The automotive, aerospace, medical device and construction industries rely heavily on the use of fasteners to secure component assemblies. For example, medical devices (e.g. dental & orthopedic implants, instruments) employ literally hundreds of different types 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.
Fasteners for Construction
Oval head, tapered head, round head (counter sunk) bolts, square head bolts (Fig. 1) and hex head bolts (Fig. 2) are typical products for the construction industry, manufactured from various grades of stainless steels (e.g., 400 series, PH grades such as 17-4, 17-7, 15-7, 13-8 and duplex grades) and alloy steels (e.g., 4140, 4340, 300M). Liner bolts are an example of applications where the bolt is designed to wear down with the liner. With standard bolts, there is often a 3mm (1/8″) clearance between the bolt and the liner; creating point contact. To stabilize the assembled joint, liner bolts are designed to conform to the tapered surface of the liner hole when assembled, creating a solid surface to tighten against.
|Figure 1 | Load of square head structural liner bolts prior to heat treatment|
Martensitic stainless steels are hardened by austenitizing, quenching and tempering much like low alloy steels. Austenitizing temperatures between 980°C (1800°F) and 1010°C (1850°F) are typical. As-quenched hardness increases with austenitizing temperature to about 980°C (1800°F) and then decreases due to the presence of retained austenite. For some grades the optimum austenitizing temperature may depend on the subsequent tempering temperature.
|Figure 2 | Load of hex head structural bolts after vacuum heat treatment (Photograph Courtesy of MET-TEK, inc.)
Slow heating rates or preheating before austenitizing is recommended to prevent cracking in high carbon grades (e.g. 440°C) and in intricate sections of low carbon types. Preheating at 790°C (1450°F), followed by heating to the austenitizing temperature is a common practice.
The hardening of martensitic grades of stainless steels because of their high hardenability and high alloy content is essentially the same as for plain carbon or low-alloyed steels. Air-cooling from the austenitizing temperature is usually adequate to produce full hardness, but oil quenching is sometimes used, particularly for larger sections. Parts should be tempered as soon as they have cooled to room temperature, particularly if oil quenching has been used, to avoid delayed cracking. Tempering at temperatures above 510°C (950°F) should be followed by relatively rapid cooling to below 400°C (750°F) to avoid embrittlement of ferritic grades at 475°C (885°F).
Precipitation hardening grades (e.g. 17-4, 17-7, 15-7, 13-8) typically require full annealing followed by austenite conditioning, transformation cooling and age (precipitation) hardening. Hardening improves strength and toughness and
typically takes place in the 480°C (900°F) to 620°C (1150°F) range.
Carburizing (Fig. 3) as well as low and high temperature nitriding processes have been developed for austenitic stainless steels and are rapidly gaining acceptance, especially for improving resistance to wear and corrosion. It should be noted that soft core hardness could limit these processes in heavily loaded applications.
|Figure 3 | Low pressure carburized and high pressure gas quenched hip repair screw (courtesy of Midwest Thermal-Vac)
Fasteners for Medical Devices
Medical devices fall into two broad categories, surgical/non-implant devices and implantable devices. Surgical and dental instruments are examples of non-implant medical devices typically manufactured from austenitic stainless steels where good corrosion resistance and moderate strength are required. Examples include canulae, dental impression trays, guide pins, hollowware, hypodermic needles, steam sterilizers, storage cabinets and 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.
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.
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 (Fig Nos. 4 – 6) 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.
Other Uses for Titanium Alloys
Titanium and its alloys have experienced rapid growth in the industrial (38%), commercial aerospace (29%) and military aerospace (23%) segments. The benefits of titanium include its strength, strength-to-weight ratio, corrosion resistance, non-toxicity, biocompatibility, excellent fatigue and fracture resistance, non-magnetic characteristics, life, cost, flexibility and elasticity that rival that of human bone.
Non-medical applications include:
- Manned and unmanned aircraft (e.g. commercial & military aircraft, rotorcraft)
- Artillery (e.g. howitzers)
- Military Vehicles (e.g. tanks, hovercraft)
- Naval and marine applications (e.g. surface vessels, submarines)
- Turbines (e.g. power generation)
- Chemical processing plants (e.g. petrochemical, oil platforms)
- Architecture (e.g. sculptures)
- Automotive (e.g. motorcycles, performance automobiles)
- Pulp and paper industry (e.g. washing & bleaching systems)
- Consumer electronics (e.g. batteries, watches)
- Sports equipment (e.g. bicycle frames, golf clubs)
|Figure 4 | Retina surgical instruments|
|Figure 5 | Surgical staples|
|Figure 6 | Dental Implant Posts (Photo Credit: Dentist in Goa via Flickr)|
Heat Treat Examples
The heat treatment of titanium and titanium alloys is complex and demands an understanding of the end use application, desired microstructure, and process variables.
Types of Titanium Alloys
Titanium alloys are classified in four (4) main groups based on the types and amounts of alloying elements they contain:
- Alpha (α) alloys– cannot be strengthened by heat treatment; low-to-medium strength, good notch toughness, and good creep resistance (superior to beta alloys) at somewhat elevated temperatures. They are formable and weldable.
- Near alpha phase alloys – medium strength and good creep resistance
- Alpha-beta (α – β) alloys –strengthened by heat treatment; medium to high strength, high formability, good creep resistance (but less than most alpha alloys), alloys with beta content less than 20% are weldable. The most familiar alloy in this category is Ti-6Al-4V.
