(1) Reduce the level of non-metallic inclusions in castings. The main purpose is to improve the purity of liquid steel. Inert gas is used to protect the gate and sand mold during pouring to prevent the secondary oxidation of liquid steel during pouring. The molding and box closing processes ensure that the sand mold is clean and free of loose sand and sundries, and the gating system is reasonably designed to prevent sand flushing. At the same time, the amount of liquid steel is sufficient during pouring to ensure that there is enough residual steel in the ladle to avoid slag. Finally, the grade of non-metallic inclusions in the casting should be lower, no matter it is coarse or fine inclusions, the grade should be less than 1.5 as far as possible to avoid foreign inclusions.
(2) In order to control the structure of casting, in addition to ensuring that the matrix structure of casting is mainly uniform tempered martensite, the internal structure is generally controlled δ- The ferrite content should be controlled below 3%. δ- Ferrite belongs to eutectoid phase produced at high temperature during solidification of liquid metal. Once it is produced, it is difficult to eliminate it by subsequent normal heat treatment. In theory, some or even all of the defects can be eliminated by high temperature diffusion annealing in the austenite single phase region δ- Ferrite, but after high temperature and long time diffusion annealing, the grain size of the material is coarse, and the casting is severely burned and deformed, which is not suitable for actual production. Therefore, for martensitic stainless steel, it is necessary to design the ratio of Nieq / Creq in the alloy to control the final microstructure δ- Ferrite content. According to the theoretical calculation and the actual statistics of the sand castings produced at present, when Nieq / Creq ≥ 0.42, there is little in the as cast material δ- Ferrite. Nieq = Ni + 30 (c + n) + 0.5mn, Creq = Cr + Mo + 1.5si.
(3) Improve the compactness of the test block and eliminate the defects. During the casting process design, the appropriate size and shape of the test block should be selected. For the attached test block, attention should be paid to the placement position of the casting, so as to avoid the test block being placed in the parts easy to accumulate slag, hang slag or wash sand. If there is no special requirement, the test block should not be placed in the hot spot, so as to avoid the test block and its surrounding parts from choking. If necessary, the test block should be added with a small air outlet pipe. In addition, the compactness of the sand in the cavity of the test block and the brushing quality of the coating should be strengthened to avoid sand sticking on the test block after pouring. Before closing the box, carefully check and clean the sand and foreign matters in the cavity of the test block. Of course, for the later sampling blocks, the appearance inspection can also be carried out in advance, and the test blocks with air holes, slag holes and serious sand sticking can be scrapped. In addition, the test block with no problem on the surface can also be used for UT and MT inspection after grinding. However, the quality of the test block is mainly controlled from the source because the small pass or linear small defect may not be identified by UT inspection.
In addition, we should try our best to ensure that the casting structure is uniform, the grain size is appropriate, the sample can be processed according to the standard requirements, the sample size, shape and surface roughness meet the requirements, and the bending test process is standardized. The above measures can significantly improve the compliance rate of bending properties.
