Improvement of cavitation erosion resistance of AISI 316 stainless steel

Surface and Coatings Technology 165(2003)258–2670257-8972/03/$-see front matter ᮊ2002Elsevier Science B.V .All rights reserved.PII:S0257-8972Ž02.00739-9Improvement of cavitation erosion resistance of AISI 316stainless steelby laser surface alloying using fine WC powderK.H.Lo ,F .T.Cheng *,C.T.Kwok ,H.C.Man a a ,a,b cDepartment of Applied Physics,The Hong Kong Polytechnic University,Hung Hom,Kowloon,Hong Kong,ChinaaDepartment of Electromechanical Engineering,Faculty of Science and Technology,University of Macau,P .O.Box 3001,Taipa,Macau,ChinabDepartment Industrial and Systems Engineering,The Hong Kong Polytechnic University,Hung Hom,Kowloon,Hong Kong,ChinacReceived 6July 2002;accepted in revised form 25September 2002AbstractFine WC powder of approximately 1m m size was employed as a convenient source of tungsten and carbon in the laser surface alloying of AISI 316stainless steel for improving the cavitation erosion resistance.A slurry containing WC powder was preplaced on the substrate by pasting and processed with a high-power CW Nd:YAG laser to achieve surface alloying.The composition and microstructure of the alloyed layer and the phases formed were investigated by energy-dispersive X-ray spectroscopy,optical microscopy,scanning electron microscopy,and X-ray diffractometry,respectively.The cavitation erosion behavior of the laser surface-alloyed samples in 3.5%NaCl solution was studied with a vibratory cavitation erosion tester.The microhardness of the alloyed layer increases with the total W content in the layer.By employing proper processing parameters,an alloyed layer that is hard but not too brittle can be formed,with a cavitation erosion resistance that may reach more than 30times that of the as-received 316.The improvement in cavitation erosion resistance may be attributed to the increase of W in solid solution and to the precipitation of dendritic carbides,both resulting from the dissociation of the fine WC powder during laser processing.ᮊ2002Elsevier Science B.V .All rights reserved.Keywords:Laser surface alloying;AISI 316stainless steel;Cavitation erosion;Tungsten carbide1.IntroductionCavitation in a fluid is defined as the generation and collapse of cavities (i.e.bubbles )due to local pressure fluctuation arising from sudden change in flow,or from vibration.When bubbles collapse,shock waves and micro-jets are emitted,thus exerting pressure pulses on a solid surface near by.The repetitive attack by these pressure pulses on the solid surface leads to fatigue,fracture and loss of material,and such an erosion mechanism is known as cavitation erosion w 1x .Cavita-tion erosion is a common cause of failure in engineering parts in hydraulic machinery and other liquid-handling systems.AISI 316stainless steel is a popular engineering alloy because of its excellent corrosion resistance.However,owing to the relatively low hardness (e.g.a Vickers*Corresponding author.Tel.:q 852-2766-5691;fax:q 852-2333-7629.E-mail address:apaftche@.hk (F .T.Cheng ).microhardness HV of only approximately 200kgf y mm ),high stacking-fault energy and low tendency to 2stress-induced martensitic transformation,its cavitation erosion resistance is low w 2,3x .As cavitation erosion occurs at the liquid y solid inter-face,and is thus related to surface properties rather than bulk properties,the cavitation erosion resistance of a material may be improved by surface modification.Surface modification has the advantage of consuming only a small amount of expensive material on the surface while using an inexpensive substrate for the bulk.It also allows a large number of combinations of surface and bulk properties,and thus significantly increases the number of options for the design engineer.Laser surface modification is a modern surfacing technique which is becoming more and more popular in engineering pared with other methods of surface modification,it possesses four important characteristics:(1)possibility of forming alloys of non-equilibrium compositions,(2)formation of a fine micro-259K.H.Lo et al./Surface and Coatings Technology165(2003)258–267Table1Laser processing parameters and sample designationsSample Laser power Scanning speed Spot diameter Fluence Overlapping P(kW)v(mm y s)d(mm)F(J y mm)2a ratio(%) WC-316-1 1.5025415.050WC-316-2 1.7520422.050WC-316-3 2.1020426.050WC-316-4 2.1035512.050LSM-316 1.5025415.050 The values of fluence were calculated for a single track.Owing to overlapping,the actual fluence received by a sample was twice the value ashown.structure,(3)presence of a metallurgical bond between the surface layer and the substrate,and(4)resulting in a small heat-affected zone ser surface modifica-tion is especially suitable for local treatment at locations susceptible to erosion attack.Different types of laser surface modification have been attempted on