In the FP6 research project DeSProCh Design of a Semihot Process Chain two representative long flat geometries, a connecting rod and a steering link, were chosen and evaluated regarding their requirements towards a production by warm forging. For the steering link a three step warm forging process (see Fig. 1) and for the connecting rod a four step warm forging sequence (see Fig. 2) have been developed. Because of the different requirements of the geometries towards the forging sequences the two developed processes vary a lot concerning the comprised forging operations.

Fig. 1: Forming stages steering link

The material utilisation achieved with the steering link forging process is 86% which is quite a lot higher than for common hot forging processes, which are in a range of 60 - 80%. But this ratio does not include the additional save on material which is caused by the lower height of the not machined warm forged part compared to a hot forged work piece. The lower heights are possible because the subsequent machining of the heights is saved. The material utilisation obtained for the connecting rod forging process is about 80%, which is a good result for a work piece with such a complex material distribution and again this result does not cover the save on material achieved by reduced subsequent machining.

Fig. 2: Forming stages connecting rod

The tolerances achieved in the steering link forging process were IT 10 for the width and length of the pieces. Qualities of IT 10 to IT 11 are expected for the heights of large batches forged in stable conditions. Compared to the tolerances of IT 12 to IT 16 obtainable by hot forging, this constitutes a valuable progress. The surface roughness Rz is below 20 μm as desired and the decarburisation of the work pieces is less than 50 μm deep and does not show an impact on the hardness. Due to this, subsequent shot blasting and machining operations can be saved. No severe scale formation has been observed during the experiments and no traces of FeO were detected. The micro structure of the warm forged connecting rods was even finer than for the hot forged pieces and else showed comparable properties and micro structure, so that a warm forging process with integrated heat treatment of the material C70S6BY is possible.
The two participating forging companies OMTAS and VIVA are introducing warm forging processes at the moment for a limited geometrical spectrum. The drawback of the developed processes is the comparably low production output due to the high number of operations in the press and thus the low production capacity. Additionally, the producible work piece geometries by the developed warm forging process are quite limited towards the mass deviation along the longitudinal axis.

DeVaPro

The idea for the project DeVaPro Development of a Variable warm forging Process chain evolved during the evaluation of the process sequences developed in the project DeSProCh. It is the target of the consortium to hand in a proposal in FP7 for a follow up research project.
The geometrical spectrum of the developed warm forging processes is limited concerning the mass distribution of the final work piece geometry. The more the mass distribution varies along the longitudinal axis, the more press operations are necessary, as the tool material can only withstand limited loads. Additional forming operations have a negative impact on the achievable tolerances. This is caused by the manual work piece transfer which is a prerequisite to produce small and medium sized batches as SMEs do. To overcome those limitations and to be able to produce more complex geometries and thus make the process more variable, a rolling operation will be introduced within the project DeVaPro.
The reason for the decision to avoid an initial preforming operation by forge rolling or cross wedge rolling during the project DeSProCh was to eliminate an intermediate heating, as conventional heating equipment is not capable to provide a controlled reheating in the required short time. The original plan in the project DeSProCh to heat and reheat the work pieces with a gas furnace was rejected due to the size of the furnace necessary to provide the desired output.
The process sequence to overcome the limitations of the current warm forging sequence for long flat pieces includes a preforming operation by rolling and a controlled intermediate induction heating followed by a warm forging sequence (see Fig. 3).

Fig. 3: Comparison of the warm forming process sequences of the projects DeSProCh and DeVaPro