The effect of subzero treatment on microstructure, fracture toughness, and wear resistance of Vanadis 6 tool steel

Significance Statement

Chromium–vanadium (Cr–V) ledeburitic tool steels manufactured via powder metallurgy are progressively used in applications, where high strength, hardness, and wear resistance along with sufficiently high toughness are required. Conventional heat treatment is insufficient to convert a higher portion of austenite to martensite during quenching because they contain high percentage of carbon, high amount of alloying elements and, their characteristic martensite finish (Mf) temperature lies well below room temperature. In this case, it is useful to insert the subzero treatment in between quenching and tempering. Subzero treatment (SZT) involves cooling down to the lowest temperature (the temperature of liquid nitrogen in most cases), holding at the temperature for a specific amount of time, and reheating up to room temperature. Following findings have been evidenced based on experimental investigations including fracture toughness determination:

  1. The Subzero treatment coupled with high-temperature tempering was found to be a promising method for enhancement of the wear performance of Vanadis 6 steel as well as marginal improvement of both the toughness and the fracture toughness with only a very slight reduction in hardness.
  2. The effect of Subzero treatment on the fracture toughness has been found as slightly positive. The beneficial effect of Subzero treatment on the fracture toughness is more significant for the immersion temperature of −90 °C, while only a marginal improvement in fracture toughness has been recorded for the samples immersed in liquid nitrogen.
  3. The wear rate manifested a clear decrease with the application of Subzero treatment. The higher the population density of secondary carbides (SGCs) and the shorter the interparticle spacing for all carbides including eutectic ones (ECs), the better the adhesive wear performance of the investigated material.
  4. Highest hardness does not necessarily lead to the best wear performance of the investigated material; for example, besides the hardness, the quantitative characteristics of carbides also play a very important role in the wear behaviour of the material. 

Effect of subzero treatment on microstructure, fracture toughness, and wear resistance of Vanadis 6 tool steel. Advances in Engineering

About the author

Dr. Jana Sobotova, Czech Technical University in Prague, Faculty of Mechanical Engineering, Karlovo nam. 13, 121 35 Prague 2, Czech Republic  

About the author

Prof. Peter Jurci, Slovak Technical University in Bratislava, Faculty of Material Sciences and Technology in Trnava, Paulinska 16, 917 24 Trnava, Slovakia 

 

About the author

Prof. Ivo Dlouhy, Institute of Physics of Materials(Brittle Fracture Group), Czech Academy of Sciences, Zizkova 22, 61662 Brno, and Institute of Materials Science and Engineering, NETME centre, Faculty of Mechanical Engineering, Brno University of Technology, Technicka 2, 61669 Brno, Czech Republic. 

Journal Reference

Materials Science and Engineering: A, Volume 652, 15 January 2016, Pages 192-204.

Jana Sobotova1, Petr Jurci2, Ivo Dlouhy3,4

[expand title=”Show Affiliations”] TEXT [/expand]
  1. Czech Technical University in Prague, Faculty of Mechanical Engineering, Karlovo nam. 13, 121 35 Prague 2, Czech Republic
  2. Faculty of Material Sciences and Technology of the STU in Trnava, Paulinska 16, 917 24 Trnava, Slovakia
  3. Institute of Materials Science and Engineering, NETME Centre, Brno University of Technology, Technicka 2, 61669 Brno, Czech Republic
  4. Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Zizkova 22, 61662 Brno, Czech Republic

Abstract

Vanadis 6 ledeburitic steel manufactured by powder metallurgy of rapidly solidified particles was conventionally quenched and subzero-treated at temperatures of −90 and −196 °C for 4 and 10 h, respectively, followed by standard double-tempering to a secondary hardening peak at 530 °C for 2 h. Besides quantification of standard microstructural features by using examination by scanning electron microscopy and X-ray diffraction analysis, the fracture toughness, hardness and nanohardness, flexural strength, fracture toughness, and wear resistance have been characterized. The obtained results indicate the following: (i) The microstructure of the material consists of tempered martensite and eutectic, secondary, and small globular carbides. Retained austenite is completely eliminated by application of the given heat-treatment schedules. (ii) The amount of small globular carbides is significantly increased by subzero treatments; the lower the temperature (or the longer the duration) of subzero treatment, the higher the content of these carbides. (iii) The hardness of the material is influenced slightly negatively by subzero treatments followed by tempering in the normal secondary hardening temperature range. (iv) There is no negative impact of the given treatment schedules on either toughness or fracture toughness of the steel. (v) When a hardened ball bearing is used as a counterpart, wear resistance is improved by subzero treatments despite the slightly lower hardness of the examined tool steel.

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