CORROSIVE WEAR BEHAVIOUR OF VARIOUS NITRIDING TREATMENTS ON A LOW ALLOY STEEL George KARAFYLLIAS1)*, Frazer BROWNLIE1), Lampros GIOURNTAS1), Trevor HODGKIESS2), Alexander GALLOWAY1), Alastair PEARSON3)
1)
Department of Mechanical and Aerospace Engineering, University of Strathclyde, Glasgow, Scotland, UK 2) Porthan Limited, Lochgilphead, Scotland 3 Weir Engineering Services, East Kilbride, Scotland, UK * Corresponding author:
[email protected]
Abstract: The corrosive wear behavior of three gas nitriding treatments (72h, 90h and 120h) on nitrided steel (BS 970-905M39) was investigated under erosion-corrosion conditions at two angles of submerged jet impingement i.e. 90 o and 20o. The test solution was 3.5% NaCl. Experiments were conducted with and without the presence of a burden of suspended silica sand particles. It was demonstrated that all the nitriding treatments promoted excellent corrosion resistance in the, particle-free impinging liquid. In the presence of the sand particles, however, clear mass losses were recorded on the nitrided steel but at a lower rate for 20o impingement than at perpendicular incidence. A comprehensive surface examination was conducted and facilitated measurements of wear scar depths under the impingement jet. The outcome of this approach was to demonstrate that the wear scar depths are not directly linked to total mass losses for all materials and this trend is discussed in the paper. Keywords: erosion - corrosion, nitriding, impingement, surface topography
1. Introduction Wearing of components is a major issue for many engineering applications. In particular, erosion-corrosion is a complex material degradation process which involves mechanical wear and electrochemical corrosion processes. Applications where erosion-corrosion occurs, include pipes and pump components, such as impellers, casings and valves. The occurrence of this phenomenon in such components can lead to several challenges like loss in performance, increase in maintenance cost and potential failures. To overcome this major issue, material selection has become significant. Surface engineering treatments, such as nitriding, have been investigated to establish whether they provide suitable wear characteristics. The nitriding process is an established and successful means of generating a hard surface layer on a component. The low diffusion temperature of nitriding offers low distortion and excellent dimensional control of the treated component. This is one of the advantages of the nitriding process in comparison with other diffusion processes, i.e carburizing, which requires an oil quenching operation from a high temperature to achieve the desired martensitic structure and associated high hardness. The nitriding process also induces compressive stress (within the compound layer), which results in improved fatigue strength. Significant improvements of nitriding in dry sliding wear resistance have been noted in previous studies [1–6]. For instance, the compound layer formed during nitriding has been found to be beneficial to sliding wear properties [7]. Furthermore, a comparative past study has found that nitriding treatment improved the adhesive wear properties compared to untreated, borided and vanadized specimens [8]. In previous work, plasma nitriding was considered for hydroturbine blades due to the potential benefit to high impact wear resistance [9]. Also, plasma nitrided 12Cr steel has been found to have greater water jet impingement erosion resistance than pack borided 12Cr steel as it has an ability to absorb shocks which occur during jet impingement [10]. Erosion-corrosion resistance of AISI 316 stainless steel was improved substantially by low-temperature plasma nitriding [11]. The duration of nitriding process time has also been studied. It was found that finely dispersed nitrides can be formed during shorter nitriding time and this resulted in improved abrasive wear resistance when compared to specimens with longer nitriding time [12]. However, it has also been observed that increasing nitriding time and hence 1
thickness of compound layer, has led to improved dry sliding wear resistance of AISI H13 steel and die forging durability of crankshafts [13]. Abrasive-erosive wear resistance of AISI 1050 steel has been found to be reduced by gas nitriding for 12 hours compared to 6 hours [14]. It has also been found that there is a significant link between surface hardness and case depth. Longer duration of nitriding time results in a lower surface hardness and deeper case depth which was less effective in preventing dry sliding wear [15]. The influence of nitriding process time has not been investigated significantly under erosion-corrosion conditions which instigated this research study. Three nitriding treatments on (BS-970) 905M39 nitriding steel were tested under erosion-corrosion conditions at both 90o and 20o angles of impingement. The objective of this study was to investigate the effect of different diffusion nitriding case depths on the relative corrosive wear performance. Also, by applying post - test analysis, fundamental erosion-corrosion mechanisms are identified providing more information on material degradation processes. Assessment of material performance includes mass loss, microscopic examination and surface profiling.
2. Materials and methods 2.1 Materials under study Gas nitriding treatments were conducted on 905M39 steel, which results in the diffusion of nitrogen into the surface of the steel at a temperature range of 500-590oC while the steel is in the ferritic phase. The case depth, up to about 0.75 mm [16], provides a case surface hardness circa 1050 HV. The composition of the 905M39 steel is presented in Table 1: Table 1: Chemical composition of 905M39 steel.
