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作者简介:

黄春林,男,1997年出生,硕士研究生。主要研究方向为电站高温材料。E-mail:hcl2260526304@163.com;

朱明,男,1978年出生,博士,副教授,硕士研究生导师。主要研究方向为高温结构材料腐蚀与表面改性。E-mail:mingzhu@xust.edu.cn;

鲁金涛(通信作者),男,1984年出生,博士,教授级高级工程师。主要研究方向为电站高温材料。E-mail:lujintao@tpri.com.cn

中图分类号:TG178

DOI:10.11933/j.issn.1007−9289.20211125001

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目录contents

    摘要

    在高温下长时间暴露,钢材表面的渗铝层与母材之间容易发生元素互扩散,对母材组织产生影响,影响母材的力学性能。为研究超超临界机组用 T92 钢表面铝化物涂层的抗蒸汽氧化性能及组织演化特性,采用低温粉末包埋渗铝法在 T92 钢锅炉管内壁制备铝化物涂层,并在 650 ℃饱和蒸汽环境中进行热暴露试验,结合扫描电镜(SEM)、透射电镜(TEM)观察及 X 射线衍射分析,研究铝化物涂层的氧化行为以及 T92 基体与铝化物涂层之间的扩散退化行为。研究结果表明:低温粉末包埋渗铝可在 T92 锅炉管内壁制备厚度约 30.4 μm 的双层结构铝化物涂层,各层结构连续均匀且组织稳定,与母材呈冶金结合。 在 650 ℃、3000 h 饱和蒸汽氧化过程中,涂层表面生长厚度约 0.3 μm 的 α-Al2O3氧化膜。650 ℃长时热暴露过程中,Fe-Al 金属间化合物始终是涂层的主要物相,但涂层由 FeAl 相向 FeAl2相退化。铝化物涂层经长时间热暴露后组织退化,但仍具有优异的抗氧化性能,能够对 T92 钢提供很好的保护。

    Abstract

    It is known that the interdiffusion at the aluminide coating / matrix interface during the long time exposure at high temperature would change the microstructure of the matrix and deteriorate the mechanical properties of the matrix. To analyze the steam oxidation resistance and microstructure evolution of aluminide coating on T92 steel for ultra-supercritical unit, aluminide coating is prepared on the inner wall of T92 steel boiler tube by low temperature powder embedding method, and the heat exposure test is carried out in 650 ℃ saturated steam environment. The oxidation and diffusion degradation behaviors of T92 matrix and aluminide coating are studied by combining scanning electron microscope equipped with energy dispersive spectroscopy (SEM / EDS), transmission electron microscope (TEM) and X-ray diffraction analysis (XRD). The results show that the aluminide coating prepared on the inner wall of T92 boiler tube by low temperature powder embedding aluminizing, which is metallurgically combined with the matrix, has a double-layer structure, and each layer is continuous and uniform. The total thickness of the prepared aluminide coating is about 30.4 μm. A very thin α- Al2O3 oxide film with the thickness about 0.3 μm is formed on the surface of the aluminide coating after exploring in the 650 ℃ saturated steam for 3000 h, which prevents the further oxidation of the coating. Fe-Al intermetallic compound is always the dominated phase of the coating, although the phase of the aluminide coating degenerated from FeAl to FeAl2. It can be concluded that even microstructure degradation of the aluminide coating occurres during the long-time thermal exposure process, it can still provide excellent protection for T92 steel to resistant water steam oxidation.

  • 0 前言

  • T92钢(Fe, Cr 9%, W 1.8%, Mo 0.5%)是在T91钢(Fe, Cr 9%, Mo 1%)基础上发展出来的一种新型马氏体耐热钢[1]。由于采用V、Nb元素进行微合金化,并控制N、B元素含量,T92钢的耐蚀性和抗氧化性、持久-蠕变强度、许用应力以及高温稳定性均优于T91钢,故广泛用于超超临界发电机组的高温过/再热蒸汽管道及集箱(集箱接管)等主要部件[2-4]。T92钢的Cr含量较高,在高温下可以形成具有保护性的Cr2O3 氧化膜,但使用寿命以及服役温度仍然受到其氧化性能和蠕变强度的限制[5]。因此,提高T92钢抗蒸汽氧化性能最有效的方法是在其表面制备高温防护涂层[6]

