Laser Cladding Of Stellite Research Articles

Design and performance of nitrogen-alloyed iron-based coating for enhancing galling resistance by plasma transferred arc welding

Iron-based alloys have been widely used as high-temperature galling resistant coatings. In this study, we designed and prepared a galling-resistant coating using plasma transferred arc welding with cored wires of varying nitrogen content. Our approach was based on the concept that nitrogen alloying can lead to the formation of hard precipitates and promote strain-induced phase transformations. The coating was primarily composed of austenite, and as the nitrogen content increased, the grain size decreased and the amount of nitride precipitation increased. The threshold galling stress of this cobalt-free coating was found to reach 80% of that of a laser-clad Stellite 21 coating at 300 °C. Following the galling test, the hardness of the worn area increased and a phase transformation from austenite to martensite occurred beneath the worn surface. The improvement in galling resistance under dry heavy load sliding conditions can be attributed to the synergistic effect resulting from multiple strengthening mechanisms, including fine-grain strengthening, second phase strengthening and phase transformation strengthening.

Sliding wear behavior of cold metal transfer cladded Stellite 12 hardfacings on martensitic stainless steel

Cold metal transfer (CMT) welding technique was used in the manufacturing of hypoeutectic carbide-type Co-based Stellite 12 hardfacings on martensitic stainless steel. It was discovered that the CMT process is capable of producing relatively thick (> 2.5 mm) low diluted single-layer coatings with cored Stellite 12 wire without cracks and pores. These coatings were investigated using microscopy (optical and scanning electron microscopy), X-ray diffraction, and hardness measurements. The high melting point chromium and tungsten particles inside the cored wire were relatively large and therefore remained unmelted in the clad layers. Self-mated sliding wear tests were performed using a pin-on-disc tribometer at room temperature and at 300 °C to determine the wear resistance and friction of the coatings. The coefficients of friction were relatively similar (~ 0.35) at both temperatures. Differences were exhibited in the ~ 40% greater loss of material at high temperature. The wear performance of the CMT clad Stellite 12 coatings did not, however, reach the wear performance of self-mated laser clad Stellite 6 reference material. CMT hardfacing was finally successfully demonstrated on a ring-shaped component.

High-temperature abrasive wear characteristics of H13 steel modified by laser remelting and cladded with Stellite 6 and Stellite 6/30% WC

The study is aimed to analyse the comparative behaviour of the high-temperature abrasive wear of H13 steel surfaces modified by laser melting and cladding with Stellite 6 and Stellite 6 + 30 wt% WC. 3-body abrasive tests were conducted at room temperature, 450 °C, 550 °C, and 650 °C. The microstructural evolution, microhardness, wear surface morphology and mechanisms, and various phases formed during laser surface modifications were also studied. The laser remelting of H13 steel surface increased its room temperature microhardness to 750 ± 35 HV0.01, whereas laser cladding of Stellite 6 powder yielded hardness of around 600 ± 20 HV0.01 in the clad layer; and Stellite 6/WC composite clad layer had marginally higher hardness than the Stellite 6 clad layer in the matrix and much higher hardness of ~3000 HV0.01 at the sporadically distributed WC particle sites. Though the room temperature microhardness of laser remelted H13 surface is the highest, the volumetric wear loss in it was comparable to that of the Stellite 6 cladding. However, Stellite 6/WC composite layer recorded a relatively less volumetric loss as WC particles resisted the abrasive wear. With increasing temperature, the wear loss in laser remelt surfaces increased at a fast rate, while that in Stellite 6 and composite clad layers varied marginally with no definite trend. Overall, Stellite 6/WC composite cladding performed better than others in the current temperature range.

On the Processing Feasibility, Microstructure Evolution, and Mechanical Properties of Laser Clad Stellite® 21 Alloy

Laser cladding is widely used for feature repair and manufacturing of large structural components. However, due to processing parameters and thus energy densities variation, defects can form in laser clad materials. There is a need to understand the relationship between energy densities and cladding quality. This study therefore investigates the critical parameters in producing structural Stellite® 21 (Co-Cr-MoNiC) alloy by laser cladding. Compared with conventional manufacturing, laser clad materials possess ultrafine α-Co dendrites and σ-CoCr interdendritic eutectics due to the fast cooling rate. In addition, adjustment of the energy densities can help control defects and improve corrosion resistance. The wear resistance, microhardness, and phase analysis in the laser clad materials are also discussed in this work.

