金属学报, 2026, 62(7): 1175-1188 DOI: 10.11900/0412.1961.2025.00203

综述

Nb稳定化奥氏体不锈钢在第四代核电领域的研究进展

陈胜虎,1,2, 谢昂1, 杨彬彬2, 刘涌涛2, 陈思含1, 赵明久1,2, 姜海昌1,2, 戎利建,1,2

1 中国科学院金属研究所 中国科学院核用材料与安全评价重点实验室 沈阳 110016

2 中国科学技术大学 材料科学与工程学院 沈阳 110016

Research Progress of the Niobium-Stabilized Austenitic Stainless Steels for Generation-IV Nuclear Reactors

CHEN Shenghu,1,2, XIE Ang1, YANG Binbin2, LIU Yongtao2, CHEN Sihan1, ZHAO Mingjiu1,2, JIANG Haichang1,2, RONG Lijian,1,2

1 CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China

2 School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China

通讯作者: 陈胜虎,chensh@imr.ac.cn,主要从事核用结构材料研究;戎利建,ljrong@imr.ac.cn,主要从事特种合金研究

编委: 李海兰

收稿日期: 2025-07-15   修回日期: 2025-09-03  

基金资助: 中国科学院战略性先导科技专项(XDA0410000)
中核集团领创科研项目

Corresponding authors: CHEN Shenghu, professor, Tel:(024)23971981, E-mail:chensh@imr.ac.cn;RONG Lijian, professor, Tel:(024)23971979, E-mail:ljrong@imr.ac.cn

Received: 2025-07-15   Revised: 2025-09-03  

Fund supported: Strategic Priority Research Program of Chinese Academy of Sciences(XDA0410000)
LingChuang Research Project of China National Nuclear Corporation

作者简介 About authors

陈胜虎,男,1986年生,研究员,博士

摘要

奥氏体不锈钢因其良好的综合性能、成熟的制造工艺以及在压水反应堆中数十年的服役经验,是第四代核电反应堆核心部件的候选材料。然而,在长期高温、强辐照环境下,奥氏体不锈钢暴露出蠕变强度不足、组织稳定性较差、辐照脆化敏感性较高等不足。近年来,Nb元素的合金化可有效调控奥氏体不锈钢的第二相析出行为及辐照缺陷的形成和演化过程,有望实现高温蠕变性能与抗辐照性能的同步提升,成为开发新一代耐高温、抗辐照、长寿命奥氏体不锈钢的重要途径。本文系统综述了Nb稳定化奥氏体不锈钢在四代核电领域的研究进展,重点从高温蠕变性能和抗辐照性能提升的角度,回顾了其成分设计与优化的发展历程及应用现状,分析了Nb添加对奥氏体稳定性、初生NbC形成机制以及含Nb第二相析出和强化机制的影响,探讨了其对强韧性、蠕变强度、高温组织稳定性、辐照诱发缺陷演化的调节机制,并对未来Nb稳定化奥氏体不锈钢的发展进行了展望。

关键词: Nb稳定化奥氏体不锈钢; 强韧性匹配; 蠕变强度; 高温组织稳定性; 辐照诱发缺陷; 含Nb第二相

Abstract

Austenitic stainless steels are utilized in the in-core and out-of-core structural components of various Generation-IV nuclear reactors owing to their excellent comprehensive properties, mature manufacturing processes, and decades of service experience in pressurized water reactors. However, after prolonged exposure to high temperature and high-intensity irradiation, these steels gradually exhibit some performance limitations, including lower creep strength, inadequate microstructural stabilities, and higher radiation embrittlement sensitivities. Recently, alloying with Nb has been proven to effectively regulate the precipitation behavior of secondary phases and the formation and evolution of irradiation defects in austenitic stainless steels, promising simultaneous improvements in creep strength and irradiation tolerance. Therefore, Nb alloying has emerged as a key pathway for the development of next-generation austenitic stainless steels with enhanced high-temperature performance, improved irradiation resistance, and extended service life. This paper systematically reviews the research progress of Nb-stabilized austenitic stainless steels for Generation-IV nuclear power applications, including the compositional design and optimization to improve the creep properties and irradiation resistance, the effects of Nb addition on austenite stability, the formation mechanisms of primary NbC, and the precipitation behavior of Nb-bearing secondary phases. Additionally, the mechanisms by which Nb alloying influences the strength-ductility balance, creep strength, high-temperature microstructural stability, and irradiation-induced defect evolution are discussed. Finally, some future research prospects of Nb-stabilized austenitic stainless steels for Generation-IV nuclear reactors are discussed.

Keywords: niobium-stabilized austenitic stainless steel; strength-ductility balance; creep strength; high-temperature microstructural stability; irradiation-induced defect; Nb-bearing secondary phase

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本文引用格式

陈胜虎, 谢昂, 杨彬彬, 刘涌涛, 陈思含, 赵明久, 姜海昌, 戎利建. Nb稳定化奥氏体不锈钢在第四代核电领域的研究进展[J]. 金属学报, 2026, 62(7): 1175-1188 DOI:10.11900/0412.1961.2025.00203

CHEN Shenghu, XIE Ang, YANG Binbin, LIU Yongtao, CHEN Sihan, ZHAO Mingjiu, JIANG Haichang, RONG Lijian. Research Progress of the Niobium-Stabilized Austenitic Stainless Steels for Generation-IV Nuclear Reactors[J]. Acta Metallurgica Sinica, 2026, 62(7): 1175-1188 DOI:10.11900/0412.1961.2025.00203

第四代核能系统是全球核能产业下一步发展的技术方向,具备安全和可靠性好、燃料利用率高、废物产生量小等优势[1~4]。目前全球在研的第四代核能系统主要包括超高温气冷堆、超临界水冷堆、气冷快堆、铅冷快堆、钠冷快堆、熔盐堆等六种堆型[4]。根据《中国核能科技创新发展报告(2025)》[5],我国在高温气冷堆、钠冷快堆、铅冷快堆、熔盐堆等反应堆技术方面均取得了重要进展。为了实现高燃耗、高堆芯温度、长寿命等高参数技术要求,反应堆的服役工况更加苛刻,包括更高的运行温度、更高的辐照剂量和更严重腐蚀的冷却介质,六种堆型的服役工况如表1[6~10]所示。具有更高性能的结构材料是保证第四代反应堆高效运行的基础,目前的候选结构材料包括奥氏体不锈钢、铁素体/马氏体钢、镍基合金、难熔金属、陶瓷等[10~25]。其中,奥氏体不锈钢凭借良好的综合性能、成熟的制造工艺以及在压水反应堆中约50年的运行经验,被用于钠冷快堆、铅冷快堆等四代反应堆的燃料组件、堆容器、堆内构件、热交换器、回路管道等核心设备[26~30]。随着奥氏体不锈钢在四代反应堆中服役时间的延长和辐照剂量的增加,早期使用的304H、316H等奥氏体不锈钢暴露出高温持久强度不足、组织稳定性差、辐照脆化倾向高等问题[31~40],影响了设备的服役寿命以及反应堆工艺参数的提升。

