1. Introduction
In the course of machinery operation [
1,
2,
3], the working environment can exert a multifactorial effect. The foregoing results from the potential occurrence of degradation promote factors [
4,
5,
6,
7] such as the following:
- -
Hard abrasives in the zone of the mating of machine components, which intensify abrasive processes;
- -
Saline waters, which tend to induce electrochemical corrosion processes;
- -
Dynamic excitations caused by start-up operations or sudden load changes which, in turn, can cause cracks in the microstructure, leading to material decohesion.
A coincidence of at least two of the aforementioned degradation factors is decisive of the complex form of the damage mechanisms observed in machine components; for instance, micro-scratching or micro-ridging, typical of abrasive wear, can be accompanied by surface layer cracking.
The combined effect of dynamic forces and abrasive material [
8,
9,
10] is typically referred to as impact–abrasion wear, which occurs in the operation of mining machinery (e.g., crushers, skips, and chutes). The resultant effect of both environmental factors should cause a significant wear increase, exceeding the sum of individual effects these factors trigger. This assumption has been confirmed in numerous papers, including by Kennedy et al. [
11] who addressed the impact–abrasion wear tests performed on coated and non-coated samples of aluminium, mild steel, and tool steel using a dedicated test rig. The experimental tests they conducted revealed higher wear rates under impact–abrasion conditions compared to contact-only abrasion. Similar insights have been formulated with reference to studies of diverse materials in papers [
12,
13,
14,
15,
16,
17,
18,
19].
The wear process triggered by the combined effect of the factors which intensify abrasive and corrosive wear is known as tribocorrosion [
20,
21,
22,
23]. Under such conditions, the friction-induced wear process and electrochemical corrosion interact, while the consequence of both these processes acting together is a synergy of degradation factors. The synergistic effect observed in the course of the tribocorrosion process is attributable to the removal of the oxide layer due to friction, which accelerates surface corrosion in areas exposed by the impact of the abrasive material or peaks of surface irregularities, while on the other hand, the hard particles of the oxide layers being removed can intensify deterioration of the surface layer [
24,
25,
26]. The evolution of tribocorrosion-induced damage is schematically shown in
Figure 1. Both the process of tribocorrosion and its effects have been described in numerous papers, including [
27,
28,
29,
30,
31,
32,
33].
Contemporary ADIs are used relatively frequently in the mining industry where machine components are exposed to abrasive wear. They offer a number of advantages which make them suitable as a material for machine parts [
34,
35], including the following:
- -
Possibility of using casting technologies to manufacture components from ADI [
36,
37,
38,
39];
- -
High mechanical strength of the ADI grades [
40,
41,
42];
- -
Possibility of shaping the properties of ADI by way of dedicated heat treatment [
43,
44,
45,
46] and surface treatment [
47,
48,
49,
50];
- -
Possibility of developing a nanocrystalline structure [
51,
52,
53,
54,
55];
- -
Option of cast iron surface hardening in processes such as TRIP [
56,
57,
58,
59,
60,
61] and capacity to attain high tribological wear resistance [
62,
63,
64,
65,
66,
67,
68].
Even though there are numerous studies on the properties of ausferritic cast irons, few of them have actually addressed the combined effect of numerous degradation factors (impact–abrasion and tribocorrosion) on the surface degradation of the said materials.
Wieczorek [
69] examined ADI under a combined impact of dynamic forces and abrasive grains, only to conclude that under such conditions, the surface wear of the elements subject to tests increased compared to the wear in the presence of the abrasive material alone, and demonstrated the synergy of the effects caused by both degradation factors. Navarro-Mesa et al. [
70] studied the tribological properties of austempered grey irons (AGI) by pin-on-disc testing in dry and wet environments. Under dry conditions, the wear resistance of AGIs increased as the austempering temperature dropped, while the wear resistance of AGIs under humid conditions increased along with the austempering temperature rise. Stachowiak and Wieczorek [
71] analysed the combined effect of corrosive action (water + 3.5% NaCl) and wear on the tribocorrosion of ADI, and concluded that the increase in abrasion resistance and hardness were correlated. However, the highest surface hardness did not entail the highest resistance to tribocorrosion. The lowest tribocorrosion wear was found in the case of the cast iron characterised by an austenite content of ca. 20% and a hardness of 382 HB. Having studied the hardest cast iron variant, it was established that the increased wear, compared to the cast iron variants of lower hardness, could be attributed to intense corrosion processes initiated where cracks emerged in the hard surface layer.
