SE546341C2 - Manufacturing of hardfacings - Google Patents

Abstract

A method for formation of hardfacings on an article (10) comprises providing of an article to be processed. The article is exposed to impact items (20) and to tungsten carbide particles (22), either as free tungsten carbide particles or within the impact items. The impact items are solid bodies with an average diameter within the range of 0.1 to 10 mm. Preferably, the average diameter is within the range of 0.5 to 5 mm. A velocity (V) difference between surfaces (12) of the article and the impact items is created. This causes impacts between the impact items and the surfaces of the article, providing a burnishing action. As a result of this, tungsten carbide particles get embedded (14) into a surface of the article. At least 80% of the embedded tungsten carbide particles have sizes within the range of 0.1 - 5 µm.

Description

lO Dfll\I¶IIF1\CYT¶II2II¶(} ()F"I¶1\IlI)FV\(3II¶(}Eš 'TIDCHÅPIHÉÉXL FTEÜLID The present technology relates to formation of hardfacings on an article.

BACKGROUND Metal matrix composites (MMC) are metal alloys reinforced With fibers, particulates, Whiskers, or Wires. MMC materials are broadly used to manufacture Wear-resistant hardfacings. Tungsten carbide (WC) particles are the most commonly used reinforcements in MMC hardfacings, due to their extraordinarily high hardness, see e.g. Grairia, A., Beliardouh, N.E., Zahzouh, M., et al., "Dry sliding Wear investigation on tungsten carbide particles reinforced iron matrix composites", in Materials Research Express, IOP Pubnshing Ltd, 5 (2018), Hardfacings can be divided into thin and thick ones. Thin hardfacings usually have a thickness of not more than 50 um. Typical techniques for preparing the thin hardfacings include physical vapor deposition (PVD), chemical vapor deposition (CVD), electroless/ electrical plating, chemical heat treatments such as carburizing, carbonitriding, nitriding, and boriding, surface mechanical processing, etc. On the other hand, the thick hardfacings generally have a thickness of more than 50 um, up to several millimeters, or even larger. The thick hardfacings are usually prepared by Welding or brazing methods. The main techniques include laser cladding, plasma transferred arc (PTA) Welding, consumable and non-consumable electric arc Welding, oxyacetylene flame Welding and brazing, furnace brazing, etc. The thick hardfacings are made of either uniform metals and alloys, or MMCs With discrete hard phase particles as reinforcements. MMCs reinforced With tungsten carbide particles can also be synthesized by powder metallurgy process The published patent application US 2018/ 178283 A1 describes the use of tungsten carbide particles for hardfacings.

The patent US 4,192,984 A describes embedment of hard particles by softening the surface layer of metal using Eddy current technique. The major disadvantage of this approach is that it affects the hardness of the metal.

Particle impingement can also be carried out using cold gas dynamic spray (Goos).

The published European patent application EP 0484533 A1 proposes a method for coating of metal articles using high Velocity impingement of particles accelerated in a gas floW to speeds of 300-1,200 m/ s.

There are also numerous thermal spray coating techniques, for instance high Velocity oxy fuel (HVOF) sprayed WC coatings produced using conventional or suspended WC-Co feedstock. See e.g. B. Heimann, R.B., Lehmann, H.D., "Recently Patented Work on Thermally Sprayed Coatings for Protection Against Wear and Corrosion of Engineered Structures", in Recent Patents on Materials Science 1 (2008) 41-55, or Dong, SJ., Ye, J., Zhu, L. et al., "Thermal effect of high-Velocity particle impingements on coating quality in cold gas dynamic spray operations" in J Mech Sci Technol 36, 3619-3629 (2022), or Ahmed, R., Vourlias, G., Algoburi, A., et al., "Comparative Study of Corrosion Performance of HVOF-Sprayed Coatings Produced Using Conventional and Suspension WC-Co Feedstock", in Journal of Thermal Spray Technology. 27 (2018) 1579-1593, or Ali, O., Ahmed, R., Faisal, N., et al., "Influence of Post- treatment on the Microstructural and Tribomechanical Properties of Suspension Thermally Sprayed WC-12 Wt%Co Nanocomposite Coatings", in Tribology Letters. 65, 33 (2017).

Another technique suitable for production of WC coatings is laser cladding. In contrast to surface Welding and spraying methods, laser cladding allows one to coat very locally and on highly complex shapes. Due to the local character, it is instead more difficult to provide a uniform coating. See e.g. Van Acker, K., Vanhoyweghen, D., Persoons, R., Vangrunderbeek, J., "Influence of tungsten carbide particle size and distribution on the Wear resistance of laser clad WC/Ni coatings", in Wear. 258 (2005) 194- Despite the various approaches for providing MMCs based on hard particles, such as WC, there is still a need for development of easily applicable methods, and in particular for methods that are suitable for providing uniform hardfacings on articles having complex geometrical shapes.

SUMMARY A general object is therefore to provide an easily applicable method of producing hardfacings suitable for compleX-shaped articles.

The above object is achieved by methods according to the independent claims.

Preferred embodiments are defined in dependent claims.

In general Words, in a first aspect, a method for formation of hardfacings on an article comprises providing of an article to be processed. The article is exposed to impact items and to tungsten carbide particles. The tungsten carbide particles are provided as free tungsten carbide particles and /or tungsten carbide particles comprised in the impact items. The impact items are solid bodies With an average diameter Within the range of 0.1 to 10 mm. Preferably, the average diameter Within the range of 0.5 to 5 mm. A velocity difference between surfaces of the article and the impact items is created. This causes impacts between the impact items and the surfaces of the article, giving rise to a burnishing action. At least 80% of the tungsten carbide particles have particle sizes Within the range of 0.1 - 5 um. Tungsten carbide particles are embedding into a surface of the article by use of the energy of the impacts.

