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材料研究学报, 2023, 37(11): 846-854 DOI: 10.11901/1005.3093.2022.589

研究论文

奥氏体耐热钢Sanicro25蠕变行为和断裂特征

吕德超, 曹铁山, 程从前, 周彤彤, 赵杰,

大连理工大学材料科学与工程学院 大连 116024

Creep Behavior and Fracture Characteristic of Austenitic Heat-Resistant Steel Sanicro25

LV Dechao, CAO Tieshan, CHENG Congqian, ZHOU Tongtong, ZHAO Jie,

School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China

通讯作者: 赵杰，jiezhao@dlut.edu.cn，研究方向为金属材料蠕变、组织演化、损伤及寿命预测

责任编辑: 黄青

收稿日期: 2022-11-08   修回日期: 2023-05-09  

基金资助: 国家自然科学基金(U1610256)
国家自然科学基金(51901035)

Corresponding authors: ZHAO Jie, Tel:(0411)84709076, E-mail:jiezhao@dlut.edu.cn

Received: 2022-11-08   Revised: 2023-05-09  

Fund supported: National Natural Science Foundation of China(U1610256)
National Natural Science Foundation of China(51901035)

作者简介 About authors

吕德超，男，1992年生，博士生

摘要

用OM、SEM和TEM等方法研究了超超临界电站用Sanicro25钢的蠕变机制。结果表明，这种钢的最小蠕变速率随着温度的升高和应力的增大而提高。根据最小蠕变速率特征得出表观应力指数为7.6~8.2，表观激活能为496.7~531.8 kJ/mol。在蠕变过程中在晶内弥散析出的纳米级Cu-rich相和MX相阻碍位错运动，导致蠕变门槛值应力的出现。用线性外延法求出的门槛值应力，随着温度的升高而减小。用门槛值将蠕变本构方程修正为$\dot{\varepsilon}_{\min }=A_{2}\left[\left(\sigma-\sigma_{\mathrm{th}}\right) / G\right]^{n} \exp (-Q / R T)$，可将不同温度下的最小蠕变速率归一化；同时确定真实应力指数(n=5)和真实表观激活能(Q=286.6 kJ/mol约等于γ-Fe自扩散激活能)，从而判别出实验参数下材料的蠕变机制为点阵自扩散协助的位错攀移。

关键词： 金属材料; 蠕变; 变形机制; Sanicro25钢

Abstract

The creep behavior at 130~240 MPa /973~1023 K of Sanicro25 steel for ultra-supercritical power plants were investigated by OM, SEM and TEM. The results showed that the minimum creep rate increases with the increasing temperature and applied stress. Based on the characteristics of the minimum creep rate, the stress exponents of 7.6~8.2 and the apparent activation energy of 496.7~531.8 kJ/mol can be acquired for the creep process. Nano-scale Cu-rich phase and MX phase precipitated in the matrix imped the dislocation motion, thus resulted in the emerging of creep threshold stress. The creep threshold stresses can be estimated by the linear extrapolation method, and which decrease with the increase in temperature. By invoking the concept of the threshold stresses to modify the constitutive equation, $\dot{\varepsilon}_{\min }=A_{2}\left[\left(\sigma-\sigma_{\mathrm{th}}\right) / G\right]^{n} \exp (-Q / R T)$, the normalization of the minimum creep rate can be acquired at various temperatures; Meanwhile, the true stress exponent (n=5) and the true apparent activation energy (Q=286.6kJ/mol approximately equal to the γ-Fe self-diffusion activation energy) can be identified. The creep rate-controlling mechanism was determined to be dislocation climbing mechanism assisted by lattice self-diffusion.

Keywords： metallic materials; creep; deformation mechanism; Sanicro25 steel

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本文引用格式

吕德超, 曹铁山, 程从前, 周彤彤, 赵杰. 奥氏体耐热钢Sanicro25蠕变行为和断裂特征[J]. 材料研究学报, 2023, 37(11): 846-854 DOI:10.11901/1005.3093.2022.589

LV Dechao, CAO Tieshan, CHENG Congqian, ZHOU Tongtong, ZHAO Jie. Creep Behavior and Fracture Characteristic of Austenitic Heat-Resistant Steel Sanicro25[J]. Chinese Journal of Materials Research, 2023, 37(11): 846-854 DOI:10.11901/1005.3093.2022.589

以化石能源为主对电力能源结构进行优化，可实现我国的“碳达峰-碳中和”目标。超超临界火力发电技术的较高稳定性和经济效益，使其能补充绿色电力系统[1,2]。超超临界机组服役温度的提高和压力的升高能提高发电效率和减少碳的排放[3]，因此应该高度关注服役材料在高温高压条件下的可靠性。

