Robert B. Tuttle

Feasibility Study of 316L Stainless Steel for the Ultrasonic Consolidation Process

The viability of using 316L stainless steel in the ultrasonic consolidation process was examined in this work. Ultrasonic consolidation is an additive, free-form manufacturing process that employs ultrasonic welding and machining to form a part. The process ultrasonically joins layers of metal together by welding them one at a time. Once four layers of metal foil are welded together, welding is suspended and the system machines the part outline, and repeats this cycle until a component is completed. Experiments were conducted to determine the feasibility and processing parameters for ultrasonically welding stainless steel. Mechanical testing and optical microscopy were conducted. 316L stainless steel was successfully welded. Increasing welding amplitude and decreasing welding speed were the most effective way to increase weld peel strength. Unlike work in aluminum alloys, these experiments found no relationship between horn force and peel strength. Rough processing windows for ultrasonically welding 316L were identified.

Effect of Rare Earth Additions on Grain Refinement of Plain Carbon Steels

This paper describes a set of experiments with misch metal and rare earth silicide additions in 1010 and 1030 steel to determine whether grain refinement occurs. Target rare earth (RE) contents of 0.1 and 0.2% were employed. After melting, the desired RE addition was added during tapping and then poured into green sand molds. The resulting test plates were then sectioned for tensile and metallographic testing. Yield strength increased for several of the 1010 and 1030 samples. The increase in yield strength correlated with a reduction in grain size. A dramatic increase in percent elongation was also observed in the 1010 sample with the smallest grain size. Electron microscopy found complex RE oxides. These oxides appear to act as heterogeneous nuclei. When they were coated with another slag, the grain size and mechanical properties were similar to the baseline material in both the 1010 and 1030.

Examination of Steel Castings for Potential Nucleation Phases

The production of steel castings needs to meet more stringent requirements on mechanical properties and cost. One possibility to improve mechanical properties is to develop grain refiners for steel castings. There are no industrially significant grain refiners for steels. While developing grain refiners based on theoretical models could assist development, there is likely already materials that function in current castings as nuclei for austenite dendrites. It was found that TiN inclusions may have assisted in the nucleation of dendrites during the solidification of several industrial castings.

Understanding the Mechanism of Grain Refinement in Plain Carbon Steels

This paper documents a series of plate casting experiments in 1010 steel. A set of castings with various levels of rare earth (RE) silicide and engineered grain refiner were produced via induction melting and green sand molds. One set of castings also contained a high phosphorous content to enable etching for the solidification structure. Metallographic and tensile specimens were examined to determine the resulting microstructure and property changes. Yield strength, ultimate tensile strength and percent elongation significantly increased with the RE additions. This increase appeared to be primarily a function of the cerium content of the final castings. Examination of the phosphorous containing castings revealed the solidification morphology changed from a columnar to equiaxed type. Electron microscopy revealed RE oxides formed in both sets of castings. The strength and solidification morphology changes were likely caused by the nucleation of δ-ferrite on these RE oxides.

Experimental Grain Refiners for Carbon Steels

A series of master alloys designed to make rare earth oxide, rare earth aluminum oxide, or rare earth sulfide inclusions were created to determine which inclusions acted as heterogeneous nuclei. Plate test castings having additions of the various master alloys were produced to understand the refinement produced and the mechanical properties. The plates underwent tensile testing and metallographic analysis. For comparison, no addition and rare earth silicide addition heats were also poured. The rare earth oxide and sulfide master alloys formed the intended inclusions. The rare earth aluminum oxide master alloy had very poor homogeneity and contained inclusions with far lower aluminum contents than intended. Only the rare earth silicide reduced the grain size, resulting in improved mechanical properties. This would indicate that simple oxides or sulfides do not play a significant role in the nucleation in steel, but the more complex oxides and probably oxysulfides play the dominate roles.

Characterization of Rare Earth Inclusions from Cast 1010 Steel

An electron microscopy investigation was conducted on two cast 1010 steel plate samples that had rare earth silicide additions. A scanning electron microscope (SEM) and transmission electron microscope (TEM) with energy dispersive spectrometer (EDS) characterized the inclusions found. The SEM primarily observed rare earth oxide particles. No particles less than 1 μm in diameter were found in the TEM which indicates that any possible heterogeneous nuclei are only about 1 μm in size. Electron diffraction and EDS spectra from the rare earth inclusions examined indicated that they were comprised of multiple rare earth phases. The matrix and one rare earth inclusion with a face centered cubic (FCC) like crystal structure had an eight degree alignment between crystallographic axes. This suggested that the body centered cubic (BCC) structure of ferrite and some of the FCC like rare earth compounds could align with each other. Such alignment would be necessary for any particle to act as a heterogeneous nucleus of δ-ferrite.

Thermal Analysis Study of Heterogeneous Nuclei in Stainless Steels

This paper presents work done to determine effective heterogeneous nuclei for 304 and HK stainless steels. Heats of each steel were melted in an induction furnace and then poured into a thermal analysis cup which contained the experimental powder. MgO, NbO, NiAl, and TiN powders were added to 304. Powders of ZrO2, La2O3, MgO, and NbO were introduced into HK. A data acquisition system recorded the cooling curves of the solidifying alloys. Data from these curves was then used to calculate the amount of undercooling to initiate solidification. MgO, NbO, NiAl, and TiN reduced the undercooling in 304 while La2O3, MgO, and NbO reduced undercooling in HK. The secondary dendrite arm spacing of the 304 samples were not affected by the addition of the heterogeneous nuclei. It is postulated that this was caused by the high cooling rate of the thermal analysis cup during solidification.