During the heat treatment of the tapered roller bearing ring, due to the structure of the bearing ring itself, heat treatment process, processing equipment, and human factors, the structure of the bearing ring is overheated, under heated, cracked, deformed beyond tolerance, and has defects such as bumps and bruises. These defects can directly lead to the direct scrapping of bearing rings. What measures should be taken to prevent damage to the bearing ring during heat treatment
1. Preventing the formation of overheated tissue
The microstructure of high carbon chromium steel bearing rings after quenching should be cryptocrystalline, fine crystalline, or small needle like martensite. Due to the limitations of the structure of the tapered bearing ring, when the thick walled end structure meets the requirements, coarse needle like martensite may appear at the thin-walled end, which is an obvious overheating structure.
This microstructure exceeds the standard requirements for heat treatment of bearing steel, and can lead to a decrease in toughness, impact resistance, and bearing life. Severe overheating can even cause quenching cracks. The reason is that the quenching heating temperature is too high or the heating insulation time is too long, which may also be caused by severe banding of carbides in the raw material or uneven distribution of carbide size in the annealing structure.
The measures taken should be selected according to the material standards (such as GCr15SiMn steel can be used when the effective thickness of the wall exceeds 15mm), and the heating temperature and time should be reasonably selected. Strictly control the banding of carbides. Improve annealing quality and take timely and effective measures in case of power outages, equipment failures, etc.
2. Preventing the formation of undercooled tissue
After quenching, the microstructure of high carbon chromium bearing steel bearing rings shows obvious needle like martensite, larger block like martensite, and the mixture of martensite or needle like martensite with block like martensite, resulting in strip like martensite. If it exceeds the specified limit, it is called undercooled structure.
Block like martensite is generated due to insufficient heating; Needle like martensite is caused by poor cooling. The banded carbides in bearing steel raw materials cause a banded distribution in the carbon poor zone, which leads to a decrease in hardness, a sharp decrease in wear resistance, and an impact on bearing life. The reasons for this are low quenching temperature, insufficient holding time, or poor cooling.
If there is troostite in production, its metallographic microstructure should be inspected, the cause analyzed, and corresponding measures taken. If the troostite is block like, it is necessary to increase the quenching heating temperature appropriately and extend the insulation time; If it is needle like martensite, the cooling rate should be increased. If the heating temperature, insulation, and cooling are all normal and there is a phenomenon of troostite, it is necessary to check for raw material problems, temperature control problems, equipment failures, etc., and promptly identify the cause and take measures.
3. Prevent the occurrence of quenching cracks
The cracks generated during the quenching process of parts are mostly caused by the tensile stress generated near the surface of the parts during cooling within the martensitic transformation temperature range, which exceeds the fracture strength of the steel at that temperature. The quenched cracked ferrule after pickling is shown in Figure 7; The main distinguishing feature 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 their resulting stress concentration, and decarburization of the part surface during heating may all promote the formation of cracks.
The quenching cracks of common bearing parts are as follows:
(1) The cracks formed by quenching overheating are caused by excessively high quenching heating temperature and prolonged holding time, resulting in coarse austenite grains, increased brittleness of martensite after quenching, decreased strength, and cracking. The crack feature is that the ferrule has fine cracks along the circumference, often occurring at the junction of thickness.
(2) Cracks generated by excessive cooling rate can cause parts to fall into an oil bath with water at the bottom for cooling in a medium with excessive cooling rate or during quenching. Due to the excessive cooling rate, the stress in the structure is significantly increased, leading to the formation of cracks. This type of crack often occurs at the junction of thickness.
(3) Due to the cracks generated 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, then these unrelieved stresses are superimposed with quenching to produce cracks.
(4) Cracks caused by stress concentration occur during processes such as typing too deep, turning marks too deep, oil grooves too deep (sharp), and steel ball file fatigue, resulting in the formation of cracks.
