Vacuum Heat Treatment of Fasteners

Fastener applications are demanding. Whether fasteners are being used in the petrochemical industry, in medical or mining applications, for assembly of marine or nuclear components or in the aerospace, automotive or construction world, vacuum processing allows us to repeatedly achieve the highest quality and metallurgy.

Most fastener materials, including stainless steels and superalloy grades, benefit from or actually require vacuum processing for heat treatment instead of being run under protective atmospheres. In general, there are three main sets of applications where vacuum heat treatment is used:

The absence of surface reactions or the ability to precisely control them is the main difference between vacuum heat treatment and all other forms of heat treatment. Vacuum processing can also remove contaminants from parts, and in some instances, degas or convert oxides found on the material’s surface.

Vacuum Systems

A vacuum system (Fig. 1) provides a space in which the pressure can be reduced and held below atmospheric pressure at all times. One of the primary advantages of vacuum heat treatment is its versatility. In addition to being self-contained, vacuum heat treatment provides a “safe” environment for the surface of the parts being treated and uses consistently reproduced cycles and recipes. When not in use, like an electric light, it is simply turned off saving energy. When turned back on, minimal conditioning time is required.

Vacuum Hardening

Vacuum processes are run in a variety of equipment (Fig. 2) designed to accommodate various workload sizes. Processes for vacuum hardening of fasteners will be discussed by type of material.

Hardening by Oil Quenching (Plain Carbon and Alloy Steels)

Oil quenching typically takes place in horizontal vacuum furnaces equipped with integral quench tanks (Fig. 3) as well as vertical vacuum furnaces (Fig. 4). The design of the quench tank is similar to its atmosphere counterpart; fixed or variable speed oil circulation via agitators or pumps located on one or both sides of the tank and internal baffles to guide the respective oil flow around and through the load. Cold or preheated oil, in the 50°C – 65°C (120°F – 150°F) range, are the most common and special (hot) oils, which run at 135°C – 175°C (275°F – 350°F), have been used with success. Heaters control the oil temperature and the oil is cooled via double wall construction or external heat exchangers usually employing air, for safety reasons.

Compared to normal quench oils, vacuum quench oils are distilled and fractionated to higher purity levels. This allows for a better surface appearance on quenched parts. It is common that the partial pressure of nitrogen above the quench oil is between 540 mbar (400 Torr) and 675 mbar (500 Torr). Usually, higher partial pressures above the quench oil can be advantageous in obtaining full hardness on both unalloyed or very low alloy materials. Low partial pressures above the quench oil produce higher hardness values, as well as lower distortion on parts consisting of medium or highly alloyed steels.

Medium alloy steels (Table 2) and most case hardening steels are hardened either by oil quenching or high-pressure gas quenching (up to 20 bar).

Notes:

[a]    Austenitizing temperature in vacuum is often 15°C (25°F) – 30°C (50°F) higher than atmosphere processing.
[b]    Cooling nomenclature: OQ = oil quench; PQ = pressure quench.

Hardening by Gas Quenching (Alloy Steels)

The most popular method of quenching used for hardening in vacuum furnaces is inert gas pressure quenching used at pressures of 2 – 20 bar. Nitrogen and argon are the most common quenching gases. Cooling in argon produces the slowest heat transfer rates, followed by nitrogen, then helium and finally hydrogen. Nitrogen is the most attractive gas mixture from a cost perspective, however, limitations exist with certain alloys (e.g. titanium). Theoretically, there is no limit to the improvement in cooling rate that can be obtained by increasing gas velocity and pressure. Practically, however, very high pressure and very high-velocity systems are complex and costly to construct. In particular, the power required for gas recirculation increases faster than benefits accrue.

The trend today is to “dial in” the quench pressure, that is, use only the highest pressure required to properly transform the material. This has been made possible due to recent changes in both material chemistry and pressure quench design (e.g. alternating gas flows, directionally adjustable blades, variable speed drives). Gas quenching is now being used to produce full hardness in many materials that in the past have been traditionally oil quenched.

