I. Why do the tensile strength and hardness of ductile iron parts not improve after normalizing?
1. Raw materials
Unqualified chemical composition: Too high or too low carbon and silicon content will affect the normalizing effect. For example, if the carbon content is too high, too much graphite will be produced, reducing the matrix strength; if the silicon content is too low, it will not be conducive to strengthening the ferrite and will not effectively improve the strength and hardness. In addition, abnormal contents of elements such as manganese, phosphorus, and sulfur will also interfere with the organizational transformation during normalizing.
Poor spheroidization or improper inoculation: Insufficient addition of spheroidizer or poor quality will result in poor graphite spheroidization, forming flake or worm-like graphite and reducing mechanical properties. Improper selection or use of inoculants will also affect the graphitization process and matrix organization, resulting in no performance improvement after normalizing.
2. Normalizing process
Inaccurate heating temperature: If the heating temperature is lower than the normalizing temperature range, austenitization is insufficient, organizational transformation is incomplete, and ideal troostite or pearlite organization cannot be formed, and strength and hardness are difficult to improve. If the heating temperature is too high, the austenite grains will be coarse, and the structure obtained after cooling will also be coarse, reducing the strength and hardness.
Insufficient holding time: If the holding time is too short, the carbides and other phases in the cast iron will not have time to fully dissolve and homogenize, the austenite composition will be uneven, and the structure and performance will be uneven after cooling, affecting the overall strength and hardness.
Inappropriate cooling speed: If the normalizing cooling speed is too slow, the austenite will be transformed into a mixed structure of ferrite and pearlite, and the pearlite content is small and the interlamellar spacing is large, resulting in reduced strength and hardness. If the cooling speed is too fast, internal stress may be generated, and even cracks may appear, which is not conducive to performance improvement.
3. Subsequent processing
Excessive machining allowance: If the machining allowance after normalizing is too large, the surface strengthening layer will be removed, so that the actual measured strength and hardness cannot reflect the true performance after normalizing.
Improper tempering: If the tempering temperature is too high or the time is too long, the troostite or pearlite structure formed by normalizing will be over-tempered, and the carbides will aggregate and grow, resulting in a decrease in strength and hardness.
In addition, measurement errors may also lead to the illusion that the tensile strength and hardness have not increased. For example, if the measuring instrument is not calibrated, the measuring position is improper, and the sample preparation does not meet the requirements, the measurement results will be inaccurate.
2. Reasons for deformation of ductile iron castings after normalizing
1. Casting structure design
Uneven structure: The thickness of each part of the casting varies greatly. During normalizing heating and cooling, the heat transfer speed of the thick wall and the thin wall is different, resulting in uneven thermal stress and deformation.
Complex shape: Castings with complex structures such as many protrusions, grooves, and holes restrict each other during the normalizing process, which is easy to deform.
2. Raw material factors
Uneven organization: The graphite nodules in ductile iron are unevenly distributed and the matrix organization is different. The organization transformation of different areas during normalizing is not synchronized, which will cause deformation.
Influence of impurity elements: The presence of impurity elements such as phosphorus and sulfur in the raw materials will reduce the high temperature strength and toughness of cast iron, making the casting more prone to deformation during normalizing.
2. Problems with normalizing process
Heating speed is too fast: Rapid heating causes a large temperature difference between the inside and outside of the casting, and the thermal stress increases sharply, exceeding the yield strength of the material, causing deformation of the casting.
Excessive holding time: Excessive holding time will cause the austenite grains to grow, reduce the high temperature strength of the casting, and make it easier to deform under thermal stress.
Uneven cooling: During normalizing cooling, the contact between different parts of the casting and the cooling medium is different, and the cooling speed is different, resulting in uneven shrinkage and deformation.
2. Furnace loading and operation factors
Improper furnace loading method: The casting is placed unsteadily and unevenly in the heating furnace, or squeezed against each other, which will cause uneven heating of various parts and cause deformation.
Irrational use of fixtures: The fixtures used are not rigid enough or the clamping method is improper, which cannot effectively restrain the deformation of the casting during the normalizing process, or the fixtures themselves are affected by heat deformation and affect the casting.
2. Pre-treatment of castings
Casting stress is not eliminated: The internal stress generated during the casting process is not fully eliminated through aging and other treatments, and it is superimposed with thermal stress during normalizing, causing the casting to deform.
Uneven machining allowance: Excessive and uneven machining allowance will cause different heat capacity and heat dissipation conditions in different parts of the casting during normalizing, resulting in deformation.
3. Reasons for crack defects in ductile iron parts after normalizing
1. Casting structure and design
Sudden change in wall thickness: The wall thickness of the casting changes too drastically. During normalizing, the junction between the thick wall and the thin wall produces large thermal stress due to the difference in heat transfer. When the stress exceeds the material strength limit, cracks will be caused.
Stress concentration: There are structures such as sharp corners, notches, and deep holes in the casting. These parts are prone to stress concentration during normalizing and become crack sources.
2. Raw material problems
Excessive sulfur content: Sulfur will reduce the toughness of ductile iron, increase brittleness, and make the casting prone to cracks under the action of normalizing thermal stress.
Poor spheroidization: Poor quality or improper dosage of spheroidizer will lead to poor graphite spheroidization effect, forming flake or worm-like graphite, which will reduce the strength and toughness of the casting and easily crack during normalizing.
3. Normalizing process factors
Heating speed is too fast: Too fast heating speed makes the temperature difference between the inside and outside of the casting too large, resulting in huge thermal stress, which may exceed the bearing capacity of the material, thus causing cracks.
Cooling speed is too fast: During normalizing cooling, the cooling speed is too fast, which will cause the shrinkage of the surface and the core of the casting to be inconsistent, forming a large tensile stress, causing cracks, especially for high-carbon and high-silicon ductile iron castings.
Tempering is not timely: If tempering is not timely after normalizing, the large internal stress inside the casting cannot be eliminated. During subsequent placement or use, the release of internal stress may cause cracks.
4. Problems left over from the casting process
Casting defects: There are shrinkage cavities, shrinkage porosity, pores and other defects in the casting during the casting process. These defects will become stress concentration points during normalizing, prompting crack formation and expansion.
Residual stress: The residual stress generated during the casting process is large, and the normalizing process fails to effectively eliminate it. Instead, it is superimposed with the normalizing thermal stress, causing the casting to crack.
5. Operation and equipment problems
Improper loading: The castings are placed unreasonably in the heating furnace, such as colliding, squeezing, or being too close to the heating element, resulting in uneven heating, local overheating and cracks.
Equipment failure: Inaccurate temperature control of the heating furnace, excessive temperature fluctuations or local temperature abnormalities will cause the casting normalizing process to get out of control and cause cracks.
