INFLUENCE OF MODES OF PULSED EMW ON THE QUALITY PARAMETERS OF THE SURFACE LAYER

ABSTRACT

The paper considers the influence of the modes of pulsed electromechanical processing (EMP) on the quality parameters of the surface layer of parts made of ductile iron.

АННОТАЦИЯ

В работе рассматривается влияние режимов импульсной электромеханической обработки (ЭМО) на параметры качества поверхностного слоя деталей из высокопрочного чугуна.

Keywords: surface quality, ductile iron, research modes, electromechanical processing.  

Ключевые слова: качество поверхности, высокопрочный чугун, режимы исследования, электромеханическая обработка.

Pulse electromechanical processing is based on the simultaneous thermal and force effects on the surface layer of the workpieces [1]. By varying the modes of pulsed EMP [2], it is possible to obtain a surface layer with different physical and mechanical properties and geometric parameters [3].

The modes of pulse EMP are: I, A - current strength; tuC is the duration of current pulses; tn,С - duration of pauses between current pulses; PH - processing force; V, m/min - processing speed; S , mm/rev – feed [4].

According to theoretical studies, current strength has the greatest influence on changes in the surface layer [5]. The value of the current strength can be determined based on the required depth of hardening and processing conditions [6]. The processing speed and effort, as well as the feed rate for pulsed EMP [7], will be taken based on the recommendations set out in [8] and the duration of the current pulses and the duration of the pauses between them from [9]. The minimum value of the current strength at which phase transformations begin to occur in parts made of ductile iron, hardened on the installation (at tu = 0.8 s; tn = 0.1 s; P = 75 N; V = 3 m/min; S = 0.3 mm / rev) according to the calculation is I \u003d 900 A.

To study phase transformations in the surface layer of parts made of high-strength spheroidal graphite iron (ductile iron), a metallographic analysis of hardened specimens was carried out. The samples were processed in the following modes: I = 3500 A; tu= 0.8 s; tn= 0.1 s; P = 75 N; \/ = 3 m/min; S = 0.3 mm/rev.

In the samples under study, the hardened surface layer extends to a depth of 1-1.2 mm. According to the cross section of the hardened layer, three zones can be distinguished:

1. The maximum hardened layer with a length of 0.3-0.4 mm consists of sections of an acicular structure with microhardness up to HV = 1100, characteristic of hardening structures and light-etched zones with HV - 800 (Fig. 1).
2. In the transition layer with a depth of 0.5-0.8 mm, light-etched areas with HV = 600-700 are adjacent to dark-etched areas with HV = 400-500 (Fig. 2).
3. The non-hardened core includes pearlite with HV=280-350, nodular graphite and ferrite. The analysis carried out allows us to conclude that high-temperature processes occur during pulsed EMT at a high rate and deformation of the surface layer. The finely dispersed structure with high microhardness and favorable arrangement of grains obtained as a result of these processes shows that, during pulsed EMT, transformations occur that are characteristic of high-temperature thermomechanical treatment (HTMT) and are similar in nature to transformations during laser hardening (Fig. 3).

Figure 3. Microstructure of the transition layer

The influence of the current strength on the microhardness and the depth of hardening are shown in fig. 4. Processing was carried out with a change in current strength from 2000 A to 4500 A. From fig. 4 it can be seen that the maximum microhardness that can be obtained by hardening parts from high-strength cast iron with impulse EMT is HV = 1050-1100. A decrease in the current strength leads both to a decrease in the microhardness on the surface and to a decrease in the depth of hardening.

Figure 4. Distribution of microhardness along the depth of the hardened layer:
I - I = 4500 А; 2 - I = 4000 А; 3 - I = 3500 А;

4 - I = 3000 А; 5 - I = 2500 А; 6 - I = 2000 А

The greatest influence on the geometrical parameters of the surface layer of the hardened parts is exerted by the current strength and the processing force. The influence of the current strength on the roughness and waviness of the surface during pulsed EMT are presented in table 1. The current strength varied from 2000 A to 4500 A.

Table 1.

Influence of the current strength during pulsed EMT on the geometric parameters of the quality of the surface layer

This is due to the high temperature (at I = 4000 A) in the contact zone of the tool with the workpiece and the beginning of the reflow process. However, if we return to the influence of the current strength on the depth of the hardened layer, it can be seen that at a current strength of 4000 A and 4500 A, both the surface microhardness and the depth of hardening increase. The study of the influence of the hardening force on the geometrical parameters of the surface was carried out when it changed from 25 N to 150H. The test results are presented in table 2.

Table 2.

Influence of the processing force during pulsed EMT on the geometric parameters of the quality of the surface layer

Analyzing the obtained results, it can be seen that a change in the current strength from 2000 A to 3500 A contributes to a decrease in the height parameters of roughness, and at a current strength of 4000 A, their growth begins. Thus, for parts hardened at high currents, it is necessary to introduce finishing methods of processing. As such methods, diamond smoothing and vibropolishing with elastic diamond bars can be used, which makes it possible to reduce the height parameters of the roughness of hardened surfaces by 2-2.5 times.

As can be seen from the obtained data, an increase in the processing force from 25N to 100H leads to a decrease in the height parameters of roughness, and with a further increase in effort, the height parameters of roughness increase.

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