疲劳损伤钢件延寿机理及效果

MECHANISM OF PROLONGING LIFE AND EFFECT FOR FATIGUE DAMAGED STEEL SPECIMENS

在恒应变ε=±0.5%和ε=±0.8%控制下,对淬火和600℃回火的40Cr钢试样进行疲劳循环加载,求得存活率为90%时的寿命Nf,将另一组试样疲劳加载到少于Nf的不同周次,在保护介质下550℃加热2小时(称为修复退火)后,测残余疲劳寿命。当损伤主要是微观结构变化时,退火效果随损伤周次增加而提高。其原因是存在试样中的应变能在修复退火中作为附加的驱动力,促使微观组织向均匀和稳态转变,延缓了碳化物界面裂纹的形成。但当损伤促使微观裂纹形成时,退火不能使其愈合,修复效果降低,所以有一个最佳的修复退火时机。此时进行修复退火,使疲劳总寿命提高2倍。但对高周次疲劳损伤,进行修复退火不能愈合裂纹和延长寿命。对它进行中温热静等压(hotisostaticpressure,HIP)处理,可提高总寿命2.4倍。中温热静等压使试样表面硬度、残余压应力都增加,可减缓微裂纹的萌生和扩展。试样的电阻率变化能够检测微观结构疲劳损伤程度和修复效果。

The quenched and 600℃ tempered specimens of 40Cr steel were fatigued at constant cyclic strain amplitudes ε = ± 0.5% and ε = ± 0.8% and the life with 90% survival probability, Nf,was determined statistically by a group of specimens. Other group self specimens were damaged to different fractions of Nf. After that the fatigue damaged specimens were heated to 550℃ for 2 hours in a protective environment atmosphere （called recovery annealing）. It is found that ff the main damage is caused by the microstructure variation, the annealing effect is increased as cyclic number increases, the reason is that in recovery annealing the accumulated strain energy can act as driving force to accelerate the transferring to homogeneous and stable microstructure, therefore to delay the carbide separation from the matrix. But if the micro-cracks formed in the steel, the annealing cannot eliminate the existing micro-cracks, the recovery effect was decreased, thus an optimizing approach of annealing exists. For the optimal recovery annealing, the total life was incieased as twice as the original. But for the specimens of high cycle fatigue damaged, the annealing can not increase the total life. The medium-temperature hot isostatic pressure （HIP） treatment can increase the total life to 2.4 times as that without the HTP, HIP treatment results in the increasing of both surface hardness and the residual compersive stress increases, which delay micro-cracks initiating and growing. The variation of electrical resistivity of specimens can be used to detect the degree of fatigue damage of the microstructure and the recovery effect.