Effect of sample geometry on regression rate of the melting interface for carbon steel burned in oxygen

Promoted-ignition testing on carbon steel rods of varying cross-sectional area and shape was performed in high pressure oxygen to assess the effect of sample geometry on the regression rate of the melting interface. Cylindrical and rectangular geometries and three different cross sections were tested and the regression rates of the cylinders were compared to the regression rates of the rectangular samples at test pressures around 6.9 MPa. Tests were recorded and video analysis used to determine the regression rate of the melting interface by a new method based on a drop cycle which was found to provide a good basis for statistical analysis and provide excellent agreement to the standard averaging methods used. Both geometries tested showed the typical trend of decreasing regression rate of the melting interface with increasing cross-sectional area; however, it was shown that the effect of geometry is more significant as the sample's cross sections become larger. Discussion is provided regarding the use of 3.2-mm square rods rather than 3.2-mm cylindrical rods within the standard ASTM test and any effect this may have on the observed regression rate of the melting interface.

Reflection of structural waves at a solid/liquid interface

This paper investigates the reflection characteristics of structural or guided waves in rods at a solid/liquid interface. Structural waves, whose wavelengths are much larger than the diameter of the rod, are described in a first approximation by classical one-dimensional wave theory. The reflection characteristics of such waves at a solid/liquid (melting) interface has been reported by two different ultrasonic measurement techniques: first, measuring the fast regression rate of a melting interface during the burning of metal rod samples in an oxygen-enriched environment, and second, monitoring the propagation of the solid/liquid interface during the slow melting and solidification of a rod sample in a furnace. The second work clearly shows that the major reflection occurs from the solid/liquid interface and not the liquid/gas interface as predicted by plane longitudinal wave reflectivity theory. The present work confirms this observation by reporting on the results of some specially designed experiments to identify the main interface of reflection for structural waves in rods. Hence, it helps in explaining the fundamental discrepancy between the reflection characteristics at a solid/liquid interface between low frequency structural waves and high frequency bulk waves, and confirms that the detected echo within a burning metallic rod clearly represents a reflection from the solid/liquid interface. (C) 2003 Elsevier Science B.V. All rights reserved.

Ultrasonic measurement technique of burning metals in normal and reduced gravity

The in situ real time measurement of the regression rate of a melting interface (RRMI) is performed by the ultrasonic measurement system reported here. The RRMI is the rate at which a solid/liquid interface (SLI) moves along a metallic rod while burning in an oxygen-enriched atmosphere and is an important flatnmability indicator. The ultrasonic transducer and associated equipment used to drive the transducer and record the echo signal is described, along with the process that transforms the acquired signals into a RRMI value. Test rods of various metals and geometric shapes were burned at several test conditions in different test facilities. The RRMI results with quantified errors are presented and reviewed. The effect of reduced gravity on burning metals is important to space-applications and RRMI results obtained in a reduced gravity environment are also presented.

Instantaneous RRMI of iron rods in reduced gravity

Standard upward-burning promoted ignition tests (“Standard Test Method for Determining the Combustion Behavior of Metallic Materials in Oxygen-Enriched Atmospheres,” ASTM G4-124 [1] or “Flammability, Odor, Offgassing, and Compatibility Requirements and Test Procedures for Materials in Environments that Support Combustion,” NASA-STD-6001, NASA Test 17 [2]) were performed on cylindrical iron (99.95% pure) rods in various oxygen purities (95.0–99.98%) in reduced gravity onboard NASA JSC's KC-135 to investigate the effect of gravity on the regression rate of the melting interface. Visual analysis of experiments agrees with previous published observations showing distinct motions of the molten mass attached to the solid rod during testing. Using an ultrasonic technique to record the real-time rod length, comparison of the instantaneous regression rate of the melting interface and visual recording shows a non-steady-state regression rate of the melting interface for the duration of a test. Precessional motion is associated with a higher regression rate of the melting interface than for test periods in which the molten mass does not show lateral motion. The transition between the two types of molten mass motion during a test was accompanied by a reduced regression rate of the melting interface, approximately 15–50% of the average regression rate of the melting interface for the entire test.

Method for determining reaction rate of mild steel containers during melting of magnesium-aluminium alloys and effect of aluminium content on directionally solidified microstructures

A magnesium alloy of eutectic composition (33 wt-'%Al) was directionally solidified in mild steel tubes at two growth rates, 32 and 580 mum s(-1,) in a temperature gradient between 10 and 20 K mm(-1). After directional solidification, the composition of each specimen varied dramatically, from 32'%Al in the region that had remained solid to 18%Al (32 mum s(-1) specimen) and 13%Al (580 mum s(-1) specimen) at the plane that had been quenched from the eutectic temperature. As the aluminium content decreased, the microstructure contained an increasing volume fraction of primary magnesium dendrites and the eutectic morphology gradually changed from lamellar to partially divorced. The reduction in aluminium content was caused by the growth of an Al-Fe phase ahead of the Mg-Al growth front. Most of the growth of the Al-Fe phase occurred during the remelting period before directional solidification. The thickness of the Al-Fe phase increased with increased temperature and time of contact with the molten Mg-Al alloy. (C) 2003 Maney Publishing.

Effect of solute on the growth rate and the constitutional undercooling ahead of the advancing interface during solidification of an alloy and the implications for nucleation

A framework is presented for modeling the nucleation in the constitutionally supercooled liquid ahead of the advancing solid/liquid interface. The effects of temperature gradient, imposed velocity, slope of liquidus, and initial concentration have been taken into account in this model by considering the effect of interface retardation, which is caused by solute buildup at the interface. Furthermore, the effect of solute concentration on the chemical driving force for nucleation has been considered in this model. The model is used for describing the nucleation of Al-Si and Al-Cu alloys. It was found that the solute of Si has a significant impact on the chemical driving force for nucleation in AI-Si alloys whereas Cu has almost no effect in Al-Cu alloys.