3 stepwise

Y. M. Abd-ElrhmanMechanical Engineering Department,

Faculty of Engineering,

Assiut University,

Assiut 71516, Egypt

e-mail: [email protected]

A. Abouel-Kasem1Mechanical Engineering Department,

Faculty of Engineering-Rabigh,

King Abdulaziz University,

P.O. Box 344,

Rabigh 21911, Saudi Arabia;

Department of Mechanical Engineering,

Assiut University,

Assiut 71516, Egypt


S. M. AhmedMechanical Engineering Department,


Majmaah University,

P.O. Box 66,

Majmaah 11952, Saudi Arabia


K. M. EmaraMechanical Engineering Department,


Assiut University,

Assiut 71516, Egypt


Stepwise Erosion as a Methodfor Investigating the WearMechanisms at Different ImpactAngles in Slurry ErosionIn the present work, stepwise erosion technique was carried out to investigate in detailthe influence of impact angle on the erosion process of AISI 5117 steel. The number ofimpact sites and their morphologies at different impact angles were investigated usingscanning electron microscope (SEM) examination and image analysis. The tests werecarried out with particle concentration of 1 wt. %, and the impact velocity of slurrystream was 15m/s. Silica sandwhich has a nominal size range of 250355lmwasused as an erodent, using whirling-arm test rig. The results have shown that the numberof craters, as expected, increases with the increase in the mass of erodent for all impactangles and this number decreases with the increase of the impact angle. In addition, thecounted number of craters is larger than the calculated number of particles at any stagefor all impact angles. This may be explained by the effect of the rebound effect of par-ticles, the irregular shape for these particles, and particle fragmentation. The effect ofimpact angle based on the impact crater shape can be divided into two regions; the firstregion for h 60 deg and the second region for h 75 deg. The shape of the craters isrelated to the dominant erosion mechanisms of plowing and microcutting in the firstregion and indentation and lip extrusion in the second region. In the first region, thelength of the tracks decreases with the increase of impact angle. The calculated sizeranges are from few micrometers to 100 lm for the first region and to 50lm in the secondregion. Chipping of the former impact sites by subsequent impact particles plays an im-portant role in developing erosion. [DOI: 10.1115/1.4026420]

Keywords: slurry erosion, impact angle, number of impact sites, eroded surface, erosionmechanisms, AISI 5117 steel, particles rebound effect

1 Introduction

Erosive wear is a process of progressive removal of materialfrom a target surface due to repeated impacts of solid particles.The particles suspended in the flow of solidfluid mixture erodethe wetted passages, limiting the service life of equipment used inmany industrial applications, such as oil field mechanical equip-ment, solid-liquid hydrotransportation systems, hydroelectricpower stations and coal liquefaction plants, and industrial boilerswhere coal is carried directly as a fuel in water or oil, as reportedin Refs. [16].Slurry erosion (solid-liquid erosion) is a complex phenomenon,

and it is not yet fully understood because it is influenced by manyfactors, which act simultaneously. These factors include flowfield parameters, target material properties, and erodent particlecharacteristics. Among these parameters, the impact angle andmicrostructure of the target material play an important role inthe material removal process, based on the data reported inRefs. [711]. Materials are characterized as either ductile or brittleaccording to dependence of their erosion rate on the angle ofattack curves [79]. For ideal ductile material, the erosion rateincreases from zero at 0 deg impact angle to a maximum when theangle of impact is increased to between 30 deg and 50 deg. Afterreaching the maximum, the erosion rate decreases to a minimum

value at 90 deg. The effect of impact angle erosion mechanismsof 1017 steel and high-Cr white cast iron using a slurry whirling-arm test rig were carried out by the authors [12]. Test resultsshowed that the effect of impact angle on erosion mechanisms of1017 steel has three regions. In the first region (h 15 deg) shal-low plowing and particle rolling were the dominant erosion mech-anisms, microcutting and deep plowing in the second region(15 deg< h< 75 deg), while indentations and material extrusionprevailed in the third region (h 75 deg). For high-Cr white castiron, the test results showed that the erosion mechanisms involvedboth plastic deformation of the ductile matrix and brittle fractureof the carbides. At low impact angles (up to 45 deg), observationsof microphotographs of the impacted surfaces revealed that plasticdeformation of the ductile matrix was the dominant erosion mech-anism and the carbides fracture was so negligible that it led tosmall erosion rate. Whereas at high impact angles (greater than45 deg), gross fracture and cracking of the carbides were the mainerosion mechanisms in addition to indentation with extruded lipsof the ductile matrix.Many researchers [1318] have been working on the erosion

