Effect of pressure application on microstructure evolution in a composite of Fe–Al alloy and CrMo steel

The effect of pressure applied by hot isostatic press (HIP) on microstructure evolution during diffusion bonding was studied using a couple composed of an Fe–Al alloy (Fe–Al) and CrMo steel (CrMo). Columnar grains are formed from the joint interface to the CrMo side irrespective of pressure application. The average length of the columnar grains of the couple bonded with pressure was shorter than that of the couple bonded without pressure. For the couple bonded with pressure, the microstructure of the CrMo side beyond the columnar grains exhibits fine facets with acicular precipitates of Fe3C. Further, strong hardening is found in this region. The application of pressure retards the aluminum diffusion from Fe–Al to CrMo. The growth rate of the columnar grain with respect to bonding time deviated from the parabolic law. These effects of the pressure application on the microstructure evolution are due to the suppression of aluminum diffusion and retardation of transformation during bonding.