APPLICATION OF PHYSILCAL SIMULATOR TO FORMING PROCESSES SIMULATION

Transcription

1 APPLICATION OF PHYSILCAL SIMULATOR TO FORMING PROCESSES SIMULATION Jan Džugan, Pavel Konopík, Petr Motyčka, Zbyšek Nový, Ondřej Stejskal a) COMTES FHT s.r.o., Průmyslová 995, Dobřany, ČR, Abstract Providing cheap materials with optimized properties for certain application is more and more demanded nowadays. In order to fulfil this requirement a wide range of materials in broad range of thermal or thermomechanical states have to be investigated. To keep costs for such a materials development at acceptable level a physical thermomechanical simulators are being efficiently applied. In this paper newly developed laboratory thermomechanical simulator is presented and part of the real production process is simulated. Agreement of microstructure and mechanical properties is investigated between materials treated in real and simulated process for two thermomechanical states of ferrite-pearlitic steels with Widmanstätten ferrite fraction. Newly installed physical simulator enabling cooling/ heating rate up to 150 C/s with deformation velocity up to 600mm/s is used in the current study. Heating is realized by tailor made resistive heating device reaching maximal temperatures up to 1350 C. In order to confirm comparability of the results with real process, a detailed specification of the real process is determined, the process is subsequently analyzed and transformed into the form suitable for physical simulator and process is simulated finally. Microstructure and tensile test properties are investigated for standard thermomechanical process as well as for simulated one. The results of each process are compared subsequently. Good agreement of the microstructure and mechanical properties was found between simulated and real thermomechanical process confirming applicability of the physical simulator for laboratory simulation of forming processes. 1. INTRODUCTION The aim of this work is to present applicability of newly developed thermomechnical simulator to simulation of real forming processes. The work is motivated by strong industrial demand for materials with enhanced mechanical properties that can be attained by appropriate thermomechanical treatment. In order to obtain optimized technology at acceptable price, processes have to be simulated firstly, before their application to real production. Firstly, there are presented capabilities of the physical simulator used in the current study. Subsequently, two real processes are simulated and the microstructures and mechanical properties of the materials are compared in order to verify applicability of the physical simulator to forming processes simulation. There is used in the current study newly installed physical simulator. In order to confirm applicability of the results of physical simulations, comparability of the results with real process is necessary. Detailed specification of the real process is required which is analyzed and transformed into the form suitable for physical simulator that can be simulated. Microstructure and tensile test properties of materials processed by standard thermomechanical and simulated process are investigated. 1

2 METAL PHYSICAL SIMULATOR The aim of the physical simulator was to enable laboratory investigation of the forming processes allowing simulation such as free forging, die forging and hot rolling. The physical simulation shall point out trends of the material properties related to underwent technological process and thus enabling material properties and microstructure optimization. First step in the new physical simulator development was definition of its technical specifications. On the basis of deformation processes present during free forging, die forging and rolling analyze, basic specifications of the machine were derived. Further, during all considered forming processes multiaxial stresses are present, but physical realisation of such a stress state under laboratory conditions is very difficult. Thus simplification considering that the most important factor influencing the material behaviour during the forming processes is total induced plastic deformation was used. This simplification allowed use of the system with single axis loading, significantly reducing complexity of the required system. The final technical specifications of demanded system are following: Load capacity 250 kn Actuator velocity 600 mm/s Ability to perform 30 square cycles ± 1,5mm at velocity 600 mm/s Heating and cooling rate 150 C Possibility of the simulation under inert gas atmosphere The physical simulator is based on standard servohydraulic system MTS 810. The system load capacity is 250 kn, but in order to achieve higher stiffness for considered dynamic process performance, 500kN loadframe is installed. The system is equipped with specially designed grips with resistive heating, Fig. 1. The example of the deformation process performed in the physical simulator can be seen in Fig. 2. Fig. 1. Physical simulator with grips and resistive heating 2

3 2, , , Displacement in mm 0,5 0,0-0,5-1,0 Displacement Force Force in N , , Time in s Fig. 2. Example of the deformation regime performed in the physical simulator 3. ROLLING PROCESS SIMULATION Rolling process is physically simulated in the current study. One material in two different thermomechanical states is investigated here. The material states designated 1 underwent total deformation of 0,3 and austenitization temperature was 920 C and the material 2 underwent total deformation of 1,3 and austenitization temperature was 1000 C. Abbreviated technological procedure is simulated here. Final part of the process only is considered and material of semiproduct extracted just before the final step is used as initial material for the current simulations. The final stage of the real forming process consists of heating the material up to austenitization temperature in furnace with appropriate hold time providing homogeneous temperature within the work piece. Afterwards semiproduct continues to rolling mills where the final shape is attained. This process had to be adjusted for the physical simulation. Cylindrical specimen with specimen active part dimensions of diameter 6 and length 11 mm is used for the simulation. Due to specimen size the hold time at heating temperature was reduced to 3 min. Scheme of the process temperature course is displayed in Fig. 3. Example of the adjusted forming process for the physical simulator is depicted in Fig. 4. Samples of both states were produced for subsequent tensile tests and metallographical observation. 3

