Effects of Rare Earth Metals addition and aging treatment on the corrosion resistance and mechanical properties of super duplex stainless steels

Effects of rare earth metals addition and aging treatment on corrosion resistance and mechanical properties of super duplex stainless steels were investigated using optical/SEM/TEM metallographic examination, an X-ray diffraction test, a potentiodynamic anodic polarization test and a tensile test. The performance of the experimental alloy with 0.32% REM addition was compared with commercial super duplex stainless steel such as SAF 2507 when they were exposed to solution annealing heat treatment and aging treatment. The corrosion resistance in Cl⁻ environments and mechanical properties of the experimental alloy were found superior to those of the commercial duplex stainless steel. The REM with larger atomic radii than those of Cr, Mo and W may fill vacancies inside the matrix and around the grain boundaries, retarding formation of harmful intermetallic σ and η phases. In addition, fine REM oxides/oxy-sulfides (1–3 μm) seemed to enhance the retardation effects. With REM additions, strength and ductility increased due to the phase and grain refinement caused by fine REM oxides and oxy-sulfides.     

Effects of Ce, La and Ba addition on the electrochemical behavior of super duplex stainless steels

The effects of rare earth metal (REM: Ce, La) and Ba addition on aqueous corrosion properties of super duplex stainless steels (SDSS) were investigated by electrochemical tests and surface analyses. The results of potentiodynamic test indicated that the passive range increased by the addition of Ce, La, and Ba, indicating increased relative resistance to localized corrosion. The EIS measurements showed that the Ce-La-Ba-bearing alloys exhibited higher R_ct and R_p values than the Ce-La-Ba-free alloy at the passive and breakdown states. Furthermore, the additions of REMs and Ba together promoted the formation of dense chromium-enriched passive film.     

Effects of rare earth metals addition on the resistance to pitting corrosion of super duplex stainless steel - Part 1

To elucidate the effects of rare earth metals addition on the resistance to pitting corrosion of super duplex stainless steel, a metallographic examination, potentiodynamic and potentiostatic polarization tests, a SEM-EDS and a SAM analysis of inclusion, austenite phase and ferrite phase were conducted. The addition of rare earth metals to the base alloy led to the formation of (Mn, Cr, Si, Al, Ce) oxides and (Mn, Cr, Si, Ce) oxides, which improved the resistance to pitting corrosion and caused a decrease in the preferential interface areas for the initiation of the pitting corrosion.     

Effects of copper addition on the formation of inclusions and the resistance to pitting corrosion of high performance duplex stainless steels

To elucidate the effects of copper addition on the formation of inclusions and the resistance to pitting corrosion of alloys, potentiodynamic and potentiostatic polarization tests, a SEM-EDS analysis of inclusions, and thermodynamic calculations of the formation of inclusions were conducted. The addition of copper to the base alloy increased the number and area of numerous (Mn, Cr, (Al), (Fe)) oxides and oxy-sulfides due to an increase in the activity of chromium and resulted in decreased pitting resistance. The thermodynamic prediction of the formation of inclusions was in good agreement with the experimental results.     

The resistance to cavitation erosion of CrMnN stainless steels

The resistance to cavitation erosion (CE) was measured using a magnetostrictive device and a rotating disk device for some CrMnN stainless steels (Chinese patent ZL 90 1 02197.0). The microstructural changes in the surface layer before and after CE were analyzed by use of Mossbauer spectra. Results show that the resistance to CE of duplex austenitic-martensitic CrMnN stainless steels is much better than that of ZG0Cr13Ni4-6Mo and ZG0Crl6Ni5Mo steel, which are in common use for hydraulic turbine runners. The metastable austenite and its changes in the process of CE are the key factors why the CrMnN stainless steels have excellent resistance to cavitation erosion.     

双相不锈钢焊接接头的耐腐蚀性能

根据母材临界点蚀温度(CPT)的试验结果,利用小试样的腐蚀实验方法研究了奥氏体-铁素体双相不锈钢焊接接头的耐点蚀性能.结果表明,手工电弧焊工艺过程对双相不锈钢材料的耐点蚀性能具有显著的影响,点蚀优先发生在焊缝金属或焊接热影响区中.双相不锈钢材料的耐点蚀性能与材料本身奥氏体和铁素体相比例有关.腐蚀试样的表面状态(粗糙程度)对母材金属的耐点蚀性能有明显的影响.表面越粗糙,耐点蚀性能越差,临界点蚀温度越低.     

