Hydrogen Effect on Nitriding Process of 304L Austenitic Stainless Steel

A series of plasma nitriding experiments has been conducted on AISI 304L austenitic stainless steel samples at temperatures ranging from 400 degC to 515 degC using continuous dc glow discharge plasma in an N₂-H₂ gas mixture. H₂ percentages in the gas mixture were changed from 0% to 50% relative to the total gas pressure, which is ranging from 3 to 9 torr in the nitriding reactor. The discharge current was adjusted to be 100 mA. The treatment time was varied from 15 min to 4 h. The structure and composition of the plasma nitrided surface layer were analyzed by means of X-ray diffraction and optical microscopy. Also, the microhardness technique was used for testing the surface microhardness of the nitrided samples. Effect of H ₂ percentage on the surface microhardness and on the thickness of the nitrided layer of the treated samples was studied. A pronounced increase of the surface hardness of the samples that are nitrided in N₂-H₂ gas mixture than that are nitrided in N₂ only. The hardness increases sharply when H ₂% is about 5%, then it slightly decreases again and saturates by further increasing of H₂% until it reaches 50% in the gas mixture. Also, the thickness of the nitrided layer increases by increasing H₂% content. The nitrided phases; gamma´-Fe ₄N and epsiv-Fe_2-3N, which are decomposed from the metastable supersaturated phase (S-phase), and CrN were observed and the grain size of the nitrided phases was in the order of tenth of nanometers