4 Fig. 3. Scheme of the thermomechanical process state , ,2 Temperature [ C] ,2-0,4 Deformation [mm] 790 Temperature Deformation -0, , , , , , ,5 574 Time [s] Fig. 4. Example of forming process with temperature change during deformation 4

5 4. METALLOGRAPHY AND HARDNESS TESTING Metallographical investigations were performed on samples of products from industrial plant and samples produced in the physical simulator. Samples were grinded, polished and etched by Nital 3%. The metallographical observations were done with the use of Nikon Epiphot 200 microscope. The linear image analysis for volume fractions determination was performed with the use of Lucia software Hardnesses HV30 were measured for all states of the material investigated with the use of HV Wilson-Wolpert hardness tester. Measured data are summarized in Figs There are displayed microstructures of the state 1 for both, industrially and laboratory produced in Figs. 5 and 6. Both microstructures consist of ferrite, pearlite and Widmanstätten ferrite. The amount of Widmanstätten in microstructure of industrially produced material is about 15 % and about 20 % in the case of the simulated specimen. The ferrite grains in microstructure of simulated specimen are lightly coarser. Determined average grain size was about 50 µm for industrial material and in the range 60 µm for simulated specimen. The measured hardness of both materials is very similar. a) 100x b) 500x, Nital 3% Fig. 5. Mictrostructure of industrially produced material 1, HV30 = 172 b) 100x b) 500x, Nital 3% Fig. 6. Mictrostructure of simulated material 1, HV30 = 176 5

6 METAL 2008 There are displayed microstructures of the state 2 for both, industrially and laboratory produced in Figs. 7 and 8. Both microstructures consist of ferrite, pearlite and Widmanstätten ferrite. The amount of Widmanstätten in microstructure of industrially produced material is about 5 % and about 11 % in the case of the simulated specimen. The ferrite grains in microstructure of simulated specimen are lightly coarser. Determined average grain size was about 40 µm for industrial material and in the range 50 µm for simulated specimen. The measured hardness of both materials is very similar. a) 100x b) 200x, Nital 3% Fig. 7. Mictrostructure of industrially produced material 2, HV30 = 175 a) 100x b) 200x, Nital 3% Fig. 8. Mictrostructure of simulated material 2, HV30 = 175 6

7 5. TENSILE TESTS Tensile tests according to CSN EN were performed. Due to limited size of the experimental material non-standard samples with gauge length to diameter ratio of 2,5 were used. Prior to testing, specimen dimensions were measured and original gauge length for the elongation determination was marked on each specimen. After the test a yield stress Reh/Rp0,2 was determined as well as tensile strength Rm. The final gauge length and cross section dimensions were also measured after the test so that and the cross section reduction could be evaluated. Example of tests records can be found in Fig. 9. Measured material properties are summarized in Table. 1 and Stress in MPa Simulated_1 127_10_c1 Simulated_2 127_10_c2 Simulated_3 127_10_c3 Industrial_1 127x10_b1 Industrial_2 127x10_b2 Industrial_3 127x10_b Strain extensometer in % Fig. 9. Example of tensile tests records for industrially produced and simulated material state 1. Table 1. Summary of tensile tests and hardness measurements results state 1. Specimen Rp0,2/ReH Rm Ag A 5 /A 2,5 Z [MPa] [MPa] [%] [%] [%] HV 30 industrial ,7 35,6 70,7 172 simulated ,1 30,9 68,6 176 Table 2. Summary of tensile tests and hardness measurements results state 2. Rp0,2/ReH Rm Ag A 5 /A 2,5 Z Specimen HV 30 [MPa] [MPa] [%] [%] [%] industrial simulated ,9 34,1 68,

8 7. RESULTS DISCUSSION Tensile tests, hardness tests and metallographical investigation were performed on the investigated materials in both, industrially produced and under simulated condition. In the case of hardness testing almost identical results were obtained for both conditions. Metallographical observations pointed out that in the case of simulated specimens the microstructure is very lightly coarser but it can be stated that the microstructure is the same. Very good agreement was found for tensile test properties, as it can be seen on example of records of tensile tests for industrially produced and simulated specimens. 8. CONCLUSIONS Abilities of newly developed physical simulator were presented in this study. Physical simulations of rolling processes were carried out as well as mechanical testing and metallographical investigations of conventionally produced and simulated materials. Very good agreement was found when microstructures and mechanical properties of industrially produced materials and simulated ones were compared. There was only final part of the complete technological process simulated in the current study. Simulation of complete technological process is planned to be simulated as a next step of the physical simulator applicability to forming processes simulation validation. ACKNOWLEDGEMENT The presented study was performed within the frame of the project Výzkum a vývoj simulace procesů tváření a tepelného zpracování project No. 2A-ITPl/061 of Ministry of Industry and Trade of the Czech Republic. 8