Closing of Shrinkage Cavities by Means of Open-Die Forging

Janine Hauri,Rudolf Kawalla,Birgit Awiszus,Marcel Graf

doi:10.4028/www.scientific.net/msf.918.77

Janine Hauri, Rudolf Kawalla + Show 2 more

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https://doi.org/10.4028/www.scientific.net/msf.918.77

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Abstract

Inner cavities in cast ingots have to be closed by means of open-die forging to guarantee the integrity of the forged components during usage. In the paper a differentiation is made between the macroscopic and the microscopic closing of natural inner cavities. Special attention is paid to the dendritic structure of the surfaces of inner cavities and their impact on the microscopic closing behavior. Additionally, investigation showed that a closing on the microscopic scale is only possible if the cavities do not come in contact to the atmosphere during hot forming. For the analysis of the cavity closing, ingots of a heat-treatable, hot-working and a cold-working steel were used. The ingots were cast, forged and the surface of the inner voids was analyzed with the help of light-microscopy, SEM and EDS for the microstructure and by using tensile tests on macroscopic scale. Also, numerical investigations were a part of the work, whereby the parameters void size, void shape and anvil-shape were varied. As a result of the numerical investigation, a so called closing function was formed. This function enables the user to calculate the necessary height-reduction for closing of inner cavities of the regarded ingot format.

Highlights

The common used semi-finished products in open-die forging of large forging products are cast ingots
Investigation showed that a closing on the microscopic scale is only possible if the cavities do not come in contact to the atmosphere during hot forming
Experimental investigation showed, that it is possible to close such defects with a smaller height reduction on the microscopic scale, if the cavities do not come in contact to the atmosphere and if a plastic strain can be realized, that guarantees a meta-dynamically or statically recrystallization in the break time between the forging steps

Summary

Introduction

The common used semi-finished products in open-die forging of large forging products are cast ingots. Because of contraction and shrinkage processes during solidification of the melt, inner cavities with a dendritic surface build up, mainly along the center line of such ingots. As long as they are not contaminated and have had no contact to the atmosphere, it is possible to close them during open-die forging. Only the macroscopic closing process with respect to void closure was considered in former projects. The results of experimental and numerical simulations are presented, that take the inartificial surface structure of inner cavities and its closing behavior during open-die forging into account.

Experimental Examinations

Findings

Summary