热老化效应对奥氏体—铁素体双相不锈钢力学性能的影响
发布时间：2019-06-04 06:28
【摘要】：第二代核电站一回路系统主要由奥氏体-铁素体双相不锈钢构建而成,长期在327℃的高温环境下工作。该温度会导致双相不锈钢中的铁素体发生调幅分解,从而引起材料的力学性能发生改变。为此,本文对一回路主泵泵壳制造材料CF8与主管道制造材料Z3CN20.09M双相不锈钢进行了一系列的加速老化试验,并试验考察了热老化对材料冲击韧性、低周疲劳性能以及棘轮性能的影响。研究结果对于一回路系统材料的失效评定与寿命评估等工作具有重要的参考价值。本文采用冲击试验机考察了热老化对CF8双相不锈钢的冲击韧性的影响。研究结果表明,在所有的冲击温度下,随着老化时间的延长,材料的冲击裂纹形成能与裂纹扩展能均逐渐下降,韧脆转变温度逐渐升高。通过SEM对材料的断口观察发现,原始态材料的断口有大量韧窝,表现为韧性断裂;老化1000 h时,材料断口表现为准解理断裂特征;老化时间超过3000 h后,材料的断口表现为完全解理断裂。这说明随着老化时间的增加,材料逐渐脆化。Z3C N20.09M双相不锈钢的低周疲劳试验结果表明热老化对材料的疲劳性能也有较大的影响。研究发现,随着热老化时间的增加,材料的应力响应与疲劳寿命逐渐增加。采用SEM对试样断面观察发现,原始态与老化后的材料的疲劳微裂纹均在相界萌生,之后通过相近微裂纹的联合与合并逐渐成长为宏观裂纹。宏观裂纹以穿晶形式扩展至试样完全断裂。材料疲劳寿命随老化时间增加而延长可归因于热老化使铁素体韧性降低。低周疲劳试验时老化材料中的铁素体中更易产生被相界牢牢束缚在铁素体内的滑移带,有利于减轻相界处的应力集中与变形不协调,从而延缓了疲劳裂纹在相界处的萌生。Z3C N20.09M的单轴棘轮试验结果表明,棘轮应变随平均应力与应力幅值的增加而增大,却随应力率与老化时间的增加而降低。此外,加载历史对材料的棘轮应变也有很大的影响。采用Ohno-Wang II模型与考虑老化时间影响的改进模型分别对原始态与老化后的Z3CN20.09M双相不锈钢的棘轮应变进行了预测,预测结果与试验结果吻合较好。
[Abstract]:The first loop system of the second generation nuclear power plant is mainly constructed of Austenite-Ferritic duplex stainless steel and works at high temperature of 327C for a long time. This temperature will lead to the amplitude modulation decomposition of ferrite in duplex stainless steel, which will lead to the change of mechanical properties of the material. In this paper, a series of accelerated aging tests were carried out on the main pump shell manufacturing material CF8 and the main pipeline manufacturing material Z3CN20.09M duplex stainless steel, and the impact toughness of the material was investigated. Effects of low cycle fatigue properties and ratcheting properties. The research results have important reference value for the failure assessment and life evaluation of primary loop system materials. The effect of thermal aging on the impact toughness of CF8 duplex stainless steel was investigated by impact testing machine. The results show that at all impact temperatures, with the prolongation of aging time, the impact crack formation energy and crack propagation energy of the material decrease gradually, and the ductile-brittle transition temperature increases gradually. Through the observation of the fracture surface of the material by SEM, it is found that the fracture surface of the original material has a large number of dimples, showing ductile fracture, and the fracture surface of the material shows quasi-cleavage fracture characteristics when aging for 1000 hours. When the aging time is more than 3000 h, the fracture surface of the material shows complete cleavage fracture. The results show that the material becomes brittle with the increase of aging time. The results of low cycle fatigue test of Z3C N20.09M duplex stainless steel show that thermal aging also has a great influence on the fatigue properties of the material. It is found that with the increase of thermal aging time, the stress response and fatigue life of the material increase gradually. SEM was used to observe the cross section of the sample. It was found that the fatigue microcracks of the original state and the aged materials originated at the phase boundary, and then gradually grew into macrocracks through the combination and combination of similar microcracks. The macroscopic crack propagates to the complete fracture of the specimen in transgranular form. The prolongation of fatigue life with the increase of aging time can be attributed to the decrease of ferrite toughness caused by thermal aging. In low cycle fatigue test, it is easier to produce slip band firmly bound by phase boundary in ferrite in aging material, which is beneficial to reduce the inconsistency between stress concentration and deformation at phase boundary. Thus, the initiation of fatigue crack at the phase boundary is delayed. The uniaxial ratcheting test results of Z3C N20.09M show that the ratcheting strain increases with the increase of average stress and stress amplitude, but decreases with the increase of stress rate and aging time. In addition, the loading history also has a great influence on the ratchet strain of the material. The ratcheting strain of the original state and the aged Z3CN20.09M duplex stainless steel was predicted by Ohno-Wang II model and the improved model considering the effect of aging time, and the predicted results are in good agreement with the experimental results.
