| For cryogenic equipment especially the austenitic stainless steel pressure vessels, strain-strengthening technology is usually applied to produce some plastic deformation in the whole structure in order to increase its yield strength and the allowable stress so as to decrease the thickness and weight of vessels. But compared with the adavanced countries such as America and Europe, study and application of the strain-strengthening technology in China are still at the initial stage with many technical problems need to be studied.06Cr19Ni10 is a Chinese brand for an austenitic stainless steel which is usually used to construct cryogenic pressure vessels in China. In this dissertation, an automatic control system for strain-strengthening on vertical pressure vessels has been developed and effects with the mechanisms of strain-strengthening on on the fatigue crack propagation at 06Cr19Ni10 welding joints has been studied. The main works are as follows:(1) Development of the automatic comtrol system for the strain-strengthening of pressure vessels. Based on the technical requirements for the strain-strengthening of austenitic stainless steel pressure vessels, concrete parameters for lower level computer PLC, data transformation module, executive module and sensor module were determined. PLC programs based on control flow of strain-strengthening were written and amonitoring screen using configuration software was developed. An automatic measurement equipment for circumferential strain was designed and installed which was proved to be superior to the present techniques for measuring circumferential strain.(2) Study of the effects of strain-strengthening on mixed fatigue crack propagation at 06Cr19Ni10 welding joints. By performing fatigue tests on base material specimens, HAZ specimens and weld metal specimens with inclined central cracks of various angles, the effects of strain-strengthening on mixed mode fatigue crack propagation were studied. Specifically, mode transformation and crack propagation direction were investigated according to macro-obeservation and numerical investigation. Stress intensity factor ? K I was calculated with the finite element software AYSYS . Paris-Erdogan formulations for various specimens were obtained to show the effect of strain-strengthening on fatigue crack growth rates. Differences of fatigue crack growth rates between different specimens were analyzed.(3) Study of the effects of strain-strengthening on residual stress of 06Cr19Ni10 welding joint. The mechanisms of strain-strengthening on the fatigue crack growth were investigated from the aspect of residual stress theory. Numerical simulations were conducted on the welding and strain-strengthening process of 06Cr19Ni10 welding joints using MSC.MARC software. Residual stress distributions at the welding joints with different strain-strengthening levels were obtained. The residual stresses at welding joints were also measured with X-ray method to verify the accuracy of numerical simulationresults.(4) Study of metallographic structure and fracture morphology of strain-strengthened 06Cr19Ni10 welding joints. The mechanism of strain-strengthening on the fatigue crack growth were also investigated from the aspect of phase transformation of microstructures. X-ray diffraction tests and metallography detection of various strain-strengthened base metal specimens, HAZ specimens and weld metal specimens were conducted. The effects of strain-strengthening on metallographic structure, macro fracture morphology and micro fracture morphology were analyzed. Fatigue lives of specimens were estimated by quantitative reverse deductions from fatigue fracture surface. |
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