Wear Resistance Of Nodular Cast Iron

- Aug 14, 2017 -

Nodular Graphite Ductile iron is obtained by spherical and inoculation process, which effectively improves the mechanical properties of cast iron, especially the plasticity and toughness, thus obtaining higher strength than carbon steel. Ductile iron is a kind of high-strength cast iron material developed in the the 1950s, its comprehensive performance is close to steel, it is based on its excellent performance, Ductile Iron Casting has been successfully used to cast some of the force complex, strength, toughness, wear-resistant parts of higher requirements. Ductile iron has rapidly developed to be the most widely used cast iron material after Gray iron. The so-called "iron substituting steel", mainly refers to ductile iron.

Carbon in cast iron precipitates in graphite form, Ductile Iron Casting if the precipitation of graphite in the strip-like cast iron is called gray cast iron or gray iron, a worm-like cast iron called vermicular Graphite cast iron, when the cast iron is called white cast iron or code iron, and a spherical cast iron is called ductile iron.

The chemical compositions of ductile iron except iron are usually: carbon content 3.0~4.0%, silicon content 1.8~3.2%, the total amount of manganese, phosphorus, sulphur is not more than 3% and the amount of rare earth, magnesium and other spherical elements.

In ductile iron, the graphite is spherical, weakening and destroying the matrix is weaker than flake graphite. Ductile iron performance depends mainly on the matrix structure, the influence of graphite is subordinate. Ductile Iron Casting The mechanical properties of ductile iron can be improved by different heat treatment methods to improve the matrix microstructure. Because of the influence of chemical composition, Ductile Iron Casting cooling speed and spherical agent, the mixed microstructure of ferrite + pearlite + cementite + graphite is often found in the casting structure, especially in the thin wall of castings. The purpose of heat treatment is to obtain the desired tissue, thus improving mechanical properties.

The usual heat treatment methods of ductile iron are as follows.

(1) low-temperature graphitization annealing heating temperature 720~760℃. With the furnace cooling to 500 ℃ below the air cooling. The cementite was decomposed to obtain ferrite matrix ductile iron to improve toughness.

(2) high-temperature graphitization annealing 880~930℃, turn to 720~760℃ insulation, with the furnace cooling to 500 ℃ below the air cooling. Eliminate white-mouth tissue, Ductile Iron Casting obtain ferrite matrix of ductile iron, improve plasticity, reduce hardness, increase toughness.

(3) Completely austenitic normalizing 880~930℃, cooling mode: Fog cold, air-cooled or air-cooled, in order to reduce stress, increase tempering process: 500~600℃, obtain pearlite + a small amount of ferrite + spherical graphite, improve strength, increase hardness and wear resistance.

(4) Not completely austenitic normalizing 820~860℃ heating, cooling mode: Fog cold, air-cooled or air-cooled, in order to reduce stress, increase tempering process: 500~600℃, obtain pearlite + a small amount of dispersed ferrite tissue, get good comprehensive mechanical properties.

(5) Quenching and tempering 840~880℃ heating, cooling mode: Oil or water-cooled, quenched after tempering temperature: 550~600℃, obtain tempering sorbite tissue, improve the comprehensive mechanical properties.

(6) Isothermal quenching 840~880℃ Heating, quenching in 250~350℃ salt bath, Ductile Iron Casting to obtain comprehensive mechanical properties, especially to improve strength, toughness and wear resistance.

Heat treatment heating, the casting furnace temperature is generally less than 350 ℃, heating speed depending on the size and complexity of the casting, choose between 30~120℃/h. Large size, complex parts of the furnace temperature is low, heating speed is slow. Ductile Iron Casting The heating temperature depends on the microstructure and chemical composition of the substrate. The insulation time is determined by the wall thickness of the casting.

In addition, ductile iron castings can be used as high frequency, medium-frequency, flame and other methods for surface quenching to obtain high hardness, wear resistance and anti-fatigue performance. Also can be soft nitriding treatment, improve the wear resistance of castings.

In the casting process of ductile iron, the white mouth tendency of this ordinary gray cast iron is large, the internal stress is also larger, Ductile Iron Casting the iron castings are difficult to get pure ferrite or pearlite matrix, in order to improve the ductility or toughness of cast iron parts, the iron castings are reheated to 900-950 ℃ and the heat preservation time is enough for high-temperature annealing, and the furnace cools to 600 ℃ to become cold. In the process, the cementite in the matrix decomposes graphite, and the graphite is precipitated from austenite, which is concentrated around the original spherical graphite, and the matrix is converted to ferrite.

If the cast structure is composed of (ferrite + pearlite) matrix, as well as spherical graphite, in order to improve toughness, only the cementite in Pearlite is decomposed into ferrite and spherical graphite, so that the cast iron is reheated to 700-760 ℃ and the temperature is cooled down to 600 ℃.