Defects Prevention through Proper Metal Transport and Ladling
By Steven Harker, Technical Director Acetarc Engineering. Co. Ltd
There’s an old Irish joke, probably also claimed by many other countries, where a English tourist, lost on the back roads of Ireland, asks a local for directions on how to get to a scenic village, to which the Irishman replies “Well if I were you, I wouldn’t start from here.”
I’m not sure how far around the world the joke travels, but over time I’ve come to think that behind the Irish whimsy there’s a lot of wisdom buried in the punchline. Basically if you are starting out from the wrong place you are just going to make it harder for yourself to achieve your goal.
The problem for many foundries is that they do not have a choice in where they start from when it comes to producing castings. They have to deal with what they have got, here and now, but foundries, especially if they are a long established business grow and develop over decades. Adapting existing work methods or bringing new processes to meet the demands of changing markets means that a typical foundry today is trying to achieve production targets while often working within constraints laid down many years ago. This is compounded by the need for ever greater efficiency and cost reduction.
Moving to a new purpose built production facility, with all new equipment, and where everything is designed for maximum efficiency, while a nice idea; is not an option for most foundries. Therefore, the alternative is to look at what can be done to get the most out of a foundry’s existing set-up and equipment.
Speaking as a foundry equipment supplier, I would say that it always helps to invest in new equipment whenever possible. This isn’t just self-interest as even the design of the humble ladle improves over time, and if a foundry is using worn out or obsolete equipment then they are just making life harder for themselves. However much can be achieved by first looking at what you have and seeing if it can be better used.
Since safety should be the priority when handling molten metal, assume that it’s a given in the following comments to avoid repetition. I’ve also limited the following to ladles but many of the points can be applicable to other foundry systems. I’d therefore like to take this opportunity to highlight some areas where attention to molten metal transportation and pouring, can have an impact on defects prevention and scrap/waste reduction in a foundry.
Metal Distribution & Handling
The purpose of a ladle is to transport molten metal and pour molten metal safely and efficiently. Whatever method used to transport the ladle must be able to deliver the molten metal from the melting/holding furnace to the point of discharge quickly so that temperature losses are minimized. If your metal delivery system can’t do it then you are already getting off to a bad start.
Sometimes the defects caused in castings are due to poor metal handling/ pouring will get put down to issues with the moulding process. However checking that your metal handling & pouring are working efficiently won’t do any harm and should prove cost effective.
Molten Metal Temperature
Ideally molten metal tapping temperature at the primary melter or holding furnace should be as close as possible to the required pouring temperature.
Sometimes, due to the foundry layout and/or the need to carry out other operations such as in-ladle ductile iron treatment process, unavoidable temperature loss in the ladle can become an issue and the window of time during which the metal is usable can become restricted.
As the metal cools, its fluidity decreases which reduces the ability of the metal to flow into all parts of the mold (misrun) and can lead to other defects such as cold lap (cold shut) and some types of gas porosity defects.
The obvious solution is to make sure that the molten metal is delivered to the molding line when it’s at a temperature where it can be used. The point to remember here is that it is not the first mold cast but the last one in the run that is important.
One of the most commonly listed solutions to cold metal is to “superheat” the metal to compensate for the temperature losses. However in these energy conscious times this is quite a wasteful solution and should only be done after other options have been considered and the cause of the problem fully understood.
Using excessively hot metal can in itself also be a cause of casting defects such as:
- Sand burning
- Internal shrinkage cavities
- Poor surface finish
General Ladle Handling
Firstly look at the ladle handling and see if it can be improved. Much can be achieved in the typical foundry by careful attention to the ladle handling procedures. Good organization of the foundry metal handling system can result in significant energy savings and casting defect reduction with lower scrap rates, and better consistent casting quality, without the need for capital equipment purchases.
- Are there any bottlenecks or obstructions on the route that can be removed?
- If you are moving the ladle by crane or monorail, can the travelling speeds be increased without raising safety issues?
- If the ladle is being transported by a fork lift truck, is the ladle designed for that purpose? A ladle that is to be used with a FLT should have an extended freeboard to guard against the metal slosh while enabling the FLT to still travel at a safe and reasonable speed.
- Can the route be made limited access so that other people don’t cause delays if they pass through?
- Can the exchange of metal between transfer (bull) ladles and casting ladles be avoided?
The exception is where the transfer (bull) ladles are relatively large, and transfer distances are long with the transfer ladle being used to fill several much smaller casting ladles.
- Is the ladle the right size? It’s better practice to use the largest practicable ladle size for both transfer and pouring.
