| It is inevitable that there are various forms of welding defects existing in weld joints. So weld joints have become the weakest part of weld structure. In weld joints, crack is easy to appear, and with the extension of crack, the weld structure will eventually be fractured. So the fatigue and fracture design of weld structure is mainly focused on fatigue fracture of weld joints. The newly developed low-carbon bainite steel is currently known as green steel, which is produced by using microalloyed and controlled rolling and cooling processes. The research on alloy structure steel is more and more focused on raising its reliability and usage. That is to say that in addition to meeting the strength requirements, more attention should be paid to its safety and longevity. Therefore, it is significant to investigate the fatigue crack growth reliability of weld joint for newly developed low-carbon bainite steel. In this thesis, the studies on conventional mechanical properties test, metallographic test, weld defect and fatigue crack growth of newly developed low-carbon bainite steel weld joints were carried out.The main works of this thesis are as followed:Mechanical properties test and metallographic test were carried out to study the weld properties of the newly developed low-carbon bainite ADB610steel, B610CF steel and WDB620steel by using different welding methods with different welding parameters. Research shows when using SMAW and MAG methods crack is not easy to appear in weld joints of these three steels, and the weld joints have excellent mechanical properties and anti-crack ability. And the welding parameters have little effect on strength, the plasticity of the whole weld joint, and the mechanical properties of heat affected zone (HAZ). However it has obvious effect on the impact toughness and metallographic structure of welded metal. Larger welding parameters will decrease the impact toughness of welded metal, and increase the size of crystalline grain in metallographic structure of welded metal.Research on probability distribution model of welding defect size and welding defect of newly developed low-carbon bainite ADB610steel and WDB620steel was carried out. It is concluded that welding defect size obeys exponential distribution based on the occurrence of welding defect meeting Poisson process and welding defect being generated randomly and defect ratio being a constant. Mathematical statistics analysis shows that welding defect size also obeys exponential distribution, lognormal distribution, normal distribution,2-Weibull and3-Weibull distribution, and the exponential distribution is the best one. Through the drawing and analysis of Pareto diagram, it is concluded that most of welding defect type in ADB610steel and WDB620steel is air hole and slag, and the crack is quite less.In this thesis, group method which adopts twice fitting of seven data was put forward. It is used to calculate the crack growth rate and Paris formula with different probability for more than one sample, when the record for crack length of each sample is not the same during the crack growth test. By using the group method, the fatigue crack growth rates of each sample in the same crack length can be calculated. And the fatigue crack growth rates and Paris formula with different probability were obtained.After ADB610steel was welded using SMAW when chosen CHE62CFLH electrode, the samples for base material(BM), HAZ and welded metal(WM) of weld joints were cut from the welded plate. After machining, the fatigue crack growth tests were carried out. By using group method, the crack growth rates of each sample in the same crack length were calculated. And the crack growth rates and Paris formula of these three zones with50%,90%,95%,99%and99.9%probability were obtained.Studies on C and m parameters in Paris formula of BM, HAZ and WM shows that C of BM is the largest, and C of WM is the least, and m of BM is the least, and m of WM is the largest when probability is the same. With the increase of probability, C and m of BM increase, and C and m of HAZ and WM decrease. And the decrease rate for C of BM is the least, and the increase speed for C and decrease speed for m of HAZ is the largest.Through visual analysis and calculation for relative difference value among BM, HAZ, and WM, it was concluded that the dispersibility of crack growth rate of these zones was obvious, and dispersibility of BM is the least, and the dispersibility of WM is the largest. When probability being the same, during the early stage the crack growth speed in BM is larger than the ones in HAZ and WM, and the one in HAZ is larger than the one in WM. The relative difference value of crack growth rate between HAZ and WM is less than the ones between BM and HAZ, and between BM and WM. During the middle and last stage of crack growth, the relative difference value of crack growth rate among three zones was obviously less than the one in early stage. With the increase of probability, the relative difference value between BM and HAZ, and between BM and WM decreases gradually in general.Statistical research on standard deviation and mean value of lg(da/dN) shows that the standard deviation among three zones has significant difference. In general, the standard deviation of BM is the least, and the one of WM is the largest. For the mean value of lg(da/dN), the one of BM is significant larger than the ones of HAZ and WM during the early and middle stage of crack growth, and the one of HAZ is significant larger than the ones of WM during the early stage of crack growth. It has insignificant difference in the mean value of lg(da/dN) between BM and WM during the last stage of crack growth, and between HAZ and WM during the middle and last stage of crack growth.From the aspects of metallographic structure and welding residual stress, the mechanism of crack growth for ADB610steel weld joints was studied. Studies shows that during the early and middle stage of crack growth, crack closure caused by resistance of grain boundaries and welding residual stress can increase the resistance for crack growth. So the speed of crack growth of WM having tiny grain and HAZ having net structure are slower than the one of BM. During the last stage of crack growth, when the resistance for crack growth decrease gradually, the difference among three zones is less. And the difference of metallographic structure among several area of BM is less than the ones among several area of HAZ and WM. So the dispersibility of crack growth of BM is less than the ones of HAZ and WM.By using single method which adopts once fitting of seven data combined with AK-da/dN method, the crack growth rate and Paris formula of ADB610steel weld joints with50%,90%,95%,99%and99.9%probability were obtained. The relative deviation by using single method compared to by using group method was studied through visual analysis, calculation for relative difference value and statistical research. It was found that the relative deviation by using single method is larger compared to by using group method. And it is inconformity with the process according the crack growth test data to analyze the dispersibility for crack growth property of material by using single method.The main innovations of this thesis are as followed:(1) group method which adopts twice fitting of seven data was put forward to calculate the crack growth rate in different probability. And it is pointed out that relative deviation is larger by using single method which adopts once fitting of seven data.(2) it was founded that standard deviation and mean value of lg(da/dN) among BM, HAZ and WM of ADB610steel weld joints has remarkable difference, which was explained from metallographic structure statistically.(3) for the research results, the crack growth rate and Paris formula of ADB610steel weld joints with50%,90%,95%,99%and99.9%probability were obtained. And with the increase of probability, the change rule of two parameters C and m in Paris formula was founded.(4) the probability model of weld defect size for newly developed low-carbon bainite steel was set up from Poisson process. And it was verified by statistical analysis through ADB610steel weld defect data on X-rays negative. |
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