Aluminum is a harmless residual element for cast iron, but it has a huge impact on cast iron!
Al is often found in cast iron as a harmless residual element. The main sources of Al are scrap steel, contaminated waste castings (such as cylinders containing pistons), ferrous alloys and non-metallic inclusions in the charge.
The problem often found in foundries is that excessive Al causes hydrogen pinholes. It is well known that Al has an effect on the surface tension of molten iron, and as a result, hydrogen pinhole defects are prone to occur.The following data shows the relationship between Al and hydrogen pinhole trends in the molten iron. The results show that gray iron has a lower surface tension, making it more prone to pinhole defects than ductile iron.When the aluminum content reaches a certain level (approximately 0.2%), the surface tension starts to rise again, which reduces the possibility of pinholes.The most critical Al content range is 0.05%-0.2% for ductile iron and 0.008%-0.2% for grey iron.The conclusion is:Al content should be kept low, preferably below high-risk areas.
At the same time, it must be remembered that the temperature of the molten iron will affect the surface tension, so good insulation of the molten iron is very important.Al also increases slag formation.
Al has virtually no incubation effect, but it can increase the hardness of cast iron and is harmful to the nodularity of ductile iron.It should be known that although the content of Ti in molten iron is generally lower than the content of Al, to some extent Ti has the same surface tension effect as the Al content (%) of Al, and the effects of the two elements on the molten iron are common.The foundry should detect and control the content of both elements at the same time.
Many other elements also interact with Al to affect the performance of the molten iron, or to improve the fertility, or produce harmful effects, such as the combined effect of Al and Ti.
Even if a small amount of Ti is present in the molten iron, it can significantly reduce the acceptable Al content.The figure below shows the combined effect of Al and Ti on the formation of hydrogen pinholes in ductile iron.Pinhole defects are easily formed above the curve.
In ductile iron, the allowable content of Al is 5-10 times that of gray iron. There is no data on the effect of the combined action of Al and Ti on the formation of hydrogen pinholes in gray iron, but it is reasonable to believe that their impact should be It is similar to that in ductile iron. Therefore, in gray iron, both Al and Ti content need to be detected simultaneously.
Since nodulants and inoculants contain certain amounts of Al and Ti, it is important to know their exact chemical composition.The higher the Al content in the ferrosilicon-based alloy is, the better the solubility is, but the increase of slag inclusions and pinholes should be paid attention to. Therefore, the high Al alloy can only be used in the case of low addition (such as inoculation).Special attention should be paid to the Al content when ferrosilicon is used as a charge.
Hydrogen pinholes often have similar characteristics to other types of pores, such as nitrogen pores.One of the characteristics of the hydrogen pinhole is that the inner surface of the hole is covered with a layer of graphite. The hydrogen pinhole is shown in the figure above. This feature is also the same for the nitrogen hole, so it is sometimes difficult to distinguish the hydrogen pinhole from the nitrogen hole.The comprehensive detection of nitrogen, Al, and Ti helps to distinguish the pore type, because high Al and high Ti may promote the formation of hydrogen pinholes, but at the same time, it will effectively neutralize nitrogen to form TiN and AlN inclusions.
The selection of sand core binder and the control of moisture in tide mould sand are also very important to avoid the formation of hydrogen pinholes and nitrogen holes.
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