1. Introduction
At present, when drifting underground headings, drifting methods based on blasting technologies or involving heading machines are used. In each of the above-mentioned cases, scraper conveyors can be used to transport the crushed rock from the mine face. Due to difficult operational conditions in the corridor workings of mineral resource mines, chain drums are the most critical elements of these conveyors. The most important factors responsible for a degradation of components of scraper conveyors include [
1,
2]:
a presence of rock dust in the area of mating between the drums and the chain;
a corrosive action of the water from sprinkler systems of heading machines and the water flowing from workings;
numerous successful and unsuccessful start-ups of drives of armoured face conveyors;
overloads caused, inter alia, by overloading and blocking of the conveyors.
The occurrence of the above-mentioned degradation factors during the operation of scraper conveyors determines the form of damage to the chain drums of conveyors; at the same time, this damage usually has a complex form, e.g., in addition to the abrasive or tribocorrosive wear of the teeth, their fractures or surface deformations are also observed. The most common damage to the chain drums appears in the teeth, but fractures in the bushings connecting individual chain wheels occur as well.
Particularly intensive destruction processes of conveyor components, including chain drums, are observed when drifting corridor workings in rocks of poor workability, such as sandstone, because the presence of crushed sandstone grains or other hard rocks in the area of mating between the chain wheel and the chain significantly accelerates the degradation of the surface of the seats of these wheels, which is often accompanied by fractures and chipping of the teeth as well as plastic deformations of mating areas.
Depending on the given working, the amount of accessing the excavated material may vary significantly. There are dry workings, where water in the excavated material can come only from the sprinkler system of a heading machine. In such a case, grains of the abrasive material are the only predominant destructive factor (
Figure 1A). The crushed rock from the working zone after passing through the conveyor discharge zone reaches the area of mating between the chain drum and the chain, where intensive wear of the surfaces of these components takes place. However, there are also workings in which mine water flows out. In this case, it mixes with rock dust and then enters the areas of mating between the chain wheel and the chain (
Figure 1B).
The mixture of mine water and rock abrasive material causes tribocorrosive wear that represents the combined action of abrasive wear and corrosive wear. The essence of this process is presented later on in this article. A further implication of the presence of crushed rock and a mixture of water and abrasive is the possibility of filling the seats of the chain drum seats and lifting the chain link—then the way of mating in the conveyor drive system changes.
For each combination of at least two environmental factors, the synergy of destructive processes may occur. The measure of this synergy includes the interaction components resulting from the action of a given factor. Wieczorek [
3] has considered the situation of the joint action of abrasive wear and dynamic forces, as well as demonstrated the synergy effect accompanying the action of both these factors. In addition, Wieczorek et al. [
4] showed a good consistency between the laboratory test of tribocorrosion of austempered ductile irons (ADI) with the test in operational conditions.
This paper presents the results of the interaction component, characterizing the synergy of degradation processes caused by abrasive and corrosive factors, occurring at the construction of underground corridor workings, taking into account the grades of cast steel commonly used in the construction of mining machines.
The process of wear, caused by two factors (abrasive and corrosive), is defined as tribocorrosion [
5], which is relatively well-recognised and described. Tribocorrosive wear most often occurs in the tribological systems, composed of two or three elements moving against one another (
Figure 2) in the presence of abrasive and corrosive agents [
5,
6,
7]. Thus, tribocorrosion [
8] can be defined as the process of wear determined by the simultaneous impact of mechanical inputs and a corrosive environment. In these conditions, abrasive and electrochemical processes interact and this interaction results in an additional synergistic component.
The synergistic effect, occurring during the tribocorrosion process, results from removing the layer of oxides through friction, which accelerates the surface wear in the exposed and rough places. Furthermore, the hard particles of the removed oxide layers may intensify the destruction of the outer layer [
8,
9]. Tribocorrosion may also take place in the presence of an additional abrasive, which may intensify this type of wear. The mechanism of tribocorrosive wear is presented in
Figure 2.
The mechanism of the combined impact of corrosion and mechanical wear is described by the Watson model [
10] which assumes the existence of synergy between both impacts. According to this concept, the total wear (
VC) is the sum of wear caused by the mechanical impacts without corrosion (
VM), the impact of corrosion without friction (
VK) and wear as a result of synergy (Δ
V), where this effect is the sum of the component resulting from the impact of friction on corrosion (
VM→K) and the component resulting from the impact of corrosion on friction (
VK→M). The above process may be described with the following equation:
where:
VC is the total wear,
VM is the wear caused by mechanical impacts,
VK is the wear caused by corrosion impact,
VM→K is the component resulting from the impact of friction on corrosion, and
VK→M is the component resulting from the impact of corrosion on friction.
