D Basak

Measurement of Specific Heat Capacity and Electrical Resistivity of Industrial Alloys Using Pulse Heating Techniques

The determination of the specific heat capacity and electrical resistivity of Inconel 718, Ti-6Al-4V, and CF8M stainless steel, from room temperature to near the melting temperatures of the alloys, is described. The method is based on rapid resistive self-heating of a solid cylindrical specimen by the passage of a short-duration electric current pulse through it while simultaneously measuring the pertinent experimental quantities (i.e., voltage drop, current, and specimen temperature). From room temperature to about 1300 K, the properties are measured using an intermediate-temperature pulse-heating system by supplying a constant current from a programmable power supply and measuring the temperature using a Pt-Pt:13% Rh thermocouple welded to the surface of the specimen. From 1350 K to near the melting temperatures of the alloys, the properties are measured using a millisecond-resolution high-temperature pulse-heating system by supplying the current from a set of batteries controlled by a fast-response switching system and measuring the temperature using a high-speed pyrometer in conjunction with an ellipsometer, which is used to measure the corresponding spectral emissivity. The present study extends the application of these techniques, previously applied only to pure metals, to industrial alloys.

Hemispherical total emissivity of niobium, molybdenum, and tungsten at high temperatures using a combined transient and brief steady-state technique

The hemispherical total emissivity of three refractory metals, niobium, molybdenum, and tungsten, was measured with a new method using a combined transient and brief steady-state technique. The technique is based on rapid resistive self-heating of a solid cylindrical specimen in vacuum up to a preset high temperature in a short time (about 200 ms) and then keeping the specimen at that temperature under steady-state conditions for a brief period (about 500ms) before switching off the current through the specimen. Hemispherical total emissivity is determined at the temperature plateau from the data on current through the specimen, the voltage drop across the middle portion of the specimen, and the specimen temperature using the steady-state heat balance equation based on the Stefan–Boltzmann law. Temperature of the specimen is determined from the measured surface radiance temperature and the normal spectral emissivity; the latter is obtained from laser polarimetric measurements. Experimental results on the hemispherical total emissivity of niobium (2000 to 2600 K), molybdenum (2000 to 2700 K), and tungsten (2000 to 3400 K) are reported.

Calibration of a two-color imaging pyrometer and its use for particle measurements in controlled air plasma spray experiments

Advances in digital imaging technology have enabled the development of sensors that can measure the temperature and velocity of individual thermal spray particles over a large volume of the spray plume simultaneously using imaging pyrometry (IP) and particle streak velocimetry (PSV). This paper describes calibration, uncertainty analysis, and particle measurements with a commercial IP-PSV particle sensor designed for measuring particles in an air plasma spray (APS) process. Yttria-stabilized zirconia (YSZ) and molybdenum powders were sprayed in the experiments. An energy balance model of the spray torch was used to manipulate the average particle velocity and temperature in desired ways to test the response of the sensor to changes in the spray characteristics. Time-resolved particle data were obtained by averaging particle streaks in each successive image acquired by the sensor. Frame average particle velocity and temperature were found to fluctuate by 10% during 6 s acquisition periods. These fluctuations, caused by some combination of arc instability, turbulence, and unsteady powder feeding, contribute substantially to the overall particle variability in the spray plume.

DYNAMIC PROPERTIES FOR MODELING AND SIMULATION OF MACHINING: EFFECT OF PEARLITE TO AUSTENITE PHASE TRANSITION ON FLOW STRESS IN AISI 1075 STEEL

The Pulse-Heated Kolsky Bar Laboratory at the National Institute of Standards and Technology (NIST) has been developed for the measurement of dynamic properties of metals. With this system, a small sample can be pre-heated from room temperature to several hundred degrees C in less than a second, prior to rapid loading in compression at strain rates up to the order of 104 per second. A major focus of this research program has been on investigating the influence of the heating rate and time at temperature on the flow stress of carbon steels, for application to the modeling and simulation of high-speed machining operations. The unique pulse heating capability of the NIST Kolsky bar system enables flow stress measurements to be obtained under conditions that differ significantly from those in which the test specimens have been pre-heated to a high temperature more slowly, because there is less time for thermally activated microstructural processes such as dislocation annealing, grain growth, and solid state phase transformations to take place. New experimental results are presented on AISI 1075 pearlitic steel samples that were pulse-heated up to and beyond the austenite formation temperature of the material (723 °C). The data show that the flow stress decreased by about 50 % due to a phase transformation in the microstructure of the material from the stronger pearlitic phase to the weaker austenitic phase. As a result, the constitutive response behavior of the material cannot be modeled by a fixed-parameter constitutive model, like the Johnson-Cook flow stress model that is widely used in computer simulations of high-speed machining processes.

