【作者】 张勇；

【导师】 王明家；

【作者基本信息】 燕山大学 ， 材料学， 2012， 硕士

【摘要】 第三代核电主管道是核岛部分的关键部件之一，在使用服役环境下存在热老化现象，而锻造316LN不锈钢的热老化问题是国内外尤其是中国不锈钢领域中亟待解决的实际工程问题，且具有重要的理论意义。本文属于核电主管道用锻造控氮奥氏体不锈钢热老化与应力腐蚀开裂的基础理论试验研究，以及进行了C或N元素含量变化时结构稳定性及力学性能的计算。本文主要研究锻造316LN不锈钢材料在350℃温度下固溶态-5000h的热老化行为。利用X射线、TC9000等仪器得到热老化时间对物相、导热率、电阻率、硬度、冲击韧性等性能的影响规律；通过慢应力应变试验，研究不同老化时间及冷却速度对SCC敏感性影响程度；使用透射电镜（TEM）等仪器旨在研究热老化行为产生的微观机理。得到主要的结论如下：随着热老化时间的延长，316LN的维氏硬度增大，奥氏体晶格参数逐渐减小，电阻率升高，导热率降低。同时在热老化过程中没有新相生成，只含有γ-Fe相，且晶粒度级别为5.5级。利用NA700J示波冲击试验机研究304LN奥氏体不锈钢材料在热老化过程中冲击断裂性能的变化规律，得到裂纹生成功基本不变，裂纹扩展功及总冲击功均随老化时间的延长而下降，呈现出明显的脆化迹象。通过对316LN热老化试样进行应力腐蚀开裂行为研究，得到：随着热老化时间的延长，应力腐蚀开裂敏感系数增大，延伸率与断面收缩率下降；对应力腐蚀开裂断口形貌分析发现，韧窝逐渐变小变浅，同时二次裂纹变深、增多。总之，随着热老化时间的延长，材料更容易发生应力腐蚀开裂（SCC）。随着热老化时间延长，晶粒内与晶界处的显微硬度差距越来越大，且晶界处硬度高于晶粒内部；同时，TEM分析得出随着热老化时间的延长，位错总密度减小，晶界处的扩展位错数量却增多，扩展位错宽度明显变窄，层错能升高，分析认为层错能升高与C元素在晶界附近的偏聚有关。更多还原

【Abstract】 The main piping of the third generation nuclear power is one part of the key components of the nuclear island.Once it fails, it will cause incalculable economic losses and safety problems of nuclear power systems.So, thermal aging problem of the forged316LN stainless steel is the most important practical engineering problem which is for urgent solution at home and abroad, especially in the field of Chinese stainless steel.This article studied thermal aging and stress corrosion cracking of the forged austenitic stainless steel for main piping of the nuclear power.This paper studied thermal aging behavior of316LN stainless steel when the time was ranged from Oh to5000h.The law of different aging time on the phase, the thermal conductivity, electrical resistivity, hardness and impact toughness was obtained by using X-ray, TC9000and other instruments.By slow strain rate test, it was studied how different aging time and cooling rate affected SCC susceptibility.The microscopic mechanism of thermal aging behavior was studied by transmission electron microscopy (TEM).The main conclusions are as follows:As thermal aging time increased, the Vickers hardness of316LN increased, the lattice parameter of austenite was gradually reduced, the resistivity increased, and the thermal conductivity decreased.There was no other new phase, only austenite phase, and the grain grade was maintained at about5.5.It was obtained that crack formation energy of304LN austenitic stainless steel remained substantially constant with the extension of the thermal aging time by using NA750J instrumented impact testing machine. Mean while crack propagation energy and total impact energy declined with the extension of the thermal aging time.There was obvious sign of embrittlement.Stress corrosion cracking behavior of the316LN thermal aging specimens was studied. Sensitive coefficient of stress corrosion cracking increased with the extension of the thermal aging time.That is, stress corrosion cracking was likely to occur.Meanwhile, the values of δ and ψ both declined, indicating plastic deformation capacity of316LN reduced after thermal aging.Stress corrosion cracking fracture was analyzed, it was found that the dimples were gradually becoming small and shallow, secondary cracks darkened and increased.Vickers hardness gap between the grain interior and grain boundary was increasing with the extension of the thermal aging time, and the hardness of the grain boundary was higher than that of the grain interior.TEM analysis results showed that the total dislocation density decreased, while the number of extended dislocation at the grain boundaries was increased significantly.Extended dislocation width narrowed significantly, which resulted in increase of the stacking fault energy.This maybe related to the segregation of the C atoms in the vicinity of the grain boundary.更多还原