The deformation and dimensional expansion and contraction of bearing steel rings after quenching and tempering have always affected the quality of heat treated products. In recent years, the bearing industry has applied a new composite quenching process of bainite, martensite, and bainite in heat treatment. Due to the difficulty in controlling the deformation, cracking, and expansion and contraction of the ring after quenching, there are significant differences in the expansion and contraction of the same batch of products, and the product qualification rate is low. Therefore, it is necessary to increase the remaining amount of grinding in the next process to meet the expansion and contraction of the product. This not only wastes materials but also consumes labor hours. Therefore, some enterprises can only abandon the advanced bainite quenching process and switch back to the original martensite quenching. The causes of quenching deformation, cracking, and expansion and contraction of bearing steel rings are influenced by many factors and are a quite complex problem. Below, we will discuss the causes of these defects and the solutions they should be taken.
1. Quenching deformation, cracking, and dimensional expansion and contraction
When the stress in the material of the bearing steel ring during quenching does not reach the elastic limit, the material only undergoes elastic deformation. When the stress exceeds the elastic limit but is lower than the material's strength limit, plastic deformation occurs; When the stress reaches the strength limit, the material fractures. Therefore, the deformation and cracking of materials occur under the influence of two factors: stress magnitude and material performance indicators.
The quenching process inevitably involves quenching stress, including thermal stress and structural stress. During the operation process, it may also cause mechanical collisions of parts to generate stress, which is unevenly distributed and may be many times higher than the average stress at the stress concentration point; At the same time, due to the non-uniformity and defects of the material, the strength indicators of various parts can also differ by many times, and the weak areas of the material are often the places where stress is concentrated. Therefore, when the stress exceeds the material strength limit, local fracture, also known as cracking, occurs.
Due to quenching stress and volume changes, the size of each part of the part changes uniformly without changing its shape, which is called size expansion and contraction. Due to uneven quenching stress, the shape of the part changes, commonly known as quenching deformation. The radial non-uniform deformation size of the ring (the difference between the long and short axes) is usually called the ellipticity of the ring; The axial uneven deformation of the ring is usually called warping deformation (see Figure 1), and the axial uniform deformation is called butterfly deformation (see Figure 2). So, quenching stress is the main cause of deformation and cracking.
Figure 1 Schematic diagram of warping deformation
Figure 2 Butterfly shaped deformation diagram
The factors that affect quenching stress and volume changes before and after quenching can be summarized as follows: chemical composition of steel; The purity of steel and the uniformity of its structure; The thickness and distribution uniformity of the original structure before quenching; Quenching heating speed and heating uniformity; The state of austenite (heating temperature, holding time, and austenite grain size); Cooling speed (cooling temperature, cooling uniformity, and quenching depth); The surface condition of the workpiece (smoothness, decarbonization, turning stress, etc.); The size, shape, thickness difference, and quenching operation method of the workpiece.
2. Ring quenching deformation
The deformation of the ring is related to the stiffness S/D and h/D of the ring (see Figure 3 and Figure 4). Generally, rings (non thrust rings) with an outer diameter greater than 70mm and an S/D value between 0.025 and 0.068 are prone to large ovality during quenching. As the S/D value decreases, the ovality increases. When the S/D value is greater than 0.08, the ellipticity decreases significantly after quenching.
Figure 3
Figure 4
The degree of warping after quenching of the ring is related to the size of the h/D value. General rings and thrust rings with h/D values less than or equal to 0.185 are prone to warping deformation after quenching. Improper heating and cooling during quenching result in significant warping deformation. After quenching, butterfly deformation usually occurs in thrust rings or spacers with an h/D value of 0.2.