表1   六种第四代反应堆系统的服役工况[6~10]

Table 1  Summary of the reactor core environments of the six Generation-IV reactor systems[6-10]

Reactor systemCoolantOutlet temperature / oCNeutron spectrum, maximum dose

Sodium fast reactor

Lead fast reactor

Gas fast reactor

Supercritical water reactor

Very high temperature reactor

Molten salt reactor

Sodium

Lead or lead-bismuth

Helium

Supercritical water

Helium

Molten salt

500-550

480-570

~850

510-625

900-1000

700-800

Fast, 150 dpa

Fast, 100 dpa

Fast, 90 dpa

Fast/thermal, 70 dpa

Thermal, 20 dpa

Thermal, 200 dpa

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奥氏体不锈钢通常采用固溶处理,目的是使合金元素充分溶解于奥氏体基体中。奥氏体不锈钢在高温下仍能保持稳定的奥氏体组织,因而展现出优异的高温力学性能,ASME BPVC Section III Division 5[41]针对高温反应堆的设计,规定了奥氏体不锈钢的允许使用温度范围,304H和316H不锈钢允许使用温度的上限为816 ℃。然而,奥氏体不锈钢的具体使用温度取决于设计寿命,这是由于其在高温长时服役过程中会析出有害第二相,导致力学性能退化,从而降低了允许使用温度。奥氏体不锈钢高温服役过程中产生的第二相主要包括M23C6M6C等碳化物,以及σ相、χ相、η相等金属间化合物[34~40]。此外,在四代核电反应堆的服役温度范围内,辐照离位损伤产生的空位浓度远超热平衡态浓度,过饱和空位聚集形成空洞而引起辐照肿胀[42,43],加之四代核电的辐照剂量高,辐照诱发的空洞肿胀也是影响奥氏体不锈钢使用温度和设计寿命的主要因素。因此,如何改善奥氏体不锈钢的高温组织稳定性和抗辐照肿胀性能是高性能奥氏体不锈钢研发的关键。

近年来,通过Nb添加调控奥氏体不锈钢中第二相析出及辐照缺陷形成,实现高温蠕变强度和抗辐照性能的同步提升,被视为四代反应堆用耐高温、抗辐照、长寿命奥氏体不锈钢的重要研发路线。在此背景下,本文综述了Nb稳定化奥氏体不锈钢在四代核电领域的研究进展,从Nb稳定化奥氏体不锈钢的成分优化设计出发,分析了Nb添加对奥氏体稳定性、初生NbC等微观组织特征,以及常规力学性能、蠕变性能等的影响,进而阐明了Nb添加对组织稳定性的影响机制,并介绍了Nb稳定化奥氏体不锈钢在四代核电领域的工程应用现状,以期为Nb稳定化奥氏体不锈钢的成分设计、组织调控及其在四代核电领域的工程应用提供理论支撑和技术指导。

1Nb奥氏体不锈钢的成分优化设计

20世纪30年代,为了解决不锈钢焊缝区或热影响区出现的晶间腐蚀问题,Nb作为稳定化元素广泛应用于奥氏体不锈钢中[44]。20世纪70年代,氩氧脱碳技术(argon-oxygen decarburizing,AOD)的发展实现了在非真空条件下将不锈钢深度脱碳至0.02% (质量分数,下同)以下,解决了碳化物析出导致的晶界腐蚀问题,不锈钢产业对Nb的需求持续减少。近年来,人们发现Nb添加可实现弥散分布第二相的调控,以及影响辐照缺陷的扩散与聚集,Nb添加重新被定义为改善奥氏体不锈钢高温强度和抗辐照性能的重要途径[45,46]表2[44~46]列举了国际上研发的先进核电反应堆用含Nb奥氏体不锈钢,其发展历程可以分为三个阶段。第一阶段主要是在304/316的基础上,通过添加一定含量的Nb,含量控制为(8C~1.0)%,以提高高温持久强度,降低辐照肿胀率,典型牌号包括EP302、347H、TP347HFG、316Nb和FV548,但是,高温δ-铁素体的存在会影响其高温强度。第二阶段主要以提高奥氏体稳定性、降低初生NbC形成倾向为主,一方面将Ni含量提高至15%,Cr含量降低至16%,实现了Cr当量(Creq) / Ni当量(Nieq)调控,消除了高温δ-铁素体,并进一步提高了奥氏体稳定性,典型牌号为DIN1.4981和EI847;另一方面,在Creq / Nieq比调控基础上,降低Nb含量,匹配添加Ti元素,降低了大尺寸初生NbC对性能的影响程度,典型牌号为PNC316、PNC1520和EK164。第三阶段主要是借助Nb/Ti/V/N复合添加以实现多种类型第二相的弥散分布调控,从而实现高温蠕变强度与抗辐照性能的同步提升,典型牌号为Alloy 709和高温超细沉淀强化(HT-UPS)钢。Alloy 709中的第二相主要为NbC、Nb(C, N)碳化物和Z相(CrNbN),HT-UPS钢的第二相主要为(Nb, Ti, V)C碳化物和FeTiP相。

表2   先进核电反应堆用含Nb奥氏体不锈钢的化学成分[44~46] (mass fraction / %)

Table 2  Chemical compositions of niobium-stabilized austenitic stainless steels for advanced nuclear reactors[44-46]

