Choosing the right material for military grade fasteners, like screws, bolts, and rivets, is a crucial step in any industrial design process.
Material selection determines the durability, longevity, and performance of these fasteners under different conditions.
It’s not a one-size-fits-all decision – several factors need to be considered, including load-bearing capacity, corrosion resistance, temperature tolerance, cost, and specific project requirements.
So, let’s get right to it…
Types of Fastener Alloys
Carbon Steel Alloys
Carbon steel alloys, predominantly composed of iron and carbon, are widely used due to their high tensile strength and formability.
They can be further classified into low, medium, and high-carbon steels.
While cost-effective and robust, their downfall is poor resistance to corrosion. Therefore, these are used in indoor applications or in those with limited exposure to corrosive environments.
Stainless Steel Alloys
Stainless steel alloys are well known for their exceptional corrosion resistance, courtesy of the significant presence of chromium. These are also resilient to high temperatures and have good strength.
Owing to their high corrosion resistance, stainless steel alloys are used in diverse environments – from kitchens and hospitals to marine and chemical industries.
Alloy Steel Alloys
Alloy steels incorporate other elements like nickel, chromium, and molybdenum, which enhance properties like strength, toughness, and wear resistance.
Though slightly more expensive than carbon steels, they find use in high-stress applications like automotive suspension systems and machine tool shafts.
Aluminum alloys are prized for their lightweight, strength-to-weight ratio, and good corrosion resistance. They also have excellent thermal and electrical conductivity.
These aluminum alloys are extensively used in the aerospace industry and other applications where weight is a crucial factor.
Titanium alloys are characterized by their exceptional strength, low density, and excellent resistance to corrosion.
Their high cost limits their use to specific applications, like in the aerospace industry and medical implants.
Factors to Consider for Fastener Material Selection
Here are five of the main factors you need to consider when choosing a fastener material:
The mechanical properties of a fastener can be its make-or-break factor. Here’s why they matter:
- Strength – Fasteners must be strong enough to handle the forces they’ll be subject to in a given application. This includes both tensile strength (resistance to being pulled apart) and shear strength (resistance to forces that can cause two contiguous parts to slide against each other).
- Hardness – The hardness of a fastener determines its resistance to surface deformation. A harder fastener will resist wear and tear better, making it suitable for applications where there is constant friction or movement.
- Ductility – Ductility refers to a material’s ability to deform under stress without breaking. More ductile materials can handle more deformation before failing, which can be particularly important in applications where the fastener might be subject to variable or shock loads.
- Toughness – Toughness indicates a material’s ability to absorb energy and plastically deform without fracturing. It’s essential in applications where the fastener could be subject to impact loads.
Corrosion can degrade the integrity of a fastener over time, leading to potential failure. Several factors can influence a fastener’s corrosion resistance:
- Environmental conditions – The surrounding environment can greatly affect a fastener’s resistance to corrosion. For instance, a salty marine environment or one with chemical exposure will necessitate materials with high corrosion resistance.
- Galvanic compatibility – When two different metals come into contact in the presence of an electrolyte (like water), galvanic corrosion can occur, where one metal corrodes faster than the other. Ensuring galvanic compatibility between the fastener material and the materials they are fastening can mitigate this risk.
- Coating options – Coatings can enhance a fastener’s corrosion resistance. Zinc, cadmium, nickel plating, and hot-dip galvanizing are common methods used to protect fasteners from corrosion.
Different materials respond differently to temperature changes:
- Operating temperatures – The fastener should maintain its structural integrity and mechanical properties within the temperature range of its operating environment. Some materials lose strength at high temperatures, while others may become brittle at low temperatures.
- Thermal expansion considerations – Materials expand and contract with temperature changes. The fastener material should have a similar coefficient of thermal expansion to the materials it is fastening to avoid loosening or failure due to temperature fluctuations.
Cost and Availability
The practical considerations of cost and availability cannot be ignored:
- Material cost – High-performance materials often come at a higher cost. The material choice for fasteners may thus need to strike a balance between performance and cost.
- Manufacturing availability – Some high-performance materials may be more difficult to source or have longer lead times. This could impact project timelines and needs to be considered in the selection process.
Specific Application Requirements
The specific application can dictate additional requirements:
- Weight considerations – For weight-sensitive applications (like aerospace), lighter materials may be preferred, even if they come at a higher cost.
- Electrical conductivity – In electrical systems, the fastener material may need to be a good electrical conductor or, conversely, an insulator.
- Magnetic properties – In applications sensitive to magnetic fields (like MRI machines), non-magnetic materials will be necessary.
All these factors need to be meticulously considered and balanced to select the right material for your fasteners, ensuring the safety, longevity, and optimal performance of the application.
Choosing the right alloy for the job
Choosing the right alloy for your fasteners is a critical decision with far-reaching implications. It is not only about the inherent properties of the materials but also about the interaction of the fasteners with their surrounding environment and their performance under expected operating conditions.