AISI316 stainless steel for improving the cavitation erosion resis-tance by the present authors,and varying degrees of improvement have been achieved w3,5,6x.The present study aims at laser surface alloying on AISI316stainless steel using fine WC powder for improving the cavitation erosion resistance.The incor-poration of carbide particles in the laser surface modi-fication of engineering alloys to enhance the wear resistance has been reported by a number of authors w7–9x.As coarse ceramic particles are beneficial toabrasive wear resistance,the WC used is coarse-grained and clad with a metal(Co or Ni)to minimize the degree of dissolution.Owing to a different mode of attack in cavitation erosion,coarse ceramic particles may not be advantageous as in the case of abrasive wear w10x. Rather,a uniform microstructure containing fine hard phases formed in situ may yield a high cavitation erosion resistance w11x.The present study is an attempt in this direction,with the use of fine unclad WC powder as a convenient source of alloying elements W and C.It has the additional advantage of avoiding the use of strategic or polluting elements like Co and Ni.2.Experimental details2.1.Materials and sample preparationAs-received AISI316stainless steel(designated in this paper as As-316,with composition in wt.%:18% Cr,14%Ni,3%Mo,2%Mn,1%Si,0.08%C,0.045% P,0.03%S,balance Fe)was machined to discs of12.7 mm diameter and3.4mm thickness.2.2.Preplacement of alloying powderA slurry was prepared by mixing fine WC powder (particle size f1m m)and a binder(4wt.%polyvinyl alcohol,PV A).The sample surface was polished with 220grit SiC paper and then the slurry was painted on the sample and dried at1208C for2h.The painted layer was then polished with1000grit SiC to obtain a uniform preplaced coating of thickness of0.3mm.ser surface alloyingLaser surface alloying was performed using a2.5kW CW Nd:YAG laser,with argon flowing at a rate of20 l y min as the shielding gas.Preliminary trials with different values of laser power P,scanning speed v and laser spot diameter d were carried out to determine the feasible processing conditions.Four sets of processing parameters(shown in Table1,with corresponding sam-ples designated as WC-316-X)were chosen for further investigation of the alloyed ser surfacing was achieved by parallel tracks with50%overlap.Such an overlapping ratio was chosen so as to compromise between surfacing efficiency and surface homogeneity. For comparison,laser surface-melted samples(designat-ed in this paper as LSM-316)without the addition of WC were also prepared.2.4.Metallographic and microstructural analysisAfter laser-treatment the samples were sectioned,pol-ished,and etched with acidic chloride solution(25g FeCl,25ml HCl and100ml H O).The average 32thickness of the alloyed layer was determined by image analysis.The microstructure of the alloyed layer was analyzed by scanning electron microscopy(SEM)and optical microscopy.The composition along the depth of the layer was determined by energy-dispersive X-ray spectroscopy(EDS)using a probing area of30=30 m m at each point.The phases formed in the surface 2layer was determined by X-ray diffractometry(XRD) using Cu K a as the radiation source(at40kV and35 mA,with Ni filter).The Vickers microhardness at the surface of the samples was measured at a load of200g and a loading time of15s.2.5.Cavitation erosion testAll the samples for the cavitation erosion test were polished with1-m m diamond paste to ensure consistent260K.H.Lo et al./Surface and Coatings Technology 165(2003)258–267Table 2Properties of various laser surface-alloyed samples SampleMelt depth Dilution Total W HVR eR *e D (mm )ratio,h content (h y m m )(%)a (wt.%)WC-316-10.35147012003208.2WC-316-20.4025521000137735.3WC-316-30.47364490084721.7WC-316-40.313Layer peeled off Layer peeled off Layer peeled off Layer peeled off LSM-3160.60––22045 1.2As-316–––200391The values of h were calculated based on the assumption that the preplaced layer was compact.Actual values should be higher.aFig.1.Cross-sectional views of laser surface-alloyed samples (a )WC-316-2;(b )WC-316-4,an example of cracking.surface roughness.Cavitation erosion tests were per-formed using an ultrasonic vibratory facility conforming to ASTM Standard G32-92w 12x ,with the samples in the unattachment mode.The peak-to-peak amplitude and the vibration frequency used were 100m m and 20kHz,respectively,with a separation of 0.5mm between the sample and the horn tip.The cavitating liquid was 3.5%NaCl solution kept at 238C.The sample was weighed at regular intervals of 30min and converted to a cumulative mean depth of erosion (MDE )w 6x .The mean erosion rate (MER )was calculated at the end of the test period,i.e.240min,and the reciprocal of MER was taken to be the cavitation erosion resistance R .The e surface morphology of the cavitated samples at the end of the test was