**2. Materials and Experimental Methods**

*2.1. Materials and Processing*

The tool steel Vanadis 6, with chemical composition as shown in Table 1, was used for the examinations.


**Table 1.** Chemical composition of the experimental steel.

Specimens of the studied steel with a size of 20 mm × 20 mm × 7 mm were gradually heated (step (1) in Figure 1) up to the austenitizing temperature of 1050 ◦C in a vacuum, held there for 30 min (2), and quenched by nitrogen gas to room temperature (3). Then, the specimens were divided to five batches. The first one was immediately subjected to tempering treatment (7, 8). This treatment route is called conventional heat treatment, CHT. Immediately after quenching, the other four sets were moved to the cryogenic system, where they were cooled down at a cooling rate of 1 ◦C/min to a pre-determined

SZT temperature (4). The SZTs were carried out at temperatures of −75, −140, −196, or −269 ◦C, and for 17 h each (5). After that, the material was re-heated to room temperature, at a heating rate of 1 ◦C/min (6). Immediately after re-heating, sub-zero-treated specimens were subjected to tempering. Tempering was carried out at temperatures of 170, 330, 450, or 530 ◦C, 2 times each, for 2 h (7, 8).

**Figure 1.** A schematic of the heat treatment schedules used.
