During heat treatment of tapered roller bearing rings, defects such as overheating, underheating, cracking, deformation exceeding tolerance, and collision damage may occur due to the structure of the bearing ring itself, heat treatment process, processing equipment, and human factors. These defects will directly lead to the scrapping of bearing rings. What measures should be taken to prevent damage to the bearing ring during heat treatment
1. Prevent the formation of overheated tissue
The microstructure of high carbon chromium steel bearing rings after quenching should be hidden crystals, fine crystals, or small needle like martensite. Due to the limitations of the cone bearing ring structure, when the microstructure of the thick walled end meets the requirements, coarse needle like martensite may appear on the thin walled end, which is a significant overheated microstructure.
This microstructure exceeds the standard specifications for heat treatment technology of bearing steel, which can lead to a decrease in the toughness, impact resistance, and lifespan of the bearing. Severe overheating can even cause quenching cracks. The reason may be due to excessively high quenching heating temperature or long heating and holding time, or it may be due to severe carbide banding in the raw material or uneven distribution of carbide size in the annealed structure.
The measures taken shall be selected according to the material standards (for example, when the effective thickness of the wall exceeds 15mm, GCr15SiMn steel can be used), and the heating temperature and heating time shall be reasonably selected. Strictly control the formation of carbide bands. Improve annealing quality and take timely and effective measures in case of power outages, equipment failures, etc.
2. Prevent the formation of undercooled tissue
After quenching, the microstructure of high carbon chromium bearing steel bearing rings shows more obvious needle shaped and larger block shaped martensite. The martensite or needle shaped martensite mixed with block shaped martensite forms a banded martensite, which exceeds the specified limit and is called undercooled microstructure.
The blocky martensite is generated due to insufficient heating; Needle like martensite is caused by poor cooling. And the banded martensite is caused by the banded carbides in the raw materials of bearing steel, which distribute in a banded manner in the carbon poor zone, causing a decrease in hardness, a sharp decrease in wear resistance, and affecting the bearing life. The reason for this is that the quenching temperature is too low, the holding time is insufficient, or the cooling is poor.
If martensite appears in production, its metallographic microstructure should be inspected, the cause analyzed, and corresponding measures taken. If the martensite is block shaped, the quenching heating temperature should be appropriately increased to prolong the holding time; If it is needle shaped martensite, the cooling rate should be increased. If the heating temperature, insulation, and cooling are all normal and the appearance of martensite occurs, it is necessary to check the raw material problems, temperature control problems, equipment failures, etc., and promptly identify the cause and take measures.
3. Prevent quenching cracks from occurring
The cracks generated during the quenching process of parts are mostly caused by the tensile stress generated near the surface of the part due to quenching stress exceeding the fracture strength of the steel at that temperature when cooled within the martensitic transformation temperature range. The cracked ring after pickling and quenching is shown in Figure 7; The main difference between quenching cracks, material cracks, and forging cracks is that there is no decarburization phenomenon on both sides of the quenching crack;
Generally speaking, rapid cooling below the Ms point during quenching is the main cause of quenching cracks. However, excessive initial stress before quenching, defects in the raw material and the resulting stress concentration, and decarburization of the part surface during heating may all contribute to the formation of cracks.
The quenching cracks of common bearing parts are as follows:
(1) Cracks formed by quenching overheating: Excessive heating temperature and holding time during quenching can cause coarsening of austenite grains, resulting in increased martensite brittleness, decreased strength, and cracking after quenching. Its crack characteristics are: fine cracks along the circumference of the ring, often occurring at the junction of thickness and thinness.
(2) The cracked parts generated by excessive cooling rate fall into the oil groove with water at the bottom for cooling in the medium with excessive cooling rate or during quenching. Due to the excessive cooling rate, the structural stress is significantly increased and cracks are formed. This type of crack often occurs at the junction of thickness and thinness.
(3) Due to the cracking caused by the original stress before quenching, if the cold working stress is not fully eliminated or the previous quenching stress is not removed before the part is repaired, these unresolved stresses will be combined with quenching to produce cracks.
(4) Cracks are formed during the process of stress concentration, such as deep typing of the ring, deep turning marks, deep (sharp) oil grooves, and fatigue marks from steel ball filing.
(5) Material defects such as looseness, white spots, pores, inclusions, and uneven distribution of carbides in steel can cause quenching stress concentration and result in quenching cracks.
(6) Cracks generated by surface decarburization not only reduce the surface strength of the part, but also cause different Ms point temperatures in the surface and core. The different transformation times of martensite during cooling lead to significant internal stresses, resulting in intermittent, fine, and shallow mesh quenching cracks.
