低镍和无镍奥氏体不锈钢的研究现状及进展

低镍和无镍奥氏体不锈钢以锰、氮等元素取代Cr-Ni不锈钢中的镍元素而具有较低的成本、优异的综合性能.综述了以锰代镍的Cr-Mn不锈钢,低镍Cr-Mn-Ni-N不锈钢,高氮无镍的Cr-Mn-N不锈钢及其在生物医用领域的研究和应用进展,并对低镍和无镍奥氏体不锈钢的发展进行了展望.     

氮强化双相不锈钢

正氮强化双相不锈钢的作用引起广泛重视,不仅强度高,而且耐蚀性更好,尤其能改善耐点蚀和耐晶界腐蚀。双相不锈钢的屈服强度约是不含氮常用奥氏体不锈钢的2倍,含氮的双相不锈钢抗拉强度可达700～900 MPa、断后伸长率20%。需注意:在双相不锈钢中,通过奥氏体形成元素氮和镍与铁素体形成元素铬和钼的平衡,才能获得所期望的双相组织。提高氮量并不总是提高屈服强度,因双相不锈钢的屈服强度取决     

湿法磷酸工业用典型氮合金化双相不锈钢评述

讨论了几种典型常用的氮合金化α+γ双相不锈钢的力学性能,耐腐蚀性能以及在湿法磷酸生产和相关领域的应用.氮合金化双相不锈钢具有十分优良的综合性能,在金属镍价格涨落不定的情况下,开发和应用节镍的双相不锈钢具有十分重大的经济意义.在湿法磷酸和硫酸等生产领域中,氮合金化的双相不锈钢优势明显具有广阔的应用前景.     

我国应重视低镍钼双相不锈钢及钢管的研发

从标准方面介绍了欧美双相不锈钢的制造水平,分析了列入美国ASTM标准的10种低镍钼双相不锈钢的化学成分和性能,总结了各低镍钼双相不锈钢在美国棒、板、管材标准中的入编历程,指出了研发低镍钼双相不锈钢及钢管的技术关键.分析认为:近几年以S32101为代表的新型低镍钼双相不锈钢在欧美快速发展,并作为无缝及(或)焊接钢管的新钢种列入美国不锈钢钢管标准;美国还在ASTM不锈钢标准中增列了S8XXXX系列钢;我国应加快低镍钼双相不锈钢及钢管的研发.     

TDS2101经济型双相不锈钢焊接接头性能研究

近年来,节镍经济型双相不锈钢发展很快,不少国家都在进行研制,结合国内资源情况,经济型双相不锈钢具有很好的开发价值.氮是一种强烈的形成并扩大奥氏体的元素,对双相不锈钢的力学性能具有强化作用,同时能够提高焊缝的耐点蚀性能.研究了TDS 2101经济型双相不锈钢中N合金元素、焊接工艺、焊接热输入等对焊接热影响区的力学性能和抗...     

经济型双相不锈钢焊接的研究进展

经济型双相不锈钢具有双相组织,且高氮低镍,其焊接问题成为近年来的研究热点。综述了国内外对经济型双相不锈钢的焊接研究进展及最新成果,包括所采用的焊接方法及工艺、焊接接头的微观组织、耐蚀性以及力学性能等。同时,对焊接过程中出现的问题进行了总结和讨论。     

时效热处理对新型节镍双相不锈钢组织及性能的影响

研究了时效温度对新型节镍双相不锈钢组织及性能的影响规律。结果表明：新型节镍双相不锈钢在650～850℃时效30 min可提高材料的抗拉强度、屈强比，降低材料的屈服强度、伸长率、冲击功、铁素体含量和硬度；700℃是新型节镍双相不锈钢的析出最敏感温度，新型节镍双相不锈钢在700℃时效30 min时晶界、晶内均有碳化物析出。新型节镍双相不锈钢应避开在650～850℃长时间使用。     

改善0Cr17Mn14Mo2N钢抗稀硫酸腐蚀性能的电化学研究

常用的铬镍奥氏体不锈钢含有贵缺的元素镍,这类钢代镍的问题,早就引起人们的注意。从1958年起,中国科学院金属研究所开展了以锰和氮代镍的铬锰氮奥氏体—铁素体复相不锈钢的研究。经过生产和使用部门的共同努力,已将一种0Cr17Mn14Mo2N钢作成设备和部件,应用到腐蚀性的尿素、醋酸、合成纤维和印染等工业介质。实践证明其耐蚀性和使用性良好。实验室的研究得出,这种钢在稀硝酸、磷酸、草酸和柠檬酸中具有与铬镍不锈钢相近或更高的耐蚀性。但在稀硫酸中,耐蚀性很差,不能使用。因此,有必要改善它的耐蚀性能。本文研究了合金元素在提高铬氮锰不锈钢腐蚀中的作用。     

