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 + 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 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 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 °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. 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. excessive grain coarsening in this region having a bainitic microstructure containing different amounts of various mar- tensitic/austenitic phases (M/A constituents), steel manu- facturers have made an attempt to restrict the austenite grain growth by the introduction of finely dispersed stable par- ticles 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 microstruc- ture/toughness in Ti microalloyed steels. These are as fol- lows: (1) refinement of ferrite grains achieved by the pinning ef- fect of thermally stable Ti-nitride or Ti-oxide particles distributed in austenite; (2) formation of pure Ti-nitride or Ti-oxide particles uni- formly dispersed in austenite at high temperature, which can set as nucleation sites for acicular ferrite during the - transformation during the cooling part of the weld thermal cycle; and (3) decrease of the detrimental effect of soluble nitrogen (80 ppm) in ferrite by the formation of fine nitrides. Microalloying with Al and Nb was previously considered to be beneficial in this respect, but the austenite grain bound- ary pinning effect of Al and Nb precipitates is restricted to peak temperatures below about 1100 °C. [1] Most of the fine AlN or Nb(C,N) precipitates in the matrix can dissolve, par- ticularly at high heat inputs. Hence, a part of the previously combined N is released in the CGHAZ, adversely influenc- ing the cleavage fracture resistance of this zone. Titanium is mainly being used because of its ability to form nitrides and oxides stable even at high temperatures (1350 °C). The expected role of Ti, however, can be com- plicated if the steel contains more than one microalloying element, e.g., Ti, Nb, or V. These complex Ti carbonitrides precipitated in steel influence grain growth and precipita- tion hardening during welding. [2] Due to their interactions,