Effect of microstructure on hydrogen diffusion and notch tensile strength of large steel forging

Microstructure, hydrogen diffusion coefficient and notch tensile strength (NTS) in two different parts (1/2 radius and 3/4 radius) of a large steel forging have been investigated by means of microscopy, thermal desorption spectrometry (TDS), and slow strain rate tensile tests (SSRT), respectively. The microstructure in the 1/2R part is granular pearlite while that in the 3/4R part is composed of granular pearlite and bainite with many fine M23C6 carbides. Results of TDS analysis indicate that hydrogen content in the 3/4R part is much higher than in the 1/2R part, corresponding to a hydrogen diffusion coefficient about one order lower. SSRT results showed that although NTS decreased with the increase of hydrogen content in both parts, the 3/4R part showed a higher NTS at the same H content. The higher H content, lower H diffusion coefficient and higher NTS in the 3/4R part can be attributed to the fine M23C6 carbides which can act as hydrogen trapping sites.