process modeling, using different modeling techniques. In theearly stages of the modeling of the erosion process, Finnie devel-oped mathematical model mechanisms for ductile metals [13] andsolid surfaces [14] for a single particle impingement; Bitter devel-oped erosion models [15,16] based on plastic deformation, cut-ting, Hertzian contact theory, and energy balance equationapproaches; Hutchings [17] developed a theoretical model basedon a plastic strain approach for metals by exposure to spherical

1Corresponding author.Contributed by the Tribology Division of ASME for publication in the JOURNAL OF

TRIBOLOGY. Manuscript received April 5, 2012; final manuscript received December17, 2013; published online February 19, 2014. Assoc. Editor: Zhong Min Jin.

Journal of Tribology APRIL 2014, Vol. 136 / 021608-1CopyrightVC 2014 by ASME

Downloaded From: http://tribology.asmedigitalcollection.asme.org/ on 02/24/2014 Terms of Use: http://asme.org/terms

particles at normal impingement; and Hashish [18] modifiedFinnies models to include the effect of the particle shape. Geeet al. [19] used a stepwise testing method for determining themechanism of gas-borne particulate erosion. Stepwise testing isan approach that has recently been developed as a way of provid-ing information on the buildup of damage in erosion. The essenceof the method is to incrementally expose a sample to erosion fromsmall quantities of erodent and then be able to accurately relocatethe test sample in the SEM to enable the same area to be examinedat high magnification, allowing for the development in damage tobe followed up at specific points on the sample surface.In the present work, the stepwise erosion combined with reloca-

tion SEM was used to follow up the real erosion processes withimpact angle in considerable detail. Based upon the SEM images,the size and the shape factor of the impact sites were analyzed. Aswell, the material removal processes were investigated.

2 Experimental Details

Slurry-erosion tests were carried out using a slurry whirling-arm rig, which is schematically shown in Fig. 1. The rig consistsof three main units: a specimen rotation unit, a slurry unit, and avacuum unit. Full description of this rig and how it works as wellas its dynamics are found in Refs. [12], [20], and [21]. Thespecimen-rotation unit provides impact velocity. Two specimensof 23mm 10mm 10mm are clamped to two specimen hold-ers. During slurry-erosion tests, only the upper surface of speci-men is exposed to the impinging slurry, since the sides of thespecimen are held by the specimen holder. The specimen holdershave tilting and locking facilities to adjust the required inclinationof the test specimen. As well, the position of the sample wascarefully marked so that the sample could be replaced in the sameposition in the erosion system. The impact angle can be adjusted

to any required value by rotating the specimen holder around itshorizontal axis, as shown in Fig. 2.The holders are mounted on the ends of the two arms of the

rotator, which is driven by a variable-speed motor.The slurry whirling-arm rig used in this work provides a

homogenous stable slurry stream (a mixture of tap water andSiO2). The velocity of falling slurry stream from the 3-mm-diame-ter funnel orifice is 1.62m/s at the specimen surface, impactingevery specimen at any preset angle between 0 deg and 90 deg.The impact angle (h) and impact velocity (v) are correlated toensure the intended value, which can be obtained from the veloc-ity vector diagram of particle impact, as shown in Fig. 2. The dis-tance between the funnel orifice and the specimen surface is40mm. The slurry test chamber is evacuated by a vacuum system(up to 28 cm Hg) to eliminate aerodynamic effects on slurrysystem.The test specimens were made from a commercial grade of

alloy steel, namely AISI 5117. This type of alloy steel is usedbecause it provides best machinability and behaves well during

Fig. 1 Schematic diagram of the slurry erosion whirling-arm rig [12]

Fig. 2 Schematic diagram of impact velocity and impactangle [12]

