**2.1. Investigated materials and modes of thermal treatment**

The investigated materials (Armco-Fe, 25CrMnSiNiMo steel) differ significantly by the availability and amount of alloying elements in them. The chemical composition of the materials mentioned above is checked by the equipment for automatic analysis "Spectrotest" [Table 1]. The low percentage of sulphur /< 0.015%/ in them guarantees a high level of hardness and toughness of the investigated materials.

Carbonitriding of Materials in Low Temperature Plasma 117

In order to clear out the morphological peculiarities of the nitrided and carbonitrided layers,

When defining the structure and the thickness of the obtained layers metallographic

The thickness of the nitrided and carbonitrided layer has been defined by the depth, to which hardness, equal to the core, has been achieved. Measuring the micro hardness has been done by means of a micro hardness-meter "Shimadzu" at a load of 0.98 N (100 g),

The process of defining the distribution of nitrogen and carbon in the nitrided and carbonitrided samples has been realised by means of the GDOES device GDA – 750, vom Spectruma-Analytik GmbH. The parameters of the glow discharge plasma are as follows: current - 20 mA, voltage - 800 V, plasma density - 1010-1011 cm-3, electron temperature 0.1 - 0.5 eV and plasma volume 15 cm3. The turbomolecular pump (56 ℓ⋅s-1) works constantly. The basic pressure is 10-6 Ра. The surfaces are polished beforehand in order to achieve congestion in the plasma sector of the GDOES device. The standardizing of the device has been carried out with a sample containing 7 weight per cents of nitrogen and 1 weight per cent of carbon.

The investigation of the internal stresses in the carbonitrided samples is performed by means of a Roentgen diffraction-meter SET-X ENSAM, following the "sin2Ψ" method.

The direction of measuring is characterized by the angles Ψ: 0°, 14.96°, 21.42°, 26.57°, 31.09°, 35.26°, 37.23°, -10.52°, -18.43°, -24.09°, -28.88°, -33.21°, -37.27° and two angels φ=0 and φ=90

A powdered sample is used for standardizing the Roentgen diffraction-meter. Since the powdered sample is free of residual stresses, it allows checking and easily adjusting the device. In this particular case chromium Roentgen radiation Cr – Kα with a wave-length of λ=2.29Å was used. Information about the formed stresses is obtained at a distance of 7μm from the surface of the sample in a plane α - Fe {2 1 1}, i.e. the stresses are measured in α -

By means of the Roentgen diffraction-meter the diffraction angles in the carbonitrided layers are measured. The data are introduced into the program "MATHLAB-2008". Through graphical representation of a straight line, built in the coordinates "2θ - sin2 Ψ", the value of

is defined.

The residual stresses are defined by the following dependence:

pictures taken by means of a microscope – Axioscop – have been used.

**2.3. Glow discharge optical emission spectroscopy** 

**2.2. Metallographic investigations** 

metallographic analysis has been done.

following the Vikers' method.

**2.4. Internal stresses** 

Fen - phase just under compound zone.

the diffraction angle 2θ at sin2 90°

[Fig 1].


**Table 1.** Chemical composition of the materials

The requirement for a preliminary thermal treatment is imposed mainly by the following consideration: for achieving the desired mechanical parameters and structure, enabling a favorable process of nitrogen diffusion in depth. The investigated steel is thermally treated in a chamber furnace under modes [Table 2].


**Table 2.** Modes of preliminary thermal treatment

Treated this way, the samples are then subjected to ion carbonitriding in the installation "Ion – 20", according to the modes [Table 3]. Ammonia (NH3) and corgon (82 % Ar и 18% CO2 ) in different percentages are used as saturating gases. The temperature of treatment for the process of carbonitriding is 823K (550оС).


**Table 3.** Modes of nitriding and carbonitriding
