**1. Introduction**

In conventional heat treatment of tools made of high-alloyed Cr and Cr–V ledeburitic steels, the material is gradually heated up to the austenitizing temperature (recommended by the steel manufacturers), held there for prescribed time, and cooled down rapidly to the room temperature. Afterwards the steels should be subjected to tempering immediately, to prevent the retained austenite stabilization, and to induce transformations leading to the achievement of final properties of tools or components.

Ledeburitic steels are commonly used as tool materials for cold work applications. In order to generate high abrasive wear resistance, they contain a high amount of carbide phases embedded in a metallic matrix. On the other hand, these carbides together with high overall steel hardness deteriorate the material resistance against crack initiation/propagation, expressed by the fracture toughness KIC. Also, the flexural strength of ledeburitic steels, which is often also taken as a measure of resistance against crack initiation for brittle materials, manifests relatively low values. Moreover, conventionally produced ledeburitic steels are cast and afterwards hot formed. As a consequence, they contain band-like carbides, thus they suffer from anisotropy of key mechanical properties [1].

Sub-zero treatment is defined as a supplementary process to the conventional heat treatment. Unlike conventional heat treatment (CHT), it is a process where the tools or components are immersed into suitable cryoprocessing medium, stored there for pre-determined time (usually in tens of hours), and re-heated to the room temperature. Research works conducted on application of this kind of treatment have shown that sub-zero treatments provide extra benefits to the tooling industry like increased hardness [2,3], better wear performance [3,4], and improved dimensional stability of products [5].

According to recent studies, the following crucial microstructural changes are responsible for these benefits:


However, the impact of sub-zero treatments on the toughness characteristics (fracture toughness, flexural strength, and impact toughness) is controversial as Table 1 illustrates.


**Table 1.** Toughness and fracture toughness of di fferently sub-zero treated ledeburitic steels—an overview of the obtained results to date.

The question of an optimal regime of sub-zero treatments is still under debate. In the "pioneer age" of this technique it was believed, within the professional community, that the benefits of sub-zero treatments are based only on the reduction of retained austenite amount. Therefore, the temperatures of around −75 ◦C were widely used in laboratory and industrial practice. Lower temperatures were not accepted for the treatments since their use often led to premature failure of tools, due to thermal shocks associated with the use of very low temperatures. Treatment at the boiling temperature of liquid nitrogen (−196 ◦C) was introduced into industrial practice only much later, when the devices enabled to carry out well controlled cooling down to such low temperature.

Other sub-zero treatment temperatures were suggested only by very limited number researchers. Reitz and Pendray and Gavriljuk et al., for instance, suggested the temperature of −140 ◦C [24,25]. Recent studies dealing with thorough analysis of microstructure and toughness of the Vanadis 6 steel treated in this way gave very promising results [22,26]. Alternatively, there were attempts and/or suggestions with the use of the temperature of boiling helium (−269 ◦C) [13,27]. However, the treatment temperature of −75 ◦C also deserves attention since one can expect that the phenomena being responsible for abovementioned ameliorations in properties would proceed faster at −75 ◦C than at lower temperatures. Also, practical experiences indicated that the extent of "extra" wear performance (or other property), which can be gained by the use of SZT at −196 ◦C (as compared with treatments at −75 ◦C) depends on the material chemistry. For instance, the wear performance of AISI D2 steel was improved by a factor of 2.59 by the treatment in liquid nitrogen (as compared with treatment at −75 ◦C), while only an improvement by a factor of 1.39 was recorded for CPM 10-V steel (steel with high vanadium content) [24].

The current paper is thus focused to an in-depth description of the results obtained with the sub-zero treatments of Vanadis 6 steel at the temperature of −75 ◦C, and to their careful discussion. Microstructural changes are presented, and they are related to the hardness, flexural strength, and fracture toughness of examined steel. The obtained results are also compared to what was obtained by treatments at temperatures −140, −196, and −269 ◦C, respectively.
