IV. Causes of uneven structural defects after normalizing of ductile iron
Uneven structural defects after normalizing of ductile iron are a common problem. The following is an introduction to its causes, effects and preventive measures:
1. Causes
Uneven heating: The temperature field in the heating furnace is uneven, and different parts of the casting are heated differently, resulting in different degrees of austenitization. Improper loading, different distances between the casting and the heating element, or damage to the heating element will cause local overheating or overcooling.
Insufficient holding time: The holding time is too short, and phases such as carbides do not have time to fully dissolve and homogenize, the austenite composition is uneven, and the structure is also uneven after cooling.
Uneven chemical composition: The quality of raw materials is unstable, and the alloy elements and impurity elements are unevenly distributed. For example, local segregation of elements such as silicon and manganese will cause different phase change temperatures and transformation speeds in different parts. Poor spheroidization and inoculation treatment, uneven distribution of graphite balls, will also lead to uneven matrix structure.
Uneven cooling: The cooling medium flows unevenly, and the contact between the cooling medium and the casting parts is different. For example, when air cooling is used, the cooling speed of the casting surface and the center is different, which is easy to form uneven structure. The casting structure is complex, and the heat dissipation conditions of each part are different. The thick wall cools slowly and the thin wall cools quickly, which will also cause organizational differences.
2. Impact
Unstable mechanical properties: Uneven organization will lead to inconsistent mechanical properties such as strength, hardness, and toughness of different parts of the casting. When bearing loads, fatigue cracks are prone to appear first in weak parts, reducing the overall service life of the casting.
Deterioration of processing performance: Uneven hardness will increase tool wear, difficult to ensure processing accuracy, and increase surface roughness, affecting processing efficiency and quality.
Different corrosion resistance: Uneven organization will cause different electrode potentials in different parts of the casting. In the corrosive medium, micro-batteries will be formed, which will accelerate the corrosion process and reduce the corrosion resistance of the casting.
3. Preventive measures
Optimize the heating process: Select a heating furnace with good temperature uniformity and calibrate the temperature control system regularly. Reasonably design the furnace loading method to ensure that the casting is heated evenly. If necessary, it can be fixed with fixtures.
Ensure sufficient insulation time: Determine the appropriate insulation time through experiments and calculations to ensure that the organization is fully homogenized.
Control chemical composition: Select raw materials with stable quality, strictly control the chemical composition, strengthen spheroidization and inoculation treatment, and ensure uniform distribution of graphite balls. Furnace-front rapid analysis technology can be used to adjust the chemical composition in time.
Improve cooling conditions: Optimize the flow of cooling medium, such as using circulating air cooling or controlling the stirring speed of the quenching liquid to ensure uniform cooling. For castings with complex structures, appropriate slow cooling measures or graded cooling can be adopted.
5. Causes of decarburization defects in ductile iron parts after normalizing
1. Causes
Influence of heating environment: Normalizing heating is carried out in an oxidizing atmosphere. If there is excessive air, water vapor, etc. in the furnace, the oxygen in it will react with the carbon on the surface of the cast iron, causing the carbon to escape in the form of carbon monoxide or carbon dioxide, resulting in surface decarburization.
Heating temperature and time: If the heating temperature is too high and the time is too long, the reaction rate of carbon and oxygen will be accelerated, and the degree of decarburization will be increased. Generally speaking, the decarburization rate increases by about 2-4 times for every 100°C increase in temperature.
Raw material factors: If ductile iron contains more elements that promote decarburization, such as silicon and aluminum, the tendency of surface decarburization will increase. In addition, the gas generated by the decomposition of oil stains and impurities on the surface of the raw materials during heating may also participate in the decarburization reaction.
2. Impact
Reduction in hardness: The reduction in surface carbon content will significantly reduce the surface hardness, generally by 10%-30%, resulting in poor wear resistance of the casting, and easy wear and tear during use.
Strength loss: Decarburization will reduce the strength and toughness of the surface. When bearing load, the surface is more prone to cracks and fatigue damage, reducing the service life of the casting.
Corrosion resistance changes: Decarburization changes the chemical composition and structure of the surface, which may also affect its corrosion resistance and make it more susceptible to corrosion in some corrosive environments.
3. Preventive measures
Control the heating atmosphere: Use a controlled atmosphere heating furnace to introduce protective gas, such as nitrogen, argon, etc., into the furnace, or use drip-type atmosphere control to drip methanol, ethanol and other organic liquids into the furnace to crack and produce a reducing atmosphere to inhibit decarburization.
Optimize process parameters: According to the material and size of the casting, reasonably formulate the normalizing heating temperature and time to avoid excessive temperature and excessive time. Segmented heating, rapid heating and other processes can be used to reduce the residence time at high temperature.
Surface protection treatment: Before normalizing, the surface of the casting is coated with a protective agent, such as a protective coating made of borax, glass powder, etc., to form a protective film on the surface of the casting to prevent oxygen from contacting carbon.
