Cast-iron solidification is a complex chemical-physical process; influenced by a high number of variables that foundrymen cannot always control and adjust at will, for technical and also for cost reasons.
For example, as raw materials account for almost 40% of casting costs, foundries take great care to assess the costs and benefits of each different type.
Contrary to common belief, simply controlling the chemical composition of the alloy is far from sufficient because its nucleation state and various solidification phases must also be monitored.
Let’s assume for a moment that you are working in an ‘ideal’ foundry, where you can purchase the best pig iron and steel containing the ideal elements for cast-iron solidification, with long series production to avoid stops to change patterns, fluid casting, constant mould weight, etc. Even in your ideal foundry the process will never be entirely under control because there are other variables, such as weekend stops, system faults, and temperature differences between the beginning and end of casting.
这就是为什么铸件有时会有缺陷的原因，尽管表面上是用生产合格铸件相同的铁水浇注。 This is why castings are sometimes defective despite having been apparently poured with the same iron that produced satisfactory pieces.
The most modern thermal analysis tools allow technicians to scrutinise the trends of cast iron solidification in a logical manner working back from the downstream process (i.e. from the pouring) all the way up to the start of melting.Changes are then made to the various process steps until normality is achieved. This is like seeing a stream flowing near your house change colour, analysing a sample and discovering that it contains pollutants. The solution is to follow the stream, all the way to the source.
The same can be done in a foundry using thermal analysis by checking the iron in the pouring furnace or pouring ladle and then tracing the entire process back to the melting furnaces.
What scenarios might you encounter?
Working your way back up the stream, you may see a factory which is polluting the water, or a dam near the source that makes the water stagnate turning it murky.
Matters are a little more complicated in a foundry. You may discover:•过高或太低的残留镁；•熔炉中硫含量过高或过少；•过高的温度、金属炉料或周末停产造成的成核不足；•过量或缺乏孕育；•铁碳图中的定位不正确；•过度的石墨膨胀；要么太弱，要么太早；•亚稳系凝固，形成渗碳体和D-E 型石墨；•高的收缩和缩松倾向。 • excessive or lack of residual magnesium; • either too much or too little sulphur in melting furnaces; • lack of nucleation caused by excessively high temperatures, metallic charge or weekend stops; • excessive or lack of inoculation; • incorrect positioning in the iron-carbon diagram • excessive graphitic expansion; either too weak or too early; • metastable system solidification with formation of Cementite and D-E type graphite; • high shrinkage and porosity tendency.
Obviously, intervening in these areas with the aid of thermal analysis and a spectrometer only requires good technical knowledge and lots of experience.
In the case of the stream, you can call technical personnel to purify the water and repair the leak. Similarly,foundrymen can rely on expert suppliers to install state-of-the-art thermal analysis software, have the system correctly calibrated and ask for support by expert technical personnel for interpreting cooling curves and receiving indications for the necessary adjustments.
For this purpose, FASSMET and PUDEKJ have partnered up to supply TCAST thermal analysis software and guarantee the technical support needed to calibrate the program, interpret results and analyses production in general(Figure 1).
A cooling curve is the measurement of temperature versus time, starting from the liquid phase, before solidification begins, i.e. starting from the TLiquidus temperature [TLiq] and ending at 1000°C when solidification is complete ifmeasuring the eutectic transformation only, or at 650°C if analyzing the eutectoid transformation as well (Figure 1.Cooling curve, temperature [Y-axis] versus time [X-axis]).
The first derivative shows the speed of cooling as a function of time; this is essential for the geometric calculation of the points of precipitation and transformation. The speed of cooling varies according to the latent heat of solidification emitted with the precipitation of each phase. For example, in the case of hypoeutectic iron, the first phase to“precipitate” is primary austenite, which emits latent heat and thus slows down cooling. When all the primary austenite has “precipitated”, cooling picks up speed again until the next precipitation (eutectic), when it slows downagain.Figure 4 - Graphs of the main page: cooling curve and first derivative
图4 - 主界面图：冷却曲线和一阶导数
主要参数是：The main parameters are:
T °Liquidus [TLiq] ：与曲线上的第一个点重合，表示凝固开始的温度。该值随碳当量（CEQ ）而变化，取决于下表所示的铸铁的类型（数值为近似值）：
T° Liquidus [TLiq]: coincides with the first inflexion point on the curve and indicates the temperature at which solidification begins. This value varies with the Carbon Equivalent (CEQ) and depends on the type of iron produced as shown in the table below (values are approximate):
T° [TEStart]: when eutectic solidification starts, coincides with the minimum point on the first derivative afterTLiquidus.
