Impact of L1₂-Ordered Precipitation on the Strength of Alumina-Forming Austenitic Heat-Resistant Steels

Abstract

Alumina-forming austenitic (AFA) heat-resistance steels firstly developed by Yamamoto et al. at Oak Ridge National Laboratory have been reported as a new promising class of steels with potential for use in high temperature applications in recent years. The creep resistance of AFA steels is improved mainly by precipitation strengthening. Besides modifying the typical existing precipitates, i.e. MC and M23C6 type carbides, B2-NiAl and Fe2Nb-type Laves phase, introduction of coherent L12-ordered precipitate is highly desired. L12-ordered phase gamma prime (γ’) is the most important precipitate for high-temperature strengthening in Ni-based superalloys. In the present work, we demonstrate that addition of 2.8 wt. % Cu to an AFA steel promotes the formation of an L12-ordered phase with the dominating elements Ni, Cu and Al. TEM characterization after slow rate tensile tests indicated there were the different precipitation behaviours at 700°C and 750°C. It was revealed that the occurrence of L12-ordered Ni-Cu-Al phase depends on temperature and Ni content. This opens up new opportunities to promote the formation of L12-ordered phase in Fe-based austenitic heat-resistance steels and benefit high-temperature mechanical properties.

Research Progress of Alumina-Forming Austenitic Stainless Steels: A Review

“…The study found that between 500 °C and 700 °C, the solubility of copper in Ni 3 Al is as high as 2.8% without changing the L12 structure [ 91 ]. The research in the literature [ 25 , 26 , 35 , 92 ] all indicated that adding 2.8 wt.% copper to AFA steels promoted the formation of coherent L12 ordered Ni-Cu-Al phase.…”

“…Furthermore, the combination of fine B2 and Laves phase increases the creep life of AFA steels at 700 °C [ 27 , 94 ]. In addition, the precipitate of the γ′ phase can also improve the creep performance of AFA steels [ 25 , 92 ]. Yamamoto et al [ 12 ] found that nano-sized Laves-Fe 2 Nb and dispersed Ni 3 Al, particularly around 100 nm in diameter, were effective in improving the creep resistance.…”