Influence of buffer layer on surface and tribomechanical properties of laser cladded Stellite 6

Wear resistance coatings of Stellite 6 were deposited on SS316 substrate by blending a buffer layer of Inconel 625 with varying Linear Heat Inputs (LHI). The mechanical and microstructural properties of Stellite 6 coatings are investigated to evaluate the effect of buffer layer. The surface flaws, microstructure, micro-hardness and wear resistance of Stellite 6 clads with buffer layer are studied and compared with direct deposited Stellite 6. The results show an increment in clad height by 30% and 9% at lower and higher LHIs, respectively as compared to pre-deposited Stellite 6 clads. The deposition pattern (with and without buffer) are free from surface and internal cracks. The dilution is reduced by 25% and 10% whereas the microhardness shown an improvement by 12% and 5% with addition of buffer layer. Further, a fine-grain microstructure is observed in buffer layer clads at the interface zone. Moreover, the studies on wear rate have shown improvement of 14% and 3%, respectively with low coefficient of friction (COF) in buffer layer deposition because of high microhardness and fine grain microstructure. Thus, laser cladding of Stellite 6 at lower LHI with buffer layer has shown excellent mechanical and microstructural properties.

Effect of Cryogenics-Assisted Low-Plasticity Burnishing on Laser-Clad Stellite 6 over SS420 Substrate

The influence of modern additive manufacturing methods, especially from the direct energy deposition (DED) processes to the coat-like finished components, is crucial under present industrial circumstances. DED induces several traits like enhanced mechanical, thermal properties in shorter lead time, which extend their adaptation for diverse applications including aerospace and automobile industries. Among the several DED processes, laser cladding has been a prospect that explores various capabilities of improving the wear resistance of cobalt-chromium (Co-Cr)-based alloys. Rather than fabricating the complete component using expensive alloys, laser cladding has paved an approach to deposit particles possessing superior qualities over the conventional material. This research work attempts to evaluate the surface integrity of SS420 when cladded with Stellite 6. The vertical face milling is executed on the cladded component surface to facilitate either low-plasticity burnishing (LPB) or cryogenic burnishing (CB) as sequential post-treatment processes. The effects of these post-treatments on the surface and subsurface microhardness, surface topography and residual stress profiles are elaborated. Increased surface and subsurface microhardness, as well as improved residual stress profiles, are observed with CB over LPB-processed specimen samples.

Laser cladding of Stellite 6 on EN8 steel – A fuzzy modelling approach

Laser cladding is a very popular technique for the process of providing protection to the surface of materials that are subjected to extreme working conditions. This article describes the process where Stellite 6 alloy is added on to the surface of EN8 steel in a controlled manner. EN8 steel is the material that is used as a base material as it the commonly used material in the manufacturing sector. Stellite 6 is a cobalt base super alloy which is characterized by extreme wear resistance and high hardness. The laser cladding process is done whilst considering parameters like laser scanning speed, laser focus height, laser power and powder feed. The laser cladded samples are corrosion tested in an acidic environment and the electron images of the samples are taken. From the electron microscopy images the corrosion is classified in order to be fed to a fuzzy logic model. This fuzzy logic model is used to deduce a corrosion index and this corrosion index is further optimized using Taguchi method and the results have been reported.

Studies on effect of laser processed stellite 6 material and its electrochemical behavior

Laser cladding is one of the popular surface modification processes for providing enhanced wear and corrosion resistance to steels in various applications. In this article the investigations on laser cladding of Stellite 6 on EN8 steel has been reported. Stellite 6 is an extremely wear and corrosion resistant alloy and this study reports the corrosion behaviour of the cladded samples. Nine cladded samples were developed with the combination of different process parameters. Each sample was tested for corrosion behaviour using 1 M sulphuric acid solution. Metallurgic studies on the nine samples were conducted and the results have been reported. A clear correlation between the laser cladding parameters and the corrosion behaviour has been established.

