The problem
Phosphor-bronze bearings were for many years the norm in naval bearing applications because of their excellent resistance properties and cost-effectiveness. While phosphor-bronze is still widely used in most standard rudder and steering gear bearings, more demanding applications, such as submarines and other naval vessels with active stabilization requirements, need materials with greater wear resistance.
Why it was difficult
The search for alternative alloys started nearly 40 years ago when the phosphor-bronze rudder pintle bearings of one submarine exhibited very high wear rates. Deloro Stellite engineers studied the operating conditions that these bearings were subjected to, and found that galling is the main wear mechanism in marine bearings, caused by the metal-to-metal contact at low speeds and high pressures.
In addition, cavitation erosion occurs when hydrodynamic forces on the rudder blade or stabiliser fin causes "flutter", while abrasion and sand erosion occur on vessels that operate in shallow waters. These severe wear conditions are aggravated by the corrosive effects of the marine environment, especially since environmental regulations limit the use of grease lubrication in waters of certain areas.
The Stellite solution
Based on its excellent galling properties and very good corrosion resistance, STELLITE® 6 was selected as the baseline alloy to replace the phosphor-bronze bearings. STELLITE® 6 bushes were cast and fitted on the submarine in 1967. After 8 years in service, these bushes exhibited minimal wear, with only 0.008" of material loss. The boat was withdrawn from service after 20 years with its original Stellite bearings still in operation.
This success led to the expansion of Stellite to other naval steering gear applications, such as rudders, stabilisers, and hydroplane shafts and bushes. However, as the size of vessels increased, larger bearings were required, and STELLITE® 6 castings became uneconomical for these very large bearings. To address this problem, Deloro Stellite engineers used proven submerged-arc weld hardfacing techniques to deposit a layer of STELLITE® 6 directly onto the forged steel shafts and bushes.
Deloro Stellite then improved this alloy further with a "custom-designed" alloy for these naval bearing applications. This alloy, known as STELLITE® 306, was developed as a tougher and more ductile alloy than STELLITE® 6, but with comparable bearing wear characteristics.
The result
Today STELLITE® 306 is a proven material for naval bearings, with an exemplary track record of no failures in over 30 years. These bearings have very long service lives. They are crucial to the successful performance of the boat and are only replaced if there is a risk of bearing failure.
In 2004 Deloro Stellite UK invested in a new large PTA (plasma transferred arc) welding facility designed and built by our German sister company, Hettiger Stellite. The facility includes the most up-to-date high deposition PTA torch, which has sufficient material deposition rates for the hardfacing of large components up to a weight of 10 tonnes. Deloro Stellite UK was able to switch the hardfacing of naval bearings to this state-of-the-art PTA technology to create much more reliable and controlled welding conditions.
|