Creep behavior of pack cementation aluminide coatings on Grade 91 ferritic-martensitic alloy

The creep behavior of various pack cementation aluminide coatings on Grade 91 ferritic-martensitic steel was investigated at 650 °C in laboratory air. The coatings were fabricated in two temperature regimes, i.e., 650 or 700 °C (low temperature) and 1050 °C (high temperature), and consisted of a range of Al levels and thicknesses. For comparison, uncoated specimens heat-treated at 1050 °C to simulate the high temperature coating cycle also were included in the creep test. All coated specimens showed a reduction in creep resistance, with 16-51% decrease in rupture life compared to the as-received bare substrate alloy. However, the specimens heat-treated at 1050 °C exhibited the lowest creep resistance among all tested samples, with a surprisingly short rupture time of < 25 h, much shorter than the specimen coated at 1050 °C. Factors responsible for the reduction in creep resistance of both coated and heat-treated specimens were discussed.