**2.4. Internal stresses**

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 [Fig 1].

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 - Fen - phase just under compound zone.

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 the diffraction angle 2θ at sin2 90° is defined.

The residual stresses are defined by the following dependence:

#### 118 Heat Treatment – Conventional and Novel Applications

$$\sigma\_{\phi} = \frac{E}{2\left(1+\mu\right)}.\cot\theta \Big(2\theta\_{\psi=0} - 2\theta\_{\psi=90}\Big)\frac{\pi}{180}.$$

Carbonitriding of Materials in Low Temperature Plasma 119

δ cz μm U V

strengthening the surface layer. They impede the diffusion of the nitrogen in depth, and in

HV0.1 МРа

δ tot μm

consequence, thinner layers with a thicker combined zone are obtained.

δ tot μm

25GrMnSiNiMo Armco-Fe

δ cz μm

1 8500 170 6 3800 220 5 530 2 9400 160 5 4300 260 6 470 3 8600 290 8 4200 340 7 470 4 9800 300 11 4200 350 10 530 5 9200 240 8 4400 330 7 415 6 8900 150 5 3700 210 6 415 7 9300 210 6 4800 280 6 435 8 9500 230 7 4150 290 6 530 9 7400 140 4 5400 210 4 380 10 7500 230 5 5600 320 8 380 **Table 4.** Results from carbonitriding and nitriding of 25GrMnSiNiMo steel and Armco-Fe samples

During the process of carbonitriding of 25GrMnSiNiMo steel in a medium, consisting of 90% NH3 + 8.2% Ar + 1.8 % CO2 at the pressure of 400Ра, a layer with a lower micro hardness (HV0.1= 9400 - 8600МРа), total thickness (160 - 290 μm) and combined zone thickness (5 - 8μm) is obtained, than after the process of nitriding without addition of a carbon-containing gas. This is most likely due to the small percentage of argon (8.2%) in the gas medium, since argon, because of its bigger atomic mass, has a strong pulverizing action. At the high coefficient of pulverizing the length of the free run of the pulverized atoms is bigger and the possibility for a backward diffusion of carbon and nitrogen is lower. A carbonitrided layer with a lower concentration of nitrogen and carbon is obtained. The more active pulverization does not allow the combined zone to grow and, as a result, a more deficient in nitrogen and carbon combined zone is obtained. In the diffusion zone of the carbonitrided layer of the steel detectable nitrided (carbonitrided) precipitations are not observed –

After Armco-Fe carbonitriding under the same mode there are no similar dependences established during the process of forming the layer as the ones, described for 25GrMnSiNiMo steel. The obtained carbonitrided layer has a higher surface micro hardness (HV0.1= 4200 – 4300 МРа), total thickness (260-340 μm) and combined zone thickness (6-7 μm). In the diffusion zone of a carbonitrided layer considerable amounts of nitrided (carbonitrided) precipitations are observed, mainly in the area of the grains. The precipitations originate at the boundary of the grains and propagate inward the volume. A bigger amount of precipitations is observed under the longer mode (6h) of carbonitriding –

№ of the mode

[Fig.2.a].

[Fig.2.c].

HV0.1 МРа

The values of the elasticity constants in the given formula are chosen for non - carbonitrided steel: Poisson's ratio μ = 0.29, elasticity modulus E = 210 GPA. The master diffraction angle is 2θ = 156° 30' and θ = 78° 15' . The miscount at defining stresses depends on the relative mistake ∆θ/θ at defining the angle θ. It is within 2 - 3**%.**

**Figure 1.** Characteristics of the direction of measuring by the angles Ψ and φ
