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Journal of Materials Processing Technology 171 (2006) 167–174
lloy deposited by PTA on carbon and stainless steel. Results showed that the use of pulsed current leads to a finer microstrucardness and lower dilution. The role of the substrate steel depends on the set of processing parameters used but for a same set
t determines microstructural features of the coatings.2005 Elsevier B.V. All rights reserved.
Hardfacing is a technique used to enhance surface prop-rties of a metallic component as a specially designed alloy
s surface welded in order to achieve specific wear proper-ies. Surface properties and quality depend upon the selectedlloys and deposition processes. Among the latter powder
eeding plasma transferred arc (PTA) allows for homoge-eous refined microstructure, low distortion and dilution,esulting on enhanced surface properties[1,2] when com-ared to hardfaced deposits processed with conventionalelding processes.PTA process employs the plasma principle hence it may
e considered an evolution of GTAW process, where theigh-energy concentration is due to the use of a constrictorose, which restrains the column diameter of an electric arcstablished between a tungsten electrode and the workpiece
n an inert gas atmosphere, usually argon. Feeding material
is carried to the plasma jet by a gas stream, which mbe inert, active or a mixture of active and inert gasethird gas flow is employed to protect the metal pool fratmospheric contamination. Even though there is thesibility of using mixtures of active and inert gases, aris typically employed for all three-gas systems[1,3]. Workdone by several researchers included the comparisonother hardfacing processes[4–6], wear and corrosion restance behaviour[7–9] and microstructure features[10,11]for different alloys. None of these researches referreduse of pulsed current although it is an interesting alternto continuous current either from the process or metagical standpoint. Process main advantage is the possof working with high current peaks without increasingaverage heat input (energy) to the substrate. From theallurgical standpoint, the use of pulsed current on contional surface welding processes has been associatedfiner structure due to an increase of the melt pool agitaRefinement of the solidification structure has been attribto an enhanced nucleation rate due to dendrite broken[12].
Table 2Processing parameters for continuous and pulsed current deposits
Continuous currentHardfacing current (A) 170 220Plasma gas (l/min) 2.05 2.20Shield gas (l/min) 15 15Carrier gas (l/min) 3.0 3.0Powder feeding rate (g/min) 22 22Hardfacing speed (mm/min) 100 100
Pulsed currentPeak current (Ip) (A) 170 220Peak time (tp) (ms) 5 5
Base current (Ib) (A) 75 75Base time (tb) (ms) 8 8
This work evaluates the influence of pulsed current andsubstrate material on coatings quality requirements, dilutionand microstructure, and their effect on coatings hardness. Anhigh carbon cobalt alloy, known by its restricted weldabilitybut excellent wear properties, was PTA deposited on stainlesssteel and low carbon steel substrates.
2. Experimental procedures
An atomized cobalt-based alloy, particle size within therange 45–235�m, was deposited by plasma transferred archardfacing process on AISI 304 stainless steel and AISI 1020carbon steel plates, 75 mm× 100 mm× 8 mm. As receivedchemical composition (wt%) of substrate and coating mate-rial is presented inTable 1.
Two sets of specimen were processed, on both substratesusing two current intensity levels, one with pulsed current andthe other for comparison purposes with continuous currentcorresponding to the peak currents used (Table 2). Process-ing parameters were set to obtain a significant difference indilution levels[13] and different energy densities.
Coating characteristics were evaluated by visual inspec-tion and dye penetrant for the presence of cracks and pores.Dilution was evaluated as the area ratio between the substrate
melted area and total melted area (Fig. 1). Microstructuralanalysis was performed by optical and scanning electronicmicroscopy on the transverse section of specimens. Vickersmicrohardness profiles were done under 500 g load, on thissame section. Quantitative metalography was used to deter-mine interdendritric spacing.
3. Results and discussion
3.1. Surface features
Good surface quality evaluated by visual inspection anddye penetrant showed a non-crack and non-porosity conditionon the deposits produced, exception being tracks ends.
A general view of the coatings processed is presentedin Fig. 2. It is interesting to notice that even though nopre-heating was done no cracks were observed on coatingsdeposited on both substrates with both current modes. Thisresult confirms the better quality of the processing proceduresused, as high hardness alloys typically require pre-heatingto reduce cracking susceptibility[1]. Dye penetrant analysiscompleted surface evaluation and showed a non-crack andporosity condition, confirming the good surface quality ofthe coatings. However, deposits on carbon steel showed pro-cessing defects, like undercut, on pulsed current specimens,r rthere
3
torw lutionr s ares uc-t ssedw singp er-a rrents rrenti nten-s sing,d lowerf f thes witht sity,ra steel
sed to
Fig. 1. Procedure u
esulting on the exclusion of these specimens from fuvaluation.
.2. Dilution
As mentioned before, dilution is a determining fachenever one wants to assess coating properties. Di
esults measured for the different processing conditionhown inFig. 3. For the two current intensities levels, a redion on dilution levels was measured for specimens proceith pulsed current. This is expected as for the procesarameters used[14], continuous current specimens avge heat input is more significant than for pulsed cupecimens and dilution is known to be dependent on cuntensity. In the latter, although peak current reached an iity identical to that used for continuous current procesue to the low base current the average heat input was
or these sets of specimens. Regarding the influence oubstrate material on the measured dilution, results varyhe current intensity used. For the lower current intenesults agree with the work of Yaedu[13] and Colac¸o [15],s dilution decreases for coatings deposited on carbon
evaluate dilution levels.
A.S.C.M. D’Oliveira et al. / Journal of Materials Processing Technology 171 (2006) 167–174 169
Fig. 2. General view of the deposited tracks.
Fig. 3. Dilution levels measured on the coatings deposited on different substrates (CS, carbon steel; SS, stainless steel), and processed with different currentmodes.
