INFLUENCE OF THERMOMECHANICAL TREATMENT ON THE STEEL C45 FATIGUE PROPERTIES

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1 CO-MAT-TECH 2005 TRNAVA, October 2005 INFLUENCE OF THERMOMECHANICAL TREATMENT ON THE STEEL C45 FATIGUE PROPERTIES Jiří MALINA 1+2, Hana STANKOVÁ 1+2, Jaroslav DRNEK 3, Zbyšek NOVÝ 3, Bohuslav MAŠEK 1+2, 1 University of West Bohemia, Univerzitni 22, Plzeň, Czech Republic, mali@centrum.cz 2 TU Chemnitz,Fakultät für Maschinenbau, LWM,Erfenschlager Str. 73, D Chemnitz, Germany, bohuslav.masek@wsk.tu-chemnitz.de 3 COMTES FHT s.r.o., Borská 47, Plzeň, , comtes@comtesfht.cz 1 INTRODUCTION The study shows the positive impact of thermomechanical treatment (TMT) upon the microstructural development in C45 steel. The goal of the study was to achieve better mechanical properties than those reached by conventional heat treatment. Application of thermomechanical treatment is expected to save one heating operation to quenching temperature and subsequent treatment. This should reduce the energy costs as compared to the conventional treatment. 2 EXPERIMENTAL The experiment was aimed at comparison of effects of classical heat treatment with those of thermomechanical treatment. Resulting microstructures were observed by metallographic techniques and mechanical properties were measured and compared, including fatigue properties. 2.1 Heat Treatment The workpieces to undergo heat treatment were hot-rolled circular section bars (Fig. 5) of 18 mm diameter. The thermal process was designed to match the hardening treatment (Fig. 3). It comprised heating to 850 C in a resistance furnace, holding for one hour and oil quenching. Tempering was performed at 600 C for one hour with subsequent air cooling. 2.2 Thermomechanical Treatment Thermomechanical treatment was performed on samples made from a hot-rolled bar. These were cylinders of 15 mm diameter and 30 mm length with a gripping shank. The initial microstructure (Fig. 5) contained ferrite and pearlite with grain size of about 20μm. The thermomechanical treatment (TMT) as such (Fig. 4, Tab. 1) consisted of heating in a 784

2 13 th International Scientific Conference CO-MAT-TECH 2005 resistance furnace to 900 C, soaking for 1200 s and six-step drawing out with rotations between blows alternatively by 45 and. This process was selected in order to achieve as uniform as possible strain distribution within the sample. The main focus was to suppress formation of the extensive strain fields along diagonals of specimen cross section. Forming was performed between 900 C and 700 C during free ambient air cooling. There were foursecond pauses between the blows. Forging die velocity was 60 mm/s, which corresponds to strain rate of about 4 s -1. Upon forming, the samples were cooled by pressure air. The cooling rate was measured with a pyrometer and computed with the aid of FEM simulation. It was found that at higher temperatures the cooling rate was about 8 C/s. Figure 1: Specimen dimensions Figure 2: Schematic depiction of initial 4 forming steps Table 1: Final thickness of specimen in individual forging steps Angle of sample rotation Specimen thickness upon blow [mm] Figure 3: Heat treatment diagram Figure 4: Thermomechanical treatment diagram 785

3 3 RESULTS 3.1 Metallographic observation Conventional heat treatment was performed according to specifications of the steel standard. However, the microstructure of the heat-treated specimens did not match the expected tempered martensite microstructure. Resulting microstructure contained bainite, tempered martensite, pearlite and ferrite. It can be assumed that the classical prescribed procedure did not result in sufficient cooling rate and to some extent the material was insufficiently quenched. The microstructure upon TMT (Fig. 7) was revealed in longitudinal metallographic sections. It contains very fine ferrite and pearlite grains. The average grain size as measured by means of image analyis was approximately 1.8 μm. The fraction of proeutectoid ferrite in microstructure is considerably higher about 33% than in the initial microstructure. Ferrite nucleation occurred due to the increase in number of stress-strain-related nuclei both along the grain boundaries and within the coarse austenite grains. The pearlite consists of lamellae (Fig. 8), which are very short and fine. They could be distinguished with the aid of electron microscope only. Figure 5: Initial ferrite-pearlite microstructure with 19% of proeutectoid ferrite Figure 6: Resulting microstructure upon heat treatment containing ferrite, pearlite, bainite and tempered martensite Figure 7: Resulting ferrite-pearlite microstructure upon thermomechanical treatment with 52% of ferrite (light Figure 8: Resulting ferrite-pearlite microstructure upon thermomechanical treatment with 52% of ferrite(scanning 786

4 13 th International Scientific Conference CO-MAT-TECH 2005 micrograph) electron micrograph) 3.2 Mechanical properties Tensile test, impact test (Tab. 2) and bending rotation fatigue specimens were made from the forged samples. Comparison of fatigue test results for both treatment methods is shown as S- N curves (Fig. 9). Table 2: Results of mechanical tests Mechanical properties Yield stress [MPa] UTS [MPa] Ductility [%] Reduction of area [%] Impact energy [J cm -2 ] Heat treatment Thermomechanical treatment Figure 9: S-N curves for heat-treated and thermomechanically treated specimens 4 CONCLUSION The heat treatment (Fig. 3) with 1-hour soaking at temperature of 850 C, subsequent oil quenching and tempering at 600 C produced mixed microstructure of bainite, tempered martensite, pearlite and ferrite with yield strength of 528 MPa, ultimate tensile strength of 790 MPa and fatigue limit of 425 MPa (Fig. 9). The experimental thermomechanical treatment (Fig. 4, Tab. 1) comprised soaking at 900 C for 1200 s and six-step forming between temperatures of 900 and 700 C during continuous cooling of specimens. Pressure air was used for cooling upon forming. The proof stress in experimental thermomechanical treated specimens was 788 MPa. Ultimate tensile strength in these specimens was 801 MPa and the fatigue limit reached 375 MPa (Fig. 9). Application of experimental 787

5 thermomechanical treatment to the C45 steel resulted in slight increase in tensile strength and considerable increase in yield strength as compared with classical treatment. It also improved the fatigue strength at lower numbers of cycles (Fig. 9). Fatigue limit in TMT specimens is slightly lower, which might be due to notch effects of the preserved pearlite lamellae [1]. Another possible cause might lie in the high yield stress of the TMT specimens of C45 steel, which have led to drop in ultimate strength/yield strength ratio to This is the value, at which cyclic softening of the material may occur [2]. Conversely, in the heat-treated material the ultimate strength/yield strength ratio suggests cyclic strengthening. References [1] Dieter G. E., Mechanical Metallurgy, McGraw-Hill, New York, [2] Klesnil M., Lukáš P.; Únava kovových materiálů při mechanickém namáhání, Academia, Praha, [3] Pluhař, J.; Korrita, J.: Strojírenské materiály; SNTL/Alfa, Praha, [4] Jech, J.: Tepelné zpracování ocelí, SNTL, Praha, Reviewer Ing. Pavel Suchmann., COMTES FHT s.r.o., Borská 47, Pilsen, Czech Republic. 788