**1. Introduction**

Recent developments of high strength low alloyed steels (HSLA) for energy sector focused on the need to target optimized combinations of strength, toughness, weldability on industrial scale at affordable prices [1–5]. A similar scenario also applies over other application sectors (e.g., offshore structural application and shipbuilding) with different specific requirements, which are set as a function of technological and exercise needs [6]. Vanadium, due to its own thermodynamic and kinetic ability to precipitate in form of carbide and nitride, is considered a key element in the metallurgical design of modern HSLA steels [7–9], as it enables efficient and cost-effective solution across a broad range of applications [10–13]. For example, in high strength-high toughness steels for pipelines, the

**Citation:** Stornelli, G.; Tselikova, A.; Mirabile Gattia, D.; Mortello, M.; Schmidt, R.; Sgambetterra, M.; Testani, C.; Zucca, G.; Di Schino, A. Influence of Vanadium Micro-Alloying on the Microstructure of Structural High Strength Steels Welded Joints. *Materials* **2023**, *16*, 2897. https://doi.org/10.3390/ ma16072897

Academic Editor: Abdollah Saboori

Received: 21 March 2023 Revised: 30 March 2023 Accepted: 3 April 2023 Published: 5 April 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

increase in the available strength level up to X80–X100 grade of line pipe steels, promoted in the latest decades, has produced economic advantages estimated in the billion-dollar range [14].

The evolution of the effect of microalloying on the microstructure and the properties of a girth weld is challenging as this depends on the number of inter-correlated metallurgical phenomena correlated to the steel chemical composition and the welding processing conditions [15–17].

Despite the importance of microalloying for the development of high strength steels with increased toughness, a decay of material properties in girth welded joints is reported in literature [18] which has limited an excessive use of microalloying.

Thermal cycles experienced during welding have a large impact on equilibrium set between high strength and high toughness in HSLA steels as these cycles are the main cause of toughness loss in the heat affected zone (HAZ).

Welds and heat-affected zones are critical when considering structural integrity, specifically, fracture and fatigue properties [19,20]. Weld design in thicker gauge plates requires consideration of the time required to perform the weld, which is partially controlled through the heat input. With the aim of saving time, reducing component manufacturing costs, and improving efficiency, high heat input welding technology has been widely used.

Historically, the lowest toughness was expected in the grain coarsened heat affected zone (GC HAZ), which is the part of the HAZ closest to the welding fusion line [21–24]. During welding, the GC HAZ experiences peak temperatures up to the melting point, followed by rapid cooling. The high temperatures can lead to significant austenite grain coarsening [25], the combination of a coarse austenite grain size and rapid cooling promotes brittle microstructures, which contain high proportions of ferrite side-plates and bainite [26].

In recent years, it has been found that the most degraded part in the HAZ is the intercritically reheated grain coarsened HAZ (IC GC HAZ), which is the region of the GC HAZ reheated to temperatures between the Ac1 and Ac3 by subsequent welding passes [27]. During the inter-critical thermal cycle, partial transformation to austenite occurs, particularly where austenite stabilizers, such as carbon or manganese, are segregated in the initial microstructure [28]. These areas include pearlite/bainite colonies. When cooling, these high carbon regions transform into pearlite/bainite or residual austenite (RA) depending on the hardenability of the austenite and cooling rate [29]. The presence of RA phase is generally regarded as the major factor which reduces the HAZ toughness [30,31].

However, it is also reported that the loss in toughness is not just due to the presence of RA phase, but is related to the distribution and morphology of the RA constituent, and the matrix microstructure. Cui et al. [32] reported that block residual austenite significantly deteriorates impact toughness of super-critical reheated coarse grain heat affected zone (SC CGHAZ).

Niobium is commonly added to enhance the strength of HSLA steels. However, under welding conditions, niobium adoption shows detrimental effect on the HAZ toughness, although its effect is strongly dependent on heat input [33]. At medium to high heat input despite a precipitation hardening effect via Nb(C,N), niobium has a detrimental influence on the fracture toughness of GC HAZ. Niobium reduces the grain boundary ferrite formation and promotes the nucleation of coarse structure of ferrite with aligned RA resulting in increased mechanical properties. A small addition of niobium suppresses ferrite nucleation at prior austenite grain boundaries and increase the volume fraction of either martensite or bainite. Previous studies reported that the major advantages of a niobium addition, i.e., the grain refinement and the resultant improvement of base metal mechanical properties, appear to be outweighed by the detrimental effects of martensite formation, when the steel plates are welded [34].

On the other hand, vanadium leads to grain refinement and precipitation strengthening to HSLA steels. The effect of vanadium on the GC HAZ microstructure is quite different from that of niobium. Vanadium has a beneficial effect on the toughness of the GC HAZ, because it reduces the bainitic colony size and, due to the low misfit between

vanadium nitrides (VN) and ferrite in comparison with other types of inclusions, promotes intragranular nucleation of acicular ferrite [20,35].

Moreover, alloying associated with the precipitate formation is an important consideration in weld design to achieve desirable microstructures in the HAZ [36]. For instance, Zajac et al. [37] performed HAZ simulations on 25 mm-thick HSLA plates (Fe-0.09%C-1.4%Mn-0.08%V-0.010%Ti-xN) with low (0.003%N) and high (0.013%N) nitrogen contents. They showed that for the high nitrogen steel, intragranular forms for a wider range of cooling rates as compared to the low nitrogen steel. In contrast, in high nitrogen steel for the high heat input (slowly cooled) conditions, a significant fraction of coarse ferrite grains forms at the austenite grain boundaries, which leads to poorer toughness compared to the low nitrogen steel. Zajac et al. [37] interpreted that the increased fraction of coarse grain boundary ferrite was associated with V(C,N), the amount of which would likely be increased during slow cooling; it should be noted that the mechanism for V(C,N) to accelerate the formation of grain boundary ferrite is not clearly discussed in the study. Zajac et al. [37] also pointed out that the precipitation status, which depends on the peak temperature in HAZ simulation, influences austenite grain size and amount of 'free' nitrogen, both of which affect phase transformations upon cooling.

Hu et al. [38] and Wu et al. [39] showed that V(C,N) with sizes between 20–30 nm were not detrimental on impact toughness because of their small sizes.

Following the above mentioned latest developments, the influence of vanadium on the toughness and fatigue resistance of the IC GC HAZ is not fully understood and requires further investigation. To understand the effective resistive behavior due to the addition of vanadium in the IC GC ZTA of a welded joint, it is first of all considered appropriate to investigate the real effect of the alloying elements on the microstructural characteristics.

In this regard, this study aims to assess the effect of vanadium alloying on material properties (in terms of microstructural constituent variation, RA formation and precipitation state) of an S355 steel (EN10025-2), when subjected to welding-representative thermal cycles in the IC GC HAZ.
