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    Tuesday
    Feb242015

    A story of Valves and Balls

    The earliest valves were probably devised by Egyptians as a means of controlling water flow for the diverting of streams to irrigate early crops. The Romans moved things forward as they developed the first plumbing systems, establishing canals and even basic sewage systems across the major cities of the Roman Empire.

    The modern valve can probably trace its roots to the Industrial Revolution and the development of the steam engine by Thomas Newcomen. This innovation marked the first critical application of a valve when it was used to control the pressure flow of steam. Indeed in most cases, without which the steam engine would not have played such a pivotal role.

    Valves today come in many forms, shapes and sizes, are made with almost every high specification material known to man, from plastics to Titanium, and are integral to the design and function of almost every major industrial process that we all take for granted. Indeed in some cases, some of most complex of these valves are now surgically implanted in our aging population to replicate functionality within the human heart.

    Of the many types of valves in service today, those that are probably used in the most critical applications and environments are metal seated ball valves. These valves are seen across a range of industries including Oil & Gas, Petrochemical, Power, Mining, Paper, and many more. They are used to control the flow of process liquids and gases that are critical to these industries, and which in many cases are being operated in extreme pressure and temperature regimes.

    An integral part of the reliability of the metal seated ball valve is a high performance thermal spray coating applied to the ball and seat, to improve the wear, corrosion, coefficient of friction and sealing properties of the valve. These coatings are applied via a range of technologies with materials that include Tungsten and Chromium Carbides, Ceramics and Nickel based self fluxing materials. After application they may be sealed, and are then finished via specialized grinding and lapping processing to ensure a gas tight seal, optimized for the particular type of service the valve is destined for.

    FW Gartner (a business unit of Curtiss Wright), has been in the ball valve coating business for many years, and as part of the growth of the business, has now incorporated specialized finishing technologies for the finishing of these components. So, for that next project that needs a specialized coating, finishing etc, whether it be something as sophisticated as a ball valve or as simple as a pump sleeve, we can provide you with an optimized solution.

    Stay tuned for our next article…

    Wednesday
    May082013

    Laser Cladding Well-Suited to Large Shafts and Bearing Surfaces in Ship and Other Offshore Applications

     

    Cladding is a type of “additive manufacturing” in which a new layer of material is added to the outer surface of a part.  It has long been used in a variety of industries for improving the surface and near-surface properties (e.g. wear, corrosion or heat resistance) of a new part, or to resurface a high-value, worn component, allowing it to then be machined back to original physical tolerances.  

     

    Examples of cladding used in ship and offshore applications include hydraulic cylinders and pump shafts, bearing surfaces for both propeller shafts and ship-board pumps, and tensioner shafts for oil rigs. For new parts, lower-cost materials, such as steel with low carbon content or some stainless steel, are often used to create the basic part, which is then clad with a tailored layer of a corrosion-resistant and/or wear-resistant material, to provide specific properties in critical areas.  This includes tungsten carbides (often in a NiCrBSi matrix) for wear resistance, and Inconel 625 for areas exposed to drilling mud or seawater.  In the case of worn parts, the corroded and/or worn outer surface is first undercut to expose a pristine metallic substrate.  This is then clad or rebuilt with the original substrate material, or with a mechanically or environmentally superior wearing material.  This step may be repeated multiple times to achieve thicker overlays.  The final step is usually a fine grind to restore the part to its original design specifications.

     

    There are numerous traditional techniques used for cladding, including various forms of arc welding and thermal spraying.  Recently, laser cladding has emerged as a viable alternative to these techniques.  In laser cladding, the laser acts as a precisely controllable power source that is used to create the optimum weld pool into which is fed feedstock in either wire or powder form, thereby forming the clad layer.

     

    Laser cladding has proved competitive or superior in some applications, because it delivers key benefits including minimized dilution (contamination of the cladding alloy across the base material-clad deposit interface), much lower heat input and hence minimized part distortion, precision deposition resulting in excellent dimensional control, high throughput, and efficient use of the cladding powder or wire.  An additional and very important benefit for marine applications is that the clad layer forms a true metallurgical bond with the base material.

