3.1 ABRASIVE WEAR BEHAVIOR
The weight loss graph of bare and hardfaced ploughshare blade after field trial is given in Figure 2. From the Figure, it is observed that bare steel experienced maximum weight loss with a loss of 52g, 85g, and 103g after 5 hours, 10 hours, and 15 hours of field trials respectively. The higher weight loss of bare steel indicates towards low abrasive wear resistance of the bare EN-42 steel. The higher wear rate experienced by the bare steel is due to the abrasive action of hard particles present in the soil, which rub directly on the surface of ploughshare blades. SEM analysis of the worn-out surface of the bare steel after actual field trials is given in Figure 3 (a). SEM reveals the presence of deep and continuous scratch marks and groves with lip formation, indicating the ductile wear mechanisms (Liu et al., 2013). Soil particles had adhered into the scratch marks and groves on the surface of ploughshare blades. The low abrasive wear resistance of bare steel is also attributed to the low hardness value of the steel as given in Figure 5. The microhardness of bare steel has been found to be less than the hardfaced steel. The optical microscope of bare steel given in Figure 4 (a) shows pearlite and ferritic structure, which might have contributed towards the low abrasive wear resistance of the bare steel (Efremenko et al., 2013). The microstructure reveals the presence of high concentration of pearlitic microstructure, indicating towards the high wear loss due to its soft nature and low impact strength. (Vasilescu and dobrescu 2015).
SEM analysis of the worn-out surface of H1 hardfaced steel is given in Figure 3 (b). The SEM does not reveal any continuous or deep grooves on the worn-out surface as observed in case of
bare steel. The SEM reveals the presence of small scratch marks, which are discrete and non-directional because of soft matrix is surrounded by hard matrix. The presence of complex boron-carbides, in the metal matrix, is attributed to the better abrasive wear resistance of H1 steel. The carbides present in the microstructure (Figure 4 (b)) does not allow the formation of deep, continuous scratch marks and grooves, thus providing high wear resistance (Saha et al., 2016). The presence of complex carbides is further supported by EDS analysis in Table 4. EDS analysis reveals the presence of Fe, Cr, B, Mo, and W along with Carbon, thus suggesting toward the presence of complex carbides in the hardfacing. Further, the presence of boron, tungsten, and molybdenum also adds to hardness and wear resistance, thus H1 hardfacing has given excellent wear resistance in the actual field trials due to composition, hardness, and microstructure (Saha et al., 2017).
Among the hardfacing under study, H1 hardfacing has shown minimum weight loss in the actual field trials as revealed by Figure 2. The low weight loss for H1 hardfacing indicates towards better abrasive wear resistance for H1 hardfaced steel. The optical micrograph of H1 hardfacing given in Figure 4 (b) reveals the presence of metal carbides (MC), metal borides (M2B), and boro carbides (M23(BC6)) in the matrix. The boro carbides (M23(BC6)) are present in the form of needles. The presence of these carbides in the microstructure is the reason for the better abrasive wear resistance of H1 hardfaced steel. The presence of these complex carbides improves the abrasive wear resistance and hardness of the material (Gou et al., 2015). As observed from Figure 5, H1 hardfacing exhibited higher value of hardness than the substrate steel and other hardfacings.
H2 hardfaced steel has better abrasive wear resistance than the H3 hardfaced and bare steel in actual field trials. Figure 2 shows that H2 hardfaced steel has experienced the weight loss of 24g, 83g, and 99g after 5 hours, 10 hours, and 15 hours of field trials respectively, which indicates better abrasive wear resistance of the H2 hardfaced ploughshare blades than the regular ploughshare blades. An optical micrograph of the hardfacing given in Figure 4 (c) shows a dendritic structure with the presence of dendritic arms and eutectic carbides. The dendritic structure being hard in nature has given better abrasive wear resistance. However, at the same time, the dendritic structure being brittle in nature has resulted in the fracture of the ploughshare blade tip during the actual field trials. (Abd El-Aziz et al. 2015) reported that the eutectic carbide helps to enhance the wear resistance properties. The presence of carbides of Cr and Fe in the microstructure has also provided the hardfacings with high hardness (Chatterjee et al., 2003). The Figure 5. shows H2 hardfacing has 817HV hardness. SEM analysis of the worn out surface of H2 hardfaced steel after the field trials is given in Figure 3 (c). SEM analysis reveals the presence of discontinued abrasion and scratch marks. It also reveals the presence of small pits and craters. The presence of carbide of Fe and Cr does not allow the formation of continued scratch marks, thereby providing high resistance to abrasion and wear, which might have provided better abrasive wear resistance to the H2 hardfaced steel (Zikin et.al 2012). The formation of pits and scratch marks takes place in the soft matrix present in the inter arms spacing of the dendrites. The hard particles present in the soil abrade the soft matrix, resulting in wear (Sabet et al., 2011).
However, hard carbides of Fe and Cr resist the wear thereby providing better wear behavior to the materials. EDS analysis in Table 4 reveals the presence of Fe and Cr along with carbon, indicating the formation of carbides of Fe and Cr, which are hard and wear-resistant in nature (Chung et al., 2013). The presence of oxygen indicates that the oxidation might have taken place due to moisture and salinity in the soil. EDS also reveals the presence of soil particles on the surface of the worn-out sample. Thus H2 hard facing has provided sufficient protection to the EN-42 steel against abrasion in actual field trials.
