Brake drum for automobile is a very important safety component, which has special technical requirements for the inner quality of casting. However, it is worrying that: according to the survey made by China Foundry Association, in many manufacturers of brake drum enterprises, the quality of the product requirements have a correct understanding is not many.

Take the cast carbon content as an example: in order to make the microstructure of cast iron has more graphite sheets, to ensure that it has a higher thermal conductivity, passenger car brake drum carbon content should be more than 3.4%, heavy vehicle brake drum should be more than 3.7%, and this is the primary quality requirements. In fact, most manufacturers simply control the carbon content at about 3.2% according to the intensity requirements, and few foundry enterprises have a correct understanding of this basic requirement.

As for the requirements of casting microstructure, American ASTMA159-83 "automotive gray cast iron" standard. The shape of graphite, the length of graphite sheet and the basic structure are specified in detail. China and some other countries have not yet developed relevant national or industry standards. Therefore, casting buyers often do not put forward clear requirements, only according to the grade of the tensile strength of the sample. Under these conditions, very few manufacturers carefully control the microstructure in the production process.

1. Operating conditions of brake drum

Automobile 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 truck brake drum, braking temperature can reach 850℃, so the friction surface is easy to crack. This kind of crack is usually called thermal fatigue crack, thermal crack or network crack.

Generally speaking, thermal fatigue is caused by the thermal stress cycle caused by temperature change. For the brake drum, the effect of temperature cycling changes, in addition to thermal stress, will also lead to changes in the internal structure of metal materials, thus aggravating the stress. In addition, during the operation of the brake drum, the heating of various parts is not uniform, and the thermal expansion of the components of the microstructure is different. Under the condition of repeated heating and cooling, the alternating stress will also be caused.

During use, its tissues may undergo a variety of phase transitions, such as:

(1) At 800℃ or slightly below 800℃, eutectic carbides decompress into graphite and ferrite;

(2) Pearlite and ferrite transform to austenite at 800℃;

2. Influence of graphite structure on fatigue strength

In gray cast iron, graphite microstructure has a very important effect on thermal fatigue crack formation, mainly as follows:

(1) If the graphite in gray cast iron is relatively thick and abundant, its thermal expansion coefficient is low;

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

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

Graphite has the function of buffering matrix tissue expansion, and it is also a solid lubricant.

3. Influence of graphite morphology

In both gray cast iron and ductile cast iron, thermal fatigue cracks initially occur on graphite substrates. In gray cast iron, the crack develops along the graphite base plane and its direction is not affected by the grain boundary of metal matrix or the pearlite orientation. Grey cast iron with A-type graphite has no directional graphite sheet in the structure, and the cracks produced at the early stage of fatigue are scattered and distributed, so it has A better ability to resist thermal fatigue cracks. Therefore, in the microstructure of brake drum castings, the graphite morphology should be type A. B, D and E/ type graphite all lead to the decrease of mechanical properties and deterioration of thermal fatigue properties of cast iron.

In ductile iron, the graphite base plane is in the diameter direction of the spherical graphite, but the spherical graphite can not buffer the stress generated in the matrix, and there will be cracks on the interface between graphite and ferrite. Radial cracks are then generated, propagating through the ferrite shell into the pearlite and into 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. However, in the presence of a large amount of flake graphite, the influence of matrix microstructure on thermal fatigue resistance is very small, so it can not be considered.

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

5. Quality requirements of gray iron for brake drum

Based on the above considerations, in addition to the mechanical properties and hardness requirements specified in accordance with vehicle characteristics, there are also the following special requirements for gray iron for brake drum:

A very low carbon content must be specified to ensure a volume fraction of graphite in the tissue. The carbon content of the brake drum 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 about 11-12%, and the thermal conductivity of cast iron is about 2 times that of ① its matrix structure;

(2) The graphite shape is A-type;

(3) do not allow too small graphite, the length of graphite is 2-4 or 3-5 grade;

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

Cast iron meeting the above requirements, normal temperature strength is not very high, but the ability to resist thermal fatigue is very good. The graphite sheet is too small gray cast iron, although the normal temperature strength is higher, but the thermal fatigue resistance ability is poor.

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

The results show that the heat resistance fatigue life of the ductile iron with pearlitic matrix structure is higher than that of the gray iron with similar matrix structure. However, for practical applications, other factors must be further considered. Due to the high thermal conductivity and low thermal expansion coefficient of gray cast iron, and the flake graphite can buffer the expansion of matrix structure, the maximum temperature of the gray cast iron brake drum is lower than that of the nodular cast iron brake drum under the same service conditions, and its actual service life is higher than that of the nodular cast iron brake drum. Therefore, ductile iron is generally not used to make brake drums.