Simple Basic Option Curves
⦁ Temperature Curve is the basic data gathered by the thermocouple.
⦁ Cooling Rate Curve is the instantaneous change in temperature. For mathematicians, this is the first derivative inverted. This visual occupies the lower 25% of the curve window.
⦁ The Lard Cooling option gives the Cooling Rate curve 50% of the curve window.
⦁ For maximum size of the Cooling rate curve, simply click off the Temperature curve and the Cooling rate will occupy the entire window.
These options suggest that the Cooling Rate curve is very important, and it is. Details of arrests are much clearer in the Rate of Cooling due to magnification. The Temperature curve may span 300 degrees while the rate of cooling spans only 10 degrees – a magnification of 30x.
Information and Variables of Simple Basic Microanalysis
⦁ Date is the day of the sample according to the computer’s clock that recorded it.
⦁ Time is the start time of the sample when the computer senses hot metal in the cup.
⦁ C.E. or Carbon Equivalent is determined by the temperature of the liquidus as it relates to the iron-carbon phase diagram. It may not agree with a chemistry-based calculation from a spectrometer, but it is the correct C.E. Chemistry based equations try to imitate Thermal C.E. and are not always correct.
⦁ Maximum Temperature is the highest temperature the thermocouple saw. It is usually 200 to 300 degrees F below the furnace temperature due to heat losses in sampling and the cup cooling the sample. The thermocouple is made of two nickel-based alloys. The lowest one melts at 2540 degrees F. Try to keep the maximum temperature below 2500 and at least 50 degrees above liquidus. With care, even malleable iron, which has a high liquidus, can be sampled.
⦁ Superheat is the Maximum Temperature minus the Liquidus temperature. It is best to have about 50 degrees superheat if that can be managed, but 25 degrees will work if necessary. Higher superheat will reveal oxide arrests as well as giving a good profile of the liquidus. A superheat close to zero may slightly underestimate the liquidus as it only captures the tail end of the arrest. Sampling too cold may even cause the system to miss the liquidus altogether.
⦁ Liquidus is the strongest point of heat generation in the liquidus zone. This is the lowest point of the rate of cooling. The C.E. test is based on the temperature of this strong point. If the arrest is particularly weak, the liquidus detection value may need to be increased. Current research indicates that trace amounts of titanium in the metal can weaken this arrest. This often occurs when the steel content of the charge is increased, or the foundry finds a new supply of steel scrap.
⦁ Eutectic is the larger (longer) arrest and will usually have undercooling and recalescence. These tests are offered in higher levels of MeltLab. For use at this level, the eutectic temperature is the strongest or highest temperature that occurs during recalescence. This has been referred to as the “Growth Temperature” of the Eutectic.
⦁ Solidus or End of Freezing is the strong point in the Solidus zone. This zone is where the grain boundaries turn solid. These grain boundaries are loosely and randomly packed and so the zone is heat absorbing rather than heat producing. Low melting tramp elements are pushed into these areas between the grains in a process called micro-segregation.
While this level provides only the bare essentials in understanding the material, these values are used in the higher levels to provide much more information on inoculation, physical properties, casting gas and shrinkage, and even the relative ability for the castings to be heat-treated.
Click HERE to return to Gray Microstructure