Abstract
The effects of specimen size and welded joints on the very high cycle fatigue properties of compressor blade steel KMN-I were studied by ultrasonic fatigue testing. It was found that the S-N curve of large specimens had a slow decline above 107 cycles, and fatigue failure still occurred in the very high cycle regime (>107 cycles), while the very high cycle fatigue characteristics of welded specimens was less obvious, and the fatigue limit was observed. Metallographic observation and SEM analysis were carried out on the fracture of the specimens. The results showed that surface fractures were mostly observed in the large specimens, and only a small number of cracks initiated from non-metallic inclusions above 107 cycles. The cracks of welded specimens initiated from the surface below 107 cycles and initiated from the internal matrix above 107 cycles. In addition, the formation mechanism of GBF (granular bright facet) was analyzed by the “dispersive decohesion of spherical carbide” theory, and the fatigue strength and fatigue life were predicted, which was consistent with the experimental results.
Highlights
The centrifugal compressor impeller is a typical gas delivery equipment which plays an important role in energy, petroleum, nuclear power, and other important industries
For compressor blade steel KMN-I, the cracks usually initiated from the surface, and we focused on the fatigue strength determined by surface detects and surface roughness
The very high cycle fatigue S-N curves of the compressor blade steel KMN-I needed were obtained through ultrasonic fatigue tests of large specimens and welded specimens, which provided theoretical support for the follow-up design of compressor impeller blades
Summary
The centrifugal compressor impeller is a typical gas delivery equipment which plays an important role in energy, petroleum, nuclear power, and other important industries. It is no longer reliable to design a centrifugal compressor impeller with fatigue life and fatigue strength in the traditional S-N curve, and it is very important to study the very high cycle fatigue behavior and mechanism of compressor blade steel. Some scholars studied the very high cycle fatigue behavior of welded joints using the ultrasonic fatigue test system, and the results showed that the cracks mainly initiated from the welded seam, which is the most dangerous part of the welded joint. We mainly studied the effects of specimen size and welded joints on the very high cycle fatigue properties of compressor blade steel KMN-I, and hope to provide theoretical guidance for fatigue design and safe operation of welded compressor impellers
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