Water Droplet and Cavitation Erosion Behavior of Laser...

Water Droplet and Cavitation Erosion Behavior of Laser-Treated Stainless Steel and Titanium Alloy: Their

SimilaritiesB.S. Mann

(Submitted June 1, 2013; in revised form July 5, 2013; published online August 6, 2013)

This article deals with water droplet and cavitation erosion behavior of diode laser-treated X10CrNi-MoV1222 stainless steel and Ti6Al4V alloy. After laser surface treatment, the water droplet and cavitationerosion resistance (WDER and CER) of these materials improved significantly. The main reason for theimprovement is the increased surface hardness and formation of fine-grained microstructures after lasersurface treatment. It is observed that there is a similarity in both the phenomena. The WDER and CER canbe correlated with a single mechanical property based on modified ultimate resilience (MUR) provided thelaser-treated layers are free from microcracks and interface defects. The CER and WDER behavior ofHPDL-treated X10CrNiMoV1222 stainless steel and Ti6Al4V alloy samples using different test equipmentas per ASTM G32-2003 and ASTM G73-1978, their correlation with MUR, and their damage mechanismcompared on the basis of XRD analyses, optical and scanning electron micrographs are discussed andreported in this article.

Keywords cavitation erosion, diode laser, Ti6Al4V alloy, waterdroplet erosion, X10CrNiMoV1222 stainless steel

1. Introduction

Water droplet erosion (WDE) damage occurring on theengineering components is due to the high impact of waterdroplets. When a liquid jet or droplet impacts on a flat solidboundary, a compression wave front is generated and contactperiphery expansion takes place quicker than the compressionwave front movement. The compression wave front travels atacoustic speed in the medium, whereas the contact peripherytravels much faster than the compression wave front. At a laterstage, the compression wave envelop overtakes the contactperiphery and a jetting action occurs due to microjets in asimilar way to that of jetting action of microjets in cavitation,causing erosion of the material (Ref 1–6). During impact of aliquid column on a solid boundary, the impact pressuregenerated at the liquid solid interface is sufficiently high tocause the erosion of materials. The impact pressure on a solidboundary reaches its maximum value of �3qCV, i.e., threetimes higher than that of well-known water hammer pressurewhich is given by qCV, and water jetting velocities become10-12 times higher than the water droplet impact velocities (q isthe density of the liquid, C the liquid acoustic speed, and V isthe liquid droplet impact velocity). Field et al. (Ref 6) havepresented high speed photographs, based on the laser beamtechnique, of a metallic projectile that is fired at a liquid jet

showing a large amount of cavities, their collapse, and shockwave propagation. The samples in the liquid impingementerosion test rig (ASTM G73) when rotated at high speed hit theliquid jet or droplet, producing cavities similar to that of ametallic projectile. The cavities collapsing near the surface leadto erosion. Erosion causes roughening of the surface, thesideway liquid applies shear to the roughened surface step, andliquid in crevices is pressurized, causing additional hydraulicload. Lesser et al. (Ref 1, 3) have reported that a liquid jethaving higher impact velocity produces cavitation clouds,causing material removal similar to that of cavitation erosion(CE) and the criterion for correlating a material with itsmechanical properties remains the same for both CE and WDE.

On the other hand, material damages occurring due to CEare very common in the industry, especially in pumps, shipimpellers, hydraulic machinery, and high speed mixers in thepharmaceutical industry. The CE phenomenon is understood asthe formation of bubbles due to drop in local pressure eitherdue to sudden change in flow or due to flow fluctuations(vibrations). These bubbles travel in a zone of high pressurewhere they collapse, leading to formation of high pressureshock waves and jetting of microjets. Due to repetitive highpressure shock waves and the jetting action of microjets, thesolid surfaces erode quickly. This becomes one of the maincauses of failure of engineering components.

