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Laser cladding protection of hydraulic cylinders

Umar Farooq, Global Engineering Manager, Hydraulic and Pneumatic Cylinders, Eaton Corporation.

Sandeep Birje, Lead Engineering Specialist, Hydraulic and Pneumatic Cylinders, Eaton Corporation.

Mike L Killian, Principal Engineer, Corporate Research and technology, Eaton Corporation

BAN Murthy, Global Engineering Manager, Actuation Projects and Services, Eaton Corporation

Abstract: Protection of vulnerable components of a hydraulic cylinder such as piston rods has been a concern for a

long time in the hydraulics industry predominantly due to harsh and corrosive environments in which

these cylinders operate in. Hard Chrome plating (HCP) has been a standard practice in the industry for a

long time but wherever environment is demanding or even mildly corrosive, HCP is found to be

inadequate resulting in premature failures and clearly falling short of the life expectancy. The hydraulics

industry has been searching for a solution to combat rust and corrosion in these cylinders for quite some

time now. Among the many techniques that have been employed lately to prolong service life such as

plasma or thermal sprays; Laser cladding has emerged as the latest technology with a lot of promise.

This article provides a short overview of laser cladding technology, how it differs from other techniques

and examines the benefits expected from using such technology. Some experimental results are shown

and a comparison with other coating technologies is also provided.

Introduction

Hydraulic cylinders are often called upon to do heavy lifting in harsh environments. In off-shore oil rigs, saltwater environments often subject equipment to corrosive conditions. When hydraulic cylinders are exposed to harsh operating environments, anti-corrosion protection plays a crucial role in the reliability and durability of the equipment. Without proper protection, hydraulic equipment can suffer pitting, crevice development, and other forms of corrosion, leading to inefficient operations, equipment failures, unwanted down times, and sometimes catastrophic consequences [1-3] as seen in Figure 1. Other marine applications, such as drilling and dredging, also face corrosive conditions.

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Figure 1: Offshore oil rigs operate in corrosive conditions. Chrome plating is seen to be spalling off in this picture [2].

Hydraulic equipment can also incur damage from material handling. Impacts from swinging chains and cables, falling debris, or dropped equipment can produce unwanted nicks, dents, and cracks, leading to corrosion and leakage. Depending on the application, hydraulic equipment can encounter various environmental factors [4], as summarized in Table 1, and as such needs to designed for the proper environment.

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Environment

Industrial

atmosphere

Potable

and fresh

water

Brackish and saltwater Marine atmosphere Soil

Splash

zone Submerged Open Sheltered

Corrosivity Very low to

very high

Low –

Medium

Very high High Very high High to

very high

Low

Category* C1, C2, C3,

C4, C5-I

Im1 C5-M /

Im2

Im2 C5-M C5-M Im3

Environmental

characteristic

Industrial

condition

(e.g. SO2,

salinity)

Rain

water and

treated

water

Seawater

(estuaries

and

coastal

areas)

Seawater in

the open

oceans

Seawater

salts and

varying

humidity

levels

Local

seawater

salts and

varying

humidity

levels

Salinity 1 - 300

ppm

1.5-3.5% 2.5 - 3.5% 2.5 - 7% 2.5 - 15% 2.5-3.5%

pH 6-9 7.5 - 8.5 7.5 - 8.5 6-9 3-9 7-9

Temperature (degrees C)

1-3 -2 to 30 -2 to 30 -20 to 50 -20 to 50 -2 to 30

Humidity, RH ----------- 100% ---------- 30 -100 % 10 -100 % -------

Pollution Low Low Low Low to

high

Low to

high

Low to

medium

Table 1: Environmental factors per EN-ISO 12944-2 [4]

* Categories defined as follows: C1: Very low Im1: Immersion in fresh water

C2: Low Im2: Immersion in sea or brackish water

C3: Medium Im3: Buried in soil

C4: High

C5-I: Very high (industrial)

C5-M: Very high (marine)

Land-based hydraulic cylinder applications, such as construction, manufacturing, and power plant equipment may operate in less corrosive conditions, but encounter dust, dirt, and other elements, necessitating coating protection of cylinder rods and other equipment. Figure 2 illustrates how coatings can fail under a corrosion attack.