investigated by SEM.3.Results and discussion3.1.Metallographic and microstructural analysis Under laser irradiation,the preplaced fine WC powder dissolved into the melt pool formed by melting a layer of the substrate.The melt pool rapidly solidified to form an alloyed layer.In the laser treatment,the laser power P ,scanning speed v ,and laser spot diameter d were varied in order to change the energy density (or fluence F )incident on the samples (F s P y (v =d )),and the interaction time (d y v ).Although laser surface alloying y cladding by powder injection is a one-step process and is also more versatile,laser alloying by powder preplace-ment is simple and the preplaced layer,which is inevi-tably porous,is expected to absorb laser energy more efficiently due to multiple reflections.Thus the fluence employed could be relatively low.The average thickness (D )of the alloyed layer for different samples is given in Table 2,together with the dilution ratio h calculated from h s (D y t )y D ,where t is the thickness of the preplaced layer.As pointed outabove,the preplaced layer was not a compact layer,and the actual values of h should be higher than the values obtained from this expression.With a suitable choice of processing parameters,a uniform alloyed layer free of cracks and pores was obtained,as shown in the optical261K.H.Lo et al./Surface and Coatings Technology 165(2003)258–position profiles of laser surface-alloyed samples (a )WC-316-1;(b )WC-316-2;(c )WC-316-3.micrograph of the sample WC-316-2in Fig.1a.When F was too small,an insufficient amount of the substrate was melted and the resolidified layer cracked due to rapid cooling as shown in Fig.1b,corresponding to the sample WC-316-4.On the other hand,when F was too large,an excessive amount of the substrate was melted,resulting in high dilution of the alloying elements.The composition profiles along the depth of the laser surface-alloyed samples are shown in Fig.2.It can be seen that the elements were fairly evenly distributed,with different relative amounts of the alloying elements corresponding to different processing conditions.In view of the fineness of the precipitates relative to the EDS probing area (30=30m m ),the amount of an element 2shown in Fig.2represents the total amount of that element in solid solution and in the precipitates.The phases formed in the alloyed layer were identified using the XRD spectra shown in Fig.3.As a result of the laser treatment,the fine WC powder dissolved in the melt pool and resolidified to form different types of262K.H.Lo et al./Surface and Coatings Technology 165(2003)258–267Fig.3.XRD spectra of laser surface-alloyed samples (a )WC-316-1;(b )WC-316-2;(c )WC-316-3.carbides such as complex metal carbides M C ,M C ,23673M C (M s Fe,W ,Cr ),and tungsten carbides WC and 6W C,together with the g -FeCrNiW phase.The SEM 2micrograph showing the microstructure of the laser surface-alloyed sample WC-316-2is given in Fig.4a.The microstructure,which is typical of one with high carbide content,is similar to the results reported by Ayers and Gnanamuthu w 13x and Riabkina-Fisherman w 14x .Owing to the high concentration of C resulting from WC dissolution in the melt pool,metal carbides were first precipitated out as dendrites,with the inter-dendritic region composed of a eutectic of the g -FeCrNiW phase and carbides.Absence of the original WC particles,which were angular in shape (Fig.4b ),263K.H.Lo et al./Surface and Coatings Technology 165(2003)258–267Fig.4.SEM micrographs of (a )laser surface-alloyed sample WC-316-2;(b )original WC powder.indicates complete dissolution of the preplaced WC.This is not unexpected in view of the fineness of the original WC particles,the high absorption coefficient of laser energy by carbides,and the low Gibbs free energy of 38.5kJ mol for WC.y 1The microhardness values (average of 5measure-ments for each sample )of the laser surface-alloyed samples are given in Table 2.Owing to the presence of the carbides,the extended solid solubility of W in the g phase,and the refined microstructure due to the high cooling rate typical of laser treatment,the microhardness HV of the alloyed layer is significantly increased,from a value of 200HV for as-received 316to approximately 1000HV for the laser-treated samples.The microhard-ness values in the present study are comparable to those reported by Choi and Mazumder w 15x and Nagarathnam and Komvopoulos w 16x in Fe–Cr–W–C coatings syn-thesized on steel.3.2.Cavitation erosion resistanceCurves showing the cumulative MDE as a function of time for various samples in the cavitation erosion test are given in Fig.5a.The corresponding cavitation erosion resistance R and the normalized value R *e e (relative to As-316)are given in Table 2and shown in Fig.5b.The cavitation erosion resistance is increased for all laser-treated samples,reaching a maximum of 35times that of As-316for