Modern low-carbon thermomechanically treated steels are popularly used in shipbuilding and marine structures (oil rigs, foundations for wind turbines), where the components of the product must withstand extreme environmental load and the resulting increased mechanical stress [1]. These structures are often subjected to dynamic loading at negative temperatures. Therefore, the steel producers strive to provide these steels with excellent toughness properties, against the regulations imposed quite strict requirements. Depending on the steel grades offshore steels are produced by normalizing (N), thermomechanically controlled process (TMCP), quenching and tempering (Q + T). For the TMCP steels in the medium strength range (ReH=400…500 MPa) are characterized by low carbon content and low carbon equivalent (CE) with the mixed microstructure of ferrite and bainite. Offshore steels with the highest toughness are typically manufactured by TMCP that leads to an extremely fine-grained microstructure. The presence of acicular ferrite (AF) forming across the small precipitates and inclusions has a determining role in achieving the demanded toughness [2][3]. In welded joints ferrite plates growing in many radial directions on small-sized non-metallic inclusions, as crystallization nuclei, can contribute to the improvement of toughness properties due to their heterogeneous structure [2]. However, the presence of M-A constituents can reduce the impact energy especially in the heat-affected zone [4]. Due to the favorable chemical composition, these steels have good weldability which is verified by its location in the first zone of Graville diagram [5]. Cold cracking is not expected, however, they may initiate in larger cross sections from the existing hot cracks at the end crater [6]. Deterioration of toughness in the HAZ for example is a known problem with offshore steels [3][7]. Furthermore, it is a challenge to guarantee that the impact energy level in the dendritic, mostly multi-pass weld, is the same as that in the fine-grained and micro-alloyed steel plate, which is rolled through specific rolling processes. The applied standards [8] have certain acceptance criteria for the toughness properties of the HAZ and the weld, which are typically required to be tested with Charpy V-notch (CVN) impact tests and with fracture toughness tests, such as crack-tip opening displacement (CTOD). Nowadays, numerous research deal with the role of AF in the heat-affected zone (HAZ), however, there is less focus on the weld microstructure and mechanical properties.
Tervo et al. [3] investigated low-temperature toughness properties of 500 MPa offshore steels and their simulated coarse-grained heat-affected zones (CGHAZ). It was found that both impact and fracture toughness of an S500 offshore steel with coarse titanium-based nitrides were lower and the scatter higher than in the steel with a higher quantity of calcium-based inclusions and without the coarse nitrides. In [7] three experimental steels were studied in order to find optimal conditions for the AF formation in the CGHAZ. One of the steels was Al-deoxidized, while the other two were Ti-deoxidized. The focus was to distinguish whether the deoxidation practice affected the AF formation in the simulated CGHAZ. It was found that AF formed in the simulated CGHAZ of one of the Ti-deoxidized steels and its fraction increased with increasing cooling time. In this steel, the inclusions consisted mainly of small (1–4 µm) TiOx-MnS, and the tendency for prior austenite grain coarsening was the highest. In another [9] study, the effects of varying fractions of AF (0–49 vol.%) were assessed in the simulated, unaltered CGHAZ of three experimental steels. Two steels were deoxidized using Ti and one using Al. The characterization was carried out by using electron microscopy, energy-dispersive X-ray spectrometry, electron backscatter diffraction and X-ray diffraction. The fraction of AF varied with the heat input and cooling time applied in the Gleeble thermomechanical simulator. AF was present in one of the Ti-deoxidized steels with all the applied cooling times, and its fraction increased with increasing cooling time. However, in other materials, only a small fraction (13–22%) of AF was present and only when the longest cooling time was applied. The impact toughness of the simulated specimens was evaluated using instrumented Charpy V-notch testing. Contrary to the assumption, the highest impact toughness was obtained in the conventional Al-deoxidized steel with little or no AF in the microstructure, while the variants with the highest fraction of AF had the lowest impact toughness. It was concluded that the decreased impact toughness in the CGHAZ of steels with AF was due to the coarser microstructural and inclusion features, and also the fraction of AF may not have been great enough to improve the CGHAZ toughness of the steels investigated. The effects of titanium content on the weld microstructure, mechanical properties, and inclusion characteristics were investigated by Seo et al [10] in the as-deposited bainitic gas metal arc weld metals having a nearly constant level of oxygen content. It was found that titanium addition enhanced the formation of AF with the maximum proportion being obtained at ∼0.07 wt.% Ti. The resultant change in weld microstructure with titanium content was well reflected in the Charpy impact energy showing the lowest ductile-brittle transition temperature at 0.07 wt.% Ti.
The effect of multipass welding on the weld metal has been studied previously e.g. by Kang et al. [11]. They utilized physical simulation to study the microstructures and mechanical properties in reheated zones in two different weld metals. They concluded that the toughness reduction in reheated zones of low hardenability weld metal was attributed to the increase of the grain boundary ferrite in the microstructure, whereas in the high hardenability weld metal the reason for the reduced toughness in the reheated zones was the coalesced bainite.
The goal of present research is to investigate the properties of multipass welded joint of a 500 MPa grade shipbuilding steel by physical simulation in the industrially relevant t8/5 cooling time range of arc welding processes. The experimental program is aimed at the production and analysis of physically simulated HAZ subzones (coarse-grained, intercritical, intercritically reheated coarse-grained) occurring in the weld, which can be considered critical zones in terms of toughness, by microstructural characterization, hardness test and instrumented Charpy V-notch impact toughness test.