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CGI breakthrough - An advanced third-generation compact graphite iron process, called Graphyte Flow, is transforming engine production

For years, achieving the perfect diesel engine has been discussed – the one that delivers more power despite smaller dimensions and lower weight, that consumes less fuel and emits fewer greenhouse gases. It has been said that using compacted graphite iron (CGI) will enhance the diesel engine’s attraction. So when will the time finally be right for using CGI technology to produce that cleaner and leaner diesel? The answer is, now. Novacast Foundry Solutions AB, a daughter company to Novacast Technologies AB, has developed the third generation CGI process, Graphyte Flow.

There is no filter or catalyst you can put on an engine to lower CO2 emissions, yet more demanding emission laws are coming into effect. The UN, through its Intergovernmental Panel on Climate Change has recently set a goal of a 50-80% decrease in CO2 emissions by 2050. A maximum temperature increase of 2°C is the target and it will cost 0.12% of annual world GDP, approximately US$50,000 billion.

Very soon, merely switching from gasoline fuel to diesel fuel will no longer be enough to reduce CO2 and other hazardous emissions. The diesel fuel needs to be more fully combusted, which requires higher operating temperatures and pressure – exactly what CGI delivers.

Soaring oil prices are here to stay, indefinitely. The high-pressure, high-temperature combustion that CGI makes possible in diesel engines will cut fuel consumption – as will the engine’s lower weight and smaller dimensions (enabling a reduction in the size and weight of the vehicles).

The global deployment of CGI, for example in the heavy duty trucks industry, has been delayed by the lack of a common standard for the material, with different countries applying different standards. That’s now history. Today, there is ISO 16112, an international standard that carefully specifies five CGI grades based on their minimum mechanical properties, their metallurgical properties and their target applications.

Mass-producing engines and engine parts requires mass-production processes. With earlier CGI production systems, the foundry process was exact enough to allow commercial manufacturing. Today, the third generation of CGI systems makes mass production truly viable. The process becomes fast, predictable and highly automated, with reduced downtime.

Mass production


The first generation of compacted graphite iron production never made it beyond the lab environment. The challenge of finely balancing the dosage of magnesium against the formation of oxides, sulfides and other compounds affecting the melt proved too complex for manual-control methods. With the advent of computer-based control systems, the CGI process could be mastered to some extent and brought to the automotive industry. Expectations were high all around. Unfortunately, they weren’t met completely. This second generation of CGI failed to meet the basic industrial requirements: high speed, low cost and high predictability. This is precisely what Graphyte Flow– the third generation – was developed to address (Figure 1).


Above: Figure 1

Graphyte Flow creates a streamlined industrial process throughout the foundry. The workflow is compressed and logistically efficient, with few physical movements of the melt on its way from the furnace to the mold. It is like producing grey cast iron. The melt is kept at a lower temperature compared to the second generation methods, which requires less energy and leads to less refractory wear. The reaction of iron and magnesium takes place inside the mold in a reaction chamber in an atmosphere virtually free from oxygen, not in an open ladle. This keeps the work environment free of smelly and potentially harmful fumes.

Also, the base iron is left untouched, which means the same melt can be used for different CGI specifications. By separating the magnesium reaction from the melt at the beginning of the process, concurrent engineering becomes possible. The absence of a reaction ladle means that the iron and the magnesium alloy can be prepared in parallel, saving considerable time per unit poured. Another benefit is that in the event of an unexpected shutdown of the process, both the base iron and the magnesium can be fully recovered and reused.

The fundamental concept of mass production is, of course, predictability – the assurance that each unit produced is identical to every other. Knowing this – rather than guessing or hoping it – is at the very core of mass-produced quality. Despite its many undisputed metallurgical advantages, CGI has so far not lent itself to mass production. It’s been time-consuming, difficult to control and – above all – not predictable. Graphyte Flow creates a paradigm shift in predictability – making it a true industrial manufacturing process for CGI.

The magnesium treatment of the iron melt, when using Graphyte Flow, happens inside the mold, so the process becomes easier to monitor and control. Until now, reaching the 80% compacted graphite content prescribed by ISO 16112 has required keeping track of 31 different process variables. With Graphyte Flow, these variables are 30% fewer, reducing both the complexity of the process and the risk of unexpected errors. Adding the exact same amount of magnesium in the exact same place inside the mold yields the exact same result.

Also, because the Graphyte Flow process happens more rapidly at lower temperatures, it inhibits the nucleation of slag in the iron. Result: consistent melt and even output quality.

Graphyte Flow process

CGI is a cast-iron alloy with a graphite structure between flake-type graphite shapes and spherical shapes (Figure 2).



Above: Figure 2

The graphite shape is determined by the conditions in the liquid iron during the solidification. Treatment of a base iron preferably with a carbon equivalent between 4.0 and 4.4 and with a sulphur content below 0.02wt%, with an alloy preferably containing 3-6 wt% magnesium and 0.5-1.5 wt% lanthanum achieves the compacted graphite structure. The magnesium content must be kept within very narrow limits and with a level of about 0.008-0.015 wt% depending on conditions of the base iron, and the cooling rate in the casting to be produced. CGI possesses mechanical and physical properties in between grey and nodular irons.

