stove temperature

Keeping the furnace temperature not high is not only important for keeping the refractory life long: It is also of great importance for the liquid metal to remain "alive". If we overheat the metal in the furnace or keep it at high temperatures for a long time, microscopic graphite particles in the liquid that facilitate graphite nucleation can dissolve. If this happens, we cannot get the efficiency we want from vaccination. Result: The shrinkage problem caused by carbides (freckles) and graphite, which we do not want to be present in the structure, cannot be separated.
Let's state in parentheses that we can keep the liquid metal alive by using a preconditioner in such cases, and then move on to the casting temperature.

casting temperature

We can handle the volume shrinkage that occurs after the liquid alloy is poured into the mold in 3 separate stages. As you can see on the graph on the side, first a contraction occurs in the liquid. Then we see a contraction in the solidification phase, and finally in the solidified part.
The total volume contraction occurring in the part is expressed as the sum of the contractions occurring in these 3 stages. Therefore, if  we can minimize the volumetric contraction in the liquid before solidification  , we naturally reduce the total contraction and therefore the feeding requirement of the part. As you can see on the graph, the way to achieve this is to keep the casting temperature as low as possible. In other words, approaching the TL (liquidity temperature) value.
So far it's nice. But of course there is also the question “how low?” We need to ask the question. To answer this question, we need to pay attention to how thin the sections are. Since the solidification time will decrease as the cross-sections of the part become thinner, we need to increase the casting temperature in order to achieve a healthy filling in thin-section parts.
At this point, we explained in the previous article that the section modulus is important to us rather than the section thickness. However, to keep things simple and only  provide advisory  values, in this article we will consider the casting temperature in terms of the thickness of the thinnest section in the part. You can see the recommended casting temperatures depending on the thickness of the thinnest section in the part on the chart below.

When casting on a casting line, we inevitably see the liquid metal in the crucible cool. Therefore, it may be an appropriate recommendation to start with a higher casting temperature in successive castings and to make an adjustment so that it does not fall below the recommended minimum casting temperature until the final mold is reached.
However, when making this adjustment, it is beneficial not to increase the casting temperature more than necessary, just to be on the safe side. As we mentioned above, high casting temperature increases shrinkage as it will cause more contraction in the liquid. At the same time, when we look at the technical literature, we observe that when we increase the casting temperature, the spheres we see in the microstructure grow and their number decreases [2]. This is a situation we do not want, because the low number of spheres is one of the factors that increases the tendency to shrink. Therefore, it is useful to keep in mind that the casting temperature should not only have a lower limit but also an upper limit.

We see that three different methods are used in the industry for carbon determination in cast iron and steel: Dry burning method, spectrometer analysis and thermal analysis. Since the issue of thermal analysis will be discussed under a separate heading, in this article, after an introduction to the dry combustion method, we will focus on the points we need to pay attention to when measuring carbon with a spectrometer. First, let's start with the dry burning method.

Dry burning method

As the name suggests, in this method we need to burn the sample for carbon analysis. To make combustion easy and relatively short, we heat a sample piece as small as half a gram to 1150°C and burn it in an atmosphere of pure oxygen. As a result of this combustion, the carbon in the sample is oxidized and turns into carbon dioxide (CO2) gas. Then, after collecting this gas in an aqueous solution or a solid absorber, we can determine the amount by weight ( gravimetry ) or volumetry measurement. These devices, which have been able to measure carbon dioxide gas using optical methods in recent years, can detect the amount of sulfur as well as carbon with high accuracy.

Spectrometer analysis

Another common method used in the foundry industry is spectrometer analysis, or more accurately optical emission spectrometry ( OES ). Although this method sometimes has difficulty in providing results that are as accurate as the dry burning method mentioned above in carbon measurement, it is preferred by foundries due to the different advantages it provides. First of all, you do not need to bother to extract small samples for spectrometer analysis: You can use a sample taken directly from the liquid for analysis after cleaning its surface a little with sandpaper. Another important advantage of the spectrometer is that the elements it can measure are not limited to just carbon and sulfur: The amount of many other elements such as silicon, manganese, nickel and chromium, which are of great importance for foundries, can be determined with this method.
Carbon determination with a spectrometer can be a bit problematic, especially when the amount of carbon contained in the sample is high. However, with correct sampling practice, it is possible to obtain accurate carbon measurements from the spectrometer. To understand how we need to do it, let's first look at how we measure carbon with a spectrometer.

Carbon measurement with a spectrometer

In optical emission spectrometers, sources that can produce glow or spark discharge can be used. We see that spark emission is preferred in the casting industry because it provides a faster measurement opportunity and can also determine trace amounts of elements.
For accurate carbon measurement, there are some points to consider when taking a spectrometer sample. The sample taken must be as representative of the liquid in the furnace as possible. Therefore, before taking samples with a scoop, especially in medium frequency induction furnaces, the liquid surface must be stirred a little and the elements accumulated on the surface must be dispersed. If possible, the use of immersion samplers that allow direct sampling from the furnace can be beneficial in obtaining a sample that is representative of the general situation. It is also of great importance to thoroughly clean the slag on the liquid surface while taking samples with a scoop and to be careful not to get any slag into the sample.

Classification based on how carbon is found in the cast iron structure:

• White cast iron:  Just like the sugar we put into tea, carbon dissolves completely in liquid iron. If this carbon dissolved in the liquid while the cast iron solidifies cannot be separated from the liquid iron and remains completely dissolved in the structure, we call the resulting structure white cast iron. White cast iron, which has a very brittle structure, is called white cast iron because it exhibits a bright, white color when broken.
• Gray cast iron:  While the liquid cast iron solidifies, the carbon dissolved in the liquid metal, such as the sugar in tea, can emerge as a separate phase during solidification. When we examine such a structure under a microscope, we see that the carbon has decomposed into a separate structure visible to the eye, in the form of graphite. We call this type of cast iron gray cast iron because when this structure, in which carbon appears as lamellar, that is, layers, is broken, a dull and gray color emerges.
• Spotted cast iron:  The white cast irons we mentioned above appear under rapid cooling conditions, while the gray cast irons appear under relatively slower cooling conditions. If the cooling rate of the cast part coincides with a range in which the transition from white to gray occurs, it is possible to see gray and white structures emerging together. When we break such a piece, gray islands appear on a white background, so we call these cast irons mottled (English: mottled).
• Tempered cast iron:  This type of cast iron is actually solidified as white cast iron. In other words, the solidification of the cast iron is ensured so that the carbon remains completely dissolved in the structure. Then, the solidified white cast iron is subjected to heat treatment to ensure that the carbon dissolved in the structure is separated from the structure. After this heat treatment, we see that the carbon appears in the form of irregularly shaped spheres and clustered.