T °[TEMin] 最低共晶温度：它与一阶导数和零轴之间的交点重合。该值与渗碳体的形成间接成比例，如下图所示（值为近似值）：
T° [TEMin] minimum eutectic temperature: it coincides with the point of intersection between the first derivative and the zero axis. This value is indirectly proportional to the formation of Cementite, as shown in the graph below(values are approximate):
Figure 5 – Relationship TEMin (y-axis) versus Cementite (x-axis)
图5 - TEMin（y轴）与渗碳体（x轴）的关系
This parameter is highly dependent upon the degree of nucleation of the iron and thus inoculation.
T °[TEMax] 最高共晶温度：它是曲线上的最大值。它与一阶导数和零轴之间的第二交点重合。该值由凝固期间的相的结晶潜热产生。它特别与石墨析出和膨胀有关。
T° [TEMax] maximum eutectic temperature: It is the maximum on the curve. It coincides with the second point of intersection between the first derivative and the zero axis. This value is generated by the latent heat of the phases during solidification. It is specifically linked to graphite precipitation and expansion.
T °Solidus [TSol] ：铸铁完全凝固的温度。它与一阶导数的最小点重合。
T° Solidus [TSol]: temperature at which the iron has completely solidified. It coincides with the minimum point on the first derivative.TCAST 处理上述参数以获得：TCAST processes the above parameters to obtain:
Primary Graphite [PrGraph]: the percentage of primary graphite. This is undesirable in ductile iron as its presence results in the formation of large nodules and graphite flotation, especially in large castings.
Eutectic precipitation of austenite [CellAust]: this parameter relates to the efficiency of eutectic precipitation,measured in seconds. High values are therefore ideal, as they guarantee castings with no porosity or microshrinkage.However, exceeding the maximum standard value carries the risk of creating the conditions for graphite flotation.Ideal values:
[KCond]: is the angle of the derivative that indicates the speed of passing from the semi-solid to the solid state. The lower the value, the fewer shrinkage cavities in the matrix. This parameter is essential in order to find the exact TSolidus [TSol] point, which coincides with the completion of solidification. Ideal values are:
Higher values indicate thepresence of porosity/shrinkage高值表明出现缩松缩孔
25 – 45°
Higher values indicate the risk ofshrinkage, lower values indicate theformation of degenerated graphite forms(vermicular and/ or lamellar)高值表明出现缩松缩孔，数值低表明片状或蠕墨石墨形成
The cooling curve and its first derivative are not easy to read, but the TCAST software helps foundry metallurgists by interpreting the results and displaying these on easy-to-read colored dial. The colored dials feature three color zones.The colored values in the red dials indicate a high risk of defects, those in the yellow dials indicate a medium risk of defects and those in the green zones indicate a low risk of defects.
Initial software calibration is, of course, necessary and can be completed in the first few days after installation.
The other colored dials indicate the tendency of the iron toward graphite expansion, graphite flotation, nodularity,shrinkage and porosity, cementite formation, inverse chill, actual position in the iron-carbon diagram, useful information for ductile irons such nodule counts, nodularity and final quality index.
Improvement areas can be identified after only a few hours of use.
在采集阶段，当工作站通过TCP / IP 技术远程连接时，可以实时查看处理后的数据和结果（FoundryIntranet ）。此机制用于提供交互式Web 界面，通过使用任何连网的设备如个人电脑、智能电视、平板电脑和智能手机远程监控详细信息。
During the acquisition phase, the processed data and results can be seen in real time when the working stations are remotely connected via TCP/IP technology (Foundry Intranet) This mechanism is used to provide interactive Web interfaces to monitor elaborations remotely by using any device connected to the networks as PCs, smart TVs, tablets and smartphones.
史江涛：南京谱德仪器科技总经理，高级工程师，2004年从事于分析仪器，专注于炉前铸铁热分析李明：工学硕士，毕业于武汉理工大学，高级工程师，主要从事于铸造技术工作Luca De Lissandri：意大利FASSMET技术总工