The Influence of Density of Pressed Iron Powder Samples on the Quality of Boride Layers

In this work the influence of the density of pressed iron powder on the quality of boride layers has been analysed. Examinations were performed on pressed samples of iron powder. The used granulation was 50-100, 100-150 and 150-200 μm. The samples were pressed under pressure of 200, 400, 600, 800 and 1000 MPa. Boroning was performed in a solid mixture based on boron–carbide, and was in principle the same for all samples. The obtained boride layers varied in depth and quality (porosity, the contact with metal). It has been observed that simultaneously with boroning sintering also occurred, and this fact offers a wide application possibility in the chemical-thermal treatment for sintered materials.


Introduction
As a type of chemical-thermal steel treatment, boroning has found its application worldwide [1][2][3].The chemical-thermal treatment of sintered materials with an emphasis on boroning has not yet been examined to the full degree.The mechanism and kinetics of sintered materials diffusing layers differs considerably from that of casting materials [4,[6][7].Therefore, the chemical-thermal treatment of sintered materials seems to be very delicate.
The kinetics of diffusing layers formation, the structure, properties and phase composition depend to a large extent on the structural characteristics of sintered materials.Porosity of sintered materials has a significant influence on diffusing layer quality.What should be noticed is the shape, dimensions and even contribution and what is most important, the kind of pores (open or closed).
For chemical-thermal treatment of materials with closed porosity, intrusion of active atoms from a saturating environment into the inner part of the sample is excluded, so the diffusing layer formation on sintered materials differs from the one on casting materials due to the presence of closed porosity, crystal structure defect, crystal lattice deformity, larger length of grains and subgrain boundaries and the presence of admixtures.Due to the mentioned facts, the diffusion process is accelerated in sintered materials.
The process of saturation with most elements causes lessening of sections and closing of pore channels due to diffusion layer formation on their surface and additional sintering as well.Therefore, it is possible to determine that sintered materials transit very quickly from semi contiguous porosity to closed porosity materials.
When talking of open porosity materials, active environment penetration through open pores into the inner part of the compact has a strong influence on the speed of diffusing layer formation and their structure.When examining saturation of open porosity materials, one should keep in mind that the initial porosity changes in the saturation process.Diffusing layer formation on the open pores surface goes along with volume enhancement, therefore, the section surface of the open pores channel diminishes during the chemical-thermal treatment.Over time, this hinders the penetration of saturating elements into the inner part of the compact.Articles published on these problems, allow several assumptions [5]: 1.The closing of pore channels supports saturation with elements by nature similar to Fe (Cr, V, Cu, Ni, Mn). 2. The saturation with elements by nature considerably different from Fe (Al, Si, S) does not make the complete closing of pore channels possible but only lessens the section thereof.3. Closing of pore channels is intensified by fluid phase formation during the saturation process as well as by the formation of chemical compounds.
During the process of chemical-thermal treatment, due to the vicinity of chemicalthermal treatment to the sintering regime, sintering of compacts occurs.
The atmosphere present during saturation considerably stimulates this process, due to which, with the application of gas saturation methods, a "non-porous" transitional zone can form under the diffusing layer, which has a significant influence on exploiting properties of the sample.The formation of a "non-porous" transitional zone can be explained as follows: 1. Effective sintering activation in the surface zone due to the atmosphere influence present.2. Closing of pore channels with the formation of new phases on their surfaces having a higher specific volume than the metal (alloys) treated.___________________________________________________________________________ In order to make progress and expand powder metallurgy into new areas machined parts with special properties (high skin hardness, wear resistance, corrosive stability, thermal resistance) are required.The chemical-thermal treatment of sintered materials with emphasis on boroning has attained these effects.The research presented in this article has shown that the boroning process on non-sintered samples is possible.

Experiment
The object of this article is examination of the influence of the density of pressed iron powder samples on the quality and depth of boride layers.The examinations were executed on pressed samples obtained from iron powder (Höganäs, Sweden) of the following selected granulations: 50-100, 100-150 and 150-200 µm.Samples were obtained by pressing with pressures of 200, 400, 600, 800 and 1000 MPa.
Application of the planed experiment in the choice of optimal conditions of boroning for the pressed iron powder samples is presented in the article.Our own experience was used in the selection of the boroning mixture, so the best results of boride layer depth and quality were achieved using a mixture of B 4 C and Na 2 B 4 O 7 .The boroning mixture had the following composition: 30 % (84 % of B 4 C + 16 % of Na 2 B 4 O 7 ) and 70 % of Al 2 O 3 with the addition of activators.All samples were boroned in the mixture of the mentioned composition.The boroning temperature was 950ºC and the process time was 4 hours.
The investigations were carried out by a 3 2 planed experiment [8] with two factors on three levels, schematically presented in Fig. 1.The two-factor plan of experiments includes variations of the following parameters: In symbols (Fig. 1) the first number is the level of factor x 1 and the second number is the level of factor x 2 .That is nine combinations of factorial levels.Each experiment was repeated two times.

Examination Results
Within the framework of this article, metallographical analysis of the quality of boride samples was executed and the boride layer depth was measured.Explanation of experimental data by regression analysis [8][9][10] reveals that a strong dependence of the boride layer depth on the pressed pressure and particles size iron powder (1) exists.Fig. 2 brings out more clearly this influence.The depth of boride layers is given by the regression equation (1): y ˆ1 -the pressed pressure, x 2 -the powder particle size.Isothickness lines of boride layers are drawn in the coordinate system of pressed pressure-powder particle size (Fig. 2).___________________________________________________________________________ Fig. 2 Dependence of boride layer depth on the pressed pressure and particle size of iron powder.
On the basis of the obtained results, the following can be concluded.With the increase of the value of pressure applied, the obtained sample density increased (Fig. 3).

Fig. 1 1 .
Fig. 1 Schematic presentation of the two-factorial experimental plan.1. Pressed pressure (factor x 1 ), with three levels 200, 600 and 1000 MPa. 2. Powder particle size (factor x 2 ), with three levels 50-100, 100-150 and 150-200 µm.In symbols (Fig.1) the first number is the level of factor x 1 and the second number is the level of factor x 2 .That is nine combinations of factorial levels.Each experiment was repeated two times.
represents the depth of boride layers, x

Fig. 3
Fig.3 Dependence of specimens green density (% TD) with a range of powder particle sizes and pressed pressure.