**1. Introduction**

In recent years, the application of ultrafast heating (UFH) treatment has gained considerable attention from both the academic and industrial communities [1]. Such a heat treatment is enabled by the possibility to electrically heat metallic materials by an electromagnetic induction process based on Joule heating through heat transfer passing through an induction coil, generating an electromagnetic field. The rapidly alternating magnetic field penetrates the object, generating so-called eddy currents inside the metallic component. Such currents flow through the resistance of the material, thus heating it by the Joule effect. UFH could strongly optimize industrial production processes in terms of time and consequently productivity [2–5]. Results very near to industrial applications were reported by Cola Jr. in AISI 8620 and in bainitic steels [6,7], but the most important limit appears to be related to UFH rates with respect to conventional steel-processing lines [8].

UFH is a process based on induction heating (IH) that is widely used to heat-treat specific areas more rapidly, and at a lower cost and lower amount of energy consumption compared to conventional heat treatment methods [8,9]. The inductive heat treatment offers several advantages over conventional heat treatments using a furnace, such as short

**Citation:** Gaggiotti, M.; Albini, L.; Di Nunzio, P.E.; Di Schino, A.; Stornelli, G.; Tiracorrendo, G. Ultrafast Heating Heat Treatment Effect on the Microstructure and Properties of Steels. *Metals* **2022**, *12*, 1313. https://doi.org/10.3390/ met12081313

Academic Editor: Andrey Belyakov

Received: 19 July 2022 Accepted: 3 August 2022 Published: 5 August 2022

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processing times, high flexibility, and localized annealing [10–12]. The temperature variation over time of the annealing process is represented by the parameter of heating rate (HR). Variation in HR affects the phase transformation kinetics and steel properties; an increase in terms of HR raises the α/γ transformation temperature, thus leading to austenitic grain refinement [13–16]. At the same time, an increase in phase transformation temperatures [17] reduces the carbon amount that can be dissolved in austenite, thus lowering steel hardenability [6]. In addition, during fast heating, the solubility of carbonitrides and pearlite areas is also evidently dependent on heating time and not just temperature, as what usually happens in conventional annealing processes [18,19].

Concerning carbon steels, UFH can be adopted in the austenitization process step (before quenching and tempering) [20–29]. In the case of austenitic stainless steels, UFH is a promising method to be applied after cold rolling to promote grain refinement [30], leading to an increase in terms of mechanical properties [31–33].

In carbon steels, the application of UFH inhibits the dislocation rearrangement that is typical of recovery [33–36]. Hence, recrystallization occurs in direct competition with austenitization [37–39]. The final microstructure of a steel subjected to UFH strongly depends on heating rate and peak temperature [40,41]. In the case of a high HR and inadequate peak temperature, a not fully recrystallized microstructure is achieved [42,43]. When conditions instead allow for complete recrystallization, the microstructure shows a morphology resulting in plastic deformation.

Since recovery is usually negligible in the case of austenitic stainless steels, the immediate start of recrystallization during UFH does not imply significant changes in the mechanism [44]. Ferritic stainless steels, instead, show a final microstructure comparable with that of industrial products [45,46], with a fully recrystallized structure and equiaxed grains for appropriate temperatures [47–51].

The aim of this review paper is to highlight trends in variation in the above-mentioned properties with heating rate, and to predict their behavior when possible for different classes of materials, focusing on the main metallurgical topics (phase transformation, recrystallization, and textural evolution).

#### **2. Effect of UFH on Grain Size**

In medium/low carbon steels, the application of UFH leads to a significant decrease in grain size due to the suppression of grain boundary movements, and thus the growth of austenite grains [52–58]. Figure 1 shows the prior austenite grain (PAG) size dependence of heating rate (HR) [40]. In the case of 0.25 wt. % C heat-treated specimens with a peak temperature at 850 ◦C, HR (between 10 and 1000 ◦C/s) and cooling rate of 20 ◦C/s, some authors [40] investigated the effect of HR on the variation in ferritic grain size (Figure 2). Results show an evident average PAG size refinement of about 55% (from 3.8 down to 1.7 μm) (Figure 1) that corresponds to a consequent decrease in ferritic grain size (about 20%, from 2.5 to 2 mm) (Figure 2) as the HR increased from 10 up to 1000 ◦C/s. A similar steel grade (0.2 wt. % C) was considered by Hernandez-Duran et al. [57], who studied the effect of the same HR as that in [40] on the ferritic grain size, and the results, reported in Figure 2, show a similar refinement of ferritic grain size, with a reduction from 2 to 1.6 mm.

Petrov et al. [59] studied the effect of heating rate increase on a cold-rolled high strength carbon steel grade for automotive applications. They indicated that, when varying the heating rate in the range from 140 to 1500 ◦C/s, ferritic grain refinement was achieved from 4 to 1 μm. They also showed that excellent ultimate tensile strength (higher than 1200 MPa) and acceptable fracture elongation were achieved. The above results were confirmed by an increase in heating rate of up to 1500 ◦C/s [40] that, compared to 150 ◦C/s, led to a more pronounced decrease in ferritic grain size of about 76%, from 2.6 to 0.6 μm (Figure 3).

**Figure 1.** Effect of the heating rate increase on PAG size (data from [40]). Low/medium carbon steel (0.25 wt. % C), peak temperature of 850 ◦C, and cooling rate of 20 ◦C/s.

**Figure 2.** Effect of the heating rate increase on ferritic grain size. Low/medium carbon steel (0.25 wt. % C and 0.2 wt. % C), peak temperature of 850 ◦C and 950 ◦C, cooling rate of 20 ◦C/s and 160 ◦C/s (data from [40,57]).

An example of microstructures obtained after UFH is reported in Figure 4, referring to the industrial hot-rolled strips of a low-C steel (0.08% C, 0.3% Mn) and of an HSLA steel (0.07% C, 0.5% Mn, 0.03% Nb), cold-rolled from 3 down to 0.5 mm, and processed by imposing different peak temperatures and heating rates to find the optimal processing conditions for achieving full recrystallization. No soaking was applied, and the strips were cooled down just after reaching the peak temperature.

**Figure 3.** Ferritic grain size variation as function of temperature for the heating rates of 150 and 1500 ◦C/s (data from [42]).

**Figure 4.** Microstructure of fully recrystallized steels subjected to UFH. (**a**) Low-C 0.08% C, 0.3% Mn steel; (**b**) HSLA 0.07% C, 0.5% Mn, 0.03% Nb steel.

The first steel was processed with a peak temperature of 870 ◦C and a heating rate of 285 ◦C/s, obtaining an average ferritic grain size of 6.1 μm and a hardness of 136 HV300g (Figure 4a). The second steel was processed with a peak temperature of 820 ◦C and a heating rate of 325 ◦C/s, obtaining an average ferritic grain size of 5.4 μm and a hardness of 158 HV300g (Figure 4b). The microstructures appeared to not be very refined, but the resulting tensile properties, nevertheless, complied with grades DX51D and HX340LAD.

If ferritic stainless steels are considered, an increasing heating rate leads to less marked grain refinement, as reported in [45]. Data reported in [45] showed that the average grain size of AISI 430 steel decreased from 7.4 to 6.7 μm for a heating rate of 1000 ◦C/s. Similarly, Salvatori and Moore [60] analyzed the effect of UFH on AISI 430 ferritic stainless steels, and showed them to be able to reproduce industrial standard products with a heating rate of up 1000 ◦C/s.