Corrosion and Wear Performance of Stellite Alloy Hardfacing Prepared via Laser Cladding

Stellite 6 alloy hardfacing has been popularly applied on the seat face of control valves where both wear and corrosion in various corrosive media are involved. In this research, Stellite 6 alloy hardfacing is prepared via laser cladding and the wear performance and corrosion behavior in morpholine, 3.5 wt % NaCl, and Green Death solution, of this hardfacing are investigated using a pin-on-disc tribometer and a three-electrode cell, respectively. The worn and corroded surfaces are analyzed using SEM/EDS. For comparison, cast Stellite 6 is also studied in parallel. The experimental results show that cast Stellite 6 has better wear resistance than laser clad Stellite 6 but better corrosion resistance in the corrosive media due to better retention of the oxide film on the specimen surface.

Microstructural evolution and wear behaviors of laser-clad Stellite 6/NbC/h-BN self-lubricating coatings

Cobalt-based self-lubricating coatings with the addition of NbC and h-BN were prepared by means of laser cladding on a die steel (Cr12MoV) substrates, aiming at their implementation in metal forming applications involving demanding friction reduction and anti-wear. The effect of different amounts of h-BN with constant content NbC (30 wt%) on the microstructure of the laser-clad Stellite 6 alloy was probed through measures of XRD, SEM, EDS and EBSD. Microhardness, wear properties, and antifriction mechanism of the self-lubricating coatings were also discussed as well. The results proved that with an increase in h-BN content, in addition to NbC phase with polygonal shape observed in the h-BN-free Stellite 6/NbC coating, Nb(C, B) interstitial compounds having multiple grain orientations were formed in inter equiaxed crystal regions. Following the microstructural characterization, the tribological behavior and micro-hardness were analyzed. The results exhibit considerably better anti-friction properties of Stellite 6/NbC/h-BN coating compared to subtract and its excellent tribological performance could be reflected by data of friction test. The lubrication effect of h-BN is proved by the reduction in coefficient of friction. Meantime, the addition of NbC helps in improving micro-hardness and anti-wear. The NbC solidification-growth was detailly discussed in the paper.

Reactive Fabrication and Effect of NbC on Microstructure and Tribological Properties of CrS Co-Based Self-Lubricating Coatings by Laser Cladding.

The CrS/NbC Co-based self-lubricating composite coatings were successfully fabricated on Cr12MoV steel surface by laser clad Stellite 6, WS2, and NbC mixed powders. The phase composition, microstructure, and tribological properties of the coatings ware investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectrometer (EDS), as well as dry sliding wear testing. Based on the experimental results, it was found reactions between WS2 and Co-based alloy powder had occurred, which generated solid-lubricant phase CrS, and NbC play a key role in improving CrS nuclear and refining microstructure of Co-based composite coating during laser cladding processing. The coatings were mainly composed of γ-Co, CrS, NbC, Cr23C6, and CoCx. Due to the distribution of the relatively hard phase of NbC and the solid lubricating phase CrS, the coatings had better wear resistance. Moreover, the suitable balance of CrS and NbC was favorable for further decreasing the friction and improving the stability of the contact surfaces between the WC ball and the coatings. The microhardness, friction coefficient, and wear rate of the coating 4 (Clad powders composed of 60 wt % Stellite 6, 30 wt % NbC and 10 wt % WS2) were 587.3 HV0.5, 0.426, and 5.61 × 10−5 mm3/N·m, respectively.

Microstructure and sliding wear properties of HVOF sprayed, laser remelted and laser clad Stellite 6 coatings

The microstructure and sliding wear properties of Co-Cr-W alloy (Stellite 6) coatings were studied in the dependence on the deposition technology. The HVOF as-sprayed, laser remelted HVOF sprayed and laser clad coatings were evaluated by SEM, XRD and ASTM G-133 reciprocating sliding wear test. In dependence on used deposition technology, the microstructure of coating varied namely in terms of grain size and phase composition. The XRD analyses in the wear track revealed that the strain-induced fcc-to-hcp transformation is taking part during wear of laser remelted HVOF sprayed coatings. The difference of the phase composition, grain size and strain-induced fcc-to-hcp transformation was shown to be responsible for differences in the sliding wear behaviour.