- Beta (β) alloys –strengthened by heat treatment; high strength, and fair creep resistance.
Some alloying elements (e.g. Al, Ga, Ge, C, O, N) raise the alpha-to-beta transition temperature (alpha stabilizers) while others (e.g. Mo, V, Ta, Nb, Mn, Fe, Cr, Co, Ni, Cu, Si) lower the beta transition temperature (beta stabilizers).
Types of Heat Treatments
While pure titanium is soft and relatively weak, heat treating can significantly enhance its properties. Titanium and titanium alloys are heat treated in order to:
- Reduce residual stresses developed during fabrication (stress relieving);
- Produce an optimum combination of ductility, machinability, and dimensional and structural stability (annealing);
- Increase strength (solution treating and aging);
- Optimize specific properties such as fracture toughness, fatigue strength, and high-temperature creep strength or create specific conditions in the material.
Standard heat treatments are typically done in vacuum style furnaces or in inert (argon) atmosphere furnaces and include:
- Annealing –increases fracture toughness and ductility (at room temperature) as well as dimensional stability and improved creep resistance. Annealing may be necessary following severe cold work and to enhance fabrication and machining.
- Homogenizing – for improved chemical homogeneity in castings.
- Solution Treating and Age Hardening (Aging) –a process of heating into the beta or high into the alpha-beta region, quenching, and then reheating again to the alpha-beta region. A wide range of strength levels is possible, fatigue strength increases while ductility, fracture toughness, and creep resistance is enhanced.
- Stress Relief –used to reduce residual stresses during fabrication or following severe forming or welding to avoid cracking or distortion and to improve fatigue resistance. Strength and ductility will not be adversely affected and cooling rate is not critical.
- Tempering – When titanium is quenched from an elevated temperature, reheated to a temperature below the beta transus, held for a length of time and again quenched, it is said to have been tempered. Three variables exist in tempering: the phases present, the time held, and the tempering temperature.
Custom heat treatments include:
- Beta Vacuum Annealing & Vacuum Aging – improves fatigue and yield strength as well as elongation in alloys such as Ti-5553 (Ti-5Al-5V-5Mo-3Cr).
- Brazing – induction, resistance and furnace brazing in an argon atmosphere or in vacuum; torch brazing is not applicable. Cleanliness is important to avoid contamination.
- Creep Forming – takes advantage of the fact that titanium moves and takes a set at temperature.
- Degassing – involves removing of entrapped gases such as hydrogen (to under < 50 ppm) to avoid embrittlement.
- Diffusion bonding – primarily in powder metallurgy where individual particles fuse together from intimate contact of their surfaces.
- Hydriding/Dehydriding – the deliberate addition of hydrogen to embrittle the material followed by the removal of the hydrogen after crushing the material into powder. These are the basic steps in the production of titanium powders.
- Isothermal Transformation – involves quenching an alloy from the all beta region into the alpha-beta field, holding and then continuing to quench to room temperature. Treatment in this way causes precipitation of the alpha phase from the beta.
- Sintering – typically involving hot isostatic pressing and laser sintering of powder particles to form near net shape components.
What’s Important – Practical Considerations
The heat treatment of all types of fasteners is best done in a vacuum furnace (Fig. 7). Heat treat furnace capacity is an important consideration since parts are either volume limited or weight limited. Load support is a critical issue in many applications to prevent creep or other dimensional changes, especially on intricate or complex part geometries.
|Figure 7 | Typical vacuum furnace for the heat treatment of fasteners (courtesy of Vac Aero International)
Temperature measurement and control must be exact, usually ± 5.5°C (±10°F) or better throughout the entire working zone of the furnace. Work thermocouples are needed; part temperature, not just the furnace temperature, must be known. Caution: when heating parts one must be aware of issues involving eutectics. For example heating titanium over 1730°F (943°C) in contact with a nickel alloy or stainless steels results in eutectic melting.
Vacuum pumping systems must be capable of reaching high vacuum levels, 1 x 10-5 Torr or lower before starting to heat. This vacuum level must be maintained while heating (requiring very slow ramp rates) as well as when at temperature. Diffusion pumping systems must be properly maintained for maximum efficiency and to avoid backstreaming.
Vacuum furnace interiors must be pristine when processing fasteners for the medical, nuclear and aerospace industry; for medical applications all metal hot zones (Fig. 8) are popular and dedicated furnaces are desired, but graphite lined furnaces also used for other processes are typical throughout the industry as a practical necessity. Thus, fixtures and furnaces must be “baked out” (i.e., cleaned) before use, typically at 1315°C (2400°F).
|Figure 8 | Typical Vacuum Furnace Internal (courtesy of Vac Aero International)
Fasteners are at the heart of many industries and heat treatment plays a critical role in the manufacturing process. Whether made of alloy or stainless steel, titanium, tungsten carbide or superalloys, a heat treat recipe is available to
maximum both mechanical and metallurgical properties for every application.
- Herring, Daniel H, Vacuum Heat Treatment, Volume I, BNP Media, 2012
- Herring, Daniel H, Vacuum Heat Treatment, Volume II, BNP Media (in preparation)
- Jones, Christie L., Fastening Solutions for Medical Devices, White Paper, SPIROL International Corporation.
- Herring, Daniel H., Practical Aspects Related to the Heat Treatment of Titanium and Titanium Alloys, Industrial Heating, February 2007.
- Herring, Daniel H., Vacuum Heat Treatment, BNP Custom Media, 2012.