The advantages and disadvantages of machine tool casting are closely related to the machine tool industry and machinery manufacturing industry in China, which to a certain extent represents the casting level of a country. To improve the quality of machine tool casting is a complete set of technical control process, from chemical composition, raw material quality, batching, alloying, smelting temperature, inoculation treatment to thermal aging, etc., all of which must be strictly controlled.
At present, thermal aging is still the main way in the production of machine tool castings in China. At present, there are many problems in the thermal aging process, which are often caused by the violation of the thermal aging process.
(1) If the heating speed is too fast, the worktable will break when the heating speed is more than 100 ℃ / h in a factory. The heating speed is generally (30-100) ℃ / h, but for the castings with thin wall and high strength or large difference in wall thickness, too fast heating will cause additional stress, which is generally suitable for (30-50) ℃ / h.
(2) If the annealing temperature and holding time are insufficient or too high, the casting stress will be eliminated badly, and if the annealing temperature and holding time are too high, the strength and hardness will be reduced. The annealing temperature shall be 500-550 ℃, the high-grade casting shall be 550-600 ℃, and the heat preservation time shall be calculated by burning through 25 mm per hour, such as 75 mm wall thickness casting, annealing for 3 h, high-strength and complex parts, annealing time plus 1 h.
(3) The rapid cooling rate and the little effect of stress relief make the previous work of thermal aging useless. The effect of cooling rate on stress relief.
(4) The temperature difference in the furnace is large. The fracture of casting occurred when the temperature difference was 80 ~ 120 ℃. The furnace temperature difference shall be kept ± 20 ℃. The large worktable of an enterprise broke at the temperature difference of 120 ℃.
(5) The casting support in the furnace is unreasonable, the worktable has been placed horizontally, and the support points are few, which leads to the fracture accident. The worktable shall be placed on the side, the complicated and long parts shall be supported at multiple points, and there shall be a space of 100 mm between the castings to facilitate the air flow.
(6) The sequence of thermal aging is unreasonable. The processing stress caused by rough machining can not be eliminated by heat aging before rough machining. The thermal aging process should be placed after the roughing process.
(7) The misunderstanding of natural prescription. Some factories are short of natural aging time, only 3 months. Natural aging in the room has no effect on eliminating casting stress. Requirements of natural aging: 90% of the total stress relaxation of casting is completed in 6 months, and 95% can be achieved after 9 months. Castings of natural aging must be placed outdoors, and aging can only be achieved under the temperature difference between day and night.