ing a 500 lb (225 kg) high frequency induction furnace and deoxidizes with aluminum. Samples 2, 3, and 4 were cast parts from a resin bonded sand foundry which melts with a 2,000 lb (900 kg) high frequency induction furnace. This foundry deoxidizes with ferrotitanium. A cover made from 1030 steel was Sample 2. Sample 3 was an outrigger com- ponent also manufactured from 1030 steel. A small hinge poured from 4140 steel was selected for Sample 4. The last foundry supplying samples produces large (over 14 ton or 12,500 kg) steel castings by resin bonded sand molds and an electric arc furnace. For this foundry, a sample was acquired from the steel stream during tapping. Aluminum is added to deoxidize the melt. Therefore, Samples 5 and 6 were a 1 inch (2.5 cm) diameter, 6 inch (15 cm) long bar. Sample 5 was a slightly modified 1050 steel. An ASTM A148 Grade 105- 85 steel made Sample 6. Inert cover atmospheres were not used by any of the foundries during melting or pouring. The chemistries for all the samples are listed in Table 1.
All of the castings were sectioned into several metallurgical mounts for analysis. Each was ground using 240, 320, 400, and 600 grit SiC paper. The samples were then polished with 6 µm and 1 µm diamond paste. Final polish consisted of polishing with 0.05 µm alumina. To assist with locating possible nucleation sites, the samples were etched by immers- ing the sample in a saturated aqueous picric acid solution for five minutes to reveal the prior austenite boundaries.14 microscopy was conducted to examine the as-cast structure.
Optical
After carbon coating, a scanning electron microscope (SEM) with an energy dispersive spectrometer (EDS) was employed to look for inclusions within the prior austenite structure and determine their chemical analysis. During the SEM investiga- tion, an accelerating voltage of 20 kV was employed.
Results Optical Microscopy
For most of the samples, the etching successfully revealed the prior austenite boundaries. One drawback to saturated aqueous picric acid was that it did not always reveal the prior austenite boundaries.14,15
The etched structure of Sample 1 only reveals the room tem- perature structure of proeutectoid ferrite, acicular ferrite, and small amounts of pearlite (See Figure 1). No evidence of the as-cast austenite dendrite structure was observed. The gating system for this sample resides at the thermal center of the cast- ing. Therefore, the cooling rate was relatively slow. As Sam- ple 1 cooled, the austenite dendrites would have grown into larger equiaxed grains. The shape and size of these equiaxed grains can be seen by the distribution of proeutectoid ferrite. Upon further cooling, the proeutectoid ferrite formed at the austenite grain boundaries; the boundaries act as nucleation points for ferrite formation.14
The slow cooling rate also pro- vided time for any carbon segregation to equalize throughout Table 1. Chemistries for Each Sample Examined
Figure 1. Etched microstructure of Sample 1. International Journal of Metalcasting/Summer 10
Figure 2. Micrograph of structure from Sample 2. 19
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