Boriding of In-738 Superalloy via the Pack Cementation Method and the Study of the Kinetic of Boride Layer Formation

In this study, mechanism and kinetic of formation of boride layer on In-738 superalloy were investigated via diffusion pack cementation method. Boriding was carried out at 900 °C for several short times (5, 15, 45 and 60 min). Phase study by means of X Ray defragtion (XRD) indicated that in addition to Ni3B, other phases such as Cr5B3, AlB2, and W2B were formed at the first period of process, and other compounds such as MoB2, VB, TiB, Ni6Si2B, and Mo2NiB2 were generated in the more prolonged time. SEM study also showed that not only the thickness of boride coating was increased, but also an interdiffusion zone (IDZ) was formed under the coating and it was grown by the upward diffusion of alloy elements. The kinetic study was good according to diffusion theory, confirming the two diffusion steps for IDZ. Thickness and hardness of the boride coating over 60 min process were 27.8 µm and 853 HV, respectively.