**3. Effect of UFH on Ac1–Ac3 Temperatures and on Recrystallization Phenomenon**

The quantitative effect of heating rate increase on Ac1 and Ac3 temperatures is related to the steel chemical composition, and several data are found in the literature showing in many cases an increase in both critical temperatures with increasing heating rate.

In a low/medium carbon steel, Valdes-Tabernero et al. [48] (Figure 5) showed a more pronounced increase in Ac1 than that in Ac3 in the range of heating rates from 1 to 200 ◦C/s. An increase in Si and Mn leads to the opposite effect [38]. In this case, a stronger dependence of Ac3 temperature on heating rate with respect to Ac1 was also reported (Figure 6).

**Figure 5.** Effect of HV increase on Ac1 and Ac3 temperature variation (steel chemical composition 0.19% C + 0.5% Si + 1.61% Mn + 1.06% Al), peak temperature of 1100 ◦C, and cooling rate of 300 ◦C/s (data from [48]).

**Figure 6.** Ac1 and Ac3 temperature variation with an increase in HR (steel chemical composition 0.25% C + 1.5% Si + 3.0% Mn) (data from [40]).

Concerning the recrystallization process, the increase in heating rate led to an increase in the recrystallization temperature, supported by the following evidence:

• The recrystallized fraction decreased with increasing heating rate: in a low carbon steel at 750 ◦C for a heating rate of 150 ◦C/s, the recrystallized fraction was ~0.5%. At the same temperature for a heating rate of 1500 ◦C/s, the recrystallized fraction decreased to 0.1% [42]; in [43], compared to a conventional annealing treatment (10 ◦C/s and recrystallized fraction of ~77%), during an ultrafast annealing process (778 ◦C/s), the recrystallized fraction became ~24%.


**Figure 7.** Recrystallized fraction as a function of peak temperature for two different HRs (data from [42]).

#### **4. Effect of UFH on Mechanical Properties**