Sanicro25钢兼具良好的抗氧化性能、抗腐蚀性、可焊性以及优异的高温蠕变性能，是制造下一代超超临界锅炉过热器和再热器的主要候选材料[4~8]。Sanicro25钢，是在传统奥氏体钢中加入W/Co/Cu/Nb元素制备的一种新型奥氏体耐热钢。这种新型高铬、镍奥氏体耐热钢，兼具固溶强化和沉淀强化的高温力学性能。在蠕变过程中Sanicro25钢晶内生成的大量弥散纳米级Cu-rich相和MX相沉淀物(Nb、C和N组成)对位错运动的阻碍以及生成的第二相弥散强化，使其蠕变性能提高[2,8,9]。此外，Sanicro25钢中的置换固溶原子(例如Nb、W等)和间隙固溶原子(C、N)等元素溶入γ-Fe基体产生晶格畸变和固溶强化，可提高其高温力学性能[8,10,11]。

超超临界燃煤机组的部件长期在高温高压环境中服役，因此了解材料的蠕变变形特征和机制以确保结构安全，至关重要。Song等[12]基于微观结构演变和颗粒强化机制，提出可预测Sanicro25钢的蠕变变形和长期寿命的位错模型，而且用实验数据验证了这个模型的准确性。Luboš Kloc等 [13]进行Sanicro25钢的低应力蠕变实验时发现，随着应力的增大蠕变的应力指数从n=1转变到n=7，且在不同温度下应力的转变点不同。Zhang等[10,11]表征分析了Sanicro25合金的蠕变析出行为和位错结构的演化特征，并引用蠕变门槛值概念分析出其蠕变机制是点阵自扩散下的粘滞性滑移控制蠕变。

同时，设计火力发电部件时预测蠕变断裂寿命也极为必要。根据实验的短期数据推测部件的长期蠕变寿命，常用的模型有Larson-Miller模型、Monkman-Grant模型和Zc参数[14~16]。了解蠕变机制和组织结构特征也能在一定程度上提高模型的有效性。为了确保材料服役的安全，本文分析蠕变的本构方程、组织结构和断裂特征以深入理解Sanicro25钢的蠕变变形机制。

1 实验方法

实验用Sanicro25钢(奥氏体耐热钢)的名义化学成分(质量分数，%)为：Ni(23.5-26.6)-Cr(21.5-23.5)-W(2.0-4.0)-Cu(2.0-3.5)-Co(1.0-2.0)-Nb(0.30-0.60)-N(0.15-0.30)-C(0.04-0.1)-Mn(0.6)-Si(0.4)-P(0.03)-S(0.015)(最大值)和Fe(余量)。实验用材料是经过固溶(1417~1523 K)处理的无缝钢管(外径为54 mm，壁厚为9 mm)。图1给出了Sanicro25钢的金相组织形貌，可见其由均匀的等轴奥氏体、大量的退火孪晶以及未溶解的富Nb、Cr和N的沉淀颗粒组成[9, 17]。

图1

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图1   Sanicro25钢的金相组织形貌

Fig.1   Austenitic heat-resistant steel (Sanicro25 steel) OM morphology


用机械蠕变机(RDJ 30)进行材料的一系列蠕变试验，蠕变实验参数列于表1。应力为130~240 MPa，温度为973~1023 K。沿着无缝钢管的轧制方向截取标准蠕变试样，直径为5 mm，标距为25 mm。用OM和TEM等技术表征蠕变后试样的微观结构演变。先将OM测试样品机械研磨和抛光，然后在室温下用HCl (20 mL)+H2O (20 mL)+CuSO4 (5 g)溶液腐蚀。TEM测试样品的制备：用砂纸机械研磨至厚度约50 μm，然后打成直径约为3.0 mm的圆形薄片；最后在10%的高氯酸酒精混合溶液(-25℃)中进行电解双喷。用SEM表征蠕变后断口的表面形貌。

