Safety is the most important issue in nuclear power industry, which directly affects whether nuclear power can be used on a large scale. Improving the overall performance of nuclear power heavy forgings from the perspective of materials and forming plays a crucial role in ensuring the safe use of nuclear power
The internal quality of large forgings is generally evaluated by ultrasonic nondestructive testing method, and the large fluctuation of inspection results of different batches of forgings is a major problem faced by the whole large casting and forging industry in China. According to the sampling analysis of defective forgings, it is found that the main causes of defective inspection of large forgings include:
(1) Microscopic cracks or other defects caused by excessive non-metallic inclusions entrainment into ingot during smelting;
(2) the microscopic cracks occurring in the microstructure segregation zone of the forging;
(3) The original defects such as porosity and holes in the ingot are not closed, and the defects of large forgings may occur during the solidification, forging and subsequent heat treatment of the ingot. Therefore, Regardless of which causes, the nonconformity of large forgings inspection is determined by the three process processes of ingot metallurgy, forging and heat treatment, and it is difficult to avoid the microstructure segregation in large forgings. The current solutions to the nonconformity of large forgings inspection caused by the internal cracking of the tissue segregation belt of forgings mainly include:
(1) Improve the solidification process of ingot to improve the microsegregation in ingot;
(2) Optimize the high temperature diffusion process before forging to eliminate dendrite segregation in ingot;
(3) The forging process is optimized to make the metal undergo large plastic deformation under the condition of three-way compressive stress
1. Forging defects
A SA508-3 steel large forging after heat treatment Ultrasonic testing found that the maximum equivalent of 7 mm intensive defects in order to determine the inspection defect properties of forgings Serious defects in inspection location for sampling and analysis of the physical and chemical inspection of sampling position material chemical composition analysis result basically met forgings for low power design requirements in the defect area sampling inspection Normal area and the defect of all kinds of inclusions are not overweight Its macrostructure morphology is shown in figure 1 Forgings in internal defects which is parallel to the direction of the main deformation linear crack As shown in figure 2 The crack is connected to intermittent serrated cracks non-metallic inclusions are not found in and around Of cracks and non-metallic inclusions So the crack is not inclusion crack By optical microscope and scanning electron microscopy (sem) of forging cracks in tissue morphology are analyzed As shown in figure 3 is shown in figure 4 Crack exists in segregation crack wider in the middle Both ends of the sharp Along the grain boundary extension of specimen near the crack line of EdS analysis As shown in figure 5 Discover the Mn element content is higher in segregation band Figure 3 nearby tissue morphology of crack
From what has been discussed above Forging internal defects for intermittent jagged crack defects According to the analysis of EdS line jagged crack occurred in forging the micro segregation of micro segregation belt exist that make forging hardness of local area and the volume change rate is different from the surrounding normal tissues Organizational stress and deformation stress of thermal stress under the joint action of segregation band is easy to crack initiation And gradually expanded in the subsequent forging and heat treatment
2, cone plate upsetting + tire mold rotating flattening process of finite element analysis in order to avoid forging micro cracks within the segregation in the process of cold forging of large forgings, should by optimizing the forging process to make internal metal forgings to compressive stress cases occurred in three large deformation, so as to avoid new cracks, is advantageous to the existing closed crack welded, Make forging more even O at present, large plate type forgings of basic forming process for the plate upsetting + + rotating cone plate upsetting flattening method, forging profile for free deformation area, the actual deformation effect is similar to the plate upsetting, does not favor the elimination of internal defects in forgings, and is likely to cause cracks within the segregation band O to optimize spinning flattening method, A ring constraint is added to the flattening process, namely the tire mold rotational flattening method, as shown in FIG. 6.