The Effect of Inclusions on Internal Cracks for 20Mn5N Steel Ingot after Forging

Abstract

The heavy forging ingots of steel 20Mn5N have been extensively used to fabricate the hydropower shaft steel, which plays as an extremely important part in industry. However, the quality of the heavy ingots has been commonly affected by various inner defects. One of the most typical defects is internal cracks, which has been found to be always accompanied by inclusions. In order to effectively analyze the formation of internal cracks, a 30-ton 20Mn5N ingot has been cut along the axle plane. Exceeding macroscopic cracks can be found in macro-segregation area gathered many large and grid inclusions (Al2O3). Based on the experimental results and FEM simulation method we have investigated that large and grid inclusions accompanied by other macro-visible defects such as banded structure could lead to the forging cracks easily. FEM simulation result shows that the stress concentration resulting from the large and grid inclusions (Al2O3) is relatively large. These large and grid inclusions accompanied by banded structure are the fundamental cause of cracks.