Active screen plasma nitriding of surface oxidised 722M24 steel

Samples of a 722M24 low alloy steel were oxidised in an air furnace. The oxidised samples and ground reference samples were plasma nitrided together in an active screen plasma nitriding (ASPN) furnace. It was found that the ground samples were nitrided as normal - as indicated by the formation of an iron nitride compound layer and considerably improved hardness of the nitrided surface. On the other hand, the oxidised sample did not cause any arcing during the plasma nitriding process. Surprisingly, it was found that after plasma nitriding the iron oxides (Fe₂O₃/Fe₃O ₄) on the oxidised surfaces were changed to iron nitrides (γ′-Fe₄N/ε-Fe₂-₃N) similar to those formed on the ground steel surface. The results suggest that with ASPN technology it is feasible to nitride a partly machined surface. In other words, it is not necessary to remove the oxide layer by grinding or machining throughout if only part surface of a component is to be nitrided. The results also suggest that with ASPN technology it is possible to modify the structure and properties of an oxidised surface.