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Morphology and characterization of lar clad composite NiCrBSi –WC
coatings on stainless steel
M.J.Tobar ⁎,C.Álvarez,J.M.Amado,G.Rodríguez,A.Yáñez
Dpto.de Enxeñería Industrial II.Universidade da Coruña.15403Ferrol,Spain
Available online 20December 2005
Abstract
In this work,veral mixtures of lf-fluxing NiCrBSi alloy powder and a nickel-clad WC powder (10wt.%Ni and balance WC)were lar cladded on stainless steel substrates of austenitic type (AISI 304).The aim of the study was to determine the influence of the volume fraction of the reinforced WC particles on the formation and performance of the composite layer.The effect of other parameters of the treatment,such as the lar energy,beam profile,traver speed and the mass rate of the feed powder was also investigated.Clad layers of 0.5–1.5mm height were obtained,its microhardness measured and the microscopic morphology and distribution of tungsten carbide particles within the layer characterized by scanning electron microscopy (SEM).It was found that most clad layer properties such as its porosity,microhardness and homogeneity are determined by the percentage of watch your back
WC particles in the mixture.Pores were obrved for volume fractions roughly above 50%.Below this limit,homogeneous,den and crack free clad layers were obtained,with measured hardness ranging between 600and 1000HV depending on the WC content.
©2005Elvier B.V .All rights rerved.
Keywords:Lar cladding;NiCrBSi;WC;Ceramic materials;Abrasive wear
1.Introduction
Among the wide scope of lar surface treatments [1]stands the so called lar cladding technique [2],which aims at obtain-ing high performance alloy coatings on steel substrates.In this process,a thin surface layer of the substrate is melted by the lar beam together with the additive material,such as nickel or cobalt alloy powder,to form the coating.The melting mecha-nism and the fast heating and cooling rates generated in the material result in den coatings,with a fine microstructure and metallurgically bonded to the substrate.
NiCrBSi coatings are widely employed to improve the qual-ity of components who surface is submitted to wear and corrosion,especially at high temperatures.The prence of boron and silicon o
n its composition lowers the melting point of this type of alloys and gives them the “lf-fluxing ”character which is especially suited for plasma spraying or HVOF coat-ing techniques.A remarkable aspect of this type of alloy powders is the possibility of including hard ceramic particles
such as tungsten carbides (WC)in order to increa the coating hardness and abrasive wear resistance.Compared to other carbides,tungsten carbide combines favorable properties such as high hardness,certain plasticity and a good wettability by molten metals [3].Conquently WC carbides are widely ud as a hard pha for manufacturing metal composites for com-ponents expod to high wear intensities,as cutting tools or mining machinery [4].
Several studies have investigated the characteristics and performance of thermal Ni –WC coatings [5–7]showing that,in general,the nature of the thermal spray techniques limits its application in very high demanding environments.The coating layers so attained are not free from pores and show a relatively weak (mechanical)bonding between the coat and the substrate and between the different alloy mixture components.
Such drawbacks could in principle be overcome using lar cladding and,in fact,veral studies have already been pub-lished [8–14]on this kind of lar produced Ni –WC coatings.The many parameter
s involved (lar power and scan speed,shielding gas and powder carrier gas flow,materials employed and its properties,etc.)complicate the process development and optimization and,as a conquence,different results can be en at the literature regarding the coating pha structure
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⁎Corresponding author.Tel.:+34981337400x3407;fax:+34981337416.E-mail address:cote@cdf.udc.es (M.J.Tobar).
0257-8972/$-e front matter ©2005Elvier B.V .All rights rerved.doi:10.1016/j.surfcoat.2005.11.093
performance.In summary,it is found that the WC content for the Ni-bad cladding should not exceed the45vol.%,other-wi resulting in large pores and poor bonding[8].The wear resistance of a Ni coating is improved from the moment a small concentration of tungsten carbide is added:a5vol.%,WC is already enough to lower the wear by a factor of10[10].For wear resistant applications,the WC particle distribution within the coating should be high and uniform,especially at the top of the coating,and a good bonding should exist between the WC particles and the matrix.But the coating morphology is found to be strongly dependent on the operating parameters[12].A careful
lection of the lar input power,scan velocity and powder feeding rate must be done in order to avoid effects such as the drift of WC particles to the bottom,WC particle disso-lution or dilution of the coating.As with respect to crack formation,the propensity for cracking ems quite difficult to avoid although some authors have reported[14]that this is significantly reduced by preheating the substrate.The type of WC particles employed,spherical or crushed,is another aspect of the lar coatings that has been studied[14].It is found that the spherical particles are less dissolved in the matrix.Besides, the spherical shape minimizes crack initiation on sharp edges of the WC particles.
