GRAIN BOUNDARY ENGINEERING FOR INTERGRANULAR CORROSION RESISTANT MATERIALS BY HOMOGENEOUS DISTRIBUTION OF CSL SEGMENTS IN THE GRAIN BOUNDARY NETWORK

晶界设计与控制的概念被发展为晶界工程(GBE)。GBEED材料的特点是具有高频率的重合格子(CSL)晶界，这是因为CSL晶界的晶界能很低，因此可以抵抗材料的晶间腐蚀等晶间劣化。采用GBE工艺研究了304奥氏体不锈钢的晶间腐蚀性能，结果表明，适当的形变热处理可显著提高其耐晶间腐蚀性能。形变处理增加了CSL晶界的出现频率，有效地破坏了材料中随机晶界的连续网络。CSL段在晶界网络中的均匀分布抑制了晶间退化。形变热处理也适用于316奥氏体不锈钢，在硫酸铁-硫酸试验中，GBEed 316钢的抗晶间腐蚀能力是316钢的4倍以上。GBE显著提高了304钢的抗应力腐蚀开裂(SCC)能力，这是因为在沸水核反应堆模拟环境的应力腐蚀试验中，304钢母材以典型的沿晶SCC方式断裂，而GBEed 304钢则以穿晶方式显示出小的SCC。在科里乌试验中，GBEed 304钢在过钝化环境中的腐蚀性也要小得多。GBE对304钢的焊缝腐蚀有明显的抑制作用。电化学动电位再活化(EPR)和硫酸铁-硫酸试验定量地表明防止了焊接热影响区的敏化。本研究表明，GBE极大地提高了奥氏体不锈钢的抗晶间腐蚀能力。

The concept of ‘grain boundary design and control' has been developed as grain boundary engineering (GBE). GBEed materials are characterized by high frequencies of coincidence site lattice (CSL) boundaries which are resistant to intergranular deterioration of materials, such as intergranular corrosion, because CSL boundaries are low in grain boundary energy. The authors have demonstrated that a suitable thermomechanical treatment remarkably improves the intergranular corrosion resistance of type 304 austenitic stainless steel by GBE. The thermomechanical treatment increased the frequency of CSL boundaries and effectively disrupted the continuous network of random grain boundaries in the material. The homogeneous distribution of CSL segments in the grain boundary network suppresses intergranular degradations. It was also demonstrated that the thermomechanical treatment was also applicable to type 316 austenitic stainless steel and that the GBEed 316 steel showed more than four times higher resistance to intergranular corrosion than that of the 316 base steel during ferric sulfate-sulfuric acid test. The GBE drastically improved the resistance of 304 steel to stress corrosion cracking (SCC), because the base 304 steel test fractured in typical intergranular SCC mode but the GBEed 304 steel showed a small SCC in transgranular mode during an SCC test simulating the environment in a boiling water nuclear reactor. The GBEed 304 steel was also much less corrosive in transpassive environment during Coriou test. The weld decay in 304 steel was significantly suppressed by GBE. The electrochemical potentiokinetic reactivation (EPR) and ferric sulfate-sulfuric acid tests indicated quantitatively the prevention of sensitization in the weld heat affected zone. The present research project has revealed that GBE improves the resistance to intergranular corrosion phenomena extremely in austenitic stainless steels.