TypeCNiCrMnMoSiNbTiVN

EP302

347H

TP347HFG

316Nb

FV548

DIN1.4981

EI847

PNC316

PNC1520

EK164

Alloy 709

HT-UPS

0.08-0.12

0.04-0.10

0.06-0.10

≤ 0.08

0.1

0.04-0.10

0.04-0.06

0.06

0.06

0.05-0.09

≤ 0.10

0.08

8.0-10.0

9.0-13.0

9.0-13.0

10.0-14.0

11.5

15.5-17.5

15.0-16.0

14.0

20.0

18.0-19.5

23.0-26.0

16.0

14.0-16.0

17.0-19.0

17.0-19.0

16.0-18.0

16.5

15.5-17.5

15.0-16.0

16.0

15.0

15.0-16.5

19.5-23.0

14.0

0.4-0.8

≤ 2.0

≤ 2.0

≤ 2.0

1.2

≤ 1.5

0.4-0.8

1.7

1.7

1.5-2.0

≤ 1.5

2.0

-

-

-

2.0-3.0

1.5

1.6-2.0

2.7-3.2

2.5

2.5

2.0-2.5

1.0-2.0

2.5

2.2-3.0

≤ 0.75

≤ 1.0

≤ 0.75

0.35

0.3-0.6

≤ 0.4

0.6

0.5

0.3-0.6

≤ 1.0

0.4

0.7-1.0

8C-1.0

8C-1.0

10C-1.0

0.7

10C-1.2

≤ 0.9

(Nb + Ta): 0.08

0.1

0.1-0.4

0.1-0.4

0.1

-

-

-

-

-

-

-

0.1

0.25

0.25-0.5

0.02-0.2

0.3

-

-

-

-

-

-

-

-

-

0.15

-

0.5

-

-

-

≤ 0.1

-

-

-

-

-

-

0.1-0.25

-

Note: HT-UPS—high temperature-ultrafine precipitate strengthened

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2 Nb添加对微观组织的影响

2.1 奥氏体稳定性

奥氏体不锈钢的相组成与合金元素及其含量密切相关,可由Creq和Nieq公式进行预测[47]

Creq=[Cr]+[Mo]+1.5[Si]+0.5[Nb]
Nieq=[Ni]+30[C]+0.5[Mn]

式中,[M]为元素M的质量分数。可见,作为铁素体形成元素,Nb添加会增加Creq,提高Creq / Nieq比,促进高温δ-铁素体的形成。Nb含量为0~0.9%的Fe-15Cr-10Ni奥氏体不锈钢铸态组织对比证实,Nb添加未改变凝固模式,Nb含量的提高促进了析出的δ-铁素体的数量(图1)。研究[48~52]表明,δ-铁素体在四代核电材料服役温度范围内会分解为Laves、σ相等金属间化合物,其分解速率比奥氏体基体中的相析出速率快约100倍,显著降低了钢的塑性、冲击韧性、持久强度、疲劳裂纹扩展速率及耐腐蚀性能。因此,Nb稳定化奥氏体不锈钢的发展趋势为调控Ni、Cr含量,以降低Creq / Nieq比,获得单相奥氏体组织。

图1

图1   不同Nb含量Fe-15Cr-10Ni氏体不锈钢的铸态组织

Fig.1   SEM images of as-cast Fe-15Cr-10Ni austenitic stainless steels with the Nb contents (mass fraction) of 0.01% (a), 0.3% (b), and 0.9% (c)


奥氏体不锈钢的层错能(γSFE)较低,塑性变形过程中发生形变诱发马氏体相变的倾向高,而形变诱发马氏体的难易程度是衡量奥氏体稳定性的另一指标。成分对奥氏体不锈钢γSFE影响的经验公式如下[53]

γSFE(mJm-2)=2.2+1.9[Ni]+40[C]-0.016[Cr]-
3.6[N]-2.9[Si]+0.5[Mn]+0.77[Mo]

目前,关于Nb含量对层错能的影响规律尚不确定。Martinez等[54]采用X射线衍射法发现,Nb添加会显著降低Fe-15Cr-15Ni奥氏体不锈钢的层错能,并归因于Nb添加降低电子/原子比和增加电子空位数。然而,第一性原理计算[55]表明,Nb添加会显著提高Fe-Cr-Ni奥氏体不锈钢的层错能,认为Nb通过改变局部磁矩进而影响磁性变化对层错能的贡献。本团队前期[56]研究不同Nb添加量奥氏体不锈钢的变形行为时发现,NbC的形成可以固定C元素而降低层错能,促进形变诱发马氏体相变的发生。新一代Nb稳定化奥氏体不锈钢中Ni含量增加可提高材料的层错能,从而抵消Nb添加带来的影响,降低形变诱发马氏体相变倾向。

2.2 初生NbC

Nb作为一种强碳化物形成元素,与C具有较高的亲和力,会优先与C结合形成NbC。NbC的析出行为与Nb含量密切相关,以C含量为0.1%的Fe-15Cr-10Ni奥氏体不锈钢为例,根据Thermo-Calc热力学计算结果可知,Nb含量≤ 0.1%时,NbC从奥氏体固溶体中析出;Nb含量≥ 0.3%后,NbC的析出温度略高于液相线温度,以初生相的形式从液相直接析出,且随Nb含量的增加,初生NbC的析出数量显著增加(图2a)。初生NbC的形成机制为共晶反应,这是由于在凝固过程中,正偏析倾向显著的Nb易在枝晶间处偏聚,满足了共晶反应的成分条件,从而在凝固末期发生了液相(L)→NbC + γ的共晶反应(图2b)。初生NbC的形成降低了奥氏体基体中的C含量,抑制富Cr的M23C6碳化物析出。Nb含量对奥氏体不锈钢铸态组织影响的研究[56]表明,Nb / C原子比为0.38时,无法完全固定C原子,晶界仍析出M23C6碳化物;Nb / C原子比为1.16时,基本上能固定C原子,有效抑制了M23C6碳化物的析出。

图2

图2   Nb含量对Fe-15Cr-10Ni奥氏体不锈钢中NbC析出行为的热力学模拟计算与含0.9%Nb铸态组织的OM像

Fig.2   Thermo-Calc calculation of NbC formation in Fe-15Cr-10Ni ausenitic stainless steels with different Nb contents (a) and OM image of as-cast samples with 0.9%Nb (b) (Inset in Fig.2b is the high magnification image of the primary NbC)


然而,初生NbC的尺寸较大且分布不均匀。一方面,变形过程中粗大初生NbC周围存在明显的应力集中,NbC界面处易萌生裂纹,增加了裂纹扩展速率,降低了塑性[56~58];另一方面,腐蚀过程中粗大初生NbC的腐蚀行为与基体不同,以控氧的Pb-Bi合金腐蚀性能为例,初生NbC的氧化会伴随着体积膨胀和气体产生,降低了氧化膜致密性,增加了腐蚀速率[59]。此外,粗大的初生NbC会消耗较多的Nb,不利于服役过程中细小弥散二次NbC的析出,削弱二次NbC对性能的改善作用。因此,如何调控初生NbC的尺寸和分布是改善Nb稳定化奥氏体不锈钢性能的关键。

