A Study On Procedure And Optimizing Types Of Closure Weld Joint Between Huge Penstock And Spiral Case In Hydropower Station

The trend of penstock design is canceling telescopic joint of penstock in the giant hydropower station. A section of penstock with soft cushion layer is adopted as the replacement of telescopic joint, upper reach tip and lower reach tip of the penstock with soft cushion layer respectively lie in the concrete of the dam and plant, and distorts with the concrete. Stress-strain field of the penstock with soft cushion layer is analyzed under dam displacement and welding deformation. Appropriate joint type is chosen to prevent the appearance of crack and the shift of spiral case. Research was done to discover the effects of welding after canceling telescopic joint upon the construction and running of penstock with soft cushion layer. Therefore, the paper has great importance both in academy and engineering.Based on a great number of references at home and abroad, the paper analyzes various welding procedure and joint type of the closure weld. Further research covers systematically the following aspects, such as dam displacement under routine load and temperature load, stress of penstock with soft cushion layer caused by dam deformation and welding deformation, crack resistance of highly intensive steel under high restraint, deformation controlled by restraint and peen, performance of two kinds of closure welding joints, which includes welding tests, theory models and numerical simulation study. The main work and research content is as follows:Three coordinate measuring system and digital images non-contact measuring system based on computer are developed with advance devices. Their measuring accuracy are satisfied with deformation requirement. Stresses and deformations measurement in the welding of the butt closure joint show that axial contraction of penstock consists of transversal contraction and the contraction because of preheat. Axial contraction of penstock transfers elastic strain of penstock . Negative effect caused by butt joint type is that axial contraction of penstock and axial stress is too large . Although penstock is bond by concrete, restraint intensity of closure weld is low.610F steel has good crack resistance performance by means of many tests. It is general thought that raising the preheat temperature can effectively prevent the appearances of crack in thick closure weld under high restraint and bad environment condition. Research shows that it is unapt to raise the preheat temperature to 100℃in closure weld, raising the preheat temperature is unbeneficial to reducing axial contraction. Peening can prevent the appearance of crack under lower preheat temperature and high restraint condition by flexible restraint length experiment; Peening was proved to affect the performance of the joint by raising the hardness of the weld and depressing the striking-tenacity; Raising the preheating temperature was proved to improve the hardness tenacity of weld after peening. According to performance of the joint and axial contraction, a method of adapting double preheating temperature is suggested.Factors affecting axial contraction are the type of joint, preheat temperature and root opening except for welding procedure parameters. The contraction rule of two joint type has been studied within the big restrain frame, the rules of the transverse contraction and restraint stress of different butt joint root opening and lapped joint root gap in peening and restraint are explored. Research shows that peening and Restraint have little effect on transverse contraction of lapped joint, but have great effect on that of butt joint; when in restraint peening have more effect on reducing transverse contraction than without peening in butt joint; the transverse contraction increases with the increasing of root opening of the butt joint, so the strain-stress is greater; the restraint stress of butt joint is greater than that of lapped joint. It is probed that the mechanism of reducing transverse contraction with the compound effect of restraint and peening . Research shows that reducing transverse contraction need big tension stress in welding process, peening can reduce contraction and tension stress.Finite element models is built up to calculate the restraint intensity of the restraint plate and slit-type weld, and the rule of restraint intensity is known. The method of combining restraint by restraint plate and slit-type weld was suggested. Combining restraint experiment shows that it can control transverse contraction ,and can also control the thick asymmetry contraction. It is analyzed that free and compulsive boundary cooling affects temperature field of penstock, anti-deformation in huge penstock is also studied, and the loading of penstock and root face under anti-deformation are analyzed, and the method of segmented combining restraint controlling the transverse contraction of huge penstock is put forward.With huge penstock simplified, non-liner finite element models and heat source models is set up, the transverse contraction and restraint stress of multi-pass weld is simulated and analyzed under restraint condition. Instantaneous stress and residual stress of multi-pass weld is simulated and analyzed under restraint condition.. The real measurement of the residual stress are basically agreement with the numerical results distribution patterns of using the two-dimensional models comparatively, The validity of using non-liner finite element numerical simulation to investigate the welding residual stress is verified. The stress results under restrain show that the bigger the restraint intensity is, the greater the restrain stress is; the transverse stress is great under restrain. According to the result of the stress simulation, the lower the preheating temperature is ,the greater the residual stress is, and the time that the dynamic stress reaches a fixed value become earlier; the welding residual transverse stress increases with the restraint intensity. Accord to the simulation, the dynamic stress of first pass is constantly increasing until weld totally cooled and the dynamic stress of last pass is raising to maximum in short time, then slightly decline.Relationship between peen force and plastic deformation of elastic-plastic materials is analyzed by contact mechanics. Column and globe finite element models are set up to simulate that peen produces stress and deformation. Relationship between magnitude of residual compressive stress field and peen parameters, and distribution of residual stress after peen is studied. The magnitude of peen indentation is correlated with the magnitude of hammer and energy of peen. Residual stress and transverse contraction affected by peen and restraint is studied. Lager magnitude of peen is, effective field of stress reduced increase ,but value of stress depressed lower. Research shows that residual stress in inner weld can be relieved by way of bead by bead peening .The models that consists of dam/workshop/bedrock is analyzed as three dimension finite element integral models, and displacement of penstock with soft cushion layer among dams is calculated under routine load and temperature load. It shows that relatively displacement changes with annual periodicity. Sub-models is set up to penstock with soft cushion layer, and stress of the penstock with soft cushion layer is calculated under various loads combination. Researches show that closure in winter is better than in summer, and the case of butt joint anti-deformation with segment combining restraint has the least combined stress.In despite of axial contraction of lapped joint is less than butt joint `s, models of stress concentration factor for lapped joint and butt joint is set up and are calculated, lapped joint is 12 and butt joint is 2. Fatigue performance difference between the lapped joint and butt joint is economically analyzed by means of stress concentration factor, and fatigue performance of lapped joint is testly researched after peened/welded and in different clearance of weld. The result of the calculate and experiment shows that the fatigue life of the butt joint is much longer than that of lapped joint; fatigue performance of the lapped joint is sensitive to gap of lap. Peening can improve fatigue performance of joints.In a word , the properties and stress of closure weld is relevant to welding procedure and joint type . By adopting anti-deformation and segmented restraint and peening, type of closure weld joint is butt joint that there is no crack, small contraction and high quality is got in the case of double preheat temperature and high restrain. Transverse contraction with pre-stress is necessary in the future.