UDK 519.61/.64:621.78:621.73.043	ISSN 1580-2949
Professional article/Strokovni članek	MTAEC9, 49(3)471(2015)
DEVELOPMENT OF NUMERICAL MODELS FOR THE HEAT-TREATMENT-PROCESS OPTIMISATION IN A CLOSED-DIE
FORGING PRODUCTION
RAZVOJ NUMERIČNIH MODELOV ZA OPTIMIZACIJO POSTOPKA TOPLOTNE OBDELAVE PRI PROIZVODNJI ODKOVKOV V ZAPRTIH UTOPNIH ORODJIH
Ladislav Malecek1, Mikulaš Fedorko1, Filip Vancura2, Hana Jirkova2,
Bohuslav Mašek2
1COMTES FHT a.s., Prümyslovä 995, 334 41 Dobfany, Czech Republic 2University of West Bohemia in Pilsen, Vyzkumne centrum tväfecich technologii - FORTECH, Univerzitni 22, 306 14 Plzen, Czech Republic
ladislav.malecek@comtesfht.cz
Prejem rokopisa - received: 2014-08-15; sprejem za objavo - accepted for publication: 2014-09-04
doi:10.17222/mit.2014.196
The paper describes a numerical simulation of the current technology of heat treatment of closed-die forgings made of the 25CrMoS4 steel. The aim of this simulation was to create a temperature model enabling a temperature analysis of closed-die forgings during the heating to the austenitization temperature. This model would permit the heating and soaking times to be reduced. The paper also describes a numerical simulation and material/technological modelling of the current forming technology and the subsequent still-air cooling of a selected type of closed-die forgings for the automotive industry. This numerical simulation provides information on the material flow, the part size and the deformation rate during forming and on the temperature conditions during handling, forming and subsequent still-air cooling. Using the material/technological modelling, samples corresponding to the selected locations of a forging can be obtained. By combining these two techniques, controlled cooling of closed-die steel forgings will be developed and optimized as a substitute for heat treatment. It is also possible to optimize the process in terms of both quality and energy consumption. Both numerical simulations were applied to the technology of forming and heat treatment of closed-die forgings of microalloyed steel, chromium-molybdenum 25CrMoS4, at the company of Kovärna VIVA a.s.
Keywords: 25CrMoS4, MARC, DEFORM, closed-die forging
Članek opisuje numerično simulacijo sedanje tehnologije toplotne obdelave odkovkov iz jekla 25CrMoS4 v zaprtih orodjih. Namen te simulacije je bil postavitev temperaturnega modela, ki bi omogočil temperaturno analizo odkovkov, kovanih v zaprtih utopih, med ogrevanjem na avstenitizacijo. Ta model naj bi omogočil skrajšanje ogrevanja in zadrževanja na temperaturi. Članek opisuje tudi numerično simulacijo in materialno-tehnološko modeliranje sedanje tehnologije preoblikovanja in ohlajanja na mirujočem zraku izbranih utopnih izkovkov za avtomobilsko industrijo. Ta numerična simulacija omogoča informacijo o toku materiala, o velikosti delov in hitrosti deformacije med kovanjem in o temperaturnih razmerah med manipuliranjem, preoblikovanjem in ohlajanjem na mirujočem zraku. Z materialno-tehnološkim modeliranjem se lahko dobijo vzorci, ki ustrezajo izbranemu položaju kovanja. S kombiniranjem teh dveh tehnik bo razvito in optimirano kontrolirano ohlajanje izkovkov v zaprtih utopih kot nadomestilo za toplotno obdelavo. Proces je mogoče optimirati tudi s stališča kvalitete in porabe energije. Obe numerični simulaciji sta bili uporabljeni pri tehnologiji preoblikovanja in toplotne obdelave izkovkov iz krom-molib-denovega mikrolegiranega jekla 25CrMoS4 v zaprtih utopnih orodjih v podjetju Kovärna VIVA, a. s.