Due to quenching deformation, besides the rigidity of the ring itself, there are also many other influencing factors. For example, due to uneven material composition, structure, and defects, uneven annealing structure, high quenching temperature, uneven heating, excessive cooling, and uneven cooling; Due to careless operation, mechanical collisions may occur during the heating and cooling processes of the parts. In addition, there may be internal stresses, ellipses, etc. in the ring before quenching, which can cause uneven distribution of quenching stresses and directions throughout the ring, resulting in deformation of the ring. These factors are described as follows:
(1) Heating and cooling
The same part is heated in the furnace (special offer is for box type resistance furnaces), with one side and the other side near the electric heating element, the front and rear sides near the furnace, and the contact and non-contact surfaces of the parts all affecting the heating. Keep it warm for a period of time as much as possible, and the surface temperature tends to be uniform. However, the actual temperature and holding time are different in various places, and the microstructure transformation during quenching and cooling is also different. Therefore, inconsistent quenching stresses are generated, leading to deformation of the ring.
Uneven cooling can also cause uneven stress and deformation, such as manual uneven movement, slow temperature dissipation due to lack of coolant blowing in the center of the large ring, and uneven cooling speed caused by first and then oil entering, resulting in uneven deformation.
(2) Heating temperature, holding time, and original tissue
Excessive increase in quenching temperature, prolonged holding time, and the presence of plate-like or punctate pearlite in the original structure compared to normal spherical pearlite all increase the quenching thermal stress and structural stress, thereby increasing the deformation of the ring after quenching. Therefore, in order to reduce ring deformation, it is advisable to use a lower quenching temperature and appropriate holding time, while also requiring the original structure of spherical pearlite with uniform size.
(3) Residual stress
When the quenched ring is repaired, it often produces greater deformation, and even when the quenched ring is heated to the quenching temperature and held for a period of time before being taken out for air cooling, it will also produce greater deformation, indicating that residual stress plays a role in heating.
The quenched ring is in a stress unstable state, and residual stress will not cause significant deformation at room temperature. Because the elastic limit of steel at room temperature is very high, as the temperature increases, the elastic limit rapidly decreases. If the heating speed is too fast to eliminate residual stress during the heating process, a higher temperature will be retained. At higher temperatures, if the elastic limit is lower than the residual stress, it will cause plastic deformation, which is more pronounced when the heating temperature is uneven.
The residual stress during machining also plays a similar role. When the ring is rough and turned directly to the finished product size, a large cutting amount will generate significant turning stress. If divided into coarse and fine processing, the stress will be greatly reduced.
(4) Quenching cooling medium
The deformation of the ring after quenching varies with different quenching cooling media. For example, quenching in media such as water, salt water, No. 20 engine oil, rapid cooling oil, nitrate, etc., their cooling rates are not the same. The deformation of the ring with a faster cooling rate will be greater, while the deformation of the ring with a slower cooling rate will be smaller.
(5) Quenching cooling method
The deformation of the quenched ring is not only related to thermal stress and structural stress, but also to mechanical action; Especially for thin-walled rings with poor rigidity, they are prone to elliptical shapes when subjected to impact at high temperatures. For example, in the case of quenching ring installation and movement cooling, using a hook to move cooling or using a shaking basket, the faster the rotation speed of the rotary quenching machine, the greater the cooling deformation, and the heavier series of rings are also prone to butterfly deformation.
(6) Deformation during machining
The experiment shows that the positions of the long and short axes of the quenched ring size are basically the same as those before quenching (turning). This means that the larger the turning ellipticity, the greater the ellipticity after quenching. The deformation of the ring after quenching is mostly the accumulation of machining deformation and quenching deformation.
(7) Other
To reduce quenching deformation of the ring, the following measures can be taken:
① Strictly control the quality of raw materials, and strictly control defects such as segregation, looseness, banding, network, carbide precipitation, and inclusions in steel.
② Improve the size and distribution of carbides in the annealed microstructure.
③ Pre shaping and stress relief annealing before quenching. The residual deformation of machining and the residual stress of machining have a significant impact on quenching deformation. Precision products and thin-walled, complex shaped parts should be pre shaped and undergo a stress relief annealing process at 450-670 ℃.
④ Avoid excessive heating temperature. When obtaining appropriate organization and hardness, it should not be overly emphasized to increase the alloying concentration and increase the quenching temperature. For finer original tissues (such as those that have been normalized or quenched twice), the quenching temperature should be reduced as appropriate.