  • 表面渗铝是一种常见的高温防护涂层制备技术,可以在几乎不降低合金力学性能的前提下提高合金的抗高温氧化和耐蚀性能[7-8]。常见的渗铝技术包括包埋渗铝[9-10]、热浸渗铝[11-12]、化学气相沉积渗铝[13-14]和料浆渗铝[15-16]等四种。其中,固体粉末包埋渗铝具有设备简单易操作、制造成本低及渗层效益良好等优点,在渗铝生产中应用最广泛。Fe基合金在高温渗铝时,容易在合金表面形成含铝量较高、脆性较大且和基体结合力较差的金属间化合物相如FeAl3、Fe2Al5 和FeAl2 等,需要进行退火处理。Fe-Al系金属间化合物中Al含量较低、韧性较好,不易出现裂纹[17-18],在高温环境中可以形成具有保护性的Al2O3 膜。采用低温渗铝工艺,不仅可以在金属基体表面得到FeAl层,而且可以减少渗铝工艺对基体组织和性能的影响,具有较高的应用价值。董猛等[19]采用粉末包埋渗铝的方法,在T92钢基材上制备了单一的FeAl涂层,在660℃ 90%水蒸气下,FeAl渗层仍具有优异的抗蒸汽氧化性能。

  • 渗铝层在高温服役过程中,涂层与基体之间的元素互扩散会导致涂层抗高温性能的退化[20-23],并对基体的力学性能产生不利影响。本文采用粉末包埋法在T92表面制备铝化物涂层,研究在650℃纯水蒸气中的氧化行为,分析氧化产物的相结构、微观形貌和氧化过程中的元素互扩散行为,并讨论铝化物涂层性能退化机制。本文研究结果可以为采用铝化物涂层方法改善T92钢抗蒸汽氧化性能提供理论依据。

  • 1 试验

  • 1.1 涂层制备

  • 所用T92钢锅炉管的化学成分见(质量分数) 表1。T92钢管通过线切割切成尺寸18mm× 10mm×5mm的瓦片状试样,经280 #、800 #、1200 #SiC砂纸依次打磨,确保样品表面光滑、均匀、整洁,无明显表面缺陷,用丙酮清洗并干燥备用。

  • 利用粉末包埋法制备铝化物涂层,粉末配比为2%NH4Cl+98%FeAl粉末(2%和98%为质量分数),并在640℃、氩气保护下烧结2h,然后在640℃ 下退火4。

  • 表1 T92化学成分(质量分数)

  • Table1 Chemical composition of T92

  • 1.2 性能测试及组织观察

  • 蒸汽氧化试验在流动的100%水蒸汽环境中进行,饱和蒸汽氧化装置如图1所示[20]。连续水蒸气的产生方法可以参阅本文课题组前期发表的论文[24]。氧化试验的具体参数见表2。

  • 图1 饱和蒸汽氧化试验装置[20]

  • Fig.1 Saturated steam oxidation apparatus [20]

  • 表2 炉内蒸汽参数

  • Table2 Steam parameters within the reactor

  • 使用D/MAX-RA型X射线衍射仪对涂层物相进行表征,采用自带Oxford能谱仪的扫描电子显微镜(SEM,ZEISSεigma)观察涂层氧化前后的微观形貌,并进行成分分析和测量氧化膜厚度随氧化时间的变化,绘制在650℃饱和蒸汽环境下氧化3 000h后T92钢、T92涂层的氧化动力学曲线。为深入研究铝化物涂层的组织演化机理,通过聚焦离子束(FIB)制备透射试样,采用Tecnai G2 F30TWIN型透射电子显微镜 (TEM)对铝化物涂层微观组织结构进行观察。

  • 2 试验结果

  • 2.1 涂层结构

  • 图2 为T92钢经粉末包埋渗铝后的铝化物涂层截面形貌及相应的XRD图谱。由图可知,管内壁涂层分布均匀、连续,与基体之间呈冶金结合;根据衬度不同,铝化物涂层分为外层和内层。涂层层次感分明,厚度均匀且无裂纹,外层主要为FeAl相,厚度约11.25 μm,内层主要为Fe3Al相,厚度约19.15 μm。

