| The improved process of austenitic nitriding by applying the controlled nitrogen potential theory could achieve high nitrogen austenite samples without a descend nitrogen profile from the surface to the center. The key of this nitriding process is that the pure iron foil can be thoroughly nitrided to high-N austenite which is thermally stable at room temperature by adjusting the nitrogen potential within the temperature range of 640oC-645oC. The nitrogen concentration of the austenite was determined as 10at.% by the XRD technique, much higher than the average nitrogen content of the Fe-N alloys.The intermediate temperature decomposition of high-N austenite was observed by Optical Microscopy(OM), Scanning Electron Microscopy(SEM), Transmission Electron Microscopy(TEM) et al in this article. It was found that the as-quenched high-N austenite grains appeared columnar morphology and their growth direction was perpendicular to the surface. Some periodic structures composed with nitrogen-rich and nitrogen-poor regions were found inside of the austenite grains when they were aged for short time in the salt-bath furnace at 225oC. The particle-like decomposed products were found within the grains after the austenite was aged for 1 hour. The lamellar products on the austenite grain boundaries kept to grow up during the ageing process, its morphology was much similar to those of the upper bainite in Fe-C alloys. More and more particle-like products appeared during the following ageing process, simultaneously, their grain size was kept in nano-scale dimension. After aged for 5 hours, the austenite had been decomposed into nano-scale dimension by theγ'-Fe4N andα-Fe. This kind of structure has much higher micro-hardness than those in the Fe-C bainite or martensite.In-situ high temperature XRD was utilized to investigate the entire decomposition process of high-N austenite at 225oC, especially its beginning stage. It was found that small amount ofα-Fe appeared when the austenite was heated to the tempering temperature, before the same amount ofγ'-Fe4N appeared when aged for 30 minutes. The peak shapes of the new-bornα-Fe andγ'-Fe4N were both broadened and scattered which indicated that the grains of the both phases were fine and dispersed. The terminal grain size ofα-Fe was larger than that ofγ'-Fe4N but both of them were kept in dozens to several hundreds of nanometers, which may attribute to the ultra-high micro-hardness of the decomposed products. During the isothermal decomposing process, the nitrogen content of theγphase were kept as its original level, which fit in with the reports of J.Foct et al. And the retained austenite grains would be absolutely transformed to the mixture ofγ'-Fe4N andα-Fe when aged for about 6 hours. The in-situ high temperature XRD observation verified theα-Fe phase would appear not in the quenching process after ageing but at the very beginning of the ageing process as well.Isothermal decomposition of the high-N austenite at intermediate temperature is a complicate phase transformation which is different to the bainitic transformation of Fe-C alloys. The upper bainitic transformation occurs on the austenite grains and forms the lamellar structure ofαandγ' ultimately. Someγ' particles nucleate on the dislocations and twin grains. But dominantly, the high-N austenite inside of the grains form the N-rich and N-poor regions by the nitrogen atoms'ordering and diffusing. Theγ'-Fe4N phase comes mainly from the N-rich regions with the cubic-cubic coherent relationship to the parent phase, which is not a nucleation-growth process but a continuous transformation. And theα-Fe comes mainly from the N-poor regions with a near K-S orientation relationship to the parent phase. The both decomposing products are kept in several hundred nanometers, which given very high micro-hardness to the substrate.
|
PDF Full Text Request