*Article* **Can Sub-Zero Treatment at** −**75** ◦**C Bring Any Benefits to Tools Manufacturing?**

### **Martin Kusý 1, Lýdia Rízeková-Trnková 1, Jozef Krajˇcoviˇc 1, Ivo Dlouhý 2 and Peter Jurˇci 1,\***


Received: 22 October 2019; Accepted: 20 November 2019; Published: 21 November 2019

**Abstract:** Vanadis 6 ledeburitic tool steel was subjected to sub-zero treatment at −75 ◦C for different durations, and for different subsequent tempering regimes. The impact of these treatments on the microstructure, hardness variations, and toughness characteristics of the steel was investigated. The obtained results infer that the retained austenite amount was reduced to one fourth by sub-zero treatment (SZT), and the population density of add-on carbides was increased by factor of three to seven, depending on the duration of SZT. Tempering always reduced the population density of these particles. A hardness increased by 30–60 HV10 was recorded after sub-zero treatment but tempering to the secondary hardness peak induced much more significant hardness decrease than what was established in conventionally quenched steel. The flexural strength was not negatively influenced by sub-zero treatment at −75 ◦C while the fracture toughness tests gave worse values of this quantity, except the case of steel tempered to the secondary hardness peak.

**Keywords:** vanadis 6 steel; sub-zero treatment at −75 ◦C; microstructure; hardness; fracture toughness