氮对2205双相不锈钢在NaCl溶液中耐腐蚀性能的影响

在实验室条件下,以氮代镍冶炼节约型双相不锈钢2205-N,并采用电化学阻抗技术和电化学极化曲线法研究了试验钢在3.5%Na Cl(质量分数)溶液中的腐蚀行为。试验结果表明:在N含量(质量分数) 0. 11%～0. 35%范围内,随着N含量的增大,试验钢奥氏体相逐渐增多且晶粒细化、容抗弧半径逐渐增大、极化电阻R_p逐渐增大、腐蚀电流I_(corr)逐渐减小,2205-N双相不锈钢的抗腐蚀性随着N含量的增大逐渐增强。试验证明,以氮代镍生产节约型双相不锈钢是行之有效的。     

高温渗氮工艺对高氮无镍不锈钢组织及力学性能的影响

采用高温渗氮在奥氏体/铁素体双相不锈钢表面形成了奥氏体高氮层。试验结果表明,渗氮层氮含量可达1.0%,与原材料相比氮含量增加了2倍。原始双相组织已经转变为奥氏体,渗氮层深度达到2 mm以上。采用合理优化的高温渗氮工艺,可在提高不锈钢强度、硬度的同时,其伸长率、断面收缩率仍然保持较高的水平。高温渗氮工艺制备高氮无镍不锈钢的最佳工艺参数为:加热温度1200℃、氮气压力0.3 MPa、保温时间24 h。     

Austenitic-ferritic stainless steels: A state-of-the-art review

Austenitic-ferritic stainless steels, more commonly known as duplex stainless steels, or DSS for short, consist of two basic phases. One is austenite, A, and the other is ferrite, F, present in about equal amounts (but not less than 30% each). The two phases owe their corrosion resistance to the high chromium content. Compared to austenitic stainless steels, ASS, they are stronger (without sacrificing ductility), resist corrosion better, and cost less due to their relatively low nickel content. DSS can be used in an environment where standard ASS are not durable enough, such as chloride solutions (ships, petrochemical plant, etc.). Due to their low nickel content and the presence of nickel, DSS have good weldability. However, they have a limited service temperature range (from −40 to 300°) because heating may cause them to give up objectionable excess phases and lower the threshold of cold brittleness in the heat-affected zone of welded joints. State-of-the art DSS are alloyed with nitrogen to stabilize their austenite, and in this respect the nitrogen does the job of nickel. Also, nitrogen enhances the strength and resistance to pitting and improves the structure of welds. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 10, pp. 20–29, October, 1997.     

Determination of the weaker phase in the pitting corrosion of non‐standard low‐Ni high‐Mn‐N duplex stainless steels

In this paper, the use of Energy Dispersive Spectrometry (EDS) is proposed to determine the partition coefficients of the elements of a new family of duplex stainless steels that are characterized by having low contents of nickel, together with high levels of manganese and nitrogen. From the values of the partition coefficients, the chemical compositions of the constituting phases have been determined, in order subsequently to calculate the value of the Pitting Resistance Equivalent Number (PREN) of each phase. The proposition put forward in this study is that the phase having the lower PREN determines the pitting corrosion behaviour of these types of steels. Results obtained by means of optical and scanning electron microscopy have provided confirmation that the pitting corrosion behaviour of these new materials gets determined by the resistance of the weaker phase and consequently by the phase having the lower PREN value. Lastly it has been proved possible to determine the existence of an exponential relationship between the alloys pitting potential (Ep) and the weaker phase PREN; this can be utilized for the low‐nickel duplex stainless steels design in which the pitting corrosion resistance is controlled.     

Corrosion resistance of low‐nickel duplex stainless steel rebars

The use of stainless steel bars in reinforced concrete structures may be an effective method to prevent corrosion in aggressive environments where high amounts of chlorides may penetrate in the concrete cover. For an estimation of the service life of structures where stainless steel bars are used, the chloride threshold for these rebars should be defined, and the influence of chemical composition and metallurgical factors that may affect the corrosion resistance (strengthening, welding, etc.) should be assessed. To reduce the cost of stainless steel reinforcement, duplex stainless steels with low nickel content have been recently proposed as an alternative to traditional austenitic steels, even though, few results are available regarding their corrosion performance in chloride contaminated concrete. This paper deals with the corrosion resistance of low‐nickel duplex stainless steel rebars (1.4362 and 1.4162) as a function of the chloride content. Comparison is made with traditional austenitic steels. An attempt to define a chloride threshold for the different stainless steels is made by comparing the results of several test procedures both in concrete and in solution.     

