ITSC 2015 – Int. Thermal Spray Conf., Long Beach, CA, USA, May 11-14, 2015. 1

Fe-Based Powder Alloys Deposited by HVOF and HVAF for Applications Exposed to Solid Particle Erosion

Senad Dizdar

Höganäs AB, Höganäs, Sweden senad.dizdar@hoganas.com

Manish Kumar

Höganäs India Pvt. Ltd., Pune, India manish.kumar@hoganas.com

Abstract

HVOF and HVAF deposited coatings of three commercial Fe-based powder alloys have been ranked according to ASTM G76 solid particle erosion testing. The reference was electrolytic hard chrome (EHC) plating. The test results at 30 m/s abrasive particle velocity showed that 6AB powder alloy, when HVAF sprayed, Fe SP586 when both HVOF and HVAF sprayed meet the EHC plating reference erosion rate. 6AB HVOF sprayed and Fe SP529 both HVOF and HVAF sprayed powder alloys achieved two to three times higher erosion rate but were still at the same level of magnitude as the EHC plating reference.

Introduction

Machine applications dealing with solid particle flow at velocities from approx. one up to few hundred meters per second are exposed to solid particle erosion i.e. erosive wear. Applications operating at moderate particle velocity include vacuum conveyer systems for powders, granules and bulk solids, slurry pumps and pipelines, pneumatic and hydraulic power systems. High particle velocity applications include diverse gas turbines, wind mills, helicopter and airplane propellers. For these applications, hard facing by thermal spray or overlay welding / cladding or metal plating is a common and effective way to protect from erosion. HVOF and HVAF powder spray technologies are widely used in, for example, aerospace and process industry due to their cost efficiency and ability to deposit a range of materials to meet these application demands [1]. HVOF spraying can deposit refractory grade powder materials such as tungsten and other carbides. HVAF spraying allows the use of cost effective Fe-based alloys with low risk of powder alloy oxidation in the spray flame A general chart to rank erosive resistance of HVOF/HVAF spray coatings contra metal plating and hard facing by other technologies is not yet available despite a wide use of HVOF/HVAF spraying. The reasons are numerous. Many solid particle erosive wear test method results are difficult to

compare. There are standards, such as ASTM G76 [2], but the test results are not always presented with a detailed description of the test coating specimens. Not uncommonly, the testing deviates from the standard method which again makes it difficult to compare results between separate investigations. A similar conclusion in the case of abrasive wear resistance after HVOF/HVAF spraying was presented by Dizdar et al. [3]. The abrasive wear ranking of HVOF and HVAF sprayed test samples of three commercial Fe-based powder alloys using ASTM G65 dry sand rubber wheel testing [4]. HVAF compared to HVOF spraying resulted generally in finer microstructure and less wear of the sprayed test samples. The ranking order achieved was Fe SP586, 6AB and Fe SP529. Electrolytic hard chrome (EHC) plating was used as a reference since many HVOF/HVAF spray coatings target replacing EHC plating. However, ASTM G65 test results on the EHC plating showed a dependence on the plating microstructure, a point often not well described. This investigation takes a next step and ranks the three commercial test samples HVOF and HVAF sprayed Fe-based powder alloys [3] in respect to solid particle erosive wear. The ranking method is ASTM G76 solid particle erosion testing, and the reference coating is electrolytic hard chrome (EHC) plating. It is hoped this will function as a first pre-study when prototyping erosion exposed components using HVOF/HVAF spraying of Fe-based powders.

Solid Particle Erosion and Test Method

Solid particle erosion or solid particle erosive wear is a term for unwanted material loss from a body surface exposed to successive impact actions of solid particles travelling at substantial velocities [5]. Figure 1 illustrates impact of a solid particle onto a coated surface. The particle mass is relatively small, commonly less than 100 g and the impact velocity exceeds 1 m/s. For ductile coating materials and relative small impact angles, such as 15° to 30°, the impacting particles plow the surface and material loss is mostly chip formed due to the cutting action of the particles. Here one sees similarities with abrasion but the normal contact load is due to impact and the