One advantage With the proposed technology is that a Wear-resistant low- friction surface paved by embedded tungsten carbide particles is produced, regardless of the geometrical macroscopic shape of the article being treated.

Other advantages Will be appreciated When reading the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS The invention, together With further objects and advantages thereof, may best be understood by making reference to the following description taken together With the accompanying draWings, in Which: FIG. 1 is a floW diagram of steps of an embodiment of a method for formation of hardfacings on an article; FIG. 2 is a diagram illustrating hardness and use of different WC-Co material; FIG. 3 is a SEM image of a surface of an embodiment of an impact item; FIG. 4 is a SEM image of a WC-Co bead used as an impact item; FIGS. 5A-C are SEM images of steel surfaces treated by different embodiments of a method for formation of hardfacings on an article; FIG. 6 is a diagram illustrating Weight loss of impact items; FIG. 7 is a schematic draWing of an impact item approaching an article surface; FIGS. 8A-C are sketches of different approaches for creating a velocity difference; FIG. 9 is a schematic draWing of an embodiment of a vibration barrel system; FIG. 10 is a schematic draWing of an embodiment of a gravity-based stream f1nishing system; FIG. 11A-B are diagram illustrating surface roughness measures of pistons before and after embedding of WC particles; FIG. 11C is a diagram illustrating a W content at the surface of treated pistons; FIG. 12 is a diagram illustrating coefficient of friction for an article before and after embedding of WC particles; lO FIG. 13 is a diagram illustrates surface roughness parameters of gears before and after triboconditioning treatment; FIGS. 14A-C illustrate surface characteristics of untreated pins; and FIGS 15A-C illustrate surface characteristics of pins treated according to the triboconditioning treatment.

DETAILED DESCRIPTION Throughout the drawings, the same reference numbers are used for similar or corresponding elements.

Embedment of abrasive particles into the workpiece during abrasive f1nishing is a well-known, but usually undesirable, phenomenon. Embedded abrasive particles are usually detrimental for the tribology since their presence is associated with a decrease in machining efficiency, higher friction and faster wear. Therefore, special measures are typically taken to avoid or minimize particle embedment, e.g. by putting a flushing nozzle at the exit point when grinding, carrying out continuous coolant f1ltration, redesigning the tool dressing, using a tougher or less friable grit, etc. See e.g. Badger, J .A., "How to avoid embedded particles", in Cutting Tool Engineering, March 2020, https: / /www.ctemag.com / news / articles / how-avoid-embedded-particles.

Despite the knowledge of WC particle embedment during abrasive finishing, this phenomenon does not appear to have ever been associated with improved tribological performance. At the same time, the use of WC particles in MMC hardfacings was, as presented in the background, at the contrary shown to lead to tribological improvement. However, such treatments require totally different and in general more complex and more expensive application methods.

When carrying out different mass f1nishing media screening tests, it was surprisingly discovered that the use of a specific non-abrasive burnishing media type - WC-Co cemented tungsten carbide beads - in particular with a lO specific particle size distribution, lead to unusually high uptake levels of tungsten in the surface. The SEM analysis of the treated surface revealed an unforeseen high density of embedded WC particles characteristic of MMC-type hardfacings. The possibility of manufacturing such MMC coatings using a tribological process was beforehand considered very unlikely and the outcome was therefore totally unexpected. Furthermore, as it turned out, the MMC coatings produced using the tribological process did not require additional finishing as they are "run-in" already in manufacture.

Based on these surprising findings, the inventors caught the insight that it would be highly desirable to produce MMC-like coatings using conventional finishing operations, such as lapping or mass finishing. By finding the right process conditions, the earlier detrimental particle embedment during abrasive finishing may instead be used to produce desirable MMC hardfacings and associated tribological advantages. Mass finishing was thereby found to be of particular interest.

For a better understanding of the proposed technology, it may be useful to begin with a brief overview of mass finishing.

The term "mass finishing" refers to a group of manufacturing processes that allow large quantities of parts to be simultaneously finished. Two broadly used types of mass finishing are tumble finishing, also known as barrel finishing, and vibratory finishing.

Mass finishing uses a grinding contact between the workpiece and the finishing media surfaces to achieve a desired surface finish quality for the workpiece. A variety of finishing media types can be used. Mass finishing can be performed dry or wet. Wet processes use liquid lubricants, coolants or cleaners together with abrasives. Cycle times can vary from minutes to hours depending on the process conditions, workpiece material and the finishing media used. The goal of this type of finishing is to burnish, deburr, clean, radius, de-flash, descale, remove rust, polish, brighten, surface harden, prepare parts for further f1nishing, or break off die cast runners.

