How to Produce 20# Seamless Pipes?
20# Seamless Pipes Manufacturing
Due to the uneven wall thickness of the
20# seamless pipe, effect force and tissue stress are generated during heating, cooling and phase transformation. In addition, large parts are subject to residual stresses after machining, and all these internal stresses must be eliminated. The stress-free annealing usually has a heating temperature of 500 to 550 ° C for 2 to 8 hours, and then the furnace is cooled. This process can eliminate the internal stress of the casting by 90 to 95%, but the cast iron structure does not change. If the temperature exceeds 550 ° C or the holding time is too long, it will cause graphitization, which will reduce the strength and hardness of the casting.
When the casting is cooled, the white layer is often produced at the surface layer and the thin section. The white mouth tissue is hard and brittle, has poor processing properties, and is easily peeled off. Therefore, the white mouth tissue must be eliminated by annealing. During the high temperature heat preservation period, the high cementite and eutectic cementite are decomposed into graphite, and the secondary cementite and the eutectoid cementite are also decomposed during the subsequent cold preservation process, and the graphitization process occurs. Due to the decomposition of the cementite, the hardness is lowered, thereby improving the machinability.
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