In industrial practice, especially mining, one typically deals with a combined effect of an abrasive material, a corrosive agent (water), and variable impact loading. Such a form of degradation is referred to further on in this paper as impact–abrasion–corrosion wear. Unlike impact–abrasion and tribocorrosion, multifactorial wear is a process which has been explored to a relatively limited extent, hence the need for further research in this field. At the current stage of development, the literature on the subject addressed in this paper contains no results of studies of multifactorial wear, especially with regard to the combined effect of degradation factors such as abrasive material, water, and pulse excitation, concerning ADI. In that respect, this paper will represent a novelty, given the state of the art.
The main purpose of the studies addressed in this article was to become familiar with the wear characteristics of ausferritic cast irons under the conditions of multifactorial wear, and the following problems were taken into consideration in particular:
- -
Effect of the content of retained austenite on multifactorial wear, including impact–abrasion–corrosion;
- -
Relationship between multifactorial wear and service hardness of the surface layer.
The studies in question were conducted at a dedicated test rig enabling one-, two- and three-factor wear to be generated.
4. Discussion
The analysis of the surface damage observed following wear testing under varying environmental conditions revealed that the occurrence of individual forms of damage is heavily affected by the matrix microstructure, and the austenite content in particular. Therefore, further discussion of the results obtained in the studies has been functionally related to this phase component.
Figure 14 illustrates the behaviour of the δ
MAX wear parameter in a function of austenite content in the matrix of the ductile irons studied, as determined for the variants of the combined degradation factor effect taken into consideration.
The figure clearly demonstrates the progressive nature of the wear values in a function of the austenite content in the ADI matrix for the variants of abrasive wear (Variant A) and multifactorial wear (Variant ACD). A different, degressive nature is observed in the case of impact–abrasion wear, while in the case of tribocorrosion, there is an extremum (minimum) for values of approx. 30% of austenite (which corresponds to the ADI_1200 variant). With reference to
Figure 14, one can also generally conclude that, as the number of degradation factors increases, the value of wear of the chain wheels made of ADI rises as well. What may be perceived as a departure from this rule is the relatively low wear values obtained for Variant AD observed for the ductile irons with high austenite content (ADI_800 and ADI_1000). An additional observation made with regard to the aforementioned grades of cast iron is that a lenticular deformation of graphite, arranged diagonally to the surface, emerges under the impact of friction force. Such an arrangement of graphite may facilitate the abrasion of the matrix by the abrasive grains, enabling them to penetrate deeper into the surface layer.
The observed increase in the wear of the ADI chain wheels correlating with the increasing austenite content in their microstructure is consistent with the research results discussed in other papers [
59,
64], and it generally stems from the lower technological hardness of the surface layer of cast irons with a lower austenite content (see
Table 9). However, when loaded, austenite transforms into martensite of the TRIP type in ADI, which significantly alters the operating characteristics of the surface layer in these ductile irons.
Figure 15A,B shows diverse diffraction profiles of the ADI_1400 ductile iron variant, including for the initial condition of the ductile iron and following its abrasive wear testing (the XRD test parameters differed from those applied to determine the austenite content). It clearly implies that, as a result of both wear tests, compared to the initial condition, there was a decrease in the peak associated with iron γ (it is observed, for instance, for the values of angle 2Θ ≈ 41°) and an increase in the peak associated with iron α (occurring for the values of angle 2Θ ≈ 43°). The XRD test results obtained evidence the TRIP-type transition under the pressure induced by wear testing.
