II. Fastener Manufacturing Process

Brief Introduction to Fastener Production Procedures

2.2.1 Wire Rod
Wire rod, also referred to as coil material or coiled stock, is produced by heating and rolling steel billets. (Due to inadequate dimensional accuracy and surface quality, hot-rolled wire rods cannot be directly used in fastener production and must undergo further processing.)

2.2.2 Annealing
Annealing is a metal heat treatment process that involves slowly heating the metal to a specific temperature, holding it at that temperature for a sufficient duration, and then cooling it at an appropriate rate (generally slow cooling, sometimes controlled cooling).

The purpose is to soften materials or workpieces that have undergone casting, forging, rolling, welding, or machining, thereby improving plasticity and toughness, homogenizing chemical composition, eliminating residual stress, or achieving desired physical properties. Depending on the objective, various types of annealing processes are employed, such as recrystallization annealing, isothermal annealing, homogenization annealing, spheroidizing annealing, stress relief annealing, recrystallization annealing, as well as stabilization annealing and magnetic annealing, among others.

Spheroidizing Annealing

• Objective: To spheroidize carbides, thereby enhancing material plasticity. The steel is heated to 20–30°C above the Ac1 temperature, held for a period, and then slowly cooled to obtain a structure with spherical or granular carbides uniformly distributed in a ferrite matrix.

• Comparison: After rolling or forging followed by air cooling, the resulting structure consists of lamellar pearlite and a cementite network. This structure is hard and brittle, making it difficult to machine and prone to deformation and cracking during subsequent quenching.

• Advantages: In contrast, the spheroidized pearlite structure obtained through spheroidizing annealing features cementite dispersed as spherical particles in the ferrite matrix. Compared to lamellar pearlite, it exhibits lower hardness, better machinability, reduced austenite grain growth during quenching, and minimized deformation and cracking tendencies during cooling.

• Inspection: The spheroidized structure and decarburization layer must be examined to verify the effectiveness of the spheroidizing process.