Influence of nitrogen and manganese on localized Corrosion behaviour of stainless steels in chloride environments

Electrochemical localized corrosion tests in substitute ocean water at 40°C and 70°C and ASTM G48 tests in 6% FeCl₃ solution were performed on three classes of stainless steels: Ni-austenitic (both traditional and with high nitrogen content ones), high nitrogen Mn-austenitic nickel free and duplex (both traditional and with high nitrogen content ones). The Pitting Resistance Equivalent formula, PRE_Mn = % Cr + 3.3% Mo + 30% N – 1% Mn, proposed to consider the presence of noticeable amount of manganese in some of the new high nitrogen stainless steels yields good linear correlation with experimental results. The existence of a threshold value of PRE_Mn (≥ 45) to attain excellent localized corrosion resistance has been recognized. According to this observation the high nitrogen Ni-austenitic 21Cr24Ni6MoO.24N, 24Crl8Ni4MoO.48N, 24Cr22Ni7MoO.52N and the duplex 25Cr8Ni4MoO.26N “super” stainless steels are immune to localized attack also in the most severe electrochemical test conditions. This superiority is maintained also in ASTM G48 tests. Due to their values of Critical Crevice Temperature (CCT_{ASTM G48} ≥ 35°C) these steels seem suitable for practical service in seawater environments up to about 30 °C.     

Effect of alloy element contents on caustic stress corrosion cracking of several stainless steels

The stress corrosion cracking behavior in caustic solutions (200 g/l sodium hydroxide, 10 g/l sodium chloride) of three austenitic (18Cr-10Ni-2.5Mo, 20Cr-25Ni-4.5Mo, 27Cr-31Ni-3.5Mo) and three duplex (23Cr-4Ni, 22Cr-5Ni-3Mo, 25Cr-7Ni-4Mo-N) stainless steels was examined. U-bend and Slow Strain Rate (SSR) tests were performed at 200–250°C. The negative influence of nickel in the lower range content for the 18Cr-10Ni-2.5Mo and 20Cr-25Ni-4.5Mo has been shown; when the nickel content is significantly increased (>30%), as in the case of the steel 27Cr-31Ni-3.5Mo, an increase of SCC resistance has been detected. The negative effect of molybdenum, mainly on the behaviour of duplex stainless steels, has also been evidenced. The duplex stainless steels show better caustic SCC resistance than austenitic stainless steels type 18Cr-10Ni-2.5Mo and 20Cr-25Ni-4.5Mo. The best behaviour has been found for the less-alloyed steel 23Cr-4Ni.     

Effects of Nano-Y_2O_3 and Sintering Parameters on the Fabrication of PM Duplex and Ferritic Stainless Steels

Here we report the effects of nano-Y_2O_3 addition,sintering atmosphere and time during on the fabrication of PM duplex and ferritic stainless steels composites by dual-drive planetary milling of elemental Fe,Cr and Ni powders followed by conventional pressureless sintering.Yttria-free and yttria-dispersed duplex and ferritic stainless steels are fabricated by conventional sintering at 1000,1200 and 1400 ℃ temperatures under argon atmosphere.In another set of experiment,yttria-free and yttria-dispersed duplex and ferritic stainless steels are consolidated at 1000 ℃ for 1 h under nitrogen atmosphere to study the effect of sintering atmosphere.It has been found that densities of duplex and yttriadispersed duplex stainless steel increase from 71%to 91%and 78%to 94%,respectively,with the increase in sintering temperature.Similarly,hardness value increases from 257 to 567 HV_(25) in case of duplex,and from 332 to 576 HV_(25) in yttria-dispersed duplex stainless steel.X-ray diffraction analysis shows the domination of more intense austenite phase than ferrite at higher sintering temperature and also in nitrogen atmosphere.It is also evident that addition of yttria enhances phase transformation from α-Fe to γ-Fe.Duplex and yttria-dispersed duplex stainless steels exhibit the maximum compressive yield strength of 360 and 312 MPa,respectively.     

