Controversy on the influence of metallurgical factors on shrinkage cavity and porosity of ductile iron

It is very important to select the appropriate carbon equivalent or carbon silicon content for the production of sound castings without shrinkage cavity and porosity defects, which is always a problem of great concern for ductile iron manufacturers.

Theoretically, the eutectic point of Fe-C alloy is 4.3%. Cast iron contains Si, Mn, P, s and other elements. In actual production, the eutectic point of ductile iron is very complex, not necessarily 4.3%. The main reason is: the actual production of nodular cast iron is not equilibrium solidification. Due to the influence of many factors such as molten iron oxidation, chemical composition, spheroidizing and inoculation treatment, including the influence of spheroidizing element magnesium and rare earth, the actual eutectic point of nodular cast iron solidification is not 4.3%, and often moves to the right to 4.6% or 4.7%. This is the reason why it is recommended to choose carbon equivalent between 4.6% and 4.75%. In 1976, China participated in the 43rd International Foundry conference held in Bucharest, Romania, and read out the paper “metallurgical and technological characteristics of rare earth magnesium nodular cast iron”. It was mentioned that the carbon equivalent of most Chinese nodular cast iron plants is 4.6% ~ 4.7%, and the advantages of rare earth magnesium nodular cast iron have attracted the attention of foundry workers all over the world.

After a long-term follow-up, the shrinkage tendency is the smallest when the carbon equivalent of a vertical production line of the author’s unit is 4.4% ~ 4.45%. The original carbon equivalent of nodular iron castings produced by an enterprise in Cixi, Zhejiang Province is about 4.6%, which leads to many shrinkage defects. The carbon equivalent is controlled around 4.40%, and the quality is stable.

It is understood that some enterprises in Linzhou, Henan Province produce small and medium-sized ductile iron castings with uneven wall thickness. The hot joints of these castings are separated everywhere, and the wall thickness of the structure connecting the hot joints is relatively thin. Many measures have been taken, such as adding chill iron or chrome ore to accelerate hot spot cooling, controlling pouring temperature, increasing carbon equivalent, changing pouring riser system, etc., which can not meet the requirements of grade 2 X-ray flaw detection. When the carbon equivalent is reduced from 4.4% ~ 4.6% to 4.25% ~ 4.30%, the shrinkage is reduced and the flaw detection is qualified.

At the Wuxi New Technology Forum in 2015, the article “carbon equivalent selection of nodular cast iron”, which touched the traditional view, listed in detail the carbon equivalent selection of domestic and foreign foundry enterprises and the latest research results of carbon equivalent of nodular cast iron eutectic point abroad. Although some scholars believe that carbon equivalent of 4.55% ~ 4.60% is the best to prevent shrinkage defects, However, it is also believed that it should not exceed 4.6%. When some foreign scholars study the solidification mechanism of nodular iron, they think that the eutectic point of nodular iron is 4.33%, and the nodular iron with carbon equivalent of 4.33% is the densest, followed by hypoeutectic nodular iron with carbon equivalent of 4.33%, and hypereutectic nodular iron with carbon equivalent of more than 4.33%.

In the early production of nodular cast iron in China, the purity of raw materials can not be guaranteed. The content of phosphorus and sulfur in the raw iron liquid is high, often P ≥ 0.06%, s ≥ 0.05%, or even higher. The sum of 11 trace elements is far more than 0.1%; In addition, the temperature of molten iron is relatively low and the degree of oxidation is high; Therefore, the content of rare earth in spheroidizer is high, and the addition amount of spheroidizer (mg6re2 as an example) is 1.4%, or even higher.

The focus of this issue is the selection of carbon equivalent of ductile iron. More than 30 years ago, the consensus of producing ductile iron was that the carbon equivalent of eutectic point was about 4.6% ~ 4.7%. The reason was that the eutectic point moved to the right due to the influence of spheroidizing element magnesium and rare earth. In addition to Mg and re, other elements such as Mn, Cr, V, Ti, s, Mo, Zr and w which hinder the graphitization also make the eutectic point shift to the right; However, in addition to C and Si, Al, Cu, Ni, Sn, P, CO and other elements promote the graphitization, and these elements will make the eutectic point move to the left.

