Written by: George English
EMC (Electro Magnetic Casting)
The use of EMC casting has been around now for decades and is not new to the aluminium industry.
EMC favours itself to wrought alloys that are susceptible to hot cracking in the 3xxx and 5xxx series, which is why large volume aluminium casthouses of these alloys have opted more recently for this technology.
The previous and somewhat still in use ‘conventional’ method of DC casting (Such as Wagstaff Epsilon (TM)) can require a lubrication method to act as an interface between the mould bore and the metal contact face in a bid to ensure a clean and smooth rolling face surface.
Several lubricants are available that can work such as the pre casting method of an application of grease such as Pyroteks water soluble ‘Varma’, ‘Kluber’ Metal Star 820 or ‘Mobil’ SHC230. I have experience with these methods but surely other alternatives exist.
An alternative to an application of grease is the continuous oil feed from the mould head, although this is no longer a preferred solution due to the water treatment requirements to remove emulsified oils from the cooling water and the level of contamination.
EMC does not require any lubricant since no interface contact is made between the metal surface and the mould. i.e. No primary cooling takes place. As such shell zones of typically 1-2mm can be avoided (actual measurements for EMC are around 0.1mm - 0.2mm so significantly less). The benefit of 'no' shell zone is that there is no requirement for expensive scalping to remove the inhomogeneous area between the rolling face and the bulk microstructure of the product. Since there is no contact and no drag forces on the liquid face of the ingot, casting speeds in excess of 80mm/min are achievable.
Conventional Casting (Direct Contact) EMC Technology (No direct contact)
Another major benefit of EMC over conventional tooling is the avoidance of edge cracking during hot rolling. Edge cracking can occur due to the same shell zone which exists around the periphery of the ingot, previously removed by scalping from the top and bottom of the ingot, the sides can remain unscalped. Some mills do insist that the ingot profile is altered (physical design of the mold) or that the sides to be scalped either partially or fully, however this is not always achievable given the age of the plant and the technology employed. Again to reiterate this is not required as EMC claims not to create a shell zone worthy of creating such problems.
On the downside the EMC casting technology requires a very complex automated system that requires the need for comprehensive technical knowhow, this can be an issue during greenfield start-ups for both maintenance and process engineers. The equipment itself is extremely expensive, has longer lead times from the OEM (such as Wagstaff) to manufacture and has a high energy requirement for the electrical field used to ‘control’ the metal interface and push it away from the mould bore wall. (which can be energised for over 2 hours during the entire length of the cast).
LHC (Low Head Composite) Casting
LHC technology is a fairly new technology and a successful compromise between conventional DC casting (with the use of a lubricant, continuous lube or grease) and EMC casting previously mentioned above. LHC offers the flexibility of variable widths up to 200mm!
LHC can also offer savings over conventional Wagstaff tooling when we discuss yield in respect of side trimming and scalping. In theory not as comprehensively claimed or large as EMC but midway between conventional tooling and that of EMC. The shell zone is controlled by means of the insulating graphite mould liner and the metal head and quoted as less than 1mm. LHC can also decrease casting time over conventional tooling by increasing the casting speed (for 3xxx and 5xxx series 65 and 56mm/min respectively or speeds thereof).
The ease of use of LHC technology over EMC and conventional tooling can be demonstrated in various plants across the world. It has a large operating ‘process window’ and can operate in extreme environments such as the Middle East, Siberia and Iceland! The formation of the butt can be greatly controlled by the use of split jet technology, which employs a second series of water jets that are activated to assist when required giving more or less heat transfer as required. The use of the second jets and the graphite (insulating) mould liner during steady state casting gives the ‘Low Head’ required to assist in the secondary cooling and reduce the primary (responsible for the creation of the inhomogeneous shell zone). This low head control the shell, reduces liquation, gives excellent metallurgical properties and adds to the smooth surface finish to the ingot. The improved as-cast surface is quoted similar to that of EMC ingots, and requires less scalping and edge trim than conventional DC ingots cast with such technologies like Epsilon moulds.
During the run condition the metal head is gradually lowered to reduce the primary cooling effect from that of the graphite mould liner and greatly reduce the shell zone.
Comparisons of EMC and LHC.
If we consider the comparison of EMC and LHC based solely on y own personal experience, some OEM literature, white papers and claimed facts, then EMC will produce a slightly superior final product to that of an ingot produce with LHC technology, however we know this given past exposure not necessarily to be the case. Scalping.
A significant benefit of EMC in the reduction / elimination of scalping and is highly acclaimed. Unfortunately this benefit is not being realized in full at certain production facilities, scalping is still taking place as the ingots are not purely flat, they repeatedly demonstrate a dog bone shape which requires scalping to bring them back into rectangular form. This additional scalping adds losses into the system and the complication of processing difficult run-around scrap in the form of scalper chips.
The dog bone shape may be due to casting speed and may be corrected with appropriate casting parameters, however, at the stage of writing this article it was not confirmed to be that was actually the case and further intensive process work with the OEM and customer is required. LHC technology does not exhibit this problem and comes with an OEM guarantee for the ingot profile to within specific tolerances as below.
Ingot Geometry
The ingot geometry of an LHC produced slab is regular, repeatable and comes with a Wagstaff guarantee with tolerances that are tighter than that of EMC resulting in fewer process losses and offers the ability to meet external customer demands. See table below.
