Brake drum used in automobile is a very important safety part, which has special technical requirements for the inner quality of casting. However, it is worrying that: according to the investigation made by the China Foundry Association, in many manufacturers of brake drum enterprises, the product quality requirements have a correct understanding of the few.

Take casting carbon content as an example: in order to make the microstructure of cast iron have more graphite pieces, in order to ensure its high thermal conductivity, the carbon content of passenger car brake drum should be above 3.4%, heavy vehicle brake drum should be above 3.7%, and this is the primary quality requirement. In fact, most manufacturers simply according to the strength requirements, the carbon content is controlled at about 3.2%, the correct understanding of this basic requirements of the foundry enterprise is very few.

As for the requirements of casting microstructure, American ASTMA159-83 "gray cast iron for automobile" standard. The morphology, length and basic structure of graphite are specified. China and some other countries have not formulated relevant national standards or industry standards. Therefore, buyers of castings often do not put forward clear requirements for this, but only require tensile strength of samples according to the brand. Under such conditions, few manufacturers carefully control the microstructure in the production process.

1. Working conditions of brake drum

Automotive brake drum, braking will lead to temperature rise due to friction, and then due to the thermal conductivity of the metal and rapid cooling, in the process of use to go through repeated heating and cooling, heavy vehicle brake drum, braking temperature can reach 850℃, so the friction surface is easy to crack. These cracks are usually called thermal fatigue cracks, thermal cracking or network cracks.

Generally speaking, thermal fatigue is caused by thermal stress cycling action caused by temperature change. For the brake drum, the effect of temperature cycle change, in addition to produce thermal stress, will also lead to changes in the internal organization of metal materials, so as to aggravate the stress. In addition, during the operation of the brake drum, the heating of each part is not uniform, and the thermal expansion of the components of the microstructure is different. Under the condition of repeated heating and cooling, alternating stress will also be caused.

During use, the structure may undergo various phase transitions, such as:

(1) Eutectic carbides decompose into graphite and ferrite at 800℃ or slightly below 800℃;

② Pearlite and ferrite transform into austenite above 800℃;

2. Influence of graphite structure on fatigue strength

In gray cast iron, the graphite structure has a very important influence on the generation of thermal fatigue crack, mainly as follows:

(1) If the graphite in gray cast iron is thicker and more, its thermal expansion coefficient is lower;

② The thermal conductivity of gray cast iron is directly proportional to the volume fraction of graphite in it and the size of graphite sheet.

Therefore, increasing the amount of graphite and making the size of graphite sheet larger, under the same operating conditions, can reduce the temperature reached by the body tissue, so that the overall thermal stress is small;

Graphite has the role of buffer matrix tissue expansion, but also a solid lubricant.

3. Influence of graphite morphology

Thermal fatigue cracks in both gray cast iron and nodular cast iron initially occur on the graphite base plane. In gray cast iron, the crack develops along the base plane of graphite and its direction is not affected by the grain boundary of metal matrix or pearlite orientation. The gray cast iron with type A graphite has no direction in its structure, and the cracks produced at the initial stage of fatigue are scattered, so it has good resistance to thermal fatigue cracks. Therefore, the morphology of graphite in the microstructure of brake drum casting should be A-type. B, D and E/ type graphite all lead to the deterioration of mechanical properties and thermal fatigue properties of cast iron.

In nodular cast iron, the graphite base plane is in the diameter direction of spherical graphite, but spherical graphite cannot buffer the stress generated in the matrix, and cracks will occur on the interface between graphite and ferrite. A radial crack is then generated, extending through the ferrite shell to the pearlite until another graphite sphere.

4. Influence of matrix structure

According to the results of laboratory tests, the pearlite matrix has a strong ability to resist fatigue crack, but in the presence of a large amount of flake graphite, the influence of the matrix structure on the resistance to thermal fatigue is very small and can not be considered.

Some standards limit the content of ferrite and carbide, mainly to ensure room temperature strength and hardness.

5. Quality requirements of gray iron for brake drum

Based on the above considerations, gray iron for brake drum has the following special requirements in addition to the mechanical properties and hardness requirements stipulated in accordance with the characteristics of the vehicle:

A minimum carbon content must be specified to ensure the volume fraction of graphite in the tissue. The carbon content of brake drums is typically 3.4% for small cars, 3.5% for medium cars, and 3.6% or 3.7% for heavy vehicles. Under these conditions, the volume fraction of graphite in cast iron is approximately 11-12%, and the thermal conductivity of cast iron is about twice that of its matrix structure.

(2) the graphite form is A type;

(3) too small graphite is not allowed, and the length of graphite is 2-4 or 3-5 grades;

④ The matrix is mainly pearlite, and the ferrite content or the total amount of ferrite and cementite should be limited.

The cast iron that meets the above requirements has low strength at room temperature but good resistance to thermal fatigue. Gray cast iron with too small graphite sheet has a high strength at room temperature, but a poor ability to resist thermal fatigue.

In order to make the mechanical properties of the special purpose gray cast iron meet the requirements of the specified requirements, in addition to seriously control the metallurgical quality and breeding process of cast iron, for the varieties with higher requirements of strength and hardness, sometimes have to add Cu, Cr, Mo and other alloying elements. In this case, the amount of alloying elements should be strictly controlled to meet the performance requirements. Excessive alloying elements will make graphite small and small volume fraction, which has a negative impact on the service life of the brake drum.

The experimental results show that the thermal fatigue life of nodular cast iron with pearlite matrix is higher than that of gray cast iron with similar matrix. However, for practical application, the influence of other factors must be further considered. Because gray cast iron has higher thermal conductivity and lower thermal expansion coefficient, and flake graphite can buffer the expansion of matrix structure, under the same service conditions, gray cast iron brake drum achieves higher temperature than nodular cast iron brake drum, and its actual service life is higher than nodular cast iron brake drum. Therefore, ductile iron is generally not used to manufacture brake drums.