SUCCESSFUL CONVERSION FROM CUPOLA TO INDUCTION FURNACE

Partner

Foundry Corporate News Topic Melting Shop

15. November 2023

BY Dr. MARCO RISCHE, WOLFGANG BAUMGART, SEBASTIAN HAARDT, STEFAN SCHMITT

The conversion from cupola to induction furnaces is a decisive step for foundries on the path to decarbonizing their production. An important challenge here is scrap quality, which must be better in the induction furnace than in the cupola. ABP Induction and Zorc Technologies offer specially developed tools for this purpose, which allow optimized operation of the induction furnace. This article uses practical examples to show how a conversion can be made successfully.

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The pressure from environmental policy requirements to achieve the climate targets set for industrial consumers is constantly increasing, also driven by the CO2 tax on fossil fuels. All industrial consumers of coke, oil and gas are pursuing alternative solutions to meet the medium-term environmental targets for CO2-neutral production. All companies surveyed in a study by Römheld & Moelle stated they would ask their suppliers for climate-neutral castings by 2050 at the latest, with 21% planning to do so by 2025 and 46% by 2030 [1].

ABP has recognized this paradigm shift to CO2 neutrality and has committed to this goal through its "Your Partner on the Way to Zero Emission" campaign. To meet the demand for decarbonization, ABP sees great potential in replacing fossil fuels with modern induction furnace technology for ecological, economic and technical independence. In this way, users make a significant contribution to implementing the targets for decarbonizing energy-intensive industrial applications.

The inductive heating uses electrical energy. In this process, the heat required for the process is introduced directly into the molten material. The process is effective and, when green energy is used, it is also carbon-neutral. ABP can plan the entire process chain for the replacement of the conventionally heated cupola and accompany the customer along the path to conversion to electrically operated induction furnace technology. This is rounded off by digital tools and AI solutions from ABP partner Zorc Technology.

Foundries with Cupola Melting Operation – Status Quo & Challenges

When foundries consider converting from a cupola to an induction furnace melting operation, there are two fundamental challenges to consider: First, the conversion from cupola to induction furnace also means a change in operation – from continuous supply to discontinuous operation, to batch operation. The second challenge is scrap quality. In cupola melting operations, it is not uncommon to run even poor scrap grades. The induction furnace operation cannot cope with this because the coupling then becomes significantly worse. This makes it much more difficult, if not impossible, to achieve a utilization rate of 100 %. Utilization rates of 60 % are more likely. Keeping this gap as small as possible hinges on how the electromagnetic field couples to the scrap. This is strongly dependent on the set scrap quality and the parameters of the electrical power supply.

A practical example: Thanks to the conversion from coke-fired cupola to induction furnaces (Fig. 1), a large part of the emissions at Römheld & Moelle can be dealt with by changing the energy mix. A special contract with its energy supplier enables the foundry to obtain 100% of its electricity from hydropower in 2023 and 2024 [2].

Figure 1: Römheld & Moelle Carbon Footprint in Scopes 1 & 2.

What does this mean from a metallurgical point of view? Iron melted in the cupola has a high sulfur content due to the process. For the production of spheroidal graphite cast iron, therefore, single- or two-stage processes are used to produce the ready-to-cast melt. However, the production of vermicular graphite is consistently based on two-stage processes. In these two-stage processes, the first stage is used for desulfurization, typically achieving sulfur levels of 0.06 to 0.025%. In the second stage, the morphology of the graphite phase and the composition of the metallic matrix are then adjusted to achieve the desired mechanical and physical properties of the material in the component (Fig. 2).

Fig. 2: Overview of the large number of possible process steps from the melting units to the mold.

Continuous desulfurization of cupola iron is possible using compounds containing calcium. Specifically, the use of calcium carbide as a desulfurization agent is also an option, but this is rarely used in foundry processes today due to environmental concerns.

Discontinuous desulfurization processes include the Mg wire feeding process and the modern lime injection process developed by the Institute for Technologies of Metals at the University of Duisburg-Essen under the direction of Rüdiger Deike and the materials development department of the Fritz Winter company under the direction of Marc Walz [3], [4].

