Component repair using HVOF thermal spraying
J.C. Tan, L. Looney, M.S.J. Hashmi
Journal of Materials Processing Technology
Abstract
The high velocity oxy-fuel (HVOF) thermal spraying technique has been widely adopted in many industries due to its ¯exibility and cost effectiveness. It is normally used to apply coatings to components to protect against wear, heat and/or corrosion, but also has potential to build up worn components. The main objective of this study was to optimise the HVOF spraying technique for the repair of damaged tool steel (D2) and stainless steel components. Experimental work presented involved the spraying of repair material on specimens in which `defects' had been introduced. These defects were grooves of various depths machined in the samples. Each sample was then sprayed with the repair material until the whole face of the sample was covered with the deposited material. Depth of groove and spraying parameters were varied. The machinability of each repaired component was examined. Techniques used include milling, grinding, turning and spark erosion. Metallography is used to evaluate the quality of adhesion of repair material both prior and subsequent to machining, and trends based on processing parameters are identifed.
Keywords: Thermal spraying; HVOF; HVOF repair
1. Introduction
Thermal spraying is the generic name for a family of coating processes in which a coating material is heated rapidly in a hot gaseous medium, and simultaneously projected at a high velocity onto a prepared substrate surface where it builds up to produce the desired coating. The technique has a long history beginning with work carried out in the late 19th century [1]. The earliest commercial application is attributed to Schoop in Switzerland who by 1910 had developed devices for melting tin or zinc, and projecting the molten metal with compressed air. By the mid-1920s uses had been found for metal spraying in at least 15 countries.
In the last three decades the demands of high technology industries, e.g. the aerospace industry, have led to major advances in the feld of thermal spraying. New materials used in these industries often require higher energy to be processed, and this challenge has been met with considerable success. It is now possible to spray virtually any material provided that it melts (or becomes substantially molten) without signifcant degradation during a short residence in a heat source. Another advancement has been the improvement of coating properties, in particular the reduction of coating porosity. This is being achieved through the use of new methods of post-treatment of thermally sprayed coatings, including ultrasonic compression, hot isostatic pressing and shot penning or hammering. Some thermal spraying processes also provide a very good surface fnish, and have slowly been replacing the chromium plating process, which is now the subject of severe occupational and environmental regulation. Irons et al. [2] examined the properties and cost of eight different thermal sprayed coatings and concluded that thermal spray coatings are a viable alternative to electroplated chromium in many applications.
The present study considers the application of thermal spraying to worn components. Wear of engineering compo-nents is a considerable problem in industrial applications. Wear may be caused by fretting, sliding, impact, abrasion,erosion and other service conditions. It is progressive damage, often involving considerable material loss. While inappropriate specifcation of materials for contacting com-ponents can cause a small increase in friction coeffcient (in the range 0.1±0.9), the corresponding wear rates can vary over many orders of magnitude [3].Thermal spraying is potentially a cost effective means for component dimension restoration following service induced wear. New surfaces may be provided without the material property distortion caused by welding, or the expense of special plating techniques. Furthermore, the new surface may be created using the same material as the base material,or with a more wear or corrosion resistant material.
As tight control on depth and spread of deposited material is not possible in thermal spraying, a fnishing process is required for components repaired using the technique. The machining of thermally sprayed material can be a diffcult task. Sprayed coatings are composed of well-defined particles and have poor thermal conductivity compared to the same material in wrought form. Heat transfer away from the cutting point is slow. The acceptable methods, practices and techniques used for machining materials in their wrought form do not apply to the same materials when sprayed.Intrinsically, materials which are abrasion resistant are diffcult to machine. In order that the repair `plug' does not come away from the component, the adhesion of the repair material to the substrate has to be strong enough to resist the forces involved in cutting. Also, the bond between the sprayed particles is primarily mechanical, therefore individual particles can be pulled out if cutting pressures are excessive. For certain applications where surface fnish is important, highly re¯ective fnishes are diffcult to achieve for sprayed aterials with a relatively porous structure.Factors which in¯uence the choice of fnishing method include type of material to be fnished, the shape of the part, fnish and tolerance required, and economics.
Carbide tools are generally used for machining of hard coating materials such as ceramics, carbides and cermets.Tool angles, surface speed and feeds are critical in the success of machining these coatings. Improper tool angles and tool pressure can result in excessive particle pull-out and destruction of the coating substrate bond.
