Thermal analysis is a vital part of producing high-quality metal castings, with repeatable, consistent composition and physical properties. By determining certain thermal characteristics of the heat, metallurgists can predict the chemistry, microstructure and physical properties of the finished metal, including its hardness and tensile strength.
To perform this thermal analysis, the temperature of the metal must be tracked from the time a sample is poured until it reaches the point of solidification. This thermal data can be plotted against time and compared to existing cooling curves and to find information about composition and microstructure or be used to develop new cooling curves.
Ultimately, tracking the temperature versus time during cooling can provide the data needed to calculate several important metallurgical metrics and provide valuable insight and model input parameters.
An old adage states, “you can’t control what you can’t measure.” This is especially true in iron casting, where data can be difficult to collect. However, with proper sampling techniques and equipment, thermal analysis can help the metallurgist more tightly control the composition of the alloy being cast. Tighter control over the composition means tighter control of the microstructure, and thus the physical properties of the finished product.
In iron casting, a few key quantities can be calculated if the temperature of the cooling metal can be recorded with respect to time. The percentage of carbon present in the final alloy, the percentage carbon equivalency, and the percentage of silicon are important quantities in predicting physical properties of the final melt.
Carbon content is the first parameter in establishing the likelihood of graphite formation in cast irons. For cast iron alloys, 2-4% (by weight) carbon is common, though the physical properties and suitability for a particular alloy vary greatly within that 2-4% range.
However, carbon alone is not the only important factor in alloy properties. A carbon equivalent calculation can determine how other additives impact the overall melt. In other words, the percent carbon equivalency can be used to approximate how these additives cause an alloy to look like it has a certain percentage of carbon.
Silicon is also tracked. Silicon has two major impacts on cast iron and steel alloys. It can raise the carbon equivalency and it can lower the viscosity (or raise the fluidity) of the melt. Also, too much silicon in the melt can erode ceramic firebrick more quickly, so optimizing the silicon content is important.
Besides these three quantities, the rate of cooling can be checked against a cooling curve to determine the microstructure that will form. Cooling curves plot temperature on the y-axis and time on the x-axis and highlight regions of different microstructures. Depending on the application of the alloy, certain microstructures are more desirable than others. The cooling curve can help the metallurgical engineer plan how to cool the alloy to get the most desirable microstructure.
Cooling curves can also be developed with temperature data collected during cooling. At phase changes (both liquid to solid and solid state phase transformations), there is a thermal arrest. During this thermal arrest, the temperature does not change though heat is still being removed from the sample. If temperature data is plotted against time, these thermal arrests are easily identified and can be used to identify phase changes.
The SYSCON FL-1 FerroLab Thermal Analysis System can measure and calculate several key process variables to ensure consistency and accuracy in iron casing quality. Specifically, the FL-1 can determine cooling curve temperatures, composition information (percent carbon, percent carbon equivalence, and percent silicon), predict Brinell hardness, shrinkage factors, tensile strength and up to five other user-defined parameters.
FL-1 is designed to be user-friendly, with an intuitive interface and the ability to specify user-defined parameters, making it useful for foundry-specific or product-specific applications. Furthermore, the touch screen display is easy-to-read and can be operated with gloved hands, which is essential in the foundry environment.
In terms of data collection and system integration, the FL-1 has one ethernet port and three USB ports for connectivity. For thermal measurements, there are four thermocouple ports (3 K-type and 1 S-type). It can be calibrated using IPTS 48 or IPTS 68 standards, and is ruggedized for foundry usage.
A practical use case for the FL-1 FerroLab Thermal Analysis System would be to monitor the temperature of cooling cast iron. By measuring the temperature as the alloy cools, the thermal arrests can be determined. Each of these thermal arrests indicate where phase changes are occurring. This data is compared to phase diagrams and cooling curve data to determine the composition of the alloy.
From here, the presence and morphology of graphite can also be predicted. For ductile or nodular iron, graphite should form spheres versus graphite flakes found in gray iron. This transition in morphology happens based on the percent carbon equivalency, meaning carbon content, silicon content and a few other elements have an influence over the microstructure.
Once the composition is determined, the physical properties of the alloy can be predicted, such as Brinell hardness or yield strength. These can later be verified through mechanical testing. If the test results are less desirable than the predicted data, metallurgical engineers can begin to track down sources of defects, such as inclusions, pores, or other similar issues.
Figuring out how to perform thermal analysis in foundries can seem challenging. However, the advantages of ensuring high-quality castings make it an essential part of all metal casting operations.
With nearly half a century of foundry measurement experience, SYSCON can help improve quality control and consistency heat-to-heat in virtually any foundry. Their FL-1 FerroLab Thermal Analysis System can ensure high-quality, low-defect iron castings are produced consistently.
For more information about the FL-1, developing and implementing thermal testing procedures or to learn more ways to improve quality control in the foundry, please reach out to the experts at SYSCON.