Development of high Al and Ti heat resistant alloys with balanced creep and corrosion resistance at high temperature

Abstract

The heat resistant alloys with high Al and Ti were developed to achieve balanced creep and corrosion resistance at high temperature. Model alloys with nominal composition range of Fe–(30~35)Ni–(16~18)Cr–4.5Al–(1~4)Ti–1Nb (in wt.%) were prepared and studied in cast and wrought conditions. The creep rupture life was evaluated in air at 750 oC and high temperature corrosion tests were conducted in the supercritical-CO2 (S-CO2) and steam environments. The as-cast model alloys consisted of interdendritic phases in the austenite matrix, which partially dissolved after the solution annealing treatment. Nevertheless, coarse B2-NiAl phase was remained and its fraction and size increased with Ti content. The creep rupture life increased with Ti content in the as-cast condition. The solution annealing treatment improved the creep rupture life, however no clear effect of Ti content on the creep rupture life was observed. Several creep voids and cracks appeared along the coarse B2-NiAl phases especially for the higher Ti (3 – 4 wt.%) containing alloys, which affected the creep rupture life. The high temperature corrosion test showed that the model alloys in as-cast condition were not completely protective due to inhomogeneity in the as-cast microstructure. Meanwhile, the wrought model alloy with 3 wt.% Ti also exhibited coarse B2-NiAl phases in the austenite matrix. The creep rupture life of the wrought model alloy was comparable to that of cast alloy in solution annealed condition, in which creep voids also appeared along the coarse B2-NiAl phases. The coarse B2-NiAl phases seems to be more stabilized with the presence of Zr, thus further reduction in Zr content is required for their dissolution during heat treatments. The wrought model alloy showed the formation of inner Al2O3 layer with some occurrence of Fe-oxide nodules after exposure to S-CO2 environment at 650 oC and 20 MPa. However, external Cr2O3 layer was developed with internal Al-oxide layer after the steam corrosion test at 750 oC. It was found that the pre-oxidation treatment resulted in improved corrosion resistance in both aggressive S-CO2 and steam environments with the formation of α-Al2O3 layer. Overall, the heat resistant alloys with high Al and Ti content showed comparable creep and better high temperature corrosion resistance than commercial heat resistance alloys.