To produce the ready-to-cast melt from the resulting base iron, a second step involving treatment with magnesium, rare earths and inoculants is carried out. In particular, the wire feeding and the pour-over method have become established. Modern wire feeding methods also use an inoculation wire in addition to the magnesium wire, resulting in dynamic inoculation during treatment. The final step on the way to the desired metallurgy is inoculation during the casting process. This inoculation can be carried out by means of a pouring jet inoculation or an inoculation in the casting or runner system.

Requirements for a modern induction furnace melting operation

As already described above, a modern induction furnace can only work with high quality metals and scrap. That is why induction furnace manufacturers also usually recommend a certain grade of scrap with which the equipment should be operated. Otherwise, the full output cannot be generated due to the poorer coupling. However, the goal must be a constant power range – ideally with 100% energy input over the course of the batch to minimize the melting time and thus the energy input and, of course, to maximize productivity.

However, the coupling of the induction furnace depends on the load circuit: the power of the equipment adjusts according to the bulk of the feedstock based on its electrical as well as magnetic properties. Stationary losses due to recharging must therefore be optimized. ABP makes it possible to measure and evaluate these operations and to guide the customer in batching. For this purpose, ABP uses the patented OptiCharge tool it has developed itself (Fig. 3). It measures the electrical influencing variables necessary to optimize performance. These parameters are compared to the current weight and the algorithm determines the lowest possible weight necessary to achieve full power consumption. The desired result is controlled batching for optimum adaptation of the weight to the power consumption over the complete batch duration. Whenever physical conditions permit, re-batching can be performed. This cold material couples optimally due to its ferromagnetic properties until it loses them at the Curie temperature point at 760°C.

Figure 3: Classic power curve of an induction furnace over a single batch.

The technical furnace parameters for this are recorded by the digital inverter control and converted into recommended actions for energy-efficient loading with the OptiCharge system. When starting up a batch with partial filling of ferromagnetic melting material, small portions of this material are automatically refilled. As a result, measurable energy savings and production increases can be achieved in daily production operations as compared to non-controlled batching. Surveys show that induction furnaces already produce less than half the CO2 emissions to melt one ton of cast iron compared to cupolas in today's electricity mix. If the share of electricity generated from renewable sources increases, CO2 emissions decrease accordingly.

This system is therefore eminently important for the transition from the cupola to the induction furnace in order to continue to provide a high continuous supply of the molten iron for the process. This is where OptiCharge reduces power dips to a minimum.

The physical background: The possibility to operate in the constant output range was created by the fact that there are thyristors in different sizes and it has become established that the next larger size is always used that is above the desired output power. This creates an optimal operating mode. However, this also means that current and voltage in the product can do more – the output power itself and a certain power window beyond that.

The principle works, as the load characteristics show (Fig. 4): If you run the induction furnace at full voltage and there is little scrap material in the furnace, little material couples and you run the furnace at low power. Even if the furnace operator adds more material and the furnace is half full, the point of 100% power at full voltage is reached, and even though only 60% of the electricity is used. If you add more material, it couples even better, uses more electricity, and the furnace needs less energy to drive more power in the process. This is the voltage reserve that can be used despite full power. In practice, however, material is sometimes added beyond this point, as the process then becomes simpler, since material has to be added less frequently and the workload is lower. However, this is not beneficial to the process, because if one were to stop filling the furnace sooner, the material fed in would be heated rapidly because the furnace would quickly couple up to the Curie temperature. The scrap then loses its magnetic properties, the coupling becomes worse – and the operation falls back one characteristic. Now you need more voltage again to drive the current to get to 100% power. However, as less and less material is magnetic, the current continues to decrease as the voltage is at its maximum level. The result is that the output drops. If material were to be added now instead, you would continue to have 100% power.

Fig. 4: Load characteristics for an induction furnace, design of a constant power converter.

This system is essential for those switching from cupolas to induction furnaces because these operations often must cope with poorer scrap qualities, which is precisely what causes the coupling to deteriorate. They also rely on a ready supply of molten iron for their processes, which is why a control tool like OptiCharge is so valuable.