  • 图2 T92钢铝化物涂层截面形貌及XRD图谱

  • Fig.2 Cross-sectional morphologies and XRD pattern of aluminide coating on T92steel

  • 2.2 饱和蒸汽氧化

  • 图3 为T92钢、T92钢铝化物涂层在650℃饱和蒸汽环境下的氧化动力学曲线。由图可知,在0~3 000h氧化后,T92表面氧化膜厚度逐渐在增加,氧化3 000h后氧化膜厚度为139.57 μm,这可能与氧化膜的剥落和重整有关[25]。氧化3 000h后,T92涂层表面的氧化膜厚度变化不明显,近似平行于X轴。这表明,在650℃饱和蒸汽环境下,与T92钢相比,T92钢铝化物涂层的抗高温蒸汽氧化性能得到大幅提高。

  • 图3 T92钢及渗铝涂层在650℃蒸汽环境下氧化3 000h后的氧化动力学及放大图

  • Fig.3 Oxidation kinetic curves of T92and aluminide coating in saturated steam at 650℃ for 3 000h

  • 图4 为T92钢、T92钢铝化物涂层在650℃纯水蒸汽中分别氧化100h和3 000h后的表面形貌。经过100h氧化后,T92钢表面被致密氧化膜掩盖,且氧化膜表面有微裂纹(图4a),局部放大后,氧化物形貌为颗粒状。经过3 000h氧化后(图4b),有大区域剥落出现,未剥落区有明显裂纹,而剥落区表面较为连续、平滑,且颗粒状氧化物呈现出更为疏松、尺寸更大、分布不规则的特征。在650℃ 氧化100h后,铝化物涂层氧化物不明显,但有裂纹出现(图4c),可能是由T92基材和铝化物涂层之间的热膨胀系数不匹配造成的。氧化3 000h后 (图4d),铝化物涂层表面氧化物增多,裂纹宽度没有发生明显变化,涂层整体较为平滑,未发生明显剥落现象,表明铝化物涂层与基体结合良好,局部放大后,氧化膜形貌为颗粒状,并出现少许针状氧化物。

  • 图4 T92钢在650℃纯水蒸汽环境下氧化后的表面形貌图

  • Fig.4 Surface morphologies of T92steel oxidized in saturated steam at 650℃

  • 图5 为T92钢、T92钢铝化物涂层在650℃纯水蒸汽中分别氧化100h和3 000h后的截面形貌。 T92钢在650℃纯水蒸汽中氧化膜呈双层结构,两层氧化膜间有明显的缺陷。氧化100h后(图5a),氧化膜整体均匀、致密,内层与基体结合良好。氧化3 000h后(图5b),氧化膜明显变厚,外层氧化膜发生明显脱落。氧化膜下方基体内氧化严重,但内氧化物形状无规律可循,可能是因晶粒的尺寸、取向不一致所致。此外,氧化膜与基体之间出现明显的空洞等缺陷。T92钢铝化物涂层在650℃纯水蒸汽中氧化100h后(图5c),表面形成了一层厚度约为0.3 μm的氧化膜,涂层与基体之间结合较好,未发现明显裂纹;当氧化时间延长至3 000h后(图5d),氧化膜厚度没有发生明显变化,但基体中出现了较多的白亮相,表明在氧化过程中,基体的相结构发生了明显的变化,具体原因将随后讨论。

  • 图5 T92钢在650℃纯水蒸汽环境下氧化后的截面形貌图

  • Fig.5 Cross-sectional morphologies of T92steel oxidized in saturated steam at 650℃

  • 图6 为T92钢、T92钢铝化物涂层在650℃纯水蒸汽坏境下氧化3 000h后的元素面分布图。由图可知,T92表面氧化膜由外层Fe氧化物和内层Fe、 Cr氧化物组成。分析表明,Fe2O3 为外氧化膜主要成分,FeCr2O4 为内氧化膜主要成分。渗铝层表面的氧化膜富含O和Al,研究表明氧化膜为Al2O3 膜。通过元素截面分布图可以看出,长时间氧化后,基体中白亮相处W元素富集。