X10CrNiMoV1222 stainless steel and Ti6Al4V alloy arewidely used in many areas such as the aerospace, military,chemical, and biomedical industries because of their excellentcorrosion resistance and high strength. To improve cavitationand water droplet erosion resistance (CER and WDER) of thematerials, a number of surface treatments and coating processeshave been published (Ref 7–28). Additional information onother materials is also available (Ref 29–35). Surface modifi-cation with laser can produce the properties desired for aspecific application. Ahmad et al. (Ref 21) have carried out

B.S. Mann, BHEL Corporate R&D, Vikasnagar, Hyderabad 500093,India. Contact e-mail: [email protected].

JMEPEG (2013) 22:3647–3656 �ASM InternationalDOI: 10.1007/s11665-013-0660-6 1059-9495/$19.00

Journal of Materials Engineering and Performance Volume 22(12) December 2013—3647

WDE testing of laser-treated X5CrNiCuNb16-4 stainless steelalong with untreated X20Cr13, X5CrNiCuNb14-5, andX5CrNiCuNb16-4 stainless steels and Ti6Al4V alloy. It isreported that X5CrNiCuNb16-4 stainless steel after lasertreatment has improved its WDER around 2.2 times. TheWDER of laser-treated X5CrNiCuNb16-4 and untreatedX20Cr13, X5CrNiCuNb14-5, and X5CrNiCuNb16-4 stainlesssteels and Ti6Al4V alloy was correlated with the mechanicalproperties based on microhardness and resilience (area underlinear portion of an engineering stress-strain curve). It isreported that the WDER of a material can be partiallypredetermined by its microhardness and resilience (SE).Robinson and Reed (Ref 7) have tested laser-treated Ti6Al4Valloy and reported an improvement of the order of 435% afterlaser treatment. It is concluded that the improvement in WDERof laser-treated Ti6Al4V alloy is mainly due to its increasedsurface microhardness and martensitic microstructure (�a).Materials based on ultimate resilience (UR) have already beencorrelated with CER and it is reported that UR is themechanical property required for CER (Ref 8). For surface-treated or -coated materials, UR does not hold good. Informa-tion on correlating WDER of HPDL-treated textured anduntextured X20Cr13 stainless steel and LPST blade material,with their mechanical properties (UR, MR, modified ultimateresilience (MUR), and microhardness), is available in Ref 9. Itis reported that the surface-treated materials and coatings can becorrelated with MR provided these are free from microcracksand interface defects. UR, MR, and MUR are given as follows:

UR = UTS2/2E (area of the triangle obtained when the yieldpoint is raised to the level of UTS of the engineering stress-strain curve)

MR ¼ UTS=2Eð Þsubstrate�Hardnesstop surface;

MUR ¼ URsubstrate � ðHardnesstop surface=HardnesssubstrateÞ2;

where E is the Young�s modulus and UTS is the ultimatetensile strength.

CER and WDER behavior of HPDL-treated X10CrNi-MoV1222 stainless steel and Ti6Al4V alloy samples usingdifferent test equipment as per ASTM G32-2003 and ASTMG73-1978, their correlation with MUR, and their damagemechanism compared on the basis of XRD analyses, opticaland scanning electron micrographs are discussed and reportedin this article.

2. Experimental Procedure

2.1 HPDL Surface Treatment

A 4.6-kW diode laser (Laserline, GmbH) with narrowrectangular beams, 309 3 and 209 2.8 mm2, of focal length275 mm was used for the HPDL treatment of X10CrNi-MoV1222 stainless steel and Ti6Al4V alloy samples. The laserwas mounted on a six plus two axes robot (Kuka, GmbH). ForWDE testing, round samples of size Ø 12.7 mm9 40 mm withinternal threading M 8 were made from stainless steel andtitanium alloy. A fixture was fabricated to hold and rotate theround samples while carrying out the HPDL surface treatment.Each sample was fixed in a self-centered three-jaw chuck at oneend and supported on a fixture at the other end. The fixture hada rotating seal so that the sample can rotate freely and the

