N18锆合金氢致裂纹延迟开裂临界温度研究

STUDY OF THE CRITICAL TEMPERATURES FOR DELAYED HYDRIDE CRACKING IN N18 ZIRCONIUM ALLOY

研究了N18锆合金(Zr-1Sn-0.3Nb-0.3Fe-0.1Cr)发生氢致延迟开裂(DHC)临界最大开裂温度(T_c)和临界最小止裂温度(T_h)随氢含量的变化规律;同时对裂纹尖端偏聚氢含量及静水应力和发生DHC的临界氢含量进行了理论分析,建立理论模型对临界温度进行理论计算.结果表明:N18合金发生氢致延迟开裂的临界温度介于相同氢含量下溶解固溶温度与析出固溶温度之间,且最大开裂温度小于最小止裂温度,计算的临界温度值与实验值相当吻合,因此该理论模型能够真实反映N18锆合金的氢致延迟开裂的物理过程.

Zirconium alloys are used extensively in nuclear reactor cores. During their service a part of hydrogen produced through the corrosion reaction of Zr with hot coolant is absorbed by materials. Hydride induced embrittlement significantly influences the in-service performance of the Zr-alloy components. Delayed hydride cracking （DHC） is a localized form of hydride embrittlement, consequently, hydrogen atoms in the solid solution will diffuse into this region ahead of the crack tip subjected to a triaxial state of stress, which may lower the chemical potential of the region. Once the hydrogen concentration in this region reaches the terminal solid solubility （TSS）, hydrides will start to form and grow. When the hydrides at the crack tip reach a critical size, the main crack will propagate through this hydrided region. The crack front finally is arrested at the end of the hydrided region by the ductile zirconium matrix, and the whole process repeats itself. Most of the investigations on DHC in zirconium alloys are focused on Zr-Nb alloys. Few literatures were found on the subject of DHC in Zr Sn alloys. The purpose of the present study was to investigate critical temperature for initiating and arresting delayed hydride cracking in Zr-Sn-Nb alloy. A critical temperature for DHC study was carried out to determine the critical temperature for initiating and arresting in N18 zirconium alloy （Zr-Sn-Nb alloy）. For a given hydrogen concentration of a specimen, the two critical temperatures were observed-a DHC initiation temperature, Tc, at which DHC would initiate when approaching the test temperature from above the terminal solid solubility （C^d） temperature in hydride dissolution and a DHC arrest temperature, Th, obtained by heating the same specimen from Tc after DHC had started. Tc slightly below Th. Both Tc and Th fall below the dissolution solvus temperature and above the precipitation solvus temperature. A theoretical analysis was carried out to quantitatively determine the hydrogen concentration limit and these critical temperatures using the method of Dutton and Plus, a key assumption in the method is that, while the local crack tip stress concentration causes a local enhancement of the hydrogen concentration in solution, the hydride precipitation solvus is unaffected by stress. Good agreements are obtained between measured and predicted values of critical temperatures, which support the Dutton-Plus theory.