  • 图6 T92钢在650℃纯水蒸汽环境下氧化3 000h后的元素面分布图

  • Fig.6 Cross-sectional element map-scanning of T92steel oxidized in saturated steam at 650℃ for 3 000h

  • 2.3 铝化物涂层氧化过程中的元素互扩散与组织演变

  • 涂层在650℃下经0h、1 000h、3 000h热暴露后对应的截面形貌如图7所示。由图可知,随着氧化时间的增加,因氧化造成涂层外层(OL)的退化并不明显,但与氧化前厚度略有减少。相反,涂层中IDZ-Ⅰ区、IDZ-Ⅱ区的厚度随氧化时间的延长增加明显,3 000h后IDZ-Ⅰ区厚度约为24 μm,IDZ-Ⅱ 区厚度约为27.53 μm,说明涂层与基体互扩散较为严重。同时,涂层各层连续、均匀、致密、组织稳定,与上述研究结果相对应。

  • 图7 650℃热暴露不同时间后的涂层形貌

  • Fig.7 Morphology of coating after thermal exposure at 650℃ for different time

  • 图8 为3 000h氧化后OL层(C区)和IDZ-Ⅰ 层(B区)的TEM表征。通过对物相的衍射斑点分析可知,外层晶粒为FeAl相;IDZ-Ⅰ层基体为bcc结构的铁素体,基体中存在第二相,通过对斑点标定可知,第二相为fcc结构的Fe3Al相,且基体与Fe3Al相存在取向关系[001] α-Fe//[001] Fe3Al

  • 图8 3 000h氧化后外层和IDZ-Ⅰ 层的TEM表征

  • Fig.8 TEM characterizations of outer layer and IDZ-Ⅰ after 3 000h oxidizing

  • 图9 为3 000h氧化后IDZ-Ⅱ 层的TEM表征。由图可知,IDZ-Ⅱ 层存在FeAl2相(图e C区域)、富Cr相(图a A区域)以及富W相(图a B区域),其中对Cr相的SAED衍射花样标定,发现该区域中基体与物相存在取向关系[221]M23C6//[110] α-Fe,与氧化1 000h后的结果一致。

  • 图9 3 000h氧化后IDZ-Ⅱ 层的TEM表征

  • Fig.9 TEM characterizations of IDZ-after Ⅱ 3 000h oxidizing

  • 由此可见,随着氧化时间的增加,涂层出现了明显的退化现象。氧化1 000h后,OL层仍为FeAl相,IDZ-Ⅰ 层为Fe3Al相,并出现少量FeAl2相形核与长大,基体为 α-Fe,IDZ-Ⅱ 层出现FeAl2 相、富Cr的M23C6 相、富W的Laves相及基体相 α-Fe; 氧化3 000h之后,物相种类没有明显的变化。尽管涂层发生了明显的退化,但涂层元素仍以Fe、Al为主,对涂层的使用寿命并无明显影响。

  • 3 讨论

  • T92钢在650℃饱和蒸汽环境下氧化时,Cr2O3 比Fe2O3 更容易形成且更稳定。然而,在氧化最初阶段,因为T92钢中Cr含量比较低,故不能形成完整的Cr2O3 膜。随着氧化的不断进行,O会继续通过氧化膜而氧化基材,Fe、Cr也会通过氧化膜到达水蒸气/氧化膜界面。因为Cr的扩散速度较Fe慢, Fe会优先通过氧化膜而扩散,导致富Cr内层、Fe2O3 外层在氧化膜/水蒸气界面形成,且氧化层上有孔洞,层间也出现了联贯、间隔较大的裂纹,导致Fe2O3 外氧化膜容易脱落[24]

