What is cast iron?
Articles cast with molten iron are collectively referred to as iron castings.
Due to the influence of many factors, defects such as pores, pinholes, slag inclusions, cracks and pits often occur.
Types of cast iron
1. Gray iron castings
Good fluidity, small volume shrinkage and linear shrinkage.
The comprehensive mechanical property is low, and the compressive strength is about 3-4 times higher than the tensile strength.
Good vibration absorption.
The elastic modulus is low.
Application: the shape can be complex and the structure can be asymmetric.
For example, it is used for the cylinder block, cylinder sleeve of engine, various machine bed, base, flat plate, platform and other castings.
2. Ductile iron castings
The fluidity is similar to that of gray cast iron;
The body shrinkage is larger than that of gray cast iron, and the linear shrinkage is smaller, which is easy to form shrinkage cavity and porosity.
The comprehensive mechanical properties are higher and the elastic modulus is higher than that of gray cast iron;
Good abrasion resistance;
The impact toughness and fatigue strength are good.
The vibration damping capacity is lower than that of gray cast iron.
Generally, it is designed with uniform wall thickness;
For thick and large section parts, hollow structure can be adopted, such as nodular cast iron crankshaft journal.
3. Malleable cast iron parts
The fluidity is worse than that of gray cast iron;
The bulk shrinkage is very large, and the final linear shrinkage is very small after annealing.
Before annealing, it is very brittle and the blank is easy to be damaged.
The comprehensive mechanical properties are slightly lower than that of nodular cast iron, and the impact toughness is 3-4 times larger than that of gray cast iron.
Application: due to the requirement of white mouth as cast, it is generally a thin-walled uniform part with a common thickness of 5-16mm.
In order to increase its rigidity, the cross-section shape is mostly I-shaped, T-shaped or box shaped to avoid cross-section;
The protruding part of the part shall be reinforced with ribs.
4. Steel castings
The fluidity is poor, and the sensitivity of volume shrinkage, linear shrinkage and crack is large.
High comprehensive mechanical properties;
The compressive strength is almost equal to the tensile strength.
Poor vibration absorption.
Application: the structure shall have the least thermal nodes and create conditions for sequential solidification.
The connection and transition of adjacent walls shall be more smooth;
The section of the casting shall adopt box shape, groove shape and other approximate closed structures;
Some horizontal walls should be changed into inclined walls or wavy shapes;
The integral wall shall be changed into a wall with a window. The shape of the window is preferably oval or circular.
The edge of the window shall be made with a boss to reduce the possibility of cracks.
5. Tin bronze and phosphor bronze parts
The casting performance is similar to that of gray cast iron.
However, the crystallization range is large and the shrinkage cavity is easy to occur;
Poor high temperature performance and easy to be brittle.
The strength decreases significantly with the increase of cross section.
Good wear resistance.
Application: the wall thickness shall not be too large;
The protruding part of the parts shall be reinforced with thin reinforcing ribs to avoid hot cracking;
The shape is not easy to be too complicated.
6. Tin-free bronze and brass parts
Large shrinkage, small crystallization range, easy to produce concentrated shrinkage holes;
Good wear resistance and corrosion resistance.
Application: similar to steel castings.
7. Aluminum alloy parts
The casting property is similar to that of cast steel, but the strength decreases more significantly with the increase of wall thickness.
The wall thickness shall not be too large;
Other similar steel castings.
What are the factors affecting the quality of iron castings?
1. The design process of cast iron.
During the design, in addition to determining the geometry and size of the cast iron according to the working conditions and the properties of metal materials, the rationality of the design must also be considered from the perspective of casting alloy and casting process characteristics, i.e. obvious size effect, solidification, shrinkage, stress and other problems, so as to avoid or reduce the occurrence of composition segregation, deformation, cracking and other defects of the cast iron.
2. There should be a reasonable casting process.
That is, according to the structure, weight and size of the cast iron, the characteristics of the casting alloy and the production conditions, select the appropriate parting surface, molding and core making method, and reasonably set the casting ribs, cold iron, riser and gating system to ensure the quality of the cast iron.
3. The quality of raw materials for casting.
The substandard quality of metal charge, refractory, fuel, flux, modifier, casting sand, molding sand binder, coating and other materials will cause defects such as porosity, pinhole, slag inclusion and sand sticking in the cast iron, which will affect the appearance and internal quality of the cast iron, and in serious cases, the cast iron will be scrapped.
4. Process operation.
It is necessary to formulate reasonable process operation procedures, improve the technical level of workers, and ensure the correct implementation of the process procedures.