由于后续的热加工和固溶处理对初生NbC形貌和尺寸的影响较小,调控铸态合金中的初生NbC是有效措施,主要包括降低凝固偏析以抑制NbC的共晶析出[60,61]、提高NbC的固溶度积实现其部分回溶[56,62]。凝固期间NbC形成机制的研究[60]表明,Nb偏析是导致凝固过程中NbC形成的主要原因,缩小凝固温度区间是降低凝固偏析的有效途径,常用方法是降低钢液中S、P等易偏析元素的含量,提高凝固温度,可使得凝固区间变窄,有效缓解Nb偏析倾向。同时,尽量减小锭型比(铸锭高度/直径(H / D)),以抑制铸锭1/4厚度处形成的A型偏析和铸锭中心形成的V型偏析,从而减少宏观偏析区形成的NbC的数量,当锭型比小于1时,在100 t铸锭中也未观察到大尺寸共晶形态NbC[61]。后续处理对NbC调控的研究[63]表明,利用NbC在奥氏体中的平衡固溶度积[Nb]∙[C]可实现NbC的回溶, NbC在奥氏体中的平衡固溶度积为[63]

lg{[Nb][C]}=2.96-7510 / T

式中,T为热力学温度。由 式(4)可见,NbC在奥氏体中的平衡固溶度积随温度的升高而增大。然而,当温度超过1280 ℃时,便会形成δ-铁素体,因此应选择低于δ-铁素体形成温度的温度进行均质化处理。计算可得,1250 ℃下[Nb]∙[C]平衡固溶度积为0.0107,可将0.037%的C和0.287%的Nb回溶到奥氏体中,保温处理过程中,将连续网状分布的初生NbC调控为断续分布[62]。但当Nb含量高于0.3%时,受限于平衡固溶度积,无法实现初生NbC的完全回溶。

3 Nb添加对力学性能的影响

3.1 强韧性匹配

Nb的原子半径比Ni和Fe原子大15%~18%,其晶格常数明显大于W和Mo,因而具有较强的固溶强化作用。Nb含量为0~0.9%的铸态奥氏体不锈钢的拉伸性能测试结果[56]表明,Nb含量提高至0.3%时,室温和550 ℃下的屈服强度略有增加,Nb含量提高至0.9%时,屈服强度的增加程度增大。由于Nb在奥氏体中的溶解度有限(式(4)),较多的Nb会以NbC形式存在于奥氏体钢中。初生NbC的尺寸较大且数密度较小,无法起到有效的沉淀强化效果。初生NbC与奥氏体的纳米压痕测试结果(图3a[62])表明,初生NbC的硬度(约9.75 GPa)显著高于奥氏体基体(约6.31 GPa),造成两者之间变形的不协调性。室温变形后初生NbC附近的变形组织分析(图3b[62])证实,硬度较低的奥氏体基体优先发生塑性变形,位错滑移带被初生NbC所阻滞而产生位错塞积,造成界面处微裂纹的萌生。随着变形温度的提高,初生NbC与奥氏体间的变形协调性显著提高,550 ℃变形后,初生NbC/奥氏体基体界面处形成二次裂纹的概率很低,仅发生在个别较大尺寸NbC处[56]

图3

图3   铸态试样中初生NbC和奥氏体的纳米压痕结果及室温变形后组织形貌的TEM像[62]

Fig.3   Nanoindentation load-depth curves of primary NbC and austenite (a) and TEM images near primary NbC after deformation to a true strain of 10% (b) in as-cast sample (SF—stacking fault. Inset in Fig.3b refers to the corresponding SAED pattern of NbC)[62]


在冲击载荷作用下,初生NbC分布对性能的影响更大,表现为加工态产品中冲击韧性各向异性问题。以Nb含量为0.9%的含Nb奥氏体不锈钢锻棒为例,纵向样品的室温冲击吸收功约180 J,而横向样品的冲击吸收功约60 J,仅为纵向样品的1/3。冲击韧性各向异性与初生NbC沿横纵方向的分布差异有关,沿变形方向(锻棒纵向)呈条带状分布,沿垂直于变形方向(锻棒横向)呈弥散分布(图4a)。在冲击载荷作用下,裂纹优先在条带状NbC处萌生并扩展(图4bc),从而导致横向样品冲击韧性显著下降。

图4

图4   Fe-15Cr-10Ni奥氏体不锈钢中初生NbC的三维分布形貌及室温冲击断裂后组织形貌的SEM像和Nb元素EDS面扫描图

Fig.4   Three-dimensional (3D) view of primary NbC in the as-cast Fe-15Cr-10Ni-0.9Nb ausenitic stainless steel (a) and SEM image of the fracture surface morphology (b) and corresponding EDS mapping of Nb (c)


3.2 高温蠕变性能

第四代核电反应堆的运行温度均高于500 ℃,ASME BPVC Section III Division 5将奥氏体不锈钢的“高温”界定为800 ℉ (427 ℃),当奥氏体不锈钢的设计温度超过427 ℃时,需要考虑蠕变等时间相关的性能指标[41]。因此,高温蠕变性能是评估奥氏体不锈钢在服役条件下使用寿命的关键指标之一。304和316奥氏体不锈钢是目前第四代核电反应堆的主要结构材料,ASME、RCC-MRx、JSME等国际标准均给出了两类材料的高温蠕变强度[41,64,65]图5汇总了304、316奥氏体不锈钢和各类Nb稳定化奥氏体不锈钢的蠕变强度,图中Larson-Miller参数(LMP)为T与蠕变断裂时间(tr)的组合参数,C为材料常数。其中,304和316不锈钢的数据摘取自ASME标准,TP347HFG、Alloy 709和HT-UPS钢为文献报道数据[19]。对比可见,Nb稳定化TP347HFG钢的持久强度显著高于304和316奥氏体不锈钢,新型的Nb、Ti复合稳定化的Alloy 709和HT-UPS钢的蠕变强度则更加优异。

图5

图5   不同牌号奥氏体不锈钢的高温蠕变强度

Fig.5   Creep-rupture stresses of several austenitic stainless steels (LMP—Larson-Miller parameter; T—temperature, K; tr—creep rupture time, h; C—material constant, C = 20)