Influence of cavitation on the passive behaviour of duplex stainless steels in aqueous LiBr solutions

The objective of this work is to study the influence of cavitation on the passive behaviour of EN 1.4462, its filler metal (EN 1.4462F), and the welded metal (EN 1.4462W) obtained by Gas Tungsten Arc Welding using electrochemical techniques. The hydrodynamic conditions of the medium were modified using an ultrasonic-induced cavitation facility.Potentiostatic experiments were used to study the effects of cavitation on the passive behaviour of the alloys. The experiments were carried out in 850 g/L LiBr solutions with and without an inhibitor (Lithium Chromate). The solution with Li₂CrO₄ (commercial solution) contains LiOH as the pH regulator. The potentiodynamic cyclic curves of the stainless steels under the static condition were used to set the values of the imposed potentials.In this work, the electrochemical behaviour of the alloys described by the potentiodynamic curves has been related to their passive behaviour under potentiostatic conditions when the pulses of cavitation were applied. The results demonstrate that cavitation affects the passive behaviour of the alloys; the influence depends on the potential applied and on the presence or absence of chromates in the medium. Only under certain circumstances the hydrodynamic conditions can suppose a breakdown of passive film formed under static conditions.     

SAF2507模拟焊接热影响区的组织转变行为

通过gleeble3800热模拟试验机,模拟超级双相不锈钢SAF2507焊接热影响区,研究了不同冷却速度与多次热循环对模拟焊接热影响区显微组织的变化过程,运用扫描电镜和能谱仪分析了合金元素的分布规律。试验结果表明,冷却速度和多次热循环对奥氏体形貌和尺寸有很大影响,随着热输入的增加,奥氏体含量从40.50%增加到58.35%。t8/5=60 s时可使组织中铁素体与奥氏体相比例为1:1,高热输入多道次焊接时,在铁素体和奥氏体相界处析出颗粒状的第二相。     

国内外双相不锈钢焊接的研究进展

主要介绍国内外双相不锈钢焊接在各个方面的研究进展.针对双相不锈钢焊接材料的开发,强调Ni和N元素在控制相平衡上的影响;介绍了国内外双相不锈钢焊接方法的最新动态,主要有埋弧焊、等离子弧焊和激光焊的改进、电阻对焊的尝试、新型双弧焊接技术的应用等;分析了奥氏体和铁素体两相比、有害析出物和氢脆对双相不锈钢焊接性的影响及解决措施;讨论了影响双相不锈钢焊接接头腐蚀性能和韧性的因素,对其影响机理进行了初步的解析;最后,进行了总结和展望.     

Friction stir welding of duplex stainless steels

Duplex stainless steels are successful in a variety of applications such as the food industry, petrochemicals and plants for desalination of seawater, where high corrosion resistance and high mechanical strength are required. However, the beneficial microstructure may change during fusion welding steps, and it can compromise the performance of these materials. Friction stir welding is a solid-state process avoiding typical problems concerning solidification such as solidification cracks, liquation and segregation of alloying elements. Superduplex stainless steels can avoid unbalanced proportions of ferrite and austenite, formation of secondary deleterious phases and grain growth of ferrite in the heat-affected zone. Consolidated friction stir welded joints with full penetration 6 mm thick were obtained for UNS S32101 and S32205 duplex and S32750 and S32760 superduplex stainless steels. The friction stir welds were submitted to tensile tests indicating an improvement of strength in welded joints, showing increased yield and tensile strength for all studied cases. Regarding the microstructural characterization, an outstanding grain refinement was observed in the welded joint, achieving grain sizes as small as 1 μm. This refinement was associated with the combination of microstructural restoration mechanisms in the dual-phase microstructure promoted by severe deformation associated with a high temperature during the welding process.     