Martensitic Stainless Steels.

All grades of martensitic stainless steel fastener grades can be processed in vacuum furnaces. Austenitizing temperatures and general heat treatment considerations are similar to those used in atmosphere furnaces (Table 3). Since the austenitizing temperatures are usually below 1100°C (2000°F), vacuum levels in the range of 10-3 mbar (10-3 Torr) are very often used which result in clean and bright part surfaces. To avoid evaporation of certain alloying elements, vacuum levels in the range of 0.1 – 1.3 mbar (10-1 to 1 Torr) are required, resulting in some sacrifice of brightness.

Due to the differences in the hardenability of the various martensitic stainless alloys, there is a limitation on the section sizes that can be fully hardened by recirculated nitrogen gas quenching; other types of cooling gas (e.g. helium) can be used but the economic benefits must be carefully considered. The actual values of section size limits depend on the type of cooling system and the capability of the specific furnace employed.

Notes:

[a]    Rapid heating rates can cause distortion and/or cracking. In vacuum heating rates of 8°C (15°F)/minute – 15°C (25°F)/minute are recommended for small parts or intricate shapes.
[b]    Certain parts will benefit from an initial preheat at the temperatures shown.
[c]    Cooling nomenclature: OQ = oil quench; PQ = pressure quench.
[d]    As quenched (oil) data shown.

Precipitation Hardening of Stainless Steels

Determining the heat treatment temperature for precipitation-hardened stainless steels (Fig. 5) depends on a number of factors such as the alloy grade, the type of parts being treated, and the required mechanical properties (Table 4). It is not uncommon, for multiple heat treatment steps to be specified. In other cases, the material is purchased in the so-called Condition “A” requiring only an aging operation to be performed (this is typically not done in a vacuum). For optimum creep and creep rupture properties, the high side of the solution annealing temperature range is typically used.  A low-end annealing temperature is used to obtain optimum strength during relatively short-term service at high temperatures. A final aging heat treatment produces a finely dispersed precipitate throughout the microstructure significantly increasing the room-temperature yield strength.

Notes:

[a]    Cooling nomenclature: WQ = water quench; OQ = oil quench; PQ = pressure quench; AQ = air cool.

Superalloys

Superalloys cover a wide range of materials, typically nickel, cobalt or iron-based alloys and are generally intended for high-temperature applications with most of them being hardened using a solution treating and aging process (Table 5). Solution treating involves heating the alloy to a temperature in the range of 980°C (1800°F) or higher, followed by gas quenching.  In most cases, gas quenching with nitrogen at a pressure of 2 bar or less is sufficient. Aging at an intermediate temperature for an extended period of time follows. In some cases, the complete solution treatment and aging cycles are programmed into the furnace instrumentation so that unloading is not required between cycles.  Certain superalloys, however, require other specialized treatments to develop required properties.

Notes:

[a] Cooling nomenclature: FC = furnace cooling; AC = air cooling; RAC = rapid air cool; OQ = oil quench; PQ = pressure quench.
[b] Air cooling equivalent is defined as cooling at a rate not less than 22°C (40°F) per minute to 595°C (1100°F) and not less than 8°C (15°F) per minute from 595°C to 540°C (1100°F – 1000°F). Below 540°C (1000°F) any rate may be used.
[c] To provide adequate quenching after solution heat treatment, cool below 540°C (1000°F) rapidly enough to carbide precipitation. Oil or water quenching may be required on thick sections.

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 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.

Fasteners for Construction

Oval head, tapered head, round head (countersunk) 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.

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.

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 softcore hardness could limit these processes in heavily loaded applications.

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)

The heat treatment of titanium and titanium alloys is complex and demands an understanding of the end use application, desired microstructure, and process variables.

Heat Treat Examples:

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 the 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.

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. 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 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).

In Summary

Vacuum processing of fasteners is a highly repeatable process that will produce the best surface finish and metallurgy of all the heat treatment methods. While there is always a cost premium, its benefits with respect to metallurgy, properties and repeatability make it a technology to consider. 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.

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