Irregular-shape, non-metallic inclusions, frequently on upper casting surfaces, which may occur in association with gas blow-holes.
Incidence of the defect
Slag is not only found at the microstructure grain boundaries but also on the surface of the casting. Slag may often appear in association with gas cavities.
Highly viscous slags more frequently appear in the microstruc-ture of the casting. Low-viscosity slags rise more quickly to the surface of the casting. However, in the event of severe turbulence and a short solidification time, low-viscosity slag may also be included in the casting. During the casting of iron-carbon alloys, oxide inclusions and slag with a high oxidation potential (e.g. high manganese and ferrous oxide contents) react with the graphite, resulting in the formation of CO blowholes.
Other gases from the melt may migrate into the CO cavities.
Slag and oxides occur when melting metals in the presence of oxygen. These frequently react with the furnace or ladle lining. When melting grey cast iron in a cupola, the oxides generated also react with the ash residues from the pit coal coke. In most cases this results in silicate slag. The metal flowing into the mould also forms oxides, which may react with the moulding sand and its components. Likewise, highly oxidizing additions (alloy com-ponents, inoculants) may be responsible for the formation of ox-ides and slag.
- Oxide content of the charge too high
- High impurity levels of oxides and hydroxides in charge materials
- Poor or slow dissolution of inoculants
- Ladle lining too highly reactive
- Poor deslagging of the molten metal
Gating and pouring practice
- Casting temperature too low and pouring rate too slow
- Too much turbulence when pouring, and poor slag precipi-tation
- Proportion of inert dust too high
- Proportion of bentonite too low, bentonite poorly developed or has poor binding capacity
- Oxygen contents in mould cavity too high
- Proportion of low-melting-point compounds too high
- Keep contents of elements which strongly react with oxygen as low as possible, e.g. aluminium, magnesium, manganese; if necessary reduce them; use materials without impurities
- Optimize the use of inoculants; avoid slagging; in case of failure to dissolve, reduce grain size
- Dry ladle prior to use; hold molten metal in the ladle for shortest possible time; avoid alloying in the ladle; skim ladles carefully; use slag binders; if necessary use teapot ladles
Gating and pouring practice
- Increase pouring temperature and shorten pouring times
- Improve gating, keep pouring basin full, use slag traps; avoid turbulence when pouring; install strainer core
- Reduce proportion of inert dust; if necessary add new sand and reduce dust recirculation
- Use bentonites with increased binding capacity and high montmorillonite content – if necessary increase proportion; improve development of bentonite
- Increase lustrous carbon production in the moulding sand or use carbon carriers with higher lustrous carbon producing capacity
During melting and pouring, many metals react with atmospheric oxygen, whereby alloying elements can accelerate or retard the process.1 If no complete oxide layers are generated, the quanti-ty of oxidation products will usually increase linearly with time. These processes have been the subject of early studies, principal-ly in relation to non-ferrous metals. In the case of iron-carbon al-loys, the process is modified by the creation of carbon monoxide. Solid oxides are only created below the so-called silicon isotherm; these then form into slag because other alloying elements such as iron and manganese also react with the oxygen.2 – 4
SiO2 solid + 2 C ➝ Si + 2 CO gaseous
Manganese silicate slag, the melting point of which is lowered by manganese sulphide, plays an important part in the creation of CO-slag pinholes through carbon oxidation (see chapter on pinholes).
The formation of slag can be attributed principally to reactions with atmospheric oxygen.1 It occurs increasingly when tempera-tures in the ladle are low. Likewise, oxidation starts earlier if ele-ments with higher affinity to oxygen are present, e.g. aluminium and magnesium.
The longer the time the melt has for oxidation, the stronger is the slagging of the oxides.2
As slag can form continuously and increasingly at low tempera-tures on any iron melt, it is necessary to precipitate the products of the reaction, this taking place in pouring basins and slag sepa-rators. Slag is also trapped by dross filters 5 or, more effectively, by strainer cores.