A mass f1nishing process can be run either as a batch process or as a continuous process, and may also be sequenced, which involves running the workpieces through multiple different mass f1nishing stages. See e.g. L K Gillespie, "Mass f1nishing handbook", Industrial Press, New York, 2007, pp. 781- Figure 1 is a flow diagram of steps of an embodiment of a method for formation of hardfacings on an article. In step S10, an article to be processed is provided. The article may have substantially any shape, e. g. comprising conveX and/ or concave curved surfaces and/ or surfaces with continuously and/ or intermittently changing curvatures. In step S20, the article is exposed to impact items, e.g. in the shape of beads, and to tungsten carbide particles. Preferably, the article is exposed to impacts from the impact items wetted by a process fluid, in which optionally the tungsten carbide particles are dispersed. As will be discussed further below, the tungsten carbide particles may furthermore be released by the impact items and/ or may be provided separately. In other words, the tungsten carbide particles are provided as free tungsten carbide particles and/ or tungsten carbide particles comprised in the impact items. The impact items are solid bodies with an average diameter within the range of 0.1 to 10 mm. Preferably, the average diameter is within the range of 0.5 to 5 mm. The choice of impact item size is preferably adapted to the curvatures of the article. A velocity difference is created in step S30 between surfaces of the article and the impact items. This velocity difference causes impacts between the impact items and the surfaces of the article, which gives rise to a burnishing action. In step S40 tungsten carbide particles are embedded into a surface of the article by use of the energy of the impacts mentioned above. At least 80% of the tungsten carbide particles have particle sizes within the range of 0.1 - 5 um. lO In a preferred embodiment, at least 80% of the tungsten carbide particles have particle sizes With a maximum diameter Within the range of 0.2 - 2 um.

There exist several types of tungsten carbides. Fused and crushed tungsten carbides (FTC) have a eutectic microstructure consisting of WC and WgC. The shape is irregular-blocky and the carbon content is typically 35-4 Wt%. Spherical cast tungsten carbides (SCC) are fused and crushed carbides that are subsequently spheroidized by a plasma torch. Similar to the FTC carbides SCC microstructure consists of WC/WgC eutectic phase. Macro crystalline tungsten carbides, sometimes also called mono crystalline carbide (MTC), consist of heXagonal WC carbide With irregular blocky shape and a typical carbon content of 6.1%. They are thermodynamically more stable than the eutectic WC / WgC carbides, due to their higher melting point and have lower density compared to the eutectic WC/WgC carbides. Macro crystalline WC carbides are less prone to dissolve than eutectic WC / WgC carbides. This leads to a higher volume fraction of non-degenerated WC particles and a superior resistance to abrasive Wear. Any type of such tungsten carbides is possible to use for the purpose of creating hardfacings on an article.

In one group of embodiments, the impact items comprise beads or balls made of cemented tungsten carbide.

By selecting the appropriate combination of hard phases, metallic binder phase and processing parameters, a Wide combination of microstructures With a variety of mechanical properties can be achieved.

Different binders can for instance be used, of Which cobalt and nickel are the most common. WC-Ni cemented carbides offer higher corrosion resistance but are in general softer that WC-Co cemented carbides. Also, thermally sprayed WC-Ni coatings are characterized by a higher bond strength, often in eXcess of 70 MPa, compared to 30-60 MPa for WC-Co coatings.

In one embodiment, the impact items comprise at least one of Co cemented tungsten carbide and Ni cemented tungsten carbide. Preferably, the impact items comprise Co cemented tungsten carbide.

The addition of other carbides, such as TiC, and/ or nitrides, such as TiN or Ti(C,N), allows one to create endless material combinations to f1ne-tune the product properties, see e.g. Garcia, J., Cipres, V.C., Blomqvist, A., Kaplan, B., "Cemented Carbide Microstructures: a RevieW", in Int. J. Refractory Metals and Hard Materials, 80 (2019) 40- To promote adequate particle release and embedment, oblique impact of WC- Co beads onto the Workpiece surface is preferred. The impact momentum and energy are preferably high enough to provide a high probability to rip off particles of WC from the impact item and to hammer them into the Workpiece. At the same time, the impact momentum and energy should preferably not be too high to prevent surface damage, e.g. impact craters. Harder WC-Co beads made of FTC and having a lower bond strength are preferred for this purpose.

In fact, the selection criteria are in one sense inverse compared to those used in abrasive finishing. Easily friable grit is preferred. Use of loW-viscosity and loW-lubricity coolants furthermore minimizes impact damping. While the process is also operable in dry conditions, this is not preferrable due to eXcessive heat generation and dusting. The presence of WC dust in the Workshop air poses a major health hazard. WC-Co beads With a hardness 750 to 2200 HV, and preferably 1600 to 2200 HV, modulus of 450 to 650 GPa, and preferably 550 to 650 GPa, and compressive strength of 3 to 9 GPa, and preferably 6 to 9 GPa, Were found to be very usable. Furthermore, a polydisperse particle size distribution in the range from 0.1 to 2 um, and preferably 0.2 to 1 um, and cobalt content from 2 to 20%, and preferably 5 to 15% appear to be very suitable for the presently presented process. With such impact items, process Was obtained, Which aims at producing an MMC-like hardfacing by particle embedment. This includes WC-Co materials commonly used in Wear parts, cutting tools, composite machining and Wire draWing applications.

In other Words, in one embodiment, the material of the impact items comprises 2 - 20 % by Weight of Co, preferably 5 - 15 % by Weight of Co, and most preferably 5 - 10 % by Weight of Co, and have a hardness of 750 to 2200 HV, preferably 1600 to 2200 HV, a modulus of 450 to 650 GPa, preferably 550 to 650 GPa, and a compressive strength of 3 to 9 GPa, preferably 6 to 9 GPa.

The mechanical properties of WC-Co beads strongly depend on the size of the WC particles, With smaller particle being associated With increased hardness, Wear resistance, compressive strength and transverse rupture strength. According to the particle size classification by Fachverband Pulvermetallurgie, see e.g. Ortner, H.M., Ettmayer, P., Kolaska, H., "The history of the technological progress of hardmetals", in International Journal of Refractory Metals and Hard Materials, 44 (2014) 148-159, the aforesaid preferred range of 0.2 to 1 um covers ultraf1ne, submicron and fine particle sizes.

