The technological progress of stainless steel forging production is of great significance to the development of the national economy, and its technical level, production capacity, economic and technical indicators often become one of the important indicators to measure the level of industrial development of a country.
At present, the single mass of large forgings has reached 250T, and the forging ingot has reached 600T. Stainless steel forging, not only to get a certain shape and size, more importantly, through forging to improve the as-cast structure of the ingot, improve the mechanical properties of stainless steel forging.
Improvement of ingot structure and defects by forging.
The large deformation of ingot by forging can improve the internal structure and defects of ingot in three aspects: changing the as-cast structure to refine the grain, reducing the degree of segregation and welding the internal cavity defects.
Change the as-cast structure and refine the grain: When the deformation degree of the ingot reaches a certain value in the forging process, the as-cast structure and dendritic crystal are broken, and a new equiaxed fine crystal structure is formed by subsequent recrystallization. However, the final grain size of stainless steel forgings is related to the temperature and degree of deformation during deformation. If the temperature at the final forging is high, the grain size tends to grow. If the deformation falls near the critical deformation degree, the grains of stainless steel forgings are abnormally coarse.
Reduce the degree of segregation and change the distribution of inclusions: when the ingot is heated to high temperature, the dendrite segregation and the intergranular segregation are reduced to varying degrees due to the significant inter-atomic diffusion effect and the extension of heating time, and the microscopic segregation can be basically eliminated through the forging deformation smashing dendrite and the subsequent recrystallization.
The macrosegregation, V segregation, A segregation and transition segregation in the ingot are also improved with recrystallization during forging deformation, especially when the forging ratio is large, the degree of these segregation is also greatly reduced. The larger inclusions gathered in the macro-segregation area, such as carbides, oxides, sulfides, etc., are broken during deformation, coupled with the effect of high temperature diffusion and mutual dissolution, so that they are more evenly dispersed in the metal matrix, thus improving the ingot structure and improving the performance of stainless steel forgings. This is of great significance for steels containing a large number of carbides, such as high speed steel and high carbon steel. Therefore, when forging this kind of ingot, it is necessary to repeatedly carry out large forging ratio deformation, such as cross upsetting, repeated upsetting and drawing length, which can improve the distribution of carbide and other inclusions.
The basic conditions of cavity defects in forged ingot are: the cavity surface is not oxidized, there is no nonmetallic inclusion, the forging temperature is high enough, the state of large three-way compressive stress, and a certain degree of deformation or local forging ratio is required.
When the ingot is forged, the properties of stainless steel forgings will change. The forging ratio has the greatest influence on the mechanical properties of stainless steel forgings. In order to obtain higher mechanical properties of stainless steel forgings, forging should reach a certain forging ratio, and the forging ratio of stainless steel forgings is generally in the range of 2-6.
In addition, forging can also improve the fatigue properties of stainless steel forgings. Through forging, the ingot can improve the density and uniformity of the structure, and the macro and micro defects are improved and eliminated, which is undoubtedly conducive to reducing the stress concentration source, so that the fatigue resistance of stainless steel forgings can be improved.
Therefore, controlling the forging ratio of stainless steel forgings during forging is also one of the important ways to improve the fatigue limit of steel.