The aforementioned synergism of tribocorrosion wear was experimentally demonstrated by many researchers [
11,
12,
13,
14,
15,
16,
17,
18]. However, not many research projects have investigated the process of synergistic tribocorrosive wear of real tribological. In the research works by Tyczewski [
19,
20,
21], possible combinations of factors intensifying the destructive processes were analyzed and the modified Watson model adapted to the specific character of the sugar industry was presented. Complex multi-type abrasive, corrosive and mechanical wear is observed in it. Other results of the research on tribocorrosion of real objects are presented in [
22,
23,
24].
In the case discussed herein, the tribocorrosion process was initiated in the variant intensified by a quartz abrasive which caused deep scratches on the surface. These damages were subject to accelerated corrosion. The corrosion products were also removed from the steel surfaces and mixed with the quartz abrasive. The results of this action demonstrate the characteristics of the synergism of the damaging processes and are the subject of this analysis.
2. Experimental Details
The research activities presented in this paper used laboratory measurements of the mass corrosion rate of the investigated steel elements and stand tests of wear under the combined impact of abrasive and corrosive agents. In order to determine the mass corrosion rate, the value of the corrosion current density needs to be known. This parameter was determined using the Solartron 1287 potentiostat with an FRA 1255A impedance system. The object of the tests included cubic samples with the side length of 10 mm, made of the investigated steel, taken from the zone of interaction between the sprocket teeth and chain links.
The stand tests of tribocorrosive wear of the steel alloys were performed using a test stand presented graphically in
Figure 3A.
The quartz abrasive or its mixture with water and salt was placed in the test chamber (
Figure 3B,C) with the tested sprockets. This way, an abrasive or abrasive–corrosive layer was always present between the surface of the sprocket bottom and the chain link. The wear testing method is described in detail in [
16]. The wear tests were carried out for 200 h, 100 h for each direction of the rotation of the sprockets. Their peripheral velocity was v = 0.7 m/s and the input power of each motor was PM
1 = PM
2 = 7.5 kW. The determined surface pressures between the wheel surface and the chain surface were equal to 48.9 MPa, whereas the maximum reduced stress at the base of the tooth was 2.18 MPa. The values for surface pressure and the maximum reduced stress by conducting numerical simulations. Sprockets (
Figure 3D) made from flame-hardened cast steel alloys: GS42CrMo-4, L35GSM, L30GS, L20HGSNM and A6 were the object of the tests. The mechanical properties of the considered steels are presented in
Table 1, while the chemical composition of cast steel is presented in
Table 2.
Three sets of sprockets were used for the investigations. Once the tests were completed, samples were taken to study the microstructure, determine the hardness of the top layer and observe the surface damage. Each of the sets was subject to a full wear cycle and the only difference between the testing variants was the type of abrasive used.
Table 3 presents the combinations of the abrasive or tribocorrosive agents.
Microscopic studies were performed after the wear tests. Samples for the micro-structural tests were cut out from the area of interaction between the sprocket and the chain. The samples were then ground, polished and etched with the 2% Nital solution. Observations of the microstructure were carried our using the OLYMPUS IX70 microscope with 50 to 1000x magnification and scanning electron microscopes: HITACHI S-3500N and Supra 35 ZEISS with an EDS spectrometer.
The top layer of the cast steel alloys L20HGSNM, GS42CrMo-4, A6, L30GS and L35GSM contained tempered martensite formed in the result of surface hardening, whereas in the core of the sprockets, fine-grained sorbitol was noticed (an example of the microstructure of the top layer of the L20HGSNM steel is presented in
Figure 4A.
Figure 4B shows an exemplary cross-section through the top layer of the L35GSM steel in the zone located between 0 and 0.05 mm from the surface. In the presented top layer, some non-metallic precipitates and manganese sulphides are also visible. The non-metallic precipitates were also found in all the tested steels and were usually situated in the areas where the sand came into contact with the melted metal.
In order to determine the abrasive wear of the sprockets, the surfaces of interaction between the sprocket and the chain, before and after the wear test, were measured using a coordinate measuring machine (CMM). The sprocket-chain collaboration area was reflected with at least 300 points which created the path of the probe (the method of determining the wear parameters is presented in [
25]).
To determine the wear, the δ
i,N parameter was used, defined as the distance between the positions of the probe before and after the test for the i-th point on the path and N-th tooth of the sprocket. The determined values of the δ
i,N wear parameter were averaged for all the measured teeth of the given sprocket, using the following equation:
where x
1_i,N is the x coordinate of the i-th point of N-th teeth before the test, x
2_i,N is the x coordinate of the i-th point of N-th teeth after the test, y
1_i,N is the y coordinate of the i-th point of N-th teeth before the test, y
2_i,N is the y coordinate of the i-th point of N-th teeth after the test, z
1_i,N is the z coordinate of the i-th point of N-th teeth before the test, z
2_i,N is the z coordinate of the i-th point of N-th teeth after the test, and N is the number of measured teeth.
