Why is Weld Cladding a Good Wear Solution?

Cladding is advantageous for the simple fact that it is often desirable to have different functional performance of the surface of a component compared to the core or bulk of that same component. Traditionally designers were forced to consider factors such as strength, toughness, corrosion and wear resistance, and cost together, but, through weld cladding, it is possible to consider some of these factors independently. Weld cladding in principle is simply the application of a different material to surface of a component that has some different functional properties that can be leveraged to save time, money, and improve performance for a target application. Welding is a preferred method of application of this secondary surfacing material because it creates bonds at the atomic level between the coating and the base material. This metallurgical bonding of the coating or overlay is typically necessary to keep these dissimilar materials joined in service in what are often demanding environments. 

Joining of different materials is easy to say but much harder to do. Those familiar with dissimilar metal welding for the purposes of joining know that it is not trivial to combine different materials and maintain the integrity of both materials and the region between them. The chemistry of the clad material responsible for the difference in functional performance can be wildly different from that of the surface it is being applied to, and with melting associated with welding thrown into the mix, it can be incredibly difficult to control the resulting chemistry and structure at the microscopic level (or microstructure) that develops. Cracking, precipitation of brittle phases, and all the risks associated with formation of welding defects exist when pursing weld cladding as a solution. Not even just the weld region itself, but the heat of the process can impact the existing microstructure adjacent to region that was molten in the heat affected zone (HAZ) potentially damaging the carefully crafted properties of the base material. As a best practice, limiting mixing and overall process heat is desirable to avoid some of these complex metallurgical challenges, which can be accomplished through rigorous procedure development or enabled through low heat input processes for overlay applications such as plasma transferred arc welding (PTAW) or laser cladding. Even in the face of these challenges, it is still often worthwhile to employ weld cladding because of the opportunity to achieve target performance, commonly with only a few millimeters worth of material, in applications that would be impractical or financially unfeasible considering an entire component comprised of the coating material. 

A great example of this from the drilling industry is a component called a stabilizer, which functionally serves to contact the formation while drilling and, as the name suggest, provide stability to the tool assembly. The stabilizer must be made of strong material, and commonly has an additional requirement of being non-magnetic to avoid interfering with sensors carefully tuned to the earth’s magnetic field to enable steering of the tool underground. With these tough requirements, the industry has turned to a niche grade of stainless steels called non-magnetic austenitic stainless steels that satisfies both requirements, but there is a catch. The edges of these stabilizers, referred to as blades, are in contact with the underground formation as the tool is drilling and are subject to extreme abrasive wear. Non-mag stainless steels just don’t cut it under these aggressive conditions, so designers have turned to weld cladding, specifically laser cladding, to combat this issue. 

The hardfacing layer is a wear resistant metal-matrix composite clad material (ceramic tungsten carbide spheres embedded in a mostly nickel matrix with the combination referred to as a nickel-tungsten carbide alloy), which enables the functional requirements of the stabilizer to be non-magnetic, strong, and locally wear resistant at the edges of the blade. Due to highly different chemistries of base material and hardfacing layer, an intermediate buffer layer, typically Inconel 625, is applied for its compatibility to both the substrate and hardfacing layer and to prevent stress-relief cracks that form in the hardfacing layer during deposition to propagate into the base material. Cracks in this context, allow the hardfacing material to relieve stresses that accumulate due to differences in shrinkage of the base material and coating (helped somewhat by the intermediate nickel-based buffer material) but are not detrimental to the wear properties of the coating in these abrasive environments. 

Though not a direct correlation, the hardness of the coating can generally be taken as a proxy for wear resistance with harder coatings resulting in improved resistance. For Ni-WC, which excels in high abrasion and/or erosion environments common to drilling applications, the hardness of the coating is typically 500-620HV (50-55HRC) for the nickel matrix of the hardfacing layer, but a whopping 2500-3000HV (miles above the top of the Rockwell C scale) for the spherical ceramic tungsten carbide particles. The net effect is a coating with tightly packed, ultra-hard spheres protected themselves in a hard matrix that is compatible with the buffer layer. Compared to the 350-440HV (37-45HRC) of the non-mag base material, the multicomponent Ni-WC coating makes possible this targeted performance where it is needed without the cost, or perhaps the impossible/impractical nature, of a purely Ni-WC stabilizer. 

It is important that designers are aware of the opportunities of weld cladding as one more tool in the toolbelt to push the limits of performance whilst maintaining functionality and minimizing cost. The possibilities of this technology are endless, and as the materials and the processes used to apply them improve, it is likely that we will see a greater uptake of weld cladding in a more diverse range of applications into the future.