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Metal Casting Technologies : March 2009
? Pearlite ? Ferrite + Graphite The second reaction, resulting in graphitization of the pearlite, is a permanent non-reversible change which occurs gradually over a period of time at temperatures above 500o Reducing growth The following structure control effects can be used to reduce growth: ? C. During service, e.g. as fire-bars, these volume changes cause craze cracking of the oxide film formed on the surface of the iron exposing the underlying material to further oxidation. The graphite flakes are oxidized away leading to internal oxidation of the metal matrix. This internal oxidation together with the graphitization of the pearlite causes a volume expansion leading to gradual distortion of the part and further cracking of the oxide film thus encouraging even more internal oxidation. This overall effect is known as “growth of cast iron”. ? The eutectic graphite structure can be made finer and/or less continuous to reduce internal oxidation via the graphite phase. Alloying additions can be used to produce a stable matrix structure and to raise oxidation resistance by improving the protective nature and adherence of the oxide film. Good adherence will reduce the cracking away (spalling) of the oxide film that results from mismatch in stresses between oxide and the underlying metal. The first effect can be achieved in one of the following three ways by: ? ? Effective inoculation to refine the flake graphite and reduce eutectic cell size Producing a fine undercooled graphite structure: this also encourages a stable ferrite matrix. ? Nodularization of the graphite. The second effect can be achieved by: ? Adding carbide forming elements to stabilize a pearlite matrix (up to the eutectoid temperature) and hence reduce graphitization ? Producing a stable non-pearlitic matrix: - either by the use of Silicon to stabilize a ferrite matrix as in Silal - or by the use of Nickel to stabilize an austenite matrix as in Ni-Resist Adding small amounts of Chromium and Molybdenum produces low alloy pearlitic grey irons which can withstand temperatures up to 700o C. During service the lamellar pearlitic carbides will become globular but they do not decompose to ferrite and graphite. Typical alloys are: 3.0%C - 1.6%Si - 0.5%Cr - 0.4%Mo - 0.1%P 3.4%C - 2.8%Si - 2.0%Cr - 0.4%P METAL Casting Technologies March 2009 37