2. HVOF spraying
Of the thermal spraying processes, high velocity oxy-fuel (HVOF) thermal spraying has been widely adopted in many industries due to its ¯exibility and cost effectiveness.Table 1 details possible applications of HVOF coatings in various branches of industry [4].The high velocity oxy-fuel process is based on a combination of thermal and kinetic energy transfer, i.e. the melting and accelerating of powder particles, to deposit desired coatings. Powder particles of the desired coating material are fed axially into a hot gas stream, then into a spray gun,are melted, and propelled to the surface of the workpiece to be coated. Carbon±hydrogen gases (propane, propylene, acetylene) or pure hydrogen are used as fuel gases and the gas temperature depends on the choice of fuel gas.
The gun consists of three sections: a mixing zone, combustion zone and the nozzle. During operation the body of the gun is cooled by air or water. The fuel and oxygen are mixed by means of co-axial jets and guided to the combus-tion zone where a pilot ¯ame or external igniter initiates combustion. During combustion, the gas is allowed to expand in the nozzle, where it is accelerated. The powder is accelerated by a carrier gas and injected into the ¯ame. The powder has the same direction of ¯ow as the surround-ing expanded gas. On entering the combustion zone through the nozzle the powder particles are heated and are further accelerated. Due to the high velocity and high impact of the sprayed powder, the coating produced is less porous and has higher bond strength than that produced by other methods[5±8]. Fig. 1 indicates the characteristics of HVOF coatings compared with those produced using the standard plasma spraying process [9].
HVOF thermally sprayed components contain residual stresses that result from contraction during cooling and solidifcation. The magnitude of the stresses vary depending upon process parameters used in spraying the coating. The coated material will crack if the magnitude of its tensile residual stresses exceeds its adhesion strength to the sub-strate. Methods which are generally used to reduce residual stresses in the coatings include: expanding the substrate prior to spraying by pre-heating, selecting a coating material with matching properties to the substrate, and macro-roughening of the substrate surface [10].
3. Experimental work
The selection of a proper coating material involves more than choosing the desired properties of a deposit. Consideration must be given to conditions such as coating function and service environment, in addition to the physical and chemical properties of both the coating and the substrate. Choosing a thermal spray material for an application is more complex than selecting a wrought or cast material for the same application. The properties of conventional materials are well understood, and their service performance is pre-dictable. This is not true of thermal spray coatings for which the mechanical and corrosion resistant properties of sprayed materials usually differ from solid or powder metal parts of the same chemistry.
Austenitic stainless steel was chosen initially as a repair material for this study in order to match a repair material to a substrate of reasonable toughness, restoring the damaged component to its original state. The experimental scope was then extended to using hard D2 tool steel and nitrided D2 tool steel substrates. These were repaired using either a `tool steel matching' powder (commercial name: Diamalloy 4010), or tungsten carbide±cobalt repair materials. D2 steel is a common material used for industrial tools. Nitrided tool steel samples were also included in the tests as industrial components are often case-hardened by the nitriding process to improve wear and corrosion resistance, and this hardening is expected to make repair adhesion more diffcult.
All stainless steel samples had the same overall dimen-sions as shown in Fig. 2. They were cylindrical blocks of dimension 25.4 mm in diameter25.4 mm in height. Tool steel samples, however, were rectangular blocks of dimen-sion 25.4 mm 2.4 mm. A groove of certain depth (mini-mum 1 mm) was machined on each sample, which were then degreased and sand blasted with aluminium oxide grit prior to coating. Immediately before spraying, they were pre-heated to 250 8 C using the ¯ame from the HVOF gun to reduce the thermal expansion difference between the sprayed repair material and the substrate. The groove was then sprayed with the repair material until the whole top face of the sample was covered with the deposited material up to thicknesses in the range 1.5±4.2 mm. The repaired samples were then fnal fnished using either grinding, milling or EDM.
The spraying processes were carried out according to thespraying parameters listed in Table 2.
Table 3 details the experimental matrix. All `A' samples are stainless steel components repaired with stainless steel material and samples `B' are D2 tool steel components repaired with matched tool steel powder. `C' samples are nitrided tool steel components also repaired with tool steel powder.
4. Results and discussion
Discussion of results from all the repaired components is presented under the type of fnal fnish machining process used on the repaired area. As the tungsten carbide±cobolt repair material failed to adhere to any of the substrates, these samples were not available for machining, and are therefore not included in the body of the discussion of results. The 1 mm depth of the groove introduced in samples exceeded the coating thickness limit for tungsten carbide±cobolt material (thickness limit 0.64 mm [11]). The tensile residual stresses within the built-up material caused it to debond from the substrate component. The substrate and repair material had different expansion rate due to their dissimilar physical properties.
