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Fig. 7. This shows the as-cast microstructure 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.


a steel rolling mill was made using 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 eliminating 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 processing technologies considered, including machining from steel stock and creating a custom green sand casting followed by machining. Table 1 lists the as-cast chem-


istry used. Te microstructure is in Fig. 1 and 2 at 200X and 400X,


respectively. Te chemistry results are in the standard range. In addi- tion, 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 equipment. 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


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 %)


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 As Cast


Weight % 0.076 17.45 0.54 2.54 12.56 0.018 0.58 0.02


foam machining. Figure 4 shows the gear after shot blasting with S550 grit blast 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 stainless 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 compo- nent 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


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 %)


0.08 max 18.0-20.0 2.0 max -


8.0-10.5


0.045 max 1.0 max


0.03 max As Cast


Weight % 0.078 18.2 1.2


0.02 8.56


0.023 0.69 0.02


Nov/Dec 2016 | METAL CASTING DESIGN & PURCHASING | 43


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