Ključne besede: 25CrMoS4, MARC, DEFORM, zaprto utopno kovanje
1 INTRODUCTION	tions. The present paper focuses on two possible
applications of a numerical simulation to optimise the
The production of closed-die steel forgings involves a	production of closed-die forgings. The first one aims at
series of forming operations and the subsequent heat	optimising the heating and soaking of forged parts prior
treatment. The formmg process typically consists of	to quenching. The other uses a numerical simulation for several operations. The ones most frequently used are
upsetting, preforming, finish-forging and trimming.	cons;ructing a matenal/technological model in order to
Trimmed forgings are transferred with a conveyor to a	develop a new method of the thermomechanical treat-
container where they cool down to the ambient tempe-	ment of forged parts.
rature. In order to attain the desired mechanical proper- The goal of the first application was to construct a
ties, the cooling is followed by re-heating the concerned	temperature model. It would be used for predicting the
parts in a continuous-tunnel furnace and by quenching	temperature fields in the forged parts during heating and
them. Today's closed-die-forging plants strive to shorten	soaking at the quenching temperature in the existing
this cycle or even omit some of the operations.	heat-treatment process. Knowing the temperature distri-
Several approaches are available for achieving this	bution, it is possible to adjust the process and potentially
goal. One of them involves the use of numerical simula-	reduce the tact time in the production.
Table 1: Chemical composition of 25CrMoS4 steel in volume fractions, Tabela 1: Kemijska sestava 25CrMoS4 jekla v prostorninskih deležih, (p/%c
Element	C	Mn	Si max.	P max.	S	Cr max.	Mo max.
Content	0.22-0.29	0.60-0.90	0.40	0.035	0.02-0.04	0.90-1.20	0.15-0.30
With respect to the second application, the development of the material/technological model, the paper describes a comprehensive numerical simulation of a forming process, including the subsequent still-air cooling. The forged part in question belongs to a larger group of products of a similar shape. The paper also presents the results of physical modelling of the forging process on a thermomechanical simulator. It compares the properties of the resulting specimens with the conditions of the corresponding locations within the actual forged part.
2 NUMERICAL MODELLING OF HEAT TREATING A FORGED PART
The objective of the numerical simulation of the heat treatment was to map the effects of the radiant heat from the furnace lining on the forgings and the effects of the radiant heat between the forgings themselves. The forged parts were made of the 25CrMoS4 material (Figure 1 and Table 1). In the process, these forgings were arranged in a charging basket passing through a continuous heating furnace.
The model was constructed with the use of the data obtained from the heat-treatment lines. The computation was carried out using the MSC.MARC/MENTAT software. This software employs the finite-element method and is suitable for solving multiphysical problems1.
The simulation of the heat treatment was an iterative process. The goal was to fine-tune the simulation to match the data obtained from the heat-treatment lines. The iterative approach consisted of a gradual refinement and an addition of the input data to the computational model. The computational model comprised three types
of bodies (groups of objects forming a single entity): the furnace, the charging basket and the forgings (Figure 2). The CAD models provided the input data for generating the mesh in individual bodies.
The meshes used for solving the problem consisted of hexagonal elements for the furnace and the basket and tetragonal elements for the complex-shaped forgings. The element size was changing in all the bodies as the computation was gradually made more accurate. There were two reasons for it. One was related to the total number of the elements and the other to the element size ratio with respect to the view-factor setting. The view factor is used in analysing the heat transfer by radiation. It defines the proportion of the radiation from surface A that reaches surface B. In the model, the view factor indicates the visibility of the face elements of the individual bodies in the furnace to one another and to the elements of the inner surface of the furnace chamber. As a rule, the more elements there are in a computational model, the more accurate the results are - and the more face surfaces of the elements there are. With these numbers increasing, the computation time of the furnace heating simulation increases as well. For this reason, the analysis was first tried out using a simplified thermal model shown in Figure 3. The goal was to examine the effect of the view factor on the heat transfer by radiation between two simplified objects.
