Quality requirements for gray cast iron for brake drum

In general, gray cast iron is suitable for the manufacture of brake drum material, but because the brake drum working conditions are extremely harsh, there must be some special requirements for the gray cast iron used.

1. Carbon content and the volume fraction of graphite in the tissue

Because the temperature of the brake drum rises instantaneously during the braking process, the heat generated must escape as soon as possible, and the thermal conductivity is an extremely important performance. High thermal conductivity can reduce the temperature gradient in the brake drum, so as to reduce the thermal stress and avoid damage caused by thermal stress and thermal fatigue.

Under laboratory conditions, THE test results of different materials with the same cycle temperature change show that the thermal fatigue life of pearlite matrix nodular cast iron is higher than that of gray cast iron with similar matrix structure. However, due to the high thermal conductivity of gray cast iron, under the same service conditions, the maximum temperature reached by the gray cast iron brake drum is lower than that of the ductile cast iron brake drum. At the same time, flake graphite also has the function of buffering matrix structure expansion, so its actual service life is higher than that of ductile iron brake drum. Considering the effect of practical application, the brake drum is generally made of gray cast iron instead of nodular cast iron.

Thermal conductivity of cast iron and graphite of volume fraction and there is a close relationship between the size of the flake graphite, which increase the amount of flake graphite and graphite piece of the larger size, thermal conductivity of gray cast iron is high, in the same operating conditions, can reduce the temperature of the substrate by the organization to achieve, so that the overall thermal stress is small.

In order to make the iron microstructure has more graphite sheet, to ensure that it has a higher thermal conductivity, the carbon content of passenger car brake drum should be above 3.4%, above 3.5% in heavy vehicles, heavy vehicles brake drum should be above 3.7%, and this is the primary quality requirements. Under these conditions, the volume fraction of graphite in cast iron is approximately 11 ~ 13%, and the thermal conductivity of cast iron is about twice that of its matrix material.

Currently, there are no relevant national standards or industrial standards formulated in our country. Some castings buyers do not require clear requirements for the carbon content of gray cast iron for braking drum. Many castings simply press the gray cast iron brand number and ensure the tensile strength of sample.

It is of course appropriate to control the carbon content between 3.2% and 3.3% to ensure the qualified strength of cast iron. However, this practice cannot guarantee the basic quality requirements of the brake drum and is not allowed.

2. Distribution shape and length of graphite sheet

The cracks in gray cast iron develop along the graphite base plane, and their direction is not affected by the grain boundary of the metal matrix or the orientation of the pearlite. The distribution shape of graphite sheet is A-type gray cast iron, the graphite sheet in the structure has no directionality, and the small cracks produced at the beginning of fatigue are scattered and distributed, so it has A better ability to resist thermal fatigue cracks. Therefore, in the microstructure of brake drum casting, the graphite morphology should be type A. Type B, D and E graphite will lead to the deterioration of thermal fatigue properties, but also reduce the mechanical properties of cast iron.

If the flake graphite is too fine, the thermal conductivity of gray cast iron is low, and the buffering effect of graphite is poor when the matrix is thermally expanded. Therefore, the gray cast iron used in the brake drum, the graphite sheet in the organization should be relatively thick, should not be too small, the length of graphite should be 2 ~ 4 or 3 ~ 5.

In ASTM A159 "gray cast iron for automobile" standard, the distribution shape of flake graphite and the length of graphite sheet are specified in detail. The distribution shape of flake graphite should be type A. For gray cast iron with low strength requirements, the length of graphite sheet is 2 ~ 4 grade; For gray cast iron with higher strength, the length of graphite sheet is 3 ~ 5 grades.

3. Matrix organization

Due to the large volume fraction of graphite in cast iron, and the graphite sheet is relatively large, the mechanical properties of cast iron are of course low, so. Gray cast iron used as brake parts has not too high requirements for mechanical properties. The lowest value of tensile strength is generally 175 MPa, 200 MPa or 250 MPa. So far, there is no brand that requires tensile strength higher than 300 MPa.

In ORDER TO MAKE THE NORMAL temperature MECHANICAL PROPERTIES OF THIS SPECIAL gray CAST IRON MEET THE SPECIFICATION REQUIREMENTS, IT IS necessary to carefully control the metallurgical quality and breeding process of cast iron, so that the matrix structure of cast iron is fine pearlite. The tensile strength is required to be more than 200 MPa, and the amount of ferrite in the microstructure shall not be more than 5%. Ferrite content is too high, not only affect the strength, and will make the brake drum processing performance deterioration, wear resistance and service life reduction.

For the varieties with higher strength and hardness requirements, it is sometimes necessary to add a small amount of alloying elements such as Cu, Cr and Mo. In this case, the amount of alloy elements should be strictly controlled to meet the performance requirements for the degree, too much alloy content will make graphite fine, volume fraction reduction, but has a negative impact on the service life of the brake drum.