Low density high manganese and aluminum steels are being consid- ered for use in tough and wear resis- tant automotive components and ballistic armor plating. These steels could be considered lightweight alternatives for components in the mining industry as well as ground- engaging components. For example, if Fe-30Mn-9Al-0.9C alloys with 15% lower density were substituted directly for the SAE 8620 steel track shoes of the Bradley Fighting Vehicle (BFV), the weight savings would be approximately 800 lbs. However, mechanical properties of cast Fe-Mn-Al-C alloys vary with composition, degree of age-harden- ing and steel cleanliness. Age hard- ening greatly increases strength in cast alloys but sharply reduces work hardening, toughness and abrasive wear resistance. Surface modification often is used to greatly improve the wear, fatigue and corrosion resistance of both ferritic and austenitic steels. One of the most effective sur- face treatments to increase wear resistance is the nitriding process which produces a hard “white layer” consisting of Ɛ – Fe2 ý – Fe4
(C,N) and/or N. Nitriding often is used
to greatly improve the wear and corrosion resistance of both ferritic and austenitic steels. The traditional gas and salt bath nitriding processes cause the release of toxic fumes and environmental pollution. Plasma nitriding, while much cleaner, is very costly and requires the use of expensive equipment. Nitriding of high manganese and aluminum steel in gaseous nitrogen may be a cost effective method to produce a hard and wear resistant layer of aluminum nitride. Manga- nese steels are brittle in the as-cast condition, and common practice is to solution treat these steels at temperatures up to 2,012F (1,100C). Tis solution treatment could be performed in a nitrogen atmosphere to produce a wear resistant surface nitride layer at little additional cost. In the case of high manganese and aluminum steels, aluminum nitride forms instead of Fe3
N. Te hard-
Looking at the Results In the study, steel test samples were
Fig. 1. The optical micrograph of Steel A after solution treatment shows a micro- structure of nearly 100% austenite. The specimens were etched with 10% nital followed by Klemm’s reagent.
Te furnace was purged for 20 minutes to eliminate residual oxygen prior to loading the specimens. Te cross sec- tions of the specimens were character- ized utilizing optical metallography. A field emission scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDS) was used to characterize the morphol- ogy and chemical composition of the reaction layers. Te results of the chemical analysis
solution treated to 1,922F (1,050C) for two hours and then rapidly quenched in ice water. Te surface of the specimens was polished to a 0.3 µm finish and washed in ethanol directly prior to nitriding. Nitriding experiments were conducted in the temperature range of 1,652-2,012F (900-1,100C) under 99.9% pure N2
,
Fig. 2. The optical micrograph of Steel B after solution treatment shows a micro- structure of austenite dendrites.
Fig. 3. The optical micrograph of Steel C after solution treatment is similar to Steel A and B and is completely austenitic.
ness of nitrides is known to increase with the amount of nitrogen, and the hardness of aluminum nitride has been reported to be 25.6 GPa, which is much higher than that of Fe3N (11.2-12.4 GPa). In a current study, the effects of aluminum and silicon on the kinetics of aluminum nitride coating formation in a Fe-30Mn steel was determined.
show that the compositions of the steels vary mainly with regard to aluminum and silicon content. Te microstructures of the respective steels after solution treatment for two hours at 1,922F (1,050C) and before the nitriding process are shown in Figures 1-3. Te microstructures are similar and all steels were nearly 100% austenitic with only a few isolated islands of primary ferrite noted in Steel C. Te second- ary dendrite arm spacing (SDAS), was also similar between the steels and measured between 50 and 75µm. Te nitriding process was carried out for up to eight hours at 1,652, 1,832 and 2,012F (900, 1,000 and 1,100C) for the three different aluminum and silicon containing steels. Figures 4a-d show the optical micrographs of the polished cross sections of Steel B (8.8% Al and 1.6% Si) and Steel C (6% Al and 1.6% Si) after nitriding for two to six hours at 1,652F (900C). After two hours at 1,652F (900C), Steel C is shown to develop twice the depth of aluminum nitride of Steel B.
Te depth of the aluminum nitride
layer increased with nitriding time for both aluminum containing steels, and after six hours at 1,652F (900C), the depth of the aluminum nitride layer was measured to be an average of 170 µm and 230 µm for Steels B and C. Increasing the process temperature to
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