Figure 2 is a diagram of different compositions of WC-Co materials in terms of cobalt content and WC particle size that are of interest for the present technology. The straight lines represent a hardness of the material. The dotted areas indicate different typical application areas of WC-Co materials. Area 100 represents resource extraction and construction, area 101 represents rolls, area 102 represents can tooling, area 103 represents Wear parts, area 104 represents metal cutting, area 105 represents composite machining, area 106 represents Wood Working and area 107 represents Wire draWing. The area 110 indicates the most preferred choices of compositions for manufacturing of MMC-like hardfacings by particle embedment.

Polydispersity is also important With the larger particles bonded in the WC-Co beads acting as hammer faces When hammering loosened smaller particles into the Workpiece surface.Figure 3 is a scanning electron microscope (SEM) image of the microstructure of WC-Co material in impact items used in the manufacturing of MMC-like hardfacings by particle embedment. Figure 4 is an electron microscope image of an entire WC-Co bead used as an impact item in the manufacturing of MMC-like hardfacings by particle embedment. Note the impact craters on the bead surface.

A number of tests have been performed with different settings and different treatment times. Figures 5A-C are SEM images of three of the treated surfaces. The microstructure of an MMC-like hardfacing on a steel surface manufactured using the above-described process is shown. Light areas show the embedded WC particles. The chemical identity of the latter has also been confirmed by EDX analysis. Figure 5A shows a surface processed for 5 minutes in a centrifugal barrel finisher, and in which 2mm WC-Co beads in an oil-based process fluid were used as impact items. Figure 5B shows a surface processed for 15 minutes in a centrifugal barrel finisher, and in which 2mm WC-Co beads in a water-based process fluid were used as impact items. Figure 5C shows a surface processed for 30 minutes in a vibratory tub finisher, and in which a mix of 1 mm and 2 mm WC-Co beads in an oil-based process fluid were used as impact items. Note that these images just are a few examples of tests with different process parameters and matter content. From these SEM images, it is concluded that embedded tungsten carbide particles are present in all cases, in different amounts. The method is thereby shown to be used in different finishing platforms and with different process parameters.

The coverage of the embedded tungsten carbide differed, but for each set of parameters it typically increased with time. In the Figure 5A-C, the coverage ranged from 10 - 52%. However, improved tribological properties were detected already for relatively small coverages.

So far, the WC particles have been described as being provided as a component in the impact items. However, alternatively or as a complement, WC particlesmay be provided directly in the process fluid or coolant as a dispersion or slurry. In this case, other non-abrasive impact items can be used, such as steel or ceramic balls Therefore, in one embodiment, the tungsten carbide particles are suspended in the process fluid. The impact items then preferably comprise WC-Co, WC-Ni, steel, and/ or ceramic items.

If WC-Co impact items are used as a source of WC particles to be transferred to the Workpiece, impact item Wear occurs With a gradual Weight loss. This is illustrated in Figure 6, Where the Weight loss of the impact items is presented as a function of time in use. At some point, the impact items Will inevitably become too light to deliver suff1cient impact energy and need to be replaced. Besides that, some impact items may start to disintegrate due to fatigue. To extend the useful service life of media, external WC particulate matter can be fed into the system, as indicated above. This is an advantageous Way since it offers a high degree of flexibility and control over the particle size. Furthermore, WC-Co impact items With the highest degree of abrasion resistance can then be used since no friable grit is needed in this case. Without using an external WC particulate feed, the media life can be limited to 100- 200 hours in a high intensity centrifugal finishing process, While With an external WC particulate source, it may Well exceed 500 hours.

Impact item Approx. number of Surface area of impact size, mm impact items in items in 1 dm3 1 dm3 1 1,300,000 4.0 m2 170,000 2.1 m2 50,000 1.4 mTable 1. Sizes of WC-Co impact items useful in the present technology.

One can easily evaluate the maximum number of parts that can be treated With a given impact item load. WC-Co beads of three practical sizes; 1, 2, and3 mm in diameter, are considered. Table 1 presents size, number of impact items per volume and total surface are of a certain volume of impact items.

By removing 1 um of material from the impact items in 1 dm~°> of packed bed, one should be able to cover between 1.4 to 4 m2 of component area with a micron-thick MMC layer. Impact item can go down in size at least 10% without any significant change on process performance. Therefore, even without an external particle supply, 1 dm3 of impact items should at least suff1ce to treat approximately 200 m2 of component area. The volume of impact items used in mass-finishing machines can vary from a few dm3 to as much as a few hundred dm3, depending on the machine size.

As preferred impact item size is concerned, the practical size range useable in a wet process carried out with neat oil or water-based process fluid is 0.5 to 5 mm. The smallest impact item size is determined by fluid velocity. Since the G-force is counteracted by the viscous drag, the following scaling relationship is to prefer: G-im act item densit -r3 = luid viscosit -im act item velocit -r 1 P Y Y P Y where G is the acceleration, e.g. due to centrifugal action or vibration, and r is the radius of the impact item.

Hence, maximum impact velocity of the impact items will scale as G - density - rz/viscosity. Therefore, the impact energy decreases rapidly with reducing the impact item size. The practical viscosity range for process fluids used in the process is 1 to 10 cP, which makes it difficult to achieve suff1cient impact energy for impact item sizes below 0.5 mm in diameter when using standard mass-f1nishing platforms. In other words, in one embodiment, the process fluid with the impact items has a viscosity of 1 to 10 cP at room temperature.In Figure 7, a Workpiece or article 10 to be treated With a surface 12 to be treated is illustrated together With an impact item 20. The impact item 20 is Wetted With a process fluid 24 and the impact item 20 incorporates tungsten carbide particles 22, Which may be released from the impact item 20 to become embedded tungsten carbide particles 14 When the impact item 20 hits the surface 12 With a velocity V. As illustrated, the maximum impact item 20 size, defined by the radius r, should not exceed the minimum curvature radius min(RC) of concave portions of the surfaces 12, r < min(RC), of the Workpiece 10. OtherWise, parts of the surface 12 having a smaller curvature radius Will be inaccessible for the impact of the impact items. This means that the choice of impact items may be Workpiece-dependent and no general upper limit, as such, can be set based on such considerations.

