**2.2. Metallographic investigations**

116 Heat Treatment – Conventional and Novel Applications

**2. Methodology of investigation** 

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

in a chamber furnace under modes [Table 2].

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

the process of carbonitriding is 823K (550оС).

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

( o C )

Treatment τ

Material thard

№ of the mode

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

level of hardness and toughness of the investigated materials.

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

Armco-Fe 0.02 0.02 0.02 0.03 0.002 0.01 0.07 0.002 25CrMnSiNiMo 0.24 0.87 0.12 1.36 0.002 1.45 1.28 0.002

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

> Cooling medium

25CrMnSiNiMo 900 Oil 873 600 Air

h

1 nitriding 2 400 - 400 530 2 carbonitriding 2 360 40 400 470 3 carbonitriding 6 360 40 400 470 4 nitriding 6 400 - 400 530 5. carbonitriding 6 200 200 400 415 6 carbonitriding 2 200 200 400 415 7 carbonitriding 4 280 120 400 435 8 nitriding 4 400 - 400 530 9 carbonitriding 2 350 350 700 380 10 carbonitriding 6 350 350 700 380

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

> P1 NH3 Pa

С Сr Мо Ni P Si Мn S

ttemp. (K)

> P2 corgon Ра

ttemp. (°С)

> Р total Ра

Cooling medium

U

V

Material Chemical elements, weight percentage

In order to clear out the morphological peculiarities of the nitrided and carbonitrided layers, metallographic analysis has been done.

When defining the structure and the thickness of the obtained layers metallographic pictures taken by means of a microscope – Axioscop – have been used.

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), following the Vikers' method.
