LDX 2101中微观组织、应力与耐蚀性的关联性研究

发布时间：2018-05-11 20:03

本文选题：经济型双相不锈钢 + 固溶处理　；参考：《太原理工大学》2017年硕士论文

【摘要】：经济节约是双相不锈钢未来的发展趋势之一,LDX 2101一经问世便被广泛地应用在石油、化工、桥梁建筑等领域。但是,腐蚀一直是制约双相不锈钢应用的主要因素。固溶态的双相不锈钢的两相含量越接近、合金含量分布越均匀或者无有害相的析出,则其综合性能越好。实际应用中,双相不锈钢的两相平衡会被焊接等热处理过程破坏掉,合金元素也会受温度的影响重新分布。除此之外,双相不锈钢作为临海结构材料,在使用过程中往往承受力的作用,故应力腐蚀开裂也是双相不锈钢常见的腐蚀失效形式。可见,很有必要探究固溶温度和拉应力对LDX 2101的耐腐蚀性的影响。本文结合目前仍存在的问题,例如:固溶温度变化时,未溶的小尺寸的奥氏体相对双相不锈钢耐蚀性能的影响,以及应力对耐蚀性的影响,然后根据现有的研究结果以LDX 2101为研究对象,利用电化学阻抗谱、动电位极化曲线和Mott Schottky等表征材料在3.5 wt.%NaCl溶液中的耐蚀性,并结合金相显微镜和扫描电镜等微观表征手段探究温度、拉应力对LDX 2101耐蚀性能的影响机理。LDX 2101分别在950°C~1200°C温度范围内保温60 min,固溶温度升高时,γ相逐渐溶解,α相的含量逐渐增多。温度在950°C~1100°C范围内变化时,点蚀发生在α/γ相界处或者铁素体内部并向铁素体相内扩展,然而,固溶温度为1000°C、1050°C、1100°C时,由于γ相溶解,在奥氏体相的晶界处出现新转变而来的α相,此时点蚀更倾向于发生在新转变的α相与周围的γ相形成的相界处。固溶温度继续升高至1150°C时,LDX 2101耐蚀性最差,此时铁素体相基体内有许多溶解成极限尺寸γ相,由于极限尺寸的γ相与周围的α相形成的两个相界面很接近,故极限尺寸的奥氏体很容易从α/γ相界处被整个腐蚀掉。而固溶温度为1200°C时,由于极限尺寸的未溶γ相几乎溶解完全,LDX 2101的耐蚀性能变好。而且1150°C固溶不同时间后的研究结果进一步证实,固溶温度相对较高时,极限尺寸的γ相对LDX2101的耐蚀性影响较大。在大小分别为0 MPa、89.5 MPa、101.3 MPa、139.4 MPa、150.8 MPa、172.6 MPa的力的作用下,LDX 2101表面的钝化膜的掺杂浓度逐渐增大,这说明试样表面的钝化膜的稳定性随着应力的增大而逐渐下降,但是拉应力对点蚀电位(Epit)影响不大,其值主要受试样的表层或次表层的夹杂物的影响,并且腐蚀形貌呈现出夹杂物形状特征相一致。当腐蚀发生到一定程度时,可观察到腐蚀沿着轧制方向扩展并呈现选择性腐蚀铁素体相的特征,尤其是当拉应力增大至172.6 MPa时,选择性腐蚀铁素体相的特征越明显。可见,拉应力对耐蚀性的作用主要体现在对腐蚀的扩展的影响,这是因为两相界面受到张力的作用时,腐蚀更容易在相界处发生并向较弱的α相内扩展。
[Abstract]:Economic saving is one of the developing trends of duplex stainless steel in the future. Once it comes out, LDX 2101 has been widely used in petroleum, chemical industry, bridge construction and so on. However, corrosion has been the main factor restricting the application of duplex stainless steel. The more close the two-phase content of solid solution duplex stainless steel is, the more uniform the alloy content distribution is or the less harmful phase precipitation is, the better the comprehensive properties of the two-phase stainless steel are. In practical application, the two-phase equilibrium of duplex stainless steel will be destroyed by heat treatment such as welding, and the alloying elements will be redistributed by temperature. In addition, duplex stainless steel, as a structural material near the sea, is often subjected to stress in the process of use, so stress corrosion cracking is also a common corrosion failure form of duplex stainless steel. Therefore, it is necessary to explore the effect of solution temperature and tensile stress on the corrosion resistance of LDX 2101. In this paper, some problems still exist, such as the effect of insoluble austenite on the corrosion resistance of duplex stainless steel and the effect of stress on corrosion resistance of duplex stainless steel with the change of solution temperature. Based on the existing results, the corrosion resistance of LDX 2101 in 3.5 wt.%NaCl solution was characterized by electrochemical impedance spectroscopy, potentiodynamic polarization curve and Mott Schottky. And by means of microscopic characterization, such as metallographic microscope and scanning electron microscope, The effect of tensile stress on the corrosion resistance of LDX 2101. LDX 2101 is kept in the range of 950 掳C ~ 1 200 掳C for 60 min. When the solution temperature increases, the 纬 phase dissolves gradually and the 伪 phase content increases. When the temperature ranges from 950 掳C to 1100 掳C, the pitting corrosion occurs at the 伪 / 纬 phase boundary or inside the ferrite and extends to the ferrite phase. However, when the solution temperature is 1000 掳C, 1050 掳C 1100 掳C, the 伪 phase is changed at the grain boundary of the austenitic phase due to the dissolution of 纬 phase. The pitting tends to occur at the boundary between the newly transformed 伪 phase and the surrounding 纬 phase. When the solution temperature continues to rise to 1150 掳C, the corrosion resistance of LDX 2101 is the worst. At this time, there are many 纬 phase dissolved into the limit size 纬 phase in the ferrite phase group. The interface between the 纬 phase of the limit size and the 伪 phase around the phase is very close. Therefore, the austenite of the limit size is easily corroded from the 伪 / 纬 phase boundary. When the solution temperature is 1200 掳C, the corrosion resistance of LDX 2101 becomes better due to the insoluble 纬 phase with the limit size. Moreover, the results of 1150 掳C solution at different time further confirmed that the corrosion resistance of LDX2101 was greatly affected by 纬 with the limit size when the solution temperature was relatively high. The doping concentration of the passivation film on the surface of LDX 2101 increases gradually under the action of the force of 101.3 MPa1, 139.4 MPa1, 150.8 MPA, 172.6 MPa, respectively, which indicates that the stability of the passivated film on the surface of the sample decreases with the increase of the stress, and the stability of the passivation film decreases with the increase of the stress. However, the tensile stress has little effect on the pitting potential Epit.Its value is mainly affected by the inclusions in the surface or subsurface of the samples, and the corrosion morphology is consistent with the shape of the inclusions. When the corrosion occurs to a certain extent, it can be observed that the corrosion propagates along the rolling direction and exhibits the characteristics of selective corrosion of ferrite phase, especially when the tensile stress increases to 172.6 MPa, the characteristic of selective corrosion of ferrite phase becomes more obvious. It can be seen that the effect of tensile stress on corrosion resistance is mainly reflected in the effect of corrosion propagation, which is due to the fact that corrosion occurs more easily at the phase boundary and extends to the weaker 伪 phase when the interface of the two phases is affected by tension.
【学位授予单位】：太原理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG142.71;TG161

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