Another limitation of the size comes from excessive edge erosion caused by heavy impact items. Therefore, it is typically impractical to use WC-Co impact items With a diameter over 5 mm. Preferably, impact items should be 3 mm or smaller.

Different non-abrasive impact item shapes can also be used if it is expedient. Spherical forms, such as beads are useful for most applications. HoWever, also other shapes may be of use, such as e.g. tubes, parallelepiped and pyramids as non-exclusive examples. When using impact items With a high aspect ratio also rotations may influence the impact energy.

In other Words, in one embodiment, the impact items have an average diameter Within the range of 0.5 - 5 mm, and preferably Within the range of-3mm.

Different mass finishing platforms use different Ways to create relative motion between the impact items and the article to be processed. The article can be fixed, e.g. attached to a fixture, or free, e.g. buried in the impact item bulk.

For most f1nishing platforms, it has been found that the velocity difference preferably is within the range of 0.5 - 5 m/ s, to achieve a good embedding action.

Vibratory f1nishers use periodic mechanic motion of a container containing the impact items 20 and the article 10. This principle is schematically illustrated in Figure 8A. G-forces are controlled by the amplitude and frequency of the vibrations.

There exist many different designs of vibratory finishing machines. One of the simplest designs, schematically illustrated in Figure 9, uses a bowl 30 mounted on a shaft 33 with an eccentric weight 34 driven by motor 32. The impact items 20, the article 10 and the process fluid 24 are placed inside of the bowl 30. The bowl 30 is supported by springs 36 to create vibrations of the impact items 20 due to the inertia of the eccentric weight 34. See e.g. Zhang, C., Liu, W. Wang, S., et al., "Dynamic modeling and trajectory measurement on vibratory finishing", in The International Journal of Advanced Manufacturing Technology 106 (2020) 253- The process kinetics depend on the bowl dimensions, and the amplitude and frequency of vibrations. The resulting impact item velocity is proportional to the product Amplitude - Frequency. The forces are proportional to the product Amplitude - Frequencyz. The vibration amplitude depends on the eccentricity, i.e. the distance between the mass centroid of the eccentric weight and the shaft, and the weight ratio eccentric weight/ bowl weight, including the impact items, fluid and article. The friction between the impact items, the article and the bowl acts as a damping factor. The energy supplied to the system is largely dissipated as friction heat. The vibration amplitude may vary from a few mm to a few cm. The typical impact item velocity range is typically 0.1 to 1 m /s. With a vibratory finisher operating at 50 Hz frequency and with a 5 mm amplitude, when using 2 mm WC-Co beads, the formation of a MMC-like hardfacing can be accomplished within 1-2 hours. A high intensity vibratory f1nishing process can provide the same impact energy as a low intensity stream finishing process. See e.g. Kacaras, A., Gibmeier, J., Zanger, F.,Schulze, V., "Influence of rotational speed on surface states after stream f1nishing", in Proc. CIRP 71 (2018) 221- Stream- and drag-finishing differ mostly by What is moving: the impact items or the article. In stream finishing, as illustrated schematically in Figure 8B, the impact items 20 moves against the fixed, or possibly rotated, article 10. In drag finishing, the article 10 is dragged through the immobile impact items 20 bed. The article 10 has to be attached to a holder and can eventually be rotated or tilted to prioritize treatment of specific areas.

Stream finishing operations are usually carried out using media velocity from 0.1 to 5 m/s. See e.g. Kacaras, A., Gibmeier, J., Zanger, F., Schulze, V., "Influence of rotational speed on surface states after stream f1nishing", in Proc. CIRP 71 (2018) 221-226. To promote adequate particle embedment and at the same time avoid article damage When using WC-Co beads as the finishing impact items, the range from 0.5 to 5 m/ s is presently considered as being more appropriate. Smaller bead size requiring higher velocity to achieve the desired effect. This velocity range can be easily covered by free falling impact items. The fall height is determined as V2 / 2g, neglecting air resistance, Where V is the impact item velocity When hitting the article, and g is the acceleration of gravity. This gives typically the fall height range from 0.01 to 1.2 m. The simplest design concept for implementing this technique is shown in Figure 10. A conveyor belt 38 transports impact items 20 from a barrel 37 containing impact items 20 and drops the impact items onto the article Various commercially available stream finishing systems capable to handle high density media types can also be used. When using 2 mm WC-Co beads, the typical treatment time ranges from a minute to a half hour.

In drag finishing, the impact items and the process fluid are typically placed inside a bowl. The article is attached to a holder, submerged among the impact items bed and dragged a certain speed. To guarantee adequate particle embedment and at the same time avoid article damage When using WC-Cobeads as the finishing media, the range from 0.5 to 5 m/ s is typically appropriate, the same as for stream finishing. When using 2 mm WC-Co beads, the typical treatment time ranges from a few minutes to a half hour.

