Comparison of Impact Toughness in Simulated Coarse-Grained Heat-Affected Zone of Al-Deoxidized and Ti-Deoxidized Offshore Steels

Henri Tervo,Jukka Kömi,Vahid Javaheri,Tuomas Alatarvas,Antti Kaijalainen,Mikko Mehtonen,Severi Anttila,Mohammed Ali

doi:10.3390/met11111783

Abstract

The presence of acicular ferrite (AF) in the heat-affected zone (HAZ) of steels used offshore is generally seen as beneficial for toughness. In this study, the effects of varying fractions of AF (0–49 vol.%) were assessed in the simulated, unaltered and coarse-grained heat-affected zones (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 coarser microstructural and inclusion features of the steels with AF and also the fraction of AF may not have been great enough to improve the CGHAZ toughness of the steels investigated.

Highlights

The demand for steels that can withstand harsh environmental conditions is increasing due to the opening of new oil fields in ever colder climates
In the most typically vulnerable zones, such as the coarse-grained heat-affected zone (CGHAZ), the intercritical heat-affected zone (ICHAZ) and the intercritically reheated coarse-grained heat-affected zone (ICCGHAZ), the toughness degradation should be minimized by design
The results for Tihigh and Tilow were similar for t8/5 of 5 and 24 s, but it was considerably lower for Tilow at 64 s. These results suggest that the CGHAZ microstructure of the Ti-killed steels is less effective in impeding the propagation of the crack than that of the Ref steel

Summary

Introduction

The demand for steels that can withstand harsh environmental conditions is increasing due to the opening of new oil fields in ever colder climates. The fine-grained microstructure of the steels used offshore is typically provided by thermomechanically controlled hot rolling processes (TMCP). Both high strength and high toughness in the base plate are obtained by utilizing this approach. The combination of strength, toughness and weldability provided by TMCP in these steels is beneficial for other structural uses [1]. Standards such as, for example, EN 10225-1 require offshore steels to be weldable using submerged arc welding (SAW), gas-metal arc welding (GMAW) or flux-cored arc welding (FCAW) processes [2]. In the most typically vulnerable zones, such as the coarse-grained heat-affected zone (CGHAZ), the intercritical heat-affected zone (ICHAZ) and the intercritically reheated coarse-grained heat-affected zone (ICCGHAZ), the toughness degradation should be minimized by design

Methods

Results

Conclusion