1. (a) Define the term stainless steel, making reference to the percentage quantities of its two main constituents.
Stainless steel is a steel alloy with a minimum of 10.5% chromium by mass. Chromium is the key element that gives stainless steel its resistance to corrosion, staining, and pitting. Other elements, such as nickel, molybdenum, and carbon, are also often added to improve the mechanical properties and corrosion resistance of stainless steel.
Here is a table showing the percentage ranges of the two main constituents of stainless steel:
| Element | Percentage Range |
| Chromium | 10.5% – 30% |
| Carbon | 0.015% – 1.0% |
Q3
1. (b) With reference to EACH of the following grades of stainless steel, list ONE of its unique properties and a common use that utilises this property:
(i) Ferritic
- Unique property: Magnetic
- Common use: Cutlery
Ferritic stainless steels are the least expensive type of stainless steel. They are magnetic and have good corrosion resistance, but they are not as strong as other types of stainless steel. They are often used in cutlery, appliances, and automotive trim.
Ferritic stainless steel, while not as widely used as its Austenitic cousin on ships, still plays a key role in certain applications due to its unique properties and cost advantages. Here’s how it finds its place in the maritime world:
Advantages of Ferritic Stainless Steel on Ships:
- Lower cost: Compared to Austenitic steels, Ferritic varieties are significantly cheaper, making them an attractive option for budget-conscious applications.
- Magnetic: Unlike Austenitic steel, Ferritic steel is magnetic, which can be an advantage for certain situations like attaching magnetic signs or equipment.
- Good mechanical properties: Although not as strong as Austenitic varieties, Ferritic steel still offers decent strength and ductility for specific applications.
- Good corrosion resistance: While not as stellar as Austenitic steel, Ferritic steel possesses reasonable corrosion resistance against mild freshwater and atmospheric conditions.
Specific Applications of Ferritic Stainless Steel on Ships:
- Interiors and decorative elements: Bulkheads, panelling, trim, and interior furnishings can benefit from Ferritic steel’s cost-effectiveness and aesthetic appeal.
- Exhaust systems and flues: The heat resistance and decent corrosion resistance of Ferritic steel make it suitable for certain exhaust components.
- Freshwater tanks and piping: In freshwater systems with controlled environments, Ferritic steel can function effectively due to its corrosion resistance and affordability.
- Ballast water tanks: Some ship designs utilise Ferritic steel in ballast water tanks due to its lower cost, though careful considerations regarding corrosion protection are necessary.
Limitations of Ferritic Stainless Steel on Ships:
- Lower corrosion resistance: Compared to Austenitic steel, Ferritic steel is less resistant to seawater and aggressive environments, limiting its use in exposed areas.
- Brittle at low temperatures: Ferritic steel exhibits increased brittleness at low temperatures, requiring consideration in cold regions or cryogenic applications.
- Susceptibility to stress corrosion cracking: Similar to Austenitic steel, Ferritic steel can be susceptible to stress corrosion cracking under specific conditions, necessitating proper design and material selection.
(ii) Austenitic
- Unique property: Non-magnetic and ductile
- Common use: Food processing equipment
Austenitic stainless steels are the most common type of stainless steel. They are non-magnetic, ductile, and have excellent corrosion resistance. They are often used in food processing equipment, medical devices, and chemical tanks.
Austenitic stainless steel plays a significant role in various applications on ships due to its unique combination of properties:
Corrosion Resistance: Austenitic steel’s high chromium content (typically 18-20%) makes it highly resistant to corrosion, which is crucial for a marine environment. It stands up to saltwater, chlorides, and harsh weather conditions, minimizing rust and wear.
Strength and Ductility: Despite its excellent corrosion resistance, Austenitic steel maintains good strength and ductility. This allows it to withstand the stresses and impacts encountered at sea, ensuring structural integrity and safety.
Ease of Fabrication and Maintenance: Austenitic steel is relatively easy to work with, being weldable and formable. This makes it adaptable to various ship components and simplifies repairs or modifications.
