Tribological Interactions

Galling - risks and mitigation

What would happen if we built seals out of a highly corrosion resistent SS316 or Duplex Stainless Steel?

In a perfect world, we could have our cake and eat it too. We could enjoy the benefits of highly corrosion resitent materils such as SS316, in a high performance rod scraper. Unfortunately, the world is not entirely perfect (pretty close though)

If a 316 stainless steel seal runs against a hardened chrome hydraulic rod, the likelihood of galling is very high without proper lubrication or intervention. This is because of the properties of the two materials in this specific application. 

Why galling is likely

• 316 stainless steel is highly prone to galling. As an austenitic stainless steel, 316 is a material that work-hardens quickly. When sliding against another surface under pressure, the passive oxide layer that protects it from corrosion is sheared away. The newly exposed metal is highly reactive and susceptible to "cold welding" to the opposing surface. Studies show that austenitic stainless steels have one of the highest risk factors for galling.

• The chrome rod creates a hard counter-surface. The hydraulic rod has a very hard, smooth surface due to the hard chrome plating. When the softer, more ductile 316 stainless steel is rubbed against this surface, the harder chrome asperities (microscopic surface roughness) act like a file, scraping off the protective oxide layer on the stainless steel. This initiates the galling process.

• High contact pressure and sliding motion. This combination is a perfect recipe for galling. As the seal is pressed against the rod, the high contact pressure promotes adhesive wear. The continuous, cyclical sliding motion of a hydraulic rod then exacerbates the issue, causing material transfer and buildup—the very definition of galling. 

How to mitigate the risk

Given the application, preventing galling is critical to avoid hydraulic system failure.

• Appropriate seal material: 304, 316 and Duplex stainless steel alloys are not standard materials for this application due to its galling risk. Hydraulic rod seals are typically made from softer, low-friction metals less prone to work-hardening, including brass, bronze, 400-series and 303 Stainless Steel. These materials are engineered specifically to slide smoothly against a chrome rod with minimal risk of wear or galling.

• Use surface treatments on the seal: If a metal-on-metal sliding surface is unavoidable, other surface treatments can be applied to the stainless steel to reduce friction and wear. This could include specialized coatings or proprietary surface-hardening processes.

• Ensure optimal surface finishes: Both the seal surface and the hydraulic rod's finish must be specified correctly. A surface that is too smooth can increase adhesion, while one that is too rough will increase friction. The specific roughness (Ra) needs to be carefully controlled to work with the seal and lubricant.

Materials with reduced potential to work-harden.

The addition of sulfur to 400-series and 303 stainless steel reduces friction and the risk of galling by promoting the formation of manganese sulfide (MnS) inclusions. During contact with a hardened chrome hydraulic rod, these inclusions act as a lubricant and break away from the surface, preventing the adhesion that causes galling. 

How sulfur prevents galling

Galling is a form of adhesive wear that occurs when two sliding metal surfaces seize or cold-weld together under pressure. Stainless steel is particularly susceptible to galling because its protective oxide layer can be broken by the compressive force of contact, exposing the reactive metal beneath. 

The sulfur in 303 stainless steel creates microscopic "stress risers" that prevent this material transfer from happening. 

1. Creation of manganese sulfide inclusions: Sulfur has a high affinity for manganese, and the two elements combine to form manganese sulfide (MnS) inclusions during steel production.

2. Chip-breaking effect: During sliding contact, these relatively soft MnS inclusions act as built-in chip-breakers. They create sites where small, brittle fragments of the stainless steel can break away, allowing for cleaner separation of the two surfaces.

3. Lubrication: The MnS inclusions and the small chips that break away act as a solid lubricant between the two sliding surfaces. This lubricating effect reduces both friction and the buildup of heat, which is a major trigger for galling.

4. Prevents cold welding: By interrupting the metal-to-metal contact, the MnS inclusions prevent the cohesive, high-energy bonds from forming between the stainless steel seal and the hardened chrome rod. 

Trade-offs of using 303 Stainless Steel

While the addition of sulfur is highly effective for improving wear characteristics in sliding applications, it comes with a few trade-offs. 

• Reduced corrosion resistance: The MnS inclusions create microscopic weak points in the steel's passive chromium oxide layer, making it more susceptible to localized corrosion.

• Lower ductility and toughness: Sulfur slightly reduces the steel's ductility and impact resistance, meaning it's not ideal for applications that require high strength and toughness.