibility of lean duplex stainless steel UNS S32101 in caustic and chloride solutions was investigated. Results show that changes in the microstructure (ferrite phase/austenite phase ratio and inter-metallic precipitation) of the duplex stainless steel (DSS), due to different annealing and aging treatments, affect its general and pitting corrosion resistance. The role of DSS microstructure in localized corrosion susceptibility was found to be significantly different in the chloride environments as compared to caustic environments.