Conversion of the melting operation

To better evaluate how processes that use induction melting differ from those that incorporate a cupola as the melting unit, we will explain the specifics using three processes from actual practice.

Example 1: Flake-graphite cast iron brake discs

In the production of cast iron brake discs with lamellar graphite, the cupola can benefit from its advantageous nucleation state. However, the requirements for brake discs have increased considerably in recent years. As a result, the material is required to have high thermal conductivity, which can only be achieved if the cast material solidifies hypereutectically. In many cases, the carbon levels required for this cannot be produced directly by the cupola, so carburizing becomes necessary. In addition, the sound specifications to prevent squeaking of modern brake discs require carbon contents in narrow tolerances, which can only be produced by duplication in an induction furnace. All this is eliminated by melting in an electric induction furnace (Fig. 5).

Fig. 5: Comparative process diagram for the production of cast iron brake discs with lamellar graphite – above with the cupola as melting unit, below with the electric induction furnace.

The challenge in using an inductive melting method is, on the one hand, dynamic inoculation, which is technically easy to realize by adding an inoculation wire to the casting trough. Strontium as an element effective in inoculations is the agent of choice here. Another difficulty is the control of microporosities, which often occur at the transitions from cap to disc. Microporosities form between the arms of large austenite dendrites. In this context, the targeted use of lanthanum in the final inoculation step has proven helpful.

Example 2: Nodular graphite casting cast iron pipes

Nodular graphite cast iron pipes are manufactured using the centrifugal casting process. In the process, the cast iron pipe cools on the mold wall and forms iron carbides. Since iron carbide has a carbon content of 6.7%, the amount of free graphite produced during solidification is greatly reduced. As a result, the free graphite cannot counteract shrinkage when the liquid phase solidifies. This has the desired effect: the cast iron pipe shrinks and can be removed from the mold. The pipe is then passed through the annealing furnace, where the iron carbide dissolves and the graphite diffuses to the spherulites. The pipe expands and reaches its final dimensions. The original nucleation state from the melt aggregates directly influences the dimensions of the component.

A cupola produces a fluctuating metallurgical condition due to varying levels in the forehearth as well as changing metallurgy during start-up and shutdown cycles. The resulting dynamics in the nucleation state require a complex control loop when using a cupola, whereas this control is much simpler when melting in an electric induction furnace (Fig. 6).

Fig. 6: Comparison of process chains for the production of nodular graphite cast iron pipes. When using the inductive melting method, nucleation state changes of the subsequent taps are compensated by means of dynamic inoculation via an inoculation wire as an additive to the treatment wire. This results in a small variation in measurement tolerance.

Example 3: Vermicular graphite

Cast iron with vermicular graphite is found in the powertrain in the cylinder head, in the cylinder crankcases and in the clutch disks. This is a material for the mass market in heavy goods vehicle traffic. The material is also used in brake discs for high-speed trains.

When vermicular graphite is produced by adding desulfurized cupola iron, it is limited because the accompanying elements from the cupola can often exceed the final analytical specifications. The proportion of desulfurized iron can vary between 30 and 70% depending on the specification. When using this material, the cupola cannot effectively use its strong point, which is its naturally good nucleation state. The formation of vermiculites occurs with high levels of undercooling, while a good nucleation state automatically gives rise to spherulites due to metallurgical factors.

The control loop for vermicular graphite production modifies the melt by adding magnesium, cerium mixed metal and inoculant. This control loop uses the measured values from the thermal analysis of the ready-to-pour melt and retroactively corrects the control variables of the next ladle via a feedback loop. The fluctuations in the base iron caused by adding cupola iron are a disturbance variable here. A switch to iron completely melted in the induction furnace simplifies this process considerably (Fig. 7).

Fig. 7: Comparison of process chains for the production of components made of cast iron with vermicular graphite. The inductive melting method introduces significantly less interference into the control loop for producing the desired graphite morphology using Mg/Cer mixed metal and inoculation treatment.

