Surface treatment – the complete guide

  Where Are Surface Treatments Based On?

Primarily, a rod must meet the functional requirements in its application.
The most common functional requirements are:

• Corrosion resistance
• Wear resistance
• Pliability / adhesion
• Repair possibility

A “pancake” cylinder (short and thick) to reposition a bridge or viaduct is used once. The cylinder is often poured into concrete. For those applications a teflon coating on the cylinder rod is sufficient. Corrosion resistance, wear resistance and ductility play no role here. Perhaps not even any coating needs to be applied to such cylinders.

However, cylinders used in mining, road salt distribution or battery processing have to deal with an aggressive environment. Cylinders used offshore are in a less aggressive environment (seawater), but are often placed in the extended position for a longer period of time, which still entails a corrosion risk. The larger lengths of these cylinders also limit the choice of the protective layer to be chosen. For certain types of rod covering, the rod length is limited. The maximum possible length for galvanic nickel/chrome (Ni/Cr) is about 11 meters, for laser cladding about 35 meters is currently possible.

Cylinders that are used in 24/7 applications such as presses with centric load, are subject to different requirements than cylinders that are used occasionally, but are subject to lateral load (e.g. due to horizontal use). In short, it is often custom made.

In the paragraphs below, the functional requirements for each type of surface treatment are discussed.

  What Other Factors Can Play a Role Than Functional Only?

• Possibility of (quick) repairs
• ‘’Total Cost of Ownership’’ (TCO)
• Environmental impact (use of chemicals and energy)
• Continuity (regulations, technical developments)

If the rod or roller is relatively small and there are no special requirements in terms of mechanical or anti-corrosion properties, it may be obvious to go for a stainless steel quality of steel without further surface treatments. However, stainless steel grades can be pricey and because they are often heavier than carbon steel has to be constructed (carbon steel usually has better mechanical properties), the higher price and the extra kilos of material can be disadvantageous for stainless steel. That is why usually a rod or roller is constructed from carbon steel, with a rod covering over it that meets the conditions of use.

As long as the carbon content in the steel is not too high, all types of coverings are possible. However, steel with a high carbon content is not suitable for classical rolling techniques. The heat input is too large for this and cracks will occur. In addition, a large “Heat Affected Zone” is created where structural changes lead to other (poorer) mechanical properties. This is not the case with laser cladding, HVOF thermal spraying and galvanic Ni/Cr.

  Functional Requirement: Corrosion Resistance.

Corrosion resistance is obtained by coating with a corrosion resistant layer that is free of pores, so that the underlying metal is shielded from the environment in which the rod or roller is located.

Classical welding techniques and laser cladding give a layer that is absolutely pore free. Laser cladding is better than welding because the underlying steel mixes up more during the welding process.

In the case of galvanic Ni/Cr, especially the nickel layer closes off the underlying material and chromium provides the wear resistance.

The extent to which galvanic Ni/Cr is corrosion resistant depends on the quality of the applied layer, the degree of contamination in the underlying material and the environment in which the rod or roller is used.

This galvanically applied Ni/Cr is not resistant to all kinds of (aggressive) environments. And the resistance it offers to corrosion also depends on the nature of its use. A hydraulic rod in the “splash” zone of salt water (alternating between salt water and air) can have a reasonable service life if the cylinder rod is regularly moistened with oil by in- and outgoing movements.

The hydraulic oil provides a temporary protective layer. But if this is not the case, corrosion can be visible within 24 hours.Contrary to the galvanic Ni/Cr, with HVOF, laser cladding and classical welding there is a wide choice of welding material, the choice of which can be adjusted to the nature of the use.

There are dozens of alloys available, mostly based on nickel (Inconel variants), cobalt (Stellite variants) and stainless steel.At the moment, Inconel alloys are most commonly used in lasercladding because these alloys are affordable and very corrosion resistant in various types of aggressive environments.

  Functional Requirement: Wear resistance

If the choice of a surface treatment is determined on the basis of corrosion resistance, its degree of wear resistance is then important for the service life. Hardness is often taken as a measure of wear resistance because it is easy for the eye to measure. Appearances are deceptive here! Scientific research shows that the correlation between wear resistance and hardness is often far off. Moreover, the hardness is not easy to measure at all.

With a hardness tester it is not the hardness of the applied layer that is measured, but rather the hardness of the underlying layer. This is because the applied layer is too thin to measure as a stand-alone layer. And furthermore, more and more layers are applied with a certain ratio of very hard carbides. When measuring, the hardness of the matrix in which the carbides are located is mainly measured, but not the hardness of the carbides themselves.

In short; do not rely on the measured hardness but look at the wear resistance as a measure for the service life (life span) of the layer.

The galvanic Ni/Cr is known to have a very good wear resistance. (However, this is seldom the cause that ends the service life of galvanic Ni/Cr. That is corrosion!)

