Influence of the forming conditions on the mechanical properties of the final product, when using the flow forming process.

Malina, Jiri ; Jirkova, Hana ; Masek, Bohuslav 等

1. INTRODUCTION

Flow forming is a method using three rollers for reducing diameter (Nagarajan et al. 1981). The rollers are not the drive (Ufer et al., 2006) and its rotation is attained through the friction between wrought rotating semi-product and roller shape (Fig. 1). The advantage of this method lies in the surface hardening, the structural refinement and, in contrast with conventional cutting, in the reduction of the amount of unused material (Roy et al., 2009; Neugebauer et al., 2001; Neugebauer et al., 2002).

The goal of this experiment was to observe the possibility of utilizing the flow forming process to produce stepped hollow shafts and to find suitable technological parameters for reaching given objectives. The variable parameters include the amount of reduction of the hollow semi-product from the diameter of 54 mm to 48 and 44 mm and the forming temperature of 20[degrees]C, 320[degrees]C, 420[degrees]C, 520[degrees]C, 620[degrees]C and 720[degrees]C.

2. THE INITIAL SEMI-PRODUCT

A hot rolled thick-walled 20MnCrS5 steel tube with a diameter of 54 mm and wall thickness of 14 mm was used for the experiment (Tab. 1).

The 20MnCrS5 steel is low alloyed manganese-chromic steel with good hardening capacity intended for cementation. It is primarily used for medium stressed motor vehicle components. The initial ferrite pearlite microstructure had an average grain size of about 9 [+ or -] 5 [micro]m (Fig. 2)

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

3. EXPERIMENT

The final products (Fig. 3) obtained by using different parameters were underwent metallographic analysis and micro-hardness measurement. The microstructure analysis showed big differences in the surveyed microstructure deposition in relation to 1) the location in the final product e.g. in the transition from the initial diameter to final diameter. Different distribution of ferrite grain size was observed in ferrite-pearlite structure of this area. Grain size varied from cca. 1 [micro]m in the area of rolling beginning to cca 4.5 [micro]m in the area remote from the surface of final product and in the same time from the forming zone. The microstructure also depended on 2) temperature e.g. the grain size was cca. 1 [micro]m in the area of rolling beginning in the semi-product formed without preheating (Fig. 4) in contrast with 2.5 [micro]m in the product preheated to 720[degrees]C (Fig. 5), and also on the last parameter 3) the reduction size. These results were consequently confirmed with micro-hardness measurement.

The results were divided into the three most significant temperature groups. The first group, 20[degrees]C without preheating, in the second, where the tubes were preheated to 320[degrees]C and 420[degrees]C and in the last one including the tubes preheated to relatively higher temperatures 520[degrees]C, 620[degrees]C and 720[degrees]C.

After the selection of three efficient temperature groups, four areas with huge structural diversity were defined. With the help of mini tensile specimens, it was possible to look at various locations and subsequently describe the influences of structural diversity on the mechanical properties.

The material was without deformation in the area 1 (Fig. 6). The area 2 (Fig. 6) was 0.3 mm beneath the surface in the transition between the deformed and undeformed material in the semi-product. The area 3 (Fig. 6) was 0.3 mm in the undersurface area of the final shape and the area 4 (Fig. 6) was 6 mm under the surface in the middle of the wall thickness of the final shape.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

4. RESULTS

The results of mechanical tests showed (Tab. 2) that in the 4th area the material was only slightly influenced by flow forming and the highest values of tensile strength were reached for the material reduced to diameter 48 mm at 420[degrees]C. In this case the value [R.sub.m] increased by about 40 MPa and [R.sub.e] about 60 MPa in comparison with the material that was neither preheated nor deformed.

The best result was gained in the third position, at 420[degrees]C and reduction to [empty set] 48 mm. The highest yield strength 775 MPa was gained under these conditions what is the surge cca. 350 MPa. This all by decreased ductility on 21%.

In the second position, the biggest differences were observed in the mechanical properties. This may be caused by material anisotropy in the transition from the initial to the final diameter.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

The best result was for the semi-product reduced without preheating. In this case, maximum tensile strength was 835 MPa, which was the maximum gained during this experiment. In the first area, which responds to the initial stand, differences caused by reheating were detected.

5. CONCLUSION

From our experiments, it can be observed that forming this material with a reduction from [empty set] 54 to [empty set] 48 or 0 44 mm is not suitable at higher temperatures. The analyzed results of the mechanical test showed that when compared with temperature, deformation does not have a big influence on mechanical properties in the centre of the wall of the semi-product.

The size of ductility, which in most cases was higher than 30 %, provides sufficient range with the possibility of further forming.

The best values of mechanical properties were gained with a reduction to [empty set] 48 mm made on the final shape at 420[degrees]C. And the best results in the transition area were with forming without preheating.

The next experiments will be focused on the determination of feed speed on micro-structure of the final product.

6. ACKNOWLEDGEMENTS

This paper includes results obtained within the project 1M06032 Research Centre of Forming Technology.

7. REFERENCES

Nagarajan H.N., Kotrappa H., Mallanna C. & Venkatesh V.C. (1981) Mechanics of Flow Forming, CIRP Annals Manufacturing Technology, Volume 30, Issue 1, pp 159-162, ISSN: 0007-8506

Neugebauer R.; Kolbe M. & Glass R. (2001). New warm forming processes to produce hollow shafts. Journal of Materials Processing Technology,, Volume 119, Issues 13, pp 277-282, ISSN: 0924-0136

Neugebauer R.; Glass R.; Kolbe M. & Hoffmann M. (2002) Optimisation ofprocessing routes for cross rolling and spin extrusion. Journal of Materials Processing Technology, Volumes 125-126, pp 856-862, ISSN: 0924-0136

Roy M.J., Klassen R.J. & Wood E. (2009) Evolution of plastic strain during a flow forming process Journal of Materials Processing Technology, Volume 209, Issue 2, 19 January, pp 1018-1025

Ufer R., (2006). Modellierung und Simulation von Druckwalzprozessen, (Modelling and Simulation of Flow forming process), ISBN 3-937524-43-6, Zwickau, Germany

Tab. 1. Chemical composition of 20MnCrS5 steel

element C Si Mn Cr S P
% 0.195 0.26 1.2 1.21 0.032 0.008

Tab. 2. Mechanical properties of 20MnCrS5 steel after cross
rolling

Preheat temp./ Rm Re [A.sub.5mm] Z
Outer [MPa] [MPa] [%] [%]
diam. [mm]

 1--The initial state

RT/48 581 418 41 73
420/48 585 382 40 74
720/48 530 339 40 70

 2--The area of deformation

RT/48 692 535 37 15
420/48 580 450 38 71
720/48 555 445 18 28

 3--The surface area

RT/48 612 598 36 66
420/48 792 775 21 52
720/48 588 500 24 57

 4--The centre of wall

RT/48 564 470 38 68
420/48 622 484 35 70
720/48 540 405 38 55