sample WC-316-2.It is obvious from the discussion above that the presence of the element W plays an essential role in strengthening the alloyed layer via solid–solution hard-ening and the formation of complex carbides.Thus the total amount of W in the layer,as estimated from the composition profile in Fig.2,is an important parameter and is included in Table 2.To ascertain this point,the relationship between HV and the total W content in the alloyed layer is shown in Fig.6a.The microhardness of the alloyed layer increases monotonically with the W content,as is clear from Fig.6a.On the other hand,the cavitation erosion resistance R *is not a monotonic e function of the total W content,as is depicted by Fig.6b.The cavitation erosion resistance of sample WC-316-1is incommensurate with its high W content and hardness.The optical micrographs of the Vickers inden-tations in Fig.7provide some information on the fracture toughness of the alloyed layers w 17,18x .The alloyed layer in sample WC-316-1is too brittle,as is evidenced by the cracking around the indentation in Fig.7a,while no cracks are present in the indentation for WC-316-2.This indicates that an appropriate compro-mise between hardness and fracture toughness or ductil-ity (opposite to brittleness )would lead to high cavitation erosion resistance,as has been pointed out by Zum Gahr w 19x and Wang et al.w 20x .The higher brittleness of WC-316-1could be attributed to a relatively higher W C content as shown in the XRD spectra in Fig.3.2The SEM micrographs in Fig.8a and b showing the cavitated surface of the as-received sample (As-316)and the laser surface-alloyed sample WC-316-2reveal entirely different morphologies of damage.In As-316sample,the surface was severely eroded after the 4-h test,with a morphology typical of ductile fracture.On the other hand,the damage was much milder in WC-316-2sample,with the interdendritic region preferen-tially eroded away,leaving behind a delineated dendritic microstructure.An additional point to note is that both the hardness and the cavitation erosion resistance of the laser surface-melted sample are only slightly higher than the as-received sample.This suggests that in the present case,the improvement in the laser surface-alloyed sam-ples mainly results from W solid–solution strengthening and second-phase (carbides )strengthening,and not from grain refinement.264K.H.Lo et al./Surface and Coatings Technology165(2003)258–267Fig.5.(a)Cumulative MDE as a function of time in cavitation erosion test in3.5%NaCl solution at238C;(b)Relative cavitation erosion resistance of various samples.265K.H.Lo et al./Surface and Coatings Technology 165(2003)258–267Fig.6.(a )Relationship between microhardness HV and total W content in alloyed layer;(b )Relationship between cavitation erosion resistance R *and total W content in alloyed layer.e In a previous study,laser surfacing of AISI 316L stainless steel has been attempted using WC powder of approximately 40m m diameter w 6x .The use of coarse-grained WC powder as the added material led to the formation of an MMC layer with unmelted carbide particles in a metal matrix.An increase of 8.5times in cavitation erosion resistance was achieved,which is lower than that in the present case.Thus,it seems that complete dissolution of WC followed by in situ forma-tion of carbides by precipitation from the melt poolwould lead to a more homogeneous microstructure and a stronger interface at the carbides,both being respon-sible for a higher erosion resistance,in addition to the presence of W in solid solution.4.ConclusionsLaser surface alloying on AISI 316stainless steel for enhancing cavitation erosion resistance by employing fine WC powder has been attempted and the following conclusions are drawn.266K.H.Lo et al./Surface and Coatings Technology165(2003)258–267Fig.7.Optical micrographs of Vickers indentation(a)WC-316-1;(b) WC-316-2(both at500-g load).Fig.8.SEM micrographs of cavitated surface of(a)As-316;(b)WC-316-2,both after240min in cavitation erosion test.(1)The cavitation erosion resistance of the lasersurface-alloyed samples is significantly improved,andmay reach more than30times that of as-received316in the most favorable case.(2)The high cavitation erosion resistance of the laser surface-alloyed samples could be attributed to a micro-structure composed of carbide dendrites and interden-dritic carbide y g-FeCrNiW eutectic.(3)The microhardness of the alloyed layer increases with the total W content in the layer.W plays an essential role in strengthening the alloyed layer via the formation of precipitated complex carbides and solution hardening.(4)The maximum cavitation erosion resistance occurs at a moderate microhardness of approximately1000HV and then decreases because the deleterious effect of brittleness becomes prominent at higher hardness. 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