also improve wear resistance. For steels nitrided at times greater than six hours, an approximately 10-20 µm complex oxide layer was noted on the surface above the nitride layer, as shown in Figure 5d. Te presence of an oxide layer
indicates that not all of the oxygen was eliminated from the furnace atmosphere. However, the oxide layer did not develop until after extended times during the nitriding process, and aluminum nitride formation was always favored because of the much higher solubility and diffusion of interstitial nitrogen in austenite over oxygen. Figure 7 shows the depth of the aluminum nitride layer as a function of time and temperature. Depending on the time and tem- perature, aluminum nitride coating thicknesses between 200 and 550 µm can be achieved. Steels A and B with 8.8% Al and 1.1% Si to 1.6% Si show similar coating thicknesses for all times and temperatures. Although silicon is known to
decrease the solubility of nitrogen in steels, increasing the silicon level from 1.1% to 1.6% had little effect on the depth of the aluminum nitride layer or the apparent density of the plates precipitated. However, increasing the amount of aluminum from 6% to 8.8% produced a denser array of very fine plates but sharply decreased the depth of the aluminum nitride layer. It also produced a 50% reduction in the diffusivity of nitrogen and increased the activation energy from 64 to 78 kJ/mol in the temperature range of 1,652-2,012F (900-1,100C). Te results of the study show that high manganese and aluminum austenitic steels can be nitrided in a gaseous nitrogen atmosphere to produce a hard and wear-resistant layer of AlN at depths of up to 550 µm. Tis would broaden the use of low density, high manganese and aluminum steels to include some ap- plications that require better wear resistance. ■
Tis article is based on the paper “Nitrid- ing of Lightweight High Manganese and Aluminum Steels” (15-036), originally pre- sented at the 119th Metalcasting Congress.
Fig. 6. Shown are the secondary electron micrographs of (a and b) Steel B and (c and d) Steel C after nitriding for eight hours at 1,100C. In Steel B, the AlN consists as a high density of longer and typically thiner plates that grow in parallel packets in specific crystallographic directions within the austenite (a and b). In Steel C, the density of AlN in the reaction layer is much less (c and d). However the plate thickness is greater and case depth of AlN is almost 200 µm greater than in Steel B.
Jul/Aug 2015 | METAL CASTING DESIGN & PURCHASING | 35
Fig. 5. Shown here are the secondary electron images of the AlN coating in (a and b) Steel A (8.8% Al and 1.1% Si) and (c and d) Steel B (8.8% Al and 1.6% Si) after nitriding for eight hours at 900C. The case depth and plate-like structure of the AlN is similar between the two different silicon containing steels, and the AlN is shown to precipitate and grow along specific crystallographic directions in the austenite (b and d). (d) Oxide layers that were determined to be most consistent with MnAl2O4 and Al2O3 developed on the surface of the 8.8% Al steels at an average depth of 10-15 µm after eight hours at 900C.
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