Research progress of high strength grey cast iron material for automobile cylinder head

1. Microstructure of high strength gray iron

The strength of gray cast iron is mainly determined by its microstructure. Through the analysis of the transformation law of gray cast iron’s microstructure, the microstructure of high strength gray cast iron generally includes the following aspects: there are more primary austenite dendritic crystals;The graphite is fine  of type A;The matrix is mainly composed of pearlite  the pearlite is fine.

(1) Primary austenite: The proportion  distribution of primary austenite have an important effect on the performance of gray cast iron.

(2) Graphite structure: The distribution form  size of graphite are important factors to determine the mechanical properties of gray cast iron. The more uniform  fine bending graphite is, the blunter the end is, the higher the mechanical properties of gray cast iron are,  vice versa.

(3) Matrix: generally, it mainly contains pearlite, ferrite, a small amount of cementite  phosphorus eutectic.The more pearlite content in the matrix, the smaller the pearlite slice spacing  the higher the strength  hardness.

2. Adjust chemical composition

The thin-walled complex castings, such as cylinder block  cylinder head, have certain requirements on the casting process performance  the compactness of parts,  need to choose a higher carbon equivalent.Under a certain carbon equivalent condition, the material properties can be improved by changing Si/C ratio.Compared with the addition of conventional alloying elements, microalloying strengthening can produce better economic benefits, but there are also risks such as work hardening  stomatal defects, which should be controlled through strict technological process.

3. Gestation treatment

Inoculation technology plays an important role in improving the mechanical properties, reducing the tendency of whiteness, improving the structure  machinability,  is an indispensable process for improving the performance of gray cast iron.Silicon inoculants are the most commonly used inoculants. In recent years, some special inoculants have been developed for cylinder block  cylinder head parts.

4. Smelting process

With the same chemical composition, different smelting process  different ingredients, the metallurgical quality of molten iron is completely different.The main reason for the difference in performance is the difference in graphite morphology,  the most fundamental reason is the different way of carbon acquisition.In the smelting process of molten iron in cylinder block  cylinder head, temperature control is very important. Overheat  heat preservation at a certain temperature can purify molten iron, refine graphite  reduce the adverse effect of charge.In order to obtain the required iron liquid stably, the foreign automobile fountainers generally adopt the double smelting, melting cast iron most uses cupola, has the advantages of fast melting speed, convenient slagging, low energy consumption, reducing the chilling tendency  shrinkage tendency, etc.By using induction furnace to heat up  modulate the molten iron, the thick graphite can be refined, the impurities of molten iron can be melted  removed,  the genetic damage of charge can be reduced.