Abstract
Deterioration of the toughness in heat-affected zones (HAZs) due to the thermal cycles caused by welding is a known problem in offshore steels. Acicular ferrite (AF) in the HAZ is generally considered beneficial regarding the toughness. Three experimental steels were studied in order to find optimal conditions for the AF formation in the coarse-grained heat-affected zone (CGHAZ). One of the steels was Al-deoxidized, while the other two were Ti-deoxidized. The main focus was to distinguish whether the deoxidation practice affected the AF formation in the simulated CGHAZ. First, two different peak temperatures and prolonged annealing were used to study the prior austenite grain coarsening. Then, the effect of welding heat input was studied by applying three cooling times from 800 °C to 500 °C in a Gleeble thermomechanical simulator. The materials were characterized using electron microscopy, energy-dispersive X-ray spectrometry, and electron backscatter diffraction. The Mn depletion along the matrix-particle interface was modelled and measured. 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.
Highlights
The opening of new oil fields in ever colder climates increases demands for steels that can withstand the harsh environmental conditions
The toughness degradation occurring in the coarse-grained heat-affected zone (CGHAZ), the intercritical heat-affected zone (ICHAZ), and the Metals 2020, 10, 1096; doi:10.3390/met10081096
The following conclusions can be drawn from the results
Summary
The opening of new oil fields in ever colder climates increases demands for steels that can withstand the harsh environmental conditions. The steels used offshore are typically manufactured by thermomechanically controlled hot rolling processes (TMCP) that ensure fine-grained microstructure. By utilizing this approach, both high strength and high toughness in the base plate are obtained. The use of moderately low amounts of carbon and other alloying elements enhances weldability. This good combination of strength, toughness and weldability makes the TMCP steels suitable for other structural use [1]. Offshore steels are typically required to be weldable using submerged arc welding (SAW), gas-metal arc welding (GMAW) or flux-cored arc welding (FCAW) processes, e.g., as specified by the EN 10225-1 [2]. The toughness degradation occurring in the coarse-grained heat-affected zone (CGHAZ), the intercritical heat-affected zone (ICHAZ), and the Metals 2020, 10, 1096; doi:10.3390/met10081096 www.mdpi.com/journal/metals
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