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Metal Casting Technologies : March 2011
Common alloying elements in ductile iron Ductile iron is commonly alloyed at moderate levels for: ● Promotion of pearlite ● Enhancing elevated temperature properties Pearlite promoting elements Pearlite promoting elements generally work by one of two fundamentally different mechanisms. One mechanism is by segregation of certain elements to the graphite/matrix phase boundary during solidification. These elements effectively act as barriers which inhibit diffusion of carbon from the matrix to the graphite spheroid. The second mechanism is by retarding the onset of the eutectoid transformation and decreasing the rate of diffusion of carbon in ferrite. Most trace elements found in ductile irons have some pearlite stabilising effect, the potency of eight common elements are ranked in decreasing order in the following table. Of these elements, pearlite promotion by either Mo or Ni is too expensive and the use of P, Ti, Mn and Cr is definitely advised against. Tin and copper are most commonly used to stabiles pearlite in ductile iron. The pearlite promotion effect by the two elements is additive with tin being about ten times stronger than that of copper. Tin additions in excess of about 0.07% can cause the iron to be embrittled due to the precipitation of tin to the grain boundaries. Copper is widely used in amounts from about 0.2% to 1.5% depending on the amount of pearlite required, the section size, and the levels of other pearlite stabilizing elements. Enhanced elevated temperature properties Molybdenum is particularly useful in increasing elevated temperature tensile strength, stress-rupture and creep strength and thermal fatigue resistance and is added in amounts up to 2% for this purpose. Nickel acts in a similar manner and often accompanies molybdenum alloying. Deleterious elements in ductile iron The production of quality ductile iron involves treatment of the molten iron with sufficient magnesium in appropriate METAL Casting Technologies March 2011 43 Elements Relative pearlite promoting effect Sn 39.00 Mo 7.90 P 5.60 Cu 4.90 Ti 4.40 Mn 0.44 Ni&Cr 0.37 Table 1. Relative pearlite promoting potency of common elements Belec Lab 3000s Spectrometer The quick and easy solution for your Alloy Analysis G & C Instrument Services P/L Ph: +61 2 4981 7255 Email: firstname.lastname@example.org www.gcinstrument.com.au