Table 2. Chemistry Limits and As-Cast Chemistry Sample (for the Microstructure Shown in Figs. 7 and 8) for ASTM A743 grade CF8M Stainless Steel.
Element C
Cr
Mn Mo Ni P
Si S
Required Range (Weight %) As Cast Weight % 0.08 max
16.0-18.0 2.0 max 2.0-3.0
11.0-14.0 0.040 max 1.0 max 0.03 max
0.076 17.45 0.54 2.54
12.56 0.018 0.58 0.02
Table 3. Chemistry Limits and As-Cast Chemistry Sample (for the microstructure in Figs. 10 and 11) for the ASTM A743 grade CF8 Stainless Steel Casting
Element C
Cr
Mn Mo Ni P
Si S
Required Range (Weight %) As Cast Weight % 0.08 max
18.0-20.0 2.0 max -
8.0-10.5
0.045 max 1.0 max 0.03 max
0.078 18.2 1.2
0.02 8.56
0.023 0.69 0.02
SAE 1060 plain carbon steel. Te gear weighed 120 lbs. (54.4 kg) and was 15 in. (381 mm) in diameter and 4 in. (102 mm) thick. Tis gear was cast in lost foam primarily to reduce the lead- time, but it also benefited from elimi- nating most of the machining. In this case, the only machining required was to remove 0.001 in. (0.025 mm) from the center hole in to fit the gear onto the shaft. Te lead time was reduced by nine weeks, and the component was 20% cheaper than alternative process- ing technologies considered, includ- ing machining from steel stock and creating a custom green sand casting followed by machining. Table 1 lists the as-cast chemistry
used. Te microstructure is in Fig. 1 and 2 at 200X and 400X, respec- tively. Te chemistry results are in the standard range. In addition, the microstructure is uniform throughout and appears standard for SAE 1060 steel. Tests for carbon were performed on samples from the surface, 0.125 in. (0.635 mm) from the surface and from the interior. Te carbon values from the surface to the interior were 0.63%, 0.61%, and 0.62%, which is within the expected error of the testing equip- ment. Tere is no evidence of areas of higher than average carbon from the surface to the middle of the casting. Figure 3 shows the foam for the SAE 1060 gear. No die was used to manufacture the foam because this was a replacement part. Rather, the foam was itself machined from stock foam, which means that the surface finish depended on the quality of the foam machining. Figure 4 shows the gear after shot blasting with S550 grit blast
40 | MODERN CASTING October 2016
Fig. 7. This shows the as-cast microstruc- ture of ASTM A753 grade CF8M stainless steel cast via lost foam with Oxalic acid etchant at 200X.
Fig. 8. This shows ASTM A743 grade CF8M stainless steel cast microstructure with Oxalic acid etchant at 400X.
media. Te gear teeth show the as-cast surface quality of the teeth matches the surface roughness of the foam pattern. ASTM A743 grade CF8M stain- less steel (316 SS) also has been cast successfully using the lost foam casting method. In one example, the component was designed for use in a corrosive environment to control fluid flow. Originally, this component was gray iron and lasted in situ 1 hour in an acid environment. Te part was converted from gray iron to CF8M stainless (316 SS) to improve the component life. Only 0.04 in. (1.02 mm) was machined on the face of the part in order to produce a seal, and the part lasted more than four months. Tis component was cast via lost foam because most other metalcasting pro- cesses would not be able to obtain the necessary part geometry at the price required. In addition, the elimination of most machining is important as CF8M stainless (316 SS) is somewhat difficult to machine. Figure 5 shows the foam pattern
and Fig. 6 shows the as-cast compo- nent after shot blasting. In this case, the foam was “blown” in a process similar to injection molding. Again, the surface finish matches that of the foam and depends on the quality of the foam beading. Figures 7 and 8 show the resulting
Fig. 9. The surface finish of the ASTM A743 grade CF8M stainless steel casting matched the roughness of the foam pattern.
microstructure of the ASTM A743 grade CF8M (316 SS) component at 200X and 400X. Again, this as-cast microstructure appears consistent throughout and typical of this grade of steel. Table 2 lists the chemistry results that are also in the standard range for CF8M stainless steel (316 SS).
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