Muffle furnace bearing tempering

The muffle furnace plays a key role in bearing tempering, achieving the core goals of stress relief, stable structure, and performance adjustment through precise temperature control. Its application can be systematically summarized into five dimensions: process objectives, temperature control, cooling methods, application scenarios, and process optimization：

1. Process purpose
Eliminate quenching stress
After quenching, residual stress will form inside the bearing, leading to an increased risk of cracking. The muffle furnace decomposes martensite through tempering (150-650 ℃), relaxes internal stress through atomic diffusion, and significantly improves toughness. For example, after low-temperature tempering at 150-180 ℃, the impact toughness of GCr15 bearing steel is improved while maintaining a hardness of HRC60 or higher.
Stable organization and size
Tempering promotes the transformation of residual austenite into a stable structure (such as tempered martensite or martensite), reducing the dimensional changes of bearings during storage or use. For precision bearings, the dimensional stability can be improved after tempering, meeting the requirements of high-precision transmission.
Adjust mechanical properties
By controlling the tempering temperature, a balance can be achieved between hardness, strength, and toughness. For example:
Low temperature tempering (150-250 ℃): retains high hardness (HRC58-64), used for wear-resistant parts such as cutting tools and rolling bearings.
Medium temperature tempering (350-500 ℃): Obtain elasticity (HRC35-45), suitable for springs and springs.
High temperature tempering (500-650 ℃): achieving a “strong toughness combination” (HRC25-35), used for shafts and gears (commonly known as “quenching and tempering treatment”).

2. Temperature control
Low temperature tempering (150-250 ℃)
Applications: cutting tools, measuring tools, rolling bearings, carburized parts.
Effect: Small change in hardness, reduced internal stress, and slightly improved toughness. For example, after low-temperature tempering at 250 ℃, the fatigue life of self-aligning roller bearings is improved.
Medium temperature tempering (350-500 ℃)
Applications: springs, hot forging dies.
Effect: Obtain high elasticity and necessary toughness. For example, after tempering at 450 ℃, the elastic limit of spring steel changes while maintaining its tensile strength.
High temperature tempering (500-650 ℃)
Application: shafts, gears, bolts and other complex components under stress.
Effect: The best comprehensive mechanical performance. For example, after tempering at 500 ℃, the tensile strength of 40Cr steel decreases, but the impact toughness increases.

3. Cooling method
air cooling
Process: After tempering, the workpiece is taken out of the furnace and naturally cooled in air.
Advantage: The cooling speed has a minimal impact on performance and is suitable for most bearing components.
Case: GCr15 bearing ring is air cooled after tempering at 180 ℃, with a hardness of HRC60-65 and dimensional stability meeting the P2 level accuracy requirements.
Oil cooled/water-cooled
Application: A few high alloy steels (such as high-speed steel) need to prevent secondary hardening.
Effect: Rapid cooling can suppress carbide precipitation and maintain high hardness. For example, W18Cr4V high-speed steel, after tempering at 560 ℃ and oil cooling, has a hardness of HRC67 and a red hardness (at 600 ℃) of HRC52.

4. Application scenarios
Conventional bearing
Process: Tempering at 180-200 ℃ and holding for 3-5 hours.
Effect: The hardness reaches HRC60-65, meeting the general transmission requirements.
High temperature bearings
Process: Select 200 ℃, 250 ℃, 300 ℃, 350 ℃ or 400 ℃ tempering according to the operating temperature.
Effect: It can maintain high hardness and dimensional stability even at high temperatures. For example, Cr4Mo4V high-temperature bearing can be used for a long time in an environment of 500 ℃ after secondary tempering at 520 ℃.
Precision bearings
Process: After grinding, stabilize the material at 120-160 ℃ and hold it for 3-4 hours.
Effect: Eliminate grinding stress, further stabilize the structure, and improve dimensional accuracy. For example, after stabilization treatment, the size change of P4 grade bearing rings is smaller.

5. Process optimization direction
Gradient tempering
Application: Adopt segmented tempering for large bearings (such as rollers with a diameter>50mm), first relieve stress at low temperature, and then adjust the structure at high temperature.
Effect: Avoid cracking and improve overall performance. For example, after pre tempering at 150 ℃ and high-temperature tempering at 500 ℃, the fatigue life of large self-aligning roller bearings is improved.
Cold treatment assistance
Process: Immediately after quenching, perform -70 ℃ cold treatment for 1-1.5 hours, and then temper.
Effect: Promote the transformation of residual austenite and reduce size changes. For example, precision ball bearings have improved hardness and dimensional stability after cold treatment.
Intelligent control
Technology: Adopting Internet of Things technology, remote monitoring of temperature curves, gas flow rates, and other parameters through a mobile app.
Advantage: Realize historical data playback and abnormal alarm, ensuring process repeatability. For example, a certain model of muffle furnace supports the function of “± 1 ℃ temperature control accuracy+automatic alarm”, which improves the yield rate.