170 A.S.C.M. D’Oliveira et al. / Journal of Materials Processing Technology 171 (2006) 167–174
substrates compared to the measured results on the alloyedsteel. However, for the higher current intensity dilution isnot influenced by the chemical composition of the substrate,suggesting that the heat input plays a major role, overcomingsubstrate features such as its thermal conductivity.
3.3. Solidification kinetics
Solidification kinetics were evaluated by microstructureanalysis.Fig. 4shows coatings microstructures, as observedunder optical microscopy, near the external surface and thefusion line. A typical solidification structure is observed withdendrites of a Co-rich matrix and a carbide interdendriticregion.
Current mode effect on microstructure is shown inFigs. 4 and 5. A structure refinement occurs after processingwith pulsed current. This was observed through the coatingthickness from the interface with the base material to the
external surface. Literature has attributed structure refine-ment after processing with pulsed current to an increase onnucleation rate, as dendrites arms break due to the melt poolagitation[12]. In the present study, the increase on nucle-ation rate cannot be attributed exclusively to molten poolagitation. Compared to powder particles size, dendrites armsare too fine to survive in the melt pool, and are expected toremelt. Previous research work[16] has shown that depositsobtained using powder as feeding material exhibited finerstructures compared to those obtained using feeding materialin the wire form. This difference was attributed to a change onsolidification kinetics, where an increase on nucleation ratewas a consequence of the clustering of powder particles actingas solidification nuclei. Following the same theory structurerefinement can be attributed to the formation of smaller clus-ters of powder particles due to the current pulses, resulting onincreasing nucleation rates. During pulsed current process-ing, undercooling is larger due to the increased temperature
Fig. 4. Microstructure features coatings n
ear the external surface and fusion line.
A.S.C.M. D’Oliveira et al. / Journal of Materials Processing Technology 171 (2006) 167–174 171
Fig. 5. Coatings microstructures near external surface as observed under SEM.
gradient in the melt, as a consequence smaller nuclei areallowed to survive. This higher nucleation rates should leadto an enhanced growing rate, favouring segregation, ratifyingthe higher amounts of interdendritic region observed near thefusion line on pulsed current deposits (Fig. 4). As growth ini-tiates, due to the planar solidification front, one could expecta rapid build up of solute ahead of the solid. After solidifica-tion growth stabilizes away from the fusion line, a steady stateis expected to be reached, accounting for the more homoge-neous structures observed through the melt pool[17].
This behaviour was observed for both current intensitylevels, the major difference being a general structure coars-ening as current intensity is raised, as confirmed by scanningmicroscopy analysis (Fig. 5) and quantitative metalography(Figs. 6 and 7).
The influence of substrate properties on solidificationstructure depended on the current intensity used (Fig. 4). Infact for the lower current intensity, 170A, coatings depositedon carbon steel substrates exhibited a finer structure that those Fig. 6. Secondary arm spacing near surface.
172 A.S.C.M. D’Oliveira et al. / Journal of Materials Processing Technology 171 (2006) 167–174
Fig. 7. Secondary arm spacing near fusion line.
deposited on stainless steel. As described by Yaedu[13],the higher thermal conductivity of the former responds fora faster cooling rate leading to a finer structure. However, forthe higher current intensity used, 220A, the influence of sub-strate properties is overcome by the increase on heat input.As a consequence, final microstructures depended mainly onthe processing parameters.
3.4. Surface hardness
Coatings mechanical properties assessed through theirhardness profiles confirmed the determining role of
solidification kinetics and dilution, for the two currentintensity levels, respectively. As described, higher cur-rent intensities resulted on coarser structures and higherdilution levels, the expected lower hardness is confirmedon the measured profiles (Figs. 8 and 9). Regarding pro-cessing with pulsed current, mechanical behaviour wasexpected to be superior in these coatings when comparedto deposits obtained with continuous current as dilutionlevel was reduced and a structure refinement observed.Hardness profiles confirm this behaviour as a higher hard-ness was measured on deposits processed with pulsed cur-rent.
As to the influence of substrate material, it wasobserved that its role depended on the current inten-sity, again hardness profiles agree with the previousanalysis. For the lower current intensity level, coatingsdeposited on carbon steel substrate exhibited a higherhardness than those deposited on stainless steel sub-strate. As dilution levels increased and structure coars-ened, on the higher current intensity level used inthis work, the role of the substrate properties wasexpected to be less important than before. However,although a general reduction on the magnitude of hard-ness was measured, hardness profiles are still higherfor deposits obtained on carbon steel substrates. Hard-ening differences between coatings deposited on car-b uredf sityl
Fig. 8. Hardness profiles for coatings deposit
on and stainless steels are now half of that measor specimen processed with the lower current intenevel.
ed with 170A, continuous and pulsed current.
A.S.C.M. D’Oliveira et al. / Journal of Materials Processing Technology 171 (2006) 167–174 173
Fig. 9. Hardness profiles measured on coatings processed with the higher current intensity level.
4. Conclusions
• Pulsed current processing resulted on finer and morehomegenous solidification structures and lower dilutionlevels, and as a consequence on coatings exhibiting higherhardness.
• Current intensity determined the role of the chemicalcomposition of the substrate. Higher current intensitiesreduced substrate influence on dilution and structure fea-tures, but for both current intensities used coatings hard-ness was higher for deposits produced on carbon steelsubstrates.
• Increasing current intensity levels resulted on higher dilu-tion and lower hardness.
Acknowledgments
The authors would like to thank to Agencia Nacional doPetroleo (ANP) for funding this work and for the scholarshipof Mr. R.L.C. dos Santos. Thanks are also due to Delore Stel-lite for material supply, in particularly to Mr. Sergio Simoesfor helpful discussions.
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