EFFECT OF DEFORMATION CHANGES ON MICROSTRUCTURE OF FORMING STEELS

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1 EFFECT OF DEFORMATION CHANGES ON MICROSTRUCTURE OF FORMING STEELS Milan Forejt a Miroslav Jopek b Jan Krejčí c a) Brno University of Technology, Faculty of Mechanical Engineering, Technická 2, Brno, Forejt@ust.fme.vutbr.cz Abstract Mechanical properties of forming steels (e.g. strength, deformation) change with the rate and time of contact between the piece being formed and the tool. One of the experimental methods that simulate real forming processes is the Taylor Anvil Test. The present paper gives a comparison of deformation changes in test specimens stressed by impact force in dependence on impact velocity with structural changes in a selection of steels currently used in the manufacture of joining parts and mechanical engineering components. Comparing experimental results with the results obtained by the LS DYNA 3D simulation program performs the comparison of deformation changes in dependence on impact velocity. 1. INTRODUCTION Mathematical description of behaviour of material undergoing forming is main premise for successful plastic flow simulation at different strain rates. Available models are based on the quasistatic forming tests performed at high and low temperatures. Therefore mathematical models must be created based on compressive experiments performed under well defined temperature and rate conditions (corresponding to the real situation during forming process) and accompanying simulations. Plastic deformation (and hardening) is mainly the consequence of dislocation motion. It is strongly affected by grain boundaries that hinder their movement. Dislocation motion is also influenced (depending on the material) by vacancies and interstitial atoms, substitution atoms, precipitates, other dislocations configuration and density, stacking fault energy, ordering, etc. Plastic deformation of solid materials is complicated process that depends on many factors. In the course of plastic deformation various factors change considerably - dislocation density and configuration, grain shape, stored energy increases. As a result, physical and above all mechanical properties of material vary considerably. Essential assumption for mathematical simulation is real time physical simulation under authentic time, temperature and mechanical conditions which should lead values of basic variables (stress, strain, temperature), their gradients and relations. Simultaneously, the evolution of structure is being evaluated. Computer simulation utilising FEM and its reliability depends not only on correct definition of boundary conditions (loading, temperatures, friction) but requires outputs of physical tests accomplished under conditions valid for forming processes

2 2. EXPERIMENTAL TESTS The specimens were made from carbon steel marketed as steel TRISTAR, Czech standard equivalent [1]. It is a structural steel with guaranteed content of C, Mn, Si, P, S, see Tab.1. This steel is assigned for cementation, it has lower core strength after quenching and its weldability is guaranteed by producer. Its advantage is high degree of formability either cold or heated to lower temperature. The steel is used for production of machine parts by cold volume forming. Table 1 Composition [wt.%] Material C % Mn % Si % P % S % Cr % Ni % W % Cu % TRISTAL 0,1 0,38 0,07 0,012 0,008 0,05 0,04 0,01 0,1 ČSN ,06 0,45 0,15 0,02 0,02 Compressive impact tests TAT (Taylor Anvil Test) were performed on air gun in Laboratory of High Strain Rate Processes of IMT FME TU of Brno on cylindrical specimens Ø 5 25 mm with polished faces. Impact velocity was from range 25 to 240 ms METALOGRAPHY Initial hardness of TRISTAL steel, measured on the specimen faces was HV10 = 187. Fig.1 shows the initial structure on the axial section. It is typical structure of hypoeutectoid steel: ferrite with pearlitic colonies mainly in triple points. Grain size ranges from 15 to 60 µm, mean around 50 µm. Structure and hardness were uniform across the specimen. Fig.1 As-received structure ~70 x Fig.2 After soft annealing (700 C/24 hrs)~70x After soft annealing (700 C/24hrs, furnace cooling) mean hardness drops to HV10 =

3 Pearlite lamellar cementite was spheroidised and mean grain size increased to µm, Fig. 3. The structure was then studied on tested specimens, again on axial sections. Even at lowest impact velocities, twins appeared near the impact face. Fig. 4 shows the structure near the impact face of specimen with impact velocity ms -1. Near the opposite face of the same specimen, Fig.5, structure bears no traces of deformation. Fig. 3 Soft annealed ~350 x Fig.4 Impact side of specimen, impact velocity ms -1, ~150 x Fig.5 Opposite side of the same specimen, ~150 x Fig.6 Impact side of specimen, Fig.7 Impact side of specimen, impact velocity 43.1 ms -1, ~150 x impact velocity 89.9 ms -1, ~150 x - 3 -

