Due to the harsh working conditions of the product, the large working load and the complicated stress, the main failure mode of the material during service is the fracture caused by low temperature brittleness. The failure of the workpiece will undoubtedly have a great impact. Therefore, the comprehensive quality and process control of the product is relatively strict.
Compared with other products, the material properties of the shaft casting not only have the requirement of -40 ℃ low temperature impact AKV≥27 J , but also the tensile strength requirement is greater than 780 MPa , and the yield strength is ≥630 MPa . The product requires both high strength and good low-temperature impact toughness.
Through the material simulation software, within the designed internal control composition range, input the simulation software according to the set target composition, and obtain the CCT curve analysis of the material, and obtain the structure transformation and the transformation temperature of the material at different cooling speeds after continuous cooling , And the quenching M transition temperature and end temperature. At the same time, it can be seen that the critical cooling rate of quenching of this material is very large, and the requirement for quenching cooling rate is very high. Otherwise, if the quenching cooling rate is not fast, there will be P or B structure This will affect the strength and reduce the low temperature toughness.
The casting adopts the quenching + tempering scheme, and the focus is on the implementation of quenching. Because the casting structure is a horn shape, the inner cavity is tapered, and the minimum opening diameter of the hole is 200 mm . In order to ensure that the test blocks on the disc flange can be heat treated under the same conditions, and to ensure the full use of heat treatment furnace resources, two pieces of one furnace are installed on the quenching tray for heating and quenching. The rotating shaft adopts vertical mounting, and the disc flange contacts the quenching tray.
Before quenching and installing the furnace, ensure that the hole in the shaft is cut and cleaned of oxide slag and debris, so that water vapor can be sprayed from the hole during quenching, so that the inner cavity can also be cooled.
In view of the above difficulties and product quality requirements, the heat treatment process needs to study the influence of the chemical composition on the low-temperature material, and formulate the best internal control of the material; study the influence mechanism of the heat treatment process on the low-temperature performance and high strength of the material to obtain the mechanical properties At the same time, research the best heat treatment furnace installation and quenching plan and process control measures in line with the actual on-site, and finally achieve a material performance compliance rate of 100% .