Cause analysis and improvement measures for perforation crack of GCr15 bearing steel
Aiming at the problem of perforation crack in GCr15 bearing steel during break-down rolling,the causes were analyzed by the means of metallographic microscope,scanning electron microscope and FEI metal quality analyzer,and the improvement measures were put forward.The results showed that the perforation crack of GCr15 bearing steel originated from the core of the billet,and there were no inclusions or slag inclusions around the core crack.The microstructure was normal and there was no o-verheating and over burning phenomenon,indicating that the billet heating temperature was normal,while the obvious decarburi-zation of the core crack indicated that it had expanded to the surface of the billet before leaving the heating furnace.Combined with the shot blasting test before the billet was put into the furnace,it was concluded that the core crack was generated during the heating process.Based on the macro-morphology of the section and microstructure of the defective billet,the perforation crack of GCr15 bearing steel is caused by supplementing gas supply when it cools down to below 1 000 ℃ and then heats up to 1 280 ℃ for rolling quickly.The billet is heated too fast,and the frequency of perforation crack greatly increase.And based on this,the heating process is optimized.For cooling problems such as insufficient gas supply of GCr15 bearing steel,it is necessary to wait for the heating furnace to regain its heating capacity.For the first heating,the temperature should be raised to over 1 000 ℃ according to the heating speed of 30-50 ℃/h in preheating stage,so that the billet can be fully austenitized.And then the billet can be heated rapidly.The adoption of this improved process can completely eliminate the problem of perforation crack during the hreak-down rolling process of GCr15 bearing steel.
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