     

    F.W. Gartner Thermal Spray (www.fwgts.com), a business unit of Curtiss-Wright Technologies, is a leading contractor supplying both laser and traditional thermal cladding services to marine and other demanding applications, including cladding of new parts and reclamation of worn parts. Michael Breitsameter, Director of Sales & Marketing at FWGTS, explains, “Thermal spraying is a low-cost method of reclaiming the original dimensions for a part, but this should not be confused with restoring original mechanical integrity.  The bond between the coating and substrate is a mechanical bond only, so the coating is less robust, potentially compromising any use involving impact or point loading.  In contrast, the laser process melts both the cladding material and the very outer surface of the substrate resulting in a true metallic bond.  This integrity delivers excellent physical properties, in some cases superior to those of the original bulk material.”

     

    Earlier laser sources for cladding, such as carbon dioxide (CO2) and solid state lasers, were not ideal for some applications in terms of their optical characteristics, cost of ownership and physical size.  To better meet the needs of laser cladding, Coherent developed the Highlight™ series of direct-diode laser systems.  Two primary advantages of direct diode lasers are their inherent electrical efficiency and physically compact size.  Furthermore, their near infrared output is efficiently absorbed by all cladding materials, and their large “effective” clad width (up to 30mm) is geometrically well matched to most cladding applications for the off-shore industry – see figure 1.  (Traditional lasers produce a narrow beam and hence small spot size on the worksurface).  All this translates into low cost of ownership, and the ability to clad at deposition rates that are competitive and in many cases superior to traditional overlay technologies – see figure 2.

     

    Figure  1.  The line-shaped beam profile from a direct diode laser system is well-matched to support both powder and wire-feed cladding.

     

    Breitsameter notes that, “A direct-diode laser is an industrially mature tool that is truly optimized for everyday use in terms of reliability, repeatability and operational simplicity.  The rapid development of this technology has significantly expanded the range of applications for laser cladding.”  To clarify which marine applications are best suited to laser processing, Breitsameter explains, “At FWGTS we use both laser and traditional cladding methods.  The laser method is always a first choice for point loading applications, where thermally sprayed claddings might suffer from early failure.  It’s also best for applications where thicker (e.g., several millimeters) coatings or reclamation are required, where thermal spraying coatings become impractical due to internal stresses.”

     

    Figure 2.  High-throughput laser cladding in action. This image show the output of a 4 kilowatt direct diode laser system (Coherent 4000L) applying a Tungsten Carbide blend on a seal area to provide a highly wear resistant surface.  Image courtesy of F.W. Gartner Thermal Spray.

     

     

     

    Wednesday
    Jan302013

    Galvanic Corrosion of Cermet Coatings

    In recent years, cermet coatings have gained popularity as a means of providing wear protection to critical engineering components. Using optimized thermal spray processes, including high velocity oxyfuel (HVOF) and plasma spray, it is possible to create high-quality coatings of these materials that have good bond strength and low porosity.

    Since many cermet-coated components, including pumps and valves, are also frequently used in environments that require corrosion protection, a number of studies have investigated the ability of these coatings to resist corrosion. These studies suggest a complex mechanism, involving not only interaction between the coating and the corrosive/erosive environment and/or substrate, but also galvanic processes occurring between constituents within the coating itself.

    Studies by de Souza & Neville [i] , [ii] have concluded that localized corrosion occurs first at the carbide/metal matrix interface within the coating, leading to loss of support for and dissolution of the hard phase particles. This in turn compromises the coating, making it more vulnerable to both corrosion and erosion.

    Shmyreva et al [iii] found that the corrosion mechanism of WC-cermet coatings involves three distinct stages, as follows:

    “Localized corrosion takes places in three stages in the carbide-metal cermets and Ni-based alloy coatings. At first, pits appear on the coating surface. Secondly, galvanic corrosion occurs between phases with different chemistry and with different corrosion potentials. Finally, galvanic and crevice corrosion take place between the coating and the substrate.”

    Neville & Hodgkiess [iv] have reported a similar phenomenon:

    “The first observation, previously reported by the authors on these and related HVOF cermet coatings, is that the coating itself, without contribution from the substrate, is susceptible to corrosion attack. Hence, although the level of interconnected porosity of the coating is important for structural integrity, it does not solely determine whether corrosion will occur or not. Penetration of the aqueous electrolyte into the coating and to the coating/substrate interface may destabilize the bond integrity and will undoubtedly accentuate the coating corrosion; however, pore-free, high-quality coatings are still susceptible to corrosion attack, because of their metallic matrix constituent.”

