An aluminide（AlFe and α-（FeAl）） surface layer containing lower-Al was formed on ferritic-martensitic steel P92 by means of surface mechanical attrition treatment（SMAT） combined with a duplex aluminization process at lower temperatures,i.e.a packed aluminization followed by a diffusion annealing treatment below its tempering temperature.Indentation tests indicated that the lower-Al surface layer formed on the SMAT sample is more resistant to cracking and has better adhesion to the substrate in comparison with the Al 5Fe 2 layer formed on the as-received sample after the duplex aluminization process.Isothermal steam oxidation measurements showed that the oxidation resistance is increased significantly by the lower-Al surface layer due to the formation of a protective（Fe,Cr）Al 2O 4 layer.The rate constant of oxidation was estimated to decrease from-0.849 mg^2 cm^-4h^-1 of the as-received material to^0.011 mg^2 cm^-4 h^-1 of the AlFe layer at 700 ℃.
In order to investigate the effect of N on the microstructure and room temperature mechanical properties of new-type high silicon martensitic heat-resistant steels,three steels containing the same total content of C and N but different N contents have been designed and prepared according to the thermo-calc calculation.The thermodynamic calculation and experiments indicate that the replacing of C by N changes the kind and volume fraction of precipitates of the high Si martensitic steel significantly.Along with the N content increasing,the precipitates in the samples after 750 °C tempering change from（Cr23C6？ VN ？ TaC） to（Cr23C6？ VN ？ TaC ？ TaN） and finally to（Cr23C6？ VN ？ Cr2N）according to both experimental results and thermodynamic calculations.The room temperature mechanical tests show that the strength of the steel decreases as the N content increases.However,the Charpy impact toughness increases with N content increasing.According to the calculation and SEM observation,it is inferred that the decrease of amount and size of precipitates accounts for the changes of the mechanical properties.
