HOT FORMING of CAST STEEL CYLINDERS 1Jonathan URSINUS
Total Page：16
File Type：pdf, Size：1020Kb
a . / w 9 HOT FOR4ING OF CAST STEEL CYLINDERS 1Jonathan UR1INU1, 1Martin BONHAGE, 12hristoph BzDENBENDER, 2Florian NzRNBERGER, 2E gen DEMLER, 1Bernd-Arno BEHREN1 1Institute of Forming Technology and ,achines (IFU,L, Leibni Universitt Dannover, Garbsen, Germany, EU ursinus%ifum.uni8hannover.de, bonhage%ifum.uni8hannover.de, behrens%ifum.uni8hannover.de .Institut für Werkstoffkunde (,aterials ScienceL, Leibni Universitt Dannover, Garbsen, Germany, EU, nürnberger%iw.uni8hannover.de, demler%iw.uni8hannover.de https://doi.org/10.37904/metal.2019.820 Abstract Regarding tool wear and energy cons mption when forging steel parts, tailored preform geometries are beneficial. In partic lar, the n mber of forging steps can be red ced, in comparison to conventionally rolled cylindrical stock material, if preformed billets are sed. In order to assess the potential offered by cast preforms as semi-finished prod cts for a s bse3 ent forging process, cylindrical steel billets .G422rMo40 were cast by sand casting and then pset with different degrees of deformation m .0.7-1.,0, forging temperat re .A00-1200 720 and ram speed .30-700 mm/s0. Forging of conventional rolled bar material nder the same forming conditions was sed as a reference. After forming, the specimens were heat treated and the mechanical properties were determined by tensile tests .DIN EN I1O A892-10 and notch impact tests .similar to DIN EN I1O 148-10. The microstr ct res were e6amined by metallographic analysis. For the investigated process variables, no significant infl ences on the tensile strengths or impact energies of the cast and forged specimens were fo nd. While the tensile strengths of the cast and forged specimens meet the val es of conventionally rolled and forged specimens, the impact energies of the cast and forged specimens s rpass those of the reference. This is attrib ted to compressed pores, which were incompletely closed d ring forging. A criterion for the design of a die forging process of cast preforms will be derived based on the obtained res lts. Keywords Hot forming, casting, steel, pore clos re 1. INTRODUCTION To f rther increase efficiency in the a tomotive and transport ind stries, forged high performance parts help to save weight and red ce the cons mption of f el or electrical energy. Altho gh enhanced mechanical properties can be achieved by thermo-mechanical processing d ring forging, many intrinsic challenges e6ist. Mainly, flawless material flow leading to an appropriate form filling has to be ens red 91:. 1tarting from a simple semi-finished form with constant cross-sectional geometry, comple6 geometries are s ally forged in several progressive steps. The design of the intermediate steps re3 ires e6pertise and is limited by the capability of the forming e3 ipment. The limited prod ct design fle6ibility may have many conse3 ences, incl ding .b t not limited to0M low material tilisation, defective prod ct prod ction and longer r n- p times. The red ction of the n mber of forging steps also bears potential to increase the economics of the prod ction process itself. Fewer forging steps means red ced tooling costs, less wear and less machinery 92:. Therefore, the technology of forging comple6 shaped cast preforms and providing greater design fle6ibility is an attractive alternative to conventional die forging. For non-ferro s metals, specifically al mini m, an economical motivation for a cast-forging process co ld be fo nd, as the ind strial 2obapress process demonstrates 93:. The combined casting and forging process of non-ferro s metals is fre3 ently e6amined in the scientific literat re. The investigated processes incl de single step cast-forging or rheo-forming 94:, two step cast forging of solidified materials and the press re casting with s bse3 ent densification .known as s3 ee8ing0 9,:. The idea of diverting form conventional steel rod material with constant cross-section to specifically cast preforms was first investigated and disc ssed in the 1980s by ,01 a . / w 9 Iîleib et al. 9A,7:. 1ince then, most of the research concerning the hot forming of cast steel deals with the free forming or open die forging of large ingots 98,9:. The application of this knowledge on pore clos re or microstr ct re evol tion to more conventionally si8ed die forging processes is missing in recent scientific literat re. The aim of this st dy is to investigate the pore clos re and elimination of casting defects in steel, and to provide a feasible process window for the cast-forging of more comple6 parts. 