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.     

Corrosion behaviour of duplex stainless steels sintered in nitrogen

Duplex stainless steels obtained through powder metallurgy (PM) technology from austenitic AISI 316L and ferritic AISI 430L powders were mixed on different amounts to obtain biphasic structures with austenite/ferrite ratio of 50/50, 65/35 and 85/15. Prepared mixes of powders have been compacted at 750 MPa and sintered in N₂-H₂ (95% and 5%) at 1250 °C for 1 h. Corrosion behaviour, using electrochemical techniques such as anodic polarization measurement, cyclic anodic polarization scan and electrochemical potentio-kinetic reactivation test and double loop electrochemical potentio-kinetic reactivation double loop test were evaluated. For duplex stainless steels, when austenite/ferrite ratio increases the corrosion potential shifts to more noble potential and passive current density decreases. The beneficial effect of annealing solution heat treatment on corrosion behaviour was established and was compared with corrosion behaviour of vacuum sintered duplex stainless steels. The results were correlated with the microstructural features.     

Electrochemical behaviour of duplex stainless steels in caustic environment

Recent studies have shown that duplex stainless steels can be susceptible to general corrosion and stress corrosion cracking in high pH caustic environments. This difference in the corrosion resistance can be attributed to changes in the electrochemical behaviour of steels. The present study has shown that the corrosion rates of duplex stainless steels increase with an increase in temperature and sulphide addition to caustic environments. Moreover, alloying Fe with Cr and Ni helps to raise the corrosion potential and lower critical current density of DSS in an alkaline environment whereas Mo can be detrimental to the corrosion resistance of the steel.     

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。     

焊后热处理对双相钢焊条电弧堆焊层组织性能的影响

双相不锈钢在室温下固溶体中奥氏体和铁素体约各占一半且兼有两相组织特征.它保留了铁素体不锈钢强度高、导热系数大、线膨胀系数小、耐点蚀,缝蚀及氯化物应力腐蚀性能的特点;又具备奥氏体钢不锈钢韧性好、脆性转变温度较低、抗晶间腐蚀、力学性能和焊接性能好的优点.通过对2209型双相钢焊条电弧堆焊层的在焊态及焊后经过690℃下不同时间的热处理态后堆焊层的组织和性能进行了试验.试验结果表明,经焊后热处理的堆焊层,铁素体含量降低、力学性能下降、硬度增高、腐蚀性能下降,这主要是由于堆焊层经热处理后,未能保持适当的相比例和σ相的析出,导致堆焊层的力学性能和耐蚀性能下降.     

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.     

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.     

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.     

Galvanic corrosion between the constituent phases in duplex stainless steel

The exclusive single-phase with the exact chemical composition of the constituent phase in 2205 duplex stainless steel (DSS) could be prepared using selective dissolution method. The respective electrochemical behavior of each constituent phase could then be measured. The experimental results showed that the two distinct peaks in the active-to-passive transition region of the polarization curve determined in 2 M H₂SO₄ + 0.5 M HCl mixed solution were actually corresponded to the respective anodic peaks of the single austenite and ferrite phases. A polarity reversion was found between austenite and ferrite phases in mixed H₂SO₄ + HCl solution and HNO₃ solution. Galvanic current measurements also revealed that austenite was anode in HNO₃ solution, but became cathode when exposed in 2 M H₂SO₄ + 0.5 M HCl mixed solution.     

Nitrogen distribution on rapid solidification in laser welded duplex stainless steels

Summary

Although several surfacing processes have been proposed and applied, none has been able to apply a 1 mm thick hard layer. This paper describes fundamental research to establish friction surfacing as a technique for applying a 1 mm thick hard layer.

Experiments were run to clarify the effects of operating parameters on surface quality using a conventional friction welder fitted with a sliding stage. The materials selected were a 1C‐17Cr martensitic stainless steel coating material and a low‐carbon structural steel substrate.

A stable hard layer of 20 mm width and 1 mm thickness was obtained, with no dilution being found. The dimensions of the coating depend on the friction speed (rotational speed) of the coating material. Thicker and wider coatings are produced at a lower friction speed. The deposition rate is slightly lower than that of conventional arc welding processes under the experimental conditions presently adopted.     

Microstructural features and corrosion properties in laser welded duplex stainless steels

Synopsis

The most common techniques of underwater welding and cutting used in practice are described. In spite of the adverse effect of pressure, present‐day technology enables good quality joints to be made. The local dry chamber, and wet‐welding with a shielded electrode methods, characterised by their low cost, are used at shallow depths down to 60 m. The hyperbaric, dry method of mechanised TIG and MIG welding is used at depths down to 200 m. Results of underwater automatic cutting by an arc‐oxygen method at depths down to 600 m, and the equipment used are discussed.     

