Treatment methods for bearing cracks

The common quality defects of bearing parts after heat treatment include overheated quenching microstructure, under heating, quenching cracks, insufficient hardness, heat treatment deformation, surface decarburization, soft spots, etc.

1. Overheating can be observed on the rough surface of the self-aligning roller bearing parts, and the microstructure after quenching is overheated.

But to accurately determine the degree of overheating, it is necessary to observe the microstructure.

If coarse needle like martensite appears in the quenched structure of GCr15 steel, it is considered as a quenched overheated structure.

The cause of formation may be comprehensive overheating caused by excessive quenching heating temperature or prolonged heating and insulation time; It may also be due to the severe banded carbides in the original structure, which form local martensite needle like coarseness in the low-carbon zone between the two bands, resulting in local overheating.

The residual austenite in the overheated structure increases and the dimensional stability decreases.

Due to overheating of the quenched structure, the coarse crystals of the steel can lead to a decrease in the toughness of the parts, a decrease in impact resistance, and a decrease in the lifespan of the bearings.

Severe overheating can even cause quenching cracks.

2. If the undercooling quenching temperature is too low or the cooling is poor, it will produce more than the standard specified troostite structure in the microstructure, called undercooling structure. It causes a decrease in hardness, a sharp decrease in wear resistance, and affects the bearing life.

3. Quenching cracks: Cracks formed during the quenching and cooling process of bearing parts due to internal stress are called quenching cracks.

The reasons for this type of crack are: due to high quenching heating temperature or rapid cooling, the thermal stress and the structural stress caused by changes in metal mass volume are greater than the fracture strength of the steel; The original defects on the working surface (such as surface microcracks or scratches) or internal defects in the steel (such as slag inclusion, severe non-metallic inclusions, white spots, shrinkage residue, etc.) form stress concentration during quenching; Severe surface decarburization and carbide segregation; Insufficient or delayed tempering of parts after quenching; The previous process caused excessive cold stamping stress, forging folding, deep turning tool marks, sharp edges and corners of oil grooves, etc.

In short, the causes of quenching cracks may be one or more of the above factors, and the presence of internal stress is the main reason for forming quenching cracks.

Quenching cracks are deep and slender, with straight fractures and no oxidation color on the fracture surface.

It often has longitudinal straight cracks or circular cracks on the bearing ring; There are S-shaped, T-shaped, or ring shaped shapes on the bearing steel ball.

The structural feature of quenching cracks is that there is no decarburization phenomenon on both sides of the crack, which is significantly different from forging cracks and material cracks.

4. During heat treatment, there are thermal and structural stresses in the deformation of bearing parts. These internal stresses can overlap or partially offset each other, and are complex and variable because they can change with changes in heating temperature, heating speed, cooling method, cooling speed, and part shape and size. Therefore, heat treatment deformation is inevitable.

Understanding and mastering its variation patterns can place the deformation of bearing parts (such as the ellipticity of the ring, size increase, etc.) within a controllable range, which is beneficial for production.

Of course, mechanical collisions during heat treatment can also cause deformation of the parts, but this deformation can be reduced and avoided through improved operations.

5. Surface decarburization

During the heat treatment process of bearing parts, if heated in an oxidizing medium, oxidation will occur on the surface, reducing the mass fraction of carbon on the surface of the parts and causing surface decarburization.

If the depth of the surface decarburization layer exceeds the final machining allowance, the part will be scrapped.

The determination of the depth of surface decarburization layer can be done using metallographic and microhardness methods in metallographic examination.

Based on the measurement method of surface layer microhardness distribution curve, it can be used as an arbitration criterion.

6. Soft spots

The phenomenon of insufficient local hardness on the surface of bearing parts caused by insufficient heating, poor cooling, and improper quenching operation is called quenching soft spots.

It can cause a serious decrease in surface wear resistance and fatigue strength, just like surface decarburization.

Introduction to the Technology of High Speed Spindle Bearings for Machine Tools (Machine Tool Bearings): Improving the speed of spindle bearings is the key to achieving high-speed practical spindles.

With the development of high-speed technology, for efficient machining machines.

The rolling element is made of ceramic material with a dmN value of 20 × Around 104, with a dmN value of 100 under grease lubrication conditions × The number of CNC machining centers around 104 is increasing and has become a development trend. The use of oil gas and oil mist lubrication methods will also become the development direction of high-speed machine tool spindle bearing lubrication in the future.

1. Development of high-speed spindle bearings: Generally, angular contact ball bearings with excellent rigidity and high-speed performance are used to meet the high-speed requirements of machine tool spindles.

Secondly, cylindrical roller bearings are used.

Under grease lubrication conditions, the dmN value is 50 × Below 104.

After the development of oil and gas lubrication, a corresponding lubrication system was gradually developed at the same time.

From the dmN value expressing the high-speed performance of the spindle bearing (dm is the diameter of the bearing pitch circle, mm × N represents the rotational speed r/min.

The dmN value has reached 100 × Above 104.

Subsequently, in terms of bearings, angular contact ball bearings with ceramic rolling elements were developed, achieving a dmN value of 200 × one hundred and four × After developing spray lubrication in the 1990s, the dmN value can reach 300 × 104.