On the basis of the wear value of the i-th point of the measuring route δ
i,N determined for each N-th tooth, the values δ
i_AVG averaged in relation to all the measured tooth surfaces of a given chain wheel were determined with the use of the following relationship:
where
n is the number of seat surfaces of a given chain wheel (
n = 24).
Then, a single number maximum wear ratio (δ
MAX) for the given sprocket was determined, using the equation:
In addition, based on the δ
i_AVG averaged wear values, the logs of this parameter along the adopted measurement path were identified (the logs of this parameter in the function of the number of the measuring point are marked as δ
i_AVG(i)). The values of δ
i_AVG(i) express the thickness of the surface layer removed as in the result of the action wear factors at a given point of contact between the chain and the seat. The method of determining the wear parameters δ
i_AVG and δ
MAX is shown in
Figure 5.
4. Interaction Components of the Tribocorrosive Wear Process in Chain Drums
The interactive component, resulting from the synergistic impact of the corrosive and abrasive agents on the wear of the surface of the sprockets (Δ
V) may be determined from the following equation:
The wear component, caused by electrochemical corrosion without friction (V
K), represents the thickness of the material removed due to electrochemical corrosion in the conditions without the synergistic impact of the abrasive wear. To determine the V
K factor, it was assumed that corrosive wear would not exceed the thickness of the material dissolved in the 3.5% NaCl solution. Based on
Table 4, this factor was determined assuming the estimated time of the corrosive wear equal to the duration of the stand test, that is, 200 h. The values calculated for the analyzed materials are compared in
Table 5.
The total wear VC value is equal to the δMAX wear value after the tribocorrosive test (Variant C or Variant C + NaCl), while the VM component is equal to the δMAX wear value after the abrasive test (Variant A).
Similarly, the curves of the synergistic component in the function of the location on the measured path ΔV(i) will take the following form:
Figure 15 presents examples of curves of the ΔV(i) interactive component against the location on the measured path for three selected steels and both tribocorrosive wear variants. When analyzing
Figure 15, it can be concluded that adding water or a water–salt mixture to the abrasive caused an additional synergistic effect (discussed in the Introduction), that is, destructive processes affecting the surface of the sprockets. The ΔV(i) component assumes values up to 1 mm (for the L20HGSNM steel) and, given the relatively short testing time (200 h), it should be considered very significant. For the GS42CrMo-4 cast steel, the ΔV(i) component reached high values of up to 0.8 mm, but lower than those for the L20HGSNM cast steel. For both cast steels, it can be seen that the addition of NaCl to the abrasive mixture had a significant impact on the synergy of the process responsible for a destruction of surfaces of the chain wheels. As it has already been mentioned, the presence of hard products of corrosion had the highest impact on the increase in the total wear in the Variant C + NaCl. In the case of the L35GSM cast steel, the lowest values of the synergistic component, associated with the combined action of abrasive and corrosive factors (not exceeding 0.5 mm) and a relatively small increase in this component in conditions of the wear intensified by the presence of NaCl in the abrasive mixture were observed.
Figure 15 also shows how much the synergistic factor differs depending on the type of steel.
Based on the curves, presented in
Figure 16, it is apparent that the higher the maximum strength, the greater the share of the synergistic component in the total wear. However, this observation should be taken critically because in the case of the manganese steels (L30GS, A6), there were unexpectedly high abrasive wear values (Variant A) and the low ΔV/V
M relative ratio might result from the effect of a high reference.
5. Conclusions
1. Based on the wear test, which simulated different abrasive and tribocorrosive wear applied on the sprockets, it can be concluded that the presence of agents intensifying the electrochemical corrosion significantly increases the total wear of the area of collaboration between the sprocket and the chain link.
2. The impact of an additional environmental factor (water), present during the exploitation in the presence of the quartz abrasive, is synergistic and affects the value of the abrasive wear in the area where the wheels interact with the chain links in all the investigated cases.
3. Adding the 3.5% NaCl solution to the quartz abrasive causes a stronger synergistic effect as compared to the mixture of water and quartz abrasive (this relation was confirmed for all the analyzed materials).
4. For the analyzed materials and in tribocorrosive wear conditions, the ΔV/VM relative wear increment was 4.8 ÷ 59% for Variant C and 15.9 ÷ 88.3% for Variant C + NaCl.
5. The lowest increments of the synergistic component were recorded for the L30GS and A6 steels, but this resulted from their high level of the VM abrasive wear reference.