Causes and Preventive Measures of Quenching Soft Spots of Slewing Bearings

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In the production process of the slewing bearing, the hardness of the steel ring is locally lower after the quenching process, which usually occurs after the quenching. This situation is called the quenching soft point, and when the pieces appear, it is called the quenching soft belt. The location where the soft spot occurs is that the bearing has low hardness, which will affect the quality of subsequent grinding. Fatigue damage is also prone to occur during use, which affects the service life of the bearing. So what is the reason for the quenched soft spot of the slewing ring?

Reasons for the soft spot of slewing bearing quenching

1. Decarburization: When the metal is heated in the furnace or the atmosphere in the furnace is not good, it is easy to cause decarburization. The process of decarburization is that the carbon element in the bearing steel reacts with clear or oxygen at high temperature to generate methane or carbon monoxide. This situation will reduce the surface carbon content and the hardness to meet the requirements.

2. Insufficient temperature: Insufficient heating temperature and insufficient heat preservation make the bearing material matrix not fully austenitized. After quenching, it cannot completely transform into martensite, resulting in local soft spots. In addition, the local cooling rate is too low, the bearing surface has bubbles or contaminants, or the critical cooling rate is not reached in the close contact of the workpiece, and it cannot all become martensitic.

3. Uneven steel structure: If the original structure of the workpiece material used in the production of the bearing is not uniform, such as carbide segregation, carbide aggregation, etc., it will affect the quenching effect of the bearing.

4. Inductor problem: The structure of the inductor and the positioning fixture during quenching are not good, which affects the heating effect, or the center line of the spray cooling hole on the inductor and the quenched surface are perpendicular or angled improperly, resulting in uneven heating, heating, and cooling. The situation arises. In addition, the unequal spacing between the bearing and the inductor can also cause uneven heating.

5. Poor hardenability: The material of the slewing bearing is made of steel with poor hardenability, such as carbon steel. However, if the cross section of the workpiece is large or the thickness difference is large, soft spots may appear at the large cross section.

6. Poor quenching medium: the cooling rate of the quenching medium is low or too old will cause soft spots in the bearing.

After understanding the reasons for the soft spots or soft bands of the slewing bearing, the method to improve the existing soft spots can be through annealing, normalizing, and high temperature tempering, followed by re-quenching according to the normal process, or re-quenching after recarburizing. Or, after quenching, the hardness can be improved by cold treatment, and then tempering treatment will try to solve it. So how do we avoid the recurrence of quenched soft spots in the subsequent bearing production and processing process?

Slewing Bearings

Measures to prevent the occurrence of soft belts on slewing bearings

1. Reasonable material selection: pre-homogenize the defective steel before quenching to remove the problems of carbide segregation and aggregation.

2. Control the heating temperature and time: For insufficient austenitization due to insufficient heating temperature and time, we need to select the appropriate heating temperature, heating time, and holding time for the steam during processing.

3. Reasonable selection of quenching medium: strengthen the relative movement between the workpiece and the medium, or stir the medium to keep the quenching medium clean. Quenching of carbon steel in salt water can effectively prevent the generation of soft spots.

4. Strictly follow the process: perform the quenching operation correctly, correctly control the pre-cooling time, the residence time in water or brine during quenching, the residence time in water or brine during dual-medium quenching, and the residence time of graded quenching.

The above is the reason for the soft spot of slewing bearing quenched by Lunda editor and the measures to prevent the soft band. I hope it can be helpful to everyone. The soft spots of the bearing have an impact on the later grinding process and performance of the bearing. I hope everyone will pay attention to it.

Why should the slewing bearing be pickled? Pickling process

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In our production process, the process of cleaning and degreasing the metal parts and soaking them in an acid solution is usually called pickling. Bearings also need to be pickled, so why should slewing bearings be pickled?

There are two main purposes, one is to remove oxide scale or burrs on the metal surface by pickling, and the other is to pickling to check metal surface defects to see if there is burn corrosion. The editor of Lunda will introduce to you the specific operation process of slewing bearing pickling.

The purpose of pickling of slewing bearings

slewing bearings

1. Surface purification: Use pickling or chemical cleaning to clean the bearing surface, such as pickling before electroplating, pickling before phosphating, pickling to remove oxide skin burrs, etc. Use pickling liquid to choose cold pickling or hot pickling according to the condition of the metal surface. It is widely used to remove oxide scale on the surface of the cage.

