THERMOMECHANICAL TREATMENT IN FORGING PROCESS OF MEDIUM-CARBON MICRO ALLOYED STEEL

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1 Piotr Skubisz*, Jan Sińczak**, Marcin Kowalski*** THERMOMECHANICAL TREATMENT IN FORGING PROCESS OF MEDIUM-CARBON MICRO ALLOYED STEEL ABSTRACT Utilization of thermomechanical treatment in forging medium-carbon alloyed steel with microaddition of titanium is presented. Results of forging tests carried out in industrial conditions with a use of a high-speed forging press in conventional cooling conditions are compared with those involving controlled forging and direct cooling after forging. Significant grain refinement accompanied by increase in tensile properties was observed. KEY WORDS: Thermomechanical treatment, controlled cooling, direct cooling, microalloyed steels, medium-carbon steels, titanium alloying l. INTRODUCTION Synergic combination of plastic deformation and heat treatment, referred as controlled processing or thermomechanical treatment, is an effective way to improve mechanical properties of steel [1]. As such, it forms the basis for technologies which provide favourable microstructure for optimal combination of high strength and ductility, better toughness, improved corrosion and fatigue resistance, which altogether result in superior technological characteristics, higher quality of products and better performance of parts and structural components [2, 3]. Although benefits of thermomechanical processing over conventional forming with subsequent heat treatment are commonly known, and controlled processing with accelerated cooling is to an ever-increasing extent well established technological process nowadays, constant development of materials science and emerging new grades of steels, with microalloyed steels at the forehead, brings about further researches in the range of determination of thermomechanical treatment conditions to take advantage of possibilities of further improvement of mechanical properties and minimization of manufacturing costs. In this respect, new technologies of metal processing are developed, which involve control of deformation conditions for required austenite grain structure and cooling conditions for proper run of transformation kinetics. Most of them depend on the use of microalloyed steels [4, 5] which allow reducing carbon content by using low or medium carbon steels to obtain higher strength. Solution hardening is provided with silicon and manganese additions. Alloying low or medium carbon steels with aluminum, vanadium, niobium or titanium provides fine-grained austenite, hampering its growth with undissolved carbonitrides (which further strengthen the material by precipitation hardening mechanisms) to obtain fine-grained ferrite-pearlite, bainite and/or martensite and retained austenite microstructure at room temperature [6, 7, 8, 9], M.Sc, Ph.D.,D.Sc.: Faculty of Metals Engineering and Industrial Computer Science, AGH - the University of Science and Technology, Cracow, Poland M.Sc.: Institute of Technology, Pedagogical University of Cracow

2 Nowadays, benefits growing out of thermomechanical treatment are widely exploited in rolling technologies. On the contrary, still advantages of these benefits taken in forging industry are very rare. However, significant savings to be gained, make interest in thermomechanical treatment among forging plants grow, and replacing conventional ways of improvement of mechanical properties (e.g. quenching and tempering heat treatment) become increasingly popular problems in scientific literature [5, 10, 11, 12]. The aim of the research is to investigate the effect of application of controlled forging with regard to forged parts manufactured on high-speed mechanical presses. The analyzed part was a small size forging of a flange, characterized by complex geometry with large width in relation to thickness of the flange, and high central boss. Geometry and major dimensions of the flange are shown in Fig. 1. Significant plan view area in the parting plane and relatively big surface area are reason why the forging is formed on fast action mechanical presses or hammers. Short dwelling and deformation times make it convenient to make use of the heat of the forging to perform direct cooling, which in combination with proper temperature regime and controlled cooling rate allow improvement of achievable final properties of the part. 2. EXPERIMENTAL PROCEDURES Fig. 1. Major dimensions of the analyzed forging of a flange The material selected for research was medium-carbon structural steel (C=0,3, Mn=0,94, Si=l,05, Cr=0,9, Ni=0,05, Ti=0,05, P=0,022, S=0,018). Deformation temperature of 950 C was established. A scheme of thermomechanical treatment regime is presented in Fig. 2a). Transition points for this steel grade are: Aci 750 C, Ac3 840 C, Ms 340 C. After heating to 950 C and homogenization at this temperature, temperature was decreased to deformation temperature of 890 C. After deformation followed by 3 seconds stand, the forging was cooled with a cooling rate high enough to omit bainitic transformation. Temperatures and cooling rate was determined on the strength of continuous cooling diagram derived from literature [13] and estimated with TTSteel software [14] (Fig. 2b). Although the forging of the analyzed part was typically carried out on a hammer, to provide consistency of deformation and temperature, as well as better control of the process parameters, technology of forging on a mechanical press was adopted. The equipment employed in the forging tests was mechanical press LZK250. For technological reasons forging stock of diameter 70 mm, and resultant height of 125 mm was used. The experiment, carried out in industrial conditions, was complemented with Finite Element Analysis with a use of commercial code Qform3D. Boundary conditions corresponding to those from the experiment were assumed: friction factor 0.4, active heat transfer coefficient 3500 W/K/m (graphite-water lubrication), tool temperature 300 C, constant thermal conductivity

