汽轮机低压转子用钢的组织结构分析及低频疲劳裂纹扩展特性研究

MICROSTRUCTURE ANALYSIS AND THE INVESTIGATION OF CRACK PROPAGATION IN LOW FREQUENCY FATIGUE FOR 30Cr2Ni4MoV STEEL

30Cr2Ni4MoV 钢是一种汽轮机低压转子用钢.在工作过程中,因汽轮机反复停机,低压转子内的裂纹将会发生缓慢扩展,因此有必要对疲劳裂纹的低频扩展特性进行研究.通过对试验钢的组织结构分析及低频疲劳试验,得出了如下结果:(1)正常热处理状态下,钢的组织是回火马氏体,第二相碳化物都是渗碳体,无高钼碳化物存在;(2)在本试验条件下,低频疲劳裂纹的扩展特性与频率升高至 f=15时的疲劳试验结果并无区别;(3)随着ΔK 增大,疲劳条纹间距变大.二次裂纹的方向和最大切应力方向有关,而二次裂纹的间膈和钢的组织有关.

30Cr2Ni4MoV steel is used in the power industry for low pressure steam turbine rotors.Because of the repeated stop and start in service time cracks in rotor will lead to slow propagating.In order to ensure the safety of steam tur- bine,it is necessary to investigate the propagating behaviour of fatigue crack in low frequency testing.The following conclusions have been drawn through the experiment:(1)The microstructure of 30Cr2Ni4MoV steel after high temperature tempering is tempered martensite.The secondary phase formed in tempering is oementite.No high Me carbides such as Mo_2C Mo_6C have been found in the microstructure;(2) The propagating behaviour of crack in low frequency fatigue is similar to that in medium frequency testing;(3)On fatigue fracture surface the striation space increases with the increase of the ⊿K value.The direction of the secondary cracks is influenced by the orientation of maxium shear stress and the separating distance between two secondary cracks is correlative with the microstructure of the steel.