021608-2 / Vol. 136, APRIL 2014 Transactions of the ASME


heat treatment and quenching with respect to distortion, internalstresses, and mechanical properties of surface and core. Thechemical composition and mechanical properties of the specimenmaterial are listed in Tables 1 and 2, respectively [22].To achieve identical initial condition for each test, the speci-

mens working faces were polished with SiC paper successivelydown to 4000 grit. The resulting average surface roughness, Ra,was 0.03lm. Wear specimens were cleaned with acetone anddried with air blower before and after each test. Mass loss of thewear specimens was measured in an electronic balance of100 g6 0.1mg.Natural silica sand sieved to a nominal size range of

250355 lm was used as an erodent. A SEM photograph of typi-cal sand particles is shown in Fig. 3. These particles were charac-terized [23] using an image analysis method in terms of the aspectratio (W/L) and roundness factor (P2/4pA), where W is the particlewidth, L is the particle length, A is the projected area of the parti-cle, and P is its perimeter. The statistical values of the particle pa-rameters are given in Table 3.Since the properties of solid particles are of great importance, a

single source of erodent particles was used throughout the experi-ments. Also, fresh particles were used in each test to avoid anydegradation of impacting particles during erosion tests. In theseseries of tests, the particles concentration was held at 1wt. % andthe impact velocity of slurry stream was 15m/s.The difference between the apparatus used in the current study

(slurry whirling-arm rig) and the other apparatus used in this field

of study is the absence of dependence on time in the present appa-ratus regarding the comparison among the different impact angles.As shown in Fig. 2, the amount of particles that impacts the sur-face of specimen differs from one angle to another. Consequently,comparison of the mass loss with the impact angle curves at thesame test time will give misleading results. Therefore, in studyingthe effect of impact angle on slurry-erosion processes, the testswill be performed at the same amount of particles that impacts thespecimens at different impact angles. The amount of particles thatimpact the surface of specimen as a function of the impact angleis derived from the geometry of the impacting process, as shownin Fig. 2 [12].The mass of particles striking each specimen per revolution is

given by

mp l sinho An lcoshoQpDN

Cwqw (1)

where:ho is the angle between the surface plane of the specimen and

the horizontal plane; l is the length of wear specimen surface inm; An is the area of orifice in m

2; Cw is the weight fraction of solidparticles in the water; qw is the water density in kg/m

3; D is therotational diameter of the wear specimen m; Q is the volume flowrate of slurry in m3/min; and N is the rotational speed of the wearspecimen in rpm.To implement the stepwise erosion approach, the specimens

were exposed to aliquots of erodent. Preliminary tests showed thatthe suitable size of aliquot was 1.3 g, since it was able to makevisible and isolated individual impact events at different impactangles. This small aliquot means that solid particles were mixedwith the tap water to form the slurry stream. Then, this slurrystream was left to strike the upper surface of specimen(23mm 10mm) for a specific time depending on the value ofimpact angle. Finally, the total amount of the solid particles,which was left to strike the surface of specimen, was the same atall the impact angles and equal to 1.3 g. The tests were carried outfor four aliquots and at impact angles of 30 deg, 45 deg, 60 deg,and 90 deg. At the start of the experiment, the specimen wasexposed to the first aliquot of erodent. It was then taken from thetest rig, cleaned as described earlier, and examined in SEM. Afterthe appropriate images had been taken, the specimen was exposedto further aliquot of erodent.The relocation SEM depends on marking the specimen so that

the same area on the specimen could be found repeatedly. Forexamining the eroded surface morphology features, SEM photo-graphs with high magnification were taken. For quantitative mor-phological analysis of impact sites, such as number, size,perimeter, etc., a low magnification was used.

