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Metal Casting Technologies : Whos who September 2011
METAL Casting Technologies September 2011 65 cast iron produced from a charge consisting of less than 15% steel scrap will have nitrogen levels less than 50 ppm whilst a charge consisting of more than 50% steel scrap can result in nitrogen level of more than 100 ppm. Some low quality carburisers can contain high levels of nitrogenous compounds and can easily increase the nitrogen level in the melt to dangerous levels. This is particularly the case if a high percentage of steel scrap in the charge necessitates large additions of carburising materials. The absorption of nitrogen from the atmosphere is unlikely, due to the high bonding energy of the nitrogen molecule. The absorption of nitrogen from mould and core binders is far more significant. The use of high nitrogen containing mould or core binders or the build-up of nitrogen in reclaimed sand can contribute to fissure defects as can inadequate mixing that results in poor distribution of the binder components. Cold-setting and hot-setting resin binders frequently contain proportions of urea resin with a high nitrogen content. The hot-box shell process uses hexamethylene tetramine as the hardener and the isocyanate components of the cold-box hardener also contain nitrogen. Furane binder systems are basically composed of furfuryl alcohol which is nitrogen free, however, these resins may be modified with urea, urea-formaldehyde and phenolic resins. The urea and urea- formaldehyde components contain nitrogen so the percentage of nitrogen in the binder system can vary significantly depending on the proportion of these components. The presence of even a small amount of hydrogen will serve to lower the overall solubility of nitrogen and enhance porosity formation. Remedies Limit the nitrogen level in cast iron to less than 80 ppm in heavy casting sections and less than 120 ppm in thin sections. Limit the amount of steel scrap in the charge, particularly with heavy section castings. Use low nitrogen mould and core binder systems and limit the addition. Ensure that the sand mixer is well maintained so that binder components are evenly distributed throughout the mix. Small additions of red iron oxide to moulding sand or the use of mould coatings containing red iron oxide can be effective in limiting pick-up of nitrogen from binders. Vent mould and cores generously. If possible increase carbon equivalent as carbon and silicon increase the solubility of nitrogen in iron. The harmful effects of free nitrogen can be neutralised by the addition of stabilising elements such as titanium or zirconium. Proprietary inoculants containing these elements are very effective in this regard and typically titanium additions of 0.025% or zirconium additions 0.05% are sufficient to suppress porosity. ■ Figure 3. Severe nitrogen fissure defects at the surface and sub- surface of an iron casting Figure 4. Micrograph of high nitrogen cast iron showing compac- tion of graphite flakes References 1. Control and Prevention of Casting Defects, BCIRA Publication 2. Common Metallurgical Defects in Grey Iron, Elkem Publication 3. Porosity Defects in Iron Castings, Dr. R. L. Naro, AFS Casting Congress 1999
Whos who September 2012