Cavitation erosion of metal parts is an unavoidable hazard in some industrial environments. When a liquid flows through a low-pressure atmosphere, gas bubbles form within it, eventually damaging the surrounding materials. How can we prevent this phenomenon, which destroys propellers, turbines, centrifugal pumps and motors?
What is cavitation?
From the Latin cavus, the term cavitation refers to the formation of holes. This is a physico-chemical reaction that is inevitable in certain contexts: under the action of mechanical agitation, a drop in local pressure can cause a liquid to evaporate, even outside its boiling point. Gas bubbles form, creating shock waves. These cause considerable damage to the surface of nearby metal parts.
For example, diesel engines and marine propellers are constantly under attack: mechanical movement in a liquid medium generates a drop in pressure in the fluid, resulting in the formation of steam, which attacks the walls of the engine or the metal surface of the propeller.
Although cavitation is a natural and unavoidable reaction , it causes a great deal of damage, particularly to high-pressure oil-hydraulic pumps, for example. Fortunately, modern heat-treatment methods for metals enable parts to be better protected against this phenomenon, thus extending their service life.
What are the mechanisms of cavitation erosion?
Cavitation erosion is the result of a complex interaction between moving fluids and metal surfaces. When bubbles of gas or vapour form in a fluid, they undergo what is known as a "sudden collapse" under the effect of increased pressure, causing an implosion. This implosion generates powerful shockwaves that damage the metal structure, creating micro-cracks and material delamination. The repeatedshock wave weakens the surface, and can be aggravated by factors such as corrosion or the presence of solid particles. These processes lead to significant structural damage in critical areas such as motors and pumps.
Nitriding to combat cavitation wear
Although cavitation erosion seems inevitable, it is now possible toact on metal parts to make them more resistant. Among the treatments available to combat this phenomenon is nitriding. A nitrogen-based surface treatment hardens the metal part, protecting it more effectively and for longer.
The nitriding process
This surface diffusion treatment involves heating the metal part in the presence of other gases to generate a high concentration of nitrogen on the surface of the part.
This element then reacts with iron toform iron nitrideswhich "cover" the surface layer of the metal part. This is known as the combination layer. Underneath is the diffusion layer. Together, these two layers form the nitrided layer.
The thickness of this nitrided layer depends in particular on the time, temperature and atmosphere in which the part is immersed. This reaction significantly improves the hardness and resistance of the surface to cavitation erosion. The THERMILYON Group offers three nitriding processes: ionic, gas and vacuum. Post-oxidation treatment is also available to improve corrosion resistance (THERMI®-NITROX).
The advantages of nitriding
Nitriding metal parts greatly extends their service life. This treatment slows down wear and improves resistance to seizure, fatigue andcavitation erosion.
The risk of deformation is also reduced. Nitriding also offers a clear economic advantage: parts last much longer, eliminating the need for frequent repairs, replacements or ongoing maintenance. As a result, nitriding benefits overall productivity over the long term.
For certain applications involving stainless steel parts, surface treatments such as THERMI®-SP offer interesting solutions.
Modeling and prediction of cavitation erosion
CFD (Computational Fluid Dynamics) modeling and simulation have become essential approaches for predictingcavitation erosion. Thanks to these tools, it is possible to anticipate potential risk zones on metal parts, by analyzing precise flows and different local pressures. At the same time, laboratory tests complying with international standards validate the numerical predictions. These combined methods offer a comprehensive vision for preventing erosion, optimizing component life and reducing maintenance costs.
Groupe ThermiLyon has recognized expertise in heat treatment and nitriding for all types of metal parts. Let's talk about your project!
