The aim of primary working (ingot breakdown) is threefold: (1) to eliminate the residual porosity, (2) to convert the original massive form (sinter ingot) to the desired preshape (sheet, rod, tube), and (3) to refine the grain size in order to improve the formability during subsequent cold forming.
The first forming step is usually carried out at 1500-1700℃. As a result of the low specific heat and high thermal conductivity of tungsten, several reheating stages are necessary in the first stages of shaping, because the heat is lost rapidly at these temperatures and the ingot cools down rap idly. If the metal is worked at too low a temperature, cracks and splits will easily develop. As the forming process continues, the forming temperature is reduced progressively since the recrystallization temperature decreases as deformation proceeds. Tungsten is generally worked below its recrystallization temperature, because recrystallization is combined with grain boundary embrittlement. With increasing work hardening during deformation, both the hardness and strength of the worked part increase significantly, and intermediate stress relief annealings are necessary to minimize the hazards of cracking (laminating) and to avoid overstraining the working tool.
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