By James D. Mullins
Mullins Professional Services
This was a phrase quite frequently used by old time foundry men to describe the often-misunderstood science of metallurgy and founding of cast irons. At first glance this seems a contradiction in terms, but as one realizes what happens to the metal during melting and the casting, following these guidelines, it should begin to make good sense.
How can you melt cold?
What this really means is that the metal should never be superheated any more than necessary to dissolve the carbon, whether it is in the form of a carbon raiser or from virgin charge materials, and other alloying elements. The reasons for this are several; superheating destroys nucleation in the melt, which in turn allows the carbon to form as carbide rather than graphite. This changes the nature of a cast iron from a softer graphitic material with less shrinkage to one that is more brittle and very sensitive to the cooling rate (section sensitivity) of the casting. This all happens without any appreciable change in the chemistry, so looking at chemistry alone will not
give a clue to nucleation changes. Secondly, at higher superheat temperatures oxidizable elements will be lost, the reaction of the metal to inoculation can be poor or erratic, while refractory and energy costs increase. These same changes will happen at longer holding times even though the metal temperature does not seem to be excessively high. Therefore holding should always be done at as low a temperature as possible, especially over weekend and holiday periods. This holding temperature may be as low as 2400 F for some foundries.
Measurement of changes to the nucleation condition should be done for each heat of metal on a continuous basis. Not only does this give information about the melting process, but can also indicate changes coming from charge materials and alloy additions. Using chill wedges or some type of thermal analysis for measurement of the nucleation condition of the melt, should be done when the chemistry is checked, just prior to readying the metal for pouring. In either case we are looking for is the amount of undercooling present in the melt. Undercooling is defined as cooling below the normal solidification starting temperature. Even a small amount of undercooling can change
the type of graphite formed, while more will increase the likelihood that carbides may form. This is easily seen as an increase in chill value of a wedge. Variable undercooling and nucleation values can be the reason why response to inoculation can be erratic. Certainly more undercooling in the melt requires increased inoculation to achieve the same end result. Also note that undercooling can be increased, by increasing certain elements such as Cr, V, etc. in the melt.
So how do we pour hot, when we melt cold?
Of course preheating refractory lined ladles is important to pouring hot, but so is removing impediments and other time delays in the metal transfer system. Another way is to use insulated ladle linings and definitely cover all ladles to retain as much heat as possible.
Pouring hot usually reduces or totally eliminates any cold iron type defect. These are low fluidity problems like misrun, cold shuts, cold shots, some slag defects, and short pours due to cold metal and backpressure. Other metal defects aggravated by low pouring temperature are most gas blows, pinholes and improper feeding from risers. Cold metal can trap gases at or near the upper surfaces. Cold metal and gating only into the heavy sections can produce large temperature gradients in a casting, causing feeding problems and certainly microstructure variations.
You have probably noticed that the exact temperatures of superheating and pouring have not been defined. This is because these temperatures are specific only to a single foundry/line or even each separate casting operation in one consolidated foundry.
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Most of the above discussion has been primarily about gray iron, but the rules certainly apply to most ductile iron operations as well. Additionally, tapping colder (from lower melt temperatures), when making ductile iron, will increase the magnesium recovery from the treatment reaction and when using MgFeSi, increases the amount of nucleation sites. The treatment reaction especially with pure Mg removes many of the nucleating particles and increases undercooling. This iron is then most often in need of serious inoculation, so temperatures must be higher to insure dissolving larger additions. Lower temperatures have one benefit though and this is that the fading of the
Mg and inoculation is usually reduced. One more issue to keep in mind when making ductile iron is that colder pour temperatures invite more slag and dross to form. This slag can react with carbon in the iron forming small CO gas holes.
So melting cold and pouring hot requires balancing the good with the bad. However the benefits of following these rules - increasing quality, while reducing scrap and processing costs are obvious. See if your operation is doing all that can be done to follow this old axiom.
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