*4.1. Carbon Steels*

Directly connected to the effect of ultrafast heating on grain size refinement, the mechanical properties of carbon steels subjected to the UFH process showed a clear improvement. Specifically, the evolution of mechanical characteristics was reported for low/medium carbon steel in the case of conventional annealing + soaking (10 ◦C/s + soak.), conventional annealing (10 ◦C/s) and ultrafast annealing (500–1000 ◦C/s) prior to quenching and partitioning (Q and P) heat treatment [40]. Results showed that the Ultimate Tensile Strength (UTS)increased for a higher HR (Figure 8); from conventional annealing (HR of CA = 10 ◦C/s) to ultrafast heating (HR of UFH = 1000 ◦C/s), the UTS shifted from 1097 to 1318 MPa (an increase of ~20%). The yield stress (YS), in the same range of heating rate, decreased from 837 to 811 MPa (−3.2%), and uniform elongation ε<sup>c</sup> increased from 2.6% to 12.8%.

A comparison of CA (5 ◦C/s) and UFH (500 ◦C/s) on dual-phase steel [39] showed the following results: the UTS improved from 625.0 ± 3.6 MPa to 666.0 ± 2.6 MPa (~6.5%), the YS increased from 277.0 ± 8.1 MPa to 372.0 ± 3.0 MPa (~34%), and the uniform elongation and total elongation increased from 16.5 ± 0.2% to 18 ± 0.5% and from 23.3 ± 0.8% to 26.6 ± 0.5%, respectively (Figures 9 and 10).

**Figure 8.** Effect of HR increase on UTS and Vicker hardness value of a low/medium carbon steel undergone the UFH process compared to CA + soaking process (steel chemical composition 0.25% C + 1.5% Si + 3.0% Mn)). Peak temperature of 850 ◦C, and cooling rate of 20 ◦C/s (data from [40]).

**Figure 9.** YS and UTS increase with increase in heating rate between CA (5 ◦C/s) and UFH (500 ◦C/s), on dual-phase steel (data from [41]).

**Figure 10.** Uniform and total elongation variation with increasing heating rate between CA (5 ◦C/s) and UFH (500 ◦C/s) on dual-phase steel (data from [41]).

#### *4.2. Stainless Steels*

Regarding the mechanical properties in ferritic stainless steels [60]:


Instead, from data reported in [45]:


In austenitic stainless steels, data from [44] show that:

