What is the process of final heat treatment such as quenching and tempering of bearing parts using new high-frequency induction heating equipment?

Heat treatment of the workpiece is necessary. However, when traditional equipment performs heat treatments such as quenching and tempering on workpieces, there will always be defects such as inconvenient operation, time-consuming power consumption, and air pollution. Currently, a new type of induction heating equipment is popular on the market. The equipment solves the shortcomings of traditional equipment and its functions are more complete than traditional equipment. Today, we will take the heat treatment of parts by new high-frequency induction heating equipment as an example to briefly introduce the final heat treatment process of bearing parts.

1. Quenching can make the parts obtain high hardness and wear resistance, high contact fatigue life and reliability, and high ruler stability. The process parameters of the quenching process include quenching heating temperature. Heating method, holding time, cooling medium and cooling method, etc.

(1) Heating time, quenching heating time includes heating, soaking and heat preservation. Total heating time = heating time + (heating + equalizing heat)) × 0.3-0.5. The heating time is related to the quenching heating temperature.

(2) Quenching heating temperature. The optimal heating temperature for quenching should make the austenite contain appropriate carbon content and dissolve a large amount of Mn and Mo alloy elements without causing grain growth and overheating structure.

(3)Quenching cooling medium and cooling method

1) Quenching cooling medium. The quenching cooling medium should ensure that the bearing parts have a sufficient cooling rate in the most unstable zone of austenite during the cooling process, without non-martensitic transformation occurring, and cooling slowly within the martensite transformation range. , in order to achieve the effect of reducing tissue transformation stress, thereby reducing the deformation and cracking of the ferrule.

Bearing steel has sufficient hardenability, and quenching oils with different cooling characteristics are usually used according to the size of the parts. Commonly used quenching oils include: ordinary quenching oil, rapid quenching oil, super-speed quenching oil, bright quenching oil, vacuum quenching oil and graded quenching oil.

2) The cooling method varies according to the different quality requirements, shape, wall thickness and size of the bearing parts. Usually martensite quenching cooling methods include: free drop cooling of small parts, up and down vibration cooling, die quenching, graded quenching, isothermal quenching and rotating machine cooling (speed <1.5m/s), etc. Its purpose is to reduce the deformation of the ferrule and obtain uniform structure and performance.

2. Cold treatment Generally, bearing steel contains about 15% retained austenite (volume fraction) after quenching. Since these austenites exist in the quenched structure, although conventional tempering treatment, they still cannot be fully transformed and stabilized. , so when parts are stored for a long time at room temperature, their dimensions will change due to austenite transformation. The purpose of the cold place is mainly to reduce the residual austenite content in the quenched structure and stabilize the remaining small amount of austenite, thereby increasing dimensional stability and improving hardness.

(1) The holding time of cold treatment, only from the perspective of martensite transformation, austenite transformation is completed when cooling to a certain temperature range. Due to different loading volumes. Therefore, the cold treatment and heat preservation time in production is usually 1 to 1.5 hours. However, it should be pointed out that cryogenic treatment cannot completely transform the retained austenite.

(2) Cold treatment temperature. The cold treatment temperature is mainly determined by the martensitic transformation termination point of the steel, the residual austenite content in the quenching structure, the impact of cold treatment on the mechanical properties, the technical requirements of the parts and the complex shape. When GCr15 steel is continuously cooled to low temperature after quenching temperature, the termination point of martensitic transformation is around -70 degrees. Cold treatment lower than Mf has no significant effect on reducing retained austenite.

(3) For the operation method of cold treatment, the residence time between quenching and cold treatment should not be too long, generally no more than 2 hours. During production, after the parts are quenched to room temperature, they are immediately cold treated in a cryogenic box or dry ice alcohol. The shorter the residence time between quenching and cold treatment, the better the cold treatment effect. If the residence time is too long, the aging and stabilization of retained austenite will easily occur and the cold treatment effect will be reduced. Parts with complex shapes that are quenched to room temperature and then cold treated immediately will cause cracking. Therefore, after quenching and cooling to room temperature, such parts can be pre-tempered at 110~130 degrees for 30~40 minutes, and then cold treated. , but tempering will stabilize the aging of retained austenite. After cold treatment, the parts should be placed in the air to return to room temperature and tempered immediately. Otherwise, the parts will crack. Generally, the residence time from cold treatment to tempering should not exceed 4 hours.

3. Tempering. There are two metastable structures in the quenched structure of GCr15 and GCr15SiMn steel - martensite and retained austenite, which have a tendency to transform spontaneously or induced into a stable structure. At the same time, parts are in a state of high stress after quenching, which can easily cause dimensional changes, loss of accuracy, and even cracking during long-term storage or use. Tempering can eliminate residual stress, prevent cracking, and transform the metastable structure into a relatively stable structure, thereby stabilizing dimensions, improving toughness, and obtaining good comprehensive mechanical properties.

(1) Tempering process. The tempering process of chromium bearing steel should be determined according to the bearing service conditions and technical quality requirements. Generally speaking, it can be divided into three processes: conventional tempering - tempering of general bearing parts; stabilized tempering - tempering of precision bearing parts; high temperature tempering - tempering of some aviation bearings and other special bearing parts. No matter which process is used, in order to ensure the stability of the size, hardness and performance of the bearing under the conditions of use, the tempering temperature should be 30 to 50 degrees higher than the operating temperature of the bearing. Generally, the operating temperature of bearings is below 120 degrees, so the conventional tempering temperature Use 150-180 degrees. For bearings with light loads and high dimensional stability requirements, their parts can be tempered at 200--250 degrees. For bearings operating at high temperatures, the tempering temperature of the parts can be 200, 250, 300 or 400 degrees depending on the operating temperature.

(2) Technical requirements and quality inspection after tempering

The microstructure, fracture surface, decarburization, carbon depletion and distortion of bearing parts after tempering are all in accordance with the technical requirements after quenching. The tempering stability of bearing parts should be less than lHRC. The crushing load of steel balls shall not be lower than specified.

Tempering quality inspection and post-tempering quality inspection. In addition to the inspection items after quenching and tempering, the steel ball crushing load and tempering stability must be inspected. The tempering stability check is mainly to check whether the tempering is sufficient. The method is to re-temper the tempered parts at the original tempering temperature for 3 hours, and retest near the original tempering hardness measuring point. Those whose hardness does not drop more than 1HRC are qualified. . The steel ball crushing load test can be carried out in accordance with the regulations of GB308.

4. Stabilization treatment (additional tempering) Stabilization treatment is mainly to eliminate part of the grinding stress, further stabilize the structure, and improve the dimensional stability of bearing parts. The stabilization temperature is 20 to 30"C lower than the original tempering temperature, generally 120 to 160 degrees, and the holding time is 3 to 4 hours. The holding time for stabilization of various parts should generally be based on the bearing accuracy grade, size and shape to choose.

5. Quality control after quenching and tempering of bearing parts. Quality control after quenching and tempering of bearing parts shall be carried out in accordance with the heat treatment technical conditions of JB1255 high carbon chromium bearing steel rolling bearing parts.

After reading the above description, we understand that the final heat treatment of bearing parts probably includes quenching, cooling, tempering and other processes. We have made some brief descriptions of these processes. We have also learned that the new high-frequency induction heating equipment has many advantages such as easy operation and small size. If your factory does not have such equipment yet, then what are you waiting for? Take action quickly.