KOYO High-quality Bearing Steel Heat Treatment Process

Writer: Eric Bearing Limited

Bearing steel is a steel with strict quality requirements. The current requirements for bearing steel are: user-free processing and inspection, quality improvement, specification refinement and dimensional accuracy, etc., and the importance of these requirements is getting higher and higher. To meet these requirements, JFE Steel uses a variety of equipment to ensure product quality and finish machining to produce bearing steel. These devices, combined with newly developed quality-enhancing technologies, can produce heat-treated and hot-rolled bearing steels with a wide range of sizes, high quality, and high added value.

The characteristics of JFE bearing steel manufacturing technology are:

1. Surface quality fine processing and quality inspection system

The method of flame cleaning the steel billet and rolling the continuous casting billet into small round billets can evenly remove surface defects, subcutaneous inclusions and decarburized layers. For materials with particularly high quality requirements, the billet peeling operation is carried out to remove defects. In order to ensure the surface quality of small round billets, automatic eddy current flaw detector and magnetic particle flaw detector are used for inspection; for internal defects, round billet full-section ultrasonic flaw detector is used to detect internal pores and inclusions. KOYO 6407 C3 bearings online , pls click here :

2. Fine manufacturing technology and quality assurance of bearing steel

In the wire rod-bar factory, a wire rod rolling line is added to the bar rolling line for joint rolling. Both the bar and wire are finished by a 4-roll finishing mill. The dimensional accuracy of the bar steel is below 0.01mm, and users can omit the peeling and drawing processes. The wire can be rolled in free size, and can produce small size wire with Φ4.2mm. Since the wire has been rolled to the forged size, users can omit wire drawing, heat treatment and surface treatment.

The heat treatment process of bearing steel includes normal heat treatment, annealing and other pre-heat treatment and final heat treatment.

1. Heat treatment in advance

(1) Normalizing: chrome bearing steel normalizing process, the workpiece is kept warm for 40~60min after diathermy, the cooling needs to be faster, and it is changed to spheroidizing annealing immediately after normalizing.

(2) Spheroidizing annealing: GCr15 chromium bearing steel often adopts isothermal spheroidizing annealing process, 790℃ is considered to be the best spheroidizing heating temperature. It needs to be heated to 900~920℃ before annealing, and normalized after holding 2/3~1h.

The holding time depends on the size of the workpiece, the uniformity of the heating furnace, the method and amount of furnace installation, and the uniformity of the original structure before annealing.

Low temperature spheroidizing annealing is mainly suitable for recrystallization annealing of cold punching balls and cold extrusion ferrules.

Ordinary spheroidizing annealing and isothermal spheroidizing annealing are mainly suitable for the annealing of forged ferrules, hot punching balls and horizontal forging balls. Spheroidizing annealing process of chromium bearing steel.

2. Final heat treatment

(1) Bearing parts: Quenching and low temperature tempering are generally adopted, and its purpose is to improve the strength, hardness, wear resistance and fatigue resistance of steel. The quenching temperature of GCr15 steel is 820~860℃, and the critical diameter of oil quenching is 25mm. Generally, oil cooling quenching is adopted. The heating and holding time is longer than that of alloy tool steel, and the salt bath heating coefficient is 0.8~1.5min/mm. The heating coefficient of the air furnace is 1.5~2min/mm.

Low temperature tempering at 160℃±10℃, tempering time is generally 2~4h.

KOYO bearing parts are of stable dimensions. After quenching, they should be cold-treated at -60 to 80°C, and the holding time is 2 to 4 hours. After cold treatment, the parts are returned to room temperature and tempered within 4 hours to prevent the parts from cracking.

Residual stresses that are not completely eliminated during low temperature tempering will be redistributed after grinding. These two kinds of stress will cause the size of the part to change, and even crack. To this end, supplementary tempering should be performed again, the tempering temperature is 120 ~ 160 ℃, heat preservation 5 ~ 10h or longer.

(2) Tool and mold GCr15 heat treatment: Because this steel is prone to white spot defects, heat treatment of large tool and mold is easy to crack. Slow heating or 690 ℃ long time (more than 5h) section isothermal can reduce the probability of cracking, austenitizing temperature selection 810℃±10℃, heat preservation coefficient a=1.6~0.9min/mm. Workpieces larger than 60mm in diameter require water-oil dual-liquid quenching.

For GCr15 tool steel, multiple tempering is required to fully eliminate residual stress, reduce the amount of residual austenite, reduce cracking, and prevent surface cracks during the use of the workpiece. 

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