During the first use of a hydraulic rod that is coated with galvanic Ni/Cr, tracks can become visible that cause a small oil collar. This is due to deformation of the softer nickel underneath the chrome layer. During further use this deformation leads to an even “cold” hardening of the nickel and thus to an excellent tread without oil loss.

The same phenomenon occurs with a laserclad layer of Inconel. The nickel used here has a composition that allows itself to be “cold deformed” more strongly in advance during use and then after the first blows it creates an excellent wear resistance itself.

Despite the fact that the chrome layer of the galvanic Ni/Cr shows an excellent wear resistance, the wear resistance of a laserclad layer based on Inconel625 is often more than sufficient. To further increase the wear resistance of an Inconel layer, alloying elements and finishing techniques can be applied in that Inconel layer.

In the HVOF process based on ceramic material (alu based), an excellent wear resistance can be achieved in the base. HVOF thermal spraying and laser cladding do not differ much from each other in this respect.

For even better wear resistance it is possible to add “hard facings” to the alloy to be applied in both laser cladding and HVOF thermal spraying. These increase the wear resistance to any desired level. The advantage of a laserclad layer is that there is no brittleness. The ductility and impact values are not affected.

The same result can ultimately be achieved with the classic welding techniques as with the laserclad, on the understanding that the layer is much thicker (more expensive), gives more undesirable build-up and a larger (unwanted) “Heat Affected Zone”.

In particular it should be mentioned that laserclad layers with “hardfacings” are often used in bearing constructions for e.g. ship drive shafts and hinges for large, heavily loaded bridges and locks. This concerns rotating instead of translating wear properties. Practice shows that the wear resistance of a laserclad layer with “hardfacings” is superior to that of other layers. For bearing construction AluBronse can also be applied via the laserclad layer, which has good properties against fretting.

  Functional Requirement: Flexibility

Because the base material of a hydraulic rod or a roller is made of elastic material, the applied surface treatment must also have that elasticity. If this is not the case, the protective layer will show cracks (which will lead to under-corrosion) and/or delamination will occur, i.e. the protective layer will loosen as a result of the material stresses. This also inexorably leads to corrosion.

The laser cladding and the classical welding techniques differ strongly from the HVOF technique and the galvanic Ni/Cr. The first group gives a metallic bond between the top layer and the underlying layer. These have become a whole.

This is not the case with HVOF and galvanic Ni/Cr. Here there is a mechanical adhesion of particles and that adhesion is broken at some degree of tension by, for example, bending. The layers then let go of each other. It should also be noted that the ceramic HVOF layers made of aluminum oxide are not very elastic and will soon show cracking.

The degree of adhesion is particularly important for kink-sensitive hydraulic rods. This is the case when the length/diameter ratio is high and when there is an eccentric pressure load or deflection due to own weight.

For special circumstances, the indentation test is also cited in this context to demonstrate (undesirable) brittleness, especially appendix temperatures. Friability is an equivalent for fatigue.

Brittleness is mainly caused by structural change as a result of heat input into the Heat Affected Zone (HAZ). In the classical welding method, this structural change in the HAZ is significant, causing the impact value to drop or the brittleness to increase.

Incidentally, the ceramic HVOF material is also reasonably brittle.

  Functional Requirement: Repair possibilities

A part of the service life is the impact that damages with an external cause have. If these damages to the protective layer can be repaired, on site and without substantial interruption of the business processes, it reduces the TCO (Total Cost of Ownership). If, on the other hand, there is no possibility of short-term repair and an action has to be taken until the completely new part, then not only the cost of the part is important, but also the downtime costs.

Damage to galvanic Ni/Cr can to a limited extent be repaired in the field by means of a so-called taping process. The depth and surface of the damage should not be too large and this is a temporary repair. The quality of the repair is not considered to be complete and that means a complete replacement of the part is needed in the short term.

With HVOF layers only limited repair is also possible in the field. The damage to be repaired should not be too great.

In the case of layers applied using the classic welding method or laser cladding, full and definitive repair in the field is possible without the need to expand the repaired part. Only a welding machine is needed for this.

  Other requirements: Total Cost Of Ownership of Protective Layers (Surface treatments)

We distinguish the following main components:

• Cost of initial manufacture
• Durability of the corresponding protective layer
• Cost of exchange downtime at end of service life or repair

Cost of initial manufacture:
A laserclad layer or an HVOF layer are broadly at the same price level, which is about 35% higher than galvanic Ni/Cr. A classic welded-on layer is about double that of a galvanic Ni/Cr layer.

Price ratios:

• Galvanic Cr/Ni layer : 1.0
• Ceramic HVOF layer: : 1.35
• Laserclad layer : 1.35
• Classic overlay : 2.0

The cost of downtime per case is set at the same amount for all rod coverings.

The service life is determined by the weakest element of the applied layer. The majority of hydraulic rods are in offshore and dredging installations where HCl and H2S are the cause of failure. The Cr/Ni layers are pressed off by under-corrosion. To a lesser extent this happens with ceramic HVOF layers. At HVOF ductility plays a role in this. Due to mechanical stress cracks occur where the source of corrosion lies.

