超超临界锅炉用奥氏体耐热钢HR3C的脆化机理

Embrittlement mechanism of austenitic heat resistant steel HR3C for ultra supercritical boiler

结合室温、高温冲击试验及700℃时效试验,利用光镜、扫描电镜、透射电镜、电子探针等手段对服役态及供货态典型奥氏体耐热钢HR3C的脆化机理进行了研究。结果表明,服役近4万小时后HR3C钢室温及650℃均出现了明显脆化现象,冲击韧性大幅度下降,700℃时效试验结果与此类似。组织分析表明,服役近4万小时的HR3C钢在晶界析出连续片状分布的M_(23)C_6相、沿晶界向晶内生长出针(条)状M_(23)C_6相、晶界周边析出纳米级立方状M_(23)C_6相及弥散Nb Cr N相,Cr、C,P、S 4种元素在晶界明显偏聚。在700℃时效不同时间后相关试验表明HR3C钢在冲击韧性快速下降阶段微观组织中碳化物在晶界已成连续片状分布,未见其余析出相,晶界仅有微量S元素偏聚。M_(23)C_6在晶界的连续片状析出是造成HR3C钢时效脆化的主要原因。

Combined with room temperature, high temperature impact test and aging test at 700~C , the impact toughness, microstructure, grain boundary element of as-received austenitic heat resistant steel HR3C and serviced for long time were studied by means of OM, SEM, TEM and EPMA. The results show that after 40 thousand hours of service, the HR3C steel has obvious brittleness at room temperature and at 650℃ , and its impact toughnessdecreases sharply, the results of aging test at 700℃ are similar. Microstructure analysis shows that after 40 thousand hours of service, the microstructure of the HR3C steel consists of continuous lamellar distribution of M23C6 phase at the grain boundary, needle like M23C6 phase in the grain boundary, nanocrystalline cubic M23 C6 phase precipitates around grain boundaries and dispersed NbCrN phase. Cr, C, P and S elements segregate in the grain boundaries obviously. The results of aging test at 700℃ show that during the impact toughness of the HR3C steel decreases rapidly, the carbides in the microstructure of the HR3C steelhave beeneontinuously distributed at the grain boundary, no other precipitated phase is observed,andonly S is found slightly segregated at the grain boundaries. The continuous precipitation of lamellar M23 C6 phase at the grain boundaries is the main cause of aging embrittlement of the HR3C steel.