Influence of surface integrity on the plastic flow in analogous testing of 42CrMo4

Kirsten Bobzin,Dennis C Hoffmann,F Vollertsen,Rafael Hild,T.A Dean,Fritz Klocke,Andreas Feuerhack,S.J Yuan,Daniel Trauth,Tobias Brögelmann,Y Qin,Nathan C Kruppe

doi:10.1051/matecconf/201819014009

Abstract

Surface modification of workpieces for cold forming processes is common in industrial practice. Specific surface topographies manufactured by means of shot peening form lubricant pockets, which provide a continuous lubricating film throughout the process. Furthermore, hydrostatic as well as hydrodynamic lubrication effects emerge, which further reduce the friction during the process and protect the tool from wear effects. These effects are well researched and applied in order to enable cold forming processes. In dry forming processes, no lubricants are used, and the positive lubrication effects do not appear. Surface modifications by shot peening or knurling change surface integrity in terms of hardness, residual stresses and geometry. The effects of the changed surface integrity on the forming process are not researched. In tribometer tests, a hard and rough surface texture lead to a lower frictional shear stress, whereas a rather smooth and hard surface texture leads to a lower punch force in an extrusion process. The mechanisms between surface textures and the smoothing and forming process are unknown. Furthermore, findings of the tribometer tests cannot be fully transferred to the extrusion process. As in tribometer tests a pin is used as tool and in extrusion a whole die, a discrepancy between the most suitable surface texture for a friction reduction results out of different flow conditions. Still, the tribometer tests give a first reference on a surface pretreament. The paper deals with an investigation of surface textures in order to distinguish characteristics of surface textures respectively a friction reduction. By means of indentation tests of the quenched and tempered steel 42CrMo4 the plastic flow in dependency of different surface integrities are researched. The results show which characteristics help to lower the punch force the most, so for further investigations an optimized surface modification is set up in order to enable a dry metal forming process.

Highlights

Cold forming processes are highly effective due to a high ratio of material utilization, high product quality and short production cycles
The results showed, that the lowest punch force was achieved from the combination of ceramic shot peened surface texture with self-lubricating tool coating, Fig. 4
The other specimens are shot peened by steel casks (St), ceramic casks (Ce), and by corundum particles (Co)

Summary

Introduction

Cold forming processes are highly effective due to a high ratio of material utilization, high product quality and short production cycles. Groche et al investigated the influence of surface textures on forming tools in order to reduce friction and wear. The understanding of the mechanisms between a textured workpiece and the forming tool helps to enable a dry metal forming process, see Fig. 1 [13]. In former investigation the influence of surface textured workpieces on the use of self-lubricating PVD-coated tools is done [19]. The boundary conditions of the tribosystem are either changed by the use of a self-lubricating coating (Cr,Al)N+X:S (X = Mo,W) on tools or surface textures by means of shot peening. The paper demonstrates how a dry metal forming process using surface textures on workpieces and self-lubricating tool coatings can be realized, see Fig. 1

Previous Work

Development and Use of a Pin-on-Cylinder Tribometer

Effect of surface textures on the punch force in an extrusion process

Experimental set-up

Results and Discussion

Summary and Outlook