溶接後水冷法によるオーステナイト系ステンレス鋼の応力腐食割れ防止 -応力腐食割れにおける溶接残留二軸応力の評価法-

Developed was a new method to control stress corrosion cracking (SCC) of weldments, named PWC (Post Weld Cooling) method. In applying PWC method to butt-welded pipes, the welding joint is cooled by water from its inner side alone immediately after all the welding works are finished. SCC can possibly be controlled by the method, because residual tensile stresses in the weldment are suppressed and, in some cases, converted to compressive stresses.<br>The details of adequate practices of PWC method were determined and the superior immunity from SCC of the weldment treated by PWC method as compared with those made by several conventional methods was evidenced by the following two tests. That is, the residual stress measurement by X-ray diffraction method and the SCC tests in the boiling 30wt% MgCl₂ solutions were performed on the welded test-pipes which include those in as welded conditions, those with PWHT applied and those made by the Heat Sink Weld method as well as those made by PWC method.<br>It proved that the susceptibility of the weldments to SCC was affected by the biaxiality of residual stresses and how to evaluate the biaxiality was discussed.<br>The following results were obtained;<br>1) Residual stresses resulted from the temperature difference between the inner and outer surfaces are considered to be prevailing in the weldments treated by PWC method.<br>2) It is possible to induce compression as far as the weldment is cooled by water before the temperature difference between the inner and outer surfaces becomes smaller than the value which produces the thermal stress equal to the proof stress of the base material.<br>3) PWC method has proved as effective in controlling SCC as the Heat Sink Weld and characterized by the following items:<br>i) No additional care for welding before applying PWC method is required other than the conventional one.<br>ii) More efficiency in welding procedure will be attained because high heat input and high welding speed are recommendable and the weldment can be cooled down to the ambient temperature as soon as the welding work is completed.<br>iii) It does not cost too much because no special equipment is used.<br>iv) It is applicable to weld repair, too.<br>4) Biaxiality of residual stresses, which depends on the pipe-size, affects on the mode of cracks. Cracks are parallel to the welding line in the small-diameter (1B) pipes whose stress condition is σ₂<0<σ₁, while they are perpendicular to the welding line in the large-diameter (900∅) pipes, in which σ₁>>σ₂. Cracks are network-like in the medium-diameter (4B, 12B) pipes, in which σ₁≈σ₂.<br>5) The threshold stresses for SCC is ruled by the equivalent stress difined in the Total Strain Energy Theory.