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an air atmosphere. The heat was brought to 3000F (1650 C), and an oxygen activity measurement was made. Based on the oxygen level and a target residual aluminum content of 0.04%, the heat was deoxidized with aluminum shot. A chemical sample was then extracted for optical emission spectroscopy. Table 1 lists the average chemical composi- tion for each stainless steel composition. Five grams of the test powder were placed in the thermal analysis (TA) cup (See Figure 1). Test powders were added in the TA cup to ensure no cross contamination between powders would occur. The furnace was tapped into a 2.3 kg hand ladle. To maintain deoxidation, an additional 2.7g of aluminum wire were added per ladle. Liquid steel was poured into the TA cup and a data acquisition system (DAQ) recorded the cooling curve of the solidifying steel at a sampling rate of 12 Hz. Sampling ceased once the sample reached 2550F (1400C). Then, the TA cup was removed and the next cup and powder addition was inserted onto the TA stand. Ten TA cups were run per heat. The first and last cups contained no powder additions. This ensured that no significant alloy or deoxidation state changes that might occur while hold- ing the melt in the furnace caused a change in the solidi- fication behavior. When the undercooling measurements between the first and last TA cups differed drastically, the entire heat was repeated. The experimental powder addi- tions were then randomly ordered between the second and eighth TA cups. Each experimental powder addition was replicated three times. A total of fifteen TA cups were ana- lyzed for each alloy.

The TA cups for this experimental setup consisted of a shell core cup with an S Type (Pt/10%Rh) thermocouple in the bottom of the cup (See Figure 1 B). A quartz tube covered the thermocouple wire to protect it during fill- ing. The cups were placed on a metal stand that connected them to the computer based DAQ system via S Type com- pensation wire (See Figure 1A). These TA cups are com- mercially available.

Powders for these experiments were selected based on their ability to nucleate either delta ferrite or austenite. Thermo- dynamic calculations were used to predict the initial phase to form upon solidification. 304 was expected to initially form delta ferrite at the start of solidification while HK formed austenite as the primary phase. Crystallographic data from the Pauling File was utilized to select candidate phases that would nucleate each alloy.12

were selected for 304. La2 for HK. ZrO2

was also added for HK because some authors O3

theorized it acted as a heterogeneous nuclei for austenite.13,14 The author’s own calculations found poor crystallographic matching between ZrO2

purity, -325 mesh powders of each selected material were acquired for addition into the TA cups.

The reader may notice that MgO and NbO were included in the experimental plan for both steels despite the differ-

30 Figure 2. Crystal structure of NbO. International Journal of Metalcasting/Winter 2012 and austenite or delta ferrite. High

MgO, NbO, NiAl, and TiC , MgO, and NbO were chosen

ent primary forming phase. Reviewing work in the literature and closely examining the crystal structure of both materi- als, the author found research indicating both could nucleate either primary phase.13,14

Both materials have the rock salt

crystal structure (See Figure 2). The exterior planes of the cubic structure have similar atom placement and spacing to the face centered cubic (FCC) crystal structure of austen- ite. Close examination of the mid-plane within the crystal structure revealed a square arrangement of either Mg or Nb atoms (See Figure 2). The sides of this square are close to the 0.2932 nm lattice parameter of ferrite. It is possible that some particles of MgO or NbO could have this plane ex- posed at their surface, allowing them to act as heterogeneous nuclei for delta ferrite. For this reason, both were included in the 304 experimental runs.

The cooling curves were analyzed for the degree of under- cooling required to initiate solidification. It has been well documented that effective heterogeneous nuclei reduce the undercooling required to initiate solidification.4,6, 13, 15


reduction in undercooling is attributed to the decreased en- ergy barrier due to the presence of an existing liquid-solid interface.4,6

Bramfitt’s orginal work on heterogeneous nucle-

ation theory found that undercooling decreased as the lat- tice disregistry became smaller between the solid metal and nucleation phase.6

Determining the undercooling required

calculating the equilibrium liquidus temperature for each steel composition and then subtracting the liquidus tempera- ture from the cooling curves. This approach was necessary since the recalescence trough that is typically used in other alloy systems was not detectable. A lack of a recalescence trough has been commonly observed by the author in other work related to steel.16

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