Application of Heat Treatment in Stainless Steel Fasteners

Stainless steel fasteners, including bolts, nuts, screws, and washers, are widely used in aerospace, automotive, marine engineering, petrochemical, food processing, and construction industries due to their excellent corrosion resistance, mechanical properties, and durability. However, the as-cast or as-formed stainless steel often fails to meet the strict performance requirements of complex working environments, such as high strength, wear resistance, dimensional stability, and resistance to cold welding (galling). Heat treatment, as a crucial processing technology, can regulate the microstructure of stainless steel fasteners without changing their shape significantly, thereby optimizing their mechanical properties and service life. This article elaborates on the common heat treatment processes for stainless steel fasteners, their application principles, material-specific adaptations, and industrial application practices.

1. Overview of Heat Treatment for Stainless Steel Fasteners

The core purpose of heat treatment for stainless steel fasteners is to adjust the internal crystal structure, eliminate internal stresses generated during forming (such as cold heading and machining), and balance the relationship between strength, hardness, ductility, toughness, and corrosion resistance. Unlike carbon steel fasteners, stainless steel contains alloying elements such as chromium (Cr), nickel (Ni), molybdenum (Mo), and titanium (Ti), which form a passive oxide film on the surface to resist corrosion. Therefore, the heat treatment of stainless steel fasteners must not only improve mechanical properties but also avoid damaging this passive film or reducing corrosion resistance.

The selection of heat treatment processes depends on the type of stainless steel (austenitic, martensitic, ferritic, duplex), the performance requirements of the fasteners, and their application scenarios. Common processes include solution treatment, annealing, quenching and tempering, precipitation hardening, surface hardening, and stress relieving, each with distinct characteristics and application scopes.

2. Common Heat Treatment Processes and Their Applications

2.1 Solution Treatment (Solution Annealing)

Solution treatment is the most basic and widely used heat treatment process for austenitic stainless steel fasteners (such as 304, 316, 321 grades) and duplex stainless steel fasteners. The process involves heating the fasteners to a temperature above the solubility limit of carbides (typically 1000–1200°C for austenitic stainless steel), holding them at this temperature for a certain period to ensure the complete dissolution of carbides (such as chromium carbides) into the austenite matrix, and then rapidly cooling (quenching) them in water or air to obtain a uniform austenitic microstructure.

The key function of solution treatment is to eliminate intergranular precipitation of carbides, which prevents intergranular corrosion—a common failure mode of stainless steel fasteners in corrosive environments such as marine and chemical processing industries. Additionally, solution treatment improves the ductility and toughness of fasteners, facilitating subsequent forming processes (such as thread rolling) and ensuring dimensional stability during service. For example, 316L stainless steel fasteners used in marine equipment must undergo solution treatment to enhance their resistance to chloride corrosion and maintain structural integrity in salty, humid environments.

2.2 Annealing

Annealing is a heat treatment process that aims to soften stainless steel fasteners, relieve internal stresses, improve machinability, and homogenize the microstructure. The process varies slightly according to the type of stainless steel: for austenitic stainless steel, annealing is often combined with solution treatment (solution annealing); for ferritic stainless steel, annealing is conducted at 700–900°C followed by slow cooling to eliminate residual stresses and improve ductility; for martensitic stainless steel, annealing is used to soften the material after quenching, reducing brittleness and facilitating machining.

In the production of stainless steel fasteners, annealing is often applied before cold heading or thread machining. For instance, before cold heading 17-4 PH martensitic stainless steel bolts, annealing at 800–850°C can soften the material, reduce the resistance during cold forming, and avoid cracking of the fasteners or excessive wear of molds, thereby reducing production costs. Additionally, stress-relief annealing (a type of annealing) is used to treat fasteners after welding or machining, heating them to 600–800°C and slowly cooling to prevent deformation or cracking caused by residual stresses during service.

2.3 Quenching and Tempering

Quenching and tempering are mainly applied to martensitic stainless steel fasteners (such as 410, 420 grades) and some precipitation-hardening stainless steel fasteners that require high strength and hardness, such as fasteners used in high-stress mechanical parts and aerospace components. The process consists of two stages: quenching and tempering.

Quenching involves heating the fasteners to the austenitizing temperature (850–1050°C for martensitic stainless steel), holding them for a certain period, and then rapidly cooling them in oil or water to transform the austenite matrix into martensite, thereby significantly increasing the hardness and strength of the fasteners. However, quenched martensitic stainless steel fasteners are highly brittle and contain significant residual stresses, so tempering must be performed immediately after quenching.

Tempering involves reheating the quenched fasteners to a lower temperature (150–650°C), holding them for a certain period, and then cooling them to room temperature. This process reduces brittleness, relieves residual stresses, and adjusts the balance between hardness, strength, and toughness of the fasteners. For example, 410 stainless steel bolts used in automotive engine components undergo quenching and tempering to achieve a hardness of HRC 35–45, ensuring they can withstand high loads and repeated vibrations without deformation or fracture.

2.4 Precipitation Hardening

Precipitation hardening (also known as age hardening) is a specialized heat treatment process for precipitation-hardening stainless steel fasteners, such as 17-4 PH, 17-7 PH grades, which are widely used in aerospace, petrochemical, and power generation industries due to their high strength and good corrosion resistance. The process consists of three stages: solution treatment, cooling (to obtain a supersaturated solid solution), and aging (precipitation of strengthening phases).

First, the fasteners are subjected to solution treatment at 1020–1060°C to dissolve alloying elements (such as copper, niobium) into the austenite matrix, then rapidly cooled to room temperature to obtain a supersaturated martensite or austenite solid solution. Subsequently, aging treatment is performed at 480–620°C, during which fine, uniform strengthening phases (such as copper-rich precipitates) are precipitated from the supersaturated solid solution, significantly improving the strength and hardness of the fasteners without sacrificing corrosion resistance. For example, 17-4 PH stainless steel fasteners after precipitation hardening can achieve a tensile strength of up to 1170 MPa and a hardness of Rockwell C38, making them suitable for aircraft components and high-pressure petrochemical equipment.

2.5 Surface Hardening

Stainless steel fasteners often suffer from galling (cold welding) during assembly and disassembly, especially in metal-on-metal contact scenarios with high tightening torques. Galling is mainly caused by low surface hardness, excessive surface roughness, or lack of lubrication, and it can lead to fastener jamming, making removal extremely difficult without damaging the components. Surface hardening is an effective solution to this problem, as it hardens only the surface layer (few to tens of microns) of the fasteners, improving wear resistance and galling resistance while maintaining the corrosion resistance and toughness of the base material.

Common surface hardening processes for stainless steel fasteners include nitriding, carburizing, and proprietary diffusion-based processes such as Kolsterising® and Super Expanite®. Kolsterising®, a low-temperature diffusion process, significantly increases the surface hardness of fasteners (typically above 1000 HV) without changing their dimensions or surface finish, making it suitable for austenitic and duplex stainless steel fasteners used in precision equipment. Super Expanite® solves the shortcomings of traditional surface hardening processes (such as reduced corrosion resistance and shallow hardening depth) by hardening both the outermost layer and the underlying material, further improving corrosion resistance and wear resistance. These surface hardening processes are widely used in fasteners for medical equipment, precision machinery, and chemical processing, where galling resistance and corrosion resistance are critical