Resultados Fase Uno – Descriptiva

Melt treatment to produce ductile iron involves the addition of magne-sium to change the form of the graphite, followed by or combined with inoculation of a ferrosilicon material to ensure a graphitic structure with freedom from carbides. There are three major types of magnesium-containing nodulizing (or spheroidizing) agents: unalloyed magnesium, nickel-base nodulizers, and magnesium-containing ferrosilicons.

Unalloyed magnesium metal has been added to molten iron as wire, ingots, or pellets; as briquets in combination with sponge iron; as pellets in combination with granular lime; or in the cellular pores of metallurgi-cal coke.

Magnesium-Containing Alloys. A nickel alloy with 14 to 16% Mg can be added to the ladle during filling or by plunging. The reaction is spec-tacular but not violent, and a very consistent recovery is obtained. A dis-advantage lies in the accompanying increase in nickel and the cost of the alloy. Other nickel alloys containing much lower magnesium contents (down to 4%) have also been used and involve a much quieter reaction.

Most alloys used to introduce magnesium into molten iron are based on ferrosilicon containing 3 to 10% Mg. The reaction varies from fairly vio-lent (with 10% Mg) to quiet (with 3% Mg). The alloy can be plunged in a refractory bell or added in the ladle using a number of different techniques, including pouring the molten iron onto the alloy in the bottom of the ladle.

Effects of Inoculation on Properties

Following magnesium treatment, the iron is usually subjected to fi-nal inoculation, sometimes referred to as postinoculation. Inoculation is

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commonly carried out in the ladle using a granular inoculant, which may be commercial ferrosilicon containing 75% Si or one of a wide range of proprietary alloys, usually containing 60 to 80% Si. The amount of inoc-ulant added usually ranges from about 0.25 to 1.0%. A higher percentage of silicon in the magnesium-addition alloy may permit less inoculation.

The inoculant can be added during reladling, stirred into the metal, placed on the bottom of the ladle before filling, or plunged in a refractory bell, as late as possible before casting. Effective stirring is necessary, and one way of achieving this is by bubbling air or nitrogen through the melt using a porous plug in the bottom of the ladle.

Inoculation reduces undercooling during solidification and helps to pre-vent carbides in the structure, especially in thin sections. It increases the number of graphite nodules, thus improving homogeneity, assisting in the formation of ferrite, and promoting ductility. It assists in reducing anneal-ing time and reduces hardness.

Effects of Alloying on Hardenability

The hardenability of ductile cast iron is an important parameter for deter-mining the response of a specific iron to normalizing, quenching and tem-pering, or austempering. Hardenability is normally measured by the Jominy test described in ASTM A 255 and SAE J406, in which a standard-sized bar (1 in. diameter by 4 in. in length) is austenitized and water quenched from one end. The variation in cooling rate results in micro-structural variations, giving hardness changes that are measured and recorded.

Figure 6 shows Jominy curves from an unalloyed ductile iron (3.9%

C, 2.2% Si, 0.04% Mg, and residual Mn, Ni, Cu, Cr, V, Ti) that has been austenitized at 870 and 925 °C (1600 and 1700 °F). The higher carbon content in the matrix, resulting from the higher austenitizing tempera-ture, increases the hardenability (the Jominy curve is shifted to larger distances from the quenched end) and a greater maximum hardness. The purpose of adding alloy elements to ductile cast irons is to increase hardenability.

Figure 7 shows Jominy curves for ductile irons containing variable quantities of manganese, molybdenum, nickel, and copper (Ref 6). It is clear from Fig. 7 that manganese and molybdenum are much more effec-tive in increasing hardenability, per weight percent added, than nickel or copper. However, as is the case with steel, combinations of nickel and molybdenum, or copper and molybdenum, or copper, nickel, and man-ganese are more effective than the separate elements. The synergistic effects of nickel and molybdenum are shown in the Jominy curves in Fig. 8 and Table 5. Thus, heavy-section castings that require through hardening or austempering usually contain combinations of these elements. Silicon

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does not have a large effect on hardenability, apart from its effect on matrix carbon content.

Effects of Alloying on Normalizing

Normalizing (air cooling following austenitizing) can considerably improve tensile strength and may be used in the production of ductile iron of ASTM type 100-70-03. The microstructure obtained by normalizing depends on the composition of the castings and the cooling rate. As described earlier, the composition of the casting dictates its hardenability.

The cooling rate depends on the mass of the casting, but it also may be influenced by the temperature and movement of the surrounding air dur-ing cooldur-ing. Normalizdur-ing generally produces a homogeneous structure of fine pearlite, if the iron is not too high in silicon content and has at least a moderate manganese content (0.3 to 0.5% or higher). Heavier castings that require normalizing usually contain alloying elements, such as nickel, molybdenum, and additional manganese, for higher hardenability to ensure the development of a fully pearlitic structure after normalizing.

Lighter castings made of alloyed iron may be martensitic or may contain an acicular structure after normalizing. Figure 9 shows the influence of various nickel contents and combinations of alloying elements on hard-nesses after normalizing of different section thickhard-nesses.

Fig. 6 Jominy curves from a ductile iron (3.9% C, 2.2% Si, 0.04% Mg, residual Mn, Ni, Cu, Cr, V, Ti), austenitized at 870 and 925 °C (1600 and 1700 °F). Source: Ref 5

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Fig. 7 Jominy curves for ductile irons containing variable quanities of (a) nickel, (b) copper, (c) manganese, and (d) molybdenum.

Austenitized at 870 °C (1600 °F). Source: Ref 6

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Fig. 7 (continued) (c) manganese, and (d) molybdenum. Austenitized at 870 °C (1600 °F). Source: Ref 6

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Fig. 8 Jominy curves for ductile irons containing variable combinations of (a) copper and nickel and (b) molybdenum, copper, and nickel.

Austenitized at 870 °C (1600 °F). Source: Ref 6

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The normalizing temperature is usually between 870 and 940 °C (1600 and 1725 °F). The standard time at temperature of 1 h per inch of section thickness or 1 h minimum is usually satisfactory. Longer times may be required for alloys containing elements that retard carbon diffusion in the austenite. For example, tin and antimony segregate to the nodules, effec-tively preventing the solution of carbon from the nodule sites (Ref 7).

Normalizing is sometimes followed by tempering to attain the desired hardness and relieve residual stresses that develop upon air cooling when various parts of a casting with different section sizes cool at different rates. Tempering after normalizing is also used to obtain high toughness and impact resistance. The effect of tempering on hardness and tensile properties depends on the composition of the iron and the hardness level obtained in normalizing. Tempering usually consists of reheating to tem-peratures of 425 to 650 °C (800 to 1200 °F) and holding at the desired

Fig. 9 Effect of alloy content and section thickness on hardness after normal-izing

Table 5 Examples of alloying combinations used to increase the hardenability of ductile iron

Maximum diameter of bar that could be Alloying elements used, % hardened by oil quenching

C Si Mn Ni Mo mm in.

3.4 2.0 0.3 … … 25 1

3.4 2.5 0.3 … … 28 1.1

3.4 2.0 0.3 1.0 … 30 1.2

3.4 2.0 1.3 … … 38 1.5

3.4 2.0 0.3 … 0.5 51 2.0

3.4 2.0 0.9 1.5 0.25 63 2.5

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temperature for 1 h per inch of cross section. These temperatures are var-ied within the above range to meet specification limits.