C
Si
Mn
Al
P
S
Mo
Cr
0.35-0.43%
0.10-0.40%
0.40-0.65%
0.90-1.30%
0.025% (max)
0.025% (max)
0.15-0.25%
1.40-1.80%
Three different gas nitriding case studies were investigated, as presented in Table 2:
Duration (h) Temperature (oC)
Table 2: Details of the gas nitriding process. Case study 1 Case study 2 72 90 520 520
Case study 3 120 590
2.2 Testing apparatus The experimental equipment comprised of two closed loop re-circulating rigs that have the same design as illustrated in Figure 1. Rig 1 was used for solid liquid erosion-corrosion impingement (SLEC) and Rig 2 was used for liquid (solids free) erosion corrosion impingement (LEC). The flow velocity on SLEC Rig was 20 m/s whilst on LEC Rig was 18 m/s. The testing medium for SLEC experiments contained an aqueous solution with 3.5 % NaCl and round silica sand loaded with 0.7g/l concentration whereas on the LEC experiments the sand was excluded. The LEC tests were carried out for 2 hours with a temperature range of 18°C -35°C. The SLEC tests were carried out for 1 hour duration with at temperature range of 18°C to 26°C. Through an assembly of piping the water was circulated by a pump to the nozzle of 3mm diameter. The circular sections of the test pieces were 38.1 mm diameter. Each specimen was submerged in the aqueous solution and, as a matter of consistency; it was decided to keep a fixed distance of 5 mm between the nozzle and the specimen. This research study involved tests at 90 o and 20o angle of impingement. A typical representation of a 90o and a 20o angle of impingement specimen after 1h SLEC test, is shown in Figures 2-3 respectively. Also, in Figure 4, the sand size distribution is presented.
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Figure 1: Schematic diagram of erosion-corrosion rig.
Figure 2: Wear scar on the 72h nitrided steel after 1h SLEC test at 90o impingement.
Figure 3: Wear scar on the 72h nitrided steel after 1h SLEC test at 20o impingement.
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Figure 4: Sand size distribution.
Cross sections of polished and etched nitrided specimens were examined on an Olympus optical microscope and Image J software on cross sectioned specimens which were also polished (3 μm) and etched. A Mitutoyo MVK-G1 hardness tester was employed to measure the micro-hardness profiles of the nitrided specimens under 200gf load. The erosion corrosion performance of tested materials was assessed as follows: measurement of mass loss after 1h and 2h SLEC and LEC experiments respectively. post-experimental analysis which was facilitated with an Alicona Infinite Focus 3D measurement scanner from which wear scar depths were determined.
3. Results 3.1 Microscopic examination All specimens were cross sectioned, ground and polished to 3μm diamond suspension and thereafter etched in 2% nital solution. The cross sections showed different case depths for each of the materials and therefore enabled a detailed analysis of metallurgical structures. Figures 5, 6 and 7 show the structure of the nitrided specimens under the three differing heat treatment conditions. All three samples exhibited three distinct zones - the outermost white layer, the diffusion zone and the core material. The total thicknesses of the compound layers that were obtained through microscopic views are demonstrated in Table 3:
Table 3: Compound layer thickness of nitriding treatments. Nitriding treatment Thickness (μm) 72h 27 90h 35 120h 72
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Figure 5: Compound layer on 72h nitrided 905M39 steel.
Figure 6: Compound layer on 90h nitrided 905M39 steel.
Figure 7: Compound layer on 120h nitrided 905M39 steel.
3.2 Micro-hardness measurements Figure 8 illustrates the micro-hardness measurements against the depth for the three different nitriding treatments under study.
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Figure 8: Micro-Hardness values of tested materials against the surface depth.
The highest hardness levels in the top surface regions were displayed by the nitrided steels for 72h and 90h whilst the 120h nitriding treatment achieved a hardness of 800 HV. A rapid fall of hardness was observed after 0.2 mm from the top surface for the 72h and 90h nitriding treatments. The latter observation was not evident for the 120h nitriding treatment as a result of the thick compound layer which maintained the hardness on higher corresponding levels than the other two nitriding cases. At 1.2 mm distance from the top surface, all materials displayed the hardness of the bulk 905M39 steel (280 HV).
3.3 Mass losses in LEC environment and static conditions exposure A number of tests were conducted under LEC conditions (solids free). The aim was to obtain information on the corrosion behavior of the materials in the flowing condition and compare the three nitriding treatments with the bulk steel. Three replicates were performed for each material and the average mass losses are presented in Table 4:
Table 4: Mass loss results in LEC environment. Material Mass loss (mg) 72h Nitrided steel 0.0 90h Nitrided steel 0.0 120h Nitrided steel 0.0 Untreated steel 30.0
It was obvious that the untreated steel performed poorly under LEC conditions, which is typical behavior for low alloy steels. On the other hand, all nitriding cases exhibited extremely high corrosion resistance due to the presence of the compound layer. Figure 9 illustrates the corrosion product of bulk steel, compared with the nitrided steel which showed no evidence of corrosion.