Maximum weight loss was observed for H3 hardfacing as shown in Figure 2. It indicates the low abrasive wear resistance of H3 hardfacing as compared to H1 and H2 hardfacings. Figure 3(d) shows the SEM analysis of H3 hardfacing, which reveals the crack formation, which might be due to its brittle nature, indicating the brittle failure of hardfacing. EDS analysis indicates the high content of iron carbides in the metal matrix which is attributed to the brittle behavior. The microstructure of the H3 hardfacing shown in Figure 4 (d) reveals the presence of iron-carbides and lath martensite in the metal matrix. Iron Carbide provides high hardness and good wear resistance to the hardfacing, but due to low impact strength and brittleness, the ploughshare tip got fractured during the field trials. H3 harfacing hardness is 726 HV as shown in Figure 5.. The presence of iron-carbide is attributed to the hardness of the metal matrix.
Among the single layer and double layer hardfacings, the double layer hardfacing had not shown any encouraging result with regards to abrasive wear resistance. The double-layer hardfacing experienced significantly higher weight loss than the single-layer hardfaced steel. Except for the H1 hardfacings in which the double layer has shown slightly better wear resistance than the single layer hardfacing. The higher weight loss for double-layered HH2 and HH3 hardfacing might be due to the reason that the double layer of overlaid material has made the ploughshare blade brittle, which got fractured during the actual field trails. As reported by Gualco et al., (2015) the double layer increases the density of hardfacing material so the crystal structure size increases which decrease the hardness of the substrate. This might be the reason for the low performance of double layer HH2 and HH3 hardfacings.
3.2 MICROHARDNESS ANALYSIS
The Microhardness test was conducted by a Vicker hardness tester. The load applied on the specimen was kept as 1kg for 10 seconds. The hardness of various regions such as base, interface, and hardfacing is shown in Figure 5. The hardness of the H1, H2, and H3 is found to be 1080HV, 816HV, and 726HV respectively. The bare sample indicated lower hardness (372HV) than the hardfacings. The optical microstructure of the bare sample revealed the presence of ferrite and pearlitic structure, which attributed to lower hardness. Among the hardfacings, H1 hardfacing has indicated higher hardness (1080HV), which might be attributed to its chemical composition and microstructure. The microstructure of H1 hardfacing in Figure 6 reveals the presence of complex carbides, which have been attributed to its high hardness. As reported by Kim et al., (2003) the presence of complex carbides in the microstructure provides good wear resistance and hardness to the material. From Figure 5. it is observed that H2 hardfacing has also given higher level of hardness, which might be attributed to the presence of chromium and carbon in the hardfacing. As suggested by Yuksel and Sahin (2014) the volume fraction of chromium carbide increases the hardness and wear resistance. The double layer hardfacings in Figure 5. indicates lower hardness than single layer hardfacings might be the reason for the lower wear resistance of double layer hardfacings.
3.3 WEAR RATE INDICES (WRI)
Wear rate indices (WRI) is used to determine the wear resistance of blades. It is defined as the wear rate of the bare ploughshare blade to the hardfaced blade (Kang et al., 2017). It is observed from Table 5 that among the double layer and single layer hardfacings, HH1 (1.53) and H1 (1.43) hardfacings indicating higher wear resistance. HH1 and H1 hardfcaing has shown higher wear rate indices attributed to the presence of complex carbides in the metal matrix which is hard in nature. HH3 hardfaced blade got fractured after initial hours of field trial so it indicates the lowest wear rate indices (0.81). The WRI of HH2 (1.03) is found not significantly different than the bare ploughshare blade. The WRI of H1 hardfaced blade is 1.45 times, H2 is 1.25 times, and H3 is 1.28 times better wear resistance than the bare ploughshare blade. Except for HH1 hardfacing, single layer hardfacings have given better WRI than the double layer hadfacings and it might be associated to the lower hardness of double layer than single layer deposition.
3.4 LENGTH LOSS ANALYSIS
Before and after completing the field trials, the length of blades was measured to examine the intensity of wear. From Figure 6 it is observed that the bare ploughshare blade suffered severe length loss (31mm), indicating high material loss due to wear, which might be attributed to its low hardness and soft metal matrix. The edges of the bare (EN-42) ploughshares blades become ineffective and required replacement after completing 15 hours of field trials.
The minimum length loss observed for HH1 (6mm) and H1hardfacing (7mm) indicates good abrasive wear resistance for HH1 and H1 hardfacing during field operation. So H1 hardfacing is suggested for field trial operation to improve the durability and reliability of the ploughshare blades. H1 hardfacing is associated to its high hardness (1080HV) as shown in Figure 5 and complex carbide microstructure, which enhances the wear resistance. Gou et al., 2015 also reported that complex carbides improve the hardness and fracture toughness of the hardfacing. Maximum length loss was observed for HH3 (38mm) hardfacing because the leading edge of HH3 hardfaced blade got fractured during the first 5 hours of field operation. SEM (Figure10) analysis also reveals the crack formations in the hardfacing, which is attributed to its brittle nature and is the reason for the higher length loss of HH3 hardfaced blade. Chatterjee and Pal (2006) suggested that the double layer deposition of iron carbides enhances the concentration of carbide volume fraction which is brittle in nature. Further, it is observed that double layer hardfacings have not shown any significant difference in improving the wear resistance of ploughshare blades than the single layer hardfacings. Therefore single layer hardfacing is recommended for practical applications.