3-19. Investigation on stress corrosion cracking of 304 ss in high-temperature water by in-situ acoustic emission technique

Jibo Tan, Zhen Zhang, Jian Xu, Xinqiang Wu*, En-Hou Han, Wei Ke

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

Abstract: Austenitic stainless steels (ASSs) are widely used in key components of nuclear power plants (NPPs), such as main pipelines and internal components, due to the good mechanical properties and anti-corrosion resistance. During long-term service in high-temperature and high-pressure water environment, ASSs may suffer from environmentally assisted cracking (EAC), especially stress corrosion cracking (SCC). The SCC of nuclear power components seriously threatens the operational safety of NPPs, and on-line monitoring is a cost-effective way to prevent these accidents. Direct current potential drop (DCPD) technique is commonly used in SCC experimental research. However, a DC must be applied to monitor the crack length by DCPD. Therefore, it is difficult to on-line monitor SCC. Acoustic emission (AE) is a non-destructive testing technique based on the rapid release of internal energy during material damage. In this paper, The SCC behavior and AE signal characteristic of 304 stainless steel in high-temperature and high-pressure water were studied by the SCC set-up equipped with in-situ AE and DCPD monitoring system (Figure 1), and the material state (annealed state and sensitized state), loading mode (slow strain rate loading and constant load loading) and dissolved oxygen (DO) were considered. Depending on the material state and loading mode, 304 stainless steel shows different cracking modes. The cracking mode of annealed 304 stainless steel is transgranular SCC (TGSCC). The cracking mode of sensitized 304 stainless steel is the mixture of TGSCC and intergranular SCC (IGSCC) under slow strain rate loading, and it is the IGSCC under constant load loading. Sensitized treatment and increasing DO concentration increase the crack growth rate of 304 stainless steel. The AE characteristic parameters of the SCC process of 304 stainless steel in high-temperature and high-pressure water were obtained, and both burst and continuous AE waveform were monitored. The burst AE signals originate from the residual ligament tearing, while the continuous AE signals generated by the plastic deformation of the crack tip. Through relating the AE signal with SCC process, it is found that the AE waveforms can relate to the SCC modes. A discriminant factor (λ) of SCC mode is proposed, which is defined as the ratio of burst and continuous signals. It is found that the λ value of TGSCC is close to 1. With the ratio of IGSCC increasing, the λ value gradually approaches 0. The λ value of the mixture of TGSCC and IGSCC is about 0.5. A random forest model based on traditional AE characteristic parameters is proposed to distinguish different AE waveforms, and the classification accuracy rate is 98.6%. Combined with λ and random forest model, qualitative automatic monitoring of SCC in high-temperature and high-pressure water can be realized. Combined with the characteristic parameters of AE and DCPD in the SCC process, it is found that there is a linear relationship between the SCC crack growth rate and the AE cumulative hits rate (Fig. 2), which proves that AE has the potential to quantitatively evaluate the SCC crack growth rate in NPPs.

Keywords: Austenitic stainless steels; Stress corrosion; Acoustic emission; High-temperature and high-pressure water

304不锈钢高温高压水应力腐蚀开裂过程的原位声发射研究 

谭季波，张震，徐健，吴欣强*，韩恩厚，柯伟

中国科学院核用结构材料与安全性评价重点实验室，中国科学院金属研究所，沈阳，110016 

摘要:奥氏体不锈钢由于良好的力学性能与抗腐蚀性能，广泛用于核电站关键构件，如主管道、 堆内构件。然而，长期服役于高温高压水环境中奥氏体不锈钢会遭受环境致裂，特别是应力腐 蚀开裂(SCC)。核电关键构件的 SCC 严重威胁核电站的运行安全，在线监测是预防事故发生 经济有效的方法。直流电位降(DCPD)技术常用于 SCC 实验研究，但需要对试样施加电流。 因此，DCPD 技术难以应用于核电站关键构件的在线监测。声发射(AE)是一种基于材料损伤 过程中内部能量快速释放的无损监测技术，应用前景广泛。本文利用带有 AE 和 DCPD 原位监 测系统的 SCC 实验装置(图 1)研究了 304 不锈钢在高温高压水环境中的 SCC 行为和 AE 信号 特征，主要考虑了材料状态(固溶态和敏化态)、加载方式(慢拉伸和恒载)和溶解氧(DO) 的影响。依赖于材料状态和加载模式，304 不锈钢表现出不同的开裂模式:固溶态 304 不锈钢是 穿晶 SCC(TGSCC)开裂模式;敏化态 304 不锈钢在慢应变速率加载条件下是 TGSCC 和沿晶 SCC(IGSCC)混合开裂模式，而在恒载条件下是 IGSCC 开裂模式。敏化处理和提高 DO 浓度 增加 304 不锈钢裂纹扩展速率。获得了 304 不锈钢高温高压水 SCC 开裂过程中的 AE 特征参数， 监测到爆发型和连续型两种波形。爆发型信号来源于裂纹扩展残余韧带撕裂，而连续型信号来 源于裂纹尖端塑性变形。结合 AE 信号与 SCC 开裂过程，发现波形类型与 SCC 开裂模式相关， 提出了 SCC 开裂模式的判别因子(λ)，定义为爆发型信号和连续的比率。发现 TGSCC 开裂的 λ 值接近于 1;随 IGSCC 开裂比例增加，λ 值逐渐接近于 0;TGSCC 与 IGSCC 混合开裂时 λ 值 约为 0.5。提出基于传统的 AE 特征参数的随机森林模型以区分不同 AE 波形，分类准确率达 98.6%。结合 λ 和随机森林模型可以实现高温高压水中 SCC 开裂模式的定性自动监测。结合 SCC 过程中 AE 与 DCPD 的特征参数，发现 SCC 裂纹扩展速率与 AE 累积撞击速率之间存在线性关 系(图 2)，证明 AE 技术具有定量评价核电站 SCC 裂纹扩展速率的应用潜力。

1(左)带有 AE 和 DCPD 原位监测系统的 SCC 实验装置
图 2(右)304 不锈钢 SCC 过程中 DCPD 监测裂纹长度和 AE 行为演变规律:(a)裂纹长度，(b)AE 累积撞击

关键词:奥氏体不锈钢;应力腐蚀;声发射;高温高压水