    Most studies have focused on WC coatings with metallic binders such as Co, Cr or Ni. These studies tend to suggest that WC-Co-Cr coatings may provide better corrosion or erosion-corrosion resistance that WC-Co alone [v] , [vi] . This appears to be attributable to the superior passivation ability of the coating when chromium is present. In a study by Cho et al.v, the following conclusions were drawn:

    “(2) A considerable microgalvanic corrosion occurred between WC particles and binder metal and the general corrosion occurred in binder materials of WC–Co, WC–Ni coatings in the aerated 5 wt.% H2SO4 solution.

    (3) The corrosion resistance of the coatings containing Cr was better than that of the coatings without Cr. This is attributed to the fact that the formation of Cr passive film in the form of surface oxide in the coatings containing Cr suppressed the binder material dissolution into the solution.

    In spite of the presence of Cr, the overall corrosion resistance of the WC–CrC–Ni coating was inferior to that of the WC–Co–Cr coating due to the existence of microcracks which act as the infiltration paths of the solution.

    […]

    (6) It was concluded that chemical composition of metallic binder materials and the occurrence of microcracks were the most important factors influencing the corrosion resistance of the HVOF sprayed WC cermet coatings in the strong acidic environment.”

    Cermet coatings remain an important means of providing wear and corrosion protection to metal components. The above studies, however, indicate that each possible constituent within the cermet coating must be identified and their electrochemical effects with the environment and with each other must be considered for each application.



    [i] V.A. de Souza, A. Neville, Assessing the Corrosion Characteristics of Metal/Ceramic Composites in Saline Environments – Aspects of the Interaction Between Phases, Paper No. STG4403251, NACE International, San Diego, CA, 2003.

    [ii] V.A.D. Souza, A. Neville, Corrosion and synergy in a WC Co Cr HVOF thermal spray coating—understanding their role in erosion–corrosion degradation, Wear 259 (2005) 171–180.

    [iii] T. Shmyreva, D. Wang, R. Thorpe, Relationship of Corrosion Resistance - Coating Structure of HP/HVOF Carbide and Metallic Coatings, Thermal Spray 2001: New Surfaces for a New Millenium: Proceedings of the International Thermal Spray Conference, p 1143-1148, at p. 1148.

    [iv] A. Neville and T. Hodgkiess, Electrochemical Study of the Localized Corrosion of Vacuum-Furnace-Fused Cermet Coatings, J. Am. Ceram. Soc., 82 [8] 2133–44 (1999), at p.2140.

    [v] J.E. Cho, S.Y. Hwang, and K.Y. Kim, Corrosion behavior of thermal sprayed WC cermet coatings having various metallic binders in strong acidic environment, Surface & Coatings Technology 200 (2006) 2653– 2662.

    [vi] A. Lekatou, D. Zois , A.E. Karantzalis , D. Grimanelis, Electrochemical behaviour of cermet coatings with a bond coat on Al7075: Pseudopassivity, localized corrosion and galvanic effect considerations in a saline environment, Corrosion Science 52 (2010) 2616–2635.

    Tuesday
    May082012

    THERMAL SPRAY FOR STEEL INDUSTRY – Part I: Furnace Rolls

    The steel production process exposes much of its hardware to very arduous environments.  High wear, corrosion and/or high temperatures are often encountered in many segments of steel production. Since the early 1970s, thermal spray coatings have apparently been used to minimize maintenance, extend service life, and to enhance performance/quality of steel facilities.  Although there are many references to different thermal spray applications in steel mills , there is limited mention of the specifics relating to these applications beyond patents.  The exception to this is the thermal spray protection of processing rolls which is known to be widely used around the world .  This and the next two blogs will discuss thermal spray applications of furnace (Part I), continuous galvanizing (Part II), and cold forming (Part III) rolls.

