Keywords

1 Introduction

Solid particle erosion has been reported as the cause of material degradation in many mechanical systems, such as aircraft gas turbines, thermal power plants, industrial waste incinerators, and many others[1], and superalloys have been found to be a better option for the components of mechanical systems that will be exposed to severe material degradation conditions, as it has been a major concern in recent research topics. From the observations made in the literature, there are mainly two categories of erosion mechanisms, namely brittle and ductile erosion, and they are further classified as deformation mechanisms and cutting mechanisms [2,3,4]. Under oblique impact conditions, the cutting mechanism is the most effective mode of erosion, whereas, under normal impact conditions, the deformation mechanism is the major cause of the erosion [5]. The effectiveness of each of the mechanisms is directly proportional to the tangential and normal components of the impact energy of the erodent, which also depends on the angle of impact [6]. Many parts of the turbine are likely to be subjected to intense mechanical and thermal loads; they will also be exposed to erosion. Hence, for a single material, it would be tough to cope with the desired properties. Hence, composite material is the best solution for satisfying demanding requirements [7,8,9,10]. Prashanth Kumar Singh et al. used a detonation gun spray technique to coat SAE213-T12 boiler steel with WC-12CO, Stellite 21, and Stellite 6 coatings, and then tested them for solid particle erosion. The cobalt component of the coating was revealed to play a significant influence in erosion resistance, with Stellite 21 and Stellite 6 coatings exhibiting 50 to 60% greater erosion resistance than WC–Co [1].

Aravind Nagaraj et al. [11] reported studies on solid particle erosion of the nickel-based superalloy CY5SnBiM at particle velocities of 59, 92, and 124 m/s and at four different angles of impact, namely 30°, 45°, 60°, and 90°. The results show that the mode of erosion is ductile, with deep pits and craters formed by rebounding erodent particles at higher angles of impact. Mustafa Kaplan et al. [12] in an investigation reported the erosion behaviour of Inconel718 superalloy using two different coatings: CoNiCrAlY and ZrO2 + 8% Y2O3. Following the air jet erosion test, it was discovered that the largest erosion rate occurred at a 60° impact angle, indicating a semi-ductile/semi-brittle mode of erosion. Mayank Patel et al. [13] reported solid particle erosion investigations of boiler tube steel SS304 at room temperature with alumina of size 50 m, impact velocity of 40 m/s, and impact angles of 30° and 90°. And results indicate the erosion rate is higher at 30° angle of impact than at 90° confirming the ductile mode of erosion. SB Mishra et al. [14] in a study employed three types of coatings on Nickel based superalloys in their study: NiCrAlY, Ni-20Cr, and Ni3Al, and subjected them to solid particle erosion tests. The results showed that the coating Ni3Al had the lowest rate of erosion at 30° and 90° angles of impact. The coating Ni-20Cr has the highest rate of erosion.

Shibe et al. [15] investigated the erosion properties of three distinct coating types, including WC-12%CO, Cr3C2-25%NiCr, and Al2O3-13%TiO2, on ASTM36 steel and uncoated ASTM36 steel, and found that all three types of coatings were effective in protecting the base metal from solid particle erosion at 45° impact angles, while WC-12%CO, Cr3C2-25%NiCr coatings were effective in protecting the substrate. Sidhu et al. [16] investigated the erosion behaviour of bare and HVOF spray coated boiler tube steel (GrA1) at 250 °C and results indicate that the coating's hardness is better than bare steel, and that the coating's material loss is greater than uncoated boiler tube steel. Pauzi et al. [17] reported the wear effects in hot-gas-path components and different hard face coatings are discussed in depth. Wear is identified as a major issue in gas-turbine hot-gas-path components. Erosion-resistant coatings, especially on turbine blades, may extend the life of hot-gas-path components. In the present work, an attempt has been made to compare and evaluate the solid particle erosion behaviour of special alloy Ti-31 and Superco-605 superalloy.

2 Experimental Details

2.1 Superalloy Materials

In present experimental work, Superco-605 (a cobalt based superalloy, ASTM B338 grade 5) and Ti-31 (ASTM 9009) superalloy are chosen as the workpiece material. The material was procured from Mishra Dhatu Nigam Limited (MIDHANI), Hyderabad, India. The material is selected due to its extensive application in the manufacturing of turbine components which demands a very high resistance to erosion wear. The chemical composition of the material is given in Table 1. The material was cut in the dimension of 25mmW × 5mmH × 25 mmL to perform the experiment.

