Determination and Control of Spheroidizing Annealing Structure of Low and Medium Carbon Steel

For most fastener products (bolts, screws, studs, rivets, etc.), soft annealing or spheroidizing annealing is required.

Spheroidizing annealing is a process method to obtain fine-grained (spherical) pearlite structure dispersed in the ferrite matrix. Its purpose is to improve the cold heading, cold forging, cold punching or cutting processing performance, reduce the tendency of deformation and cracking during quenching and tempering, and make the fasteners obtain fairly uniform final mechanical properties.

The original structure of cold heading steel or low, medium carbon steel, and low alloy steel is lamellar pearlite during steel rolling. At this time, the finer the ferrite and cementite structure, the more its phase interface, the more austenite nuclei are formed, and the faster the growth rate of the crystal nuclei, so the formation process of austenite can be accelerated. If the cementite in the original structure is spherically distributed after spheroidizing treatment in advance, the phase interface between ferrite and cementite is reduced, so the formation rate of austenite will be slowed down.

There are two types of spheroidizing annealing processes: one is to austenitize the steel, and by controlling the austenitizing temperature and time, the carbon content of the austenite is unevenly distributed or a large number of undissolved cementite particles are retained, and slowly cooled in a higher temperature range below the A1 point to obtain granular pearlite; the other is to heat the steel to a temperature below the A1 point and keep it warm for a long time to obtain granular pearlite. At this time, the driving force for the spheroidization of lamellar pearlite is the reduction of the phase interface (or interface energy) between ferrite and cementite.

Compared with lamellar pearlite, granular pearlite has lower hardness and strength, and better plasticity and toughness. Therefore, many important mechanical parts, especially high-strength bolts and nuts, must be heat treated to transform them into tempered troostite structures with granular carbides, which have high strength and toughness and excellent comprehensive mechanical properties.