    Furnace rolls are used in continuous annealing lines where the steel strip is heat-treated to enhance ductility .  Ideally, these rolls should resist high-temperature corrosion, thermal cycling, wear, and foreign particle pick-up.  In addition, a certain degree of surface roughness is required to mitigate slip and strip meandering.  For working temperatures of up to 850°C, CrC-NiCr coatings are often used; however, higher temperature environments incorporate materials often seen in gas turbine engine applications - namely, MCrAlYs.  At temperatures up to 1100 °C, MCrAlYs are combined with oxides such as Al2O3 to provide simultaneous wear, hot corrosion, and oxidation resistance .  For even greater temperatures, monolithic oxide ceramic top coats are applied over an MCrAlY bond coat.  The top coats are composed of one or more combinations of ZrO2, SiO2, Cr2O3, TiO2, Al2O3, Y2O3, CeO2, etc.

    There is an obvious need to develop more reliable and high performing coatings for the harsh environment found in the steel industry.  Unless new materials that can handle these harsh conditions are developed, advanced coating materials and coating processes will be required to meet the challenge.

    The steel production process exposes much of its hardware to very arduous environments.  High wear, corrosion and/or high temperatures are often encountered in many segments of steel production. Since the early 1970s, thermal spray coatings have apparently been used to minimize maintenance, extend service life, and to enhance performance/quality of steel facilities.  Although there are many references to different thermal spray applications in steel mills , there is limited mention of the specifics relating to these applications beyond patents.  The exception to this is the thermal spray protection of processing rolls which is known to be widely used around the world .  This and the next two blogs will discuss thermal spray applications of furnace (Part I), continuous galvanizing (Part II), and cold forming (Part III) rolls.

    Furnace rolls are used in continuous annealing lines where the steel strip is heat-treated to enhance ductility .  Ideally, these rolls should resist high-temperature corrosion, thermal cycling, wear, and foreign particle pick-up.  In addition, a certain degree of surface roughness is required to mitigate slip and strip meandering.  For working temperatures of up to 850°C, CrC-NiCr coatings are often used; however, higher temperature environments incorporate materials often seen in gas turbine engine applications - namely, MCrAlYs.  At temperatures up to 1100 °C, MCrAlYs are combined with oxides such as Al2O3 to provide simultaneous wear, hot corrosion, and oxidation resistance .  For even greater temperatures, monolithic oxide ceramic top coats are applied over an MCrAlY bond coat.  The top coats are composed of one or more combinations of ZrO2, SiO2, Cr2O3, TiO2, Al2O3, Y2O3, CeO2, etc.

    There is an obvious need to develop more reliable and high performing coatings for the harsh environment found in the steel industry.  Unless new materials that can handle these harsh conditions are developed, advanced coating materials and coating processes will be required to meet the challenge.

    Coated Hearth Roll

     

    REFERENCES


     

    [i] Introduction to Thermal Spray Processing, Handbook of Thermal Spray Technology, 2004, pp. 3-13.

    [ii] R.L. Hao, Thermal Spraying Technology and Its Applications in the Iron & Steel Industry in China, International Thermal Spray Conference and Exposition (May 14 - 16, 2007) Beijing.

    [iii] S. Matthews, B. James, Review of Thermal Spray Coating Applications in the Steel Industry: Part 1—Hardware in Steel Making to the Continuous Annealing Process, Journal of Thermal Spray Technology Volume 19(6) December 2010, pp. 1267-1276.

    [iv] T.-S. Huang, C.-S. Yu, J.-R. Wu, H.-Y. Liou, S.-H. Hsieh, T.-K. Huang, Effect of Mn on the Formation of Pickups on Thermal Spray Coatings, thermal Spray 2009: Proceedings of the International Thermal Spray Conference, pp. 595-600.

    [v] Bender US website - http://www.benderus.com/Steel%20Coatings/SteelCoat.html.

    Thursday
    Apr262012

    MANUAL AND AUTOMATED THERMAL SPRAYING 

    In most cases, thermal spraying is carried out in spray booths either through a manual (handheld) or automated (robotic, x-y manipulators, etc.) method.  There are several key reasons for using automated systems.  These include: health and safety; quality and reproducibility; and productivity and economics.

    For obvious reasons, the use of automated methods can notably reduce risks to workers by limiting exposures[i],[ii],[iii],[iv] to:

    • ·                Gases and vapors;
    • ·                Dust/powder;
    • ·                Noise;
    • ·                Radiation;
    • ·                Heat;
    • ·                Electric Shock.