In centrifugal barrel finishing, turret rotation is used to control the G-force. The local situation will be similar as in drag finishing, Figure SC, but with the article following a rotation path. In a typical application, the article is placed inside a barrel, together with impact items and process fluid. The combined fill rate of impact items and fluid inside the barrel is typically 50-90%, of which the impact item fill rate typically is 20-80%, but preferably 40-60%. The barrels sit in cradles which are mounted on a turret. The turret rotates around a horizontal axis, creating a Ferris wheel-like motion with a one-to-one ratio of barrel rotation to turret rotation. Inside the barrels, the rotating motion induces collisions between impact items and article, altering the surface finish of the article and causes tungsten carbide particles to be embedded. The process kinetics depends on barrel and turret dimensions. For barrels of 20 cm diameter, and turret of 60 cm diameter, the turret should typically rotate at 150-220 rpm. 160-180 rpm has been found to be the optimal window. The speed scales up with dimensions to keep the factor diameter - rpmz constant. The treatment time can vary from a few minutes up to an hour, but in most cases the 10-30-minute interval is targeted.

A lot of hybrid solutions also exist. For instance, one can fix the article and induce the media flow in a vibratory finisher by choosing the right vibration mode.

Besides mechanical action, the impact items can also be forced to move by using gravity (as mentioned above), fluid flow, magnetic field, etc. For the process success, one needs to achieve the right impact energy. A too high energy will lead to the article damage and excessive impact item wear. Too low energy will not achieve the requested particle embedment.Shot-peening and abrasive floW machining (AFM) systems may also be adapted for the purpose of the current invention, the AFM technique being suitable for the treatment of internal cavities. In shot peening, the impact items are accelerated using compressed air. In abrasive floW machining, the impact items are set into motion by hydrodynamic forces. HoWever, an impact item speed of 10-100 m/ s, Which may be considered as a standard speed in conventional shot-peening, may be too high at least if dense media are used.

In such cases, it is thus preferred to use less dense media or lower speed.

In one embodiment of a method for formation of hardfacings on an article, the step of creating a velocity difference is achieved using centrifugal barrel finishing, vibratory finishing, stream finishing or drag finishing as a platform.

Unlike conventional abrasive finishing, the presence of a certain amount of WC particles in the process fluid is benef1cial When particle embedment is prioritized. HoWever, if particle concentration increases too much, fluid rheology changes significantly. The fluid then starts to look like a slurry and damp impacts of impact items. Furthermore, particle composition may change With time as metal particles from the components being treated are accumulating in the machine.

To ensure process stability, a variety of fluid management systems can be used. The simplest is a cascaded system comprised of a sedimentation tank or a multiWeir system complemented by a cyclone and a magnetic separator. Such a system is efficient in removing excess WC and magnetic metallic particles from the process fluid. After end-of-life for the process fluid is reached, burnishing media should be reconditioned and Washed, and fresh fluid charged in the unit. The latter operation is scheduled as a planned maintenance.

While particle embedment can also be achieved in dry conditions Without using any process fluid, this is not desirable due to eXcessive dusting, With air borne WC dispersions posing a major health safety hazard.In general, two types of process fluids are most suitable for use according to the present technology: neat oils and water-borne synthetic fluids. The following examples show two possible formulations: Neat Oil Naphtha (Exxol D100, ExxonMobil) .................................... .. 96 wt.% Sulfurized olef1n (Additin RC 2540, Lubrizol) ............................. .. 2 wt.% Zinc dialkyl dithiophosphate (Lubrizol 1371, Lubrizol) ................ .. 2 wt.% Antioxidant (Rianox 1135, Rianlon) ........................................ .. 500 ppm Antifoam (Viscoplex 14-520, Evonik) .................................. .. 200 ppm This composition gives a kinetic viscosity at 25°C of 2.4 cSt, a specific gravity of 0.82 g/cm3 and a flash point of 90°C.

Water-borne sunthetic fluid Water .................................................................................... .. 60 wt.% Propylene glycol ..................................................................... .. 30 wt.% Water soluble polyalkylene glycol (Breox 50 A 20, BASF) ....... .. 5 wt.% Corrosion inhibitor (SR1, Chemworld) ......................................... .. 5 wt% Antifoam (DOWSILTM AFE-1267, Dow) ....................................... .. 500 ppm This composition gives a kinetic viscosity at 25°C of 1.7 cSt and a specific gravity of 1.0 g/cm It has been found that similar mass finishing equipment also can be utilized to run a mechanochemical surface finishing process known as Triboconditioning®, on which a patent application PCT/ SE2022 / 050859 is based. By replacing fixed tools by a swarm of non-abrasive dispersed impact items, such as cemented metal carbide balls, ceramic beads, aluminum oxide beads, zirconium oxide beads or the like, the process overcomes certain difficulties with treating components having a complex geometrical shape or treating dissimilar components in the same batch. lO Centrifugal barrel finishing, stream finishing, and vibratory finishing equipment proved to be suitable also for running the Triboconditioning treatment using small balls / beads made of cemented metal carbides, nitrides, zirconium oXide, or ceramics. Preferred ball size is from 1 to 5 mm. Smaller size is required to treat concave surfaces, for instance, to access tooth flanks and bottom lands in the case of large diametral pitch gears. A Wet f1nishing process is preferred, Where f1nishing media is used together With reactive fluids providing reagents for the tribochemical reaction. In contrast to Well- known chemical-mechanical polishing (CMP) processes used in the semiconductor industry, the function and composition of reactive fluids used by the Triboconditioning process are totally different. Unlike CMP fluids, Triboconditioning fluids are not expected to chemically etch the surface to speed up the process. Their primary function is to provide chemistries for the tribof1lm generation, as Well as complimentary corrosion protection and detergent functions required for process stability.