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velocity is substantially higher. On the other hand, for brittle coating materials and impact angles close to 90°, the impact results in brittle fracture of the surface followed by detachment of sharp surface fragments. These two examples illustrate the large erosion loss that occurs at low impact angles for ductile materials respective 90° impact angles for brittle materials. Accordingly, it is a suggestion on how to select material for erosion exposed applications. Work such as Finnie [6][7][8] for example can provide some deeper understanding of the solid particle erosion phenomenon. Hutchings [9] reviews almost 40 papers on erosion and shows that erosion is much more complex than simplified ductile - brittle behavior. Hard materials can behave as brittle at room temperature but as ductile at higher temperatures. This is typical for ceramics. Shape, size and hardness of the erodent particles also play a role. The said factors and these interactions determine if the particle impact will cause elastic rebound from the body surface or its plastic indentation, Hertzian fracture, lateral fracture, or erodent particle fracture or a combination of those. Solid particle erosive wear testing was performed according to ASTM G76 “Standard Test Method for Conducting Erosion Tests by Solid Particle Impingement Using Gas Jet“[2]. Figure 2 shows a schematic of the testing. Alumina (Al2O3) sand with mean particle size of approx. 50 µm (240 mesh) flows by gravity or a feed system to a mixing chamber where the sand is added to a pressurized air stream. The alumina stream with a rate for 2 g/min is then supplied to a 1.5 mm ID, 50 mm long tubular nozzle for blasting of a test specimen at a working distance of 10±1 mm. The testing is by default performed at an impact angle of 90° and velocities of 30 and 70 m/s. The latter velocity inserts high kinetic energy into the impacting particles and is accordingly suitable for material with very high erosion resistance. The chart in Figure 2 shows mass loss as a function of eroding time for a typical hard coating erosion test trial. Mass loss is commonly directly proportional to eroding time, as long as the mass loss occurs in the coating layer. When the erosion fully removes (or penetrates) the coating, the mass loss rises due to the lower erosion resistance of the softer base metal. The mass loss versus time relationship is in common first order line, and its slope is taken as (steady state) erosion rate. Observe that the line is virtually not starting from the origin. The erosion rate at zero eroding time is of course zero, but the erosion process stabilizes after approximately two minutes [2]. Accordingly, there is an intercept on the volume loss axis. Erosion test results can also be reported as average erosion value in mm3/g – this is erosion rate divided by the abrasive flow rate and by the specimen density. In other words, the average erosion value is volume loss of the tested material per unit mass of the erodent.

Figure 1. Impact schematic for ductile and brittle coating

materials.

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Figure 2. An illustration of ASTM G76 particle erosion testing.

Experimental Method

Test specimens in this investigation were manufactured at the same time as specimens from Dizdar et al. [3], only the specimen length and width are adjusted to fit another test rig. Table 1 lists Fe-based powder alloys used in this investigation, Table 2 lists test specimens and their manufacturing routes. Figure 3 shows test specimens’ hardness HV0.3. The test specimens were manufactured using HVOF/HVAF spraying onto shot-peened 8 mm thick plane blanks made of EN S355 MC, high strength cold-formed steel, followed by water jet cutting to 25x58 mm coupons and plane grinding in a knife grinding machine with cup formed grinding wheel. The top layer in this investigation was removed by fine grinding using a movable head grinder to achieve HVOF/HVAF sprayed coatings with reasonably equal bonding between all sprayed layers for the purpose of a first model erosion testing.

Table 1. Typical composition of Fe-based powder grades

Designation FeSP529 FeSP586 6AB

Ele

men

tal c

om

posi

tion

(w

t%)

C 1.9 0.7 2.1 Mo 5.8 - - Cu 1.5 - Ni 4.2 12.3 5.9 V 0.2 Fe (bal) (70.7) (52.3) (70.9) Mn 0.2 - 0.3 Cr 9.7 30.6 14.4 Si 2.1 0.6 3 B 3.7 3.5 3.4

Sieve cut: HVOF-DJH, -45+20µm, HVAF-M3, -36+20µm Table 2: Test specimens and their manufacturing route

Powder alloy* Ref.** 1 2 3 4

Fe SP529 Fe SP586 6AB EHC Deposition HVOF (by Tampere Univ.) EHC

Bath Plating HVAF (by Univ. West)

Substrate Hot rolled structural steel EN S355 MC

Steel EN C45

Coating thickness

Approx. 350 µm 11 layers x 15 µm

250 µm

Test surface finish

Fine ground on a moving head grinder. Top 15 µm removed from the coating. Surface roughness Ra 0.5 to 1 µm.

* Höganäs AB, Höganäs, Sweden ** Swechrom AB, Borlänge, Sweden

Figure 3. Hardness HV0.3 of the test samples. HVOF spraying was performed at Tampere University using DJH2700 system. HVAF test spraying of the samples was performed at University West using HVAF-M3 system by Unique-coat Technology. HVOF spraying is considered more suitable for WC–Co–Cr, Co– and Ni–based consumables because of the particle flame temperature of around 3000°C so that refractory constituents will soften enough to adhere to the

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target surface and the matrix will not oxidize. HVAF spraying on the other hand has higher particle velocities and lower particle flame temperature so that Fe-based consumables can be used with reduced risk of oxidizing. The top coating layer sprayed with HVOF/HVAF is known to be less homogeneous in comparison to the layers below due to the lack of so called “peening effect“ The top layer was therefore removed by plane-grinding on a moving head /knife grinding machine. A sharp diamond grinding cup wheel was used with circulation of cooling fluid to minimize heat impact on the sample structure. This grinding removed about 15 µm from the top of the coating but resulted in no additional densification or homogenization of the coated sample surface.

