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Bearing ring surface "black spots" cause analysis!

release time:2018-05-26
1 Test materials and methods
The raw material is GCr15 bearing steel produced by Xingcheng Special Steel.
Process route: 100t EBT furnace smelting - LF refining - VD vacuum processing - CCM300 billet temperature (or slow pit) - continuous rolling - sawing length - finishing - inspection storage.
Samples were taken at “black spots” and observed with JSM-5600 LV scanning electron microscope and NORAN spectrometer and the corresponding phase analysis was performed.
2 test results
The sample was washed with anhydrous alcohol and observed by scanning electron microscopy. The “black spots” were large and small corrosion pits and pitted pits, and the parallel stripes were grinding strips.
Corrosion pits were observed to have corrosion products after amplification. High levels of oxygen, sulfur, chlorine, sodium, potassium, and other elements were detected by energy spectrum analysis. No inclusions were identified. It was not the problem of the steel itself. It may be processed. Caused by the process. Because there is chlorine in the acid solution, the coolant used in grinding contains sodium, potassium and other elements.
The corroded pits are high-magnified and covered with a layer of corrosion products. Their appearance is a cracked mud-like pattern. The white granules are carbides in the steel because the carbides are not easily eroded and remain on the surface.
3 Discussion and Analysis
Some literatures described the occurrence of “black spot” defects as aggregated inclusions. It was known by scanning electron microscopy and energy spectrum analysis that the “black spot” on the bearing surface was mainly due to the corrosion of chlorine, sodium, and potassium ions, and no aggregation was found. Inclusions. Chloride ion is brought in by the pickling solution during pickling. When the partial oxide film on the metal surface is damaged for some reason (possibly scratches or bumps), Cl- in the residual acid solution is caused. The invading metal reacts with the surface to become the anode while the rest is the cathode, forming a microbattery.
As a result, the metal surface is corroded into many etch pits, and Cr-containing carbides are not easily corroded. The alkali generated by the cathode reaction around the pit has promoted the passivation, thus inhibiting the corrosion. The sodium and potassium ions are brought into the bearing tube by the cooling liquid during the turning process, and its invasion accelerates the progress of corrosion. In order to prevent the occurrence of similar phenomena, it is necessary to strictly prevent the contact of the workpiece with the medium of chlorine, sodium, and potassium ions in the manufacturing process, and to wash the residual acid during the acid washing, and to clean the surface after turning. liquid. According to this measure, no similar situation has occurred in the subsequent production.
4 Conclusion 
Through electron microscopy and energy spectrum analysis, it was found that the "black spot" defect was caused by the corrosion of the pickling solution during the process of cleaning, and was then caused by the promotion of Na, K cations in the cooling liquid.