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 treatment -CCM300 square billet temperature feeding (or into the slow cooling pit)- continuous rolling - saw cutting fixed length - finishing - inspection storage. 

Interception of sample in produce "black spots" place, use JMS - 5600 lv sem and NORAN spectrometer for observation and correspondence analysis.

2 test results

The samples were cleaned by anhydrous alcohol and observed by scanning electron microscope. The "black spots" were large and small corrosion pits and pitting pitts, and the parallel stripes were the cutting stripes of grinding. 

Corrosion pit after magnification to observe the corrosion products, with energy spectrum analysis detected a higher content of oxygen, sulfur, chlorine, sodium, potassium and other elements, no inclusion, can be determined is not the steel itself between the problem, may be caused in the process of processing.Because there is chlorine in the pickling fluid, and the cooling fluid used in grinding contains elements such as sodium and potassium. 

The corrosion pitting will be high magnification, there is a layer of corrosion product inkstone cover, its morphology for crack shape of mud pattern.The small white glume is carbide in steel, which remains on the surface because the carbide is not easily corroded. 

3. Discussion and analysis

Some literatures have introduced that "black spot" defect is caused by aggregated inclusions. According to sem observation and energy spectrum analysis, "black spot" on the bearing surface is mainly caused by pitting corrosion caused by chlorine, sodium and potassium ions, and no aggregated inclusions have been found.Chlorine ions are brought into the waste tube by the pickling solution during pickling. When part of the oxide film on the metal surface is damaged for some reason (possibly due to scratch or bruising), Cl- in the residual acid solution will invade and react with the metal on the surface to become the anode, while the rest is the cathode, forming a micro battery. 

As a result, the metal surface was corroded into a lot of corrosion pits, while the carbide containing Cr was not easy to be corroded and was preserved (figure 3).The alkali generated by the cathode reaction around the pit promoted the passivation and thus inhibited the corrosion. Sodium and potassium ions were brought in by the coolant during the turning process of the bearing tube, and their invasion accelerated the corrosion.In order to prevent the occurrence of similar phenomenon, the workpiece must be strictly prevented from contacting with the media of chlorine, sodium and potassium plasma in the manufacturing process. The residual acid should be cleaned when pickling the surface oxidation skin, and the surface coolant should be cleaned after turning.According to this measure, the similar situation did not happen again in the later production. 

4 conclusion

By means of electron microscope and energy spectrum analysis, it was found that the "black spot" defect was caused by pitting corrosion in some parts which were not cleaned by the acid washing liquid during the processing, and then by the promotion of Na and K cation in the cooling liquid.