The quality of ready–to-test samples is defined by microstructure cross cut of the samples shown in Figure 4, with XRD analysis in Figure 5. The microstructure in Figure 4, shows typical voids and oxide stringers for HVOF FeSP529 and 6AB samples. HVAF samples of the same powder show the same features but to a lower extent. Samples of FeSP586 powder, show the best overall microstructure with lowest content of voids and oxide stringers. XRD analysis in Figure 5 shows matrix and hard phase constituents of the powder and sprayed coatings. Taking into account the limitations of XRD analysis and Thermocalc simulation, there is good agreement on the austenitic matrix for FeSP586 powder. The ferritic/austenitic matrix of 6AB powders contains hard phase constituents in the form of carbides and borides.

Figure 4. Typical SEM BSE microstructure cross cut photographs of test samples [3].

Figure 5. XRD analysis of the powder and sprayed specimens [3].

ITSC 2015 – Int. Thermal Spray Conf., Long Beach, CA, USA, May 11-14, 2015. 5

Results

Figure 6 presents erosion rate and erosion intercept ranking achieved by ASTM G76 solid particle erosion testing at 30 m/s erosive particle velocity. Both erosion rate and intercept were evaluated by plotting mass loss versus eroding time. For EHC plated samples, unlike those sprayed, erosion penetrates the coating after 20 minutes test run. Three specimen batches, HVAF sprayed 6AB test samples with 0.02 mg/min, and both HVOF and HVAF sprayed Fe SP586 samples respectively with 0.03 and 0.04 achieved the same order as the reference EHC plated samples with 0.03 mm/min in erosion rate. On the other side, HVOF sprayed 6AB samples with 0.07 mm/min, and HVOF/HVAF sprayed Fe SP529 samples with respectively 0.08 and 0.10 mg/min achieved roughly double to triple the erosion rate in comparison to the reference and the low erosion rate samples. Considering the intercept, the outcome is different. This is most likely due to a large influence of local topography of the top coating layer where the erosion crater took place. These results can therefore have a lower confidence level than erosion rate. The reference EHC plated samples achieved 0.1 mg intercept value and one has to note that these samples unlike sprayed samples have a relatively high cohesion of the coating. The low erosion rate samples, 6AB HVOF and Fe SP586 HVOF sprayed achieved relatively high intercept values of 0.6 mg in contrast to 0.3 mg for Fe SP586 HVOF sprayed. The high erosion rate sprayed samples achieved intercept values of 0.3 mg and 0.2 mg. Figure 7 illustrates the profile of the eroded hole of the selected test samples eroded for 10 minutes. Note, the profile depth is the results of both erosion rate and erosion intercept. The erosion rate and intercept outcome needs to be related to the specimens’ microstructure (Figure 4). Fe SP586 HVAF and HVOF sprayed samples achieved the finest microstructure and low erosion rate. The former achieved low but the latter high intercept. For 6AB samples it was the opposite, HVAF sprayed low erosion rate samples achieved high intercept and vice versa. Fe SP529 both HVAF and HVAF sprayed samples achieved high erosion rate but low intercept. The bottom of the erosion crater was analysed by SEM, both in the top and metallographic cross-cut (Figure 8). All samples shown were tested for 10 minutes. All top view SEM photographs show evidence of plastic indentation and cutting chips, but size, distribution and number of scratches vary between the samples. Looking at the metallographic cross-cut photographs, one gets an explanation for the top view appearance of the samples. Fe SP529 samples include relatively many oxide stringers and these initiate and accelerate the growth of lateral cracks. In contrast, 6AB, HVAF sample includes much less severe oxide stringers and this resulted in a relatively flat crater bottom.

Figure 6. ASTM G76 solid particle erosion testing – erosion rate for coated test samples.

Figure 7. ASTM G76 solid particle erosion testing – profile of the eroded hole of selected test samples eroded for 10 minutes. The crater bottom of reference EHC plated specimen shows, of course, a very different appearance. The plating includes small vertical cracks of about 2 to 4 µm in length. V-shaped crater in-crater grew along the cracks and is going to reach the base metal quite soon if the erosion continues. The crater-in-crater growth is also assisted by a lack of bonding between the plating and the base metal

ITSC 2015 – Int. Thermal Spray Conf., Long Beach, CA, USA, May 11-14, 2015. 6

Fe SP529, HVAF

6AB, HVAF

EHC plating

Fe SP529, HVAF

6AB, HVAF

EHC plating

Figure 8. SEM analysis of the eroded crater bottoms with top view (left column) and in metallographic cross-cut (right column) and All samples were tested according to ASTM G76, for 10 minutes under 30 m/s.

