1.Technical requirements
For the technical requirements of metallographic structure, most customers’ products are basically consistent with those recommended in technical conditions for camshaft of automobile engine.
Fig. 1 is a schematic diagram of metallographic cutting of all cold shock cam. From the surface layer to the center of the cam, the cross-section structure of the cam is composed of the outer cold shock zone, the transition zone (between the outer layer and the center) and the non cold shock zone such as the center and the journal.
(1) the microstructure of the outer cold shock zone is white, and its microstructure is ledeburite + acicular carbon + flake (or granular) pearlite. A small amount of free graphite is allowed.
(2) the transition zone is pockmarked structure, and its microstructure is ledeburite + pearlite + carbide + a small amount of flake graphite.
(3) the center of cam (the diameter of the center should be more than or equal to 10 mm) or the center and base circle of semi chilled cam are pearlite gray cast iron, and its microstructure is fine flake (or medium flake) pearlite + graphite (graphite shape a or a + e). The length of graphite is 4-7 grade, the quantity of carbide is 1-5 grade, and the quantity of pearlite is 1-2 grade.
(4) the non quenched parts such as journal are pearlite gray cast iron, and its microstructure is fine flake (or medium flake) pearlite + graphite (graphite shape a or a + e). The length of graphite is 4-7 grade, the quantity of carbide is 1-4 grade, and the quantity of pearlite is 1-2 grade.
2.Control points
After the internal quality control of chemical composition, matrix hardness, depth of cold shock layer and cam tip hardness is qualified, the metallographic structure is usually easy to meet the technical requirements. In the actual casting production process, occasionally the carbide in the non quenched part exceeds the standard, which affects the machining cutting. The reduction of carbides can be achieved by taking the following measures: avoiding premature opening of individual box, avoiding low pouring temperature in later stage, improving inoculation effect, avoiding inoculation failure, controlling Cr / Mn alloy content not exceeding the upper limit, etc.