The repair of all samples with the other two types of repair material (stainless steel and tool steel) were carried out with success, with the built-up thicknesses ranging from 2.2 to 5.5 mm. The repair built-up thickness of 5.5 mm was achieved with the help of a cool air jet stream to dissipate the heat during the deposition. Interruption of spraying is also needed in order to achieve thicknesses of this magni-tude. Figs. 3 and 4 show the pictures of some of the repaired samples produced in this study.
Two identical damaged samples (samples A1(a) and A1(b)), both stainless steel substrates repaired with stainless steel repair material, were sprayed to the same built-up thickness of 3 mm. Both samples were section and mounted for inspection under microscope, one without any fnal fnishing, and the other being fnal fnished using a milling machine. The frst sample was used as a comparison for any damage on the repaired area that might be induced by the milling machining. Sample A2, also stainless steel sprayed onto stainless steel, was machined from an original built-up thickness of 5.5 mm to a depth of 1 mm into the substrate. Observation of the cross-section micrograph shows good adherence of the repair material to the substrate, even though a large volume of the repaired material was machined away by the milling process (Fig. 6). This test proves that the repaired area can withstand an aggressive machining process.
Pictures of the stainless steel pre-machining and post-machining repaired samples were taken and compared. The cross-sections indicates very little difference under micro-scope ( 100 magnifcation). There are no signs of damage to the coating caused by the milling process (Figs. 5 and 6).Observation of the cross-section micrograph indicates good adherence of coating to the substrate. A dark zone,possibly an oxide layer between the coating and the sub-strate, can be seen on the surface of the repaired area. This is a layer of oxide inclusion which was probably caused by the ¯ame from the gun when the sample was heated to the temperature of 2508 C before the repaired material was sprayed onto it. This pre-spray heat treatment of the sample is to reduce the thermal expansion difference of the coated layer and the substrate, and hence reducing the tensile stress on the coated material which might lead to coating failure. Samples B1(a)±B1(d) all damaged D2 samples repaired with matched tool steel material, underwent different types of milling processes after being built up to 3 mm thick. Top face of sample B1(a) was machined off; whereas one side of sample B1(b) was machined in steps, and sample B1(c) was machined at an angle of 458. Some of the repaired samples are shown in Fig. 3. These samples prove that the area repaired with D2 material could withstand machining to different geometries and depth of cut. Observation of the cross-section of the repaired samples indicated good adher-ence of coating to the substrate. All the repaired materials on sample B1(d) was machined away completely, the original dimensions prior to the repair process were restored. This test proves that it is possible to reverse any repair action if desirable. The substrate surface integrity was not affected by the heat from the HVOF process. A good repair result was also observed on the nitrided D2 sample (sample C1) under similar tests.
Sample A3 was machined by turning; the top face of the sample was turned until the repaired area and the substrate were both exposed. Two dark lines were observed on the turned surface, coinciding with the edge of the repaired groove. These lines were investigated under higher magnifcation and were found to be areas where the repaired material consists of smaller grains and more oxide inclusion compared with the rest of the area. This is because some of the deposited particles were de¯ected by the angled (458 ) wall. It is generally recommended to have the direction of deposition perpendicular to the substrate for best spraying result for the thermal spraying process.
Samples A4, A5, B2(a), B2(b) and C2 were machined with a diamond grinding wheel, and all samples showed good adherence of repaired material on the substrate. Sam-ple B2(b) was ground to remove all the repaired material.
Observation of the cross-section and top face of these samples under microscope revealed them to be quite similar
to the other samples. Samples A6, B3 and C3 were all machined using the spark erosion process, this is quite a different machining process to those used on the other samples, but aside from the typical EDM heat affected area, the quality of the repaired area when inspected under microscope appeared to be as good as those which underwent other machining processes. Fig. 7 shows the cross-section view of the sample A6.
5. Summary
The HVOF thermal spraying process has successfully been used to repair stainless steel and D2 tool steel sub-strates with different depths of damage to a built-up thick-ness of up to 5.5 mm. This thickness can only be achieved if
the substrate and the repair material have similar or match-ing physical properties. Repair was not possible using WC±Co repair material on either of the substrates within the range of spraying parameters used. For all the successful repairs, sprayed material shows good adherence to the substrate when inspected under microscope, even following various types of aggressive machining processes. This is true even when a large section of the repaired area is removed.
The repair action can also be reversed, if necessary. A longer spraying time is required to accomplish repair work on the nitrided components compared to others due to the hardened nitrided surface de¯ecting some of the deposited material away from its surface during the spraying process.
References(略)
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