表1   Sanicro25钢的实验参数

Table 1  Experimental parameters of Sanicro25 steel

MaterialTemperature, T / K

Applied stress,

σ / MPa

Sanicro25 steel973180
220
240
998150
180
220
1023130
180
220
240

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2 实验结果

2.1 Sanicro25钢的蠕变变形行为

图2给出了Sanicro25钢的蠕变和速率曲线。按照蠕变速率的变化趋势，可将蠕变分为三个阶段。第一阶段为瞬时变形以后的应变阶段，蠕变开始时蠕变速率非常大，随后加工硬化产生的变形抗力增大使蠕变速率逐渐降低。随着加工硬化程度的提高高温动态回复速度随之提高，最后加工硬化与回复软化过程达到平衡，蠕变速度达到一个短暂的稳定，此即最小蠕变速率阶段。第三阶段为加速蠕变阶段，蠕变速率加速提高，试样塑性变形增大的同时其内部产生大量蠕变损伤，产生应力集中、空洞以及试样颈缩等不均匀变形[18]。随着加载应力的增大和温度的提高材料的应变速率随之提高，断裂寿命降低。

图2

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图2   Sanicro 25钢的蠕变曲线

Fig.2   Creep curve characteristics for Sanicro 25 steel (a) (b) at different applied stresses (c) (d) at different temperatures


2.2 最小蠕变速率与应力、温度的经验关系

材料的蠕变特征，可用最小蠕变速率表征。最小蠕变速率的高低除了与材料的特性相关，还与试验用应力和温度有关。图3给出了Sanicro25钢的蠕变最小速率与应力和温度的关系。在相同温度下最小应变速率与加载应力符合幂律关系(图3a)

图3

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图3   Sanicro25钢蠕变最小速率与应力和温度的关系

Fig.3   Minimum creep rate as a function of stress and temperature for Sanicro25 steel (a) Minimum creep rate vs. applied stress; (b) Minimum creep rate vs. the reciprocal of temperature


ε˙min=Aσnaexp(-QaRT)

其中ε˙min为最小蠕变速率，A为常数，σ为加载应力，T为绝对温度，na为表观应力指数，Qa为蠕变表观激活能，R为气体常数。蠕变的表观应力指数范围为7.6~8.2。在应力不变的条件下，最小蠕变速率的对数lnε˙min与绝对温度的倒数1/T成线性关系(图3b)。根据 式(1)可计算出蠕变的表观激活能为496.7~531.8 kJ/mol。根据本文的实验结果计算出的Sanicro25钢的蠕变表观应力指数和激活能，与各种奥氏体不锈钢的数值大致相同[10,19,20]。

2.3 蠕变断裂的特征

在不同温度下材料的断裂寿命均随着加载应力增加而下降(图4a)，而且在双对数坐标下符合线性关系

图4

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图4   Sanicro25钢蠕变断裂寿命与加载应力的关系

Fig.4   Creep fracture life as a function of applied stress for Sanicro25 steel (a) Log-log plot of ruptured time vs. applied stress; (b) Larson-Miller parameter curve


tr=A1σ-n1

其中A1为常数，n1为线性指数,tr为蠕变寿命。使用Larson-Miller参数(LMP)关系(P(σ)=T(20+lgtr))拟合材料的断裂寿命数据，可得主曲线(图4b)。可以看出，材料700℃蠕变10万小时的持久强度约为101 MPa (σ105700=101 MPa)。在工程服役中，很多高温构件(例如石化工业的高温反应装置、电站锅炉和蒸汽轮机等)的使用寿命都按服役材料10万小时的持久强度设计，所以预测Sanicro25钢的10万小时寿命有重要的工程意义。

图5a给出了Sanicro25钢在不同温度下的断裂寿命与最小蠕变率的关系。可以看出，断裂寿命数据符合Monkman-Grant(M-G)方程

图5

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图5   Sanicro25钢蠕变断裂寿命与最小蠕变速率的关系及其损伤特征

Fig.5   Relationship between fracture life and minimum creep rate and damage characteristics of Sanicro25 steel (a) Monkman-Grant relationship (b) Damage parameters characteristics


ε˙minα⋅tr=cMG

其中α和cMG是常数。损伤累积参数λ=εfε˙min⋅tr定义为断裂应变除以最小蠕变速率与蠕变断裂寿命的乘积[19]，可用于确定材料蠕变的损伤断裂模式。研究表明[13,19]，不同损伤累计参数值(1~20)可表明工程材料损伤方式。λ由小变到大，表明材料由不带明显塑性变形的脆性断裂转化为显著塑性变形后的韧性断裂。Sanicro25钢的损伤累计参数集中在4~7(图5b)，表明蠕变的断裂主要是结构不稳定和颈缩所致[13]。

