Abstract
The fatigue crack growth (FCG) behavior of 34CrMo4 steel, a typical material for gas cylinders, has been investigated. Specimens were taken from the base material (BM) as well as the hot-drawn (HD) cylinder and cold-flow (CF) formed cylinder along the longitudinal and transverse directions. The FCG tests were conducted under different stress ratios for different materials and directions. The main purpose of this research was to explore the influences of the mechanical and thermal processes, sampling direction and stress ratio on the FCG behavior of 34CrMo4 steel. To further reveal the mechanism of crack propagation at different stages, the microstructures and fracture modes of FCG specimens were analyzed by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD), respectively. The results showed that HD and CF materials exhibited better resistance to fatigue crack propagation than BM. The FCG rates of investigated materials can be accelerated by the increase in stress ratio. However, the sampling direction had little effect on the FCG rate. Finally, a driving force parameter (DFP) model was used to fit the experimental FCG data of three materials with different mechanical and thermal processes. A unified transition stage between the stable and unstable FCG stages of three materials under various experimental conditions was revealed by DFP model, playing an important role on the early warning of fatigue fracture for different types of 34CrMo4 steel.
Highlights
Gas cylinders are often used as special equipment to provide medical oxygen or as fuel storage containers
No obvious change of fatigue crack growth (FCG) rate in the stable region was caused by the increase in stress ratio
The FCG behavior of 34CrMo4 cylinder formed by hot drawing and cold flow was studied
Summary
Gas cylinders are often used as special equipment to provide medical oxygen or as fuel storage containers. Studies have shown that the failure of gas cylinders is mostly directly related to these defects, especially surface crack defects. Arola and Williams [6] investigated the effect of surface texture on the fatigue life of AISI 4130. Higher hydrogen pressure and lower loading frequency would lead to faster crack growth. The fracture observation results showed that the small number of inclusions had no effect on the fatigue crack growth (FCG) rate. AISI 4130 steel exhibited a high hydrogen embrittlement sensitivity, and the FCG rate was two or three orders of magnitude higher in specimens filled with hydrogen. Due to the numerous influencing factors, it was still necessary to explore the fatigue crack growth characteristics from different perspectives
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