    Among the numerous dangers associated with thermal spray application, respiratory issues relating to exposure to gases and vapors, dust particles, and toxic metals are one of the most significant concernsii.  By contrast, the principal danger associated with robots is that a robot does not have any detection capability and a collision with a worker in its pre-defined trajectory can occurii

    Automation can also help enhance quality and reproducibility through its ability to accurately control several aspects of the gun/torch positioning and movement, i.e., working distance, translational speed, pitch/increment between passes, and spray anglei,iv,[v],[vi].  This is most important for complex-shaped components that require high thermal and mass transfer tolerancesiv,v,vi,[vii],[viii].  McDonald et al.[ix] provide helpful general information on robot programming for thermal spraying. 

    Example of F.W. Gartner automated thermal spray application.

    In addition to enhancing coating quality and reproducibility, incorporating automated thermal spraying can also increase productivity and makes good economic sense, especially in high-volume productioni,iv,[x].  Kutay[xi] at Carnegie Mellon University provides some insight into the economic impact of automation technology.

    Even with the advantages of incorporating automation into a thermal spray process, numerous applications require manual spraying with protective personal gear, i.e., bridges, boilers, artwork, architecture, etc.  Also, an automated system can require considerable preplanning and programming which can take time and affect profitability.  Hence, it is not always feasible to remove the human factor from all applications.


    Example of F.W. Gartner manual thermal spray application.

    REFERENCES


    [i] D.E. Crawmer, Process Control Equipment, Handbook of Thermal Spray Technology, J.R. Davis-editor, ASM International and the Thermal Spray Society, 2004, pp. 85-98

    [ii] H. Heriaud-Kraemer, G. Montavon, S. Hertert, H. Robin, C. Coddet, Harmful Risks for Workers in Thermal Spraying: A Review Completed by a Survey in a French Company, Journal of Thermal Spray Technology Volume 12(4) December 2003, pp. 542-554

    [iii] C.P. Howes, Thermal spray safety and OSHA compliance, THEFABRICATOR.COM, July 2001, http://www.irmaassociates.com/articles/Thermal_Spray_Safety_and_OSHA_Compliance.pdf

    [iv] Z. He, B. Lu, J. Hong, Y. Wang, Y. Tang, A novel arc-spraying robot for rapid tooling, Int J Adv Manuf Technol (2007) 31: 1012-1020

    [v] A. Kutay, L. Weiss, Economic Impact of Automation: The Case of Robotic Thermal Spraying, The Robotics Institute – Carnegie Mellon University, March 1990, http://www.cs.cmu.edu/~lew/PUBLICATION%20PDFs/TECH%20REPORTS/CMU-RI-TR-90-07.pdf

    [vi] A. Frutos, Numerical analysis of the temperature distribution and Offline programming of industrial robot for thermal spraying, Final Thesis, University of Stuttgart, January 2009

    [vii] P.D.A. Jones, S.R. Duncan, R. Rayment, P.S. Grant, Optimal Robot Path for Minimizing Thermal Variations in a Spray Deposition  Process, IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, VOL. 15, NO. 1, JANUARY 2007, pp. 1-11

    [viii] W. Xia, H. Zhang, G.-L. Wang, Y. Yang, G. Han, H. Zou, Integrated Robotic Plasma Spraying System for Advanced Materials Processing, PIERS Online, Vol. 4, No. 8, 2008, pp. 876-880

    [ix] A. McDonald, K. Schoof, B. Harvey, Thermal Spraying Training Module: For As-sprayed (un-polished) Coatings, http://www.ualberta.ca/~andre2/files/McDonald_Training%20Module.pdf

    [x] D. Breen, Thermal Robotic Arm Controlled Spraying via Robotic Arm and Vision System, Doctoral Thesis, Dublin Institute of Technology, Ireland, January 2010

    [xi] A. Kutay, The Economic Impact of Automation Technology, The Robotics Institute – Carnegie Mellon University, July 1989, http://www.ri.cmu.edu/pub_files/pub3/kutay_aydan_1989_1/kutay_aydan_1989_1.pdf