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Metal Casting Technologies : December 2008
Internal blowholes Internal blowholes in steel castings are not always apparent to the naked eye but are often revealed after machining, radiography or ultrasonic testing. See Figure 2. An indicator of the occurrence of internal blowholes may be observed as the “mushrooming” or “cauliflowering” of metal in feeders caused by the internal gas pressure forcing metal back through the casting. Gases such as oxygen, hydrogen and nitrogen dissolved in the melt are expelled as the casting cools causing internal porosity. See Figure 3. Chemical analysis can reveal high oxygen or nitrogen levels in castings; however hydrogen analysis is not so easily analysed from a casting and requires special sampling techniques from molten steel for analysis. Control of oxygen is achieved by deoxidation of the melt usually with aluminium. There must be sufficient residual aluminium to avoid surface pinholes, usually 0.03% residual is sufficient, however, steels poured into green sand moulds may require residuals as high as 0.06% to safeguard against surface pinholes. Hydrogen and nitrogen are readily absorbed into molten steel and there solubility decreases markedly as the steel cools and solidifies. Hydrogen levels of 7ppm and nitrogen levels of 150ppm are generally regarded as being the maximum safe levels in plain carbon and low alloy steels. High chromium steels can tolerate much higher levels of both gases. The main source of nitrogen and hydrogen is from the atmosphere and an occurrence survey may reveal a higher incidence during periods of high humidity. Nitrogen can also be accumulated from returns cast into moulds bonded with nitrogen containing binders. Control of nitrogen and hydrogen is achieved by the steel making process and ensuring all late additions to the melt are dry and refractory lining materials in the launder and ladle are dry. In the case of dead melted steels, charges should be diluted with new scrap so as to prevent build up which can slowly accumulate to critical levels. The adverse effects of nitrogen can be suppressed by the addition of Figure 2. Internal blowhole in steel casting caused by high nitrogen level. Figure 3. Mushrooming of metal in feeders indicating the presence of internal blowholes. elements that form stable nitrides. Small additions of titanium and zirconium may be advantageous in this regard since they will form stable nitrides of low solubility in molten steel. ¦ “ONCE A GAS HOLE FORMS, OTHER GASES SUCH AS HYDROGEN AND NITROGEN CAN DIFFUSE INTO IT CAUSING THE VOID TO BE ENLARGED.” METAL Casting Technologies December 2008 45