图6给出了Sanicro25钢的蠕变断裂塑性。由图6可见，随着加载应力的增大和温度的提高断后伸长率和断面收缩率的变化没有规律性。同时，从图6b可见，近断口区域的收缩率明显高于均匀变形段，进一步验证Sanicro25钢的蠕变断裂是明显的塑性变形后的颈缩断裂。图7给出了Sanicro25钢的横截面和断口形貌。由图7a可见，在近断口区域出现混合开裂(穿晶和少量沿晶)的特征。根据对断口表面的观察(图7b)，材料是以准解理的方式沿着中心向周围扩展。扩展到晶界出现少量的沿晶界面，实现了穿晶界面扩展结合。综上所述，Sanicro25钢的蠕变断裂模式是以穿晶为主的混合断裂模式。

图6

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图6   Sanicro25钢的蠕变断裂塑性

Fig.6   Creep fracture plasticity characteristics of Sanicro25 steel (a) Fracture strain (b) Section shrinkage of different region


图7

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图7   Sanicro25钢蠕变的截面和断口形貌

Fig.7   Creep cross-sectional and fracture morphology of Sanicro25 steel (T=973 K; σ=240 MPa) (a) OM cross-sectional morphology (b) SEM fracture morphology


3 讨论

根据固溶体合金蠕变的应力指数和激活能，可推定材料的蠕变变形机制。但是，实际应用的奥氏体耐热钢中析出相的弥散强化使蠕变抗力提高。第二相弥散强化材料的最小蠕变速率与应力仍然满足幂律关系，但是应力指数显著提高，而且蠕变的表观激活能也明显高于材料的自扩散和互扩散激活能[20~22]。

在温度为1023 K和应力为220 MPa的条件下，在蠕变后的基体中出现了细小的沉淀物，其尺度为20~50 nm(图8a)。图8b、c分别给出了细小沉淀物相的高分辨形貌和相应的傅里叶转化衍射斑点。这些细小沉淀物的类型和特征与在Sanicro25钢中观察到的Cu-rich相和Nb(C，N)碳化物相同[9~12, 23]。另外，位错线与细小析出相之间存在交互作用。其他学者对Sanicro25钢的研究结果表明，在蠕变后的基体内也析出细小的沉淀颗粒并与位错发生交互作用[10,12]。

图8

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图8   Sanicro 25钢在1023K和220MPa蠕变后的TEM结构特征

Fig.8   TEM microstructural characteristics of Sanicro 25 steel after creep at 1023 K and 220 MPa (a) Interaction of dislocations with fine precipitates. HRTEM micrograph and corresponding to SAED pattern; (b) Cu-rich phase (c) Nb (C, N)


实验得出的材料的表观应力指数(na=7.2~8.2)和表观激活能(Qa=496.7~531.8 kJ/mol)明显高于其本征应力指数和本征激活能，因此不能直接用于判断变形机制。其原因是，奥氏体耐热钢在高温蠕变过程中基体内弥散析出的沉淀相(Cu-rich相和Nb(C、N)相)阻碍位错的运动(图8a)，导致蠕变门槛值的出现[21, 24]。研究表明[19,25~27]，靠近颗粒的位错段在热激活协助下攀移越过颗粒，而其他位错段仍然在原滑移面上滑移。位错线长度的增加是位错攀移的阻力，这是出现应力门槛值的原因。在位错的运动过程中，当施加的应力所做的功大于位错线的额外长度引起的线能量的增加时，位错才能攀移过颗粒而使蠕变继续进行。

除了第二相弥散强化，Sanicro25钢还有固溶强化。溶质原子的溶入引起基体金属的晶格畸变，从而产生了内应力。蠕变时溶质原子偏聚在位错周围形成的溶质气团(例如柯氏气团)，使溶质原子与位错的弹性交互作用能增大。位错运动时，溶质气团扩散随之运动，限制了位错的自由滑移使其滑移而出现牛顿粘滞性。另外，如果溶质与位错的弹性交互作用能较小，则在位错周围不能形成溶质气团。这时位错运动的阻力很小，滑移速度较高，而位错的攀移较慢。如果位错的粘滞性滑移速度高于位错攀移速度，则蠕变被位错攀移控制；反之则被位错的粘滞性滑移控制[28]。

蠕变的控制机制为粘滞性滑移时：(1)应力指数n=3；(2)本征激活能等于固溶体的互扩散的激活能；(3)位错均匀分布，不形成亚结构。当蠕变的控制机制为位错攀移时：(1)应力指数n=5；(2)本征激活能等于基体金属的点阵自扩散激活能；(3)稳态时易形成稳定的亚结构[19,28~31]。