(5) Cracks caused by material defects in steel, such as porosity, white spots, pores, inclusions, and uneven distribution of carbides, can cause quenching stress concentration and produce quenching cracks.
(6) The cracks generated by surface decarburization not only reduce the surface strength of the parts, but also result in different temperatures at the Ms points on the surface and center. During cooling, the transformation of martensite takes place at different times, causing significant internal stress, resulting in intermittent, small, and not deep network quenching cracks.
(7) Cracks generated by delayed tempering after quenching. Under the long-term effect of quenching stress, the fracture strength of quenched martensite decreases with the extension of time. Therefore, if the quenched parts are not tempered in a timely manner, it will cause cracks.
(8) After quenching, the oil outlet temperature of the crack ring caused by impact is relatively high. If it is immediately cleaned or tempered before being collided, due to excessive quenching stress and mechanical collision force, wide and neat penetrating cracks will be generated along the longitudinal direction.
Preventive measures
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) Choose a reasonable quenching temperature and holding time to strictly prevent overheating of the workpiece, especially for parts with too fine annealing structure and secondary quenching, pay more attention to this point.
(3) Select appropriate cooling medium and cooling method, strictly prevent water mixing in the quenching oil (the water content in the quenching oil is less than 0.1%), and control the temperature of the quenching cooling medium (the quenching oil temperature is around 90 ℃); For complex parts with wall thicknesses 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 tempered thoroughly.
4. Controlling Carbon Potential to Prevent Surface Decarbonization
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 decarburization layer on the surface exceeds the machining allowance, the parts 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.
After quenching, tempering, and polishing of the bearing ring, obvious pits were found on the surface. The decarburization layer of the bearing parts 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 carbon poor layer to a normal structural area. The surface of the severely decarburized bearing ring with obvious pitting after polishing is shown in Figure 9. After wire cutting, it was observed that the depth of the decarburization layer far exceeded the standard requirements on the longitudinal section. The reason is that the carbon potential in the quenching furnace was low during the quenching and heating process of the ring. After investigation, it was found that one of the holes where methanol was dropped into the furnace top was blocked, resulting in a low amount of methanol dropped into the furnace. The measures taken to prevent carbon deposition in the top inlet pipe of the furnace, which affects the carbon potential of the protective atmosphere, require operators to clear 1-2 times per shift
5. Take measures to prevent collisions and injuries
After quenching and tempering, it was found that there were obvious scratches 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 on the production line, the interface (such as between cold and hot cleaning agents, between cold and clean cleaning machines and tempering furnaces), and the discharge port of the tempering furnace, as well as the collision between the rings during the polishing process, resulting in bumps and bruises.
The measures taken are to install heat-resistant rubber at various interfaces of the heat treatment production line (such as between cold and hot cleaning agents, between cold and clean cleaning machines and tempering furnaces) and at the discharge port of the tempering furnace to prevent scratches. For heavier rings, a hanging polishing machine is used to polish them. During the polishing process, gently take them in and out with your hands to prevent scratches from occurring.
6. Control quenching deformation to prevent dimensional abnormalities
During the quenching, heating, cooling, and structural transformation process of the bearing ring, thermal stress and structural stress are inevitably generated. This stress change causes deformation of the ring, resulting in changes in the size of the ring.
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 leather or lathe tool lines left after grinding, leading to scrapping. If the deformation is too large, such as warping and deformation of the surface, after the surface grinding is completed, there will be black leather or lathe tool lines left on the surface, resulting in scrapping.
In addition to its own stiffness, the quenching deformation of the ferrule is also related to the following factors: uneven raw material composition and structure, uneven annealing structure, large furnace loading, high quenching heating temperature, and uneven quenching heating; Uneven cooling and collisions during the cooling process. Therefore, in order to reduce deformation, lower quenching heating temperature and appropriate insulation time should be used as much as possible, and the annealing structure should be uniform carbide particles, and the temperature of quenching cooling oil should be appropriately increased.