Non-magnetic: Unlike other steel types, Austenitic steel is non-magnetic. This is useful for applications where interference with compass readings or sensitive equipment needs to be avoided.
Specific Applications of Austenitic Stainless Steel on Ships:
- Deck fittings and railings: Handrails, ladders, stanchions, and other deck components benefit from Austenitic steel’s corrosion resistance and durability for enhanced safety and aesthetics.
- Galley equipment: Austenitic steel’s hygiene and non-toxicity make it ideal for food processing equipment, utensils, and tanks in kitchens and pantries.
- Piping and tanks: From freshwater systems to waste systems, Austenitic steel pipes and tanks offer longevity and resistance to aggressive fluids.
- Propeller shafts and rudders: The high strength and corrosion resistance of Austenitic steel are essential for these critical components, ensuring smooth operation and minimising maintenance.
- Marine hardware: Fasteners, shackles, and other hardware benefit from Austenitic steel’s strength and corrosion resistance, particularly in saltwater environments.
Further Considerations:
While Austenitic steel is a versatile material for ships, some limitations require consideration:
- Higher cost: Compared to other steel types, Austenitic steel can be more expensive, influencing material choices based on specific applications and budget constraints.
- Susceptibility to stress corrosion cracking: In certain environments and under high stress, Austenitic steel can be susceptible to cracking. Careful design and material selection are necessary to mitigate this risk.
(iii) Martensitic
- Unique property: Hard and strong
- Common use: Surgical instruments
Martensitic stainless steels are the strongest type of stainless steel. They are magnetic and have good corrosion resistance, but they are not as ductile as other types of stainless steel. They are often used in cutlery, surgical instruments, and tools.
Martensitic stainless steel, known for its exceptional strength and hardness, finds some niche applications on ships, where its unique properties offer valuable advantages despite certain limitations. Here’s a closer look at how it plays its part in the maritime world:
Strengths of Martensitic Stainless Steel on Ships:
- High strength and hardness: Compared to other stainless steel types, Martensitic steel boasts superior strength and hardness, making it ideal for demanding applications.
- Good wear resistance: Its hardness translates to excellent wear resistance, ideal for components facing friction and abrasion.
- Magnetic: Similar to Ferritic steel, Martensitic steel’s magnetic nature can be beneficial for specific situations like attaching magnetic tools or equipment.
- Affordable compared to other high-strength materials: While not as cheap as Ferritic steel, Martensitic steel can be more cost-effective than other high-strength materials like certain alloys or titanium.
Specific Applications of Martensitic Stainless Steel on Ships:
- Marine hardware: Shackles, pins, and other high-wear hardware pieces benefit from Martensitic steel’s strength and wear resistance, especially for anchor chains and winches.
- Pumps and valves: Internal components of critical pumps and valves can utilize Martensitic steel for its strength and wear resistance, particularly in abrasive or high-pressure environments.
- Cutting tools and blades: Propeller blades and other cutting tools can benefit from Martensitic steel’s hardness and strength for prolonged sharpness and durability.
- Springs and fasteners: In situations requiring high strength and fatigue resistance, Martensitic steel springs and fasteners can provide reliable performance.
Limitations of Martensitic Stainless Steel on Ships:
- Lower corrosion resistance: Compared to Austenitic and Ferritic varieties, Martensitic steel offers lower corrosion resistance, limiting its use in saltwater and harsh environments.
- Brittle at low temperatures: Similar to Ferritic steel, Martensitic steel becomes brittle at lower temperatures, requiring careful consideration in cold regions or cryogenic applications.
- Susceptible to stress corrosion cracking: Under specific conditions and high stress, Martensitic steel can be prone to stress corrosion cracking, necessitating proper design and material selection.
- Difficult to weld and fabricate: Compared to other stainless steel types, Martensitic steel presents challenges in welding and fabrication, requiring specialised techniques and expertise.