Further to the specification outlined generally any OEM supplier of mold sets would have the following acceptance criteria associated with the order for a new LHC mould set.
Side Trimming
As large bore EMC casting for both 3xxx and 5xxx series alloys is relatively new, it should be stressed that some new (wider) bores are somewhat a more difficult to predict when it comes to the design of the mould and the actual physical size of the ingot. Although at the time of writing this article, this was the case I believe this process should be more robust and the OEM should now be able to provide a guarantee similar to such technologies as Epsilon and LHC. This should be requested during the design stages of the project.
Capital Investment and Delivery requirements
The capital required to procure the EMC moulds is almost twice that required for a set of LHC moulds. With lead times for both EMC and LHC in excess of 30 weeks (again please contact the OEM for an accurate assessment). EMC technology may actually exceed 52 weeks.
Challenges and Opportunities :
During the commissioning of the project it would be suggested to look at the analytics of the ingot against the design of the mould bore and to understand and optimize the EMC moulds to minimise production losses at the customers hot mill.
This would be in relation to:
Ingot Geometry
Butt Swell
Concavity / (Convexity)
Bow and Twist
Surface finish
Casting Speed
Metallurgical Testing Shell zone Average Grain and Cell size Inclusion level Hydrogen content Shrinkage cavity depth It would be a good idea to produce process control charts depicting KPI’s regarding all aspects of geometry and metallurgical performance to understand the ‘nominal’ average(s) and deviations about this norm. Water Quality Water quality plays a significant role when it comes to casting with LHC technology, far more than it does with conventional tooling and EMC. Stability and consistency are key to success in terms of water treatment, with target values were specified by the OEM as below.
Water temperature
For commissioning purposes the OEM requires consistent water temperature. The system shall maintain the water temperature within +/‑ 3°C (+/‑ 5.4°F) between drops. Water temperatures between 20° and 30°C will be maintained yearly with a target of 25˚C.
Chemicals used to control water chemistry affect quenchability and shall be added on a continuous basis.
All solids >1.0 mm in any dimension shall be removed from the cooling water, prior to the casting table, with a filtration system.
It has been noted from previous experience that some MENA casthouses employ contract hire to perform continuous water monitoring and treatment to get the best from the system and maintain a constant and stable water process. This is really a practical idea and comes at a cost, nevertheless the cost is justified in the event of long delays and downtime due to water related problems. and it has happened!
Conclusion It would be my conclusion that It is still relatively unclear with which technology is best suited for new and developing casthouses going forward casting the 3xxx and 5xxx series alloys. Should the casthouse, or sales and marketing team look towards external sales of rolling slab in 3xxx and 5xxx series alloys then the casthouse must get tighter in terms of geometrical specifications on the ingots cast whether that is with LHC (and the guarantees associated with this tried and established method), or with EMC.
Previous experience suggests that EMC at the site we monitored and supported was not capable to offer external sales due to geometrical differences and dog bone shaped ingots. The cast ingot thickness nor widths that they can readily use in European, American or Asian rolling mills.
Presently at one site the EMC is not achieving the desired geometrical specification. In particular the widths (with a +10-12mm addition to the ordered specification) are a cause for concern. The additional width creates additional run-around scrap at the hot mill, the volume to which is again yet to be determined, but does come with additional handling risk, remelt costs and adds another process step in the route to rolling.
Slabs have been measured with an irregular profile (dog bone) which requires scalping prior to hot rolling, this additional step completely negates any benefit that the expensive EMC technology which was the desired intention and implemented to do. Scalping should only be required for any surface damage to salvage a previously unusable ingot, not as a routine process for irregular / concave shaped ingots.
The major attraction and selling point for EMC moulds and technology, based on topics previously discussed, is that it produces virtually no shell zone as there is no primary cooling, therefore to scalp for geometry makes no sense at all and the benefit is not realised.
Predicted casting time using EMC is significantly less than LHC, however correcting the profile and dog bone shape may result in an increase in casting time as the run speed would require lowering from 81mm/min to something nearer that of LHC.
Current speeds employed for LHC for AA3104 vary but as a rule of thumb we can say around 63mm/min and 56mm/min for AA5182.
Surface finish is comparable for both EMC and LHC with shell zone on LHC less likely to produce as much scalping waste as the concave ingots presently do.
The procurement of LHC moulds would give any sales and marketing team the opportunity to sell slab with a variable width of up to 200mm in increments of 50mm, the only requirement would be the purchase of starting heads to match the mould opening, an inexpensive alternative to ordering a full set of EMC moulds and heads for each new product. Of course depending upon the other dimensions and in particular width. (520mm or 600mm).
The demands of water treatment are far more challenging for LHC technology, a full water treatment evaluation would be required if a conversion from Epsilon or EMC to LHC was requested. This should be conducted before further investment in time and money is demonstrated to conclude the opportunities to cast with LHC. Your contractor monitoring the water quality may be able to help out here and give technical justification before any change be made.
It would be my recommendation that prior to any plant implementing LHC or EMC a full evaluation be made on scalping scrap generation, ingot shape geometry, water treatment and cost comparisons before your final decision is made.
Many thanks for taking the time to read this blog, please feel free to share with your friends and colleagues.
Should you still require further help on this particular subject reach out and please contact : albergtech@gmail.com
George
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