In recent years, compared with 30 years ago, the quality of ductile iron melt in China has been greatly improved. In 2007, a small amount of high-purity pig iron began to be produced in China. In 2010, high-purity pig iron was produced on a large scale by direct reduction of blast furnace with “three refining methods”, and the sum of 11 trace elements was less than 0.05%; Then in 2017, ultra-high purity pig iron for casting was successfully developed and produced by the process of “three refining methods + refining and deep purification”, with the sum of 11 trace elements ≤ 0.02%. The mass production and application of high purity pig iron and ultra-high purity pig iron, as well as the development and application of high-quality nodularizing agent and high-efficiency inoculant, greatly reduce the content of harmful elements that hinder graphite nodularizing in liquid ductile iron. 015%, or even lower; The addition amount of spheroidizer (mg6re2 as an example) is 1.0%, even lower; The pearlite content of QT450-10 produced by many enterprises is less than 5%. These progress and changes show that with the improvement of the quality of raw and auxiliary materials, the metallurgical quality of ductile iron has been greatly improved, and its actual eutectic point may shift to the left.

Ecken (China) has studied in recent years that the eutectic point of ductile iron moves to the right due to the magnesium content in molten iron. When the magnesium content is 0.035% ~ 0.045%, the actual eutectic point is about 4.4% ~ 4.5%.

It has become the consensus of most domestic technicians to “increase the carbon content as much as possible without graphite floating and primary graphite precipitation”. No graphite floating and no primary graphite means that the carbon equivalent should not be too high and hypereutectic.

It is found that the shrinkage tendency of cast iron increases from hypoeutectic to near eutectic, eutectic and slightly hypereutectic decreases. In fact, these slightly hypereutectic alloys show complete eutectic solidification, that is, there is no pre eutectic precipitate on the thermal analysis curve. When the carbon equivalent is further increased, the degree of Hypereutectic is higher, the pre eutectic reaction occurs, that is, the pre eutectic graphite ball, and the shrinkage porosity increases. It is also considered that the formation and growth of austenite halo around the graphite ball before eutectic is the reason for the observed increase of porosity in hypereutectic alloy. There is also thermal analysis

The results show that when CE is higher than eutectic composition, the solidification starts from graphitization and floatation, and the fluidity of molten iron decreases, which also leads to the occurrence of cold shut defects. Graphitization floating will reduce the amount of graphite generated during eutectic solidification and reduce the graphitization expansion, so the risk of shrinkage porosity of Hypereutectic liquid iron is high.

The range of carbon equivalent or carbon silicon recommended by American Foundry Association and German foundry encyclopedia is shown in the figure.

According to the figure, most of the recommended amounts of carbon and silicon are located in the micro hypereutectic region. For thin section castings (less than 12 mm), 4.55% Ce is recommended; For medium section castings (12-40 mm), CE of 4.35% to 4.45% can be used; For thick section (more than 50 mm), CE should not exceed 4.30% to avoid graphite floating, but C + 1 / 7si should not be less than 3.9% to prevent excessive shrinkage defects.

At present, enterprises with good metallurgical quality control of ductile iron in China choose carbon equivalent far less than 4.6% ~ 4.7%, which is about 4.35% ~ 4.45% through thermal analysis and subcooling verification. However, for the enterprises with poor metallurgical quality control, their carbon equivalent may still remain at 4.6% ~ 4.7%. In the selection of carbon equivalent, it should be increased or decreased according to the specific product process characteristics.

There are many factors that will affect the choice of carbon equivalent of ductile iron, especially the different casting structure, the carbon equivalent should be different. Therefore, every enterprise should do meticulous technical work and carry out meticulous management, and should not blindly adopt too high carbon equivalent.