forced air used for cooling the sample does not leak. Fastcooling of the sample was carried out during HPDL treatmentby introducing compressed air having volumetric flow rate ofaround 15 m3/h through the M 8 tapped hole. The air is capableof removing heat at a rate of around 180 J/s which iscomparable to the heat removed in a bulk stainless steel duringHPDL surface treatment. The samples were thoroughly cleanedusing acetone before the start of the experiment to make thesurface free from dust, oil, etc. The complete setup details andHPDL facility used for the experimentation are alreadyavailable (Ref 11–18). Laser beam power was controlled in aclosed loop using a two-color pyrometer and a uniform surfacetemperature was maintained. The complete system was con-trolled by a robot controller. The robot was programed in such away that the laser beam tracked the sample at a scan speedranging from 1 to 5 mm/s ensuring complete hardening of thesamples in one pass. Thus, a wide area having a span equivalentto the width of the beam on the outer periphery of the samplewas laser hardened in one pass. The HPDL treatment of the testsamples was carried out at 1550 �C. Corresponding laser powerdensities were in the range of 1725-1975 J/cm2. Lower HPDLpower densities were observed for the titanium alloy. TheX10CrNiMoV1222 stainless steel and Ti6Al4V rectangularblocks of size 1009 509 25 mm3 were HPDL treated using209 2.8 mm2 laser beam and CE test samples as per ASTMG32-2003 were made from these blocks. The robot wasprogramed in such a way that the laser beam tracked the blocksat an optimum speed ranging from 1 to 5 mm/s ensuringuniform heat treatment. Thus, a wide area equivalent to thewidth of the laser beam was HPDL treated in one pass. Tocover the complete area, a number of passes were given. Thelaser on/off was also controlled by robot at appropriate positions.Laser power densities in the range of 2300-2530 J/cm2 weremaintained for HPDL treating of these blocks. Lower HPDLpower densities were observed for the titanium alloy. Thefacility details are given in Fig. 1. The untreated and HPDL-treated X10CrNiMoV1222 stainless steel and Ti6Al4V alloyare designated as X10CrNiMoV1222 AS, X10CrNiMoV1222LH and Ti6Al4V AS, Ti6Al4V LH, respectively.

2.2 Microstructural and Metallographic Characterization

The HPDL-treated X10CrNiMoV1222 stainless steel andTi6Al4V alloy specimens were sectioned and polished. Thesewere studied by optical and scanning electron microscopy. Themicrohardness across the specimens was measured using aTukon 2100 Macro/Microhardness tester (Wolpert, USA).

2.3 X-Ray Diffraction Analysis

The x-ray diffraction analyses of HPDL-treated anduntreated samples were carried out by an x-ray diffractionsystem (Philips X-pert system, Philips, Netherlands). Copper Kalpha radiation and a nickel filter were used for XRD analysis.

2.4 CE Testing

2.4.1 CE Testing as per ASTM G32-2003. CE test setupdetails are given in Fig. 2. It consists of a high frequency–highvoltage signal generator which energizes the piezo-electricelement in the converter via the transducer RF cable to oscillateat 20 kHz with amplitude of 50 lm. This mechanical vibrationis transmitted to the horn made of titanium alloy of matchingimpedance. The sample to be tested is screwed into the horn tip

3648—Volume 22(12) December 2013 Journal of Materials Engineering and Performance

and securely tightened to ensure that there is no loss of energytransfer from the horn tip to the sample. The sample alsooscillates at a frequency of 20 kHz, thereby simulating thephenomena of cavitation on its surface.

2.4.2 CE Testing Using a Rotating Disk Apparatus. Arotating disk apparatus simulates the dynamic conditions ofrotating machines such as hydro turbine and pumps. Materialscan be tested at a similar cavitation coefficient (r) simulatingdynamic conditions. In brief, the rotating disk apparatusconsists of a test chamber in which an aluminum disk of sizeØ 330 mm9 4.5 mm is rotated in water under controlledconditions of disk rotation speed, water pressure, and temper-ature. Grooves of size Ø 64 mm9 3 mm are made in the diskfor mounting the samples at 125 mm radius. Brass CE inducersof size Ø 25 mm9 3 mm are fixed ahead of the grooves toprovide a source of cavity formation in the water. A sectionalview of the test rig is given in Fig. 3. The disk is rotated in awater-filled chamber at 48 Hz to provide linear peripheralvelocity of 37.30 m/s. The water pressure is varied from 0.1 to0.15 MPa. The test varies from a few hours (incubation) tocomplete damage of the materials. The CE test results of13Cr4Ni stainless steel, a most common material used in hydroturbines and high speed pumps, up to 55 h at r = 0.210, arealready available in Ref 10. This material has been tested forCE using a vibratory test setup as per ASTM G32-2003. Thedamage intensities of both the testing equipment have beencompared.