Laser cladding of Stellite 6 on stainless steel to enhance solid particle erosion and cavitation resistance

Laser cladding of Stellite 6 on stainless steel 13Cr–4Ni has been performed to study the performance of clad on solid particle erosion (SPE) and cavitation erosion at varied energy densities (from 32 to 52J/mm2). Results are also compared with the AISI 304 stainless steel. The cladding geometry, dilution, microstructure and variation in microhardness were also investigated with laser energy inputs. The performance of cladded surfaces was studied for solid particle erosion and cavitation erosion resistance in 3.5% NaCl solution according to ASTM standard G76-07 and ASTM G32-07 methods respectively. Results indicated that clad dilution was 3–6% (geometrically) and 4.48% (compositionally) at 32J/mm2 that increased further with laser energy density. This accompanied compositional changes in the clad such that the Fe and Ni contents increased and Co, Cr, and W were observed to reduce with variation of laser energy density from 32 to 52J/mm2. The highest hardness (705Hv) of the clad was obtained at 32J/mm2 which reduced further by enhancing the laser energy density. Stellite 6 cladding has significantly enhanced the solid particle erosion resistance of stainless steel. Cladding at 32J/mm2 showed SPE and cavitation resistance than the cladding performed at higher laser energy densities. Cavitation erosion resistance of the stainless steel in 3.5% sodium chloride solution was enhanced by >90% by laser cladding. Lower corrosion current density of 13Cr–4Ni is observed after laser cladding which further increased with laser energy density. The erosion resistance obtained can be explained on the basis of dimensionless parameter related to kinetic energy. Cavitation resistance appears related to elastic recovery after cladding.

Study of Laser Cladding of Stellite 6 on Nickel Superalloy Substrate with Two Different Energy Inputs

Stellite 6 was deposited by laser cladding on a nickel superalloy substrate (NIS) with energy inputs of 1 kW (NIS 1) and 1.8 kW (NIS 1.8). The chemical compositions and microstructures of these coatings were characterized by atomic absorption spectroscopy, optical microscopy and scanning electron microscopy. The microhardness of the coatings was measured and the wear mechanism of the coatings was examined using a pin-on-plate (reciprocating) wear testing machine. The results showed less cracking and pore development for Stellite 6 coatings applied to the nickel superalloy substrate with the lower heat input (NIS 1). Further, the Stellite coating for NIS 1 was significantly harder than that obtained for NIS 1.8. The wear test results showed that the weight loss for NIS 1 was much lower than for NIS 1.8. It is concluded that the lower hardness of the coating for NIS 1.8, together with the softer underlying substrate structure, markedly reduced the wear resistance of the Stellite 6 coating.

Evaluation of Microstructure and Mechanical Properties at the Interface Region of Laser-Clad Stellite 6 on Steel Using Nanoindentation

The interface area in laser cladding, microstructures and mechanical properties of which determine the structural integrity of the laser-deposited coating, consists of two distinctive zones: the dilution and heat-affected zones (HAZs). The dilution region is the region where melted substrate forms a mixture with coating material and possesses microstructure similar to the coating. The HAZ is the region that lies in the substrate but possesses different microstructures compared with original substrate due to heat transfer from melt pool during processing. There are limited data available on mechanical properties of the dilution and HAZ and, in this study, the mechanical properties of the two regions have been evaluated using nanoindentation. This testing technique has the ability to resolve the mechanical properties at fine spacings, unlike microhardness testing and conventional mechanical testing. Stellite 6 powder was deposited onto a round bar AISI 4130 steel using a continuous wave Nd:YAG laser. The effect of laser cladding process on the microstructure, hardness, and elastic modulus of the coating, at the interface has been examined by nanoindentation testing. The elastic modulus of the substrate measured was found to be 198.76 ± 24.96 GPa, which is in a good agreement with the standard elastic modulus of AISI 4130 reported in literature. For the laser clad specimen, dilution area showed elastic modulus of 192.62 ± 7.67 GPa, slightly higher than the average elastic modulus of the HAZ, 189.94 ± 14.75. This is due to the dilution area containing a mixture mainly of Fe, Cr and Co. The increased level of Fe in this dilution area leads the tendency of this region to behave as substrate.

The effect of localized dynamic surface preheating in laser cladding of Stellite 1

In laser cladding, high cooling rates create outcomes with superior mechanical and metallurgical properties. However, this characteristic along with the additive nature of the process significantly contributes to the formation of thermal stresses which are the main cause of any potential delamination and crack formation across the deposited layers. This drawback is more prominent for additive materials such as Stellite 1 which are by nature crack-sensitive during the hardfacing process. In this work, parallel to the experimental investigation, a numerical model is used to study the temperature distributions and thermal stresses throughout the deposition of Stellite 1 for hardfacing application. To manage the thermal stresses, the effect of preheating the substrate in a localized dynamic fashion is investigated. The numerical and experimental analyses are conducted by the deposition of Stellite 1 powder on the substrate of AISI-SAE 4340 alloy steel using a 1.1 kW fiber laser. Experimental results confirm that by preheating the substrate a crack-free coating layer of Stellite 1 well-bonded to the substrate with a uniform dendritic structure, well-distributed throughout the deposited layer, can be obtained contrary to non-uniform structures formed in the coating of the non-preheated substrate with several cracks.