(7) Cracks caused by failure to temper in a timely manner after quenching. Under long-term quenching stress, the fracture strength of quenched martensite decreases with the prolongation of time. Therefore, if the quenched parts are not tempered in time, it will cause cracks.
(8) After quenching, the oil outlet temperature of the cracked ring caused by impact is relatively high. If it is immediately cleaned or subjected to impact before tempering, large and uniform penetrating cracks will be generated along the longitudinal direction due to excessive quenching stress and mechanical collision force.
Preventive measures
To prevent the occurrence of quenching cracks, the following measures are taken to address the causes:
(1) Strengthen the acceptance inspection of raw materials and strictly control the quality of steel.
(2) Choose a reasonable quenching temperature and holding time to prevent overheating of the workpiece, especially for parts with fine annealing structure and secondary quenching, which should be paid more attention to.
(3) Choose appropriate cooling medium and cooling method, strictly prevent water mixing in the quenching oil (the moisture content in the quenching oil is less than 0.1%), and control the temperature of the quenching cooling medium (the temperature of the quenching oil is around 90 ℃); For complex parts with wall thickness that are prone to cracking, graded quenching is used.
(4) After quenching or cold treatment, it should not stay, especially for parts that have undergone secondary quenching, they should be immediately tempered and fully tempered.
4. Control carbon potential to prevent 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 decarburization. If the depth of the decarburization layer on the surface exceeds the allowance for post-processing, the part will be scrapped. The determination of the depth of surface decarburization layer can be carried out using metallographic examination and microhardness method. The surface layer microhardness distribution curve measurement method can be used as an arbitration criterion.
After quenching, tempering, and polishing of the bearing ring, obvious pitting was found on the surface. The decarburization layer of the bearing part is a typical morphology under the metallographic microstructure: the outer layer is white and bright ferrite, and the lower layer is a transition from a lean carbon layer to a normal tissue area. The surface of the bearing ring with severe decarburization after polishing shows obvious pockmarks as shown in Figure 9. After observing the longitudinal section through wire cutting, it was found that the depth of the decarburization layer far exceeded the standard requirements. The reason is that the carbon potential in the quenching furnace was low during the quenching heating process of the ring. After investigation, it was found that one of the holes for methanol dripping into the furnace top was blocked, resulting in a small amount of methanol dripping into the furnace. The measures taken to prevent carbon deposition in the furnace top inlet pipe, which affects the carbon potential of the protective atmosphere, require operators to clear it 1-2 times per shift
5. Take measures to prevent collision injuries from occurring
After quenching and tempering, obvious scratches were found on the ring, resulting in the ring being scrapped. The reason is that during the heat treatment process, the workpiece falls into the oil groove, interface (such as between cold and hot cleaning agents, between cold and cold cleaning machines and tempering furnaces), and the discharge port of the tempering furnace, as well as collisions between the rings during the polishing process, resulting in scratches.
The measures taken are to install heat-resistant rubber at various interfaces of the heat treatment production line (such as between hot and cold cleaning agents, between the cold cleaning machine and the tempering furnace) and the discharge port of the tempering furnace to prevent collision damage. For heavier rings, a hanging polishing machine is used to polish the rings. During the polishing process, the rings are gently taken in and out by hand to prevent collision damage.
6. Control quenching deformation to prevent dimensional abnormalities
During the quenching, heating, cooling, and microstructure transformation process of bearing rings, thermal stress and microstructure stress are inevitably generated, which causes deformation of the rings and changes in their size.
The deformation caused by quenching of bearing rings, including dimensional expansion and contraction and changes in geometric shape. For the expansion and contraction of dimensions, if the expansion and contraction amount is too large and the grinding allowance is too small, it will result in black skin or tool marks left after grinding, leading to scrap. If the deformation is too large, such as warping and deformation of the surface, after the surface grinding is completed, there will be black skin or tool marks left on the surface, resulting in scrap.
The quenching deformation of the ring is not only related to its own stiffness, but also to the following factors: uneven composition and structure of the raw material, uneven annealing structure, large furnace load, excessively high quenching heating temperature, and uneven quenching heating; Uneven cooling during the cooling process and collisions occurring during the cooling process. Therefore, in order to reduce deformation, lower quenching heating temperatures and appropriate holding times should be used as much as possible. At the same time, it is required that the annealed structure has uniform carbide particles, and the temperature of the quenching cooling oil should be appropriately increased.

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