6. Uneven hardness of ductile iron after normalizing
Uneven hardness of ductile iron after normalizing is a common problem. The following is an introduction to its causes and solutions:
Causes
Difference in cooling rate: During the cooling process, the contact conditions between different parts of the casting and the cooling medium are different, resulting in different cooling rates. For example, the surface of the casting cools quickly, the core cools slowly, the surface hardness is relatively high, and the core hardness is low. Uneven flow of the cooling medium will also cause uneven cooling rates in different parts of the casting, resulting in uneven hardness.
Chemical composition segregation: During the casting process, alloy elements and impurity elements are unevenly distributed, forming segregation. For example, if the local content of elements such as silicon and manganese is too high or too low, the phase change temperature and transformation rate of different parts will be different, resulting in different hardness.
Uneven organization: Uneven heating or insufficient holding time will result in inconsistent austenitization degree, uneven organization after cooling, and different hardness. Poor spheroidization and inoculation treatment, uneven distribution of graphite balls, will also cause differences in matrix organization and cause uneven hardness.
Casting structure factors: The casting structure is complex, the wall thickness difference is large, the thick wall cools slowly, and it is easy to form coarse pearlite or ferrite structure with low hardness; the thin wall cools quickly, and fine pearlite or bainite structure with high hardness may be formed.
Solutions
Improve cooling conditions: Optimize the flow of cooling medium, such as using circulating air cooling or controlling the stirring speed of quenching liquid to ensure uniform cooling. For complex structure castings, appropriate slow cooling measures or graded cooling can be adopted.
Control chemical composition: Select raw materials with stable quality, strictly control chemical composition, strengthen spheroidization and inoculation treatment, and ensure uniform distribution of graphite balls. Furnace front rapid analysis technology can be used to adjust chemical composition in time.
Optimize normalizing process: Select a heating furnace with good temperature uniformity and calibrate the temperature control system regularly. Reasonably design the furnace loading method to ensure uniform heating of the casting. Through experiments and calculations, determine the appropriate insulation time to ensure that the structure is fully homogenized.
Perform homogenization treatment: For castings with uneven hardness, high-temperature tempering or secondary normalizing and other homogenization treatments can be performed to eliminate differences in structure and hardness.
Seven. Widmanstatten structure appears after normalizing of ductile iron
The appearance of Widmanstatten structure after normalizing of ductile iron is an issue that needs attention. The following is an introduction to its causes, effects and solutions:
Causes
The heating temperature is too high: When the normalizing heating temperature is too high, the austenite grains will grow significantly, providing favorable conditions for the formation of Widmanstatten structure. In the subsequent cooling process, the supersaturated carbon will precipitate from the austenite in the form of needles or flakes to form the Widmanstatten structure.
Improper cooling rate: If the cooling rate is too fast, the driving force of austenite transformation will increase, promote the rapid precipitation and diffusion of carbon, and easily lead to the formation of Widmanstatten structure. For example, under cooling methods such as air cooling or water cooling, if the cooling rate is not well controlled, this situation may occur.
Influence of chemical composition: The content and ratio of alloy elements in ductile iron are not appropriate. For example, if the content of elements such as silicon and manganese is too high, the supersaturation of carbon in austenite will increase, which will promote the formation of Widmanstatten structure.
Impact
Reduced toughness and plasticity: The presence of Widmanstatten structure will significantly reduce the toughness and plasticity of ductile iron, making the material brittle and hard. When subjected to impact loads or alternating loads, brittle fracture is likely to occur, reducing the reliability and service life of the casting.
Deterioration of processing performance: Due to the high hardness of the Widmanstatten structure, tool wear will be aggravated, processing difficulty will increase, and processing surface quality will be difficult to guarantee, increasing processing costs and processing time.
Solutions
Adjust the heating process: Strictly control the normalizing heating temperature, and reasonably select the heating temperature range according to the material and size of the casting to avoid excessive temperature. Segmented heating, preheating and other methods can be used to heat the casting evenly to prevent excessive growth of austenite grains.
Optimize cooling method: Select the appropriate cooling rate according to the specific situation of the casting. For castings that are prone to Widmanstatten structure, the cooling rate can be appropriately reduced, such as air cooling or a combination of air cooling and air cooling. At the same time, the uniformity of the cooling medium must be ensured to avoid excessive local cooling.
Control chemical composition: strictly control the quality of raw materials, ensure that the content of alloy elements meets the requirements, adjust the ratio of silicon, manganese and other elements, and reduce the factors that promote the formation of Widmanstatten structure. If necessary, some elements that can inhibit the formation of Widmanstatten structure, such as molybdenum and vanadium, can be added.