2. E23ERI4ENTAL 1teel cylinders .G422rMo40 were cast in a sand casting process. After the removal of the casting system, the cylinders were then hot compressed. The forming parameters .temperat re, ram speed, plastic strain0 were varied and their infl ence on tensile strength and impact energy investigated. All psetting e6periments were repeated with a conventional rod material as a reference. 2.1. Casting For determining the casting parameters, according to 1eo et al. 910:, the sand casting system was set p based on sim lation res lts achieved with MAGMA1OFT_ . Figure 1 0. The designed system allows the casting of 12 cylindrical parts with a diameter of 44 mm and a length of 74., mm. Each part is e3 ipped with a riser to ens re that the melt flows in to the casting part after filling the system. 1ince low-alloyed steels have high melting temperat res and poor casting properties compared to cast iron, different casting temperat res .1A,0- 1720 720 and casting d rations .8-20 s0 were sim lated. The calc lation res lts revealed that with a casting temperat re of 1A80 72 and a casting d ration of 12 s the system can be filled witho t a premat re solidification. Figure 1 2AD model .left0 and sim lation model .right0 of the casting system for casting of cylindrical parts A polymer .PLA0 cast model was man fact red by additive man fact ring .3-D printer, Ultimaker0. A sand mo ld was made sing this model and prepared for the casting e6periments. For the melting of the steel 422rMo4, a tilt-pot f rnace .Ind ctotherm0 with a ma6im m filling capacity of 22 kg was sed. The monitoring of the temperat re d ring the melting process was carried o t with a pyrometer. After reaching a temperat re of abo t 1700 72, the molten steel was cast within 10 s into the sand mo ld. After casting, the cylindrical parts were separated from the casting mo ld and the s rface was cleaned by sandblasting. The chemical composition of the casting parts was meas red by optical emission spectrometry .OE10 and is given in Table 1 . D e to the casting, a lowering of the content of 2 and Mn occ rred. However, the meas red val es correspond to the re3 ired val es according to DIN EN 10293 for the alloy 422rMo4. The microstr ct ral analysis showed a pearlitic str ct re. ,02 a . / w 9 Table 1 2hemical composition in wt.5 as determined sing OE1 Elements C Si 4n P S Cr 4o Fe before casting .422rMo40 0.,2 0.2, 0.78 0.01 0.03 1.1 0.23 Rest after casting .G422rMo40 0.3, 0.18 0.,3 0.01 0.03 1.1 0.23 Rest Val es according to Min. 0.38 - 0.A0 - - 0.9 0.1, - DIN EN 10293 Ma6. 0.4, 0.4 0.90 0.3, 0.3, 1.2 0.30 - 2.2. Upsetting The deformation of the cast specimens was achieved by psetting e6periments. The e6perimental design was based on a two-step plan of the three variablesM temperat re, plastic strain and ram speed. Additionally, a central point with medi m temperat re and deformation was sed. The global tr e strains of m N 0.7-1., correspond to slightly different local deformations in the centre of the specimens. The mean deformation of the investigated vol me is similar to the global tr e strain and th s these val es have contin ed to be sed. ,0 cast and ,0 conventional cylinders were tested. The cast specimens were sorted by their density and then evenly distrib ted on the 10 parameter sets to minimise the effect of casting batch deviations. The cylinders were heated for 30 min in a batch f rnace to their respective forming temperat res. The temperat re was verified by thermoco ple meas rements. Two flat dies then applied the psetting force. Their s rfaces were l bricated by graphite spray .F chs L britech 2on Traer G3000. The se of two different presses allowed for two different ram speeds. The slower deformation took place on a hydra lic press .1chirmer nd Plate0 with a ram speed of 30 mm/s, while the fast psetting was carried o t on a screw press .Lasco0 with abo t 400 mm/s ram speed on contact. 2.0. 4echanical Testing The compressed cylinders were c t with a band saw to e6tract the ro gh shapes of the later specimens for mechanical testing. These ro gh specimens of similar cross-section were then identically heat treated .LT to 2, HR20 to eliminate the infl ence of the heating history d ring hot forming. After heat treatment, a specimen for tensile testing and one for notch impact energy testing were machined. Th s, every compressed cylinder was represented by both tests. The geometry of the tensile specimens was chosen according to DIN ,012, type B , 6 2,.