Fastener applications are as diverse as the industries they service, driven primarily by the aerospace, construction and automotive industries with petrochemical, nuclear, medical, marine and mining being important niche markets. It is estimated that some 350 manufacturing plants produce more than 200 billion fasteners per year in the U.S. alone.
Fastener materials also cover the gambit from ferrous to nonferrous materials, including steel, stainless steel, tool steel, aluminum, titanium and exotic specialty grades. Coatings are also in common use and include zinc, cadmium, nickel, galvanized (hot dip, sherardizing), phosphate and PTFE (polytetrafluoroethylene) coatings to name a few.
Heat treatment plays a critical role in the manufacture of fasteners in order for them to achieve the desired performance properties. Heat treatment is typically conducted after the forming processes and before any coating or finishing process in both captive and commercial shops. The equipment necessary to effectively heat treat steel fasteners includes well-controlled atmosphere furnaces with temperature control, atmosphere control, quenching tanks and cleaning equipment. This equipment typically requires a large capital expense as well as continuing maintenance costs.
Heat-treating systems for fasteners come in a variety of shapes and sizes, but they all have one thing in common – flexibility, or the capability to produce large or small quantities of fasteners “on demand.” It is not uncommon to see production lots as small as 4.5 kg/hour (10 pounds/hour) and as large as 4,535 kg/hour (10,000 pounds/hour) or more.
The most common heat treatments for steel fasteners involve annealing of the incoming material, through or selective hardening, and case hardening such as carbonitriding and carburizing. Case depths are shallow, typically 0.0038-0.038 mm (0.0015-0.015 inch), specified in ranges of 0.127 mm (0.005 inch). Quench media runs the gambit from brine, water, polymer, oil and molten salt, depending on engineering requirements. Stainless steels and many nonferrous fasteners are solution heat treated and age hardened.
The world market for industrial fasteners represents a $67 billion industry divided between Asia (38.2%), Europe (25.3%), North America (21.9%) and the rest of the world (14.6%). Of this total, the Asian market is dominated by China (14.8%) and Japan (10.1%). Strong growth prevails in the automotive, aerospace and energy sectors. The industry is expected to show continued growth in the near future based on rapid global industrialization and rising demand for durable goods.
Global Industry Growth
The demand for fasteners is strong. The aerospace industry is currently the largest user of fasteners, accounting for over 30% of the total market. Going forward, the use of fasteners by the construction industry is poised to overtake aerospace, based on recent activity worldwide. The construction industry’s use of fasteners is expected to grow at a compound annual growth rate of just over 9% from 2012 to 2018. Other OEM segments (e.g., fabricated metal products, electronic/electrical) have also exhibited strong growth in the last few years. The automotive sector is expected to continue to show above-average growth as well in the next five years.
The Asia Pacific region accounted for the highest demand for industrial fasteners last year and is expected to account for over 45% of the market by 2018. This trend is due to factors such as rapid industrialization and favorable economic conditions, which is expected to boost the demand for durable goods and other manufacturing and development activities.
The heat treatment of fasteners is also highly application- and industry-specific. Here are some highlights of major markets.
Aerospace applications (Fig. 1) include aircraft (manned and unmanned, fixed and flex wing), rotorcraft (helicopters, gyrocopters) and space vehicles (shuttles, space stations, satellites).
Fasteners are one of the most critical components used in all of these applications and are required to meet the most demanding performance characteristics. The types of fasteners in the aerospace industry are quite diverse and include screws, rivets, bolts, nuts, pins, collars and washers.
Solution annealing and aging of aerospace fasteners (Fig. 2) in vacuum is an example of a thermal treatment. Stainless steel fasteners of Carpenter Customâ 630 (17Cr-4Ni), a martensitic precipitation/age-hardening stainless steel with high strength and excellent corrosion resistance, are processed using the following cycle:
Step 1: Solution heat treatment
1. Pump to 10-4 torr range
2. Heat to 1040°C (1900°F)
3. Hold for 30 minutes (based on a workload thermocouple)
4. Quench in nitrogen to 32°C (90°F)
Step 2: Age hardening
1. Load furnace and pump down to mid-10-5 torr range
2. Heat to 480°C (900°F)
3. Hold for 65 minutes (based on a workload thermocouple)
4. Nitrogen fan cool to 52°C (125°F)
The result is bright, shiny parts having a hardness of 42-44 HRC.
Fasteners account for a significant number of parts in aircraft and directly affect strength characteristics and weight of structural assemblies. According to The Boeing Company, the 747 includes over six million parts, half of which are fasteners. On average, for example, 2,400,000 fasteners are used to assemble a Boeing 787 aircraft. On average, 22% are structural bolts (mostly titanium) and the rest are aluminum rivets.
As the industry evolves to incorporate newer, more-exotic materials, fasteners continue to figure prominently in the manufacturing and assembly processes. Fasteners play a critical role in defining the longevity, structural integrity and design philosophy of most metallic aircraft structures.
Typical aerospace fastener materials include aluminum, steel (e.g., A286, H-11) superalloys (e.g., Waspaloy, Hastalloy, Inconel 718), nickel alloys (e.g., Monel, K-Monel) and titanium.
The medical industry relies heavily on the use of fasteners. For example, medical devices (e.g., dental and orthopedic implants, instruments) employ literally hundreds of different types of fasteners to hold their assemblies together. Even though the components in the medical devices are small or even tiny, when a fastener fails, the device will almost always fail as well.
The correct fastener ensures that the device goes together and stays together for the intended life of the assembly and that the device performs as desired. Fasteners can overcome challenges in assembly, solve quality problems and significantly reduce the total cost of the device.