⑤ Quenching heating should be slow and uniform. Therefore, the parts should be evenly placed in the isothermal zone in the furnace, not too close to the heating element. To avoid warping and compression; If necessary, preheating at 400-500 ℃ before heating can be adopted to avoid rapid heating and uneven heating.
⑥ Avoid excessive cooling. To achieve this, it is necessary to choose a reasonable cooling medium and control the temperature of the medium. Strive to slow down the cooling process at a rate not lower than the critical cooling rate, especially after the temperature drops below 450 ℃. For parts that are prone to deformation, such as large-diameter thin-walled collars. Graded oil quenching or nitrate isothermal quenching can be selected.
⑦ Strive for even cooling. When quenching and cooling, it is necessary to consider uniform cooling of all parts of the part. Adopt cooling measures such as compressed air or mechanical stirring. The selection of a rotary quenching machine should adopt different rotation speeds according to the size of the ring diameter.
⑧ Avoid mechanical collisions with the collar. During transportation, furnace loading, heating, and cooling, collisions should be avoided as much as possible, especially in hot and red conditions. For example, when heating in a box type electric furnace, be more careful when removing the hook collar from the furnace. The iron hook often causes the collar to deform when hooked out.
3. Quenching cracks
There are many types of cracks that may not necessarily be quenching cracks, such as raw material cracks, forging cracks, stamping folds, and grinding cracks, which are often discovered during heat treatment, grinding, or assembly. Correctly distinguishing between other types of cracks and quenching cracks is an important condition for identifying the cause of cracking and taking effective preventive measures.
The shape of raw material cracks is generally straight, penetrating the entire workpiece along the direction of material fibers. The workpiece undergoes high-temperature heating such as rolling, forging, and annealing before quenching, resulting in severe decarburization around the cracks in the raw material.
Cracks caused by excessive cooling rate after forging due to contact with water or other reasons are mostly located at the end face or wall thickness difference of the ring. After annealing, there is a decarburization layer around this crack; Sometimes these cracks are small and undetected, but they expand during quenching, so there may be no obvious decarburization around them.
The two ends of the forged folding crack are relatively aligned, without any fine tail, and there is severe decarburization around the crack.
Grinding cracks are often accompanied by severe burns on the surface of the workpiece. The cracks are relatively fine and form a network, which is a typical grinding crack.
The morphology and size of quenching cracks are quite complex. Generally speaking, except for surface decarburization and knife shaped cracks, other quenching cracks are relatively deep, with no decarburization layer around the cracks. The end of the crack has a pointed tail, and small cracks are often distributed slightly near the pointed tail under the microscope.
The causes of quenching cracks include:
(1) The segregation of raw material components and uneven banded structure increase the local strength of thermal stress.
(2) Inclusions or other material defects in steel cause a slight decrease in local strength and stress concentration in the steel.
(3) Excessive quenching temperature or prolonged holding time can cause overheating of the structure, resulting in high quenching stress and a decrease in the strength limit of the steel.
(4) The impure cooling medium causes uneven cooling; Or if the medium temperature is too low and the oil outlet temperature is too high, immediate cold cleaning may occur, resulting in excessive quenching stress.
(5) Stress concentration is easily formed at parts chamfers, grooves, oil grooves, oil holes, and drilling roller top pin holes.
(6) The surface poverty alleviation carbon causes a significant decrease in the strength limit.
(7) The parts with significant wall thickness differences experience significant quenching stress due to slow and uneven cooling.
(8) To prevent the occurrence of quenching cracks, the following measures are taken to address their causes:
① Strengthen the acceptance inspection of raw materials and strictly control the quality of steel.
② Reasonably choose the quenching temperature and holding time according to the above principles, and strictly prevent the workpiece from overheating; For parts with excessively fine annealing structure and those that have undergone secondary quenching, this point should be paid more attention to.
③ According to the above principles, choose the correct cooling medium and cooling method, strictly prevent water from entering the quenching oil, and control the temperature of the quenching medium; If the oil outlet temperature is high and cannot be immediately rinsed with cold water after oil outlet, graded quenching can also be chosen for complex parts that are prone to cracking.