4 Fig.8 Impact side of specimen, Fig.9 Impact side of specimen, impact velocity ms -1, ~ 150 x impact velocity ms -1, ~ 60 x Fig.10 Impact velocity ms -1, detail, ~120x Fig.11 "Corner" of impact face ~120 Detail of the Corner of impact face at the Fig.11 is very interesting. It is clear that the material near edge does not exhibit signs of deformation. 4. DISCUSSION Structure shows that even at lowest strain rates (impact velocities) used, main mechanism of deformation is twinning. We must bear on mind those deformation parameters (stress, strain rate, and temperature) in these test vary in place and time considerably. The progress of plastic deformation front from impact face into specimen means that not only the maximum strain but also the strain rate gradually decreases and, especially in most severely loaded specimens we cannot exclude repeated passage stress wave (with changed sign) through the same region. The influence of finite length of the plastic "wave" (and all other waves) could be seen in bending of deformation twins, Fig.10. This effect is especially visible at highest impact velocities near the impact face. Also "undeformed" corners of impact face (Fig.11) ask for some additional experiments and simulation, especially with respect to the friction between impact face and anvil

5 It will require further thorough study of structure of this material and comparison with similar materials with BCC lattice to uncover deformation pattern, i.e., supply quantitative (if possible) data concerning deformation hardening vs. strain rate, for forming technology. The results of simulation- the final shape of the deformed specimen - are compared with experimental test. The simulation results and the practically obtained shape are in good agreement (5-8%) that is evident from Fig.12. Fig.12 Comparison of the experiment with the simulation results On the other hand, various simulations of different materials exhibit similar disagreement between experiment and simulation - either a match is obtained on most deformed part of specimen or the relations fit to each other near the specimen's free end. It means that simulation is based on correct coefficients for high or low strains and strain rates. Simulations should be modified in such away that they enable simulation of both strain rate ranges (both ranges are higher than quasistatic strain rates). Acknowledgment: This work was supported by the Grant Agency of Czech Republic, grant No 101/99/0373 and by Ministry of Education CEZ research intention MSM (CZ /2201). REFERENCES [1] ČSN (TRISTAL) [2] SMALLMEN R. E.: Modern Physical Metallurgy. Butterworths London,1962 [3] MAYERS Marc A.: Dynamic Behaviour of Materials. A Wiley-Interscience Publication. ISBN X. John Wiley & Sons, Inc. New York, Toronto, 1994, pp 668 [4] MEYER L.W., SEIFERT K., MAEK S. Ab.: Behaviour of Quenched and Tempered Steels under High Strain Rate Compression Loading. Journal Physique III, Colloquia C3, France - 5 -

6 1997, pp [5] FOREJT M., JOPEK M., BUCHAR J.: Constitutive Equations for the Behaviour of BCC Steels at High Strain Rates. In.: 9 th International Metallurgical Conference METAL 2000 Ostrava, Symposium B. CD ROM, ISBN TANGER s.r.o. Ostrava May , pp 214/1-214/6 [6] FOREJT M., JOPEK M., BUCHAR J.: Plastic deformation at real compression rates. In. 8 th Intern.Conference on METAL FORMING 2000, Krakow/Poland, 3-7 September Pietrzyk at al.(eds). ISBN Published by Balkema, Rotterdam, 2000, p [7] FOREJT M., BUCHAR J., JOPEK M.: High strain rates compression loading of BCC steels. In.8 th Intern. Research Conference CO-MAT-TECH 2000, Trnava, Slovakia, October 19-20, ISBN Published by STU Bratislava, 2000, p [8] BUCHAR J., FOREJT M., JOPEK M., KŘIVÁNEK I.: Evaluation of constitutive relations for high strain rate behaviour using the Taylor Test. Journal Phys. IV France 10 (2000), p. Pr Pr9-80. [ [9] JOPEK M., FOREJT M.: Vliv rychlosti deformace na strukturni změny uhlíkových ocelí. In. XV. Miedzynarodove Sympozium Metody oceny struktury oraz wlasnośti materialów i wyrobów, (Struktura a vlastnosti konstrukčních materiálů. Brno, Mechanika Nr.263/2000, z.63, ISSN , Politechnika Opolska, OPOLE 2000, s