Nb稳定化奥氏体不锈钢较好的持久蠕变性能源于基体内弥散分布第二相对位错的有效钉扎作用,已报道的第二相类型包括NbC、Nb(C, N)、(Nb, Ti)C、(Nb, Ti)(C, N)、CrNbN等。如何促进含Nb第二相的弥散析出是提高Nb稳定化奥氏体不锈钢蠕变性能的关键,为此,研究人员提出了多种处理方法[66~77]。本团队近期研究[56]表明,均质化处理大幅提高了Nb在奥氏体基体中的固溶度,特别是对于Nb含量较高(≥ 0.5%)的奥氏体钢,通常存在一定数量的初生NbC,均质化处理可显著促进初生NbC的回溶,从而有利于在后续热处理、热时效以及持久蠕变过程中含Nb第二相的弥散析出。Solenthaler等[66]在固溶处理的基础上,提出了短时硬化热处理工艺,具体工艺为:1050 ℃保温处理后随即连续冷却至950 ℃,使得TP347钢中析出大量弥散细小的二次Nb(C, N),显著提高了沉淀强化效果。Shin等[67,68]采用热机械处理(thermo-mechanical processing,TMP)方法,首先通过在再结晶温度以下的热轧变形,提高奥氏体基体的位错密度,随后的时效处理促进了NbC在位错处的沉淀析出,基于热轧变形过程中的位错形貌调控,可实现时效处理过程中NbC的均匀析出。Zhao等[69]采用0.1%Ti的微合金化和10%预应变相结合的方法,显著促进了Fe-25Ni-18Cr-3Al-0.5Nb奥氏体钢中纳米级(Nb, Ti)C的析出,一方面,由于Ti-C间较高的结合能以及较快的Ti扩散速率,Ti会进入早期形成的NbC晶核而减小了NbC与奥氏体基体间的错配度,有效降低了(Nb, Ti)C的形核能垒;另一方面,预应变引入的高密度位错增加了(Nb, Ti)C的形核位点,并提高了Nb原子的扩散速率。Mu等[70]基于含Nb第二相易在位错处形核的机理,在Fe-21Cr-10Ni-0.2Nb-0.08Ti奥氏体钢铸态样品中引入高密度位错,促进了后续时效过程中纳米级(Nb, Ti)C的均匀弥散析出。Ha等[71]在Nb稳定化15Cr-15Ni奥氏体钢的基础上,采用高氮合金化(0.236%)与700 ℃时效-淬火的多循环热处理相结合的方法,制备出纳米级(Nb, Cr, Fe)(C, N)弥散强化型合金。此后,高氮的Nb稳定化奥氏体钢在高温时效/蠕变过程中可形成细小弥散的Z相(CrNbN)。Z相的粗化速率低,被证实为一种有效的沉淀强化相[72~75]。已有研究证实,Z相的形成机制包括两种:一种是在奥氏体基体中直接脱溶析出,易在位错处形核[72,73];另一种是由优先形核的NbN转变形成,NbN逐渐溶解提供了Z相形成所需的Nb和N[74,75]

研究人员通过合金成分调整和处理工艺创新,一方面开发出更多类型的含Nb第二相;另一方面不断提高含Nb第二相的高温稳定性。利用多种类型第二相的复合沉淀强化作用,是提高高温蠕变强度的主要途径,典型代表是Alloy 709和HT-UPS奥氏体钢。图6[76,77]为Alloy 709和HT-UPS奥氏体钢经高温时效或蠕变作用后基体中析出的第二相形貌。两种材料的基体中均析出了弥散分布的第二相,Alloy 709钢中的第二相主要是在位错线处析出的NbC、Nb(C, N)和Z相(图6a[76]),HT-UPS钢中的第二相以弥散分布的(Nb, Ti, V)C碳化物为主,还有少量的针状FeTiP相(图6b[77])。

图6

图6   Alloy 709和HT-UPS奥氏体不锈钢经高温服役后的第二相形貌[76,77]MC碳化物EDS分析

Fig.6   TEM images of secondary phase in Alloy 709 after aging at 650 oC for 500 h (Inset is the high magnification image of the dislocation (arrowed in Fig.6a)) (a)[76] and HT-UPS steel after creep-rupture test at 700 oC, 170 MPa for 18000 h (Inset is the analytical electron microscopy of MC carbides (arrowed in Fig.6b)) (b)[77]


4 Nb添加对高温组织稳定性的影响

在第四代核电反应堆的运行温度范围内,奥氏体不锈钢经长时服役后,易脱溶析出第二相,包括碳化物(如M23C6M6C)以及金属间化合物(如σ相、χ相、η相),关于常规奥氏体不锈钢中第二相晶体结构和化学成分的研究较为充分[29,30,78,79]。已有Nb稳定化奥氏体不锈钢研究更多关注的是基体中MX碳化物的析出行为,而对其他析出相的研究相对较少。

通常采用温度-时间-析出 (temperature-time-precipitation,TTP)曲线描述不同温度下二次析出相的析出序列以及相互之间的竞争关系。在常规奥氏体不锈钢500~900 ℃热老化过程中,M23C6碳化物优先析出,随后在更长热老化时间后析出多种类型的金属间化合物。以图7a[80]的316L奥氏体不锈钢为例,M23C6碳化物最先析出,孕育期最短;富(Cr, Mo)的η相、χ相和σ相的孕育期较长,析出顺序与温度密切相关,主要取决于Cr和Mo的扩散速率以及金属间化合物与奥氏体基体间的位相关系。相比之下,关于Nb稳定化奥氏体不锈钢的TTP曲线的报道较少,图7b[81]为Nb稳定化20Cr25Ni奥氏体不锈钢的相析出曲线。除基体析出Nb(C, N)碳化物外,Nb添加显著改变了相析出行为,具体表现为第二相化学成分改变、种类增多以及析出顺序改变等。第一,Nb添加延长了M23C6碳化物析出的孕育时间。以常用的650 ℃敏化处理温度为例,316L钢的M23C6碳化物析出孕育期为0.15 h,Nb稳定化20Cr25Ni钢的孕育期延长至35 h,这是由于Nb对C的亲和力高于Cr,降低了M23C6碳化物析出倾向。第二,Nb添加促进了G相的析出,G相为fcc结构,化学表达式为Nb6Ni16Si7,晶界G相析出的孕育期短于M23C6碳化物,这是由于晶界处的残余Nb(C, N)会快速转变为G相。晶内存在的Nb(C, N)也会转变为G相,但需要更长的孕育期。Nb(C, N)转变为G相的机理示意图如图8ab所示,Nb(C, N)中的Nb原子向外扩散以及基体中的Ni和Si原子扩散至Nb(C, N)界面处,满足了G相形核所需的成分条件(图8a);G相的生长机制分为两种:一种是由Nb(C, N)原始界面向外生长,另一种是由Nb(C, N)转变而来,其生长速率受控于Nb(C, N)中Nb的外扩散速率(图8b)。G相向外生长过程中,向外扩散的C原子与Cr结合形成碳化物,形成了G相+ M23C6的双相结构[81,82]。此外,辐照会加速G相的析出,针对Nb稳定化FV548奥氏体钢的研究[20,45]发现,400 ℃以上的中子辐照可促进G相的单独析出,形核位置包括晶界和空洞。本团队近期研究[50,83~85]发现,在Si含量较高的Nb稳定化奥氏体不锈钢中,G相析出也会加剧,甚至诱发奥氏体向铁素体的转变。第三,Nb添加降低了σ相的析出温度,316L钢中σ相的析出温度在700 ℃以上,且需要上千小时的孕育期;然而,Nb稳定化20Cr25Ni钢中σ相的开始析出温度降至约550 ℃,峰值析出温度下(约675 ℃)的孕育期仅约100 h。研究[81,85~87]表明,Nb稳定化奥氏体钢中的Nb(C, N)和G相可促进σ相的析出。Nb(C, N)周围形成σ相的机理示意图如图8cd所示,Nb(C, N)长大消耗C、N原子而导致局部贫C/N微区的出现,G相析出消耗Ni原子而降低了基体中的Ni含量,满足σ相形核所需的成分条件,依附Nb(C, N)形核(图8c);σ相不断长大,最终形成了σ相包裹Nb(C, N)的复相结构(图8d)。

