Brake drum quality requirements
Brake drum used in automobile is very important security parts, which has special technical requirements on the inner quality of castings. But, make a worry is: according to the investigation that Chinese foundry association makes, in numerous brake drum business, have correct understanding to product quality requirement however a few.
Take the carbon content of cast iron as an example: in order to make the microstructure of cast iron have more graphite sheets, in order to ensure its high thermal conductivity, the carbon content of passenger car brake drum should be 3. Above 4%, the heavy vehicle brake drum should be at 3. More than 7%, and this is the first quality requirement. In fact, most manufacturers simply limit their carbon content to 3. About 2%, the correct understanding of this basic requirements of casting enterprises are very few.
As for the microstructure requirements of cast iron, astm mal59-83 "gray cast iron for automotive use" standard. The morphology, length and matrix structure of graphite are specified. China and some other countries have not formulated relevant national or industrial standards. Therefore, casting buyers often do not make clear requirements for this. Under such conditions, there are fewer manufacturers who carefully control microstructure in the production process.
1. Working conditions of brake drum
Automotive brake drum, brake due to friction and lead to temperature rise, and then due to the high thermal conductivity of the metal and rapid cooling, in the process of use to undergo repeated heating and cooling, heavy vehicle brake drum, brake temperature can reach 850~C, so the friction surface is prone to cracks. This kind of crack is usually called thermal fatigue crack, thermal crack or reticular crack.
Generally speaking, thermal fatigue is fatigue caused by thermal stress cycling due to temperature changes. For brake drum, the role of temperature cycle change, in addition to the thermal stress, but also lead to the internal structure of the metal material changes, so as to aggravate the stress. In addition, brake drum operation, each part is not uniform heat, the thermal expansion of each micro component is different, in the repeated heating, cooling conditions, will also cause alternating stress.
2. Influence of graphite morphology
Thermal fatigue cracks in gray cast iron or nodular cast iron originated on the graphite substrate. In gray cast iron, cracks develop along the graphite substrate and are not affected by grain boundary or pearlite orientation. Gray cast iron with type A graphite has no directionality of graphite sheets in the structure, and cracks generated in the early stage of fatigue are scattered and distributed, so it has A good resistance to thermal fatigue cracks. Therefore, the morphology of graphite in the microstructure of brake drum casting should be type A. Type B, type D and type E lead to the decrease of mechanical properties and deterioration of thermal fatigue properties of cast iron.
In nodular graphite cast iron, the base surface of graphite is radial to nodular graphite, but nodular graphite cannot buffer the stress generated in the matrix, and cracks will occur on the interface between graphite and ferrite. The radial cracks are then propagated through the ferrite shell to the pearlite until another graphite ball is formed.
3. Influence of matrix tissue
According to the laboratory test results, the pearlite matrix has the strongest resistance to thermal fatigue crack, but in the presence of a large number of flake graphite, the matrix structure has little influence on thermal fatigue performance, which can be ignored. Some standards limit ferrite and carbide content, mainly in order to ensure room temperature strength and hardness. The quality requirements of grey iron for brake drum are based on the above considerations.