The failure of welded tubes with various diameters is mainly in the form of Stress Corrosion Cracking (SCC) nearby the welded joints where high stress exists when they are employed in the transportation of corrosive media in the petroleum and chemical industry. In most cases, one of the most effective resolutions to this problem is to change the stress state in the joints. Many methods are available. Among them, the temperature-difference compatible deformation method has more obviously advantages than others. Its main principle is as follow: 1) Welding residual stresses are relieved through the radial deformation between the weld and base metal by a temperature difference produced by the positive temperature field on both sides away from the weld. The heating sources inducing positive temperature field are located in the compressive residual stress region. 2) Welding stresses on inner surface is reduced by producing an negative temperature difference between the inner and outer surface of the weld. Differences between the two ideas lie in that: the former one is based on the rule of compatible deformation between the heating regions with the welding region, while the later one based on the rule of forced plastic extension in the inner weld.
Lin, etc. proposed a method that temperature difference could be made by means of torch heating on the both sides of a weld (in the way torch heating offered positive temperature fields). The experimental and numerical results show that although the tensile residual stresses in the weld transferred into compressive stresses, high tensile stresses come out in the heating zones. Meanwhile, the method is difficult to control, so it is not convenient for industrial application.
To overcome the shortage of above method, a new technique producing temperature difference by induction heating was proposed and studied. The induction heating treatment on a girth-weld tube was numerically studied by the non-linear finite element analysis software - MSC.Marc. The main contents are as follows:
1.By FEM analysis, welding thermal field and welding residual stresses field were simulated on 90mm diameter of tube with different materials(20# steel and 1Cr18Ni9Ti stainless steel)and different wall thick(3mm~10mm). The results show that the distribution and its variation of temperature field in the tube were fit to the common rule of welding. The welding residual stresses in the model are well compatible with the experimental results.
2.By FEM analysis, the process of induction heating for stress relief was simulated on the same tubes. It indicates the effects of some main factors such as the location (L) and gap (d) of heating coil, heating frequency (f) , heating current (I) and heating time (t) on the stress relief. The results show that the temperature differenceDT and location are main factors affecting effect of stress relief. The temperature difference is dependent on the gap, the frequency, current and heating time. Among them, the heating time has the most obviously impact, the gap and current have some impact, and the frequency has a little action. The induction heating method may effectively cover the shortage of the flame heating method. Apart from rapid and convenient, the compressive stresses may be obtained on the inner surface of the girth-weld as lower tensile residual stresses (not more than 1/2 yield strength) existed in the heating area.
3.The choosing principle of the optimal parameter was proposed: The stresses on the inner surface of welds are between ±50MPa, the stresses on the inner surface of heating zones are below a half of the yield stress. The optimal parameters referring to the above principles were given for the tubes.
4.The mechanism of stress-relief by the induction heating temperature-difference deformation technique was analyzed. It is considered that the weld region with necked-down was significantly extended due to the expansion from both heated zones. Thus the action of stress overlap in weld region results in some plastic deformation, the original welding residual stresses have got relaxed.
5.A new heating way from this method was proposed—asynchronously alternative heating technique on both sides of the weld. The results show that the technique shows rather stress relief effect.
The results presented above are useful for future experimental proof, and it can be considered guide to industrial application of this new method.
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