Table 3. Calculated rate constants, diffusion coefficients, and activation energies Steel
Steel A Steel B Steel C
Temperature, C 900
1000 1100 900
1000 1100 900
1000 1100
1,832F (1,000C) increased the kinet- ics of the nitriding process, and after six hours, the average depth of the aluminum nitride layer increased to 200 µm in Steel B and 370 µm in Steel C, as shown in Figures 4 (e and f). Tus, for a constant silicon content of 1.6%, increasing the aluminum content from 6% to 8.8% lead to a decrease in the depth of the aluminum nitride layer for all times and temperatures. However, for a constant aluminum content of 8.8%, increasing silicon from 1.1% to 1.6% showed only a slight decrease in the depth of the aluminum nitride layer. Te morphology of the aluminum
nitride appears similar between the respective steels, and it appears to precipitate and grow as plates along <111> crystallographic directions within the austenite. Te second- ary electron images of the aluminum nitride coating in Steel A (8.8% Al and 1.1% Si) and Steel B (8.8% Al and 1.6% Si) after nitriding for eight hours at 1,652F (900C) are shown in Figures 5 (a and b) and Figures 5 (c and d). Te case depth and plate-like structure of the aluminum nitride are similar between the two different silicon con- taining steels (Figure 5). Te secondary electron micro-
graphs of Steel B and Steel C are shown in Figure 6 after nitriding for eight hours at 2,012F (1,100C). In Steel B, the nitrided layer consists of a high density of longer and typically thinner plates with an average spacing of less than 5 µm (depending on the plane of polish) as shown in Figure 6 (a and b). In Steel C, the density of aluminum nitride in the reaction layer is much less, and the average spacing between the plates is greater than 10 µm as shown in Figure 6 (c and d). However, the plate thickness in Steel C is greater, and the case depth is almost 200 µm greater than in Steel
30 | MODERN CASTING June 2015 k, m/s
7.93E-13 2.00E-12 2.56E-12 6.75E-13 1.30E-12 2.16E-12 2.56E-12 3.29E-12 6.77E-12
DN, m2 /s
1.49E-11 3.76E-11 4.79E-11 1.27E-11 2.43E-11 4.04E-11 3.32E-11 4.26E-11 8.77E-11
79 78 64
B after nitriding for eight hours at 2,012F (1,100C). A less lamellar arrangement of aluminum nitride is observed in Steel C as shown in Figure 6 (d). Te average composition of the aluminum nitride
Q, kJ/mol
precipitates was found to be invariant with temperature and steel composition. Te stoichiometric ratio of aluminum to nitrogen was close to one for all individ- ual plates sampled. Te austenite matrix chemistry between the plates for Steel A and C after nitriding for six and eight hours at 1,832F (1,000C) and 2,012F (1,100C) was almost completely depleted of aluminum and enriched in manganese and silicon, and this may also improve wear resistance. For steels nitrided at times greater than six hours, an approxi- mately 10–20 µm complex oxide layer was noted on the surface above the nitride layer, as shown in Figure 5d.
a
b
c
d
e
f
Fig. 4. The optical micrographs of nitrided specimens from Steel B with 8.8% Al (a,c and e) and Steel C with 6% Al (b, d and f) show the acicular structure of the aluminum nitride layer. (a and b) After nitriding for two hours at 900C, the 6% Al specimen in (b) is shown to have an AlN case depth that is almost twice the case depth in the 8.8% Al specimen in (a). (c and d) At 900C, a case depth greater than 100 µm develops after six hours for both aluminum containing steels. (e and f) For steels nitrided for six hours, increasing the temperature to 1,000C increases the kinet- ics of the nitriding process and the depth of the AlN layer to over 300 µm.
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