Figure 9: Corroded surface of 905M39 steel (left) and nitrided 905M39 steel under 2h LEC conditions.
The results shown in Table 4 and Figure 9 demonstrated the influence of gas nitriding in providing good corrosion resistance in severely impinging water as well as in static water (Figure 10). 6
Figure 10: A non-nitrided (left) and a nitrided specimen (right) in 3.5%NaCl solution for 24h. Notice corrosion is absent in the nitrided specimen.
3.4 Mass losses in SLEC environment The overall erosion-corrosion performance of materials under study was assessed by mass losses. A minimum amount of four replicates was undertaken for SLEC experiments at 90 o impingement whilst at 20o angle of attack the corresponding number was three. Figure 11 demonstrates the average mass losses of the three nitriding cases at both angles of impingement.
Figure 11: Average mass losses for materials under study at 90o and 20o angle of impingement. Comparison of the mass losses between the two impingement angles reveals a clear and substantial reduction at the lower angle for all nitriding treatments. At both angles of attack, the highest mass loss was clearly on the 90h nitrided steel with the other two treatments yielding rather similar mass losses
3.5 Surface topography Three dimensional representations of the wear scars of the tested specimens were obtained by using an Alicona Infinite Focus 3D machine. Several scans were conducted in order to identify the deepest wear scar in each specimen. Figure 12 shows the average wear scar depth of materials under study at both angle of impingement.
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Figure 12: Average wear scar depths of materials under study at 90 o and 20o angle of impingement. Wear scar measurements of the mass loss for the two impingement angles showed that, at 90o the wear scar was deeper for all nitriding test pieces (72h, 90h & 120h) when compared to 20o impingement tests. The 72h nitrided steel displayed the lowest wear scar depth at both angles of attack, followed by 90h nitrided steel and 120h nitrided steel.
4. Discussion In past studies, it has been found that the increase of the temperature of the nitriding treatments from 350 oC to 550 C provides lower hardness close to the top surface (0.05 mm distance from the top surface), but, the reduction of hardness with depth was at a lower rate for the high temperature nitriding treatment [17]. According to Figure 8, a similar finding was obtained in the present work. o
In aspect of corrosion resistance, it is sometimes stated that gas nitriding of carbon steel, or low alloy steel, leads to a porous/cracked surface which might be expected to impair corrosion resistance. On the other hand, electrochemical studies have shown significant benefits of low corrosion rates for gas nitrided carbon steels exposed to quiescent 3.5% NaCl [18] and plasma nitrided carbon steel in acidic and saline water [19,20]. The present study has extended the scope of these previous works by demonstrating that gas nitriding on alloy steel provides excellent resistance to a high velocity impinging jet and this feature does not appear to be dependent on the nitriding conditions. However, in terms of SLEC, the behavior is clearly affected by the nitriding times and temperatures. The erosion-corrosion damage at 90o angle of impingement was substantially higher than at 20o angle of attack. This could be related to the notion that brittle materials, such as nitrided steels, are more vulnerable strikethrough to perpendicular impingement than at lower angles in terms of erosion resistance [21,22]. The 72h and 120h nitrided steels depicted lower mass loss than the 90h nitrided steel at both impinging angles. As shown in Figure 12, the 72h nitrided steel demonstrated the lowest wear scar depth at both angles of impingement implying superior erosion-corrosion resistance over the 90h and 120h nitrided steels. Also, the 90h nitrided steel showed slightly lower scar depth than the 120h nitrided steel under both angles of attack.
5. Conclusions 1) Gas nitriding has been shown to provide substantial increases in corrosion resistance. This beneficial effect does not simply relate to static water but extends to corrosion resistance under high velocity impinging conditions. The benefits are obtained over all of the nitriding conditions studied in this work. 2) A more detailed study under solid liquid impingement conditions has shown that the behavior is dependent on the gas nitriding conditions. 3) At 20o angle of impingement the mass losses and the wear scar depths of all studied materials were lower compared with perpendicular incidence. 4) At 90o the wear scar depths which is associated with erosion-corrosion damage under direct impingement conditions, demonstrated that 72h nitrided steel is superior over the other nitriding treatments. 5) Overall, in the present study, the 72h nitrided steel has been shown to be the most effective and also the most economic.
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6. Acknowledgement The authors would like to acknowledge the support for this study, which was provided by the Weir Group PLC via its establishment of the Weir Advanced Research Center (WARC) at the University of Strathclyde.
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