Defects of Large Forgings and Countermeasures
Large forgings usually refer to complex and large-scale forgings that weigh more than 10 tons and are difficult to process and inspect due to their large size, complex shape, high strength and low toughness. They are mainly used in petrochemical, power station, shipping, aerospace and other industries. During the production process of large forgings, some defects may occur.
1. Inclusions and Harmful Trace Elements
Inclusions are impurity particles in steel and castings. Inclusions mainly refer to a whitish crystalline subject matter, which has a variety of shapes and sizes. Its main body is composed of oxides, sulfides, nitrides and other non-metallic inclusions. Inclusions are usually distributed in the form of granules or rods along the grain boundaries and directions. Due to their small size and low hardness, they are difficult to detect during the pouring and heat treatment. Inclusion defects can cause harmful trace elements such as sulfur and phosphorus to gather on the surface of forgings and form scab.
Sulfur and phosphorus are very harmful to steel and may result in reduced mechanical properties and poor weldability. In addition, inclusions also reduce the plasticity, machinability and fatigue strength of steel, resulting in crack tendency and poor quality of product appearance.
2. Segregation
The segregation of chemical compositions caused by improper casting process or excessive temperature difference between the center and the corner of the heated forgings will lead to the inhomogeneity of chemical compositions. The mechanical properties of the inner and outer parts of the forgings are obviously different, and the fatigue strength, crack tendency, brittleness and wear resistance weaken.
3. Hot Spots
Improper control of temperature during the preheating process of large forgings may form hot spots with unequal concentration of heat energy. The two parts of hot spots are: coarse grains and fine grains, respectively, resulting in the decrease of metal structure and the reduction of strength.
4. Ferrite Strands
The phenomenon of ferrite strand usually occurs in large cylindrical forgings. It is often caused by poor thermal conductivity of the molten metal, uneven heating or slow furnace temperature, insufficient pouring time and so on. If a large number of ferrite strands are present, the hardness, strength and plasticity of the forgings will be reduced, resulting in shrunken forges, crack and brittleness.
Countermeasures
1. Reasonably Control Chemical Composition and Clean Steel
For large forgings, the chemical composition of the material should be reasonably determined according to the specific requirements of the product, such as strength, toughness, weldability and corrosion resistance. The austenitizing temperature of large forgings should be strictly controlled, and a certain amount of alloying element should be added to make the surface of the forgings homogeneous and free from oxygen, sulfur and other harmful elements.
2. Adopt Appropriate Pour System
The pouring system should be designed reasonably and the pour system should be agreed upon before manufacturing and according to the size and shape of the large forgings. Simultaneous pouring is usually used to avoid temperature differentials.
3. Strengthen Heating and Heat Preservation
Before the preheating of large forgings, some carbon elements can be added to the metal liquid to promote the formation of flocs of rich carbon particles. The preheating temperature of large forgings should be controlled in the range of 500-800℃, depending on the size and chemical composition of the forgings. In addition, for large forgings, sanding core plate should be used for heat preservation, and it should be completely sealed and timely replaced after preheating.
4. Strictly Implement Heat Treatment Process
The heat treatment process of large forgings should be strictly implemented in each array, and the temperature should not be too high or too low during the austenitizing process. The time of heat preservation of large forgings should comply with the process requirements and should not be too short or too long. After quenching, proper tempering can eliminate and reduce ferrite strands.
Conclusion
Large forgings generally have more defects, and the defects are relatively difficult to detect. The common defects of large forgings include inclusions, segregation, hot spots and ferrite strands. It is inevitable to prevent the occurrence of these defects in the production process of large forgings, we need to take some countermeasures including controlling chemical composition, adopting appropriate pouring system, strengthening heating and heat preservation and strictly implementing heat treatment process.
