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Prevention and Treatment of Uneven Spheroidization During Spheroidizing Annealing

The spheroidizing annealing process is mainly used for cold heading and pre-heat treatment before cold extrusion of low carbon steel, medium carbon steel, low carbon alloy structural steel and medium carbon alloy structural steel. Its purpose is to reduce the hardness, increase the plastic index of the matrix, and prevent the parts from cracking during cold heading and cold extrusion. At present, it is widely used in the initial forming of threaded fasteners, small and medium-sized carburized parts, and carbonitriding parts.

For high-carbon steel and high-carbon alloy tool steel, spheroidizing annealing can be used to reduce hardness, facilitate cutting and improve surface accuracy. And can reduce the deformation and cracking of the final heat treatment.

The reason for the uneven spheroidization is: the annealing heating temperature is low, the holding time is short, and the fine flake pearlite and punctate carbides are finally formed; the heating temperature is high, and the heating time is too long, resulting in large particles of carbides and coarse flake pearlite ; Before spheroidizing annealing, normalizing is used to eliminate the network structure or bulk carbides.

The annealing heating temperature is low, which makes the austenitization uneven, the cementite in the original structure is not completely broken, and the carbon atom diffusion is insufficient. When passing the eutectoid transformation temperature, in the carbon-rich zone, the undissolved carbides grow up quickly and precipitate in a spherical shape, accompanied by the formation of ferrite around the spherical carbides; in the carbon-poor zone, flake pearlite is first formed, if The residence time below Ar1 is too short, so that the cementite cannot be fully spheroidized, and after the final cooling, a striped structure, a polygonal structure and a herringbone structure are formed.

When the heating temperature is too high, the cementite is completely dissolved, and the core of cementite is lost during spheroidization, which is not conducive to spheroidization and causes uneven spheroidization.

During spheroidizing annealing, low-carbon steel has less carbides and large diffusion distance of carbon atoms during spheroidization. It is difficult to spheroidize and easy to form a flaky structure. Therefore, attention should be paid to controlling the temperature and holding time during operation.

There are large blocks of ferrite and pearlite in hypoeutectoid steel. After spheroidizing and annealing, the carbide distribution is extremely uneven. When the original structure is lamellar pearlite, it is difficult to spheroidize even if the temperature is lower than A1 and low temperature spheroidizing annealing is used.

Under the same carbon content, when the spheroidizing temperature is constant, the longer the spheroidizing annealing time, the coarser the carbide grains and the lower the hardness. In the same spheroidizing time, the spheroidizing temperature is low and the hardness is high. The faster the cooling rate when cooling from the austenitizing temperature to the isothermal temperature, the lower the transformation temperature, the more difficult it is to diffuse ferrite and cementite, and the less conducive to the formation of a spherical structure. Therefore, it should be cooled slowly at a cooling rate of 10-20°C/h.

Hypereutectoid carbon steel or high-carbon alloy steel often presents underheated structure (undissolved flaky carbides). The structure has high hardness and is easily dissolved by quenching heating, which increases the tendency of quenching to crack and increases the amount of retained austenite. It can be improved by supplementing low-temperature spheroidizing annealing. At the same time, the austenitizing temperature, time and cooling specifications of spheroidizing annealing should be strictly controlled.

Refer to JB/T 5074-2007 standard for the rating of low-carbon steel and medium-carbon steel spheroids. The structure of cold heading and cold extrusion is generally qualified at grade 4 to 6.

Rolling bearing steel is graded according to the GB/T 7813-1998 standard after spheroidizing and annealing, and it is generally qualified in grades 2 to 4.

Carbon tool steel is graded according to GB 1298-2008 after spheroidizing annealing, alloy tool steel is graded according to GB 1299-2000 after spheroidizing annealing, and high-speed steel is graded according to GB 9943-2008 standard after spheroidizing annealing.