Grey Iron Castings VS Ductile Iron Castings

The graphite in the gray cast iron structure is in the form of flakes, and the graphite in the spheroidal graphite cast iron structure is in the form of spherulites. If the spheroidal graphite cast iron has a sound similar to that of carbon steel, it means that the spheroidization rate of the spheroidal graphite cast iron is relatively high. And the sound of gray iron percussion sounds dull. From the appearance of the fracture (not the machined surface), ductile iron and gray iron can be easily distinguished. Observed by naked eyes, the crystal grains of the gray iron fracture are relatively coarse, gray-white, and the crystal surface has metallic light. The fracture crystal grains of ductile iron are very small, black-gray. If the spheroidization rate is high, there is almost no metallic luster. If there is whitish luster, the general situation is that white mouth tissue appears. In terms of smelting method, nodular iron is added with inoculant and spheroidizing agent to gray iron during smelting to make graphite change from dispersed flocculent to spherical, in order to reduce the fragmentation of ferrite and increase the strength.

The difference in the structure of gray iron and ductile cast iron leads to huge differences in their properties. The strength and plasticity of gray cast iron castings are low. This is because the flake graphite splits the matrix and causes stress concentration. At the same time, gray iron castings are brittle and have good vibration damping properties. Therefore, gray iron is mainly used to produce various boxes and bases that have low strength requirements and mainly bear compressive stress. For spheroidal graphite cast iron castings, the spheroidal graphite has the lowest cracking effect on the matrix, and the stress concentration is the smallest, so its strength is very high, which can be comparable to medium carbon steel. At the same time, ductile cast iron can give full play to the performance of the matrix, and has a certain degree of plasticity and good toughness. Moreover, the casting performance of ductile iron is better than that of medium carbon steel. Therefore, ductile cast iron is often used to produce parts with high toughness requirements and complex shapes, such as internal combustion engine crankshafts, connecting rods, and so on. Nodular cast iron can generally be strengthened by heat treatment, while gray cast iron generally cannot be strengthened by heat treatment due to the influence of flake graphite.

Ductile Iron Comparison Chemical Composition(%)

Matrix Structure

China Grade ASTM A536 EN 1563:-1997 C Si Mn P S Mg Re others
QT400-18 60-40-18① F32800 GJS-400-18 3.6-3.8 2.3-2.7 <0.5 <0.08 <0.025 0.03-0.05 0.02-0.03 Annealed Ferrite
QT400-15 60-42-10 F32900 GJS-400-15 3.5-3.6 3.0-3.2 <0.5 <0.07 <0.02 0.04 0.02 Annealed Ferrite
QT450-10 65-45-12 F33100 GJS-450-10 3.4-3.9 2.7-3.0 0.2-0.5 <0.07 <0.03 0.06-0.1 0.03-0.1 Annealed Ferrite
QT500-7 70-50-05 GJS-500-7 3.6-3.8 2.5-2.9 <0.6 <0.08 <0.025 0.03-0.05 0.03-0.05 Pearlite + Ferrite
QT600-3 80-60-03② F34100 GJS-600-3 3.6-3.8 2.0-2.4 0.5-0.7 <0.08 <0.025 0.035-0.05 0.025-0.045 Normalized Pearlite
QT700-2 100-70-03 F34800 GJS-700-2 3.7-4.0 2.3-2.6 0.5-0.8 <0.08 <0.02 0.035-0.065 0.035-0.065 Mo0.15-0.4 Cu0.4-0.8 Mix Microstructure
QT800-2 - GJS-800-2 3.7-4.0 <2.5 <0.5 <0.07 <0.03 Mo0.39 Cu0.82 Mix Microstructure
QT900-2 120-90-02 F36200 GJS-900-2 3.5-3.7 2.7-3.0 <0.5 <0.08 <0.025 0.03-0.05 0.025-0.045 Mo0.15-0.25 Cu0.5-0.7 Lower Bainite
① from ASTM A716-2003. ② from ASTM A476/A476M-2000。

Nodular cast iron has an absolute advantage in strength. The tensile strength and yield strength of ductile iron castings are much higher than gray iron castings. The tensile strength range of gray cast iron is 100-350 MPa, while the tensile strength range of ductile iron is 350-700 MPa. Ductile cast iron also has requirements for elongation, while gray cast iron has few requirements in this regard. The strength ratio of ductile iron is much better than that of cast iron. Nodular cast iron is the same as cast iron in terms of corrosion resistance.

Mechanical Properties of Gray Cast Iron
Item according to DIN EN 1561 Measure Unit EN-GJL-150 EN-GJL-200 EN-GJL-250 EN-GJL-300 EN-GJL-350
EN-JL 1020 EN-JL 1030 EN-JL 1040 EN-JL 1050 EN-JL 1060
Tensile Strength Rm MPA 150-250 200-300 250-350 300-400 350-450
0.1% Yield Strength Rp0,1 MPA 98-165 130-195 165-228 195-260 228-285
Elongation Strength A % 0,3 – 0,8 0,3 – 0,8 0,3 – 0,8 0,3 – 0,8 0,3 – 0,8
Compressive Strength σdB MPa 600 720 840 960 1080
0,1% Compressive Strength σd0,1 MPa 195 260 325 390 455
Flexural Strength σbB MPa 250 290 340 390 490
Schuifspanning σaB MPa 170 230 290 345 400
Shear Stress TtB MPa 170 230 290 345 400
Modules of elasticity E GPa 78 – 103 88 – 113 103 – 118 108 – 137 123 – 143
Poisson number v 0,26 0,26 0,26 0,26 0,26
Brinell hardness HB 160 – 190 180 – 220 190 – 230 200 – 240 210 – 250
Ductility σbW MPa 70 90 120 140 145
Tension and pressure change σzdW MPa 40 50 60 75 85
Breaking Strength Klc N/mm3/2 320 400 480 560 650
Density g/cm3 7,10 7,15 7,20 7,25 7,30

The strength of ductile iron is comparable to that of cast steel. Nodular cast iron has a higher yield strength. In most municipal applications, such as water, salt water, steam, etc., the corrosion resistance and oxidation resistance of ductile iron exceed those of cast steel. Due to the nodular graphite microstructure of nodular cast iron, nodular cast iron is better than cast steel in terms of reducing vibration ability, so it is more conducive to reducing stress. Nodular cast iron is called the "best in the two worlds" metal, which means that nodular cast iron has the strength of cast steel and the excellent corrosion resistance of cast iron. The cost difference between cast iron and ductile iron castings is $ 500 per ton.

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