During the heat treatment of tapered roller bearing rings, defects such as overheated structure, underheated structure, cracking, excessive deformation and bump damage are likely to occur on the rings due to the influence of the ring's own structure, heat treatment process, processing equipment and human factors. These defects will directly lead to the scrapping of bearing rings. So, what measures should be taken to prevent damage to bearing rings during heat treatment?
1. Prevention of Overheated Structure Formation
The microstructure of high-carbon chromium steel bearing rings after quenching should be cryptocrystalline martensite, fine-grained martensite or fine acicular martensite. Due to the structural particularity of tapered bearing rings, when the structure at the thick-walled end meets the requirements, coarse acicular martensite may appear at the thin-walled end, which is a typical overheated structure.
This structure exceeds the standard requirements for the heat treatment of bearing steel, which will reduce the toughness and impact resistance of the rings, thereby shortening the service life of bearings; severe overheating may even cause quenching cracking. Its causes include excessively high quenching heating temperature, too long heating and holding time, severe banded segregation of carbides in raw materials, or uneven carbide particle size distribution in the annealed structure.
It is necessary to select suitable materials in accordance with material standards (e.g., GCr15SiMn steel can be used when the effective wall thickness of the ring exceeds 15mm), set the heating temperature and holding time reasonably; strictly control the degree of banded segregation of carbides and improve the annealing quality; in case of sudden situations such as power failure and equipment failure, effective countermeasures should be taken in a timely manner.
2. Prevention of Underheated Structure Formation
After quenching of high-carbon chromium bearing steel bearing rings, if obvious acicular martensite, massive martensite, or a mixed structure of martensite with acicular martensite and massive martensite forms lath martensite beyond the specified limit, it is called underheated structure.
Massive martensite is usually caused by insufficient heating, while acicular martensite is induced by poor cooling. Banded carbides in the raw bearing steel will cause the carbon-poor areas to distribute in bands, ultimately leading to a decrease in the hardness of the rings and a sharp reduction in wear resistance, which affects the service life of bearings. The main causes of underheated structure include excessively low quenching temperature, insufficient holding time or poor cooling effect.
If troostite appears during production, its metallographic structure should be inspected to analyze the causes and take corresponding measures: in the case of massive troostite, the quenching heating temperature should be appropriately increased and the holding time prolonged; for acicular martensite, the cooling rate should be raised. If troostite still appears when the heating temperature, holding time and cooling conditions are all normal, it is necessary to investigate problems such as raw material quality, temperature control and equipment failure, and find and solve the causes in a timely manner.
3. Prevention of Quenching Cracking
Most cracks generated in parts during quenching are caused by the tensile stress generated near the part surface when cooling to the martensitic transformation temperature range exceeding the fracture strength of the steel at that temperature. After pickling, the morphology of rings with quenching cracks is shown in Figure 7; the main difference between quenching cracks and material cracks and forging cracks is that there is no decarburization on both sides of quenching cracks.
In general, rapid cooling below the Ms point (martensite start temperature) during quenching is the main cause of quenching cracking. In addition, excessive initial stress of parts before quenching, defects in raw materials and the resulting stress concentration, as well as surface decarburization of parts during heating, may all promote crack formation.
The common types of quenching cracks in bearing parts are as follows:
(1) Quenching overheating cracks: Excessively high quenching heating temperature and too long holding time lead to coarse austenite grains, increased brittleness and decreased strength of martensite after quenching, and ultimately cracks. Such cracks are characterized by fine cracks appearing on the ring along the circumferential direction, mostly occurring at the junction of thick and thin parts.
(2) Cracks caused by excessively fast cooling rate: Quenching in a medium with an excessively fast cooling rate, or when parts fall into an oil tank with water at the bottom for cooling, the internal stress of the structure increases significantly due to the excessively fast cooling rate, thus forming cracks. Such cracks also often appear at the junction of thick and thin parts of the ring.
(3) Cracks caused by initial stress before quenching: If the cold working stress of parts is not fully eliminated, or the stress from the previous quenching is not removed before rework, the superposition of these unreleased stresses and the stress generated during quenching will lead to cracking.
(4) Cracks caused by stress concentration: Stress concentration is likely to occur during processing such as over-deep marking, over-deep turning marks, over-deep oil grooves (sharp edges and corners) and steel ball filing fatigue, ultimately forming cracks.
(5) Cracks caused by steel material defects: Defects such as porosity, flakes, blowholes, inclusions and uneven carbide distribution in steel will cause quenching stress concentration, thus generating quenching cracks.