3 Results and Discussion

3.1 Impact Craters

3.1.1 Number of Impact Craters. At first, three positions wereidentified on the eroded specimen surfaces: two were at the lead-ing and trailing edges and the third was near the midway of thespecimen. Then, the aspects of the damaged area characteristics,i.e., the number, shape, and size of erosion sites, were systemati-cally investigated for successive stages and different impact

Table 1 Chemical composition of low alloy steel AISI 5117 [22]

Element C Si Cr Mn S P Fe

wt. % 0.17 0.3 0.9 1.2 0.003 0.005 balance

Table 2 Mechanical properties of low alloy steel AISI 5117 [22]

Yieldstrength(MPa)

Tensilestrength(MPa)

Modulus ofelasticity(GPa)

Hardness,Hv

(200 g)Density(kg/m3)

600 950 210 200 7850

Fig. 3 Scanning electron microphotograph of silica sand(mean diameter5 302lm)

Table 3 Statistical values of particle size and shape as obtained by image analysis of SiO2 particles

Particle sizerange (lm)

Statisticalparameters

Area(lm2)

Averagediameter (lm)

Length,L (lm)

Width,W (lm)

Aspectratio,W/L

Perimeter,P (lm) P2/(4pA)

250355 mean 76,336.88 301.10 387.08 272.76 0.7180 1117.48 1.36median 76,040.1 300.99 375.81 276.32 0.736 1108.79 1.25

standard deviation 20,507.5 43.60 64.29 44.68 0.14 161.34 0.38

Journal of Tribology APRIL 2014, Vol. 136 / 021608-3


angles. It is worth noting in this work that all impact sites devel-oped on eroded surface are called craters. The counting processwas carried out on a large digital screen to facilitate the countingprocess. The counting process was a very exhausting process indetermining whether this crater occurred because of one particleor more. In this process, some uncertainty associated with the

Fig. 4 Scanning electron microphotographs of 5117 alloy steel eroded surfaces atdifferent impact angles and mass of erodent during the four successive stages atthe midway of the specimens

Fig. 5 Number of accumulative impact craters versus mass oferodent for different impact angles

Fig. 6 Cumulative mass loss of low alloy steel 5117 versusmass of erodent for different impact angles

Fig. 7 Damages at the midway of the specimens surfaces atmass of erodent of 2.6 g and different impact angles: (a) 15 deg,(b) 30 deg, (c) 45 deg, (d) 60 deg, (e) 75 deg, and (f) 90 deg; sym-bols A and B refer to the particle body impacting and more thanone protrusion impacting, respectively, and the scatter arrowsillustrate the divergence of the particles



number of impacts might be found. However, this uncertaintywould be very small because of the small exposure. Due to smallexposure, the number of impacts was limited, and to some extent,the impact sites were isolated and clear. The number of impactswas counted manually for each specimen at three positions men-tioned above, and then the average was calculated.

The midway location was used as all illustrations in the manu-script and this was representative for all examined locations.Figure 4 presents a series of relocated areas that resulted from thestepwise erosion tests on the carbon steel for impact angles of30 deg, 45 deg, 60 deg, and 90 deg. The inclined vertical lines arethe traces of polishing lines. The counted number of craters is

Fig. 8 (a) Crater-size distributions with different impact angles and after exposure of (a) 1.3 g, (b) 2.6 g, (c) 3.9g, and (d) 5.2g. (b)Crater-size distributions with different impact angles and after exposure of (a) 1.3 g, (b) 2.6 g, (c) 3.9g, and (d) 5.2g.



plotted versus the mass of erodent for different impact angles, asshown in Fig. 5. It can be observed that the number of cratersincreases with the increase in the mass of erodent for all testedangles. These increases, in this work, are fairly linear. It can alsobe observed that the number of craters decreases with the increaseof angle. In order to see whether these data correspond with theresults of loss in mass of the sample against the erodent mass,

cumulative mass loss of steel 5117 versus mass of erodent at dif-ferent impact angles was obtained experimentally and presentedgraphically in Fig. 6. Curves in Fig. 6 have the same trend as thosein Fig. 5. Moreover, from Fig. 6, it is obvious that the mass lossdue to erosion increases with the increase of impact angle from30 deg up to 45 deg, where it reaches its maximum value. Furtherincrease of the impact angle beyond 45 deg up to 90 deg leads to

