Table 3. Calculated rate constants, diffusion coefficients, and activation energies Steel
Steel A
µm and 230 µm for Steels B and C. Increasing the process temperature to 1,832F (1,000C) increased the kinetics 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%
Steel B Steel C
Temperature, C 900
1000 1100 900
1000 1100 900
1000 1100
to 1.6% showed only a slight de- crease in the depth of the aluminum nitride layer. Te morphology of the aluminum
nitride appears similar between the respective steels, and it appears to
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
Q, kJ/mol 79 78 64
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-
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. 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). At 900C, a case depth greater than 100 µm develops after six hours for both aluminum containing steels (c and d). For steels nitrided for six hours, increasing the temperature to 1,000C increases the kinetics of the nitriding process and the depth of the AlN layer to over 300 µm (e and f).
34 | METAL CASTING DESIGN & PURCHASING | Jul/Aug 2015
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 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 precipitates was found to be invariant with temperature and steel composition. Te stoichiometric ratio of aluminum to nitrogen was close to one for all individual plate samples. Te austenite matrix chemistry between the plates for Steels A and C after nitriding for six and eight hours at 1,832F (1,000C) and 2,012F (1,100C) were almost completely depleted of aluminum and enriched in manganese and silicon, which may
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62