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Figure 2: Illustration of how coatings fail on cylinder rods. Cross-sectional view of rod coating indicates large cracks (red arrows), porosity (blue arrows), and corrosion in deeply penetrated bond line (black circle).

Many different coating systems have been tried to improve and prolong the service life of these piston rods as examined in a review study by Tucker [5]. Thermal sprays gained popularity in mid-90’s due to improved corrosion resistance over Chrome plating and better surface retention characteristics for sealing systems. Other advantages include a wide variety of materials that can be applied as coatings, lack of significant heat to the base material and ability to strip and recoat without changing the base material properties or dimensions. Thermals sprays are still a popular method of coating piston rods. However, there are some disadvantages as well. These include weak bond strengths, susceptibility to porosity, weaker bending resistance and weaker strain to deflection characteristics.

In a recent study [6], Touminen et. al examined the piston rods made of carbon, quenched and tempered (QT) and stainless steels coated with methods such as thermal spraying, hard chrome plating, and overlay welding. The laser welding sometimes also called cladding outperformed the other methods in corrosion, impact and other tests.

The paper is organized as follows: first a short overview of laser cladding technology with differences from other coating techniques is presented. Then the benefits expected from using such technology including some experimental findings are shown ending with comparison against other coatings.

How Laser Cladding Works

Laser cladding [7-8] produces a metallic coating with a strong metallurgical bond between the coating layer and a substrate material such as carbon steel in case of piston rods. Using precise, state-of-the-art lasers such as the high power diode laser (HPDL) as a controllable heat source, metallic powder is injected into the system by nozzles. Energy from the laser beam produces a shallow, molten cladding puddle. Filler material powder is injected into the beam and the puddle. As the laser beam passes through the area, the cladding puddle solidifies rapidly, leaving the desired build-up of cladding material

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with minimal dilution of the base material. Care must be taken to keep the heat-affected zone (HAZ) soft enough to avoid brittleness and potential delamination with the substrate.

The result is a protective coating with high ductility, resistance to bending, high strength, and impact resistance characteristics, as shown in Figure 3.

In contrast, most thermal sprayed coatings use processes in which divided molten metallic or nonmetallic material is sprayed onto a prepared substrate to form a coating. The sprayed material can be in the form of wire, powder or rod with preference normally given to the first two forms. These materials when fed through spray unit get heated to a molten state and then get propelled through a stream of compressed gas onto the base material substrate where they stick and conform to the surface of substrate. The substrate temperature remains low ~ 200°C thus eliminating change in metallurgical properties of the substrate. Thus the bond formed is of mechanical type rather than fusion type. Extra care must be ensured to prepare the base surface else the bond strength is lost during processing and the coating can delaminate[9].

In general when compared with other types of coatings, laser cladding offers the following:

Optimized corrosion, wear, scratch, and impact resistance

Consistent coating depth

Strong adhesion due to the metallurgical bond

Hardness throughout the depth and length without cracking

Optimized ductility and toughness

Unique seals designed for longer life

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Figure 3: Laser cladding exhibits no cracks or pores, smooth substrate adhesion, uniform coating thickness, and zero dilution.

A 100X Image of (Eatonite®) courtesy of Eatonite Corporation.

In terms of industries served, Laser cladding provides protection for a variety of applications, including:

Energy (oil, power stations, windmills)

Marine (cranes, drilling, dredging)

Industry (process machinery, milling, mining)

Civil works (dams, locks, ferry berths)

Laser cladding can have both short-term and long-term economic benefits. As a field-repairable technique, laser cladding can reduce repair cost and down time. The increased durability of properly coated equipment can improve reliability, reducing operating expenses and total cost of equipment ownership over the life of the equipment. For example, a day rate in oil production (the amount a drilling contractor gets paid for a day of operating a drilling rig) can range from $200,000 to $600,000 for a floating rig [10]. For each day of down time, costs can quickly skyrocket, emphasizing the importance of equipment reliability.

Laser Cladding benefits in resisting Cracks, Wear, and Other Failures

Prevention of cracks, wear, and fatigue in hydraulic equipment is critical in many fields. With a typical hydraulic cylinder, a piston connected to a piston rod reciprocates back and forth in a cylindrical barrel.