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Discussion

Solid particle erosion ASTM G76 testing of the HVOF and HVAF sprayed coatings with Fe-based powder highlights the importance of both erosion rate and erosion intercept since both determine erosion resistance of a sprayed coating. Erosion intercept stands for erosive wear before erosion reaches steady state. In some way it corresponds to running-in or breaking-in sliding adhesive wear contacts. Both can represent a significant part of total wear and even lead to severe wear if improperly handled. To reduce and control the erosion intercept one should consider use of surface finishing methods which will increase the bonding of the top coating layer to layers below. Shot-peening, ball-point burnishing, or even polishing will increase the bonding strength. As previously stated, the top layer, in this investigation, was removed by fine grinding using a movable head grinder with the intention to achieve HVOF/HVAF sprayed coatings with reasonably equal bonding between all sprayed layers for the purpose of a first model erosion testing. However, it motivates further investigation to evaluate erosion performance of the sprayed coatings with surface finishing aimed to increase the bonding. Testing performed in this investigation is model testing ASTM G76 which serves as a base for the next step – prototype testing. However, prototype testing deals with real components often more opportune for thermal spraying than the plane specimens. Particular attention needs to be put on surface finish of the sprayed coatings as discussed here. Erosion rate and HV0.3 hardness are not in agreement. The hardest sprayed sample, 6AB HVAF, achieved lowest erosion rate too, but this is the only common point. The rankings match for specimens made of one alloy for Fe SP529 and 6AB but not for Fe SP586 alloy, which otherwise shows smallest difference in structure between HVOF and HVAF spraying.

Conclusions

Three Fe-based powder alloys have been deposited by HVOF and HVAF spraying onto plane EN 355 MC steel specimens. Test samples were machined from these coated plates and tested for solid particle erosive wear according to ASTM G76. The following conclusions were drawn:

HVAF sprayed 6AB test samples with 0.02 mg/min, and both HVOF and HVAF sprayed Fe SP586 samples respectively with 0.03 and 0.04 achieved the same order as the reference EHC plated samples with 0.03 mm/min in erosion rate.

HVOF sprayed 6AB samples with 0.07 mm/min, and HVOF/HVAF sprayed Fe SP529 samples with 0.08 and 0.10 mg/min respectively achieved roughly double to triple the erosion rate in comparison to the reference EHC plated samples.

Erosion intercept i.e. “running-in” showed another ranking order than erosion rate due to influence of local topography in the top coating layer where the erosion crater occurs. These results are therefore considered to have a lower confidence level than erosion rate.

Further Work

This investigation indicates further study is required to understand:

Effect of surface finish to increase the layer bonding strength by shot peening, ball-point burnishing or polishing on erosion performance.

Erosion mechanisms before erosion reach steady state.

Erosion testing with particle impact angle 15 to 30°.

Acknowledgments

The author appreciates efforts of Christophe Lyphout, University West, Trollhättan, Sweden, Andrea Milanti, Heli Koivuluoto and Petri Vuoristo, Tampere University of Technology, Tampere, Finland, for cooperation in manufacture of test specimens and valuable discussions.

References

[1] Fauchais, P.L., V.R. Heberlein, J.V.R, Boulos, M., Thermal Spray Fundamentals: From Powder to Part., Springer, New York, 2014.

[2] ASTM G76 - Standard Test Method for Conducting Erosion Tests by Solid Particle Impingement Using Gas Jet; ASTM Int. 2010.

[3] Dizdar, S., Lyphout C., Milanti A., Koivuluoto H., Vuoristo, P., “Fe-Based Powder Alloys Deposited by HVOF and HVAF for Abrasive Wear Applications” ITSC 2014 – 2014 Int. Thermal Spray Conf., Barcelona, Spain, May 2007.

[4] ASTM G65 - Standard Test Method for Measuring Abrasion Using the Dry Sand/Rubber Wheel Apparatus, ASTM Int., 2010

[5] Kosel, T.H., Solid Particle Erosion, Friction, Lubrication and Wear Technology, Vol. 18, ASM Handbook, ASM Int., 1992, 199-213

[6] Finnie, I., “Erosion of Surfaces by Solid Particles”, Wear, 3 (1960) 87–103.

[7] Finnie, I., Kabil, Y. H., “On the Formation of Surface Ripples during Erosion”, Wear, 8 (1965) 60–69.

[8] Finnie, I., “Some Reflections on the Past and Future of Erosion”, Wear, 186-187 (1995) 1–10.

[9] Hutchings I.M., Transitions, Threshold Effects and Erosion Maps, Key Eng. Mater., 71 (1992) 75-92.

[10] Hutchings, I.M., “Abrasive and Erosive Wear Tests for Thin Coatings: A Unified Approach”, Trib. Int. 31,(1998) 5–15.