Internal nitriding of sintered iron in nitrogen–low ammonia atmospheres

Sintered iron specimens with porosities of 27, 12, and 8% were subjected to internal gas nitriding at 610°C for 1 h in a gas mixture consisting of nitrogen and 1, 5, and 9% ammonia and then quenched. Precipitation of Fe₁₆N₂ nitrides in the ferritic matrix was found after processing. The diffusion zone without coarse Fe₄N nitrides and ϵ phase was formed throughout specimens with a porosity of 12% or higher when nitrided in an atmosphere containing over 5% ammonia. The reaction surface area formed by the surface area of interconnected pores and by the geometrical surface area of the specimen played a decisive role in the process. An increase in matrix grain microhardness, tensile strength, and hardness of the nitrided specimens without significant embrittlement, as compared with the plain iron properties, was achieved. The specimens with a porosity of 12% provided the best combination of mechanical properties without dimensional changes. The process can be applied for sintered iron based components for loading under adhesive wear conditions with high contact stresses or dynamic stresses, or under fatigue loading.