ADDING IT ALL UP 2 Procedure
Te test casting was designed and produced based on several criteria: • Mold filling must be
quiescent, and the velocity at the gate should not be higher than 5 cm/second (2 in./second).
• The cooling rate must be compa- rable to industrial applications. The target was 34.7F (1.5C) per second. In addition, the cooling rate in the test casting should be uniform.
• The test casting must be free of shrinkage porosity or any other internal defects. Te test casting design was opti-
mized using a commercial casting simulation software package. Te final design is shown in Fig. 1. Two 120-lb. heats of CGI (with
a carbon equivalent of 4.3-4.4%) were poured. Te sand used was silica sand with 55-grain fineness number. Sodium silicate was used as a binder. Te liquid iron was treated at 2,642F (1,450C) in the reaction chamber. Te four-point bending test pro- vided better stress distribution in the gage area. Tis setup generally deliv- ers more consistency than three-point bending, which has stress concen- trated on a contact point. In addition, the probability of picking up the surface defect of the four-point bend- ing is higher due to the larger gage area. Terefore, the test was expected to be able to monitor the presence of surface defects. Both as-cast and machined sam-
ples were tested. Machined samples were prepared on all six surfaces and polished to minimize the effect of surface roughness. Te as-cast sam- ples were machined and polished on three sides in the same manner, but the lower side was left as-cast during testing. Fig. 2 shows the location of the machined and as-cast samples in the test plates. Te machined samples were located approximately 0.08 in. (2 mm) from the casting surface. Te roughness average was measured in the gage area for both as-cast and machined samples. During the fatigue testing, the samples were subjected to bending stresses that caused tensile stress on
March 2012 MODERN CASTING | 47
“On the Effect of the Casting Skin on the Fatigue Properties of CGI” S. Boonmee, M.K. Moran, D.M. Stefanescu, The Ohio State Univ., Columbus, Ohio
1 2 3
Background—Casting skin has been shown to have a negative effect on iron castings. However, little work has been done to explore the effect on fatigue properties. Procedure—The study compared the results of fatigue testing on as-cast samples with the casting skin left on with those of samples machined on all sides. Results and Conclusions—The machined samples exhibited better fatigue performance, highlighting a direct relationship between greater amounts of casting skin and reduced fatigue strength.
the bottom face and compressive stress on the top face. Te test- ing mode was force-controlled. Te alternating stress was varied to find the fatigue life, while the mean stress was kept constant. Te samples were left running until frac- ture occurred, or until 5 x 106
cycles
were completed. Te skin thickness of the as-cast samples was evaluated using image analysis and optical microscopy.
Fig. 1. The test casting design was opti- mized with casting simulation software.
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