2. Pickling inspection: Use pickling to check the bearing surface defects, which is called pickling inspection or burn corrosion. It is a kind of detection method, commonly used in cold pickling with nitric acid. If the metal surface has cracks, decarburization, peeling, burns and other defects, it can be manifested by cold pickling. In foreign countries, this pickling inspection technology is often classified into the NDT (no inspection) method. Most domestic factories use this inspection method to detect the presence of surface defects.

According to different application purposes, processing requirements and different raw materials, select the correct pickling technology and use the appropriate pickling process (including the selection of acid solution, corrosion inhibitor, process steps, pickling time, etc.), and a special The pickling staff conduct operations and inspections to ensure the pickling effect and the quality of the bearing products. Let’s take a look at the operation process of pickling with Ronda editor.

The process flow of slewing bearing pickling

slewing bearings

1. Preparation before pickling: the bearing parts need to be cleaned before pickling, and the surface should not be dirty with oil, magnetic powder, dust, etc.; if the surface is severely rusted, it needs to be derusted and then pickled.

2. The pickling process:

(1) Put the bearing into a metal cleaner solution for soaking. The concentration of the solution is required to be 4%~5%, and the temperature should be controlled at 75~90℃. If it is pickling, use normal temperature water to dilute the solution. The soaking time is not less than 1 minute.

(2) During the process, the bearing should be shaken or moved to make it fully contact with the solution, and the solution should be kept clean, and the pickling time should be controlled within 5 minutes.

(3) Rinse the soaked bearing in flowing cold water and cool it to room temperature.

3. Inspection after pickling: Check the appearance of the bearing. Its surface should be consistent silver gray or light gray. Generally, the burned position will appear black or bright white, the soft spot position will be dark gray or black, and the decarburized position will be off-white. During the inspection process, the bearing surface is not allowed to be wiped. If in doubt, use absorbent cotton dipped in ethanol or anti-rust solution to wipe.

Slewing Bearings

After the inspection is completed, the bearing should be sent to the anti-rust process in time for treatment to avoid re-oxidation and corrosion of the surface.

3. Precautions for pickling of slewing bearings

1. During the pickling process, the operation should be carried out strictly in accordance with the process sequence, and the process sequence should not be randomly selected.

2. Keep Lingshui flowing and clean during the pickling process and avoid turbidity.

3. The whole process should be carried out under the conditions of good ventilation and safety protection.

4. When replacing the pickling solution, the cleaning tank needs to be cleaned, rinsed with clean water 2-4 times and then the solution is prepared.

5. When preparing the diluted solution, pour the concentrated acid into water or alcohol and keep stirring. If there is solid, stir the solid first and then add the concentrated acid. It is forbidden to pour water into the concentrated acid, which is prone to splashing or danger. accident.

The above is a specific introduction on why the slewing bearing should be pickled and the pickling process. I hope to help everyone better understand the purpose and meaning of pickling. At the same time pickling can help the bearing achieve better performance

Why should the slewing bearing be annealed?

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Why should slewing bearings be annealed? In fact, annealing of slewing ring bearings is a process of bearing heat treatment. Generally, the bearing metal is heated to a certain temperature and kept for a period of time, and then the metal is cooled at a suitable speed. This method improves the toughness of the bearing steel. , Reducing bearing hardness and residual stress can reduce the probability of deformation and cracks, and enhance the stability of bearing materials.

Why should the slewing bearing be annealed?

There may be three reasons for the purpose of annealing the slewing bearing:

1. The annealing process can reduce the hardness of the bearing steel, improve the plasticity, and facilitate the later cutting and deformation processing.

2. Annealing can refine the grains of bearing materials, eliminate structural defects caused during bearing casting, forging, and welding, and uniform the structural structure and steel composition of the steel, which can improve the performance of the steel and prepare for the later heat treatment.

3. The annealing process can eliminate the internal stress in the steel and prevent or reduce the possibility of deformation and cracking of the bearing. In one case, the annealing process is not only for metal materials but also for annealing of non-metal materials. After understanding why slewing bearings should be annealed, the following editor will introduce the types of annealing processes for slewing bearings. In fact, there are many types of annealing processes, and different steel structures are suitable for different processes.

Annealing process type of slewing bearing

Slewing Bearings

1. Recrystallization annealing

Recrystallization annealing is also called full annealing, which is an annealing process in which iron-carbon alloys are austenitized and then slowly cooled or close to equilibrium. The heating temperature of carbon steel is generally Ac3+ (30～50℃); alloy steel is Ac3+ (500～70℃); the holding time depends on the type of steel, the size of the workpiece, the amount of furnace installed, the selected equipment model, etc. The factors are determined. In order to ensure that the undercooled austenite undergoes the pearlite transformation, the cooling of the recrystallization annealing must be slow, and the furnace is cooled to about 500°C and then air-cooled.