2.5 WDE Testing

The details of WDE test facility are already available(Ref 11–14, 17, 18). In short, the test facility consists of a700-mm-diameter chamber and a round stainless steel diskwhere the test samples are positioned. Test samples,Ø 12.7 mm9 40 mm, are affixed on the periphery of the disk.The disk is rotated at 79.166 Hz to obtain the test sampletangential velocity of 147.0 m/s. Two water jets impinge on theround test samples and cause impingement erosion. As such, arelative velocity of 147.6 m/s is obtained. The WDE tests werecarried out as per ASTM G73-1978. The details of the test rigare given in Fig. 4. Characterizing materials in WDE using the

rotating wheel as per ASTM G73-1978 is cumbersome,expensive, and time consuming, whereas evaluating thematerials in CE as per ASTM G32-2003 or ASTM G134-95(2006) is simple, quick, and does not require elaboratearrangements. The revised versions of these standards, as ondate, are available as ASTM G73-2010 and ASTM G32-2009.The HPDL-treated test samples were WDE tested for a longduration up to 6.0399 106 cycles as per the test parametersgiven in Table 1. A precision balance (±0.1 mg) was used formeasurement of mass loss in the samples after certain testduration. The test duration, depending upon energy and massfluxes, was selected in such a way as to achieve steady stateerosion in a limited number of cycles. The accuracy andrepeatability of the tests have been established on X10CrNi-MoV1222 stainless steel samples before start of the experi-mentation. The extent of erosion damage is calculated from the

Fig. 1 HPDL facility showing laser surface treatment being carried out on (a) round and (b) flat sample

Fig. 2 CE test facility as per ASTM G32-2003


mass loss divided by the density of the material. The resultshave been plotted in the form of cumulative volume loss versusnumber of cycles.

3. Results and Discussion

3.1 HPDL Surface Treatment

In laser hardening, the surface temperature plays animportant role for phase transformation. Moreover, the laserpower is controlled by a pyrometer which is integrated withlaser in a closed loop. It records the surface temperature andcontrols the pre-fixed temperature by varying the laser power.The scan speed of the laser optics against the job along with thelaser power decides the depth of hardening and distortion in thecomponents. All these parameters make the operation very

complex. The integration of the laser beam with a six plus twoaxes robot through programing simplifies the operation. Therobot is programed in such a way that the laser beam tracks thecomponent at a uniform speed ensuring hardening of a requiredlength in one pass. Typical surface temperature and laser powertime cycle (STLPTC) of the laser-treated Ti6Al4V alloy testsamples (round and flat) at 1550 �C are given in Fig. 5. It canbe seen from Fig. 5 that the STLPTC fluctuations are at aminimum for the rectangular sample. The temperature varia-tions on the round sample were as high as ±50 �C compared to±12.5 �C on the flat sample. The STLPTC fluctuations onX10CrNiMoV1222 stainless steel samples were observed onthe lower side as compared to that of titanium alloy, possiblybecause of its higher thermal conductivity.

3.2 Microstructural and Metallographic Examination

3.2.1 Microstructure. Optical micrographs of a HPDL-treated Ti6AL4Valloy sample showing formation of martensitic(�a) phase are given in Fig. 6 and that of X10CrNi MoV1222

Fig. 3 Sectional view of a rotating disk CE apparatus

Fig. 4 WDE test facility as per ASTM G73-1978

Table 1 WDE test conditions

Conditions Test parameters

Volume of water impacted per cycle 0.035 mLWater impact energy (1/2 mV2) 0.380 JWater energy flux, J/m2Æs 57.1679 106 J/m2ÆsWater mass flux 4.0 m/sRelative water velocity 147.6 m/sTest sample size Ø 12.709 40 mmTest duration 6.0399 106 cyclesAngle of impact 0-90�Impact frequency 79.166 cycles/sExperimental accuracy ±15.5%Salt concentrations As per Ref 12