As a direct result of the austenite transition into martensite, the service hardness of the surface layer increased (
Table 9), and subsequently, its wear resistance improved. By the effect of the abrasive quartz grains, the initial hardness of the ADI increased by 143–231 HB. However, as
Figure 16 implies, the increase in the hardness of this layer depends, to a considerable extent, on the wear testing conditions (which is also confirmed by the information provided in
Table 10, listing the values of Pearson’s r coefficient of correlation between the austenite content and the service hardness of the surface layer), and consequently on the combination of degradation factors, but also on the austenite content in the cast iron microstructure.
To recapitulate on the foregoing, the wear decrease observed for Variant A, correlating with the decrease in the austenite content in the ADI microstructure is mainly attributable to the higher hardness of the surface layer which has developed under load.
In the case of a pulse-type abrasion, one can observe increasing wear as the austenite content decreases in the structure of ADI (
Figure 14). The most favourable anti-wear properties were displayed by the ADI_800 ductile iron, the ADI_1000 material being ranked second in this respect, while the lowest resistance to this type of wear was established for the ADI_1400 material variant. At the same time,
Figure 16 implies that the service hardness of the surface layer attained in Variant AD is the lowest among all the wear variants taken into consideration.
The most probable reason for the foregoing is the reduced intensity of the TRIP-type phase transition, caused by the occurrence of surface layer cracks, thereby leading to a reduction in the forces required to detach material in the course of abrasion. As argued in the papers by Putatunda and Bingi [
74], Yang and Putatunda [
40,
41], and Ravishankar et al. [
75], what happens during the TRIP-type phase transition is the formation of martensite, responsible for wear resistance enhancement, but at the same time, the resistance to brittle cracking (K
1C) of ADI declines as the ausferritising temperature drops, causing the austenite content in the microstructure to decrease as well. What can be concluded with reference to the test results obtained is that the chain wheels made of ADI with an upper ausferrite structure are better suited for operation under the conditions of abrasive wear and variable dynamic forces.
The behaviour of wear of ADI under the conditions of tribocorrosion in a function of austenite content (
Figure 14) is generally similar in nature to that of impact–abrasion wear; however, the slope of the regression line is smaller, which indicates certain similarity between the damage mechanisms observed for both wear forms. As aforementioned, the reason for the relationship identified with regard to impact–abrasion wear was recognised in the occurrence of cracks in the surface layer and the reduced intensity of the austenite transition into martensite. In the case of tribocorrosion wear, on the account of the absence of dynamic forces, no cracks developed in the surface layer during the tests, which was confirmed by damage analysis (
Figure 10). However, the second factor taken into consideration, namely the weakening of the structure consolidation process, was undoubtedly at play. Attributable to the impact of water, the corrosion process caused the formation of hard oxide layers and facilitated their abrasion by quartz sand grains, which should have reduced the intensity of the TRIP-type process. The above finding was confirmed by the service hardness decrease compared to the abrasive wear and its increase compared to the impact–abrasion process (
Table 9 and
Figure 16), as well as the reduced fraction of the α phase, as observed in
Figure 15 following XRD testing.
The nature of the wear behaviour observed in the multifactorial (impact–abrasion–corrosion) tests is identical to that of abrasive wear; however, the intensity of this process is higher. The combined wear processes induce intense surface micro-scratching (
Figure 11), surface layer cracking (
Figure 13), and considerable surface oxidation. The former of these processes should exert a progressive impact on the TRIP-type transformation, while the other two should act in a degressive manner. Ultimately, it can be noticed that the parameters of surface hardening under load are lower than those obtained following abrasive wear, while being higher than those measured after impact–abrasion testing and those attributable to tribocorrosion wear (for most of the ADI tested). The wear progression observed to be dependent on the increasing austenite content in the microstructure implies that multifactorial wear is predominantly abrasive in nature. The foregoing is attributable to a number of reasons, but primarily to the growing surface micro-abrasion effect of abrasive material under the impact of dynamic external forces, the higher accumulation of hard oxide particles in the abrasive, and the stronger synergistic effect associated with the interaction between environmental factors (the synergy of the surface layer degradation processes is not among the subjects discussed in this paper, but it will be addressed in the author’s subsequent studies).