Laser cladding on S355J2 carbon steel base material Mag Factor 100X

Micro-hardness indentations not Cracks

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A hydraulic pump brings in a fixed or regulated flow of hydraulic fluid, typically oil, to the hydraulic cylinder, to move the piston. Wear and corrosion resistance are particularly crucial on the outer diameter of the piston rod. Figure 4 shows a piston rod exhibiting blistering.

Figure 4: Early Blistering of Plasma Sprayed Carbon Steel Piston Rod (WRT) [2]

Different operating conditions present different challenges to hydraulic cylinder equipment. Corrosion, impact from debris, bending, high temperatures, and various combinations of these conditions may be encountered.

With a surface hardness starting at around 300 HV (Vickers Hardness Number) and approaching 500 HV, laser cladding is highly resistant to wear and scratches. A comparison of various laser cladding technologies is shown in Figure 5. The microhardness chart shows how in general laser cladding is hard at the outer surface but follows a smooth transition curve to become soft at the HAZ. Process control is the key here. Some examples of laser claddings - that may not be fully processed controlled – show that as a result cladding could either be soft at the surface- indicating low wear resistance, or reach a plateau just below the surface- indicating possible issues with disbonding/delamination.

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Figure 5: Comparison of laser cladding with other coating technologies. Image courtesy of Eaton Corporation

Laser cladding also provides high impact resistance. Coatings such as Eatonite have been tested up to 24 foot-pounds of energy without cracking. Same goes for commercially available coatings such as Ultimet [11]. Spray-type coatings, such as high-velocity oxygen fuel (HVOF) coatings typically top out at only 8 foot-pounds of impact energy without cracking. Simply put, when struck with a ballpeen hammer, laser cladding may dent, but will not crack. With a bond strength exceeding the strength of the steel base metal (>50,000 psi), laser cladding will also not spall or chip. In comparison, if a thermal spray coating is struck with the same ballpeen hammer, the HVOF or hard chrome plating or plasma sprayed ceramics will crack and spall.

Another key aspect for laser cladding is ductility. Laser cladding’s ductility provides high flexibility without cracking. The product can be applied to cylinder rods up to 21 meters long without cracking and can withstand up to 180-degree bends, as shown in Figure 6.

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Figure 6: Laser cladding’s ductility provides flexibility without cracking. Image courtesy of Eaton Corporation.

Combining strength, high ductility, and minimal porosity, laser cladding can help avoid fatigue cracks and limit the propagation of other flaws like solidification or shrink cracks, as were shown in Figure 2. Durability is another strong suit. Laser claddings have been used on offshore rigs for more than five years (over 43,800 hours) without any significant performance degradation in highly corrosive saltwater environments.

Field Repairable Solution

A key distinguishing trait of laser cladding is its field repairability. Using common welding processes, laser cladding can be repaired on site, saving significant time and money. Depending on whether the damage is impact or linear, as shown in Figure 7, different tools and techniques can be used. In general, an in-line die grinder with carbide burs, a right angle die grinder, a cleaning solvent to remove oil and grease, and appropriate welding tools are the key tools. Proper training is required prior to conducting any field repairs of laser cladding.

On the contrast, thermal or plasma sprays don’t offer such luxury. If a cylinder rod coating with HVOF or Chrome is fund to be cracked, the coating must be removed and reapplied to maintain integrity of such coatings. This might not be a big issue for some industries but a cylinder supporting a platform in oil rig or a drawbridge can cause significant downtime, cost and accrue displeasure of consumers involved.

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Figure 7: Deep linear discontinuities like cracks and gouges require weld repair as shown above in a linear impact.

Seal Compatibility of Coating

The interaction between rod surface, seals, and various fluids used in hydraulic systems can present challenges. System requirements such as pressure, temperature, velocity, friction, lubricity, fluid media, and frequency of operation all play vital roles in determining the seal choices. One seal system may work well in a certain application, but not in another. For that reason, proper seal selection with rod coating surface finish parameters is key to ensuring long-term satisfactory performance [12].

