Microstructure Property And Control Of Two Kinds Of Pressure Pipeline Structural Steel

Because of the excellent properties, pressure pipeline components have important roles in all fields, such as pipeline transportation and power plants. The purpose of failure analysis to structure components is to improve the quality of products and to increase the using life. This plays important of role on industry, livelihood and technology development.In this study, three failure components have been used. They are304stainless steel elbows,304stainless steel bellows and X70pipeline steels. The specimens, which were selected from these three failure components near the fracture and apart from the fracture, respectively, were tested and analyzed using the following measures. The microstructures of these three failure components were observed using optical microscopy. The morphologies of fractures were investigated using scanning electron microscopy. The chemical compositions of these three failure components were tested by direct-reading spectrometer. The mechanical properties of304stainless steel elbow and X70pipeline steel were investigated using hydraulic tensile testing machine. The hardness of these three failure components specimens were investigated using Vickers hardness tester. And the phases compositions of failure components were studied by X-ray diffraction analysis.The tests results show that the banded structures were precipitated in the microstructure of fracture of304stainless steel elbow. Some inclusions were observed in this alloy. According to calculation results of explosion pressure, the tensile strength of fracture of304stainless steel elbow is lower than that of the standard low-limit. A large amount of cleavage planes were both observed in the explosion failure fracture and in the tensile fracture. It means that304stainless steel has a characteristic of embrittlement cracking. The failure of304stainless steel elbow was caused by defects and inclusions in this alloy. Cr23C6phase, precipitated in304stainless steel bellows, was investigated using X-ray diffraction analysis. Fatigue crack was easily initiated at this precipitate. This finally led to the fracture of304stainless steel bellows. Fatigue property of304stainless steel bellows can be improved by optimizing heat treatment, improving microstructure of alloy and increasing the thickness of bellows to suppress the forming of damaging channel. Martensite-austenite structure groups (M-A structure groups) and cavities, found in the microstructure of X70pipeline steel weld joint, are the main reasons that led to the embrittlement of weld joint and the degradation of toughness of X70pipeline steel weld joint. Through improving welding process of X70pipeline steel and optimizing the proportions of alloy elements in weld joint, the mechanical property of X70pipeline steel weld joint can be improved.