ABSTRACT:

For arctic applications the resistance against low temperature brittle fracture of welded structures is of primary concern. Therefore, the present work addresses the mechanical properties and fracture toughness of a 50 mm thick 420 MPa steel and its weldment at −60ºC. Submerged arc welding were used to deposit weld metal in a half-V groove with one straight side for mechanical testing (tensile and Charpy V) and fracture toughness testing of fusion line with surface notch, in addition to the weld metal.

INTRODUCTION

At present, oil and gas exploration is moving towards deeper fields and harsher climate conditions. For example, a preliminary assessment by the US Geological Survey suggests the Arctic seabed may hold as much as about 30% of the world's undiscovered gas and 13% of the world's undiscovered oil (Gautier et al, 2009), mostly offshore under less than 500 meters of water. By one estimate, 400 billion barrels of oil might lie beneath the Arctic seabed. In these areas, the temperature may occasionally fall below −40°C, and design temperatures approaching −60°C may be required. This represents huge challenges to the materials which are to be used. Normally, e.g. structural steels and pipelines may easily satisfy toughness requirements at such low temperature. However, welding tends to be very harmful to low temperature fracture toughness. Both the heat affected zone (HAZ) and the weld metal may fail in providing "sufficient toughness". Although the ISO Standard 19906 has been launched for arctic areas, there will still be some discussions in the years to come concerning the test methods, test results and fitness for purpose assessments. Therefore, the present work was carried out with the objective of evaluate the HAZ and weld metal low temperature toughness of submerged arc welded (SAW) steel with specified minimum yield strength (SMYS) of 420 MPa.

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