1.Effect of aging treatment
For the engine parts of gray cast iron cylinder head, such as cylinder block, cylinder head, crankcase and other castings, there are aging treatment processes in the production process. However, with the increase of environmental protection pressure, the heat treatment resources tend to be scarce. We have been trying not to carry out aging heat treatment for the gray cast iron cylinder head in the engine, such as the crankcase blank in the engine, We have obtained great benefits through the aging cancellation treatment, while the cylinder head of gray cast iron cylinder head still retains the aging treatment. At present, the aging process is to heat the gray cast iron cylinder head to 530 ~ 630 ℃, after 2 ~ 6 hours of insulation, slowly cool it in the furnace to below 200 ℃, and then discharge the furnace for air cooling. However, when the temperature of gray cast iron cylinder head is higher than 550 ℃, the matrix structure may be changed, resulting in the reduction of strength and hardness. This shows that the temperature control of aging treatment is also a factor affecting the hardness of gray cast iron cylinder head.
2. Addition amount of silicon barium long-acting inoculant
In the initial stage of process development, 75sife is used as the main gray iron inoculant. With the development of later materials, silicon barium inoculant is introduced. Both inoculants can be used in normal production, and the use amount of silicon barium inoculant is the same as 75sife. When the inoculation effect is close, the amount of silicon barium inoculant should be reduced, and too high amount of silicon barium inoculant is easy to increase ferrite and reduce hardness. However, as a long-term inoculant, silicon barium inoculant is of great help to improve the sensitivity of wall thickness.
3. Overheating temperature of smelting is too high
During smelting, in order to purify and inoculate, the molten iron will be heated above the normal temperature, which is overheated smelting. Overheating can remove impurities and purify molten iron, mainly to reduce the amount of dissolved oxygen in molten iron. The temperature of molten iron gradually increases from 1350 ℃ to 1450 ℃, and the dissolved oxygen always rises to the highest. When it is above 1450 ℃ and below 1500 ℃, the reaction occurs to produce CO, the dissolved oxygen decreases and the nitrogen content increases. The superheated temperature of molten iron will also affect the mechanical properties of gray cast iron cylinder head. The tensile strength increases with the increase of molten iron temperature, the molten iron temperature is lower than 1460 ℃, and the hardness value increases with the increase of temperature; When the temperature of molten iron is higher than 1500 ℃, the hardness value decreases with the increase of temperature. However, when the superheat temperature exceeds 1550 ℃, the white cast iron cylinder head tends to be large, the A-type graphite decreases and the D-type graphite increases.