Aiming at the wet mold clay sand casting process of railway steel castings, the research is carried out from the aspects of the formulation development of wet mold surface sand and back sand, the computer simulation and optimization of casting process scheme, the analysis and optimization of casting defects in product trial production and mass production, and the following conclusions are obtained:
(1) According to the results and analysis of single factor experiment, orthogonal experiment and optimized formula verification experiment of surface sand and back sand, and considering the actual production conditions and the influence of seasonal temperature and humidity changes, the recommended optimized formula of surface sand and back sand laboratory is determined as follows:
1) The surface sand “100% Jinjiang sand + (8.0-8.5%) bentonite + (0.3-0.5%) starch additive + (3.0-3.6%) water” can achieve the molding sand performance compaction rate of 51-54%, air permeability of 440-450, wet compression strength of 85-95kpa, hot and wet tensile strength of 4.1-4.5kpa, crushing index of 70-72%, surface stability of 97.0-97.5%. The molding sand performance meets the factory surface sand performance requirements and is verified by factory batch production;
2) The backing sand is “100% recycled old sand + (0.2-0.3%) bentonite + (0.1-0.3%) additive + (3.3-3.6%) water”, which can achieve the molding sand performance compaction rate of 55-60%, air permeability of 200-210, wet compression strength of 90-95 kPa, hot and wet tensile strength of 3.8-4.5 kPa, crushing index of 72-75% and surface stability of 97.5-98.0%. The molding sand performance meets the factory backing sand performance requirements and is verified by factory batch production.
(2) ProCAST software is used to simulate and analyze the original process of type 17 hook body and type 16 hook tongue: type 17 hook body may have defects such as erosion, slag inclusion and sand sticking, air hole, shrinkage hole, thermal crack and so on; Type 16 coupler tongue may have defects such as slag and sand inclusion, shrinkage porosity and shrinkage cavity, thermal crack and so on. The improved process scheme is put forward and simulated and analyzed: for the hook body, change the introduction position of the inner gate and place it on one side of the hook body. The sprue adopts the formed fire-resistant sprue. Three thermal insulation risers are set at the head of the hook body, one thermal insulation riser is set at the upper part of the hook body, a small dark riser is added at the position of the inner gate, the sand mold outlet hole is staggered with the riser outlet hole, and the formed cold iron is added at the hot joint. The simulation results show that the process is improved, The probability of each casting defect is greatly reduced, and the design of gating and riser system is more reasonable; For the hook tongue, change to a box of four pieces, arrange symmetrically, change the inlet position of the inner gate, place it on the back side of the hook tongue, remove the central large riser, and place the insulation top concealed riser at the pressure push table. The improved process simulation results show that the probability of casting defects is reduced, and the feeding efficiency of riser and process yield are greatly improved.
(3) Product trial production: the final process scheme of type 17 hook body basically adopts the simulation optimization scheme, and the final process scheme of type 16 hook tongue basically adopts the original process scheme (the simulation optimization process mold must be reordered and processed, and the subsequent production will gradually replace the existing mold); The parts of coupler body and coupler tongue produced by the final process scheme have passed the appraisal at all levels, and the quality inspection items meet the standard requirements. The factory has the mass production capacity of coupler body and coupler tongue; The key defects in the trial production process are cracks, mainly thermal cracks (including heat treatment cracks), which can be solved from the perspectives of increasing the transition fillet radius of sensitive parts, setting anti crack ribs, adjusting the formed cold iron, improving smelting pouring, controlling sand falling and heat treatment process. Cold cracks can not be ignored, and the sand falling cleaning and pouring riser cutting process should be strictly controlled.
(4) Mass production of products: 17 type hook body and 16 type hook tongue products have no obvious surface defects except that a small amount of steel castings are scrapped due to sand inclusion and penetration defects caused by improper molding and box closing process. However, the crack defect of coupler still exists, but it is much less than that before the adjustment of mass production process (trial production stage), which is within the control range of processable process. On the one hand, the occurrence of cracks is due to the casting cracks formed by objective reasons such as high thermal sensitivity and poor weldability of Grade E steel. More importantly, a considerable number of crack defects have been produced in the subsequent machining, welding repair and heat treatment processes of steel castings. Therefore, further standardizing the machining and heat treatment process operation of steel castings and carrying out corresponding worker operation training are of great significance to improve crack defects and production efficiency.