Comparison of surface properties modification by direct and active screen plasma nitriding of an ASTM M2 high-speed steel in a nitrogen rich gas mixture

Leonardo Fonseca Oliveira,Alexandre Da Silva Rocha,Giovanni Rocha Dos Santos

doi:10.4322/2176-1523.20212448

Leonardo Fonseca Oliveira, Alexandre Da Silva Rocha + Show 1 more

Open Access

https://doi.org/10.4322/2176-1523.20212448

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Abstract

The active screen plasma nitriding technique has been studied in recent years as a method to optimize surface properties in steel components, avoiding the negative effects caused in the conventional Direct Current plasma treatments. In this study, ASTM M2 high-speed tool steel samples were plasma nitrided with and without the use of the active screen, in order to compare the surface properties developed by each technique. The treatments were carried out at 500 Â°C, with the gas mixture of 76 vol.% N2 + 24 vol.% H2 , for 4 and 8 h. The active screen used was built with an expanded sheet of 304 stainless steel. The results were evaluated through optical microscopy, X-ray diffraction, and microhardness measurements. The samples were subjected to dry ball-on-flat tribological tests and the wear tracks were verified by 3D optical interferometry. The active screen use provided the formation of thinner layers, however, avoided the formation of brittle phases, resulting in the lowest wear volumes observed, up to 10 times less than the other analyzed conditions (nitrided and non-nitrided samples).

Highlights

Plasma nitriding is a thermochemical treatment applied to steel parts and tools in order to develop outstanding surface properties, the main features provided by plasma nitriding are the increase in surface hardness, wear resistance and corrosion resistance [1,2]
Cross-sectional optical microscope images of the M2 steel treated by Direct Current Plasma Nitriding (DCPN) for 4h and for 8h and treated by Active Screen Plasma Nitriding (ASPN) for 4h and 8h are shown in Figure 2a to Figure 2d, respectively
The different treatment methods developed different case structures, as follow: the DCPN samples (Figure 2a and 2b) presented an evident thin compound layer followed by a diffusion zone underneath, whereas no compound layer was found on the ASPN treated samples (Figure 2c and 2d)

Summary

Introduction

Plasma nitriding is a thermochemical treatment applied to steel parts and tools in order to develop outstanding surface properties, the main features provided by plasma nitriding are the increase in surface hardness, wear resistance and corrosion resistance [1,2]. The Active Screen Plasma Nitriding (ASPN) technique was developed to bypass the restrictions of the conventional DCPN method. This technique consists of developing the glow discharge plasma in a screen structure that surrounds the nitriding parts. ASTM M2 is a high-speed tool steel widely used for cutting and forming tools as for example cold work punches and dies [7] These tools main feature is the presence of sharp and tough cutting edges, which are subjected to harsh conditions. In order to compare the surface properties developed by each technique (DCPN and ASPN), plasma nitriding treatments were carried out with and without the active screen in quenched and tempered ASTM M2 steel samples

Materials and methods

Results and discussion

Conclusions