With this version of the Triboconditioning process, the outcome depends on process ability to modify the article surface in requested Ways. The article macrogeometry is preserved Within applicable specif1cations. Surface roughness profile (as per ISO 1302) is modified by acquiring negatively skeWness, Rsk = -O.5 to -3, decreasing amplitude roughness, in particular reduced peak height and core roughness, Rpk and Rk, and gradient roughness, that can be expressed as root mean square slope, Rdq, or as variance of angular distribution of scattered light, Aq, if the angle-resolved light scattering (ARLS) technique is used. A low friction solid lubricant tribof1lm is generated by tribochemical reaction With the process fluid during the process. A compressive stress, i.e. negative residual stress, is generated in the subsurface.

Fluid formulations used in the Triboconditioning process can also be used in the present particle embedding process. The tribof1lm priming typically calls for another additivation strategy. In particular, the Triboconditioning process preferably uses high levels of eXtreme-pressure and antiWear additives in thefluid. However, the mentioned additives tend to drive fluid costs up and may have a detrimental effect on the health safety and environmental profile of the fluid. Otherwise, it is indeed possible to combine the two approaches of Triboconditioning and particle embedment.

Thus, in one embodiment, in the method for formation of hardfacings on an article, the process fluid is a chemically reactive process fluid comprising a solvent and additives of solid-lubricant precursor substances. The solvent is a low-volatile high-flash solvent. The additives of solid-lubricant precursor substances surface-reactive compounds serving as carriers of at least one of S, P, B and surface-reactive compounds serving as carriers of at least one refractory metal and/ or oil soluble metal carboxylates in combination With sulfurized additives. The method comprises the further step of forming solid lubricant substances on the surfaces of the article by chemical reactions comprising the solid-lubricant precursor substances, induced by the energy of the impacts in the presence of said chemically reactive process fluid. The chemical reactions take place at the surfaces of the Workpiece With tungsten carbide particles being embedded into it at the same time.

Example 1: Treatment of pistons for a radial piston pump Hydraulic pistons made of 1020 steel Were finished according to the present technology using a centrifugal barrel f1nishing platform. A number of randomly picked parts Were examined before and after treatment.

Figure 11A is a diagram illustrating the Ra defined as per ISO 1302 of 19 randomly picked parts before treatment as curve 120, and after the treatment as curve 121. Analogously, Figure 11B is a diagram illustrating the Rpk defined as per ISO 1302 of 19 randomly picked parts before treatment as curve 122, and after the treatment as curve 123. It is easily seen that there is a distinct change in the surface roughness parameters of pistons treated according to the disclosed method. It is thus obvious that a f1nishing action has taken place during the exposure for impact items.Figure 11C is a diagram illustrating the tungsten reading at the surface of treated pistons. The analysis is performed by XRF analysis. Since no tungsten carbide is present in the bulk of the pistons, this is a proof of that the above established f1nishing is accompanied by an embedment of tungsten carbide into the surface. The treated parts were then tested in tribological tests. An improved tribological performance was demonstrated. Both the starting friction torque and the wear of the pump were signif1cantly reduced.

Example 2: Sintered powder metal pins Sintered powder metal pins, 10 mm diameter, were made of Astaloy CrA, (Fe- 1.8%Cr). They were sintered and low pressure carburized, giving a density of 7.2 g/ cm3, and a hardness 760 HV0_1. The metal pins were finished according to the present method with a centrifugal barrel f1nishing platform with 1 mm WC-Co impact items. The treatment time was 15 min at 165 rpm.

Surface roughness parameters Ra, Rpk, Rk, Rvk and Rsk defined as per ISO 1302 were measured before and after the treatment. The results are disclosed in Table Roughness Parameter, Before treatment After treatment um Ra 0.44 0.Rpk 0.42 0.Rk 1 .45 0.Rvk 0. 54 0.Rsk -0. 17 -1.Table 2 Change in the surface roughness parameters of powder metal pins treated according to the disclosed method.XRF measurement also confirmed that treated pins feature an MMC-like layer due to embedded WC particles.

Tribological tests were also performed before and after treatment. In Figure 12, the coeff1cient of friction of a lubricated metal-metal contact for powder metal pin is shown as a function of time. A cross-cylinder friction and wear test was used at 5N load. Curve 126 corresponds to an untreated pin and curve 127 corresponds to a treated pin. As can be seen, the tribological performance has greatly improved due to treatment, both in terms of friction and wear.

Example 3: Gears A set of bevel gears made of 16MnCr5 gearing steel, with 64 HRC hardness, were finished using a centrifugal barrel finishing platform with 2 mm WC-Co beads as the f1nishing impact items. Table 3 shows the changes in the surface roughness parameters defined as per ISO Roughness Parameter, Reference Finished gears pm ground gears Ra 0.65 0.Rz 3.67 2.Rpk 0.71 0.Rk 1.70 1.Rvk 1.99 1.Table 3 Change in the surface roughness parameters of gears before and after the treatment.

The formation of an MMC-like surface coating has been confirmed by scanning electron microscopy imaging and SEM-EDX elemental analysis. The compressive residual stress in the topmost material layer increased fromto 1400 MPa, based on X-ray diffraction (XRD) analysis.The parameters are illustrated in a diagram in Figure 13, Where the boXes 128 represent surface roughness parameters of reference ground gears and the boxes 129 represent surface roughness parameters of gears treated according to the principles presented here above.

The gearset finished according to the present invention has demonstrated improved NVH behavior, With the operational sound pressure level decreasing from 68 to 63 dB, as Well as reduced Wear and improved pitting resistance.

