亚临界300MW机组高压主汽阀阀杆断裂原因分析

Reasons Analysis on Fracture of High-pressure Main Steam Valve Stem in a Subcritical 300MW Unit

以断裂的亚临界300 MW机组主汽阀阀杆为研究对象,通过结合阀杆的服役环境,从结构设计、宏观形貌、显微组织、力学性能等多个方面分析了该主汽阀阀杆断裂的可能原因。结果表明:阀杆断裂是多方面因素共同作用的结果,长期服役后材料力学性能下降,已经低于材料的标准技术要求;长期服役后阀杆的显微组织也发生弱化,不仅带状偏析仍有残留,而且晶界网状碳化物析出加剧;阀杆材料C422钢本身对缺口比较敏感,首先在阀杆加工缺陷处产生裂纹,并且由于变径处的结构形式存在一定的缺陷,过渡圆弧半径较小,造成应力高度集中,在阀杆动作过程中受力较大,使得断裂沿着阀杆最薄弱的45°斜面扩展;又由于断裂阀杆强度不足导致开裂加剧,最终断裂。

With the fractured main steam valve stem in a subcritical 300 MW unit as the study example,the possible fracture causes of the main steam valve stem were analyzed from several aspects,including the structural design,macro morphology,microstructure and mechanical properties,combined with the service environment.The results show that the fracture was due to a combined effect of many factors.The mechanical properties of the valve stem material had declined below the standard requirement after long-term service,and the microstructure had a degradation,including remaining banded segregation and precipitating network carbide at the grain boundary.In addition,the valve stem material C422 steel displayed serious notch sensitivity,so cracks first initiated at the machining defect positions.At the same time,there existed structure design defect at the diameter changing position and the transition arc radius was rather small,which resulted in large stress concentration.So,during the valve stem working,the initiated cracks propagated along the weakest 45°slope,and the lower strength of the valve stem accelerated the crack propagation,which finally led to the fracture of the valve stem.