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Metallurgy of Mo in cast irons

Molybdenum is known to act as carbide stabilizer in cast iron. At low addition levels, molybdenum has little deleterious effect on castability and chilling tendency. Free carbides are only formed with higher molybdenum additions. The amount and shape of graphite are not measurably affected by molybdenum additions below 0.5%. The main effect of molybdenum alloying to cast iron is observed during solid state transformations. With increasing molybdenum alloy content, the pearlite phase field in the continuous cooling transformation (CCT) diagram is shifted towards longer times.

Adding molybdenum up to around 0.5% acts as a very powerful pearlite stabilizer and increases strength by refining pearlite. Higher molybdenum additions, preferably in conjunction with nickel or copper, promote the transformation from austenite into acicular ferrite. Such irons usually contain at least 0.8% molybdenum and more than 1.2% nickel. The actual amount of alloying required depends on the section thickness. The tensile strength of acicular irons is in the range of 400-500 MPa. Acicular irons are more easily machinable at high levels of hardness (250-320 HB) than unalloyed irons due to the absence of free cementite.

Among the typical alloying elements, manganese and nickel coarsen the interlamellar spacing of pearlite. In contrast, an increasing content of chromium produces finer pearlite spacing. Silicon has only a slight influence on pearlite spacing. Molybdenum has the strongest effect on decreasing interlamellar spacing. The individual effects of these alloying elements on the pearlite interlamellar spacing, S0, can be described as:

log S0=−2.212+0.0514×[Mn]−0.0396×[Cr]+0.0967×[Ni]−0.002×[Si]−0.4812×[Mo]−log(ΔT/Te)

where S0 is measured in µm; manganese, chromium, nickel, silicon, and molybdenum are the different alloy contents in weight percent and ?T is the undercooling from the eutectoid temperature Te. In pearlitic microstructures, the interfaces between ferrite and cementite act as barriers to dislocation movement.

The critical stress necessary to move dislocations in ferrite lamellae is related to the macroscopic yield stress. That critical stress rises with the refinement of the pearlitic microstructure since a decrease of pearlite interlamellar spacing leads to an increase in the resistance to glide according to a Hall-Petch type relationship. Furthermore, finer pearlite microstructures comprising smaller colony size and shorter interlamellar spacing show a more ductile fracture character.

Effect of Mo alloying on pearlite lamella size and spacing