Medical devices fall into two broad categories: surgical/non-implant devices and implantable devices. The alloys and heat treatments for the fasteners involved in both are explained.
Surgical and Non-Implant Medical Devices
Surgical and dental instruments are examples of non-implant medical devices. They are typically manufactured from austenitic stainless steels where good corrosion resistance and moderate strength are required. Examples include canulae, dental-impression trays, guide pins, hollow-ware, hypodermic needles, steam sterilizers, storage cabinets, work surfaces and thoracic retractors to name a few. These applications often use a variety of stainless steels that can be easily formed into complex shapes.
Implantable Medical Devices
Specific grades of austenitic stainless steel and high-nitrogen austenitic stainless steels are used for some surgical implants. Examples include aneurysm clips; bone plates and screws; femoral fixation devices; intramedullary nails and pins; and joints for ankles, elbows, fingers, knees, hips, shoulders and wrists.
However, the vast majority of orthopedic implants worldwide are manufactured from titanium (e.g., Ti-6Al-4V alloy) or cobalt-based alloys (e.g., ASTM F75, a cobalt-based alloy or cobalt-chromium-molybdenum alloys). They are manufactured from castings, forgings or bar stock.
Medical application examples include pins, bone plates, screws, bars, rods, wires, posts, expandable rib cages, spinal fusion cages, finger and toe replacements, hip and knee replacements, and maxio-facial prosthetics.
Heat-Treat Equipment for the Fastener Industry
Fastener heat treatment can be performed in a wide variety of furnaces and ovens, and fastener designers should understand the multitude of choices available to them. Here are several examples of atmosphere furnaces used for this task.
Mesh-Belt Conveyor Furnaces
Today, mesh-belt conveyor furnaces (Fig. 3) are the dominant technology for the heat treatment of fasteners. These units are often part of a completely automated heat-treating system that includes loaders, pre- and post-washers, a hardening furnace with quench tank and a tempering furnace. Soluble oil tanks and endothermic atmosphere generators or nitrogen/methanol systems are common ancillary items.
Heat-treating systems for fasteners must be flexible enough to handle both large and small quantity demand. It is not uncommon to have production lots as small as 4.5 kg (10 pounds) and as large as 4,535 kg (10,000 pounds) or more. Standard production capacities in mesh-belt furnaces typically range from 100 kg/hour (250 pounds/hour) to 3,000 kg/hour (7,000 pounds/hour). It is also not uncommon to see fasteners loaded on the belt between 12.7-63.5 mm (0.5-2.5 inches) deep (Fig. 4).
Through hardening, selective hardening and case hardening (carbonitriding and carburizing) are typical heat-treat processes for fasteners. Case depths are typically shallow, in the order of 0.0038-0.038 mm (0.0015-0.015 inch). Quench media are more diverse, ranging from brine, water, polymer, oil and molten salt, depending on engineering requirements.
Rotary retort (Fig. 5), shaker hearth (Fig. 6) and cast-link conveyor furnaces are the most common alternatives to mesh-belt conveyors and are usually floor mounted. The systems are typically automated and completely automatic after loading parts into the hopper at the front end. Vibratory hoppers and weight-actuated skip loaders deposit precisely measured charges into the furnace to ensure the uniform loading of parts.
Typical standard system capacities for rotary-retort furnaces vary from around 225 kg/hour (500 pounds/hour) to 450 kg/hour (1,000 pounds/hour), but they can be manufactured to handle 1,800 kg/hour (4,000 pounds/hour) or more. The retorts are either cast or fabricated from high-temperature alloys. In general, cast retorts have superior mechanical-strength characteristics compared to wrought fabricated designs but often come at a cost premium. Auger flights with the retort convey the fasteners through the furnace. Variable-speed rotation of the retort provides flexibility of time-based processing cycles.
The actual quenching process of the part is typically completed within two to five seconds from the time fasteners drop into the quenchant, with 10 minutes being a typical residence time in the quench for continued cooling. The exception is austempering, which requires extended times in the order of 20 minutes or more to produce a bainitic structure. It is critical that an adequate amount of fluid flow is delivered to the active quench area because overall quench-tank capacity is not enough to ensure individual quenching of each part. The quench-tank design must eliminate any clumps and clogs to ensure that both proper technique and adequate quenching is obtained.
Applications for Vacuum Furnaces
An increasing number of vacuum furnaces (Fig. 7) are being used for the processing of fasteners and fastener-related products due to the type of material being processed (e.g., bioengineering or nuclear) and the type of surfaces produced.
Tempering furnaces and ovens, whether continuous (e.g., mesh belt, pusher) or batch, use a recirculation system designed to allow heated air to circulate through the mass of fasteners to achieve uniform temperature throughout the load. Convection heating is used to full advantage for efficient heat transfer in a minimum floor space. Typically, operating-temperature ranges vary from 150-650°C (300-1200°F).
Induction heating is also used to selectively heat treat fasteners at high production rates. For example, high-frequency (10-50 kHz) systems can temper (draw) back bolt heads after carburizing to improve toughness at rates in the order of 1.0-2.5 pieces/second. Similarly, seat and seat-belt retention bolts can be case hardened (200 kHz)to depths up to 0.508 mm (0.020 inch) and surface hardness of 40-45 HRC to impart both strength and toughness.
Fastener heat treatment in atmosphere furnaces is widely used to meet the demand for higher quality in a broad range of industries and provides a highly cost-competitive technology based on high throughput and consistent, repeatable quality.