An Evaluation of the High Strength Cast Steel Welded Joints for Offshore Industry

Abstract

Torpedo anchor piles are tubular carbon steel welded structures that are connected to the mooring lines, which prevent the oil platform from moving, through padeyes welded to the structural pipes. Due to the complex geometry of these padeyes, cast pieces are an important option in the manufacturing of these items, in order to meet market demand. Considering the limited availability of information about such type of welded junction in the technical literature, the present work studies the behavior of welded joints of a high strength cast steel used on offshore platforms torpedo anchor padeyes. Thus, K-type joints were used for the welding of an ASTM A 148 Gr. 105-85 steel with an ultimate tensile strength of about 700 MPa, by using shielded metal arc welding process, in flat position, with a preheating of 200 °C and an average welding energy of 1.5 kJ/mm. After welding, a post weld heat treatment (PWHT) was applied, at 620 °C for 1 hour, followed by air cooling. The properties were evaluated through tensile, hardness and Charpy-V impact tests, as well as metallographic examination by optical and scanning electron microscopy with the use of EBSD (electron backscatter diffraction) technique, in both as welded and after PWHT conditions. The results shown that, although the mechanical properties obtained for the welded joint achieved the requirements for the application in all analyzed conditions, the execution of PWHT is recommended for the relief of residual stresses and promotion of a tempering of the high hardness martensite of the last welding pass. Additionally, it was verified that it is possible to obtain properties that are similar to those of welded joints of rolled steels, which are usually used on mooring components.

Key-words:
Cast steel; Welding; Mechanical properties