The meshed objects were assigned material properties. The properties (the thermal conductivity and the specific heat) were measured for the forgings using thermophysical measurement methods. The material properties of the basket and the furnace were retrieved from the material data library of the software. The computation was fine-tuned by defining a permanent thermal contact between the basket and the forged parts. The
Figure 1: Shape of a forging - a 3D view Slika 1: Oblika izkovka - 3D-pogled
Figure 2: Bodies used in computing a temperature model in the MARC software environment
Slika 2: Telesa, uporabljena za izračun temperaturnega modela v okolju programske opreme MARC
Figure 3: Trial model (examination of the effect of the view factor between two simple objects: cuboids)
Slika 3: Preizkusni model (preiskava vpliva faktorja videza med dvema enostavnima kvadratastima objektoma)
initial temperature of the forgings was 20 °C. At the start of the simulation, the furnace temperature was 690 °C. It changed during the simulation in accordance with the schedule used. The furnace heating and soaking schedule was constructed in accordance with the real-world conditions. It was applied to the side walls and the top wall of the furnace chamber. Heating by radiation was first modelled using the MONTE-CARLO method which, however, did not yield adequate results. Therefore, the HEMI_CUBE method was employed. This method uses a pre-defined hollow space, within which the heat is reflected from or absorbed by the objects. The hollow space is a numerical zone where the outer elements of the bodies constitute a working space within which the view factor is computed.
Due to the increasing computation time, some aspects that substantially complicated the simulation were neglected and certain preconditions were defined. The variation in the position of the basket inside the
Figure 4: Layout of forged parts in the furnace with thermocouple locations. The layout was also used for the simulation of heat treatment (quenching).
Slika 4: Razporeditev izkovkov v peči s položajem termoelementov. Razporeditev je bila uporabljena tudi za simulacijo toplotne obdelave (kaljenja).
Figure 5: Comparison between the temperatures found with the numerical simulation and the field measurement (curve 1 shows the furnace temperature)
Slika 5: Primerjava med temperaturami, dobljenimi z numerično simulacijo in z meritvami (krivulja 1 prikazuje temperaturo peči)
furnace was neglected, as it can be taken into account by adjusting the thermal schedule. The wire basket was substituted with a solid metal-sheet container in order to shorten the computation of the view factor. The temperature field inside the furnace was considered to be uniform, although the actual temperature field is not constant. It is affected by opening the furnace door, by the transitions between its zones with different temperatures, the types of heating and the temperature-measurement methods. The results of the FEM simulation were compared with the temperature curves obtained in the selected locations of the real-world forgings in the production (Figure 4).
Simulation results (Figure 5) were in agreement with the temperature curves obtained for the forged parts in the continuous furnace. Therefore, the numerical model is suitable for this type of analysis. It can be used for predicting the temperatures of the forged parts during heating and soaking before quenching.
3 NUMERICAL SIMULATION OF FORMING AND COOLING A SPECIFIC TYPE OF FORGED PARTS
A numerical simulation of forging a selected type of forged part (Figure 1) was carried out using DEFORM 3D, a program developed for modelling forging processes. The input data for the simulation was obtained by measuring the mechanical and thermophysical properties of the 25CrMoS4 steel, the material of the forged part. The goal of the measurement was to obtain an accurate description of the plastic and temperature behaviours of the material for the numerical simulation. The plastic behaviour of the forged material was described with the flow stress/temperature (T), flow stress/strain (e) and flow stress/strain rate (e) relationships in the form of curves. The flow-stress levels were found using the Rastegaev test2,3. The temperature behaviour of the work-piece, i.e. the changes in the temperature field within the
Figure 6: Block diagram of the manufacturing process of the forged part Slika 6: Blok-diagram izdelavnega procesa izkovkov
Figure 7: Tracked points on the cross-section of the FEM model of the forged part (left) and a micrograph of the P1 location on the real-world forged part (right)
Slika 7: Spremljane točke na prerezu FEM-modela izkovka (levo) in mikrostruktura položaja P1 v realnem izkovku (desno)
forged part during forming, was described using the measured specific-heat and thermal-conductivity values, as in the previous simulation of heat treatment. A kinematic model of the LMZ 2500 press, in which the actual forged part was made, was developed. The simulation was based on the forging-sequence description provided by the company of Kovarna VIVA a.s., as well as on the manufacturing-route analysis and on the field measurement (Figure 6).
The model comprised all the forming operations. Their sequence consisted of: upsetting - preforming -finish-forging - trimming. After the trimming, the forging cooled in still air to the ambient temperature. All the relevant handling times were taken into consideration, including the duration of the transfer of the forged part by the conveyor to the container.
