Why can ductile iron be heat treated to improve mechanical properties, but gray cast iron cannot?

Gray iron. According to the shape of graphite, it is divided into gray cast iron, malleable cast iron, and ductile iron.

The shape of graphite in these types of cast iron is flaked, flocculent, spherical and worm-like. Due to the sharp shape of the ends, flake graphite and flocculent graphite have a great effect on the destruction and fragmentation of the cast iron matrix, causing stress concentration.

The degree is obvious, so that the role of the matrix can not be played, so gray cast iron and malleable cast iron generally do not carry out heat treatment to improve the comprehensive performance. Due to its spherical shape, spherical graphite has little damage and fragmentation effect on the collective, and the degree of stress concentration is not large, and the role of the matrix can be fully exerted, so the required performance can be obtained by changing the matrix structure through heat treatment. The properties of ductile iron after heat treatment are comparable to those of medium carbon quenched and tempered steel.

The properties of compacted graphite iron are slightly lower than those of ductile iron. Thank you.

Both types of cast iron have higher strength and toughness, the difference between the two is that malleable cast iron has higher impact toughness, while ductile iron has higher wear resistance. The graphite in the gray cast iron structure exists in flakes, and the graphite in the ductile iron structure exists in the form of balls.

Ductile iron: gray, the luster is brighter than gray iron, and the surface roughness is like gray iron. The mechanical properties of ductile iron are further improved after spheroidization treatment, and the casting performance and processing performance are better, while the matrix structure of gray cast iron cannot change the shape and distribution of graphite.

As a result, grey cast iron is not as reinforcing as steel and ductile iron after heat treatment. So far, the purpose of heat treatment of gray cast iron has been mainly limited to eliminating internal stresses and changing the hardness of castings. Ductile Iron:

Spherical graphite has the lowest splitting effect on the matrix, and the least stress concentration, so its strength is very high, and it can be compared with medium carbon steel, and can give full play to the performance of the matrix, and has a certain plasticity and good toughness. It is often used to make some workpieces with high toughness requirements and complex shapes (better casting performance than steel, but worse than gray cast iron), such as internal combustion engine crankshaft connecting rods and other parts. Ductile iron can generally be strengthened by heat treatment, while gray cast iron generally cannot be heat treated to improve strength.

The tensile strength, plasticity and toughness of gray cast iron are lower than that of carbon cast steel, especially plasticity and toughness Compared with gray cast iron, ductile iron has higher tensile strength and bending fatigue limit, and it is almost zero. Low strength of ferrite matrix gray cast iron; And because the graphite sheet splits the metal matrix, it has quite good plasticity and toughness. This is due to the fact that spherical graphite weakens the cross-section of the metal matrix, resulting in low elongation and impact toughness.

Pearlite has high strength, hardness and wear resistance, and the effect is small, so that the matrix is relatively continuous, and the strength utilization rate of the matrix can reach 70% 90%, and the strength, hardness and wear resistance of the pearlite matrix gray cast iron are better than the effect of ferrite matrix gray casting causing stress concentration during tensile.

Summary. Performance ductile iron castings are used in almost all major industrial sectors that require high strength, plasticity, toughness, wear resistance, severe thermal and mechanical shock resistance, high or low temperature resistance, corrosion resistance, and dimensional stability. In order to meet these variations in the conditions of use, ductile iron is available in a number of grades, offering a wide range of mechanical and physical properties.

Taking the continuous improvement of the performance and application of ductile iron as an example, the application of "Mechanics of Materials" in modern industry is expounded.

Performance of ductile iron castings has been used in almost all major industrial sectors that require high strength, plasticity, toughness, wear resistance, severe thermal and mechanical shock resistance, high temperature or low temperature resistance, corrosion resistance and dimensional stability. In order to meet these variations in the conditions of use, many grades of ductile iron are available, offering a wide range of mechanical and physical properties. With a large ......

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Cast iron is an iron-carbon alloy with a carbon content greater than that, which is obtained by high-temperature melting and casting of steel and its alloy materials such as industrial pig iron and scrap steel, and in addition to Fe, it also contains carbon in other cast iron precipitated in the form of graphite. Because ductile iron graphite is spheroidal, the fragmentation between the grains is minimized, the stress concentration is minimized, and its strength is high. Therefore, the matrix can give full play to the tensile strength and elongation.

Therefore, the mechanical properties of ductile iron are higher than those of gray cast iron and malleable cast iron.

Performance analysis of three types of cast iron:

Ductile iron has the characteristics of medium and high strength, medium toughness and plasticity, high comprehensive performance, good wear resistance and vibration damping, and good casting process performance. Its properties can be changed by various heat treatments. It is mainly used for various power machinery crankshafts, camshafts, connecting shafts, connecting rods, gears, clutch plates, hydraulic cylinders and other parts.