Planning and implementing the conversion

The engineering team

"What should the dimensions of our melting plant be?" "How can I increase productivity?" "Which upgrades will pay for themselves most quickly in terms of ROI?" – These are all questions foundry operators ask themselves when converting from cupolas to induction furnaces. And they are important questions, as they often entail large investments or can boost earnings. In order to be able to perfectly design, scale and plan a melting plant, ABP has developed the Meltshop Designer. This determines which solution is the best when it comes to material flow in the foundry. ABP experts can develop simulations for different foundry situations in close coordination with the customer's staff involved in the process, present alternatives when setting up the furnace, and include different configurations from the ladles to the filling of the molding system.

The metallurgical team

The metallurgical team must have expertise in cast iron production using the induction furnace as well as skills in producing the desired nucleation state. In contrast to the cupola, this is largely produced in a synthetic manner in the induction furnace. The metallurgical team creates a digital twin of the complete metallurgical process chain to evaluate and optimize all process steps in advance of the conversion from cupola to induction melting operation.

The energy efficiency of induction melting depends crucially on the sequence of materials to be batched and the timing of the recharging. These factors are incorporated into the Digital Twin's knowledge base, so the Zorc Genesis AI calculates batch composition and melt sequence based on this expertise. To determine the optimal time for charging, based on the current coupling and Curie temperature, ABP has developed the patented Opticharge tool mentioned earlier.

The synthetic nucleation state is generated in several stages. It starts in the furnace via the adding of silicon carbide (SiC), whose special physics during the dissolution process forms the basis for the nucleation state [5]. The next step in achieving the desired nucleation state for nodular graphite casting and vermicular graphite alloys involves adding cerium mixed metal (CerMM) and carbon prior to treatment. Cerium compounds have a high density and are not lost during the treatment process. In the production of thick-walled cast iron, it must of course be considered that cerium compounds can result in undesirable graphite shapes, up to and including chunky graphite. This requires a metallurgical balancing act in which the addition of elements such as antimony or bismuth creates an equilibrium that has a positive effect on graphite precipitation.

The interlinking of consulting services and production operations is a core feature of the metallurgical team. The service team's broad experience base enables them to provide comprehensive training to the production team. The production team, in turn, has precise knowledge of the foundry processes and brings with it a significant wealth of experience that must be incorporated into the process planning.

The specific implementation

Scheduling

The timeline for the conversion from a cupola to an induction melting unit is the biggest challenge and requires a strategic approach. It is a challenge for the planning team, the executing engineering team responsible for the implementation and the production team to use the newly established process chain. In the process, the condition of the existing production environment, including all integrated aggregates, must be converted to the newly designed process and plant technology, considering all parameters and influencing variables.

Engineering phase

The Meltshop Designer is used in the engineering phase, which is what makes its variability so valuable. ABP experts can simulate all materials by accessing or incorporating new materials into a large database. Various one-off or periodic events can also be simulated, for example power supply limitation, a not exactly uncommon problem in which energy providers reduce power supply at certain times when more power is currently being drawn elsewhere in the power grid. The basis for the simulation is data: The added value comes from the analysis performed by the ABP experts based on the linkage of the information. The simulation results in the recommended actions for moving from the cupola to the induction furnace.

Zorc's Genesis AI also works with data and uses the digital twin as a basis for controlling process flows. In the planning phase, it is used to simulate and optimize the processes in the newly designed melting operation. This helps to identify and avoid bottlenecks early in the planning process.

Transition of metallurgy from the cupola process to induction melting

While the cupola is still active, measurements of the metallurgical state are performed using thermal analysis, spectrometry and combustion analysis. Here, the precise mapping of the data is important to establish a link to the microstructural and mechanical properties obtained. This is provided by the service-oriented software Foundry Cloud developed by Zorc. Modern thermal analysis uses double-chamber crucibles to simulate the final stream inoculation or casting stone inoculation.

