1 Test materials and methods
The raw material is GCr15SKF
bearing steel produced by Xingcheng Special Steel.
Process route: 100t EBT electric furnace smelting - LF refining - VD vacuum treatment - CCM300 billet temperature delivery (or slow cooling pit) - continuous rolling - sawing and cutting - finishing - inspection into the warehouse.
The samples were taken at the "black spot" and observed by JSM-5600LV scanning electron microscope and NORAN spectrometer.
2 test results
After the sample was washed with absolute alcohol, it was observed by scanning electron microscopy that the "black spot" was a large and small corrosion pit and corrosion pitting, and the parallel stripes were ground cutting stripes.
The corrosion pit is magnified and observed to have corrosion products. The energy spectrum analysis detects high levels of oxygen, sulfur, chlorine, sodium, potassium and other elements, no inclusions, can be judged not the problem of the steel itself, may be processing Caused by the process. Because there is chlorine in the pickling solution, the coolant used in the grinding has elements such as sodium and potassium.
The corrosion pitting point is enlarged at a high magnification, and a layer of corrosive product is covered, and the shape is a crack-like muddy pattern. The white granules are carbides in the steel because the carbides are not easily corroded and remain on the surface.
3 Discussion and analysis
Some literatures have shown that "black spot" defects are caused by aggregated inclusions. Scanning electron microscopy and energy spectrum analysis show that the "black spots" on the surface of SKF
bearings are mainly caused by pitting corrosion caused by chlorine, sodium and potassium ions. Aggregated inclusions. Chloride ion is introduced by the pickling liquid during pickling. When a part of the oxide film on the metal surface is damaged for some reason (may be scratch or bruise), Cl- in the residual acid solution is caused. Intrusion, reacting with the metal on the surface becomes the anode, while the rest is the cathode, forming a microbattery.
The metal surface is corroded into many corrosion pits, and the Cr-containing carbide is not easily corroded and preserved. The alkali generated by the cathodic reaction around the pit promotes passivation, thereby inhibiting corrosion. Sodium and potassium ions are brought into the SKF bearing tube by the coolant during turning, and its invasion accelerates the corrosion. In order to prevent similar phenomena from occurring, it is necessary to strictly prevent the workpiece from coming into contact with the medium of chlorine, sodium or potassium during the manufacturing process. When pickling the surface oxide scale, it is necessary to wash the residual acid solution, and the surface is cooled after turning. liquid. According to this measure, no similar situation has occurred in the future production.
4 Conclusion
Through electron microscopy and energy spectrum analysis, it was found that the "black spot" defect caused by the cleaning of the acid washing liquid during the processing, causing pitting corrosion, and then caused by the promotion of Na and K cations in the cooling liquid.