Table 1 Chemical composition of material
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2.2 Solid Particle Erosion Test

The erosion test is carried out as per ASTM G76 standard with the help of Air jet Erosion rig shown in Fig. 1. The process parameter used to perform the test is listed in Table 2. To start, the sample is first cleaned using acetone to remove any dirt and dust. Using an electronic weight balance (accuracy: 0.001gm), the sample's initial weight is recorded. The sample is then located in the test rig's sample holder. The erosion test is performed at different impact angles 30°, 60°, and 90° with constant particle velocity of 40 m/s. The pressure of air jet was maintained to 1 bar throughout the test. After each cycle of operation, the weight loss and cumulative weight are recorded. This procedure is repeated until the erosion rate reaches to a steady state. From the recorded data, the following formulas are used to compute erosion rate and steady state volumetric erosion rate.

Fig. 1
A diagram of the air jet erosion rig. The labeled components are air intake, mass flow, particle feeder, conveyor belt, sample, and sample holder.

Schematic diagram of air jet erosion test setup

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Table 2 Process parameter for erosion test
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Erosion rate (g/g) = Cumulative weight loss of sample/ Mass of erodent

Steady state volumetric rate of erosion (cm3/g) = Average of erosion rate/ Density

3 Results and Discussion

3.1 Studies of Erosion Mechanism of Ti-31

Figure 2 shows an optical micrograph of the scar formed by erosion of a Ti-31 sample at various impact angles. The enclosed region of the micrograph depicts the material loss on account of erosion which is immediately followed by elastically loaded material zone. Figure 2 shows the erosion rate and steady state volumetric erosion rate graphs. Figure 2a shows an increasing erosion rate at a 30° impact angle and a minimum at a 90° impact angle, Similarly, as demonstrated in Fig. 2b, steady state volumetric erosion rate is highest at 30° and lowest at 90° angle of impact, which is the trend generally observed in materials that exhibit ductile mode of erosion. The variation in erosion rate with cumulative mass of erodent is shown in Fig. 3a. Also, the variation in steady state volumetric rate of erosion at different impact angles is shown in Fig. 3.

Fig. 2
3 micrographs of coarse light-shaded surfaces of T i 31 samples that have central depressions with areas A and B marked. a. An oval depression. b. A large circular depression. c. A small circular depression.

Images of scar developed due to erosion on Ti-31 sample at a 30° angle of impact, b 60° angle of Impact, c 90° angle of Impact

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Fig. 3
A multiline and a bar graph for T i 31. a. Erosion rate versus cumulative mass of silica erodent. Lines for 30 and 60 degrees decrease while 90-degree impact angle increases. b. Steady-state volumetric erosion rate versus impact angles. 90 degrees has the highest rate followed by 60 and 30 degrees.

Plots for Ti-31 special alloy exposed to erosion at three different angles of impact, a erosion rate curves; b bar chart of steady state volumetric erosion rate

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3.2 SEM/EDS Analysis

The SEM micrograph of the encircled region is taken to depict the mechanism of material removal involved in the erosion process and is shown in Fig. 4. SEM micrographs of scars developed on Ti-31 samples due to erosion at a 30° impingement angle are shown in Fig. 4a, and material removal is seen as actions like ploughing, formation of loose debris, and grooves. SEM images of the erosion scar obtained on special alloy Ti-31 for a 60° impingement angle are shown in Fig. 4b, and material removal is seen as ploughing and crater formation, and the crater is formed when embedded sand particles are dropped from the surfaces. When the samples are subjected to erosion at a 90° angle of impact, the micro structural features are as shown in Fig. 4c. It is obvious from the micro graph that ploughing action, erodent entrapment, and groove development occur. EDS examination (ref Fig. 5) on the erosion scar developed on a sample subjected to erosion at 30° impact angle reveals that on the surface there is 46.01 wt % Ti as the major phase and 10.45% of C, 4.85% Al as minor constituents, and the presence of 7.59% Si confirms the entrapment of sand particles in the substrate.

Fig. 4
9 S E M micrographs present coarse surfaces of superalloy T i 31. a. Cracks, craters, and plowing are indicated for the 30-degree angle of impingement. b. Plowing, up, and grooves are indicated for the 60-degree angle of impingement. c. Plowing, grooves, and craters are indicated for 90 degrees.

