Fig 9 Experimental and predicted increase in forming pressure with density during hot repressing of steel powder

The speed of forging also affects the quality of parts produced by powder forging. Since the preform is porous, rapid cooling of the hot preform surface by contact with the colder die surface will lead to residual porosity at these surfaces. To minimize this effect, the contact time between the preform and die must be as short as possible. For this purpose, screw presses with a very stiff frame are widely used for powder forging. The screw press motion ensures rapid withdrawal of the forging punch from the die, and the stiff frame minimizes contact time between the part and die at the peak load. The total contact time for forging by a screw press is —30 ms, while the contact time for a typical mechanical crank press is 80 ms. This difference has a very large effect on local cooling of the preform and, consequently, residual surface porosity in the forged part.

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