Metallurgical Defects of Forgings and Heat Treatment Techniques
Forging is an ancient metal forming process, which utilizes the plastic deformation of metal to obtain forgings with improved mechanical properties and ideal microstructure. With the opening of automobile, aerospace and other high-end manufacturing industries, the application requirements of forgings are getting higher and higher, and the performance requirements of precision forgings are becoming higher and higher. The requirements for the metallurgy of forgings and the quality of heat treatment are also increasing. The metallurgical defects of forgings, such as severe concentrations of non-metallic inclusions, microcracks and unfavorable microstructures have become the number one enemy of precision forgings.
First of all, the low purity of raw materials and serious non-metallic inclusions are the main reasons for the occurrence of serious concentration of non-metallic inclusions in forging. If these inclusions are directly transferred from raw materials and collected in certain locations during the plastic deformation of forgings, the defects of non-metallic inclusions will appear. The formation of non-metallic inclusions not only affects the structure and properties of forgings, but also increases the difficulty of processing and causes the difference in size and surface quality of forgings.
In addition, poor heat treatment causes the formation of many micro-cracks. When subjected to heat treatment, the chemical composition of steels varies, and when the temperature difference is too large or too fast, the internal stress of forgings will be excessive and cause micro-cracks. In addition, softening and recrystallization need to be carried out during heat treatment. The improper control of the parameters will cause too large atoms such as carbon, sulfur and phosphorus to be precipitated and generate micro-cracks in the forgings.
Finally, improper heat treatment will lead to unqualified microstructure. When the heating temperature is higher or the holding time is longer, the grain size of the forgings will be too large, leading to poor mechanical properties. At the same time, improper heat treatment will produce coarse grains, graphite and other non uniform microstructure that will further worsen the mechanical properties of forgings.
Therefore, in order to ensure that forgings meet the requirements of precision forming, it is necessary to carry out strict selection of raw materials and careful process control during the production process of forgings, and solve the problems due to unreasonable process parameters through strict heat treatment process adjustment.
Firstly, the raw materials should be selected strictly. The selection of raw materials should meet the requirements of the international standard of steel grades, and the physical and chemical properties should basically meet the requirements of the standard. The carbon, sulfur, phosphorus and other harmful elements in the molten steel should be strictly controlled to ensure that the harmful elements do not exist in the molten steel and heat-affected zone in excess of the standard requirements. In addition, the temperature of the molten steel should be controlled, preferably to a temperature not exceeding 1450 degrees Celsius, in order to prevent the enrichment of harmful elements in the molten steel.
Secondly, strictly control the forging parameters. The forging temperature of the forgings should be suitable, and the heating temperature of the forgings should be strictly controlled to prevent the forgings from overheating. In addition, in order to prevent the uneven heating of the forgings, the forging is usually preheated.
Finally, heat treatment should be strict. The heat treatment temperature should be strictly controlled and the furnace temperature equipped should be calibrated regularly to ensure that the heating temperature is accurate. In addition, the specifications of heat treatment should be strictly observed, and the thermal temperature rise of the heat treatment should be appropriate. The holding time of each temperature segment should be sufficient, and the embrittlement after heat treatment should be eliminated.
Overall, the metallurgical defects of forgings and the quality of heat treatment should be strictly controlled in order to improve the quality of forgings and its properties. With the improvement of the process quality of forgings and the advances in heat treatment technology, it is expected that the metallurgical defects of forgings have become gradually eliminated.