This annealing process is mainly used for hypoeutectoid steels, generally medium-carbon steel and low- and medium-carbon alloy structural steel forgings, castings and hot-rolled sections, and sometimes also used for their welding components; it is not suitable for hypereutectoid steels, Because the recrystallization annealing of hypereutectoid steel needs to be heated to above Acm, during slow cooling, cementite will precipitate along the austenite grain boundary and present a network distribution, which will increase the brittleness of the material and leave hidden dangers for heat treatment.

2. Uniform annealing

Uniform annealing is also called diffusion annealing, in order to reduce the segregation of the chemical composition of metal castings or forging billets and the unevenness of the structure. It is heated to a high temperature, maintained for a long time, and then slowly cooled to homogenize the chemical composition and structure. Purpose of annealing process. The heating temperature of homogenization annealing is generally Ac3+ (150～200℃), that is, 1050～1150℃, and the holding time is generally 10～15h to ensure the full progress of diffusion and the purpose of eliminating or reducing the uneven composition or organization. Because the heating temperature of diffusion annealing is high, the time is long, and the crystal grains are coarse, for this reason, recrystallization annealing or normalizing is performed after diffusion annealing to re-fine the structure.

Slewing Bearings

3. Incomplete annealing

Partial annealing is an annealing process in which the iron-carbon alloy is heated to a temperature between Ac1 and Ac3 to achieve partial austenitization, followed by slow cooling. Partial annealing is mainly suitable for medium and high carbon steel and low alloy steel forgings, etc. Its purpose is to refine the structure and reduce the hardness. The heating temperature is Ac1+ (40-60) ℃, and the temperature is slowly cooled after heat preservation.

4. Stress relief annealing

One of the key reasons why slewing bearings should be annealed is the annealing process in order to eliminate the residual stress caused by plastic deformation processing, welding, etc. and the residual stress in the casting. There are internal stresses in the workpiece after forging, casting, welding and cutting. If it is not eliminated in time, the workpiece will be deformed during processing and use, which will affect the accuracy of the workpiece. It is very important to use stress relief annealing to eliminate internal stress generated during processing.

5. Isothermal annealing

Isothermal annealing is to heat the steel or blank to a certain temperature and keep it for a period of time, and then quickly cool it to a certain temperature in the pearlite temperature range and keep it isothermally, so that the austenite is transformed into a pearlite structure, and then in the air Cooling annealing process. This process is suitable for medium carbon alloy steel and low alloy steel, and its purpose is to refine the structure and reduce the hardness. The heating temperature of hypoeutectoid steel is Ac3+(30～50)℃, and the heating temperature of hypereutectoid steel is Ac3+(20～40)℃. Keep it for a certain period of time, and then perform isothermal transformation with furnace cooling to slightly lower than Ar3 temperature, and then air cooling out of the furnace . The isothermal annealing structure and hardness are more uniform than recrystallization annealing.

6. Spheroidizing annealing

Spheroidizing annealing is an annealing process to spheroidize carbides in steel. Heat it to 20-30°C above Ac1, keep it for a period of time, and then slowly cool it to obtain a structure of spherical or granular carbides uniformly distributed on the ferrite matrix.

Spheroidizing annealing is mainly used for hypereutectoid steel and alloy tool steel (such as cutting tools, measuring tools, molds, bearings and other steel grades). The main purpose is to reduce the hardness, improve the machinability, and prepare for later quenching. There are many spheroidizing annealing process methods, and the two commonly used processes are ordinary spheroidizing annealing and isothermal spheroidizing annealing. Compared with ordinary annealing methods, spheroidizing annealing can not only shorten the cycle, but also make the spheroidized structure uniform, and can strictly control the hardness after annealing.

7. Recrystallization annealing

Recrystallization annealing is also called intermediate annealing. It is a heat treatment process in which the cold-deformed metal is heated to a temperature above the recrystallization temperature and kept for an appropriate time to recrystallize the deformed grains into uniform equiaxed grains to eliminate deformation strengthening and residual stress. .

The above is an introduction to the reasons why slewing bearings should be annealed, which also includes the types of annealing processes and their respective characteristics and process introductions. I hope to help everyone have a clearer understanding of the annealing process.