In this work we aim at making protective layers of a mixture of NiCrBSi with WC ceramics on AISI304stainless steel by means of lar cladding.WC particles are added to the alloy powder so as to obtain coatings with better abrasive wear resistance than that of NiCrBSi.In order to evaluate the effect of the WC content in the mixture,experiments were performed with different mixing ratios.Changes in the coating morphology on different operating parameters were investigated.The results obtained and the main conclusions are prented in this paper.
2.Experimental
Box shaped workpieces with dimensions200mm×50 mm×5mm were machined out of austenitic stain
less steel AISI304and ud as substrate material.A mixture of a NiCrBSi alloy powder(METCO16C)and a nickel-clad WC powder(WOKA3303)was ud as coating material.The composition for both powders and the steel substrate is listed in Table1.The size of the powder particles was in the range of 45μm to135μm for the NiCrBSi alloy and11μm to45μm for the WC clad powder.In Fig.1a magnified view of the WOKA 3303powder particles is shown,which conform to nickel-bound spherical agglomerates of WC grains.
The cladding process was achieved by means of a2.2kW industrial CO2lar(Rofin Sinar)and off-axis powder feeding. The feeding system compris of a feeding unit(Sulzer Metco)and a powder exit asmbly attached to the lar head.A cooper nozzle of1mm diameter is mounted on this asmbly for delivering the powder at about10–15mm distance from the meltpool and with an angle of60°with respect to the lar scan direction.
In the experiments,single lar tracks were produced on the substrate material,corresponding to a lar spot size of∼4mm. The output power and scan velocity of beam,as well as the powder mass flow rate were varied in order to investigate their influence on the height,morphology and bonding to substrate quality of the resulting clads.Range for the variations was500W to2250W for the beam power P,100to1100mm/min for the scan velocity v and5to20g/min for the power mass flow rate q.All one of us
experiments were performed under N2atmosphere(10l/min).Nitrogen was also ud as powder delivering gas.
All specimens were cut transversally,mechanically grounded, polished in diamond paste and etched with a mixture of acetic, nitric and chloride acids.The cross-ctional view of the resulting clads was examined by scanning electron microscopy(SEM)and its composition measured by energy dispersive X-ray(EDX) analysis.Microhardness measurements were done along clad cross-ction from substrate to clad surface.
3.Results and discussion
For the range of parameters ud in the experiments,single clad layers were obtained of2mm to4mm width and0.5mm to2mm height.Metallurgical bonding with the substrate was obtained for most of the samples,except for tho cor-responding to the lowest power energy densities,where clad detachment was obrved.The SEM/EDX inspection of the cross-ction of the NiCrBSi clad layers obtained using only the METCO16C powder shows a den,uniform and crack free structure.From substrate to surface,the analysis reveals a plane front with diffusion columnar grains,associated to the high temperature gradients ahead of the interface.A dendritic matrix follows,which is mainly badecimeter
d on gamma-nickel plus an interdendritic eutectic with mixed borides and carbides in be-tween.This microstructure profile agrees with that obtained by other authors for similar NiCrBSi alloys[15].
Prior to investigation of the morphology effects of adding WC particles to the NiCrBSi alloy,tests with the WOKA3303 (10wt.%Ni and balance WC)ceramic powder were performed. During the experiments,meltpool instabilities were obrved, resulting in a toothedge appearance along the clad.Such effect was partially addresd to the high melting point of the cera-mics and its high concentration in the powder,resulting in a low fluent material in the meltpool and thereby promoting the formation of ripples[16,17].A smoother surface was obtained
Table1
Composition(in wt.%)of the alloy powders and steel substrate ud in the experiments
B C Cr Cu Fe Mn Mo Ni P S Si W AISI304b0.0818–20bal.b28–10.5b0.045b0.03b1
METCO16C40.5163 2.53bal.4
WOKA3303 5.50.0310.90bal. 6314M.J.Tobar et al./Surface&Coatings Technology200(2006)6313–6317
for tho tests performed at the larger scan speeds,but even in tho cas big pores were encountered on the SEM view.If the concentration of WC particles in the clad powder is decread to a 50\wt.%by mixing the NiCrBSi and the WC10Ni powders in suitable proportions,the situation ameliorates significantly though some porosity still persists.
If the WC ratio in the NiCrBSi –WC mixture is further decread to a 25wt.%pore free coatings are obtained.Cracks could not be completely avoided but the rate for cracking was significantly reduced.Best results were achieved by tting the beam power to its maximum (P =2250W)and scanning speeds between 300and 700mm/min.Coatings of different height and WC particle density are obtained depending on the powder feeding rate.The morphology of the 75%NiCrBSi –25%WC coatings is shown in Fig.2.Parameters for this particular ca were:P =2250W,v =700mm/min and g =20g/min.The dis-tribution of the WC particles across the whole ctional area is rather uniform,although a higher concentration is en near the interface.Probably convection currents within the meltpool are not efficient enough to avoid the ceramic particles from sinking to the bottom,as explained in [9].Such effect can be avoided
by reducing the scan speed,as in this way the lar energy density increas and so does the Marangoni convection cur-rents.Fig.3shows a magnified view at the bottom part of the coat obtained
with P =2250W,v =300mm/min and g =20g/min,where an isolated WC ceramics can be obrved at the clad –substrate interface.The melting mechanism has not been able to melt the ceramic particle and disaggregate the WC grains,so it appears with the initial agglomerate WOKA 3303appearance.Blocks are visible at the particle boundaries which composition was measured by EDX yielding 27C –30Cr –24W –10Ni –4Fe –5Mo (at.%).This indicates that the outer part of the WC grains must have melt and diffud into the liquid alloy,solidifying in mixed carbides.The same pattern is repeated throughout the coating area,as shown in Fig.4,ensuring a good bonding between the WC particles and the Ni matrix.