图7

图7   316L钢和Nb稳定化20Cr25Ni奥氏体不锈钢的温度-时间-析出(TTP)曲线[80,81]

Fig.7   Temperature-time-precipitation phase diagrams of 316L steel (a)[80] and Fe-20Cr-25Ni-Nb stabilized austenitic stainless steel (b)[81]


图8

图8   Nb稳定化奥氏体不锈钢中G相和σ相形成机理示意图

Fig.8   Schematics of formation mechanisms of G-phase and σ-phase in Nb-stabilized austenitic stainless steel (a, b) transformation mechanism of Nb(C, N) to G-phase (c, d) formation mechanism of σ-phase adjacent to Nb(C, N)


5 Nb添加对辐照损伤性能的影响

与现役压水堆相比,第四代核电反应堆的辐照剂量更高。以快中子反应堆为例,气冷快堆、铅冷快堆和钠冷快堆的辐照剂量均超过50 dpa[7~9]。316奥氏体不锈钢被最早用于钠冷快堆燃料组件包壳,可承受的最大辐照剂量仅约10 dpa,辐照诱发的空洞肿胀是主要限制因素[88]。第四代核电反应堆的服役温度处于fcc金属材料发生肿胀的温度区间(0.35~0.6TMTM为熔点温度)[42,43],这是由于该温度区间内辐照离位损伤产生的空位浓度远超热平衡态浓度,过饱和空位聚集形成空洞,进而引起肿胀。因此,如何改善奥氏体不锈钢的抗辐照肿胀性能是国际上的关注重点。

Nb稳定化奥氏体不锈钢中Nb的存在形式主要包括固溶态Nb元素以及含Nb析出相,均会对辐照损伤行为产生影响。奥氏体不锈钢中添加Nb可有效降低辐照诱发元素偏聚倾向,这是由于大原子半径元素Nb可有效捕获点缺陷,降低点缺陷的可动性[89~91]。为证实固溶态Nb对辐照肿胀行为的影响,Kawanishi等[92]设计了无C的Fe-16Ni-15Cr合金,以避免NbC带来的影响,考察了50 dpa离子辐照条件下Nb添加对辐照空洞密度和直径的影响规律,如图9[92]所示。与不含Nb合金相比,含0.6%Nb合金的辐照空洞密度略有下降(图9a[92]),辐照空洞平均直径减小,且随辐照温度的提高,平均直径减小越明显(图9b[92])。

图9

图9   Nb添加对Fe-16Ni-15Cr合金50 dpa离子辐照条件下空洞密度和直径的影响[92]

Fig.9   Variation in void number density (a) and mean void diameter (b) in Fe-16Ni-15Cr and Fe-16Ni-15Cr-0.6Nb alloys after irradiation to 50 dpa[92]


相较于固溶态Nb元素,NbC可显著抑制辐照肿胀,通过引入细小弥散的NbC是发展耐辐照奥氏体不锈钢的重要方向[93,94]。Williams等[45]在Dounreay快堆中对比分析了316不锈钢和Nb稳定化FV548不锈钢经30 dpa中子辐照后的肿胀行为,FV548不锈钢的辐照肿胀率显著低于316不锈钢,主要原因是晶内细小弥散分布的NbC可充当捕获点缺陷的陷阱,加速点缺陷的复合。Shin等[95,96]对比分析了316不锈钢和纳米NbC强化型ARES-6P不锈钢经重离子辐照高达200 dpa后的肿胀行为,ARES-6P不锈钢中大量纳米级NbC的存在使得其肿胀率显著低于316不锈钢,进而提出了NbC/基体界面捕获点缺陷的微观机制:辐照产生的点缺陷扩散至NbC/基体界面处并聚集形成团簇,点缺陷在界面处交互作用发生湮灭(图10[96])。此外,Maziasz[97]和Tanaka等[98]研究了Nb/Ti复合稳定化奥氏体不锈钢在300~500 ℃范围内、高通量同位素反应堆(HFIR)中经中子辐照至34~57 dpa后的肿胀行为,结果表明,(Nb, Ti)C纳米颗粒可以优化He泡行为,包括作为He泡异质形核位点、细化He泡尺寸以及阻碍He泡迁移和粗化,从而显著提高合金的抗辐照肿胀性能。

图10

图10   NbC/基体界面捕获点缺陷的微观机制[96]

Fig.10   Schematic illustration of the 3D structure after irradiation (a) and evolution of irradiation-induced defects along the NbC/matrix interface (b)[96] (SIA—self-interstitial atom)


6 Nb稳定化奥氏体不锈钢在第四代核电领域的应用现状

目前,部分Nb稳化奥氏体不锈钢已实现工程应用,EP302不锈钢应用于俄罗斯BREST-OD-300铅铋冷却快堆的堆容器和堆内构件[99],EI847不锈钢应用于俄罗斯BN-350、BN-600钠冷快堆的燃料包壳[100,101],PNC316不锈钢应用于日本JOYO、MONJU钠冷实验快堆[102,103]。Nb稳定化奥氏体不锈钢在第四代核电反应堆中的应用需要综合考虑各种因素,以钠冷快堆为例,需要考虑力学性能(强度、蠕变、蠕变-疲劳)、高温组织稳定性、钠相容性以及焊接性能。两种新型Nb稳定化奥氏体不锈钢HT-UPS和Alloy 709被视为取代现役316H不锈钢的候选材料,可以支撑反应堆运行温度提高、设计寿命延长的技术发展。图11[104]对比了HT-UPS、Alloy 709以及316H钢的综合性能。与316H不锈钢相比,HT-UPS和Alloy 709钢通过在奥氏体基体中引入细小弥散的第二相,显著提高了高温强度、蠕变强度、蠕变-疲劳性能以及高温组织稳定性,同时保持了良好的耐腐蚀性能和焊接性能。相比之下,Alloy 709钢的综合性能更加优异,正在进行纳入ASME III Rules for Construction of Nuclear Facility Components: D5 High Temperature Reactors标准的数据测试工作。美国核管理委员会(Nuclear Regulatory Commission)正在开展将Alloy 709钢用于先进长寿命(60~100 a)钠冷快堆堆容器、管道、堆芯支撑的评估工作[105,106]