Application of laser polarimetry to the measurement of specific heat capacity and enthalpy of the alloy 53Nb-47Ti (mass%) in the temperature range 1600 to 2000 K by a millisecond-resolution pulse heating technique

The determination of the specific heat capacity, enthalpy, and electrical resistivity of the alloy 53Nb–47Ti (mass%) in the temperature range 1600 to 2000 K is described. The method is based on rapid resistive self-heating of a solid cylindrical specimen from room temperature to the maximum temperature of interest by the passage of a subsecond-duration electric current pulse through the specimen and on simultaneously measuring the pertinent experimental quantities. The experimental quantities measured are the current through the specimen, voltage drop across the effective specimen, specimen radiance temperature at two wavelengths, and normal spectral emissivity of the specimen. The present study extends this technique, previously applied to pure metals, to the determination of specific heat capacity and enthalpy of the alloy, 53Nb–47Ti. The measured properties were compared to those calculated from a thermodynamic description of the Nb–Ti system.

Microsecond Time‐Resolved Pyrometry during Rapid Resistive Heating of Samples in a Kolsky Bar Apparatus

Analysis of machining processes is important in the understanding and improving of manufacturing methods. The modeling of machining processes relies on high‐strain rate, high‐temperature material properties. A split‐Hopkinson pressure bar (or Kolsky bar) is being installed in a NIST high‐current pulse‐heating facility. By heating the material sample rapidly with a controlled current pulse immediately before the mechanical impact in the bar, structural changes in the sample are inhibited, thus better simulating conditions during machining. A stress‐strain relationship can be determined at various temperatures for test materials. We describe the design and the development of a millisecond‐resolution split‐Hopkinson apparatus, where the sample is resistively heated by the passage of a sub‐second‐duration electric current pulse. The impact bar is constructed out of maraging steel and the sample is a cylinder of AISI 1045 steel. The current is transmitted through the oiled‐bronze sleeve bushing of the impact b...

Thermal imaging of metals in a Kolsky-bar apparatus

Since the modeling of machining processes relies on high-strain-rate, high-temperature material properties, NIST has built a split-Hopkinson (or Kolsky) bar to determine the stress-strain behavior of rapidly heated materials at high temperatures. Our Kolsky bar has been constructed in the NIST high current pulse-heating facility, which enables electrically heating the samples within ~ 100 milliseconds time duration, immediately before the mechanical impact in the bar. Due to the rapid heating, we avoid possible structural changes in the sample, and a stress-strain relationship can be determined at different temperatures for various test materials. We describe the design and the development of the resistively-heated Kolsky-bar apparatus. The incident and the transmitted bars are constructed of 1.5 m long, 15 mm diameter maraging steel, and a typical sample is a 4 mm-diameter, 2 mm-long cylinder of 1045 steel. The sample is placed between the bars and held by friction. The current is transmitted through the graphite-sleeve bushings of the two bars. The non-contact temperatures are measured using an InGaAs near-infrared micro-pyrometer (NIMPY) and an InSb focal-plane (320 by 256) array (thermal camera). The NIMPY and the thermal camera are both calibrated using a variable-temperature blackbody, and the thermodynamic temperature of the metal is determined using the emissivity determined from the measured infrared spectral reflectance of the metal. Thermal videos of the electrically-heated and the room-temperature impacts will be shown with 1 kHz frame rates, and the changes in the stress-strain curves with the temperature of the samples will be discussed.

Moving the pulsed heating technique beyond monolithic specimens : experiments with coated wires

Note: Aug 2001 Reference LSMX-ARTICLE-2001-007View record in Web of Science Record created on 2005-11-22, modified on 2017-05-10

Effect on Flow Stress of a Rapid Phase Transition in AISI 1045 Steel

New experimental data on AISI 1045 steel from the NIST pulse-heated Kolsky Bar Laboratory are presented. The material is shown to exhibit a nonequilibrium phase transformation at high strain rate. An interesting feature of these data is that the material has a stiffer response to compressive loading when it has been preheated to a testing temperature that is below the eutectoid temperature using pulse-heating than it does when it has been preheated using a slower heating method. On the other hand, when the material has been pulse-heated to a temperature that exceeds the eutectoid temperature prior to compressive loading on the Kolsky bar, it is shown to exhibit a significant loss of strength. A consequence of this behavior is that fixed-parameter constitutive models, such as the well-known Johnson-Cook model, cannot be used to describe this constitutive response behavior. An argument is made that the phase transition does not occur during high-speed machining operations, and suggestions are made as to how to modify the Johnson-Cook model of Jaspers and Dauzenberg for this material in order to obtain improved temperature predictions in finite-element simulations of high-speed machining processes.Copyright

Application of High-Speed Laser Polarimetry to Noncontact Detection of Phase Transformations in Metals and Alloys at High Temperatures

A high-speed laser polarimetry technique, developed recently for the measurement of normal spectral emissivity of materials at high temperatures, was used to detect solid–solid and solid–liquid phase transformations in metals and alloys in millisecond-resolution pulse-heating experiments. Experiments were performed where normal spectral emissivity at 633 nm was measured simultaneously with surface radiance temperature, resistance, and/or voltage drop across the specimen. It was observed that a phase transformation, as indicated either by an arrest in the specimen radiance temperature or changes in the resistance and/or voltage drop, generally caused a change in normal spectral emissivity. Experiments were conducted on cobalt, iron, hafnium, titanium, and zirconium to detect solid–solid phase transformations. Similar experiments were also performed on niobium, titanium, and the alloy 85titanium–15molybdenum (mass%) to detect solid–liquid phase transformations (melting).