Heat Treatment of Fasteners
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Time：
2024-10-21
Fasteners are manufactured from a wide variety of materials. The choice of material depends on the application and the operating environment. Steel and stainless steel are the most commonly used, while aluminum and other non‑ferritic alloys are also widely employed. Depending on the selected material, appropriate mechanical properties must be achieved through various processing techniques. Heat treatment is a method used to enhance mechanical strength, ductility, and toughness, and for certain alloys, it can even improve corrosion resistance. The following sections will discuss several cases where materials require heat treatment prior to use.
Steel
The most commonly used materials for manufacturing fasteners are carbon steel and alloy steel. Carbon steel is classified into low‑carbon, medium‑carbon, and high‑carbon grades, while alloy steel is divided into low‑alloy and high‑alloy steels.
Prior to manufacturing fasteners, the cold-heading wire should undergo a subsequent heat treatment. This heat‑treatment process is known as spheroidizing. It softens the steel, enhances its formability in cold working, and reduces tooling costs. Once the fastener has been formed, the steel’s strength increases slightly due to mechanical deformation; however, this increase does not meet the technical specifications required for high‑strength grades (such as Grade 8, 10.9, or 12.9).
The formed fasteners will undergo two heat-treatment processes: austenitizing and tempering.
This is the most widely used heat‑treatment process in industry. The austenitizing step typically involves heating the fastener to 815–870°C, holding it at that temperature for a specified period, and then quenching to produce a harder, higher‑strength microstructure known as martensite. Oil or water is the most common quenching medium. The martensitic microstructure thus formed can confer the following effects:
- Increase strength
- Reduce residual stress
- Reduce ductility
- Reduce toughness
Next, the part undergoes a subsequent tempering heat treatment. Tempering slightly reduces the fastener’s hardness and relieves residual stresses, thereby enhancing its ductility and toughness while improving dimensional stability. By adjusting the tempering temperature and holding time, the desired hardness can be achieved.
Stainless steel
Stainless steels are defined as iron-based alloys containing at least 10.5% chromium. They can be classified into the following five types:
- Austenitic stainless steel grades, such as 304 (18-8, A2) and 316 (A4), cannot be hardened by heat treatment. This type of stainless steel can only be strengthened during manufacturing processes like cold forming and thread rolling. This hardening process is referred to as cold working. However, in certain cases, heat treatment can help restore corrosion resistance and relieve residual stresses introduced during other manufacturing operations, such as welding. Cold working also increases magnetic permeability; when low magnetic permeability is required, heat treatment can be used to mitigate the effects of cold working.
- Ferritic stainless steels, such as 430 stainless steel, can only be slightly hardened through cold working. Heat treatment does not increase the strength of these steels. However, the cold‑working processes employed during manufacturing reduce their ductility, necessitating a subsequent annealing heat treatment to restore it.
- Martensitic stainless steels, such as 410, 431, and 440A/B/C/F, are heat-treated in the same manner as carbon steels and alloy steels. Quenching is typically performed using oil or air as the quenching medium.
- Duplex stainless steels, such as SAF2205® (UNS S31803), are used in high‑strength applications—where the yield strength is typically twice that of conventional austenitic stainless steels—and in environments demanding superior corrosion resistance to chloride‑induced stress corrosion cracking. This grade is characterized by a dual microstructure consisting of ferrite and austenite, and it is heat‑treated by annealing.
- Precipitation‑hardening stainless steel grades fall into three categories: low‑carbon martensitic, semi‑austenitic, and austenitic. Each category employs a distinct heat‑treatment process to achieve the desired mechanical properties and corrosion resistance. Precipitation hardening is a heat‑treatment step that enhances a material’s strength. This type of heat‑treatment process can also be applied to other alloys, such as aluminum and titanium.
Aluminum
Aluminum alloys can be classified into two categories: non‑heat‑treatable and heat‑treatable. The non‑heat‑treatable grades include the 1XXX, 3XXX, 4XXX, and 5XXX series. In most of these series, strength is enhanced through work hardening during cold forging and thread forming. Moreover, as the strain rate increases, the material’s strength also rises.
Heat-treatable aluminum alloys in the 2XXX, 6XXX, and 7XXX series require solution heat treatment followed by quenching, after which age hardening (aging) is performed to achieve the desired mechanical properties. Solution heat treatment allows alloying elements to dissolve into the aluminum matrix without forming intermetallic compounds.
Titanium
Titanium alloys are used for corrosion resistance, and their strength and lightweight nature are of paramount importance. For a given level of strength, titanium alloys can be 40% lighter than steel. These properties have made them indispensable in the aerospace and motorsport industries. Grade 5 is a commonly used titanium alloy in fastener manufacturing.
Nickel alloy
Nickel-based alloys are well suited for applications requiring resistance to severe corrosion and high temperatures. They are used in aircraft and land-based gas-turbine engines, as well as in cryogenic storage tanks. The following is an example of the use of Inconel 718 in fastener manufacturing. This material can operate over a temperature range from –250 to 700°C.
Different alloys require distinct heat treatments to achieve the desired properties. In fastener manufacturing, certain inherent characteristics of these alloys can be leveraged; however, when subjected to high‑strength service conditions, appropriate heat treatment is essential.
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