Role of nitrite addition in chloride stress corrosion cracking of a super duplex stainless steel

This paper presents the role of addition of nitrite ions in susceptibility of a super duplex stainless steel, SAF 2507 to stress corrosion cracking (SCC) in chloride environment, which has a particular industrial relevance. Slow strain rate testing (SSRT) in 30 wt.% MgCl₂ solution established SCC susceptibility, as evidenced by post-SSRT fractography. However, the addition of nitrite has interesting influence. At their lower concentrations, nitrite additions seem to decrease SCC susceptibility, whereas, at a higher concentration, it has an accelerating effect on SCC. Attempts have been made to understand this behaviour on the basis of the role of nitrite in passivation and pitting characteristics of SAF 2507 in chloride solution.     

Improvement of the cavitation erosion resistance of UNS S31803 stainless steel by duplex treatment

A duplex surface treatment consisting of High Temperature Gas Nitriding (HTGN) followed by Low Temperature Plasma Nitriding (LTPN) was carried out in an UNS S31803 duplex stainless steel. The HTGN treatment was intended to produce a relatively thick and hard fully austenitic layer giving mechanical support to the thinner and much harder expanded austenite layer. HTGN was performed at 1200 °C for 3 h, in a 0.1 MPa N₂ atmosphere while LTPN, was carried out in a 75% N₂ + 25% H₂ atmosphere, at 400 °C for 12 h, under a 250 Pa pressure, and 450 V. An expanded austenite γ_N layer, 2.3 μm thick, 1500 HV0.025 hard, was formed on top of a 100 μm thick, 330 HV 0.1 hard, fully austenitic layer, containing 0.9 wt% N. For comparison purposes LTPN was carried out with UNS S30403 stainless steel specimens obtaining a 4.0 μm thick, 1500 HV 0.025 hard, expanded austenite layer formed on top of a fully austenitic matrix having 190 HV 0.1. The nitrided specimens were tested in a 20 kHz vibratory cavitation-erosion testing equipment. Comparison between the duplex treated UNS S31803 steel and the low temperature plasma nitrided UNS S30403 steel, resulted in incubation times almost 9 times greater. The maximum cavitation wear rate of the LTPN UNS S30403 was 5.5 g/m²h, 180 times greater than the one measured for the duplex treated UNS S31803 steel. The greater cavitation wear resistance of the duplex treated UNS S31803 steel, compared to the LTPN treated UNS S30403 steel was explained by the greater mechanical support the fully austenitic, 330 HV 0.1 hard, 100 μm layer gives to the expanded austenite layer formed on top of the specimen after LTPN. A strong crystallographic textured surface, inherited from the fully austenitic layer formed during HTGN, with the expanded austenite layer showing {101} crystallographic planes//surface contributed also to improve the cavitation resistance o f the duplex treated steel.     

Influence of chemical composition on the pitting corrosion resistance of non-standard low-Ni high-Mn-N duplex stainless steels

The pitting corrosion resistance of a new family of duplex stainless steels has been evaluated. These non-standard duplex stainless steels are characterised by low Ni content and high N and Mn levels. Potentiodynamic polarisation scans in NaCl solution have been carried out to determine pitting potentials. A crevice-free cell has been used to perform the electrochemical tests.An exponential equation is obtained in the regression analysis between the pitting potential and chemical composition which allows an estimate of the pitting resistance of these new duplex stainless steels.     

双相不锈钢热老化现象的研究进展

双相不锈钢兼具优异的力学性能、耐腐蚀性以及抗辐照能力,是核电站一回路主管道的关键结构材料。然而,在服役环境下长期工作,双相不锈钢中铁素体会发生调幅分解,生成富Fe的α相和富Cr的α′相,即产生热老化脆化现象,从而恶化合金的力学性能。本文综述了双相不锈钢的热老化机制,探索不同因素对合金相分解的影响,进而分析其微观组织及动力学演化规律。此外,利用计算机模拟平台对合金的相分解过程进行预测,可以缩短材料的研发周期和降低成本,对迫切解决双相不锈钢的热老化问题具有重要帮助。     

