Weldability and toughness assessment of Ti-microalloyed offshore steel
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I.
INTRODUCTION
MOST 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 weldability and Charpy-V notch impact toughness of the weld joints made by the then used welding procedures. Nowadays, because of higher heat inputs, the coarse-grained heataffected zone (CGHAZ) adjacent to the fusion line of this steel grade represents a region of pronounced low toughness. 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 significance of LBZs characterized by their low crack tip opening displacement (CTOD) toughness has recently been studied extensively and debated, it still remains a controversial 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 7C being experienced by the microstructure adjacent to the molten weld metal can lead to pronounced precipitate dissolution. The consequences are austenite grain growth and the formation of hardened transformation products during cooling, which result in rather low toughness zones susceptible to brittle fracture initiation, LBZ. To prevent embrittlement that is the consequence of the I. RAK, Head of Institute for Design and Machine Manufacturing, and V. GLIHA, Researcher at the Welding Laboratory, are with the Faculty of Mechanical Engineering, University of Maribor, 2000 Maribor, Slovenia. M. KOC¸AK, Head of Welding Group, is with the GKSS Research Center Geesthacht, D-21502 Geesthacht, Germany. Manuscript submitted July 20, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
excessive grain coarsening in this region having a bainitic microstructure containing different amounts of various martensitic/austenitic phases (M/A constituents), steel manufacturers have made an attempt to restrict the austenite grain growth by the introduction of finely dispersed stable particles such as Ti nitrides or Ti oxides into the different steel grades. Surveyed open literature[1–26] indicates that for over a decade continuing efforts have been made in different countries to achieve fine-grained heat-affected zones (HAZs) in high heat input welds by using Ti-microalloyed steels. There is general agreement about three inter-related main mechanisms for the improvement of HAZ microstructure/toughness in Ti microalloyed steels. These are as follows: (1) refinement of ferrite grains achieved by the pinning effect of thermally stable Ti-nitride or Ti-oxide particles distributed in austenit
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