依据不同的变形机制(应力指数n为3或者5)，用线性外延法可计算出不同温度下材料蠕变的门槛值。图9给出了基于假定的蠕变机制计算出的不同温度下Sanicro25钢的门槛应力。图9a、b给出了假定不同应力指数蠕变门槛值的求解过程。同时，用所求的门槛值计算了相应的真实激活能和最佳线性拟合参数，结果列于表2。分析结果表明，n=3时线性拟合参数R2为0.986,计算出真实激活能为206.8 kJ/mol，明显高于间隙原子(C，N(132-136 kJ/mol)[32])的互扩散激活能，而明显低于置换原子(Cr，246 kJ/mol)、(Ni，282 kJ/mol)、(Mn，247 kJ/mol)、(Nb，268 kJ/mol)的互扩散激活能[19]。而当n=5时线性拟合参数R2更高达到0.995，求得的真实激活能为286.5±8.5kJ/mol，与奥氏体钢的点阵自扩散激活能(270±20kJ/mol)[10,33,34]基本相同。综合分析结果表明，温度为700、725和750℃时材料的蠕变门槛值应力分别为82.9±3.9，69.7±1.4，52.2±2.4 MPa，其应力随着温度的升高而减小。图10表明，材料在不同温度蠕变后具有不同的位错组态。随着温度的升高材料中的位错由波浪状的位错线转化为不规则网格位错，表明更高的温度有助于位错攀移越过颗粒。

图9

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图9   基于假定的蠕变机制计算的不同温度下Sanicro25钢的门槛应力

Fig.9   Threshold stresses of Sanicro25 steel calculated based on assumed creep mechanism at different temperatures (a) Viscous glide mechanism: n=3； (b) Dislocation climbing control mechanism: n=5


表2   Sanicro25钢的蠕变门槛值应力分析

Table 2  Threshold stress analysis for creep of Sanicro25 steel

Assumed stress exponent, n

Temperature,

T/K

Threshold stress,

σth/MPa

Avg. correlation coeffcient

R2/pct

Avg. Activation energy,

Q/kJ·mol-1

3973138.5±1.90.986206.8±6.2
998122.4±1.3
102399.8±3.0
597382.9±3.90.995286.5±8.5
99869.7±1.4
102352.2±2.4

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图10

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图10   不同温度下基体内的位错-细小沉淀物相互作用的明视场TEM图像

Fig.10   Bright-field TEM image of dislocation-fine precipitates interactions within the matrix at different temperature (a) 973 K-220 MPa; (b) 1023 K-220 MPa


ε˙min=A2σ-σthGnexp(-QRT)

其中ε˙min为最小应变速率，A2为常数，σ为加载应力，σth为门槛值应力，T为绝对温度，G为剪切模量，n为真实应力指数，Q为真实激活能，R为气体常数。在归一化有效应力相同的条件下求出的材料的真实蠕变激活能为286.5±8.5 kJ/mol (图11b)，约等于奥氏体钢的点阵自扩散激活能。分别用真实激活能和剪切模量修正最小应变速率和有效应力进行修正，则在不同温度下数据在一条斜率为5的直线上(图12)。这表明，在实验温度范围内，Sanicro25钢的蠕变变形机制为点阵自扩散协助下的位错攀移机制。

图11

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图11   不同温度下Sanicro25钢的真实应力指数和激活能

Fig.11   True stress exponents and activation energy of Sanicro25 steel at different temperatures (a) Log-log plot of the minimum creep rate vs. effective stress (b) Semi-log plot of the ε˙m vs. the reciprocal temperature


图12

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图12   不同温度下Sanicro25钢归一化的最小蠕变速率

Fig.12   minimum creep rate normalizated of Sanicro25 steel at various temperatures


4 结论

(1) Sanicro25钢的最小蠕变速率与加载应力符合幂率关系，其蠕变的表观应力指数为7.6~8.2，表观激活能为496.7~531.8 kJ/mol。

(2) 在Sanicro25钢的蠕变过程中，在晶内析出的纳米级Cu-rich相和Nb(C, N)相阻碍位错的运动，出现蠕变门槛值。随着温度的升高位错易于热激活攀移越过颗粒障碍使门槛值降低，相应的位错组态由波状的位错线转化为位错网格。

(3) 引入门槛值修正蠕变本构模型，可确定材料的真实应力指数(n=5)和真实激活能(Q=286.5±8.5 kJ/mol)。材料的蠕变变形机制是点阵自扩散协助下的位错攀移。

(4) Sanicro25钢的蠕变寿命符合Monkman-Grant关系，其断裂是以穿晶为主的混合模式。

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