After an operation of 2.1969 106 cycles, the WDE tests onX10CrNiMoV1222 AS and Ti6Al4V AS samples were discontinuedbecause of the excessive damage. These were removed and theirmetallographic analyses were carried out. The salt concentrations inwater were as follows: calcium hardness �134, magnesium hardness�166, M-alkalinity �206, P-alkalinity �nil, chlorides �97, sulfates�47, and total solids �690 having pH 7.93 and conductivity0.894 millimhos/cm


stainless steel sample are given in Fig. 7. SEM micrographstaken on the HPDL-treated X10CrNiMoV1222 stainless steeland Ti6Al4V alloy samples are given in Fig. 8, 9, and 10.

3.2.2 Microhardness and Case Depth. The Vickersmicrohardness of X10CrNiMoV1222 LH stainless steel andTi6Al4V LH alloy samples was measured by applying a load of3N with a dwell time of 13 s. The maximum microhardnessobserved on the HPDL-treated X10CrNiMoV1222 sample wasaround 550 HV0.3 compared to 370 HV0.3 observed on theHPDL-treated Ti6Al4V alloy sample. A typical microhardnessversus case depth plot of X10CrNiMoV1222 LH flat sample isshown in Fig. 7. Table 2 gives the case depth and laser powerdensity of stainless steel and titanium alloy samples. The laserpower density of a sample is decided by the shape of thecomponents, its thermal properties, and adjusted automaticallyonce temperature data from the pyrometer are made available tothe laser system. The laser power observed on round samples islower than that of flat samples, though the surface temperatureis the same (1550 �C). It can be seen from Table 2 that the casedepth of a round sample is approximately 50% less than thatof a flat sample. This is because of lower laser power density.

Fig. 5 STLPTC of Ti6Al4V alloy samples: (a) round and (b) flat. The temperature variations on a round sample are as high as ±50 �C as com-pared to ±12.5 �C on a flat sample

Fig. 6 Optical micrographs taken on the outer edge of a HPDL-treated Ti6Al4V alloy round sample, showing case depth and formation of mar-tensitic phase (�a). For comparison, an optical micrograph taken on the untreated sample is also given. The HPDL treatment was carried at1725 J/cm2

Fig. 7 Microhardness vs. case depth plot and optical micrographstaken on the X-section of a HPDL-treated X10CrNiMoV1222stainless steel flat sample. The HPDL treatment was carried at2530 J/cm2


Fig. 8 SCM of WDE-tested samples (a) Ti6Al4V LH, (b) Ti6Al4V AS, and (c) X10CrNiMoV1222 LH after WDE testing up to 2.1969 106

cycles, showing formation of pits, microholes, and tunnel-type damage

Fig. 9 SEM of the WDE-tested samples (a) X10CrNiMoV1222 AS, showing coarse grains and excessive damage, and (b) X10CrNiMoV1222LH, showing fine grains and minimum damage. The formation of pits, microholes, and tunnel-type damage are clearly seen. The WDE testswere carried out up to 2.1969 106 cycles on X10CrNiMoV1222 AS sample and up to 6.0399 106 cycles on X10CrNiMoV1222 LH sample

Fig. 10 SEM of the WDE-tested samples (a) Ti6Al4V AS, showing coarse grains, and (b) Ti6Al4V LH, showing fine grains, formation of pits,microholes, and tunnel-type damage. SEM taken on Ti6Al4V AS sample was on the lesser damaged area, whereas the one taken on Ti6Al4VLH was on an excessively damaged area. WDE tests were carried out up to 2.1969 106 cycles on Ti6Al4V AS and up to 5.499 106 cycles onTi6Al4V LH sample


The case depth of Ti6Al4V LH alloy samples is around 50%lower than that of X10CrNiMoV1222 LH samples. This couldbe because of lower thermal conductivity of Ti6Al4V alloy(7 W/mÆK) compared to that of X10CrNiMoV1222 stainlesssteel samples (25 W/mÆK). Optical micrographs of a roundTi6Al4V LH sample showing martensitic microstructure (�a)and case depth (�400 lm) are given in Fig. 6(b) and (c),respectively. An optical micrograph of Ti6Al4V AS sampleshowing a + b microstructure is also given (Fig. 6a).