When using the above presented method to produce MMC-like hardfacings, it is important to keep an eye on the geometric dimensioning and tolerancing, e.g. as per ISO 110122017, ASME Y14.5-2018, or other applicable standards. In general, microgeometry of the components to be treated according to the present technology should always be set during the prior abrasive machining, such as turning, grinding, etc. Under most conditions suitable for the present method, the tungsten carbide embedment process Will typically only affect the surface roughness profile. HoWever, the tungsten carbide embedment can be preceded by a dedicated abrasive media finishing step to craft specific surface roughness profiles as Well as to control surface Waviness. For instance, one can first run abrasive finishing to "prime" the surface by cutting deeper grooves, and then run the tungsten carbide embedment process, comprising an inherent finishing, to flatten surface peaks and create an MMC-like hardfacing. Such a two-step process layout allows one to push the tribological performance boundaries.

Figures 14A-C illustrate surface characteristics of untreated pins, serving as reference measurements. Figure 14A is s diagram illustrating a surface roughness profile 130. Figure 14B is a diagram illustrating material ratio 131 and an amplitude density curve 132. Figure 14C illustrates the Fourier transform 133 of the roughness profile. Total height of profile Wt = 0.45 um.

Figures 15A-C illustrate surface characteristics of pins treated according to the principles above. Figure 15A is s diagram illustrating a surface roughness profile 134. Figure 14B is a diagram illustrating material ratio 135 and an amplitude density curve 136. Figure 14C illustrates the Fourier transformof the roughness profile. Total height of profile Wt = 0.17 um.

The improvement of surface roughness of the items treated by the presently proposed tungsten carbide embedment process is striking. A smoother surface is obtained despite the embedment of tungsten carbide particles. In such a way, lower friction as well as increased wear resistance is achieved.

As a conclusion, a method for formation of hardfacings on an article comprises exposing an article to be processed to impact items, either as such or in the presence of free tungsten carbide particles dispersed in a process fluid. The impact items may comprise solid bodies, such as beads or balls, e.g. made of cemented tungsten carbide, with the diameter within the range of 0.1 to 10 mm. Preferably, the average diameter should be within the range of 0.5 to 5 mm. A velocity difference between surfaces of the article and the impact items is created. This causes impacts between the impact items and the surfaces of the article, providing a burnishing action. If impact items are made of cemented tungsten carbide, they will release tungsten carbide particles on impact. If dispersed tungsten carbide particles are present in the process fluid, other non-abrasive impact items can be used, such as steel or ceramic balls. As a result of this, tungsten carbide particles get embedded into a surface of the article. At least 80% of the embedded tungsten carbide particles have sizes within the range of 0.1 - 5 um.

The embodiments described above are to be understood as a few illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible. The scope of the present invention is, however, defined by the appended claims.

Claims (12)

1. A method for formation of hardfacings on an article (10), comprising the steps of: - providing (S10) an article (10) to be processed; - eXposing (S20) said article (10) to impact items (20) and to tungsten carbide particles (22); said tungsten carbide particles (22) being provided as at least one of tungsten carbide particles comprised in said impact items and tungsten carbide particles (22) provided directly in a process fluid (24) as a dispersion; Wherein said impact items (20) being solid bodies With an average diameter Within the range of 0.1 to 10 mm, and preferably 0.5 to 5 mm; - creating (S30) a velocity difference between surfaces (12) of said article (10) and said impact items (20), Which causes impacts between said impact items (20) and said surfaces (12) of said article (10), giving a burnishing action; Wherein at least 80% of said tungsten carbide particles (22) have particle sizes Within the range of 0.1 - 5 um; and - embedding (S40) tungsten carbide particles (14) into said surfaces (12) of said article (10), hammering said tungsten carbide particles (14) into said article (10) by use of the energy of said impacts.

2. The method according to claim 1, characterized in that said step of eXposing (S20) said article (10) comprises exposing said article (10) to said process fluid (24) in Which said impact items (20) are provided and in Which said tungsten carbide particles (22) are provided.

3. The method according to claim 2, characterized in that said process fluid (24) With said impact items (20) has a viscosity of 1 to 10 cP at room temperature.

4. The method according to claim 2 or 3, characterized in that said process fluid (24) comprises one of neat oils and Water-borne synthetic fluids.

5. The method according to any of the claims 1 to 4, characterized in that at least 80% of said tungsten carbide particles (22) have particle sizes Within the range of 0.2 - 2 pm.

6. The method according to any of the claims 1 to 5, characterized in that said impact items (20) comprise said tungsten carbide particles (22).

7. The method according to claim 6, characterized in that said impact items (20) are made of Co-cemented tungsten carbide.

8. The method according to claim 7, characterized in that said impact items (20) comprise 2 - 20 % by Weight of Co, preferably 5 - 15 % by Weight of Co, and most preferably 5 - 10 % by Weight of Co, and have a hardness of 750 to 2200 Hv, preferably 1600 to 2200 Hv, a modulus of 450 to 650 GPa, preferably 550 to 650 GPa, and a compressive strength of 3 to 9 GPa, preferably 6 to 9 GPa.

9. The method according to any of the claims 2 to 5, characterized in that said tungsten carbide particles (22) are suspended in said process fluid (24) and said impact items (20) comprise at least one of: WC-Co, WC-Ni, steel, and ceramic items.

10. The method according to any of the claims 1 to 9, characterized in that said impact items (20) have an average diameter Within the range of 0.- 5 mm, and preferably Within the range of 1 - 3 mm.

11. The method according to any of the claims 1 to 10, characterized in that said step of creating (S30) a velocity difference is achieved using one of the following platforms:centrifugal barrel finishing, vibratory finishing, stream finishing, drag finishing, and shot peening.

12. The method according to any of the claims 1 to 11, characterized in that said Velocity difference is Within the range of 0.5 - 5 m/ s.