Heat Treatment of Fasteners for the Medical Device Industry

The medical industry relies heavily on the use of fasteners. 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. Read on to discover the critical role heat treating plays in creating reliable fasteners in this original content article by Daniel H. Herring, also known as “The Heat Treat Doctor” ® © 2013 The HERRING GROUP, Inc.

This Technical Tuesday column appeared in Heat Treat Today’s December 2021 Medical and Energy print edition. 

Introduction

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.

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 below.

Surgical & Non-Implant Medical 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, 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. (The heat treatment of stainless steels was covered in Heat Treatment of Fasteners for the Petrochemical Industry, Fastener Technology International, October 2013.)

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 maxillofacial 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 and 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 and bleaching systems)
• Consumer electronics (e.g., batteries, watches)
• Sports equipment (e.g., bicycle frames, golf clubs)

Heat Treat Example

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 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; 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; most familiar alloy in this category is Ti-6Al-4V
• Beta (β) alloys — strengthened by heat treatment, high strength, and fair creep resistance

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 are 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 and 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 share components

Practical Considerations — What’s Important

The heat treatment of titanium and titanium alloys is most often done in a vacuum furnace (Figure 7). Heat treat furnace capacity is an important consideration since many titanium parts are volume-limited rather than 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 typical in a medical fastener.

Temperature measurement and control must be exact, usually ± 10 °F (5.5°C) 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 over 1730°F (943°C), titanium cannot be in contact with a nickel alloy or stainless steels since eutectic melting will occur.

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.

Since titanium is a strong getter material, vacuum furnace interiors must be pristine; ideally, all metal hot zones 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” (cleaned) before use typically at 2400°F (1315°C).

In Conclusion

Fasteners are at the heart of the medical device industry and heat treatment plays a critical role in the manufacturing process. Whether made of stainless steel, titanium, tungsten carbide, or superalloys, a heat treat recipe is available to maximize both mechanical and metallurgical properties for implantable and non-implantable applications.

References

[1.] Jones, Christie L., Fastening Solutions for Medical Devices, White Paper, SPIROL International Corporation.

[2.] Herring, Daniel H., Practical Aspects Related to the Heat Treatment of Titanium and Titanium Alloys, Industrial Heating, February 2007.

[3.] Herring, Daniel H., Vacuum Heat Treatment, BNP Custom Media, 2012.

About the Author:

Dan Herring, who is most well known as The Heat Treat Doctor®, has been in the industry for over 45 years. He spent the first 25 years in heat treating prior to launching The HERRING GROUP, Inc. in 1995. His vast experience in the field includes materials science, engineering, metallurgy, equipment design, process and application specialist, and new product research. Dan holds a patent (as a co-inventor), and his consulting services in heat treating and sintering, metallurgy, operations, business management, sales and marketing, and technology have benefitted a broad range of industries.

For more information

Contact Dan: Tel: 630.834.3017 Cell: 630.640.9048 Fax: 630.834.3117

E-mail: dherring@heat-treat-doctor.com or dherring@ameritech.net

Website: www.heat-treat-doctor.com