3.3 X-Ray Diffraction Test Results

The XRD analysis of HPDL-treated X10CrNiMoV1222stainless steel sample shows increased martensitic phase (�a)(Fig. 11), which has resulted in increased microhardnessaround 550 HV0.3 from the original 295 HV0.3 (Fig. 7). Theincreased martensitic phase was confirmed by comparing themicrographs of HPDL-treated and untreated areas of the sample(Fig. 7). HPDL-treated Ti6Al4V alloy sample has shownincreased alpha HCP peak height at 2h values at 35� and 40�and reduced beta peak height at 2h values at 37.8� and 69.8�.XRD could not differentiate between a and �a phases excepttheir peak counts. A significant change in alpha and beta peakcounts for the HPDL-treated sample confirms the increasedmartensitic phase (�a). Details are given in Fig. 12. Themicrohardness of Ti6Al4V alloy has increased marginally(370 HV0.3 from original 340 HV0.3). The increased martensiticphase (�a) appears to have an optimum balance of mechanicalproperties without compromising on ductility and resulted inenhanced WDER. An improved WDER of laser-meltedTi6Al4V alloy using higher laser power density (105 W/cm2)has also been reported by Robinson and Reed (Ref 7). It is alsoreported that laser melting has resulted in homogeneous casedepth, around 400 lm, with martensitic microstructure (�a).Similar findings, during laser processing of Ti6Al4V alloy, arereported in Ref 19. Surface melting generally induces tensileresidual stresses and reduces the fatigue life of the components.To avoid melting of stainless steel and titanium alloy samples,lower laser power densities, maximum up to 2530 J/cm2, wereused in the present study.

3.4 WDE and CE Test Results

The X10CrNiMoV1222 LH, Ti6Al4V LH, X10CrNi-MoV1222 AS, and Ti6Al4V AS samples were tested in WDE.After carrying out WDE tests up to 2.1969 106 cycles, themaximum improvement was observed in X10CrNiMoV1222 LHsample and this improvement continued up to 6.0399 106

cycles. The experimental test conditions and parameters aregiven in Table 1. WDE tests on X10CrNiMoV1222 AS andTi6Al4VAS samples were discontinued after 2.1969 106 cyclesbecause of excessive damage (Fig. 13). The excessive damagesamples were removed and their metallographic analyses werecarried out. After carrying out CE tests up to 8 h as per ASTMG32-2003, the damages observed on X10CrNiMoV1222 AS andTi6Al4V AS samples were also high (Fig. 14). The X10CrNi-MoV1222 AS sample was excessively damaged and removed.The CE tests on other samples continued. WDE-tested sampleswere first examined at low magnifications and their micrographsare given in Fig. 8. At higher magnifications, the SEMmicrographs taken on the HPDL-treated X10CrNiMoV1222stainless steel and Ti6Al4V alloy samples showing fine micro-structures are given in Fig. 9(b) and 10(b), respectively, whereasthose taken on untreated samples, showing coarse microstruc-tures, are given in Fig. 9(a) and 10(a). Deep tunnel-type WDEdamages in untreated and HPDL-treated stainless steel andtitanium alloy samples are clearly seen in Fig. 9(a) and 10(b) andthese damages are similar to those observed in CE-testedsamples (Ref 10).

