Experimental Investigation of Aluminum Nitride and Vanadium Nitride Solubility in Medium-Carbon SteelsWilliam Christman, Lee M. Rothleutner, Dr. Chester J. Van Tyne

Background Small additions of alloying elements such as vanadium, niobium, titanium, and aluminum are often included as "microalloyed" additions to steels in order to improve mechanical properties. At high reheat temperatures (above 1000 ) vanadium precipitates will dissolve and ℃aluminum precipitates can form in the austenite [1]. Vanadium precipitates (vanadium carbonitrides) contribute to precipitation strengthening while aluminum deoxidizes the steel and aluminum precipitates (aluminum nitrides) significantly reduce grain size through grain boundary pinning [2]. Vanadium and aluminum can precipitate simultaneously so competition can occur for the limited free nitrogen in the system although the exact process is not completely understood [1]. Solubility products have been reported for aluminum nitride and vanadium carbonitride precipitates in low carbon steels [3]. There has not been many investigations on the solubility products of higher carbon steels. Determining the solubility products for higher carbon steels presents an opportunity to examine how higher carbon content influences nitride formation and the competition for nitrogen between aluminum and vanadium in austenite.

Electrochemical Precipitate Extraction Prior to the electrochemical dissolution all of the samples were quartz encapsulated and heat treated for 96 hours at one of three temperatures (1150 , ℃1100 , and 1050 ). The samples were water quenched immediately after the heat treatment. ℃ ℃ The electrochemical cell consists of a working and counter electrode that are immersed in a non-aqueous solution. The working electrode is the heat treated sample while the counter electrode is a platinum mesh. Inert argon gas is bubbled through the solution to deoxidize the solution and prevent hydrogen build up inside the cell. A direct current power source is used so a reference electrode is not needed.

Filtration and DigestionThe electrolyte and precipitate is vacuum siphoned through a 10 nm pore size poly carbonate filter. The filtered electrolyte is acidified and diluted before being sent for ICP-MS analysis. The filter with the precipitate is digested in a four step process. •2 hours reflux in 10 mL nitric acid •15 mL of hydrogen peroxide is added at temperature and allowed to evaporate •2 hours reflux in 15 mL sulfuric acid •25 ml of hydrogen peroxide is added at temperature and allowed to evaporateThe product of the digestion is diluted before being sent for ICP-MS analysis.

References1. L. M. Rothleutner., Influence of Reheat

Temperature and Holding Time on the Interaction of V, Al, and N in Air-Cooled Forging Steels, Golden, CO: Advanced Steel Processing and Products Research Center, 2012.

2. M. D. Head, Ed., Bar Steels: Steel Products Manuel, Warrendale, PA: Association for Iron and Steel Technology, 2010.

3. T. Gladman, The Physical Metallurgy of Microalloyed Steels. Institute of Materials, 1997.

AcknowledgementsI would like to thank Lee Rothleutner and Dr. Chester Van Tyne for their help and support during the course of this project.

This material is based upon work supported by the National Science Foundation and the Air Force Office of Scientific Research under Grant No. DMR-1062797

Summary

This project is currently still on-going. After ICP-MS analysis we will determine the percentage of aluminum and vanadium in the system that formed precipitates. With these data it is possible to determine the equilibrium solubility products of the precipitates and discern the effect of higher carbon content on the solubility of aluminum and vanadium nitrides. These results should lead to a better understanding of the competition between aluminum nitride and vanadium nitride in austenite.

Figure 1: The disassembled electrochemical cell. Figure 2: The assembled electrochemical cell.

Figure 3: The filtration apparatus.