Mechanical engineering

INVESTIGATION OF STEEL 45 APPLIED LAYER PROPERTIES DURING ELECTRO-SPARK ALLOYING

S. Luzan, O. Gorbachevskaya

Kharkov National Automobile and Highway University

The dependences of the mass transfer coefficient, roughness and thickness of the coating during electro-spark alloying of the steel surface on the duration of treatment are specified. A mathematical model of surface roughness change during electro- spark alloying is obtained.

Key words: electro-spark alloying, surface, electrode, anode, cathode, mass transfer, surface roughness, coating.

I. Introduction. Adhesion strength of the flame coating with the substrate depends on the quality of sprayed surfaces preparation and to a greater extent on its roughness. Based on the analysis of technical characteristics of the main methods used for surface treatment prior to the deposition of gas-flame coating there was selected the method of spark alloying (ESA) [1]. In order to obtain the desired sprayed surface roughness providing the predetermined adhesive strength of the coating with the part, one needs to determine the dependencies of surface roughness on the modes and duration of its processing.

Scientists A. D. Verkhoturov, G.V. Samsonov, B. R. Lazarenko note that the nature of the process of electro- spark alloying is determined by the properties of electrode materials, namely their erosion resistance [2,3,4]. The intensity of anode material deposition to the cathode and the alloying layer properties under otherwise equal conditions depend on both the electrodes material properties and the duration of treatment. In addition, the dynamics of surface layers formation on the cathode is characterized by the fact that the transfer rate of the anode material on the cathode is maximal during the initial period of the process, and further – is reduced. The surface of the applied layer has developed roughness consisting of alternating projections and of the alloying material and recesses in the substrate; provided that the nature of roughness varies depending on the duration of treatment. The behavior of competing processes of formation and destruction of layers with the prevalence of the latter with increasing of the processing time leads to limiting of the formed layer thickness.

The analysis of investigations results of the ESA process showed that the roughness of the treated surface depends on a great number of factors and the most reliable quantitative data for specific treatment modes can be obtained only through experimentation.

II. Research objective: to investigate the following properties of electric-spark coatings: surface roughness, thickness of the deposited layer and micro-hardness. Determine the dependences of roughness change on the duration of treatment during steel alloying by 45 different materials. Define the rational modes and duration of spark alloying of steel substrates before flame spraying.

III. Summary of the basic material. Study of ESA modes effect on surface roughness, micro-hardness and thickness of the deposited layer was performed using specialized installation "Elitron-50." The processing of samples made of steel 45 was performed by a vibrating electrode manually, the speed of electrode moving did not exceed 0,07-0,09 m/min. The alloying electrodes were made of the following materials: nichrome H20N80, St2kp, T15K6.

Table 1 shows the results of the study of the value of finished surface roughness during 4 minutes depending on the ESA mode for different anode materials.

Table 1 – Modes of alloying and surface roughness (t = 3-4 min/sm²)

Modes	The operating voltage on the electrodes, V	Power of short-circuit current, А	Surface roughness Ra, μm
			Electrode- anode H20N80	St2kp	T15K6
Soft	9-20	3,5-4	3,63	4,33	3,56
Average	20-50	4-5,5	5,86	6,99	5,55
Hard	50-90	5,5-10	6,25	7,53	5,87

Table 1 shows that the greatest surface roughness is provided by the "hard" mode treatment, so further studies were carried out for this particular mode.

The roughness and thickness of coatings are also determined by the nature and intensity of mass transfer of anode material to the cathode and depends on the duration of treatment. To determine the nature of roughness Ra change and the thickness δ of the coating layer were applied to samples with the area of 1 cm² during 1 min, performed repeated measurements for 8-10 times to determine the time interval of treatment within which there occurs formation of a layer with maximum surface roughness. The surface roughness was evaluated using the TR-200 device, the thickness of the deposited layer – by PMT-3 micro-hardness measurement device. The mass transfer coefficient was defined as the ratio of the cathode weight gain to the anode mass change:

.

(1)

The thickness and hardness of the alloy layer at steel surface treatment are shown in Table 2. The hardness of the alloy layer is considerably higher than the hardness of the framework made of steel 45 (HB 207), the surface hardening coefficient is defined as the ratio of the alloy layer hardness to the hardness of the alloy layer to the hardness of the alloy material:

.

(2)

Table 2 – Thickness of the alloy layer and the ratio of surface hardening during alloying of steel 45 (t = 8-10 min/sm²)

The material of the anode- electrode	Coating thickness, mm	Micro-hardness of coating, kgf/ mm²	Coefficient of hardening kу
H20N80	0,061	240	6
St2kp	0,052	170	4,5
T15K6	0,068	270	8,1

Figure 1a shows the roughness dependence during steel 45 alloying by different electrodes- anodes on the duration of treatment. After fitting of the experimental curves we obtain the equations representing the polynomial functions of degree 3:

- for steel 2kp

;

(3)

- for T15K6

;

(4)

- for H20N80

.

(5)

The correlation coefficients are: R₁ = 0, 9796; R₂ = 0, 9859; R₃ = 0, 9901. Expressions (3, 4, and 5) are mathematical models that set the dependences of roughness of treated surfaces depending on the duration of electric-spark alloying.

The generalizing mathematical model of surface roughness change during ESA has the following form:

,

(6)

wherein A, B, C, D – coefficients which are determined by the material of the electrode-anode.

a)

b)

c)

Figure 1 – The dependence of the surface roughness (a), the mass transfer coefficient (b) and the thickness of coating (c) on the duration of ESA

From the analysis of experimental dependence (Fig. 1) one can see that the formation of the surface layer during ESA for the investigated electrodes is similar and consists of 3 stages. In the first stage of treatment (0-3 minutes) there occurs bedding of surfaces and formation of a discrete alloying layer with low roughness, nevertheless the increase in weight and thickness are insignificant. The alloying layer is a combination of components of the substrate material and the alloying electrode.

Next there takes place the second stage (3-6 minute of treatment) when an intensive mass transfer with formation on the surface of a uniform coating layer occurs. In this case, the anode electrode does not interact with the substrate but for the most part with the formed alloyed layer. This process continues until the third stage (6-10 minutes of treatment) when the weight gain of the cathode is hardly observed, there occurs over saturation of the surface with anode material and partial erosion of the cathode, and further the surface roughness does not change.

IV. Conclusions:

1. The dependences of the change of roughness and thickness of coating during electro-spark alloying of the steel surface with various materials on the duration of treatment are specified.

2. It was determined that the formation of the surface layer at ESA for the investigated electrode materials is similar. The rational mode of alloying before the flame spraying is the hard mode, which allows the surface roughness equal to Ra = to 6...9 mm, which meets the requirements for spraying of gas-flame coatings.

3. The most effective treatment duration is the interval of 3-6 min/sm² when there is formed a layer with high hardness and high roughness Ra avoiding erosion of the base.

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