[Basic Knowledge] Heat Treatment Processes and Microstructural Features of Common Fastener Bolts
Classification:
Time：
2025-12-22
Common fastener bolts are typically made from medium-carbon steel and medium-carbon alloy structural steel. Their heat treatment generally involves quenching followed by tempering, or through‑hardening (quenching plus high‑temperature tempering), resulting in a tempered sorbite microstructure. Due to variations in carbon content and cooling rates, the tempered sorbite may contain a small amount of ferrite; for bolt microstructures, minimizing ferrite content is desirable, with a recommended limit of no more than 5% for high‑strength bolts.
The white blocky and acicular structures in the sorbite microstructure are ferrite, as shown in the figure below.
Tempered sorbite exhibits excellent toughness and ductility, while also possessing high strength, thereby delivering superior overall mechanical properties.
Below are the heat treatment processes and microstructural features of several common carbon steel grades.
I. Carbon Steel Bolts
Material: 25 steel bolt
Heat Treatment: Quenching + Tempering
Quenching: 900°C for 40 minutes, quenched rapidly in oil.
Tempering: 420°C, 90 min, water quenching
Microstructure: Sorbite + Ferrite
Tempered microstructure of 25 steel bolts at 400× magnification.
Material: 35 steel bolt
Heat Treatment: Quenching + Tempering
Quenching: 850°C for 45 minutes, quenched rapidly in oil.
Tempering: 500°C for 90 min, water quenching
Microstructure: Sorbite + Ferrite
Microstructure of 35 steel after quenching and tempering, 500×
Material: 45 steel bolt
Heat Treatment: Quenching and Tempering
Quenching: 840°C for 40 min, quenched rapidly in oil.
Tempering: 520°C, 90 min, water quenching
Microstructure: Sorbite + Ferrite
Metallographic structure of 45 steel bolts after quenching and tempering, magnification 500×.
II. Alloy Structural Steel Bolts
Material: 35CrMo steel bolt
Bolt strength: Grade 10.9
Heat Treatment: Quenching and Tempering (Quenching + High-Temperature Tempering)
Quenching: 880°C for 40 min, quenched rapidly in oil; Tempering: 520°C for 90 min, water‑quenched.
Microstructure: Sorbite + Ferrite
Tempered sorbite microstructure of 32CrMo steel bolts, magnified 500×.
Material: ML40Cr bolt
Bolt strength: Grade 10.9
Heat Treatment: Quenching and Tempering (Quenching + High-Temperature Tempering)
Quenching: 880°C for 45 minutes, quenched rapidly in oil.
Tempering: 480°C for 90 min, water quenching
Microstructure: Sorbite + Ferrite
Tempered sorbite microstructure of ML40Cr steel bolts
Material: 40Cr steel
Bolt strength: Grade 8.8
Heat Treatment: Quenching and Tempering (Quenching + High-Temperature Tempering)
Quenching: 880°C for 45 minutes, quenched rapidly in oil.
Tempering: 520°C, 90 min, water quenching
Microstructure: Sorbite after tempering
Tempered sorbite microstructure in the core of 40Cr steel bolts
Material: 42CrMo steel bolt
Bolt strength: Grade 10.9
Heat Treatment: Quenching and Tempering (Quenching + High-Temperature Tempering)
Quenching: 860°C for 45 min, quenched rapidly in oil.
Tempering: 520°C, 90 min, water quenching
Microstructure: Sorbite + Ferrite
Tempered sorbite microstructure of 42CrMo steel grade 10.9 bolts
Material: 42CrMo steel bolt
Bolt strength: Grade 12.9
Heat Treatment: Quenching and Tempering (Quenching + High-Temperature Tempering)
Quenching: 860°C for 45 minutes, quenched rapidly in oil.
Tempering: 480°C, 90 min, water quenching
Microstructure: Sorbite after tempering
Tempered sorbite microstructure of 42CrMo steel grade 12.9 bolts
The heat‑treatment process and the resulting microstructure of fastener materials directly determine the bolt’s safety, reliability, and service life; therefore, a basic understanding of these factors is essential when selecting fasteners or designing product structures.
Latest News
Related Products
Contact Us
Address: Beigang Industrial Park, No. 28 Shucheng Road, Hongshan District, Wuhan City, Hubei Province, China
Mobile: 86-18674085456
Phone: 86-027-88137773
E-mail: liyanhe@noblifting.com
©2023 NOB Machinery Equipment (Wuhan) Co., Ltd. SEO Business license This site supports IPV6