The relationship between duplex oxidation and structural stability in austenitic stainless steels

Samples of austenitic stainless steels from various sources have been thermally cycled. Many materials exhibited a volume expansion on cooling superimposed on the normal thermal contraction, as a result of an austenite → ferrite phase transformation. Samples of materials which exhibited the ferrite transformation also show a period of rapid oxidation on re-exposure after cooling during the first autoclave shutdown of an oxidation test. It is proposed that the local volume expansion accompanying the formation of ferrite on cooling disrupts the initial protective oxide scale thereby allowing oxidation to proceed more rapidly.     

Unmixing behaviour in dissimilar laser welds for duplex and austenitic stainless steels

ABSTRACT

Metallurgical characteristics of single-mode laser dissimilar welds between super duplex (UNS S32750) and austenitic (type 316L) stainless steels is the existence of an unmixed zone that originates from each base metal. It was confirmed that the unmixed zone that flowed out from the 316L had the microstructural morphology of primary austenite with secondary δ-ferrite solidification mode, while the morphology of a δ-ferrite single phase solidification mode was observed within the unmixed zone that flowed out from the S32750. Furthermore, it could be speculated that each unmixed zone that flowed out from 316L and S32750 coexisted independently in terms of crystallographic aspect (orientation distribution function and predominant orientation relationship) analysed by electron backscatter diffraction.     

A dissolution kinetics model and its application to duplex stainless steels

This work proposes a semi-empirical model for isothermal particle dissolution. An impingement factor in the form y^c+1 is introduced to generalize the exponential law found in the recent literature and numerically obtained by means of the phase field method. A procedure is described for calibration of the model parameters that uses information from isothermal experiments. The model was applied to isothermal dissolution of σ phase in duplex stainless steels. A very good agreement was obtained between experimental and analytical results.     

Identification of phases and precipitates by AFM in duplex stainless steel unions

ABSTRACT

Atomic Force Microscopy (AFM), a characterization technique that generates topographic images of surfaces at very high resolutions, works by recording the surface relief details of the material with a cantilever which moves over the sample, while a piezoelectric sensor monitors changes in height. This technique is useful for identifying phases, particles and precipitates on a nanoscale and is therefore proposed for identifying them in the fusion zone of welded components and for early identification of faults due to cracking. This is a technique that does not require conductive samples, or special metallographic preparation of specimens as is the case with electron microscopy. The material studied is a duplex 2205 stainless steel, welded by means of gas tungsten arc welding (GTAW). The welded material was subjected to stress tests. The test specimens were characterized using optical microscopy, scanning electron microscopy, atomic force microscopy and nanoindentation tests. The results focus on the identification of the specific magnetic, topographic and hardness characteristics of each of the phases, particles and areas present in the base metal, the fusion zone and the zone deformed by the fracture.     

Corrosion behaviour of corrugated lean duplex stainless steels in simulated concrete pore solutions

This work studies the corrosion behaviour of two corrugated lean duplex stainless steels (SAF 2001 and 2304 grades) in eight alkaline solutions (carbonated and non-carbonated, saturated Ca(OH)₂ solutions with different chloride contents). 2001 stainless steel is a new grade in market because of its composition. 2304 is a grade previously studied under different conditions. However, its use as reinforcement in concrete is new. Studies are carried out by polarization curves following scanning electronic microscopy (SEM) and optical observations. Results are compared to those of carbon steel and austenitic AISI 304 and duplex SAF 2205 under similar conditions. After corrosion tests in alkaline media with chloride, ferrite tends to corrode selectively in 2304 duplex, while austenite corrodes selectively in 2001 under the same conditions. The influence of the duplex microstructure on attack development and morphology is analyzed. The electrochemical parameters obtained from the polarization curves suggest 2001 could replace 304 keeping the structure its corrosion performance (and with clear economical advantages). 2304 shows better corrosion behaviour than the more expensive 304, but somewhat lower than the excellent behaviour shown by 2205.     

Microstructure and stress corrosion cracking in simulated heat-affected zones of duplex stainless steels

The effects of nitrogen content and the cooling rate on the reformation of austenite in the Gleeble simulated heat-affected zone (HAZ) of 2205 duplex stainless steels (DSSs) were investigated. The variation of stress corrosion cracking (SCC) behavior in the HAZ of 40 wt% CaCl₂ solution at 100 °C was also studied. Grain boundary austenite (GBA), Widmanstatten austenite (WA), intergranular austenite (IGA) and partially transformed austenite (PTA) were present in the HAZ. The types and amounts of these reformed austenites varied with the cooling rate and nitrogen content in the DSS. U-bend tests revealed that pitting corrosion and selective dissolution might assist the crack initiation, while the types and amounts of reformed austenite in the HAZ affected the mode of crack propagation. The presence of GBA was found to promote the occurrence of intergranular stress corrosion cracking. WA, IGA and PTA were found to exhibit a beneficial effect on SCC resistance by deviating the crack propagation path.