  • 在氧化最初阶段,T92钢铝化物涂层中的Al元素发生选择性氧化,形成连续的Al2O3 膜。由于Al2O3 膜的生长有O元素通过氧化膜向内扩散所控制,且O的扩散速度非常慢[26],因此,即便是在水蒸气中氧化3 000h后,Al2O3 膜的厚度仍然没有超过1 μm(图6b)。一般认为,在较低温度下稳定相 α-Al2O3 很难形成,但在100%水蒸汽条件下,发现水蒸气可以减小 α-Al2O3形核的能垒,加速Al离子的扩散,从而加速向 α-Al2O3 的转变[27],故通过粉末包埋所获得的涂层在650℃较低温度下,表面也能够快速形成 α-Al2O3 [28-29],而 α-Al2O3 是最稳定的氧化铝结构,具有良好的稳定性和黏附性。

  • 在氧化过程中,涂层中的Al也会在高铝活性梯度的驱动下向内扩散,在涂层与基体之间形成扩散层(图7)。在热暴露期间,IDZ厚度继续增加,这一结果与Al浓度变化一致,表明了Al、Fe互扩散严重。基体中Ni、Cr也向外扩散到涂层中,有助于IDZ的生长[30]。但扩散层厚度的增长速度会随着时间的延长而逐渐减小,这是由涂层与基体之间的铝浓度梯度减小造成的。由于涂层在制备过程中存在互扩散,原始涂层会形成Fe3Al相的IDZ层。氧化1 000h后,随着Fe含量的增加,IDZ-Ⅰ 层出现少量FeAl2 相形核与长大,IDZ-Ⅱ 层出现FeAl2相、富Cr的M23C6 相,这是互扩散的直接结果。3 000h后由于Al在不断消耗,涂层基体进一步转变为富Fe相(α-Fe)。在此过程中,IDZ的厚度不断增加,但其物相种类基本不变。故涂层的退化可总结为: Fe从基体向FeAl层扩散,使Fe在扩散层附近富集,同时,Al从FeAl层向基体扩散,导致Al含量局部减少。此外,扩散层中存在较高浓度的Cr,这可能会破坏原始FeAl的结构[31]

  • 在氧化过程中,Fe-Al金属间化合物始终是涂层的主要物相,IDZ中富Cr和富W相的形成主要是由基体中Cr等元素向IDZ扩散迁移,导致涂层结构明显恶化。富Cr相的析出表明,Cr在Fe-Al金属间化合物中过度饱和,可提高Fe-Al金属间化合物的塑性;也可作为Al和其他基体元素的扩散屏障[32],导致更多的Al被限制在Fe-Al金属间化合物中;富Cr相的析出为Cr向涂层外层扩散提供了来源,向外扩散的Cr有助于在较低的临界铝浓度下维持Al2O3 的生长,延长了涂层的使用寿命。但随着时间的增加,富Cr相的粗化明显,这可能对铝化物涂层的塑性产生负面影响。另外在IDZ层,Cr与C的亲和性较高[21],导致碳化铬析出。IDZ-Ⅱ层出现富W的Laves相可能是从基体中析出的第二相,由于W原子半径大,在bcc相中扩散系数低。随着时效的增加,Laves相析出越多,并且极易长成大颗粒,使系统的总界面能降低,系统越稳定。

  • 4 结论

  • (1)采用低温粉末包埋法在T92钢表面制备的铝化物涂层为30.4 μm厚的双层结构,各层结构均匀且组织稳定。

  • (2)在650℃纯水蒸汽中氧化至3 000h后, T92钢外层疏松氧化物Fe2O3与内层氧化物FeCr2O4 呈双层结构,外层氧化膜发生严重剥落。T92钢铝化物涂层表面形成厚度仅为0.3 μm的 α-Al2O3 膜。

  • (3)铝化物涂层在650℃长期蒸汽氧化过程中发生了显著的结构变化,但经过3 000h氧化,也能很大程度保持涂层微观结构的稳定性和完整性。

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  • 参考文献

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    • [2] 王伟,唐丽英,李文胜,等.T92 钢内压蠕变试验组织性能老化规律[J].热力发电,2016,45(9):1-6.WANG Wei,TANG Liying,LI Wensheng,et al.Aging law of microstructure and properties of T92 steel under internal pressure creep test[J].Thermal Power Generation,2016,45(9):1-6.(in Chinese)

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