Chilling Tendency and Chill of Cast Iron

More documents

Recommendations

Info

178 where for flake graphite cast iron, or 5 3 s e 3 φ 2 ef 1/6 a ⎛ 2 T ⎞ p = ⎜ ⎟ (3) π ⎝L c ⎠ ⎡ 1 ⎛ b ⎞⎤ CT = ⎢ exp 3 8 ⎜ ⎟⎥ ⎢Ns( 1 fγ) µ T ∆T ⎣ − ∆ sc ⎝ sc ⎠⎦⎥ ⎡ 1 CT = ⎢ ⎢⎣ N ( 1 − f ) µ ∆T and for ductile cast iron, or ⎛ p = a ⎜ ⎝4 π 3 8 cr γ sc T 3 s 5 2 2 2 4 β B Lez c ⎤ ⎥ ⎥⎦ ⎞ ⎟ ⎠ 1/6 1/6 1 ⎡ 1 ⎛ b ⎞⎤ CT = exp 1/2 ⎢ 2 ⎜ ⎟⎥ D ⎢⎣Ns β ∆Tsc ⎝∆Tsc ⎠⎥⎦ 1/3 1/3 1/6 (4) (5) (6) (7) 1 ⎡ 1 ⎤ CT = 1/2 ⎢ 2 D ⎣Ncr β ∆Tsc⎦ ⎥ (8) ∆ Tsc = Ts− Tc (9) φ = cef B1+ cB2 (10) Tsinghua Science and Technology, April 2008, 13(2): 177-183 T T T B= ln , B = ln , B2= ln T T i l 1 1 s L γ cef = c+ Tlγ−Ts i T 1 (11) (12) z=0.41+0.93B (13) In these equations CT is the chilling tendency of the cast iron; Ns and b are the nucleation coefficients for flake graphite or nodular cast iron; fγ is the proeutectic austenite volume fraction; a is the material mould ability to absorb heat; cef is the effective specific heat of the proeutectic austenite; Ti is the initial metal temperature just after filling the mould; φ is the heat transfer coefficient; α is the wedge angle; ∆ T = T −T and sc s c c, D, Le , Lγ , Tl , Tlγ , Ts , Tc , µ , and β are defined in Table 1; Ncr is the critical cells or nodule count at temperature T≈Tc; and n is the wedge size coefficient which expresses the influence of wedge size on the chill width. The ProductLog [y]=x is the Lambert function, also known as the omega function which can be easily calculated by means of the instruction Product- Log [y] in Mathematica. Table 1 Selected thermophysical data Parameter Value Latent heat of graphite eutectic L e=2028.8 J/cm 3 Latent heat of austenite L γ=1904.4 J/cm 3 Specific heat of cast iron c=5.95 J/(cm 3 · ℃ ) 9.2 6.3Si 10 − Growth coefficient of graphite eutectic µ= ( ) 0.25 6 − × cm/(℃ 2 ·s) Material mould ability to absorb heat a=0.10 J/(cm 2 ·s 1/2 · ℃ ) The diffusion coefficient of carbon in austenite D = 3.9 10 −6 cm 2 /s Coefficient related with the slopes of the solubility lines JE’, E’S’, and BC’ in Fe-C system β = 0.001 55℃ –1 Liquidus temperature for austenite T l=[1636 –113(C+0.25Si+0.5P)]℃ Formation temperature for cementite eutectic Tc=[1130.56+4.06(C–3.33Si–12.58P)]℃ Graphite eutectic equilibrium temperature T s=[1154.0+5.25Si–14.88P]℃ Carbon content in graphite eutectic C e=(4.26–0.30Si–0.36P)% Maximum carbon content in austenite at Ts Liquidus temperature of austenite for austenite composition C γ C γ=(2.08–0.11Si–0.35P)% Weight fraction of austenite g γ=(C e–C)/(C e–C γ) Austenite density ρ γ=7.51 g/cm 3 Melt density ρ m=7.1 g/cm 3 T lγ=[1636–113(2.08+0.15Si+0.14P)]℃ Volume fraction of proeutectic austenite f γ=ρ m g γ/[ρ γ+g γ(ρ m–ρ γ)] Note: C, Si, and P indicate content of carbon, silicon, and phosphorus in the cast iron, %.
E. Fraś et al:Chilling Tendency and Chill of Cast Iron 179 The theoretical predictions agree qualitatively with the available data in the literature. In particular, it is well known that the chill of cast iron decreases with increasing eutectic cell or nodule count N (and in consequence Ncr) as a result of inoculation, as well as decreased heating times and decreased bath superheating temperature [11] . It is also known that increasing C and Si in the cast iron reduces the chill [11] . An increase in the carbon (1) increases the eutectic cell count in uninoculated cast iron or the number of nodules (Ncr) [11] , (2) decreases the proeutectic austenite fraction fγ (Table 1), (3) slightly narrows the ∆Tsc range (Eq. (9) and Table 1) and (4) decreases the austenite liquidus temperature Tl. Points 1 and 2 are the main reason for the reduction in the chill by carbon (Eqs. (1) and (2)). An increase in the silicon (1) reduces the growth coefficient for graphite eutectic µ (Table 1), (2) increases the cell or the number of nodules (Ncr) [11] , (3) decreases the proeutectic austenite fraction fγ (Table 1), (4) widens the ∆Tsc range (Eq. (9) and Table 1), and (5) decreases the austenite liquidus temperature Tl. The effects of increases in the number of cells or nodules and of the proeutectic austenite fraction are the most important and as a result silicon reduces the chill (Eqs. (1) and (2)). In addition, the critical plate wall thicknesses, scr, and the wedge chill widths, W, according to Eqs. (1), (2), (3), and (6) increase as the ability of the mold to absorb heat, a, increases. scr and W also depend on B, B1, and B2 (Eq. (11)) and, hence, on the initial temperature, Ti, of the cast iron just after pouring into the mould. Higher pouring temperatures, Tp, will result in higher Ti. Therefore, decreasing Tp reduces φ parameter (Eq. (10)), thus increasing scr and W (Eqs. (1) and (2)). A schematic diagram showing the role of the various factors on the chilling tendency and the chill of cast iron is given in Fig. 1. Fig. 1 Schematic representation of the effect of various factors on the chilling tendency
Page 1: TSINGHUA SCIENCE AND TECHNOLOGY ISS
Page 5 and 6: E. Fraś et al:Chilling Tendency an
Page 7: E. Fraś et al:Chilling Tendency an

Chilling Tendency and Chill of Cast Iron

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?