Did you know Cladding can increase up to 10 time an equipment Life Span?
One of the most commercially known welding operations in the oil and gas industry is the Cladding coating process. This method, with the addition of engineering materials, either in powder or in a wire form, aims to create a coating that presents higher corrosion, oxidation and wear resistance. The improvement of equipment surface properties depends, primarily, on the chemical composition of both workpiece and coating material.This overlay usually is composed of complex metal alloys based on elements like cobalt, iron, nickel, titanium and aluminum.
Among the Cladding welding processes, the most industrially used, due to its relative easiness of automation and high metallurgical quality, is the TIG (GTAW) process with preheated wire feeding, called TIG Hot Wire. Other classic welding processes are also used for Cladding purpose: from robust GMAW and SMAW to high tech PTA and Laser, each one with its inherent pros and cons.
Cladding is extensively applied to equipment exposed to severe environments that could compromise its mechanical and chemical properties leading to operation failure.
But Why the Equipment Itself is not Made of Resistant Materials?
This is an engineering choice, with economical and mechanical properties base. Not building the entire equipment with the same material brings the opportunity to reduce costs and blend properties between the different materials. There is no reason use the finest and expensive materials throughout the structure of the equipment if only certain regions will in fact be exposed to corrosion, oxidation and/or wear agents.
In general, the chemical composition of the coating will determine the final properties of the equipment. In this way, all aspects that affect this final composition should be analyzed. These are:
Chemical composition of the base material;
Chemical composition of the welding material;
Coating layer thickness;
Dilution (Metallurgical mixing of the base metal with the welding metal, due to penetration of the weld).
Understanding Cladding Dilution
Cladding dilution is one of the quality control indicators of a Cladding operation, and therefore must be always measured and analyzed.
During Cladding two different alloys are mixed in the liquid state, the workpiece alloy and the coating one. Due to the high temperature of the molten pool and the chemical composition gradient a convection current is formed, blending the two alloys until the bead solidify. This dynamic contributes critically in the dilution rates.
How to Determine Cladding Dilution
A very common Cladding operation is the coating of low alloy steels with Nickel alloys 625. In this example, the concentration of Fe element in the coating layer already provides a good indication of the dilution resulting from the operation. This can be estimated because Nickel 625 alloys have less than 1% Fe in its composition, virtually considered as 0%. Thus, it can be assumed that any concentration of Fe that is to be found in the coating originates from the workpiece base metal. This method depends on the use of equipment and techniques of chemical analysis of metals, such as Energy- Dispersive X-Ray Spectroscopy (EDXA), made possible by the Scanning Electron Microscope (SEM).
Another way to obtain an estimate for the Cladding dilution is the geometric way. Less accurate than the method using the Fe chemical composition in the above reported example, the dilution can be measured by the weld thickness and penetration. This method is possible after metallographic preparation, that will provide condition for both metallurgical analysis as well as the dilution rate calculation. After its measurements, the following equation can be used:
Effects of Cladding Dilution
The dilution, as already mentioned, is characterized by the physical mixture of base metals and welding metals. Thus, the very presence of base metal elements in the coating affects the corrosion, oxidation and wear resistance as well as other crucial properties in the operation of the equipment, such as:
Hardness decrease;
Increased susceptibility to cracking.
Effects of Mutiple Coating Layers
In order to achieve lower dilution levels, it is common to use the TIG Hot Wire welding process with multiple passes for Cladding. This process is justified by the drastic reduction of dilution between the first and second layers. For example, supposing that two welding layers with the same welding parameters were performed and 20% dilution levels were achieved in the two layers, it can be easily seen that the resulting approximate dilution is only 4%. It should be noted that this figure is only approximate.
20% x 20% = 4%
Although the multiple layer technique decreases the dilution rate, there is an eventual disadvantage: the final cladded piece must keep it projected dimensions and the multiple layers usually are not precise enough to keep it, asking for secondary steps like grinding or machining. More modern techniques such as PTA and Laser welding processes can be used to avoid such losses due to their better energy input control and inherent better surface finish.
As several equipment that uses Cladding have standards of maximum coating thickness, it is often necessary besides multiple layers, a subsequent machining operation, to remove a small layer of coated material and provide its homogenization superficial, causing an increase of time and investment necessary to manufacture the equipment. More modern techniques such as PTA and Laser welding processes can be used to avoid such losses due to their better dilution control and inherent better surface finish.
Cladding represents the application of a great rule common to all engineering sectors: using nobler materials, consequently more expensive, only when and where they are really needed.
The true welding science is not only about knowing the best materials and processes available, but knowing when to use them.
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