Based on salt spray tests and a number of practical experiences, the following life times of the different layers in a saltwater environment are assumed: (We realize that the results may be different in practice, depending on the circumstances).

• Galvanic NiCr : 8 years
• Ceramic NiCr : 15 years
• Laserclad layer : 30 years
• Classic coating : 30 years

If the service life of the installation is set at 30 years, the TCO for this comparison of layers is as follows: Number of times initial layer application cost ( IK) + number of times downtime cost ( SK)

Total Cost of Ownership (TCO):

• Galvanic Cr/Ni layer : 30/8 x 1.0 IK + 30/8 SK = 3.75 IK + 3.75 SK
• Ceramic layer : 30/15 x 1.35 IK + 30/15 SK = 2.7 IK + 2.0 SK
• Classic coating : 30/30 x 2.0 IK + 30/30 SK = 2.0 IK + 1.0 SK
• Laserclad layer : 30/30 x 1.35 IK + 30/30 SK = 1.35 IK + 1.0 SK

The result of this comparison gives an indication of the TCO and not an exact amount. This simple model tries to indicate what the initial costs, the downtime costs and downtime do with the TCO. The service life will usually be the domination factor and the initial cost the least domination. The TCO of the galvanic Ni/Cr layer and the ceramic HVOF layer are under these assumptions the highest and that of the laserclad layer the lowest.

  Other Requirements: Tax For the Environment And Future Resistant.

The environmental impact of surface treatments and the products to be used is becoming increasingly important. It is not only a social responsibility but soon also a legal one. Companies will be obliged to implement plans to reduce energy consumption.

Not only energy consumption is important in the segment of surface treatments. The use of environmentally harmful and hazardous chemicals also plays a role. And the possibility of enforcing these processes in terms of permits. Local authorities are increasingly increasing the establishment requirements for high-risk processes and the EU has focused on reducing or banning, for example, Cr6. Which processes are still future-proof with regard to regulations?

Galvanic Ni/Cr surface treatments:

The principle of applying such a galvanic layer is based on the precipitation of nickel and chromium on a product to be treated. This process itself, including pre- and post-treatments, takes place and baths with chemicals under the use of high current. The energy requirement and therefore the CO2 load is high. In order to guarantee a certain quality of the nickel and chrome layer, the chemicals need to be refreshed at a certain moment. So there is a consumption! The (external) processing of polluted chemicals is an energy-consuming process.

The local and central government has in recent years strongly tightened its requirements with regard to the maintenance and enforcement of these processes by means of BRZO regulations. This means that the safety of the environment is better guaranteed, but also that the obtaining of new business licenses is subject to stricter rules. Are the existing galvanic companies able to maintain themselves due to the stricter requirements?

The EU has been focusing on banning Cr6 for a number of years now. In short, the current rule is that processes
where Cr6 is released may only be applied when there is no alternative and the ban has too great an economic impact.
At this moment it is still possible to obtain a temporary permit for this branch of industry.
It is currently not known how long this will take.

In the branch of galvanic surface treatments, a lot of work is being done to replace chromium with a similar product without the release of Cr6.

HVOF surface treatments:

HVOF surface treatments are applied by applying a precious metal in powder form over a product. The powdered precious metal is heated and sprayed against the product at high speed. This creates a layer of powder particles bonded on top of each other. The energy required for this process is relatively small compared to the galvanic process and can be compared to the energy required for laser cladding.

The HVOF process is not environmentally harmful according to industrial standards. The process gives a high noise level, but that does not disturb the environment by processing in cabins and the use of
protective equipment by the operators.

The authorities therefore issue environmental permits without special requirements and the expectation is that these types of companies will be able to maintain themselves in this area.

The consumables, the precious metal to be applied, do not pose any special risks to people and the environment.

Laserclad surface treatments:

With laserclad, just like with the HVOF technique, a precious metal in powder form is applied over the product to be protected. The difference is that the powder is melted onto the product with a laser beam.

The amount of energy required for this is very small. The melting bath that is made is only about 0.1 mm deep. As with the HVOF technique, the energy load and therefore CO2 emissions are low.

The laserclad process does not disturb the environment. Just as with the HVOF process, it is applied in shielded cabins. Also, just as with the HVOF technique, the consumables (the powder) are not environmentally
harmful and risk-free for the environment.

Classic welding processes:

Classical welding involves welding a wire of a precious metal to the product to be protected. The amount of energy required for this process is considerably higher than with laser welding or HVOF technology.

Due to the higher energy consumption, the CO2 emission is considerably higher compared to the laserclad- and HVOF process, but lower than the galvanic NiCr. The used welding wire has no negative environmental aspects.
However, in this process the choice of material is more limited. With HVOF and laserclad many different mixtures of filler materials can be used. This process does not disturb the environment. More fumes are produced, but these can be filtered with simple techniques.

 

For more information about Laser Cladding visit the ”TopClad® Laser Cladding” page or contact us.