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| Method for welding of austenitic stainless steel pipe |
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In an attempt to overcome those difficulties in the butt welding of austenitic stainless steel pipe, it has been proposed to weld the pipe ends together while cooling the junction. However, external cooling of the welds is not effective enoughto prevent the carbide precipitation in the heat affected zones, and mere circulation of cooling water through the pipe does not reduce the residual stresses on the inner surface of the pipe, thus rendering it very difficult to preclude the stresscorrosion cracking. The present invention has for its object to provide a welding method which solves the foregoing problems in the butt welding of austenitic stainless steel piping and which prevents the development of stress corrosion cracking of the welds throughremarkable reduction of carbide precipitation as well as of tensile residual stresses due to welding. The object of the invention is achieved by either of the following two inventions. The first invention resides, in essence, in a method for butt welding austenitic stainless steel piping with forced cooling of the inside, which comprises the steps of initially welding the pipe ends together by a root pass prior to the inflow ofthe cooling medium so as to prevent leakage of the medium to the outside, introducing an insert for constraining the flow of the cooling medium into the pipe, and welding the junction while forcing the medium to flow through the space between the insertand the surrounding wall of the pipe, the insert being constructed so that it increases the flow velocity of the cooling medium to the maximum in the vicinity of the welded joint being formed. The second invention resides, in essence, in a method for butt welding austenitic stainless steel piping with forced cooling of the inside, which comprises the steps of initially welding the pipe ends together by a root pass prior to the inflowof the cooling medium so as to prevent leakage of the medium to the outside, introducing an insert for constraining the flow of the cooling medium into the pipe, and welding the junction while forcing the cooling medium, by means of the insert, toimpinge upon the welded joint being formed, at right angles thereto, and then flow through the constrained flow path formed between the insert and the surrounding wall of the pipe over distances of at least 25 mm each upstream and downstream from thewelded joint. In view of the fact that the stress corrosion cracking that develops in the welds of austenitic stainless steel piping is largely ascribable to the chromium carbide precipitation and tensile residual stresses in the weld on the inner pipesurface, extensive searches have been made for means which would reduce or relieve the two factors. As a result, it has now been found that the cooling medium proves strikingly effective when it is allowed to cool the weld on the inner surface of thepipe with the aid of means for ensuring good heat transfer. It has been clarified that, whereas natural or circulatory flow of the cooling medium through the pipe is little effective in correcting the aforesaid two causes of cracking, a substantiallyfavorable effect is attained by inserting a special jig into the pipe and thereby constraining the flow of the cooling medium and increasing the flow velocity of the medium along the junction being welded and the heat affected zone. |