Thermal Analysis: A Helpful Tool For Foundries
Typically, based on the availability of charging materials in their geographic location and on their individual practices, gray cast-iron foundries melt a charge made up of some pig iron, scrap, in-house rejects, etc., and expect that the metal that comes out from the furnace will meet their chemical and metallurgical specifications. This has been the practice the world over, with some sophistication in terms of input materials and knowing their chemistry to help in calculation of inputs.
Traditionally, the general tendency has been to believe that chemistry of the melted metal also will meet the metallurgical requirements of the castings. Though chemistry does influence the metallurgical parameters, chemistry is not metallurgy, and tools other than a spectrometer are needed to get that information.
Unlike in a machine shop, where a part being machined can be checked for the dimensions during the process and corrected until the required results (dimensions) are obtained, in an iron foundry, metal derives its properties during cooling, and measuring and making changes during this period is not possible. Thus, once the metal is poured into the mould, the die is cast, literally! The metallurgical properties are discovered when the cast tool is sent on for machining. If it is found hard to machine, what has probably happened is that the casting has grain boundary carbides (which are impossible to see under the microscope or to measure using a hardness tester). Or if the casting is found porous upon checking for water-tightness, it means the casting has microporosity.
It is possible to overcome these problems using thermal analysis techniques to determine the basic chemistry and the major metallurgical parameters of the metal in the melting furnace before pouring, and corrections can be made to the metal in the furnace before pouring.
Thermal analysis principles
A sample of molten metal is taken from a furnace and poured into a sand cup having a thermocouple, connected to a computer through an analog/digital converter. The computer software program reads the temperature in real time and plots a cooling curve on the monitor. The software also studies the rate of cooling in C per second (which is the second derivative). Once it finds that the rate of cooling at any particular time is zero, the system understands that condition to be an "arrest" point. The first arrest is the liquidus and the next is the solidus, and so on. From the second derivative, the software calculates the third and fourth derivatives and displays the various findings on the computer monitor. The entire procedure happens very fast. All the details are available within about three minutes from the sampling time.
The melter wants to know two things when the metal is molten in the furnace: (1) Does the metal meet the required specification, and can it be poured? (2) If it does not meet the specification, what needs to be done to correct it before pouring?
The faster the answer is available, the quicker will be the melt correction, which would help increase the number of melts in a given shift.
The answers to the questions posed above can be obtained using thermal analysis software systems now available for foundries. Once such system is MeltLab, a software program available in different versions. The basic MeltLab helps in checking and correcting the basic chemistry. The system uses one sand cup, with a small amount of tellurium in the form of a capsule or a tablet or coating at the bottom of the cup. A sample of the molten metal from the furnace is taken and poured into the cup.
The computer monitor then starts displaying a cooling curve in real time as the metal sample cools inside the cup. At the liquidus arrest temperature, the CE (carbon equivalent) is displayed. At the solidus arrest, the monitor displays C (wt% carbon) and Si (wt% silicon). If the required specification is input while configuring the system, then the monitor will highlight the actual CE, wt% C and wt% Si and indicate if the results are high or low compared with the specifications. A typical display as can be seen on the computer monitor is shown in Fig. 1.
Further, if the details of the furnace capacity and obtained recovery from carbon, ferrosilicon and silicon-carbide additions are input while configuring the software and the "recommended-addition" feature is turned on during configuring the system, the system will display the recommended additions to be made to the metal in the furnace-all within three minutes. This is shown in Figure 2.
The recommended additions (or corrections) also are indicated with respect to three situations:
- "Aim" indicates the correction to be made if the metal is required to meet the mid-point of the specification.
- "Rng" (or range) shows the corrections required to get the metal just within the specification range.
- "Chg" (or charge) indicates what corrections will need to be applied for the next charge.
The number "1000" next to recommended additions denotes the capacity of the furnace as input in the example illustrated in this discussion.
The Advanced MeltLab goes a step further beyond the basic version, displaying the important metallurgical properties as shown in Fig. 3. Unlike the Basic MeltLab where the system calls for use of a single cup, the Advanced MeltLab (also known as DualCup MeltLab) requires use of two cups on two stands, both connected to the computer via the A/D converter. One cup contains tellurium while the other cup is a plain sand cup containing only a thermocouple. Both cups must be filled with the same molten metal sample with a time gap between filling of only a few seconds. The tellurium cup is filled first followed by the plain sand cup.
The system monitors and plots the cooling curves of both the cups on the computer display. The plot for the tellurium cup is by default the red color curve, and the plot for the non-tellurium cup is the green color curve. The tellurium causes the metal sample to solidify as white iron while the non-tellurium cup indicates the normal cooling of the metal.
By comparing the arrest points and other parameters from both the curves, the software calculates and presents the important metallurgical parameters as mentioned above.
The basic chemistry such as CE, %C and %Si can be seen in Fig. 3. The recommended additions can be seen below as "C" for carbon addition and "SC" indicating addition of silicon carbide necessary to get the metal to the required specification. In case the "C" and/or "Si" are "Hi," then "ST" addition will appear for steel scrap or iron addition, below "SC."
The other metallurgical parameters shown including "UC," "Ch," "Df," "RC" and "IN" are explained below:
- "UC" (or undercooling) denotes the presence of grain boundary carbides in the metal that would render the metal "hard" while machining. This cannot be determined by means of a hardness tester or under the microscope. Machining a casting having such a condition either will produce a poor machined finish, which could result in scraping the casting, or will cause excessive tool breakage.
- "Ch" (or chill) denotes the chill-wedge value. This is far more accurate because it eliminates the possibility of human error when checked visually.
- "Df" or (delta freeze) denotes the presence of microporosity in the metal, which will render the metal unsuitable for pressure-tight castings.
- "RC" or (recalescence) indicates the type of graphite matrix that can be expected in the metal; for example, Type A, Type B, and so forth.
- "IN" or (inoculant) denotes the inoculant to be added based on the input data while configuring the software.
All of these parameters are studied by the software using the second, third and even the fourth derivatives and analyzing the metal cooling parameters, using the enormous amount of metallurgical information used in making of the software.
The other numbers displayed are temperatures: "MT" for maximum temperature of the metal sample, "St" for final stand temperature (red for the tellurium stand and green for the non-tellurium stand), "Lq" for the liquidus temperature, "Eu" for the eutectic temperature and "Ef" for the end-of-freeze temperature.
Because the results are indicated as numbers, it also helps in quantification; that is, as to where the defect is huge or minor. By gathering this information and over a period of time, specifications can be set up for each parameter and a "foundry drawing" of the cooling curve for each type of casting can be developed.
The software also provides for as many as 20 different combinations of capacity and metal specifications and, therefore, can cover the full gamut of foundry requirements. It also is user friendly because it is menu driven and does not need high-end computers. Specific MeltLabs are available for ductile iron, steel and aluminum, as well as for specific applications.