Melt pool temperature and its effect on clad formation in pulsed Nd:yttrium-aluminum-garnet laser cladding of Stellite 6

The effective melt pool temperature in pulsed Nd:yttrium-aluminum-garnet laser cladding of Stellite 6 was measured with a two-color pyrometer. The pulse peak temperature (Tp) and durations when the melt pool temperature is above the melting point of Stellite 6 powder (t1) and substrate (t2) in each pulse have been calculated and averaged. The effects of laser operating parameters (pulse energy, pulse frequency, spot overlap, powder mass flow rate, and pulse length) on these values have been investigated. It is found that Tp depends on the peak laser power, pulse length and determines the maximum size of melt pool (DM), whereas both t1 and t2 increase with peak power, pulse frequency (f), pulse length, and spot overlap. The prediction of the clad area, the total area of a single-track clad layer, based on Tp, t1, and t2 is given for pulsed laser cladding of Stellite 6. Comparison between the experimental data and prediction has been made with a correlation coefficient of 0.99. In order to produce a larger single-track clad area, the higher value of the product of f⋅t1 and longer beam interaction time (τ) are required.

Tribological properties of laser clad Stellite 6 coatings on steel substrates

The laser cladding of diesel engine exhaust valves is an emerging technology that provides excellent results in terms of the wear and corrosion resistance of the repaired materials. This work focuses on the deposition by laser cladding of Stellite 6 alloy coatings on two different base materials: an AISI 1045 carbon steel and an AISI 304 stainless steel. Following a process optimisation criterion, several of the fundamental laser treatment parameters were varied in order to obtain defect-free coatings with good adherence and minimal dilution. The metallographic structure of the clad material, irrespective of the substrate used, consists of a typical dendritic microstructure in a solid solution of Co. The mechanical strength of the layers was analysed by measuring HV0·1 microhardness and sliding wear behaviour. Laser coatings achieved hardness greater than 500 HV0·1, and presented excellent dry sliding wear behaviour, with a dimensional wear coefficient k one and a half orders of magnitude lower than that of the substrates. A combined oxidation and abrasion mechanism dominated the wear of the Stellite 6 layers, while in the case of AISI 1045 steel substrate, the wear was caused mainly by a surface oxidation mechanism. For the AISI 304 steel, the wear resulted from a combined adhesion, abrasion and plastic deformation mechanism.

Characteristics of thin surface layers of cobalt-based alloys deposited by laser cladding

Thin surface layers of cobalt-based alloys (Stellite 6 and Tribaloy T-900) and 410 stainless steel (SS) were deposited on mild steel substrates by a coaxial laser cladding process. This process could produce a thin surface layer of less than 0.5 mm with low energy input. Laser-clad Stellite 6 and Tribaloy T-900 specimens exhibited refined dendritic microstructure. The interdendritic eutectics consisted of either small carbides or intermetallic compounds randomly distributed in a cobalt-rich solid solution, unlike the lamellar structure as observed in conventional weld overlays. The hardness values of the laser-clad layers were generally higher than those of conventional welding deposits owing to the refined effect. The wear and corrosion resistance of S-6 and T-900 specimens were considerably better compared to those of laser-clad 410 SS specimens. In addition, the T-900 specimen had substantially lower corrosion rates than the S-6 specimen in hydrochloric acid solution. Experimental results indicated that wear and corrosion characteristics of the T-900 specimen were excellent; however, preheating was required to avoid cracking in the laser cladding process.

ステライト6及びWC粉末のレーザ肉盛における割れ挙動及び肉盛層の組織:YAGレーザロボットを用いた多層肉盛及び補修技術における材料学的研究（第3報）

ステライト6及びWC粉末のレーザ肉盛における割れ挙動及び肉盛層の組織:YAGレーザロボットを用いた多層肉盛及び補修技術における材料学的研究（第3報）