Phenomena and mechanism on superplasticity of duplex stainless steels

The superplasticity of Fe-24Cr-7Ni-3Mo-0.14N duplex stainless steel after being solution treated at 1350°C followed by 90% cold rolling was investigated at 850°C with a strain rate ranging from 10^-3-10^-1s^-1. The microstructure of duplex stainless steel consists of a matrix γ phase having low angle grain boundaries and a σ phase as second phase particles before the deformation at 850°C. It is well known that the constituent phases in duplex stainless steel is changed following α→α+γ→α+γ+σ→γ+σ through phase transformation during deformation at 850°C. The final microstructure of duplex stainless steel consisted of 70 vol.% of γ and 30 vol.% of the σ phase. A maximum elongation of 750% was obtained at 850°C with a strain rate of 3.16xl0^-3s^-1. The dislocation density within matrix γ grains was low and a significant strain-induced grain growth was observed during the deformation. The misorientation angles between the neighboring γ grains increased as the strain increased, thus the low angle grain boundaries were transformed into high angle grain boundaries suitable for sliding by dynamic recrystallization during the deformation at 850°C. The grain boundary sliding assisted by dynamic recrystallization is considered to be a controlling mechanism for superplastic deformation at 850°C.     

时效温度对2906超级双相不锈钢组织及性能的影响

对固溶态的2906超级双相不锈钢材料进行了650～950℃×6 h时效处理,利用OM、XRD、SEM、EDS和电化学工作站分析r时效温度对材料金属间相、力学性能和耐蚀性能的影响.试验结果表明:650℃时效6 h时,没有发现σ相生成,材料的抗拉强度和耐蚀性变化不大;当时效温度为750℃,组织中生成σ相含量最多,导致抗拉强度降低、塑性及耐蚀性大大下降,但硬度略有提高;随着时效温度的进一步提高到850℃及950℃,生成的口相逐渐减少,强度及耐蚀性略有回升.     

Application of the EPR method to duplex stainless steels

The present study is aimed at defining appropriate testing conditions in view of using the EPR (Electrochemical Polarization Reactivation) method for detecting the sensitization of 22 and 25% Cr duplex stainless steels at an earlier stage than conventional methods. In addition to this high sensitivity, these testing conditions should also be easy to use, for being applied not only in the laboratory (for qualification purpose) but also on production site (for Quality Control purpose, if needed). Reliable results were obtained with the required sensitivity, so the method can be used to control base materials and weld areas. Detailed testing conditions are provided in this paper for both 22 and 25% Cr stainless steels. They are based on the double loop method, with sulphuric and hydrochloric acid mixtures as testing media. Among the electrochemical parameters investigated, the importance of the initial cathodic polarisation is particularly pointed out as well as the quality of surface preparation, for getting reproducible results. Now an important piece of work remains for determining acceptance threshold levels for specific applications, since these acceptance levels are dependent both on the material (detailed composition, fabrication mode) and on the expected service conditions.     

Effects of heat input on pitting corrosion in super duplex stainless steel weld metals

Due to the difference in reheating effects depending on the heat input of subsequent weld passes, the microstructure of the weld metal varies between acicular type austenite and a mixture of polygonal type and grain boundary mixed austenite. These microstructural changes may affect the corrosion properties of duplex stainless steel welds. This result indicates that the pitting resistance of the weld can be strongly influenced by the morphology of the secondary austenite phase. In particular, the ferrite phase adjacent to the acicular type austenite phase shows a lower Pitting Resistance Equivalent (PRE) value of 25.3, due to its lower chromium and molybdenum contents, whereas the secondary austenite phase maintains a higher PRE value of more than 38. Therefore, it can be inferred that the pitting corrosion is mainly due to the formation of ferrite phase with a much lower PRE value.     

The corrosion behaviour of austenitic and duplex stainless steels in artificial saliva with the addition of fluoride

The evolution of the passive films on 2205 duplex stainless steel (2205 DSS) and AISI 316L stainless steel in artificial saliva, and with the addition of fluoride, was studied using electrochemical impedance spectroscopy (EIS) and potentiodynamic measurements. The extent of the passive range increased for the 2205 DSS compared to the AISI 316L in both solutions. The formation of the passive film was studied by EIS at the open-circuit potential (OCP). The passive layers were studied at the OCP by X-ray photoelectron spectroscopy (XPS). The passive films on both materials predominantly contained Cr-oxides, whereas the Fe species were markedly depleted.