| Summary: | The ZG40Cr20Ni20Al<i>x</i> (<i>x</i> = 0, 1.76, 3.45, and 5.34) heat-resistant steel has been newly developed on the basis of HK40 steel for aggressive oxidizing environments. The results reveal that the Al greatly enhances the oxidation resistance of ZG40Cr20Ni20 steel at high temperatures. The mass gain of ZG40Cr20Ni20 upon oxidation at 1100 °C for 480 h is up to 103.6 mg/cm<sup>2</sup>, while the values for the steels containing 1.76 and 3.45 wt% Al are sharply decreased to 6.1 and 5.4 mg/cm<sup>2</sup>, respectively. Both of their matrix phases are still austenite, which is the same as that of ZG40Cr20Ni20. Their FeCr<sub>2</sub>O<sub>4</sub> spinel oxide scales appear to be more stable under high oxygen partial pressure than that of ZG40Cr20Ni20, and the continuous Cr<sub>2</sub>O<sub>3</sub> film appears between their matrix and spinel oxide. As for the steel with 5.34 wt% Al, the mass gain is only 1.1 mg/cm<sup>2</sup>. Its matrix is compared to those of austenite and ferrite, and the oxide scale is continuous Al<sub>2</sub>O<sub>3</sub>.
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