For scan speeds below 300mm/min or above 700mm/min,deviations from the above coating structure are
clearly
Fig.2.Cross-ctional view of a single lar clad layer obtained with 25%WC content (wt.%)in the NiCrBSi –WC powder mixture.Process parameters:P =2250W,v =700mm/min;q =20
g/min.Fig.3.WC particle at the substrate –clad interface with 75%NiCrBSi –25%WC (wt.%).The ceramic appears with the initial spherical agglomerate appearance.Carbides are formed at the boundaries due to partial dissolution of the WC particles into the matrix.Process parameters:P =2250W,v =300mm/min;q =20
g/min.
Fig.4.Good bonding between WC ceramics and matrix by mixed carbides throughout the 75%NiCrBSi –25%WC (wt.%)clad cross-ction.Process parameters:P =2250W,v =300mm/min;q =20
g/min.
Fig.1.SEM view of the WOKA 3303powder particle morphology.
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obrved.In the first ca,the higher energy density produces a significant dissolution of WC particles in the matrix and a higher dilution from the substrate.The large amount of C,W,Fe and Cr dissolved in the melt material promotes the formation of mixed carbides on solidifying,resulting in a den distribu-tion of carbide blocks as shown in Fig.5.In the cond ca,at higher processing speeds,the energy density is not enough to melt and diffu the boundaries of the ceramic agglomerates into the matrix.As shown in Fig.6,no carbide blocks are thus formed around the WC particles and,conquently,the cera-mics and the matrix must be poorly bonded.In some cas,especially with the particles of bigger size,a clear detachment from the matrix is obrved.Bad wear resistance for this type of coating structure is therefore to be expected.
Fig.7shows the measured microhardness for some of the obtained coatings,corresponding to the different powder com-positions tested.The NiCrBSi alloy prents a uniform hard-
ness clor to 550HV with a uniform behaviour.WC10Ni shows significant higher hardness but with strong variations from one point to the other which can be attributed to the WC particles being agglomerate into a weak Ni-bad matrix.Finally,the tested WC –NiCrBSi mixtures have a more uni-form behaviour,with mean values proportional to the WC content.4.Conclusions
In this work,lar clad NiCrBSi –WC coatings were pro-duced by combining two materials commercially available,METCO 16C and WOKA 3303.Den and pore free layers were obtained as far as the WC content in the mixture was maintained at least below 50wt.%.Otherwi the quite differ-ent melting points of the materials yield a low fluent and unstable meltpool,making difficult the lar treatment and causing in the end important porosity and crack levels.For a 25\wt.%content optimum parameters were found which pro-duce clads with a uniform WC distribution and good bonding with the matrix.SEM inspection of the obtained coatings shows that the WOKA 3303WC particle agglomerates appear partially dissolved at the boundaries forming mixed carbides within the Ni-bad matrix.The hardness of the coating is found to increa with respect to that of the ba steel and proportionally to the WC content in the WC –NiCrBSi powder mixture.Further work is needed in order to characterize the performance of the coatings in terms of wear and corrosion resistance.
Acknowledgements
The authors would like to acknowledge financial support from Xunta de Galicia through Project Ref.PGIDT03D-PI16601PR.Support from the Spanish System of Science and Technology through the Ramon y Cajal Program is also grate-fully acknowledged by the first
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Fig.5.High density of carbides due to WC dissolution and dilution from the substrate at low scan speeds due to the higher energy density:P =2250W,v =100mm/min;q =20g/min.Powder composition:75%NiCrBSi –25%WC
(wt.%).
Fig.6.Bad bonding between the WC particles and the matrix at the higher scan speeds:P =2250W,v =900mm/min;q =20g/min.Powder composition:75%NiCrBSi –25%WC (wt.%).
–0.4–0.200.20.40.60.81 1.2 1.4
20040060080010001200
1400Distance from substrate (mm)
M i c r o h a r d n e s s (H V )
Fig.7.In-depth profile of the measured microhardness for different mixing NiCrBSi/WC ratios.The WC (NiCrBSi)label refer to the layers obtained using exclusively WOKA 3303(METCO 16C),in the clad powder.
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