Relationship between Inclusions and Internal Defect Spatial Distribution in Large Forging Piece for Wind Power Generation Gear

The spatial distribution of inclusions in a large forging piece is closely related to the fatigue life of gears. In this paper, the size, number, types, and distribution of inclusions in a large forging piece of gear steel used for wind-power generation have been systematically analyzed by the automatic scanning of inclusions, in situ analysis of inclusions, scanning electron microscopy, and energy spectrum analysis. The inclusions distribution model is established and the size of the largest inclusion in the forging piece is predicted. The distribution of the number and size of inclusions exhibits an exponential relationship. The total number of inclusions is lowest at the tooth center area, and macro-inclusions with sizes above 10 μm mainly concentrate in the tooth center, with a maximum size of 101.5 μm. The typical inclusions in forging pieces include 2.85% oxides, 80.95% sulfides and 16.2% composite inclusions of oxides and sulfides. The sulfide preferentially precipitates on the surface of oxide's core in the following order: Al2O3-MgO-CaO > Al2O3 > Al2O3-MgO > Al2O3-MgO-SiO2-CaO > Al2O3-MgO-SiO2 > Al2O3-SiO2. It is helpful to change the brittle oxides into composites of oxides and sulfides to improve the fatigue life of gear steel.