(6) Cracks caused by surface decarburization: Surface decarburization not only reduces the surface strength of parts, but also makes the Ms point temperature of the part surface different from that of the core. During cooling, the martensitic transformation occurs at different times, generating large internal stress, and ultimately forming intermittent, fine and shallow network quenching cracks.
(7) Cracks caused by delayed tempering after quenching: Under the long-term action 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, cracks are likely to occur.
(8) Cracks caused by impact: If the ring is cleaned or tempered immediately after collision when the oil discharge temperature is high, the superposition of quenching stress and mechanical collision force will produce wide and neat penetrating cracks along the longitudinal direction of the ring.
Preventive Measures:
In view of the causes of quenching cracks, the following preventive measures can be taken:
(1) Strengthen the acceptance inspection of raw materials and strictly control the quality of steel.
(2) Select a reasonable quenching temperature and holding time, and strictly prevent workpiece overheating; special attention should be paid to parts with too fine annealed structure and those requiring secondary quenching.
(3) Choose a suitable cooling medium and cooling method, strictly prevent water from mixing into quenching oil (the water content of quenching oil should be lower than 0.1%), control the temperature of quenching cooling medium (the quenching oil temperature is controlled at about 90℃); adopt the marquenching process for complex parts with uneven wall thickness and easy cracking.
(4) Parts should not be placed for a long time after quenching or cold treatment, especially parts requiring secondary quenching, which should be tempered immediately with sufficient tempering ensured.
4. Control Carbon Potential to Prevent Surface Decarburization
If bearing parts are heated in an oxidizing medium during heat treatment, an oxidation reaction will occur on the surface, resulting in a decrease in the mass fraction of carbon on the part surface and surface decarburization. If the depth of the surface decarburization layer exceeds the machining allowance, the part will be scrapped. The depth of the surface decarburization layer can be measured by the metallographic method and microhardness method in metallographic inspection, among which the measurement method based on the surface microhardness distribution curve can be used as the arbitration basis.
Obvious pitting will appear on the surface of bearing rings after quenching, tempering and polishing. The typical metallographic structure of the decarburization layer of bearing parts is as follows: the outer layer is white and bright ferrite, and the lower layer transitions from the carbon-poor area to the normal structure area. The morphology of bearing rings with obvious pitting on the surface after polishing and severe decarburization is shown in Figure 9; observation after wire cutting found that the depth of the decarburization layer on its longitudinal section far exceeds the standard requirements. The cause of this problem is the excessively low carbon potential in the quenching furnace during the quenching heating of the rings. Investigation found that one of the methanol dropping holes on the furnace top was blocked, resulting in insufficient methanol dropping into the furnace. To prevent carbon deposition in the air inlet pipe on the furnace top from affecting the carbon potential of the protective atmosphere, operators are required to clean the air inlet pipe 1-2 times per shift.
5. Take Measures to Prevent Bump Damage
If obvious scratches appear on the surface of the ring after quenching and tempering, the ring will be scrapped. The causes of scratches include: during heat treatment, workpieces are prone to collision at the oil tank of the production line, various interfaces (e.g., between hot and cold cleaning agents, between cold cleaning machine and tempering furnace) and the discharge port of the tempering furnace; in addition, collision between rings during polishing will also cause bump damage.
The following preventive measures can be taken: install heat-resistant rubber at various interfaces of the heat treatment production line (e.g., between hot and cold cleaning agents, between cold cleaning machine and tempering furnace) and the discharge port of the tempering furnace to prevent scratches; use a suspended polishing machine for polishing heavy rings, and handle them gently by hand during polishing to avoid scratches.
6. Control Quenching Deformation to Prevent Dimensional Abnormality
Thermal stress and structural stress are inevitably generated in bearing rings during quenching heating, cooling and structural transformation. The change of these stresses will cause the deformation of the rings, thereby changing the dimensions of the rings.
Quenching deformation of bearing rings includes dimensional expansion and contraction and geometric shape change: if the dimensional expansion and contraction is too large and the grinding allowance is insufficient, black skin or turning tool marks will remain after grinding, leading to the scrapping of the rings; if the deformation is too large (such as surface warpage deformation), black skin or turning tool marks will still remain on the surface after surface grinding, also resulting in the scrapping of the rings.
In addition to its own stiffness, the quenching deformation of the ring is also related to the following factors: uneven composition and structure of raw materials, uneven annealed structure, excessive furnace charge, excessively high quenching heating temperature, uneven quenching heating, uneven cooling and collision during cooling. Therefore, to reduce deformation, a relatively low quenching heating temperature and a suitable holding time should be adopted as far as possible to ensure that the annealed structure is uniform carbide particles, and the temperature of quenching cooling oil should be appropriately increased.
