ＨＩＰ処理した鉄系焼結材料の疲労き裂進展に及ぼす組織の影響

Abstract

Fatigue crack growth of the sintered ferrous materials by HIP has been investigated taking the microstructures and the pattern of fractured surfaces into account.The results obtained are summarized as follows:(1) In the case of constant stress ratio, the fatigue crack growth rate(da/dN) of sintered materials by HIP with diffusion alloyed powder had a tendency to become slower than that of sintered materials by HIP with prealloyed and mixed elemental powders under the same stress intensity factor range(ΔK). The material constant value (m) in da/dN=C(ΔK)m was found in the range of 3.8 to 5.3, which was the same as that of wrought materials. Sintered iron compacts showed higher da/dN and m values than those of sintered materials by HIP.(2) The ΔKth(da/dN<10-9m/cycle) of sintered iron compacts was higher than that of sintered materials by HIP. The influence of stress ratio was recognized with the relationship between da/dN and ΔK of any sintered materials by HIP.(3) Fatigure crack grew preferentially in the ferrites as forming the slip bands close together. The behavior of fatigue crack growth was distinctly dependent on the microstructures, the cracks propageted linearly through the pearlites, and the cracks penetrated through or bypassed the cementites.(4) The fractured surface reflected the local intergranular fracture in the region of low crack growth rate(da/dN<10-9m/cycle). On the other hand, the striation like pattern was mainly observed in the region of high crack growth rate(da/dN=10-7-10-9m/cycle). As the stress ratio was increased, unevenness of the fractured surface was more distinguished.