④ Before quenching and heating, check the surface of the parts for deep tool marks, scratches, sharp chamfers, oil grooves, oil grooves, and excessive wall thickness differences. If stress should be removed, and if it cannot be removed, efforts should be made to avoid quenching cracks from the quenching process or operation. If stress relief annealing is added before quenching, preheat the workpiece before quenching, appropriately reduce the quenching temperature, and appropriately increase the oil temperature or workpiece oil outlet temperature; For thin edges and edge holes, asbestos or other insulation fillers should be considered to prevent cracking.
⑤ Take measures to avoid decarbonization.
⑥ After quenching, the ring should not be stopped for a long time, especially for the ring that has undergone secondary quenching. After quenching, it should be immediately tempered and fully tempered.
4. Ring size expansion and contraction
The expansion and contraction of the ring size during the quenching process come from:
(1) The phases with different specific capacities before and after quenching undergo transformation, and the volume of the parts changes, causing size expansion or reduction.
(2) Quenching thermal stress and structural stress cause elastic extension or compression and plastic deformation in all directions of the workpiece.
The size of each part of the part varies greatly with the increase of the volume of the steel, for example, some rings expand towards the outer diameter, while others expand towards the height direction. The specific situation of ring size expansion and contraction can only be inferred from actual production. In most cases during production, if the direction of expansion and contraction of the outer diameter inside the ring is consistent, then both the inner and outer diameters expand outward or contract towards the center.
The outer diameter of GCr15 steel ring quenched by normal process has a larger expansion than that of GCr15SiMn steel. In many cases, both the inner and outer diameters of the ring shrink at the same time, while GCr15SiMn steel shrinks more severely than GCr15 steel.
The influence of cooling methods, for example, when the rotary quenching machine cools, the outer diameter of the ring expands more than it moves, and the shaking basket cools less. And there is a greater tendency for contraction. The higher the rotational speed. The outer diameter expands less and shrinks more.
The inner and outer diameters of the ring always shrink during secondary quenching. The more quenching times, the more shrinkage. Changes in the size of the tapered ring; The thick end swells more than the thin end, and sometimes there is also a situation where the thick end swells and the thin end shrinks. If both ends contract, the thin end shrinks more than the thick end.
During the tempering process, there are also factors that affect size expansion and contraction. One of them is that due to the decomposition of residual austenite, the volume of quenched steel will increase. On the contrary, the decrease in supersaturation of martensite during tempering leads to a decrease in its specific volume, resulting in a decrease in the volume of quenched steel. When residual austenite decomposes rapidly at 220-250 ℃, the volume of quenched steel increases, while the rest increases with the tempering temperature. The second factor plays a major role in reducing the volume of steel.
Sometimes in production, the method of adjusting the tempering process can be used to save waste caused by size expansion and contraction based on the law of tempering size expansion and contraction.
5. Conclusion
Quenching deformation cracking and dimensional expansion and contraction have different main contradictions under different conditions. Seizing the main contradiction, accumulating data in production practice, and adjusting production process parameters step by step to meet the quenching and tempering process requirements of bearing steel rings.
Enterprises can choose domestically produced environmentally friendly, energy-saving, and environmentally friendly advanced heat treatment quenching and tempering equipment with a protective atmosphere based on the variety and quantity of products produced. The use of pressure die quenching for the ring can reduce deformation, reduce machining allowance, and improve efficiency and product quality.
2024 March 4th Week Marginal Product Recommendation:
MG-TEX Steel with PTFE Fiber Fabric Bearings:
This new material uses the PTFE fibre fabric overlay on steel backing, the fabric is with high load capacity and much longer operating life comparing with conventional 3-layer bushes.The metal matrix provides the bearing with excellent load-bearing performance and can transfer the heat generated during the operation of the bearing in time, while PTFE woven material is designed to be used in completely dry friction conditions. It has a lower friction coefficient and excellent wear resistance. Compared with traditional bearings, in addition to high load performance, it can completely eliminate oil and eliminate process maintenance, and it slides smoothly. There will be no "sticky" phenomenon.