图11

图11   Nb稳定化奥氏体不锈钢的综合性能评估[104]

Fig.11   Property ranking chart of Nb stabilized austenitic stainless steels[104]


7 总结与展望

Nb添加有望实现奥氏体不锈钢高温蠕变强度与抗辐照性能的同步提升,是第四代核电用耐高温、抗辐照、长寿命奥氏体不锈钢的重要研发途径,部分牌号材料已获得工程应用。本文介绍了Nb稳定化奥氏体不锈钢的成分设计和组织特征,总结了强韧性、蠕变性能、高温组织稳定性、抗辐照性能等方面的研究现状,未来Nb稳定化奥氏体不锈钢的研究可以重点围绕以下几方面展开研究。

(1) 深化对Nb/Ti/V复合稳定化影响规律的认识,揭示Nb/Ti/V之间的耦合效应。当前研究主要聚焦于Nb、Ti的单独作用机制研究,但有关Nb、Ti之间的交互影响研究还较少,尤其针对Nb、Ti、V三者之间的交互作用的研究更加缺乏。未来研究应进一步深化Nb、Ti、V复合稳定化对第二相析出行为的影响,为发挥三者之间的耦合效应提供策略。

(2) 抑制粗大初生NbC析出,降低其带来的性能退化。较大尺寸的初生NbC本身会引起塑韧性的降低,同时在高温长时服役过程中会演变为脆性有害相,进一步损伤塑韧性,导致Nb稳定化奥氏体不锈钢的塑韧性低于常规奥氏体不锈钢。如何有效抑制粗大初生NbC析出以进一步改善塑韧性,也是后期研究的重点内容。

(3) 提高第二相稳定性,进一步提升高温蠕变强度和抗辐照肿胀性能。当前采用第二相形成元素含量优化与增加第二相形核位点相匹配的方法,实现了更多类型、更高数密度的第二相控制。但某些第二相的粗化速率较快,降低了第二相对位错的钉扎作用以及对辐照缺陷的捕获作用,对高温蠕变强度和抗辐照肿胀性能产生不利影响。未来研究应重点关注第二相与基体的界面关系,发展提高第二相稳定性的新方法,以降低第二相粗化驱动力。

(4) 澄清高温组织稳定性机制,降低有害脆性相的析出倾向。Nb稳定化奥氏体不锈钢在高温长时服役过程中,G相、σ相等有害脆性相的析出倾向较高,是其一大短板。当前关于Nb稳定化奥氏体不锈钢的高温组织稳定性研究较少,尤其是关于较长服役时间下的数据报道更少,制约了改善高温组织稳定性的技术研发。未来研究应重点关注Nb稳定化奥氏体不锈钢中第二相析出序列以及相互之间的演变规律,从根本上抑制有害脆性相的析出,实现高组织稳定性微观组织的调控。

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Effect of δ-ferrite on hot deformation and recrystallization of 316KD austenitic stainless steel for sodium-cooled fast reactor application

[J]. Acta Metall. Sin., 2024, 60: 367

陈胜虎, 王琪玉, 姜海昌 .

δ-铁素体对钠冷快堆用316KD奥氏体不锈钢热变形行为和动态再结晶的影响

[J]. 金属学报, 2024, 60: 367

DOI     

奥氏体不锈钢中δ-铁素体的存在显著影响热加工过程中奥氏体晶粒度的控制,造成晶粒不均匀现象,关于δ-铁素体对奥氏体动态再结晶行为的影响机制尚不清楚。本工作利用Gleeble-3800热力模拟试验机在1423 K、0.1 s<sup>-1</sup>条件下进行了铸态样品和均质化处理态样品的热压缩实验,结合SEM、EBSD和TEM等研究了δ-铁素体对热变形行为和动态再结晶的影响。结果表明:1473 K均质化处理14 h可基本消除δ-铁素体,同时伴随着奥氏体晶粒的显著长大,计算表明δ-铁素体向奥氏体的转变速率主要受控于Cr在奥氏体中的扩散。铸态样品变形过程中,δ-铁素体内部、δ-铁素体/奥氏体界面处的塑性变形先于奥氏体相,高温下较软δ-铁素体的存在,造成铸态样品的流变应力明显低于均质化处理态无δ-铁素体样品。随着变形的进行,δ-铁素体易发生动态回复,动态回复引起的软化导致流变应力显著下降。均质化处理态样品的动态再结晶机制是原始奥氏体晶界弓出形核的不连续动态再结晶。而δ-铁素体的存在促进了δ-铁素体/奥氏体界面附近奥氏体的动态再结晶,其机制为连续动态再结晶。铸态样品中的原始奥氏体晶界为不连续动态再结晶的形核位置,2种动态再结晶机制的耦合作用使得铸态样品的动态再结晶程度显著高于均质化处理态样品。

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[J]. Acta Metall. Sin., 2025, 61: 1035

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The corrosion and mechanical properties of austenitic stainless steels can be enhanced considerably by adding Nb. Newly developed Nb-stabilized austenitic stainless steels, such as 347HFG, 316Nb, TP310HCb, NF709, and HT-UPS, exemplify this advancement. The required Nb content varies across these steels. Prior research has indicated that in the as-cast microstructure of these steels, coarse and unevenly distributed primary NbC often forms, adversely affecting their mechanical and corrosion properties. Furthermore, this coarse primary NbC depletes the solid solution of Nb, which is counterproductive for fine secondary NbC precipitation. Notably, modifying the morphology and size of primary NbC through hot working and heat treatment is challenging. To enhance the microstructure and mechanical properties of Nb-stabilized austenitic stainless steel, this study investigated the effects of Nb content and homogenization treatment on these steels. The microstructure and tensile properties of cast austenitic stainless steel were analyzed using OM, SEM, TEM, and tensile test. The findings reveal that varying Nb content influences the precipitation of primary NbC and M23C6 carbides. In Nb-free steel, M23C6 carbides precipitate continuously at grain boundaries. This precipitation still occurs in steel with 0.30%Nb (mass fraction), alongside the formation of NbC + γ eutectic structures. Increasing Nb content to 0.90% can suppress M23C6 carbide precipitation, although the eutectic structures become more prevalent. A notable enhancement in yield strength accompanies an increase in Nb content to 0.90%. This improvement is attributed to the solid solution strengthening by Cr (due to suppressed M23C6 carbides) and Nb, grain boundary strengthening from refined grain sizes, and precipitation strengthening by secondary NbC. However, microcracks are easily nucleated at primary NbC/γ interface under plastic deformation, leading to rapid crack propagation along primary NbC networks and resulting in trench-like brittle fractures. This mechanism significantly reduces elongation. Post-homogenization treatment at 1250 oC alters the primary NbC morphology from rod-like to spherical/ellipsoid. This change increases the critical stress required for microcrack nucleation at NbC/γ interfaces, thereby inhibiting microcrack initiation. Additionally, the primary NbC networks transform from continuous to discontinuous distributions, impeding microcrack propagation. Consequently, this treatment significantly enhances elongation without compromising strength.