Chloride-induced stress corrosion cracking of Duplex stainless steels. Models,test methods and experience

Stress corrosion cracking (SCC) induced by chlorides frequently causes problems in applications where standard austenitic stainless steels are being used. Often this problem can be solved by the use of duplex stainless steels. In this report the mechanisms for SCC have been surveyed, and the cause for the high SCC resistance of duplex stainless steels has been discussed and evaluation of test methods for SCC and how duplex stainless steels respond to them, as well as practical experience of duplex stainless steels. The study shows that no single mechanism can be attributed to the good resistance to SCC of duplex stainless steels. Probably a synergistic effect of electrochemical and/or mechanical effects is responsible for the good performance. Test methods for SCC often give relatively good correspondence with real applications, but ranking is often doubtful, and comparisons of different material types should be made with caution. Numerous cases with SCC on standard austenitic stainless steels have been solved by the use of duplex stainless steels.     

Effect of nitrogen on corrosion behavior of 28Cr-7Ni duplex and microduplex stainless steels in air-saturated 3.5 wt% NaCl solution

The corrosion behavior of 28Cr-7Ni-O-0.34N duplex stainless steels in air-saturated 3.5-wt% NaCl solution at pH 2, 7, 10 and 27 °C was studied by the potentiodynamic method. Two types of microstructures were investigated: the as-forged duplex and microduplex (average austenite grain size 5-16 μm) structures. The austenite volume fractions of the tested steels were between 0.35 and 0.64. The nitrogen effect on corrosion behaviors of both duplex and microduplex stainless steels were the same. At pH 2, the corrosion potential increased when the nitrogen content increased, however, corrosion current density as well as corrosion rate decreased. At pH 7 and 10, the effect of nitrogen on corrosion potential and corrosion rate could not be observed. Corrosion potential at pH 10 was lower than at pH 7. Pitting potential increased when the nitrogen content in the tested steels increased at all tested pH. For the nitrogen effect on the passive current density, it seemed that only at pH 2, the average passive current densities reduced when the nitrogen content increased. Nitrogen may have participated in the passive film or has been involved in the reaction to build up passive film. The ammonium formation and nitrogen enrichment at the interface metal/passive film with adsorption mechanism were discussed. The dissolute nitrogen might have combined with the hydrogen ions in solution to form ammonium ions, resulting in increasing solution pH. The steel could then easily repassivate, hence the corrosion potential and pitting potential would increase. However, the ammonium formation mechanism could not explain the decrease of corrosion potential in basic solution. Nitrogen enrichment at the metal/passive film interface with adsorption mechanism seemed to be an applicable consideration in increasing pitting potential. However, this mechanism did not involve the ammonium ion formation. In general, for the duplex and microduplex stainless steels tested, nitrogen increased the general corrosion resistances in acid solution and pitting corrosion resistance at all solution pH. Metallographic observation in both tested duplex and microduplex steels after pitting corrosion at all tested pH revealed that, the corroded structure in the tested steels without nitrogen alloying was austenite, but those with nitrogen alloying was ferrite. Even though ferrite had a higher chromium content than austenite but higher dissolved nitrogen in austenite than in ferrite may have increased the pitting resistance equivalent number (PRE) of austenite to be higher than that of ferrite.     

Stainless steel reinforcing bars – reason for their high pitting corrosion resistance

In harsh chloride bearing environments stainless steel reinforcing bars offer excellent corrosion resistance and very long service life for concrete structures, but the high costs limit a more widespread use. Manganese bearing nickel‐free stainless steels could be a cost‐effective alternative. Whereas the corrosion behavior of stainless steels in alkaline solutions, mortar and concrete is quite well established, only little information on the reasons for the high pitting resistance are available. This work reports the results of pitting potential measurements in solutions simulating alkaline and carbonated concrete on black steel, stainless steel DIN 1.4301, duplex steel DIN 1.4462, and nickel‐free stainless steel DIN 1.4456. Duplex and nickel‐free stainless steels are fully resistant even in 4 M NaCl solutions with pH 13 or higher, the lower grade DIN 1.4301 shows a wide scatter between fully resistant and pitting potentials as low as +0.2 V SCE. In carbonated solutions with pH 9 the nickel‐free DIN 1.4456 shows pitting corrosion at chloride concentrations ≥3 M. This ranking of the pitting resistance can be rationalized based on XPS surface analysis results: both the increase of the Cr(III)oxy‐hydroxide and Mo(VI) contents in the passive film and a marked nickel enrichment beneath the film improve the pitting resistance. The duplex DIN 1.4462 shows the highest pitting resistance, which can be attributed to the very high Cr(III)oxy‐hydroxide, to a medium Mo(VI) content in the film and to a nickel enrichment beneath the film. Upon time, the protective properties of the surface film improve. This beneficial effect of ageing (transformation of the passive film to a less Fe²⁺ containing, more hydrated film) will lead to higher pitting potentials. It can be concluded that short‐term solution experiments give conservative results in terms of resistance to chloride‐induced corrosion in reinforced concrete structures.