The CE wear rate of 13Cr4Ni stainless steel samples, whichwere tested up to 55 h using a rotating disk apparatus, werecompared with the CE wear rate of the samples which weretested up to 5.5 h and their values were 0.09 and 14.56 mg/h,respectively. The CE wear rate of the samples which were

Fig. 11 XRD plots of untreated and HPDL-treated X10CrNi-MoV1222 stainless steel flat samples showing increased martensiticphase (�a) after HPDL treatment. The HPDL treatment was carried at2530 J/cm2

Fig. 12 XRD plots of untreated and HPDL-treated Ti6Al4V alloyflat samples showing formation of martensitic phase (�a) after HPDLtreatment. The HPDL treatment was carried at 2300 J/cm2

Table 2 Case depth and laser power density of HPDL-treated samples

MaterialsCase depth,

lmLaser powerdensity, J/cm2 Remarks

X10CrNiMoV1222flat sample

1600 2530 Ref. Fig. 7

X10CrNiMoV1222round sample

800 1975 …

Ti6Al4V flat sample 800 2300 …Ti6Al4V round sample 400 1725 Ref. Fig. 6

For CE tests, flat samples were used and for WDE tests, round sampleswere used


tested as per ASTM G32-2003 is about 162 times more thanthose tested using a rotating disk apparatus. Both processeshave shown formation of pits during the initial stage of erosion(Fig. 8 and 14) (Ref 10) and after long duration of testing, theseare converted into deep tunnels (Fig. 9 and 10) (Ref 10). Thetunnel-type damages are due to microjets, causing deepmaterial removal. The cumulative volume loss of X10CrNi-MoV1222 LH, Ti6Al4V LH, X10CrNiMoV1222 AS, andTi6Al4VAS samples due to WDE and CE are given in Fig. 13and 14, respectively. It is seen from the figures that the HPDL-treated samples have performed exceptionally well andX10CrNiMoV1222 LH is the best among these.

3.5 WDE and CE Similarities

The WDE and CE test results of HPDL-treated samples(X10CrNiMoV1222 and Ti6Al4V) are given in Fig. 13 and 14,respectively. It is seen from the figures that the HPDL-treatedX10CrNiMoV1222 stainless steel samples have performedmuch better than all other samples. The volume loss versus CE/WDE test duration trend for both materials is similar. Thematerials which have performed excellently in WDE have alsoperformed well in CE. The solidification of water, arising due tocavitation bubble collapse, generating very high pressure( ‡ 1 GPa) for less than a nanosecond, has already beenexperienced and reported in Ref 35. This could be one of thereasons for the material being removed spontaneously. Thewater droplets� impact velocities in the WDE test rig are ofthe order of 147.6 m/s and assuming an acoustic wave speed ofthe order of 1500 m/s, an impact pressure �0.664 GPa isexerted on the surfaces (calculated from 3qCV). The actualwater droplet impact velocities occurring in low pressure steamturbine blades, bypass and control valves are supersonic(approximately four to five times that of 147.6 m/s). Undersuch situations, the impact pressure generated due to 3qCV willexceed 2.5 GPa. There are chances that due to very high impactpressure, the water in the form of microjets may solidify andWDE may become a solid particle impact wear similar to CE asreported in Ref 35.

From the SEM and optical micrographs of WDE- and CE-tested samples at low magnifications, it is observed that pits andmicrocavities are being formed on the surfaces (Fig. 8 and 14).After long exposure, these are getting converted into deeptunnel-type damage (Fig. 9 and 10). Some information oncavitation and water impingement eroded 18-8 stainless steelsamples is available in Ref 30. It has been reported that plasticdeformation has occurred inside the crystal grains. Due to veryhigh impact pressure of liquid cavity or bubble collapse, deepgrooves and cracks appeared on the grain boundary resulting inspontaneous material removal. It has been concluded that thecavitation and water impingement erosion damages occurringon this material are similar. In the present investigation,material removal in the case of HPDL-treated X22Cr13Ni-MoV1222 stainless steel samples appears to be different fromthat reported in Ref 30. This may be due to the fact that theHPDL surface-treated samples are hard and brittle. The materialremoval appears to be grain by grain. The material removal inuntreated ones appears to be similar to that reported by Hattoriand Takinami (Ref 30). It can also be seen from Fig. 13 and 14that the wear rate trends (WDE and CE) of HPDL-treated anduntreated X10CrNiMoV1222 stainless steel and Ti6Al4V alloysamples are similar. The X10CrNiMoV1222 LH sample hasperformed much better than all other samples.

