The Internal Creep Rupture Strength of Joints of Steel Tubes Welded with Dissimilar Metals

Abstract

Hereunder is given an account of the uniaxial tension and internal pressure creep rupture tests conducted of the joints of 2. 25 Cr-1 Mo, 5 Cr-0.5 Mo, 9 Cr-1 Mo low alloy steel tubes, and of 18-8 Mo, 18-8 Ti, 18-8 Nb stainless steel tubes, welded with dissimilar metals, to investigate the effect of the weld defects, and of the decarburized layer, on the internal pressure creep rupture strength of the welded joints, as well as the relationship between the uniaxial and the internal pressure creep rupture strength.The results are summarized as follows:(1) When poor penetration is formed at the welded joints, fracture often occurs in the welded metal under the internal pressure, and leaves the internal pressure creep rupture strength poor.At sound joints, fracture always occurs in the base metal in the internal pressure test, and the strength of the welded joints is equal to that of plain tubes.(2) A decarburized layer is formed at all dissimillar metal joints, regardless of electrodes, stainless steels or Inconel alloys.At the joints of 2.25 Cr-1 Mo and stainless steels with 24 Cr-13 Ni stainless steel electrode, fracture occurs in the decarburized layer in uniaxial tests lasting for a longer time than 700∼2000 hours, but in the internal pressure tests, fracture occurs in the base metal even after a longer duration. At other joints, fracture always occurs in the base metal both in the uniaxial and the internal pressure tests.Thus it is evident that the internal pressure creep rupture strength of joints with dissimilar metals is not affected by the formation of a decarburized layer.(3) The uniaxial and internal creep rupture data of the plain tubes and the welded joints which fractured in the base metal, are analyged by the statistical method to determine the relationship between the uniaxial and the internal creep rupture strength.The mean diameter formula is fairly applicable to the result of 2, 25 Cr-1 Mo tubes, and of 9 Cr-1 Mo tubes, either in the received condition or in the stress relieved condition, while the ASME or the Tresca formula is applicable to the result of 5 Cr-0.5Mo steel tubes.