【学位授予单位】：天津大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG142.71
本文编号：2492534
[Abstract]:The first loop system of the second generation nuclear power plant is mainly constructed of Austenite-Ferritic duplex stainless steel and works at high temperature of 327C for a long time. This temperature will lead to the amplitude modulation decomposition of ferrite in duplex stainless steel, which will lead to the change of mechanical properties of the material. In this paper, a series of accelerated aging tests were carried out on the main pump shell manufacturing material CF8 and the main pipeline manufacturing material Z3CN20.09M duplex stainless steel, and the impact toughness of the material was investigated. Effects of low cycle fatigue properties and ratcheting properties. The research results have important reference value for the failure assessment and life evaluation of primary loop system materials. The effect of thermal aging on the impact toughness of CF8 duplex stainless steel was investigated by impact testing machine. The results show that at all impact temperatures, with the prolongation of aging time, the impact crack formation energy and crack propagation energy of the material decrease gradually, and the ductile-brittle transition temperature increases gradually. Through the observation of the fracture surface of the material by SEM, it is found that the fracture surface of the original material has a large number of dimples, showing ductile fracture, and the fracture surface of the material shows quasi-cleavage fracture characteristics when aging for 1000 hours. When the aging time is more than 3000 h, the fracture surface of the material shows complete cleavage fracture. The results show that the material becomes brittle with the increase of aging time. The results of low cycle fatigue test of Z3C N20.09M duplex stainless steel show that thermal aging also has a great influence on the fatigue properties of the material. It is found that with the increase of thermal aging time, the stress response and fatigue life of the material increase gradually. SEM was used to observe the cross section of the sample. It was found that the fatigue microcracks of the original state and the aged materials originated at the phase boundary, and then gradually grew into macrocracks through the combination and combination of similar microcracks. The macroscopic crack propagates to the complete fracture of the specimen in transgranular form. The prolongation of fatigue life with the increase of aging time can be attributed to the decrease of ferrite toughness caused by thermal aging. In low cycle fatigue test, it is easier to produce slip band firmly bound by phase boundary in ferrite in aging material, which is beneficial to reduce the inconsistency between stress concentration and deformation at phase boundary. Thus, the initiation of fatigue crack at the phase boundary is delayed. The uniaxial ratcheting test results of Z3C N20.09M show that the ratcheting strain increases with the increase of average stress and stress amplitude, but decreases with the increase of stress rate and aging time. In addition, the loading history also has a great influence on the ratchet strain of the material. The ratcheting strain of the original state and the aged Z3CN20.09M duplex stainless steel was predicted by Ohno-Wang II model and the improved model considering the effect of aging time, and the predicted results are in good agreement with the experimental results.
【学位授予单位】：天津大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG142.71
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