The flake graphite in gray cast iron has serious slicing of the matrix, and it is easy to cause stress concentration at the sharp corners of graphite, so that the tensile strength, plasticity and toughness of gray cast iron are much lower than that of steel, but the compressive strength is equivalent to that of steel, and it is also the cast iron with the worst mechanical properties in commonly used iron castings. At the same time, the matrix structure also has a certain influence on the mechanical properties of gray cast iron, and the graphite sheet of ferrite matrix gray cast iron is coarse, with the lowest strength and hardness, so it is less used.

The graphite in malleable cast iron is flocculent, and the fragmentation effect on the matrix is small, so its mechanical properties are higher than those of gray cast iron, and the plasticity and toughness are good, but the malleable cast iron cannot be forged. The matrix structure of malleable cast iron is different, and its properties are also different, among which black-hearted malleable cast iron has high plasticity and toughness, while pearlite malleable cast iron has higher strength, hardness and wear resistance.

Organizational differences lead to huge differences in their performance as well:

Gray cast iron is a flake graphite fragmented matrix, which is easy to cause stress concentration, high brittleness, and good vibration cancellation performance. It is mainly used to produce some of the strength requirements are not high, and mainly bear the compressive stress of various box bases, etc.

Ductile iron is a spherical graphite matrix, the fragmentation between the grains is minimized, and the stress concentration is minimized, so its strength is very high, and it can give full play to the performance of the matrix with medium carbon steel, and has a certain plasticity and good toughness. It is often used to make some workpieces with high toughness requirements and complex shapes, such as internal combustion engine crankshaft connecting rods and other parts. Ductile iron can generally also be strengthened by heat treatment, while gray cast iron generally cannot be heat treated to increase strength (the effect of flake graphite).

Malleable cast iron is a ferritic (F) + flocculent graphite (G) matrix, which can be used to make parts that are resistant to impact or vibration and torsional load, and is often used in the manufacture of automobile rear axles, spring supports, low-pressure valves, pipe joints, tool wrenches, etc.

1. The morphology of graphite in cast iron is different.

1. The graphite of gray cast iron is flaked;

2. The graphite of ductile iron is spheroidal;

3. The graphite of compacted graphite cast iron is insect-like;

4. The graphite of malleable cast iron is agglomerate;

Second, the formation is different.

1. Gray cast iron; It is formed by the graphitization process when the molten iron is slowly cooled, and is composed of flake graphite and matrix structure;

2. Ductile iron: obtained by graphitization of molten iron after spheroidization and inoculation treatment; High tensile strength;

3. The production of vermicular graphite cast iron is obtained by adding a certain amount of creeping agent to the molten iron for pre-furnace treatment;

4. Malleable cast iron is obtained by graphitization annealing of white cast iron of a certain composition;

Third, the application is different.

1. Gray cast iron is used in machine tool bed, guide rail, cylinder block, etc.

2. Ductile iron replaces some cast steel and forged steel parts, such as crankshafts, connecting rods, rolls, automobile rear axles, etc.;

3. The strength and plasticity of compacted graphite cast iron are better than those of gray cast iron, and are used in high-pressure heat exchangers, cylinder heads, hydraulic valves, etc.;

4. Malleable cast iron can be applied to pipe joints, low-pressure valves, automobile or tractor thin shell parts, etc.;

Graphite is spherical in ductile iron and flake in gray cast iron. Compared with flake graphite, the damage to the matrix, the fragmentation effect and the degree of stress concentration caused by spherical graphite on the matrix are relatively small, and the reduction of the effective bearing area is relatively small, and the role of the matrix can be fully exerted, so the strength and plasticity (mechanical properties) of ductile iron are higher than those of gray cast iron.

The fluidity of ductile iron is smaller than that of gray iron, the shrinkage is larger, and the process control is more difficult than that of gray iron (spheroidization, inoculation, etc.), so the castability is worse than that of gray cast iron.

Casting performance: also known as casting process performance.

Mainly include: 1) Flow performance, full of mold casting ability;

2) shrinkage, the ability of the volume shrink when the casting solidifies;

3) Segregation refers to the heterogeneity of chemical composition;

4) Absorbent, in melting;

5) The ability to absorb gas during pouring.

The graphite in the gray cast iron is flake-like, which has a splitting effect on the matrix, and the graphite tip will also cause stress concentration, so that the strength of the gray cast iron is not high, the plasticity is very low, and it is a brittle material.

The graphite in ductile iron is spherical, which has little effect on the fragmentation of the matrix, and there is no stress concentration, and the performance mainly depends on the performance of the matrix. Ferritic ductile iron has good plasticity and slightly higher strength than mild steel. Pearlite has higher strength than ferrite but has less plasticity and toughness than ferrite.

The graphite in malleable cast iron is flocculent, and the roundness is worse than that of ductile iron. The performance is also worse than that of ductile iron, but the production process is more complex and consumes more energy.

Without a heat treatment process, graphite in malleable cast iron generally exists in cementite form, and carbon is non-graphitized. If nodularization and inoculation are not carried out, the graphite in the ductile iron will also be flaked.