The second generation CGI methods apply the magnesium treatment in a ladle leading to a considerable fading effect of active magnesium, due to the relatively low magnesium boiling temperature of 1,090°C. This is a serious problem with ladle treatments and is, in case of Graphyte Flow process, addressed by moving the treatment in the mold to the reaction chamber. The yield of magnesium is consequently high, approximately 85-90% and consistent. During pouring, the iron flows into the chamber and gradually dissolves the alloy. The treated metal then fills the casting cavity. As a consequence, Graphyte Flow uses as much as 10% less magnesium compared to the second generation batch treatments to achieve the same amount of compacted graphite iron in the component. As a result, the risk of casting defects such as micro shrinkages and dross are reduced. Since the Graphyte Flow does not use any additional inoculants during the process, due to the already high inoculation effect in the reaction chamber, the process stability is increased.

For each type of casting, a well-adjusted gating system and reaction chamber are created in order to obtain an optimal amount of active magnesium. This is done by advanced simulation of the casting process using Graphyte Flow gating software. The pouring rate is controlled by choking directly after the downsprue. Figure 3 shows the patented Graphyte Flow gating system. The surface section area of the reaction chamber is varied in depth in order to achieve a continuous absorption of magnesium during the pouring cycle. The contoured reaction chamber is connected to a pressure chamber and to ceramic filters. The system assures a constant magnesium percentage throughout the pouring cycle.



Above: Figure 3


The metallurgical software of Graphyte Flow is based on chemical analysis and advanced thermal analysis. Specimens for thermal analysis are collected directly from the melting or holding furnace (Figure 4).



Above: Figure 4


The system combines thermal and chemical analysis to monitor the properties of the iron and to maintain the adequate process window (Figure 5).



Above: Figure 5


If the metallurgical parameters exceed the boundary values, the software recommends countermeasures in order to change the active carbon equivalent, the nucleation status, the chemical composition (especially sulphur) and the oxygen level. Adjustments can be made in the furnace or in the transport ladle (using rare earths containing cerium to control oxygen and sulphur levels). The method ensures that the base iron is always congruent with the potential to form compacted graphite iron for each specific casting.

The process control system driving Graphyte Flow even learns from its own imperfections. Any irregularity in any variable that’s been encountered and corrected by the operator is stored by the system and will help fine-tune the controls even further. Over time, this virtually eliminates the risk of operator errors.

The shorter and more rapid Graphyte Flow process even yields a superior CGI quality – virtually free of adverse carbides. As featured in Figure 6, only very small and harmless carbides are detected in the microstructure. This makes the material less brittle and more elastic, facilitating machining and reducing wear and tear on machining tools.

 


Above: Figure 6


Graphyte 350

Machining of castings manufactured using Graphyte Flow is considerably easier than machining of second generation material. High inoculation effect in the reaction chamber is believed to contribute to the absence of hard primary carbides that are detrimental to tool life. Results of a study of machining properties of grade Graphyte 350, recently carried out, indicate that when pearlite/ferrite ratio is kept close to 1:4 or below, the machining properties are even better then for reference grey iron material usually used for manufacturing cylinder blocks.

Automotive cylinder blocks are preferably manufactured in Graphyte 450 or Graphyte 500 containing considerably larger amounts of pearlite. The study has also concluded, as expected, that an increase of pearlite content in the material matrix leads to a decrease in tool life when machining grades 450 and 500. However, Graphyte 350 is a very suitable material for applications such as bedplates, cylinder heads and blocks for large marine and stationary diesel engines, exhaust manifolds, brackets and couplings, ingot molds and much more. It is a high strength, high ductility and high thermal conductivity CGI material with outstanding machinability.

Designers do not need to choose between ISO 16112/JV/300 and ISO16112/JV/350. They simply select Graphyte 350. Tensile strength testing of Graphyte 350 during the machining study revealed the following mechanical properties; tensile strength > 350MPa, 0.2% proof strength 300MPa, elongation 3.8%. The graphite morphology investigation showed 85% CGI and 15% nodularity. The microstructure contained 20% pearlite. No primary carbides have been detected in the microstructure.

Graphyte Flow into the future

The engine producing industry today especially the automotive industry is strongly driven by optimising costs of different components and systems in an engine. It is of a great importance for all the responsible suppliers, developers, designers and end users to engage in cost management activities.

So far the CGI has been used when replacing grey iron in different applications. The driver for such a material change is the performance requirements on the component. Inevitably, it leads to an overall manufacturing cost increase due to the material cost, process cost and of course, machining cost. It is often forgotten that CGI should be used to replace nodular iron in some components. Designers usually prescribe nodular iron for applications even in components there the usage of the CGI would be a preferable choice. This nodular iron-CGI replacement is potentially a great cost saver. It is seldom pointed out that machining of the CGI is actually somewhat easier than the machining of the nodular iron of comparable grades.

Novacast possesses 25 years of experience of advanced solutions for the lean foundry metallurgical process control and of material selection. Graphyte Flow has proven having great capabilities in employing cost saving activities. As an example of a project to replace nodular iron by Graphyte 350 in an engine component is the application of Graphyte Flow for mass production of bedplates in horizontally split molds. $120 per ton of good castings was saved.

In conclusion, the third generation CGI process Graphyte Flow features fewer variables, lower direct costs, reduced environmental impact, and improved material properties compared to the earlier generations. Graphyte Flow is a reliable mass production process suitable for casting a large diversity of components.

Contact the author:

Dr. Peter Vomacka at Novacast Foundry Solutions
+46 457 386 300
Email: peter.vomacka@novacast.se



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