Determination of the Particle Size Distribution of AlN Precipitates in Gear Steel using Ultra-low Accelerating Voltage Scanning Electron Microscopy

Abstract

Abnormal grain growth during carburizing of steel can cause problems such as increased heat treatment distortion and reduced fatigue strength. Since grain pinning using precipitates is effective for suppressing abnormal grain growth, determination of the particle size distribution of the precipitates is vital. Indirect methods such as the small-angle scattering method and direct methods such as transmission electron microscope（TEM）observation are used for analysis. However, the former has problems with accuracy in size determination, and the latter with ensuring representativeness. In this study, we investigated an evaluation method using a scanning electron microscope（SEM）, which can observe a wider area than a TEM. By using an ultra-low accelerating voltage SEM at a low accelerating voltage and selectively collecting low-energy secondary electrons, AlN can be distinguished from the matrix by its bright contrast. We applied this method to gears of two types of gear steel with different former γ grain sizes and determined the histogram of the AlN size in each gear.