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Weldability and toughness assessment of Ti-microalloyed offshore steel

Weldability and toughness assessment of Ti-microalloyed offshore steel

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METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 28A, JANUARY 1997—199 Weldability and Toughness Assessment of Ti-Microalloyed Offshore Steel I. RAK, V. GLIHA, and M. KOC ¸ AK The present study has been carried out to investigate the coarse-grained heat-affected zone (CGHAZ) microstructure and crack tip opening displacement (CTOD) toughness of grade StE 355 Ti-microal- loyed offshore steels. Three parent plates (40-mm thick) were studied, two of which had Ti microal- loying with either Nb 1 V or Nb also present. As a third steel, conventional StE 355 steel without Ti addition was welded for comparison purposes. Multipass tandem submerged arc weld (SAW) and manual metal arc weld (SMAW) welds were produced. Different heat-affected zone (HAZ) micros- tructures were simulated to ascertain the detrimental effect of welding on toughness. All HAZ mi- crostructures were examined using optical and electron microscopy. It can be concluded that Ti addition with appropriate steel processing, which disperses fine TiN precipitates uniformly, with a fine balance of other microalloying elements and with a Ti/N weight ratio of about 2.2, is beneficial for HAZ properties of StE 355 grade steel. I. INTRODUCTION M OST of the present operating offshore structures have been constructed with conventional 355 MPa yield strength steel plates that have been on the market since the early 1960s. They were generally characterized by good welda- bility and Charpy-V notch impact toughness of the weld joints made by the then used welding procedures. Nowa- days, because of higher heat inputs, the coarse-grained heat- affected zone (CGHAZ) adjacent to the fusion line of this steel grade represents a region of pronounced low tough- ness. This is often revealed by fracture toughness tests, which are being increasingly used in offshore constructions. The CGHAZ regions are often the main reason for local brittle zone (LBZ) appearance. Although the structural sig- nificance of LBZs characterized by their low crack tip opening displacement (CTOD) toughness has recently been studied extensively and debated, it still remains a contro- versial topic. Nevertheless, steel manufacturers have made improvements in conventional alloy design of this steel grade by aiming at good weldability and high CGHAZ toughness in order to meet stringent requirements of the offshore constructions. To avoid brittle fracture at low temperatures, the weld joints, in particular their CGHAZ, should have adequate toughness. Evidently, the weld thermal cycle that results in a peak temperature of about 1300 7 C being experienced by the microstructure adjacent to the molten weld metal can lead to pronounced precipitate dissolution. The conse- quences are austenite grain growth and the formation of hardened transformation products during cooling, which re- sult in rather low toughness zones susceptible to brittle frac- ture initiation, LBZ.
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