On the one hand, the data obtained during this phase are used to define the target parameters for the new melting process in such a way that the core parameters of the melt remain stable or improve during the conversion to ensure the smoothest possible transition during further processing of the components. This phase should last about four weeks to cover all metallurgical conditions. When the induction melting process is ramped up, it usually takes another four weeks for all aspects of the new process to be correctly mapped in the digital twin.

Tools for optimal planning and control of a modern production process

How is the metallurgy controlled and how are the transport logistics and the melt treatment and casting units controlled? Both can be answered in one sentence: with the Zorc Genesis AI, whose expertise is integrated into a digital twin. The Digital Twin mirrors the charging and melting process by replicating the physical processes in the furnace in a virtual environment. In doing so, it considers the pouring both in the charging troughs and in the furnace itself. It simulates the oxygen balance and the dynamics of the nucleation elements. It also describes the energy flow in the induction melting furnace, including the cooling capacity.

During the pouring and transport process, it tracks the interactions with the environment (atmospheric oxygen) and calculates the heat dissipation via convection and radiant heat. The Digital Twin has a comprehensive knowledge of the physics and chemistry of the wire feeding processes as well as the pour-over processes and simulates the associated nucleation processes. In the casting and solidification process, the twin performs simulation-based analysis to determine the effects of melt composition and nucleation state on part properties. In addition, the digital twin also keeps an eye on seemingly everyday processes: It calculates the duration of a forklift run, recognizes bottlenecks when tapping the iron and takes into account the break times of the forklift drivers.

Digital Twin Data Sources

To compare its forecasts with the current actual situation, the digital twin requires information about the current production status. The Digital Twin's "senses" are the measuring devices in the production environment. Traditional measuring instruments such as spectrometers determine the melt composition, while temperature measuring lances and pyrometers measure the bath temperature. Methods such as thermal analysis capture the dynamics of the solidification process and allow conclusions to be drawn about the nucleation state and graphite morphology.

Systems such as the Zorc Track & Trace are used to determine the position of forklifts, even in halls with "Indoor GPS", as well as to track the ladle position in the process flow. Service-oriented software such as the Foundry Cloud provides all the services required to read data sources such as measuring devices, PLC systems or IoT devices and convert them into a structured database.

And what is the AI doing? Production planning and process control

AI has the task of planning the next steps in the short, medium and long term in order to effectively solve the tasks set in the production process. Much like a chess computer, it searches for the optimal strategy. The way the task is formulated can influence the character of the production. The question is whether cost priority is higher than meeting deadlines or vice versa. All these factors can be adjusted with the help of parameters. The AI calculates optimal future trajectories while developing a plan that takes into account resources such as labor, energy consumption, electricity costs, etc.

Interaction with process owners and employees

The Zorc Genesis AI creates a workflow plan in much the same way that a navigation system suggests a route. Analogous to a navigation system, the human planner is not obliged to strictly adhere to this plan. Instead, he may deviate from it. The Genesis AI then calculates an adjusted workflow based on the new situation.

Tasks within the workflows are distributed via the Foundry Cloud to employees using mobile or stationary devices. The feedback is used by the Genesis AI to capture the current status of production and dynamically adapt workflows to deviating situations. A self-optimizing process: by following the procedure described above, production within FoundryCloud generates data that is used to refine all aspects of the Digital Twin. This, in turn, enables the Genesis AI to further optimize all aspects of production while learning from employee experience.

Conclusion

To achieve decarbonization targets, the transition of operations from fossil fuels to virtually carbon-neutral production is of great importance. How can foundries with cupola operations address the requirement for decarbonization? For the experts at ABP Induction and Zorc Technology, the solution lies in switching from cupola to induction furnace operation and controlling the processes using digital tools which incorporate AI. The induction furnace has clear advantages when operating with high-grade scrap, and additional tools such as ABP OptiCharge and the ABP Meltshop Designer can increase production capacity and throughput and improve energy efficiency. The flexibility of production with stringent quality requirements is achievable with the help of dynamic production planning and control using digitalization. The use of Zorc Genesis AI represents the next evolutionary stage of these systems and will result in increased efficiency even in complex production situations.

https://www.abpinduction.com

https://www.zorc-technology.com/

References
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