3.6 WDER and CER Correlation with Mechanical Properties

Materials based on UR have already been correlated withCER and it is reported that UR is a valid mechanical propertyrequired for CER (Ref 8). For surface-treated or -coatedmaterials, UR may not hold good. Information on correlatingWDER of a HPDL-treated textured and untextured X20Cr13stainless steel sample with mechanical properties (MR, andMUR) has already been reported (Ref 18). It is concluded thatWDER can be correlated with mechanical properties based onMR and MUR provided the laser-treated surfaces have a highdegree of integrity and good bonding with the substrate. Thevolume loss up to 2.1969 106 cycles due to WDE and up to8 h due to CE of HPDL-treated and untreated samples is given

Fig. 13 Volume loss vs. WDE tests duration of X10CrNiMoV1222 AS, Ti6Al4V AS, Ti6Al4V LH, and X10CrNiMoV1222 LH samples. TheWDE tests were carried out as per test parameters given in Table 1


in Table 3 and for longer duration, these are given in Fig. 13and 14. From Table 3, it is seen that X10CrNiMoV1222 LHsample has the highest MR and MUR resulting in minimumvolume loss. It can also be seen from Table 3 that MURprovides a better correlation with WDER and CER.

4. Conclusions

From the SEM of the samples, it is observed that microcav-ities are being formed during the initial stage of WDE and CEtesting, and after a longer test duration, these are converting intodeep tunnel-type damages. The grains in X10CrNiMoV1222LH stainless steel samples are worn-out less compared to thosein untreated samples before these are being removed.

The CE and WDE volume loss versus time trends of HPDL-treated and untreated stainless steel and titanium alloy samplesare similar. The materials, which have performed extremelywell in water droplet erosion, have also performed very well inCE. The WDE and CE damages occurring on the stainless steeland titanium alloy samples are similar.

CER and WDER of HPDL-treated stainless steel andtitanium alloy samples can be correlated with MUR, a singlemechanical property, provided the HPDL-treated surfaces are

free from microcracks and interface defects. The HPDL-treatedX10CrNiMoV1222 stainless steel and Ti6Al4V alloy samplesare free from these defects and performed extremely well inWDE and CE. X10CrNiMoV1222 LH sample has outper-formed all other samples because of its higher MUR and fine-grained microstructure.

Acknowledgments

The author is thankful to the management of BHEL CorporateR&D for giving an opportunity to work in this area. The author isalso thankful to Mr. S. M. Hussain of Quality Cell and all thecolleagues at the Centre of Excellence for Surface Engineering fortheir help during the course of the work.

References

1. M. Lesser, Thirty Years of Liquid Impact Research: ATutorial Review,Wear, 1995, 186–187, p 28–34

2. J.E. Field, J.P. Dear, and J.E. Ogren, The Effects of Target Complianceon Liquid Drop Impact, J. Appl. Phys., 1989, 65, p 540–553

3. M.B. Lesser and J.E. Field, The Impact of Compressible Liquids,Annu. Rev. Fluid Mech., 1983, 15, p 97–122

Table 3 Mechanical properties and volume loss of HPDL-treated samples due to WDE and CE

Materials UTS, MPa Hardness, HV0.3 UR, J/cm3 MR, HVMUR,J/cm3

Volume loss, mm3

due to CE(a)Volume loss, mm3

due to WDE(b)

X10CrNiMoV1222 ‘‘AS’’ 1000 295.0 2.42 0.714 2.42 10.57 41.04X10CrNiMoV1222 ‘‘LH’’ … 550.0 … 1.33 8.4 1.92 1.29Ti6Al4V ‘‘AS’’ 874 340.0 3.17 1.24 3.17 5.13 33.38Ti6Al4V ‘‘LH’’ … 370.0 … 1.35 3.752 2.93 10.64

(a) CE tests up to 8 h as per ASTM G32-2003(b) WDE tests up to 2.1969 106 cycles as per test parameters given in Table 1

Fig. 14 Volume loss vs. CE tests duration of X10CrNiMoV1222 AS, Ti6Al4V AS, Ti6Al4V LH, and X10CrNiMoV1222 LH samples


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