Modified 9Cr-1Mo steel has been applied to the parts of boiler in thermal power plants due to its excellent properties at high temperatures. In recent years, this steel also has been applied to the parts of turbine in ultra super critical power plants.
Our company has been manufactured forged products by using large ingots of Mod.9Cr-1Mo steel that exceeded 20t for ten years or more, and it has been recognized that internal defects were liable to occur in Mod.9Cr-1Mo steel ingot. Especially the generation of the casting defects becomes more serious problem with the size of ingot, therefore measures were taken to improve the quality of forgings about both of ingot making and forging procedure.
As a result of investigation of the defects in large Mod.9Cr-1Mo steel ingots, we concluded that defects formed in forgings were caused by casting defects and eutectic Nb (C,N). And the defect was proved to be influenced considerably by solidification rate and the cooling rate by means of solidification analysis that was performed because complete sticking of defect surface was difficult and the improvement of the steel ingot was necessary. Afterwards, large ingots were produced based on the analysis and the effectiveness of the analysis was confirmed by UT and the investigation of the microstructure.
As mentioned above, we were able to raise quality and the stability of the product by improving the quality of the steel ingots.