Fig. 8 (Continued)



continuous decrease in mass loss. It reaches its minimum value atnormal impact. This represents a typical behavior of ductile mate-rial subjected to slurry erosion.To understand the dynamics of particles, i.e., the efficiency of

impact, rebound effect, particle irregularity, and so on, the numberof craters is compared with the number of particles theoreticallyimpacting the specimen surface during each stage. By assumingthe efficiency of impact 100%, the number of impact particles canbe found by knowing the mass of erodent that impacts the surfaceof specimen at each exposure and for each impact angle. Since itis verified that this test rig [12] provides a homogeneous mixture,the particles will be uniformly distributed on the eroded area. Inthis study, all microphotographs for all impact angles have thesame size and magnification for each exposure. So, by knowingthe factor of magnification and the size of studied microphoto-graphs in relation to the whole eroded area, the calculated numberof particles that impacts the selected microphotographs can beestimated. For comparison between the counted number of cratersand the number of particles, the latter is plotted in Fig. 5, fromwhich one can observe that the counted number of craters is largerthan the calculated number of particles for all impact angles andat any stage.The pertinent factors that can lead to the increase in the number

of craters more than the number of calculated impact particlesmay be the rebound effect of particles, the irregular shape forthese particles, and particle fragmentation. When the particlesstrike the substrate, part of their kinetic energy is spent on remov-ing the material, part on indentation of substrate, and a part onrebounding. Stack et al. [24], in their study on the effect of elasticrebounds, reported that at velocities that might typically beencountered with slurry pipe lines. For instance, between 1m/sand 10m/s, the coefficients of restitution are significant. Thisleads to the conclusion that the effect of the rebound of the parti-cle, and the elastic forces often deemed negligible, cannot in factbe ignored. By using the whirling-arm test rig, Abouel-Kasemet al. [20], in their study on the effect of the rebound particles,reported that most experimental works for slurry used a slurry jetwhere the rebound particles have a negative effect in developingerosion, since the erodent particles rebounded from the samplesurface shield the surface by their collision with incident particles.For the whirling-arm test rig, the effect of the rebound particlescannot be neglected. In this tester, no shielding of surface occursas it occurs in the slurry jet. Particle fragmentation leading to sec-ondary erosion has been observed by Tilly [25] in his work usingquartz on H46 steel. It was assumed that the amount of fragmenta-tion and secondary erosion would be dependent of the particlevelocity, size, impact angle, and difference in hardness betweenthe erodent and target material.

3.1.2 Impact Crater Morphology. The damage developed onthe carbon steel specimen surfaces at different impact angles anda mass of erodent of 2.6 g is shown in Fig. 7. It can be generallyobserved that the impact craters formed at angles of 15 deg,30 deg, 45 deg, and 60 deg have tracks in the direction of slurrystream, which is from bottom to the up of the micrographs, whilefor angles of 75 deg and 90 deg, it is hard to distinguish suchdirectionality of slurry. However, closer observation for themicrographs at 75 deg and 90 deg, it can be seen that some cratershave a random directionality, as shown by arrows in Figs. 7(e)and 7(f).This may be attributed to the divergence of particles.Therefore, the effect of impact angle based on the impact cratershape, which is developed here for a ductile material, can bedivided into two regions: the first region for h 60 deg and thesecond region for h 75 deg. In the first region, the length of thetracks decreases with the increase of impact angle Figs. 7(a)7(c).The impact craters, having been divided according to their shapewith impact angle, can be revealed from the dominant erosionmechanisms at each stage. In the first region, microcutting andplowing are the dominant mechanisms, while indentations andmaterial extrusion are the dominant in the second region [12].On

the other hand, from these micrographs depicted in Fig. 7, it canbe inferred that the actual impact angle of particles differed fromthe nominal one. This can be observed from the relative positionof the impact craters to each other. This observation had beennoted in the literature based on fluid flow analysis [26].The development of crater size with the mass of erodent and

impact angle is shown in Fig. 8. The crater-size range was theleast for the normal impact and was from few micrometers toabout 50lm. For angles of 30 deg and 60 deg, the upper limit ofthe range exceeded more than that for normal impact and reached100 lm. The crater-size range for the angle of 45 deg was similarto that for angles of 30 deg and 60 deg, except the lower limit ofthe range, which shifted to a higher value of two digits. Generally,by comparing the lower and upper limit crater-size range with theaverage size of particle of 302.5lm, they represent about 3% and30%. This means that the impact process of particles depends onthe orientation and angularity of particles. For the small cratersizes, it may be produced by one of the particle protrusions. Asfor the large one, it may be formed by the body of particles(labeled A) or by more than one protrusion (labeled B), as shownin Fig. 7(a).