2. High speed spindle bearing technology: (1) High speed angular contact ball bearings: Single row and double row cylindrical roller bearings are inferior to angular contact ball bearings in terms of high-speed performance.

Angular contact bearings are bearings with contact angles, which can be seen as tapered roller bearings.

The larger the contact angle, the better the axial stiffness, but due to the large gyroscopic and spin slip between the ball and the raceway, the heating will also increase.

To improve speed performance, the method is to reduce the size (or mass) of the ball and change the curvature coefficient of the channel to reduce the centrifugal force of the ball, reduce the internal load generated during high-speed operation, and increase the number of balls to improve rigidity.

(2) Ceramic ball angular contact ball bearings: A hybrid ceramic ball bearing using only silicon nitride (Si3N4) ceramics for rolling systems has been introduced to reduce ball mass and improve speed.

As a high-speed spindle bearing material, ceramic (Si3N4) has the following advantages in performance comparison: the centrifugal force generated by the rolling element during high-speed rotation is small, and the weight is light.

Due to its lower density than bearing steel.

The rotational torque can be reduced, thus reducing temperature rise and improving service life.

Good thermal conductivity makes the rolling element of ceramic materials less likely to adhere to metals during high-speed operation.

Under good lubrication conditions, it is resistant to burns and can cause burns.

Low thermal expansion makes it difficult for the rolling element to increase the preload when in contact with the inner ring, resulting in a decrease in clearance.

The deformation of bearings is small due to their high hardness and good rigidity.

The rigidity of the spindle has also been improved.

In summary, using ceramic materials (Si3N4) as rolling elements can increase the speed by about 25% to 35% compared to bearing steel rolling elements, and increase the lifespan by about 3 times.

(3) New type of hybrid angular contact ball bearing: The inner ring expands due to centrifugal force and rotates at high speed.

The contact stress with the rolling element increases, resulting in an increase in internal preload, a decrease in clearance, and an increase in heat generation.

In response to this issue, a new type of hybrid ceramic ball bearing with stainless steel inner ring has recently been developed.

Due to the fact that the linear expansion coefficient of stainless steel is 20% smaller than that of bearing steel, it can further control the increase in preload caused by the expansion of the inner ring during high-speed rotation of the bearing.

Sufficient lubrication conditions can increase the dmN value by 1.2 times under fixed preload.

(4) Hybrid angular contact ball bearings with ceramic inner rings: under the condition of positioning preload, there have been recent data introductions.

The inner ring also uses a mixed type angular contact ball bearing made of ceramic material.

Because the inner ring also uses ceramic materials, the centrifugal expansion of the bearing inner diameter or raceway is small, and the increase in preload is also small. In addition, the rigidity is good, and the contact area between the ball and raceway is small, so the heating and expansion are also small, which can achieve higher speeds than bearings with only ceramic balls.

However, it is precisely due to the small centrifugal and expansion during high-speed rotation that the mating stress between the metal spindle and it may cause damage or even fragmentation if it is too large.

3. Lubrication of high-speed spindle bearings: Due to their simple and economical use, they have been widely used. The trend of high-speed development of spindle bearings has put forward higher requirements for lubrication.

Conservative dmN value at 50 × Grease lubrication below 104 is widely used due to its simple and economical use, and does not require special maintenance or subsequent supplementation. Most are lifelong lubrication.

As the speed increases, the dmN value reaches 100 × When the temperature is above 104, oil gas and oil mist lubrication are used. Compared with grease lubrication, the temperature rise is small and it can rotate at a higher speed, making it the main lubrication method.

Although the dmN value of spray lubrication can reach 250 × However, it requires a large amount of lubricating oil, which causes significant power loss due to stirring resistance, and requires complex auxiliary equipment with high interest rates. Therefore, this lubrication method is generally not used.

Inspection, analysis, and judgment of roller shaft after bearing damage

For the entire roller bearing, the inner ring has a high probability of damage, and there are many reasons for the damage of the inner ring of the roller bearing. Therefore, when dealing with such problems, it is necessary to have targeted measures. Conduct a comprehensive inspection of the roller bearing to determine the location and cause of damage before taking effective measures.

If it is known that the self aligning roller bearing is damaged, the first step is to inspect the damaged area to distinguish whether it is the complete breakage of the outer ring of the roller bearing, the severe damage of the inner ring, or the axial cracking of the roller bearing. At the same time, it is also necessary to consider the selection of lubricating oil. If there are no errors in this regard, it may be a problem with the design of the bearing.

If the roller bearing does not meet good design standards, it will still experience some adverse phenomena during use, especially the special design proposed by some users. If not handled properly, it will bring adverse reactions to customers.

In addition, it is necessary to further determine whether the damage to the inner ring of the roller bearing is caused by excessive temperature, as in many cases, the use of roller bearings is often caused by this factor. In addition, it cannot be ignored that the hardness changes of roller bearing products after damage.

Since damaged roller bearings can be so troublesome, it is necessary to take sufficient protective measures during their use to maintain good and stable operation. At the same time, strengthen the maintenance and upkeep of roller bearings to make them more durable and reduce the occurrence of problems.