SEM image of scar developed due to erosion on superalloy Ti-31 at 30°, 60°, and 90° angle of impingement

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Fig. 5
A micrograph of the coarse surface of a T i 31 sample, eroded at a 30-degree impact angle. A table lists elements and weight percentages at an indicated point on the surface. T i is at 46.01% followed by O at 30.74%, C at 10.81%, S i at 7.59%, and A l at 4.85%.

EDS analysis of Ti-31 sample eroded at 30° impact angle

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3.3 Erosion Behaviour of Superco-605 Superalloy

Figure 6a–c shows photographs of the erosion scar generated on the Superco-605 sample at 30°, 60°, and 90° impedance angles, respectively. The eroded scar's middle (A) symbolises a limited zone of material deterioration that is surrounded by an elastic loading area B). The erosion rate and steady state volumetric erosion rate are calculated; Fig. 7 shows the erosion rate curves as well as the steady state volumetric erosion rate bar chart. The steady state volume erosion rate is found to be greatest at 90° impact angles and lowest at 30° impact angles (Fig. 6b). This is the typical behaviour of materials that have a brittle mode of erosion.

Fig. 6
3 macrographic images of uncoated Superco 605 surfaces with central dark patches that have areas A and B indicated. a. A large elliptical dark patch at the center. b. A small oval patch. c. A circular patch.

Macrographs of uncoated Superco-605 being eroded at different angles of impact. a 30°, b 60°, c 90°

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Fig. 7
A multiline and a bar graph for Superco 605. a. Erosion rate versus silica erodent cumulative mass. Lines for 30 and 60 degrees decrease, and 90-degree impact angle increases. b. Steady-state volumetric erosion rate versus impact angles. 90 degrees has the highest rate followed by 60 and 30 degrees.

a Plot of erosion rate and; b steady state volumetric erosion rate of Superco-605 superalloy for all three angles of impact

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3.4 SEM with EDS Analysis

The scar generated on uncoated Superco-605 at a 30° impact angle is shown in micro structural detail in Fig. 8a. Ploughing, debris trapping, and groove creation are all visible damage in the microstructure. Similarly, Fig. 8b depicts microstructure features in a scar created by a 60° impact angle, which reveals material degradation in the form of ploughing, groove creation, and craters. Craters may occur when erodent particles are dislodged from their entrapment. Micrographs of material damage at a 90° erodent impact angle are shown in Fig. 8c. Ploughing causes material degradation, crater creation, and erodent particle trapping. EDS analysis (refer to Fig. 9) of the erosion scar on a sample that was eroded at a 30° impact angle shows that the surface contains 50.96 wt.% Co as the major phase, 19.45 wt.% C, 17.79 wt.% Cr as minor constituents, and 10.45wt.% Si, which shows that sand particles have been trapped in the substrate.

Fig. 8
9 S E M micrographs present coarse surfaces of Superco 605. a. Plowing, craters, and cracks are indicated for the 30-degree impact angle. b. Grooves, plowing, and craters are indicated for the 60-degree impact angle. c. Plowing, grooves, and craters are indicated for the 90-degree impact angle.

SEM images of erosion scar developed on Superco-605 at impact angles of 30°, 60°, and 90°

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Fig. 9
A micrograph of the coarse surface of a Superco 605 sample, eroded at a 30-degree impact angle. A table lists elements and weight percentages at an indicated point on the surface. T i is at 46.01% followed by O at 30.74%, C at 10.81%, S i at 7.59%, and A l at 4.85%.

EDS analysis of Superco- 605 sample eroded at 30° impact angle

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4 Conclusions

  1. 1.

    In the instance of Ti-31, erosion is higher at 30 degrees and less at 90 degrees. This is the typical behaviour of material that erodes in a ductile mode.

  2. 2.

    The erosion rate of Superco-605 is greater at 90° and lower at 30° angle of contact, indicating that the mechanism of erosion is brittle.

  3. 3.

    Material loss in the form of ploughing, groove, and crater development can be seen in SEM micrographs of eroded samples of Ti-31 and Superco-605.

  4. 4.

    EDS analysis reveals silica sand particle penetration into the substrate, followed by crater formation as a result of entrapped sand particle drop-off from the substrate.