- Can the ladle design be changed to reduce temperature losses? Ladle shells can be sized to accommodate both a working refractory lining plus an insulation backing layer that can result in significant reduction in temperature loss.
- Can the ladle be fitted with a cover to aid heat retention?
- If ladles are used intermittently, can they be put under a pre-heater to maintain temperature when not in use?
Ductile Iron Treatment Ladles
If the ladle is used for a treatment process, is this process causing a significant temperature loss either through the process reaction itself or through the time taken to complete the treatment process, including slagging off operations etc. If so, can you change the treatment ladle method? Covered treatment ladles such as the tundish type and teapot spout treatment (mod-tundish) designs have significantly reduced temperature loss when compared to the open top (sandwich) process. Using the wire feed treatment method can reduce temperature losses but this may be due more to using ladles specifically designed for the wire feed process rather than due to the actual process itself.
It can greatly assist the quality and consistency of the treatment process if the ladle can be charged with the correct proportion of additives with respect to the amount of metal to be treated. Throwing a couple of bags in and then one for good luck isn’t perhaps the best way. Also if a timer system can be set up to give the ladle operatives warning if the metal hasn’t been poured within a set time and there is a danger of fade occurring. This allows them to pig off the metal rather than pour suspect castings.
Correct pre-heating of the ladle prior to use and during interruptions in production, can help eliminate the risk of random ladles with “cold” metal being poured. The use of a correctly designed ladle pre-heater is also much more efficient when compared to open ended gas pipes, resulting in lower gas usage. A well designed pre-heater will have its own combustion fan which gives better efficiency and control, avoiding the need to use expensive compressed air.
Although not as common due to the popularity of modern castable refractories, if ladles are used with a lining that has a high moisture content, such as a “wet” ganister mix or a naturally bonded sand mix and are not properly dried, there is a risk of hydrogen pin-holing occurring as the metal becomes contaminated by the hydrogen given off from the moisture.
Ladle Maintenance – Slag & Dross Inclusion
Ladles should be used with a good quality refractory lining which should be kept clean with any slag/dross removed between refills.
Teapot spout ladles and bottom pouring ladles should have a lip-pour spout fitted so that any residual metal and molten slag can be poured out of the ladle without contaminating the teapot spout neck or the bottom pouring nozzle.
Any slag or dross on the external surfaces of the ladle should be removed as soon as the ladle is taken out of service and not left to build up.
Ladle Pouring & Maintenance
The ladle operator must have complete control over the rotation, and thereby the pouring, of the ladle at all times to be to ensure consistency and quality of casting. Therefore ladles should be correctly maintained with special attention paid to the gearbox and any worn or damaged parts replaced.
The pouring rate in which the molten metal is introduced into the mold has to be fully controllable If not then, at best, scrap metal will be created through over-pour and spillage while at worst the casting will have defects rendering the casting as only good for scrap.
If the ladle gearbox is badly worn or damaged it may be possible for the ladle to continue rotating a little after the operator wants to stop, resulting in uncontrolled flow of the metal into the mold, and usually all over the top of the mold as well.
If the molten metal enters the mold too quickly, and in too large amounts, turbulence can be created in the metal flow which can lead to mold erosion, sand inclusions and casting porosity.
Ladle spouts often erode through contact with the molten metal and these should be kept in good condition to maintain accuracy of pouring.
Metal Waste Reduction
Metal waste used to be seen as not too big an issue since it all went back in the furnace and got recycled but as energy and labor costs only go one way, this view is not sustainable. The old practice of “washing out” a ladle with molten metal to quickly pre-heat it and then pigging off the cooled metal would be considered very wasteful today (and frowned upon by the refractory suppliers). Much more interest is now being shown by foundries in accurately controlling the metal poured into a mold so the amount of metal in the pouring cup can be minimized with no overfill and definitely no spillage. As discussed above, reducing metal wastage due to the metal either being too cold to pour, or, in the case of ductile iron, due to fade can result in significant energy savings.
Often without realizing it, many foundries waste energy and create higher amounts of scrap than they need to by having a molten metal handling and pouring system that is not operating as well as it could, resulting in casting defects and higher than necessary wastage. Correct pouring temperature of the metal is critical to achieving good consistent result. Cold metal results in casting defects and higher than necessary metal wastage while hot metal results in casting defects and increased energy costs; a lose /lose situation.
As I hope I have outlined above, many improvements can be made, often at little or no cost to the foundry, that help minimize or eradicate many of these issues.
Steven Harker, Technical Director Acetarc Engineering. Co. Ltd