Microstructure of the Welded Joint

The study and identification of the microstructure of the heat-affected zone in multipass welding is a multi-step physicometallurgical challenge due to overlapping of multiple areas with different thermal cycles. Fig. 4 shows a panorama of the microstructure of the main zones of the investigated welded joint with a HI of 15 ± 2 kJ/cm, whose classification is provided in accordance with EN 10225-1:2019 taking into account the scheme shown in Fig. 3 and the location of Zone X in Fig. 2.

The columnar weld metal of the zone of incomplete melting of the weld metal of investigated welded joint (Fig. 4.1) has a dendritic structure and is a mixture of polygonal ferrite and Widmanstedt structures. The average radius of the columnar structure in this case is ~4198 µm (Fig. 2a, Zone X). The average conditional size of one dendrite ranges from 150 to 1200 µm in the direction parallel to the solidification front and from 40 to 105 µm in the direction transverse to the solidification front. The “beads” of each weld pass are located symmetrically relative to each other. The fusion line (Fig. 4.12) is clearly pronounced and characterizes the transition from the dendritic structure of the weld to the structure of the grain-coarsened overheated zone.

Different morphological types of ferrite and bainite, are observed in the structure of the grain overheated coarsened zone of the heat-affected zone (GCHAZ), when there was no influence of other zones during multipass welding [ 12] (Fig. 4.11). It is important to note the absence of martensitic or bainitic rude packages.

This has a positive effect on the level of low-temperature impact toughness and increased fatigue resistance of HAZ. In the fine grain zone (Fig. 4.10) where complete recrystallization occurs (FGHAZ), the structure contains a mixture of quasi-polygonal ferrite formed during the cooling process in the temperature range from 1100 ÷ 1200 °C to Ac3.

The next important area of the welded joint is the approximate front near point Ac3 (Fig. 4.4). This is the area which is heated to temperature Ac3 that separates the fine grain zone where complete recrystallization occurs from the intercritical zone where large grains are formed as a result of incomplete recrystallization.

The microstructure of this interval represents a transition state from the fine grain zone to the intercritical heat affected zone (ICHAZ) a mixture of fine and coarser grains (Fig. 4.6). This zone, without the influence of the neighbouring zones, is characterized by incomplete recrystallization, due to the achievement of heating and cooling conditions in the range between Ac1 and Ac3. This results in a mixture of fine recrystallized grains and large uncrystallized ferrite grains. Thus, irregularly shaped quasipolygonal ferrite with separate zones of coarse and finer grains is observed in the structure of this zone [ 13]. When several zones overlap, ferrite grains of different shapes are observed in the structure [ 14] (Fig. 4.3). In addition, the intercritical zone is limited to the area with a transition structure of the Ac1 line with isolated areas of degenerate pearlite (Fig. 4.2). The structure of the base metal (Fig. 4.9) is represented by a combination of quasi-polygonal and polygonal ferrite with minor amounts of pearlite and bainite.

The SEM method was used to study the morphology of ferrite and carbonaceous phases in the area of complete recrystallization (Fig. 5) and in the area of partial recrystallization (Fig. 6). These areas are primarily characterized by the presence of grains of different sizes in the microstructure. Fig. 5 clearly shows recrystallized ferrite grains with an average conventional diameter of ~5.3 µm and a maximum conditional diameter of up to 10 µm and small colonies of degenerate pearlite (Fig. 5b) with an average conditional size of up to 2 µm and a maximum of no more than 4 µm are clearly visible. In the area of partial recrystallization, coarse grains of polygonal ferrite (Fig. 6a) with an average size of ≈16 µm and a maximum size of up to 25 µm are present along with fine-dispersed ferrite-perlite microstructure. The pearlite component is also represented by areas of degenerated pearlite (Fig. 6b).

(Fig. 4.5), which, in turn, is followed by the subcritical zone (Fig. 4.7), where, in the absence of influence from other zones, the structure is represented by coarse grains of polygonal ferrite, as well as a finely dispersed ferrite-perlite mixture. In case of overlapping of several heat-affected zones during repeated heating below Ac1, recrystallization occurs repeatedly, which leads to the formation of a homogeneous ferrite structure

An increase in heat input from 15 ± 2 kJ/cm to 50 ± 2 kJ/cm changed the macroscopic contours of the welded joint in the direction of their significant enlargement, however, it practically did not lead to sensitive changes in the formation of microstructure in the HAZ of the welded joint made of the same material by multipass arc welding. Fig. 7 shows a panorama of the microstructure of the main zones of the welded joint with

HI 50 ± 2 kJ/cm. It is important to note that in the case of an increase in the heat input, the HAZ zones are less pronounced and have insufficiently clear boundaries due to an increase in the heat input of each new subsequent pass and, consequently, an increase in the mutual diffusion of zones during repeated welding cycles. The average radius of the columnar structure amounts to approx. 8000 µm.

The average conditional size of one dendrite ranges from 235 to 1020 µm in the direction parallel to the solidification front and from 50 to 190 µm in the direction transverse to the solidification front. In the structure, both at the macro- and microlevels, multiple overlapping of “beads” and their non-symmetrical arrangement is observed (Fig. 2b). It is considered, that when using multipass welding, the overlap effect has a positive impact on macrostructure formation by preserving more heat and reducing the cooling rate of the new pass. The quantitative and morphological features of the zones of the investigated welded joint with a HI of 50 ± 2 kJ/ cm remain most similar to the above welding regime. No critical microstructural components that have a sharply negative effect on the impact toughness of the welded joint have been identified.