As technologies such as laser cladding have advanced, expectations for surface finish and seal life have risen. Various seal manufacturers establish product-specific surface finish requirements, but most specify additional parameters such as Rp (peak height), Tp (material bearing ratio), and Rz (average of largest peak to valley sums). Due to these extensive research efforts, life expectancy of laser cladded rods has extended beyond millions of cycles.

Certification

The Joint Industry Project (JIP) defines the qualification criteria in Guideline document per DNV standard for Laser Cladding [13]. Established by international oil and gas manufacturers, operators, contractors, and suppliers, this standard established the recommended practices for documenting requirements for major subsea components. This is strict criteria that requires passing tests such as

Saline Droplet Test (DNV-C1)

Electrochemical Porosity Test (DNV-C2)

Resistance to Rapid Deformation (DNV-M1)

Hardness testing (DNV-M2) - Hardness & HAZ

Dynamic Bend test (DNV-M3)

Metallographic examination (composition, cracks, thickness, microstructure, porosity, content and slag-oxide content)

Surface roughness, surface finish and optical imaging dye penetrant

Wear Abrasion Testing

Anodic polarization, critical pitting temperature and critical crevice temperature tests

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The successful results can be obtained using high process control, superior powder quality and optimized laser cladding techniques.

Comparison of Coating Technologies

A summary of laser cladding properties compared with other coating technologies is shown in Table 2. Plasma sprays and HVOF type coatings are generally harder but exhibit high porosity, low bond strength, low impact resistance, and brittleness. To minimize porosity in these coatings, an additional operation such as chemical sealant is often applied, but this can lead to inconsistent pore sealing and evaporation of sealants at high temperatures. As a comparison when Eatonite laser cladding was certified by DNV, the DNV rods exhibited less than 0.02 percent porosity. In contrast, the best HVOF has porosity ranging from 0.3 to 1 percent, with most HVOF coatings containing 2 to 5 percent porosity. Also, chrome plating is often cracked as manufactured. The low bond strength and low impact resistance can lead to delamination and spall offs. Plasma Transferred Arc (PTA) is a process similar to laser cladding but with moderately high porosity content and higher dilution. Dilution is defined as the amount of mixing of the clad and the base/substrate materials. A low dilution value is preferred to obtain a surface that is similar to that of clad material, and the clad material properties are fully preserved [14].

Laser cladding is categorized as a “low hydrogen” welding process. The cladding puddle (melt zone) is protected by a shielding of the inert gas Argon. The powder flow lines feeding the laser cladding torch also deliver Argon. In some applications, the coaxial flow of Argon is augmented with an additional trailing gas shield. In this manner there is no pumping of elemental Hydrogen into the heat affected zone of the steel base metal. With laser cladding, post-cladding baking of the finished hydraulic cylinder rod is not needed. As such, hydrogen embrittlement common with chrome plating is a non-issue with laser claddings.

With the right chemistry laser cladding provides outstanding resistance to stress corrosion cracking (SCC). In this regard, laser cladding resists SCC better than other products such as Chrome plating.

PLASMA SPRAYING

HVOF PTA LASER CLADDING

Heat Source Nontransferred

Electric Arc Combustion Flame

Transferred Electric Arc

Focused Laser Beam

Coating Thickness

0.05 to 0.50 mm (0.002 to 0.020 inch)

0.12 to 0.70 mm (0.005 to 0.028 inch)

0.64 to 1.90 mm (0.025 to 0.075 inch)

0.40 to 1.90 mm (0.015 to 0.075 inch)

Bond Type Mechanical Mechanical Fusion weld Fusion weld

Bond Strength

2,500 psi typical (17.2 MPa)

<13,000 psi (89.6 MPa)

> 50,000 psi (> 344.7 MPa)

> 50,000 psi (> 344.7 MPa)

Behavior Brittle, easily cracked

Brittle, easily cracked

Ductile, no cracks Ductile, no cracks

Porosity Content

3% to 10% 0.5% to 5 % 0.5% to 2 % 0.05% to 0.5 %

Dilution None None 3% to 12% 0.5% to 2%

Topcoat Materials

Ceramics Chromia-Titania Alumina-Titania Zirconia

CoCrWC NiCrBSi NiCr-CrC

Inconel 625 Ultimet Stellite (all)

Inconel 625 Eatonite Ultimet Stellite (all)

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Bond Coat Materials

Metals NiCr80/20 NiAl MCrAlY

None None None

Table 2: Comparison of laser cladding with other coating technologies.