谢 昂, 陈胜虎, 姜海昌 .

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The lead-cooled fast reactor is considered one of the promising Generation IV nuclear energy systems. Structural materials used in the construction of pressure vessels and internals for this reactor include 300 series austenitic stainless steels. Nb-containing austenitic stainless steels are developed to improve corrosion properties, mechanical properties, and irradiation resistance. However, coarse primary NbC carbides are formed during solidification in these steels and cannot be eliminated through subsequent hot working and heat treatment. Recently, researchers have found different oxidation behaviors between secondary phase particles and the matrix, which affect the material's corrosion properties. However, the oxidation behaviors of primary NbC are rarely reported. This study analyzes the corrosion behaviors of a solution-treated Nb-containing austenitic stainless steel plate after exposure to oxygen-saturated liquid lead-bismuth eutectic (LBE) at 550 and 600 oC using SEM, EPMA, XRD, and TEM. The results show that the oxidation probability of NbC is correlated with its location in the samples at 550 oC. NbC at the initial surface is easily oxidized, while NbC within the interior is difficult to oxidize due to the low equilibrium oxygen partial pressure in the inner oxide layer, which suppresses the oxidation of NbC. However, NbC at the initial surface and within the interior are prone to be oxidized as the temperature increases to 600 oC. Compared to the matrix, NbC oxidizes into Nb2O5, resulting in a higher Pilling-Bedworth ratio (PBR). This leads to high compressive stress and resultant microcrack formation in the surrounding oxide layer. Additionally, the presence of CO2 generated during the oxidation of NbC within the interior reduces the compactness of the oxide layer, leading to a higher growth rate.

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The creep-resistant austenitic stainless steel Alloy 709 (Fe-20Cr-25Ni (wt%) based steel) is being investigated as a candidate structural material for the next generation fast neutron reactors at service temperature of 500-550 degrees C. However, the study of microstructural evolution of Alloy 709 during aging is lacking. In this study, thus, the microstructure of Alloy 709 has been investigated using electron microscopy in the as-received state and after static aging at 550, 650 and 750 degrees C. The results show that the prominent precipitate in the as-received Alloy 709 is Nb(CN), with the rod-like Z phase (CrNbN) observed very occasionally. After aging at 550 degrees C even up to 2000 h, no significant microstructure change was observed, which means that Alloy 709 is fairly stable at 550 degrees C. Aging at 650 degrees C produced globular M23C6 phase on grain boundaries, plate-like M23C6 carbides at twin boundaries and in the grain interior, and blocky M23C6 carbide nucleated on Nb(CN). Fine dispersoid Z phases were found on dislocations after aging at 650 degrees C for 500 h; their amount increases with aging time and temperature. (Cr,Mo)(3)(Ni,Fe)(2)SiN theta phase forms at grain boundaries after aging at 650 degrees C for 1000 h. After aging at 750 degrees C, theta phase nucleated on M23C6 carbide and a transformation of M23C6 to theta phase was found, which suggests that theta phase is the more stable. Aging at 550 degrees C promotes the segregation of Cr and Mo to grain boundaries whereas clear Cr depletion was observed at 650 degrees C due to the precipitation of Cr-rich M23C6 carbides at grain boundaries. Such depletion nearly disappears at 750 degrees C. The implications of aging for the subsequent mechanical behaviour of Alloy 709 are discussed briefly.

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The austenitic heat resistant-steels have been considered as important candidate materials for advanced supercritical boilers, nuclear reactors, super heaters and chemical reactors, due to their favorable combination of high strength, corrosion resistance, perfect mechanical properties, workability and low cost. Since the precipitation behavior of the steels during long-term service at elevated temperature would lead to the deterioration of mechanical properties, it is essential to clarify the evolution of secondary phases in the microstructure of the steels. Here, a summary of recent progress in the precipitation behavior and the coarsening mechanism of various precipitates during aging in austenitic steels is made. Various secondary phases are formed under service conditions, like MX carbonitrides, M23C6 carbides, Z phase, sigma phase and Laves phase. It is found that the coarsening rate of M23C6 carbides is much higher than that of MX carbonitrides. In order to understand the thermal deformation mechanism, a constitutive equation can be established, and thus obtained processing maps are beneficial to optimizing thermal processing parameters, leading to improved thermal processing properties of steels.

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DOI     

研究了一种新型25Cr-20Ni奥氏体耐热不锈钢在750 ℃不同拉应力持久实验中析出相演变及其对性能的影响。结果表明,当持久应力为180 MPa时,持久寿命为32.6 h,析出相包括M<sub>23</sub>C<sub>6</sub>和(Nb, V)(C, N)相。其中M<sub>23</sub>C<sub>6</sub>主要分布在晶界位置,(Nb, V)(C, N)相在晶内弥散析出。当持久应力为150和120 MPa时,随着持久时间延长至98.1 h以上,晶界位置的M<sub>23</sub>C<sub>6</sub>发生长大和Ostwald熟化,但(Nb, V)(C, N)相具有较好的尺寸稳定性。同时,在持久应力为120 MPa条件下,组织中出现σ相。σ相首先在晶界位置析出,当持久应力为100 MPa时,随持久时间延长至752.3 h,σ相也会在晶内析出。研究还发现,大量σ相依附于(Nb, V)(C, N)相析出,这是由于(Nb, V)(C, N)相的析出导致附近奥氏体基体中局部位置C和N元素含量减少,从而促进了σ相的形核。不同应力条件下试样断裂方式均为沿晶断裂,当持久时间较短时,裂纹在晶界M<sub>23</sub>C<sub>6</sub>处产生,引起沿晶开裂。随着持久时间延长,σ相在晶界析出后,裂纹更容易在晶界σ相处产生,导致持久延伸率随持久寿命延长而减小。

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