Irregular, small to large-area solid metallic thickening, frequently several millimetres thick. Such defects often run parallel to the casting surface, their own surfaces being rough. The edges fre-quently exhibit so-called “undercuts”. Castings with solid scab-bing often exhibit sand crust inclusions.
Incidence of the defect
Scabs may appear on all casting surfaces when pouring into green sand moulds. They occur primarily at the top of the mould cavity, although their formation on the mould bottom in the form of “rat tails” is likewise possible. Highly compacted parts of the mould cavity are also at risk. Scabs can be created when the mould wall is heated by radiant heat, but may also be caused by the direct flow of heat from the molten metal into the mould wall.
During pouring, parts of the sand mould near the surface are heated up rapidly. A dry sand crust a few millimetres thick forms on an over-wetted stratum (condensation zone). As quartz tends to significantly expand at 300°C – 575°C, this causes a high compressive stress in the sand crusts which, in the case of poor wet tensile strength in the condensation zone, can lead to them being separated from the stratum.
- Too little bentonite or poor bentonite quality in the moulding sand
- Proportion of fine quartz in the sand too high
- Moulding sand too fine
- Insufficient conditioning of bentonite
- Degree of chamottization of the sand too low
- Too much salt in the sand
- Excessive or irregular compaction
Gating and pouring practice
Sand heated up too severely by inflowing metal and long exposure to radiated heat
- Increase bentonite content in the moulding sand
- Use bentonite with a higher specific binding power
- Reduce proportion of fine quartz in moulding sand
- Reduce recirculation of dust; if necessary use bentonites with higher montmorillonite content
- Use coarser moulding sand
- Improve conditioning of bentonite; prolong mixing times; pre-wet used sand
- If practical, reduce amount of new sand to increase the de-gree of chamottization
- Improve water quality; switch from well water to public sup-plies; perform partial desalination
- Reduce compaction; reduce compacting pressure
- Ensure even distribution of sand and more uniform compac-tion of the mould
Gating and pouring practice
- Improve gates to avoid local overheating through excessive amounts of metal
- Aim for shorter pouring times and therefore shorter exposure to radiated heat
Scabs are typically found on castings from bentonite-bonded sand moulds. The inflowing metal heats the upper surface of a mould. The evaporating water is condensed in underlying layers and weakens the bond in over-wetted regions. At the same time, the compressive stress in the heated sand layers increases through expansion of the quartz. Surface crusts separate. The tendency for such defects to occur intensifies with in-creasing compressive stress and declines with increased wet ten-sile strength.1 Reference 2 deals with compressive stress and methods of its measurement.
It is known that compressive stress increases with higher packing density of the quartz grains. A dramatic increase takes place when the proportion of fine quartz in the moulding sand is in-creased.
Wet tensile strength is described in publications 3 and 4 as a pa-rameter for determining susceptibility to scabbing.
The influence of various moulding sand admixtures on the form-ation of the sand crust has been studied. In general, the risk is re-duced by the use of all materials that increase wet tensile strength and lower compressive stress.
Publication 1 examines calcium-type and activated bentonites, wood flour, powdered peat, inert fines and starches as well as carbon dust. Here it was clearly ascertained that the use of soda-activated bentonites markedly reduces susceptibility to scabbing. Another publication 5 emphasizes the impressive reduction in sand crust formation resulting from the addition of carbon carriers with low softening points, which reduce the compressive stresses. Publication 6 reports on the effects of various coal dusts on expan-sion pressure and wet tensile strength. The study determined an intensified increase in compressive stress when using pit coals with 10 – 35 % volatiles. The stress is reduced by coal mixtures which contain additional carbon carriers with low softening points.
The risk of scab formation when using bentonite-bonded moulding sands can be reduced by increasing the bentonite con-tent, reducing the fine quartz content and using coarser sand. When such defects occur, the degree of mulling of the sand should also be checked. Likewise, too much salt in the water can result in scabbing. In certain cases, such defects could have been avoided by using a partial desalination plant.
Where there is a very high packing density in the mould, a small increase in compactability may counteract the formation of defects.
Likewise, the formation of sand crusts can be avoided by modifying the gating system.