Laser cladding is essentially free of constituents deemed harmful to the environment. Conventional rod coatings such as hard chrome plating often include toxic chemicals such as hexavalent chromium. Regulations such as the European Union Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) include measures to eventually phase HCP out.

Conclusion

Laser cladding provides a high-performance, high strength, ductile, impact resistant and environmentally friendly coating solution in protecting hydraulic cylinders. This technique has emerged as a go-to solution for actuators in hydraulic cylinders. This overview paper shows various aspects of laser cladding and compares some key characteristics against the commonly used thermal sprays, plasma sprays and hard chrome plating.

In general laser cladding provides a fully proven, field-repairable coating with all the necessary ingredients to replace the traditional coatings and successfully meet the needs in the harshest of operating environments.

References

[1]. “Premature Failure of Riser Tensioner Piston Rods Exposed to Offshore Splash Zone Operation – Status and Review of Critical Multi-degradation Factors”, 2009, by C. Ohe, R. Johnsen, and N. Espallargas, paper 09199 in 2009 NACE Corrosion Conference and Expo.

[2]. “Hydraulic Cylinders for Offshore Splash Zone Operation: A Review of Piston Rod Failure Cases and Alternative Concepts”, 2009, by C. B. von der Ohe, R. Johnsen and N. Espallargas, Paper No. OMAE2009-79039, Volume 6, pp. 1-14; in 2009 ASME 28th International Conference on Ocean, Offshore and Arctic Engineering, Honolulu, Hawaii, USA, May 31–June 5.

[3]. “The piston rod — simple, yet critical”, http://hydraulicspneumatics.com, October 13, 2010. [4]. “Guideline for Qualification of Wear and Corrosion Protection Surface Materials for Piston

Rods,” DNV Report No. 2009-3295. https://www.dnvgl.com/ [5]. “Thermal Spray Coatings”, 1994, by R. C. Tucker, Jr., in ASM Handbook, Volume 5: Surface

Engineering, pp 497-509, ASM International, Materials Park, Ohio. [6]. “Wear and corrosion resistant laser coatings for hydraulic piston rods”, 2015, by J. Tuominen, J.

Näkki, H. Pajukoski, J. Miettinen, T. Peltola and P. Vuoristo, in Journal of Laser Applications, Volume 27, 022009.

[7]. Laser Cladding, 2004, by E. Toyserkani, A. Khajepour, and S. F. Corbin, 1st Edition, CRC Press. [8]. “Laser cladding”, 1999, by R. Vilar, in Journal of Laser Applications, Vol 11, Issue 64. [9]. Surface hardening of steels: Understanding the basics, 2002, ASM International, Materials Park,

Ohio. [10]. http://www.rigzone.com/data/dayrates/, November 20, 2015. [11]. http://www.haynesintl.com/ultimetalloy/ULTIMETAlloyAW.htm. Retrieved April 05, 2016. [12]. “How piston-rod coatings affect hydraulic seals,” http://machinedesign.com. Retrieved October

15, 2015. [13]. https://www.dnvgl.com/

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[14]. “FEM modeling and experimental verification for dilution control in laser cladding”, 2011, by J.T. Hofman, D.F. de Lange, B. Pathiraj, and J. Meijer in Journal of Materials Processing Technology, pp 187- 196, Vol 211, Issue 2.

About the authors

Umar Farooq is a global engineering manager for cylinder business at Eaton Hydraulics. He holds a Ph.D. degree in mechanical engineering and has experience in control and vibration systems. In his present role, he has been involved in developing and implementing surface technologies to hydraulic systems, particularly on cylinder rod surface coating technologies.

Sandeep Birje is a technical specialist for Hydraulic and Pneumatic cylinders……

Mike Killian, is a principal engineer in Eaton’s corporate research and technology group, and subject matter expert in various aspects of coating technologies.

BAN Murthy, is an engineering manager for ACTUATION PROJECT AND SERVICES at Eaton Corporation.

Acknowledgements

The authors wish to thank Stephanie Risberg in Eaton’s marketing department for images help. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and may not necessarily reflect the position of Eaton Corporation.