Cooling curve: a melter's staple.

"Back when I was running a 96-in. cupola, we pulled a cooling curve test about every half hour and ran a chill test at least once an hour. It was important because it gave us a running fix on the microstructure of our gray iron and gave us a feel for where the iron might be trending," said a former melting supervisor in a high-production gray iron foundry.

"If the iron was getting too hard," he added, "we could keep production going by changing the parts being cast to other castings requiring a harder iron until we could bring the first iron up to where it should be."

The importance of the cooling curve, or thermal analysis, of cast iron hasn't changed.

The kind of iron formed during solidification depends on the composition of the iron, the presence of nucleating agents and how rapidly the metal solidifies. Higher carbon and silicon contents and the presence of other graphitization elements favor the formation of graphite. Lower carbon and silicon contents and the presence of carbide stabilizing alloys limit the formation of graphite, causing the carbon to remain combined with the iron as iron carbide and to solidify as white iron.

The function of the cooling curve is to follow the course of solidification as a means to control the desired microstructures of cast iron. Most cast iron is specified by physical properties, such as tensile and yield strengths and hardness, each directly related to microstructure. Microstructure depends on the metal's consistent graphite structure and the formation of fine pearlite for uniform strength and hardness. The elimination of chill, carbides and free ferrite is the function of cooling curve control.

Thermal Analysis

The thermal arrest periods are measured by time and temperature on a curve described by the cooling characteristics of molten metal. These points, the liquidus, solidus and eutectic phases of solidification (shown in Fig. 1) can be correlated with the desired physical properties of the molten metal.

When the liquid iron passes through the liquidus phase, austenite begins to form. The lower the carbon equivalent, the higher the temperature at which solidification begins and the more austenite forms before the eutectic solidification starts. Because latent heat (heat that must be lost before solidification is completed) evolves during solidification, the cooling rate of the metal in a mold slows when solidification starts, marking the beginning of the time/temperature range of solidification.

As the metal passes through the austenite-graphite eutectic at the solidus temperature, the remaining liquid changes to austenite and graphite releases carbon, again causing a temperature "bounce" of the molten iron that briefly reverses the heat loss of the molten metal.

Solidification is complete when the temperature again decreases, dropping the curve below the eutectic temperature.

There are two modes of eutectic solidification possible for cast irons: the iron/graphite eutectic and the iron/carbide (cementite) eutectic reactions. The eutectic temperature for the iron/graphite reaction is higher than that of the iron/carbide reaction for gray iron chemistries.

Graphite nucleation requires undercooling below the graphite eutectic temperature. The more desirable graphite structure (Type A) is normally obtained in irons having a high state of nucleation where the eutectic reaction initiates and grows at a temperature near the equilibrium eutectic temperature.

However, should undercooling go below the carbide eutectic temperature, austenite plus carbide will result. Therefore, the ideal solidification objective for gray iron lies between the graphite and carbide eutectic temperatures.

Testing for the Cooling

Curve

Because the temperature range of solidification can be very accurately measured, modem instrumentation has made thermal analysis (determining the carbon and silicon contents of the metal at the furnace) a routine and accurate test procedure.

A small, hard sand cup - lined with tellurium or containing a tellurium button in the bottom and fitted with a thermocouple - is filled with molten iron. An instrument immediately records the changes in thermocouple potential with elapsed time and, moments after the sample has solidified, it indicates the carbon eqivalent, carbon and silicon contents and the liquidous and solidus temperatures.