INVESTIGATION ON THE STRUCTURE OF CARBONITRIDED LAYER IN SINTERED MANGANESE STEELS

The structure of carbonitrided layer in sintered Fe-C and Fe-Mn-C alloys was studied by TEM observation. The results show that the element Mn can increase the open porosity in the sintered compacts and consequently increase its depth of carbonitrided layer.After tempering of both steels at 150℃, the Fe₁₆N₂(α″)phase was precipiteted from both the carbonitrided martensite matrix. The experimental results show that hhe element Mn can significantly increase the dispersity of precipitated phase in Fe-Mn-C alloy and the average thickness of dispersoid is about 60~70Å and the length 500~600Å, while the thickness of dispersoid precipitated from the martensite of Fe-C alley is about 500Å and the length 3000Å, which is one order larger than that from the martensite of Fe-Mn-C alloy. This illustrates that the Mn is favourable to the nucleation of dispersed phase Fe₁₆N₂ and restrains its growth.
When the carbon steel was tempered at 100~200℃ the stress would be relaxed and the ε phase should be precipitated, but in this study, the Fe₁₆N₂ phase was precipitated from the martensite, and the ε phase was not observed. These results show that the nitrogen atoms not only can interrupt the odering of carbon atoms in anstensite lattice, but also interrupt the partial crdering of carbon atoms in martensite phase, which leads to restrain the precipitation of ε phase. It is shown that when the martensite is tempered below 200℃, the Fe₁₆N₂ phase is precipitated as mentioned above, tempered at 200~290℃, the FeN phase(γ')is observed, tempered at higher than 300℃ only Fe₃C phase is observed.