Fig. 9 Sequence of images at the midway of the specimensillustrating development of damage at the impact angle of45 deg after exposure of (a) 1.3g, (b) 2.6g, (c) 3.9 g, and (d) 5.2 g

Fig. 10 Sequence of images at the midway of the specimensillustrating development of damage at the impact angle of60 deg after exposure of (a) 1.3g, (b) 2.6g, (c) 3.9 g, and (d) 5.2 g



3.2 Tracking theEffects ofRemovalofMaterials. Figures 911show the sequence of images of damage developed after exposureto aliquots of erodent at impact angles of 45 deg, 60 deg, and90 deg, respectively. Due to the first exposure of the solid par-ticles on the surface of specimens, considerable damages to thesurfaces occurred. This damage depends on the mechanismsrelated to the impact angle, where the microcutting and plowingwere the dominant mechanisms for acute angle, while indenta-tions and material extrusion prevailed for the normal impact [12].The examination of slurry-erosion behavior of subsequent stagesshown in Figs. 911(b), 11(c), and 11(d) reveals that the particleimpact processes include the following events: forming newimpact sites, impacting former sites, and impacting the surfacebut without noticeable effect. The new impact sites that areformed in the subsequent stages, after stage 1, are encircled,as shown in micrographs. When new particles impact formerimpact sites, the chips and the extruded materials formed atlow- and high-impact angle, respectively, in the impact siteswill be detached. Under subsequent impacts for these sites, itwas observed that, in some of the impact sites, craters hadbeen formed and the others disappeared. From Fig. 11, it isnoteworthy that the sequence of images illustrating the devel-opment of damage at normal impact shows increase in thenumber of craters (impact sites) without noticeable detachmentof the extruded metal. This explains the well-known fact thatthe mass loss due to slurry erosion is minimal at normalimpact angle. In conclusion, the stepwise erosion of AISI5117 sample clearly showed that erosion had occurred by theaccumulation of increasing the impact crater number, damage,and the chipping of the former impact sites.There are many techniques that are used nowadays to eval-

uate and to predict the slurry erosion. One of these methodsis to use the fuzzy modeling for evaluation and prediction ofthe slurry erosion [27], as it is well known that the slurryerosion is influenced by many important factors that shouldbe taken into consideration to model the slurry-erosion pro-cess. These factors are impact angle, time, roundness factor,aspect ratio, particle size, impact velocity, and concentrationof the particles. This fuzzy rules technique depends onsome experimental observations to develop a two-layer fuzzymodel to correlate the slurry-erosion variables. Also, this

model is based on the assumption that the slurry-erosion char-acteristics of ductile materials are an imprecise complex func-tion of many interacting variables and can be described andevaluated by the theory of fuzzy sets [27]. So, the proposedmodel suggests the possibility of developing an expert real-time and integrated system using fuzzy logic technology tomonitor the slurry-erosion process.

4 Conclusions

(1) The impact sites or craters are counted using stepwise ero-sion. It was found that the number of craters increases withthe increase of erodent for all impact angles. As well, thenumber decreases with the increase of impact angle.

(2) Based upon analysis, the results have shown that the num-ber of craters is larger than the calculated number of par-ticles of erodent. This is interpreted in the light of reboundand particle fragmentation.

(3) Using image analysis for stepwise eroded surface, the cratersize was categorized into two regions according to theimpact angle: the first region for h 60 deg and the secondregion for h 75 deg. In the first region, the crater sizerange was from few micrometers to about 100lm. How-ever, in the second region, the upper limit of crater-sizerange was the least and it was about 50lm. The resultsshowed also that the lower limit of the size range for theangle of 45 deg rose to two digits.

(4) The material removal by chipping as result of subsequentparticle impacts plays an important role in slurry-erosionprocess

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Fig. 11 Sequence of images at the midway of the specimens illustrating develop-ment of damage at the impact angle of 90 deg after exposure of (a) 1.3 g, (b) 2.6g,(c) 3.9 g, and (d) 5.2 g



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faculty of engineering

number of impact sites

increase of impact angle

subsequent impact particles

assiut university

different impact angles

impact crater shape

developing erosion