Submarines are incredible machines that rely on precision and durability for seamless operation. Yet, back in 1996, the US Navy encountered a significant hurdle with their fleet: the accumulator cylinders vital for submarine function were proving to be both too costly and challenging to produce.
That’s where Spuncast’s centrifugal casting expertise and in-house metallurgy team became an invaluable part of the picture.
In this case study, with insights from Spuncast Sales Engineer Jim Schwartz, we’ll explain the complexities of producing these accumulators. We’ll then illustrate how Spuncast simplified the manufacturing process, saving the Navy time and money, and explore the resulting positive outcomes.
CHALLENGE: Build a durable, corrosive-resistant accumulator for a fleet of submarines without costly dual metal casting
US Navy submarines heavily depend on hydraulic pressure for vital system functions, with water serving as the primary working fluid. That means, beyond enduring anticipated pressures, their accumulators must also withstand emergency situations, such as exposure to seawater.
“At the time, the cylinders used in their accumulators were made by a method called dual metal casting, which aimed to create corrosion resistance inside and strength outside,” Schwartz explained.
To achieve this, they had to carefully pour one type of metal into another. For example, they would use martensitic steel (similar to a 400 series metal) for the exterior and austenitic steel (similar to a 300 series metal) for the interior. However, achieving the perfect mix was often difficult and unsuccessful.
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This approach created heightened operational costs due to the multiple alloy melts. It was also an environmental concern because chrome electroplating requires the use of extremely hazardous chemicals. The process was messy, overly complicated, and ultimately compromised product quality.
SOLUTION: Modify single-alloy material for streamlined production, minimized failure points, & assured performance
The Navy’s accumulator cylinders must meet stringent performance criteria, necessitating specific mechanical properties:
- Ultimate tensile strength: 110 KSI
- Yield strength: 70 KSI
- Elongation/strain to fracture: 20%
- Fracture toughness: 35 ft. lbs by Charpy Impact
- Hardness: 229-250 Brinell
To meet these demands, the Spuncast metallurgical team recommended ASTM A-890 Grade 1B duplex stainless steel for its durability and reliability. Utilizing a single metal would streamline manufacturing, minimize potential failure points, and ensure consistent performance.
However, initially, Grade 1B did not meet all the mechanical requirements, so the team adjusted the alloy’s composition to improve its strength and toughness.
Schwartz explained that this modification allowed the material to meet the Navy’s specifications, providing a single-alloy solution that combines strength and corrosion resistance.
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Spuncast provided the Navy with prototype parts crafted from the new material. After subjecting these parts to a series of demanding tests simulating submarine operating conditions, the Navy was impressed and expressed interest in integrating the material into their submarine accumulators.
RESULTS: Production costs cut 20%, scrap rates lowered, cylinder failures avoided & chrome electroplating eliminated
Spuncast’s thorough testing, ranging from corrosion resistance trials to meticulous impact testing, ensured that the cylinder made with the chosen duplex material met the Navy’s stringent standards.
It proved to be the optimal material for achieving a balance between ferrite and austenite phases, resulting in superior properties ideal for the harsh environment experienced by submarine accumulators.
“Most importantly, we never had a failure during the most critical and last phase of the testing, which was the X-ray testing. That was a relief for everyone involved,” Schwartz added.
Spuncast’s innovation proved highly effective across multiple aspects, as confirmed by a design case study conducted by the Steel Founders Association of America (SFAA).
This study showcased how Spuncast’s material and method optimized cylinder performance for US Navy submarines. The results marked a significant transformation in accumulator manufacturing, including:
- Reduced production costs by 20% and minimized scrap rates.
- Avoided significant wear or in-service failures of cylinders.
- Mitigated environmental hazards by eliminating chrome electroplating.
The study highlighted the improved durability, cost-effectiveness, and environmental benefits of the Spuncast solution, demonstrating compliance with stringent environmental regulations.
Why centrifugally cast cylinders made sense
Centrifugal casting offers a combination of high structural integrity, strength, precision, cost-effectiveness, and corrosion resistance. This makes it an ideal manufacturing process for the accumulator cylinders used in the Navy’s submarines, in addition to its ability to incorporate new materials.
The uniform structure and the specialized alloy improve corrosion resistance, which is an important factor for components like accumulators exposed to the marine environment.
It is also a process that provided the dimensional control and consistency to meet the required as-cast tolerances for the outer diameter (OD) and length, which needed to fall within +/- 1/16” to ensure the precise fit and function of the cylinders.
“Employing centrifugal casting with the new duplex material created consistent uniformity and durability in the cylinders, eliminating the need for chrome electroplating or intricate dual-layer casting,” Schwartz said.
Committed to making durable, cost-effective components
By leveraging centrifugally cast duplex stainless steel, the Navy not only improved the cost-effectiveness of their submarine accumulators but also maintained the top performance and reliability they required.
Despite being almost two decades old, the standards and advantages of centrifugal casting used in this case are as reliable as ever.
This example shows why centrifugal casting is great for making durable components for tough environments. It also demonstrates the benefits of teaming up with a smaller foundry with in-house expertise specializing in making unique centrifugally cast tubing parts.
Spuncast’s exceptional capacity to adapt and customize solutions to meet specific customer requirements continues to distinguish it within the casting industry.