The goal of the numerical modelling was to obtain the strain and temperature versus the time plots which were going to be used as the input data for the thermo-mechanical simulator (for the material/technological modelling). The material/technological modelling allows the entire process model to be validated using real specimens and also permits the microstructure evolution and mechanical properties to be mapped4. The point-tracking method was employed to determine the tempe-rature-versus-time and strain-versus-time curves for the selected locations during the production of the forged part (Figure 7). A single representative point (P1) was selected for the physical simulation. The information obtained for this point of the forged part, i.e. the
Figure 8: Strain and temperature plots for the tracked point Slika 8: Diagram napetosti in temperature za preiskovano točko
Figure 9: Bainite-ferrite microstructure of the forged part (HV 284) Slika 9: Bainitno-feritna mikrostruktura izkovka (HV 284)
Figure 10: Bainite-ferrite microstructure of the physical-simulation specimen (HV 270)
Slika 10: Bainitno-feritna mikrostruktura v fizikalno simuliranem vzorcu (HV 270)
strain-time and temperature-time curves, is shown in Figure 8.
Using this data, a schedule for the thermomechanical simulator was developed and applied to an actual specimen. The microstructure of the real-world part (Figure 9) was then compared with the specimen microstructure upon the physical simulation (Figure 10) conducted for the selected point (P1). In both cases, the microstructure consisted of bainite and a portion of ferrite. For the sake of comparison, the measured Vickers-hardness values are shown as well.
4 CONCLUSIONS
Finite-element-method-based simulation is a powerful tool that can provide information about the variables that are difficult to measure otherwise: the strain and temperature curves for particular points of a forged part. The knowledge of these values is the key to optimising the existing processes and developing new procedures and materials. This is, however, impossible without verified models, required for a reliable analysis of the process. The present work deals with two applications of a FEM simulation to analysing the manufacturing routes in closed-die forging.
The first application of the numerical simulation involved constructing a temperature model. It described the temperature changes in closed-die forgings during the heating to the austenitizing temperature before the quenching. Using this model, the heating and soaking times of the forgings in the furnace can be shortened, the optimum layout of the forgings in the furnace can be found and various types of problems solved.
In the model, all the heat-transfer modes were taken
into consideration. The most effective method of the
solution was sought, taking account of the accuracy of the results. Due to the complexity of the problem, the computation times of the simulation variants were on the order of hundreds of hours. The sizes of database files even exceeded 100 GB. For this reason, this model will continue to be developed in an effort to shorten the computation times and reduce the data storage requirements. Gradual improvement in the accuracy of the model is a matter of course.
The second application of the numerical simulation involved an analysis of a closed-die forging process for a selected forged part. This model was developed to obtain the temperature and strain data to be used as the input data in constructing a material/technological model. Such a model combines the findings from the numerical and physical simulations for assessing the feasibility of substituting the existing hardening process. The available alternative is the thermomechanical treatment (combining forming and the subsequent controlled cooling).
It was found that thermomechanical treatment can produce practically identical properties of a workpiece as conventional hardening. However, such results should be interpreted with caution and this finding should be supported by a larger body of statistical data. In future efforts, the FEM simulation of forming processes will be refined, e.g., using Johnson-Cook model for describing the plastic behaviour of a forged part instead of the curve plots employed so far.
Acknowledgement
This paper reports the results obtained under project TA02010390 "Innovation and Development of New Thermo-Mechanical and Heat Treatment Processes of Die Forgings by the Transfer of Findings Obtained from Material-Technological Modelling". The project runs in the framework of the ALFA programme and is funded from the specific resources of the state budget for research and development through the Technology Agency of the Czech Republic.
5 REFERENCES
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21. Polakova, T. Kubina, Flow stress determination methods for numerical modelling, 22nd International Conference on Metallurgy and Materials, METAL 2013 , Brno, Czech Republic, 2013, 273- 278 3S. L. Semiatin, T. Altan, Measurement and Interpretation of Flow Stress Data for the Simulation of Metal-Forming Processes, Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio, USA, 2010, 1-57 4 V. Pilecek, F. Vancura, H. Jirkova, B. Masek, Material-Technological Modelling of the Die Forging of 42CrMoS4 Steel, Mater. Tehnol., 48 (2014) 6,869-873