3 of the metal 80 W/K/m2, forming speed 1 m/s. From the obtained forgings specimens for tensile tests were taken out, and ultimate tensile strength, tensile yield stress, reduction area and elongation to fracture were investigated. a) f r 7vxim b) ConUnuout C»l ug Fig. 2. Diagrams of: a) thermal treatment applied, b) continuous cooling calculated with TTSteel. Condition of the material after forging and cooling was illustrated with microstructures, compared to microstructures of the material in as-received and conventional quenching and tempering condition. The location of specimens is also shown in Fig RESULTS 3.1. Numerical analysis The numerical analysis was performed with a use of commercial code Qform2D/3D, widely used for simulation of bulk forming processes [15, 16]. As the geometry of the flange is nearly axi-symmetrical, for saving of time of the simulation, axis symmetry was assumed. The correctness of the simulation is confirmed (in addition to 7% discrepancy in the forging load value) by flow lines configuration, consistent with the grain flow pattern observed in the forged part (Fig. 3). Fig. 3. Calculated flow lines compared to grain flow pattern of the forged part By means of the numerical analysis temperature profile after forging (Fig. 4a) and distribution of effective strain (Fig. 4b) was established, which was later used for the investigation and discussion on the material condition.

4 Fig. 4. Numerically estimated distribution of: a) temperature after forging, b) effective strain 3.2. Metalographic work The specimens cut out from the central zone of the forging were polished and nital etched to reveal grain structure and structural components. In Figures 5 an-c results of the metalographic work are presented. Fig. 5. Microstructures of the material in: a) as-received, b) quench-tempered, c) thermomechanically processed condition Both in result of conventional and thermomechanical treatment ferrite-pearlitic structure was turned to martensite with rest austenite, however, in case of the latter processing, it was significantly more refined. Estimated grain size before forging was 5^-5,5 ASTM (Fig. 5a). After forging and conventional quenching and tempering treatment it was estimated to be 7-^7,5 ASTM, whereas, after thermomechanical processing it was refined to 8^9 ASTM Tensile tests Tension tests were conducted on a tensile machine Instron Tensile strength, tensile yield stress, reduction area and elongation to fracture were observed. The results are depicted with graphs in Fig. 6 and Fig. 7. Tensile strength and yield stress of the steel after thermomechanical processing are, 1241 and 1103 MPa, whereas, after conventional heat treatment, respectively, 1169 and 1049 MPa. In turn, elongation and area reduction at fracture - 12,9 and 53,2 % for the first technology, and 11 and 48 % for the latter.

5 as i*r*iv rd OftT TMT CflrtrW on Cwulftoo Fig. 6. Comparison of: a) tensile strength and yield stress, b) elongation and area reduction at fracture of the material in as-received condition, after quenching-tempering (Q&T) and after thermomechanical treatment (TMT) 4. DISCUSSION The tests of forging the analyzed part of a flange, show several advantages over traditional technology of forging such parts, with a use of hammers. Usually, switching from hammer to press forging requires revision of the design of the whole technology. However, for such configurations, determined energy of blow is required. In case of analyzed size of a part, energy provided by stroke of a mechanical press is sufficient to produce demanded deformation work. In hammer forging, strain distribution, as well as time of contact with tools and time of cooling, are dependent on skills of the operator. With replacing hammer with a mechanical press, more consistency is gained, and the process is repeatable. This makes it possible to conduct forging process in accordance with predefined process conditions, which is the main condition to perform controlled processing. During forging on a mechanical press large amount of deformation heat is produced, which in combination with short time of deformation, and thereby short time of heat transfer to dies, allows keep high temperature at the end of the forging process. As shown in the results of numerical analysis, in the region of the flange, high effective strain level is observed, which generates increase in temperature of the metal by 50 C. In the surface, the temperature decreases to approximate 800 C, which enables application of any pattern of cooling, dependent on steel forged. In the presented work, after forging direct cooling was applied, to obtain martensite structure. Owing to control of the temperature of heating and deformation in the austenite range, which was lowered to about 50 C above A3 point, fine-grained martensite was obtained. In addition to the grain size of the resultant structure, in can be noticed, that the spines of martensite are also much finer. This results in higher strength of forged part, which show the tensile tests. As shown in Fig. 6, over 70 MPa increase in tensile strength and 50 MPa increase in yield stress was observed. Significant increase in elongation to fracture (2%) and area reduction at fracture (5%) proves good combination of strength and ductility. It should be noted, that the specimens were taken from the core zone of the forging, where the highest temperature, and the lowest amount of deformation was observed, which suggest higher properties in other, more responsible areas. 5. CONCLUSIONS The research on the application of thermomechanical treatment in a forging process, which involved controlled deformation and utilization of the forging heat to perform controlled cooling directly after forging brought significant improvement of mechanical properties. Compared to

6 conventional quenching-tempering heat treatment it offers costs savings associated with reduced energy consumption for reheating forged parts, and further savings resulting from the possibility of application lower carbon and low-alloy steels to obtain required properties. The mechanical properties, reported as the results of tensile tests, show that both strength and ductility of the material was improved. Ultimate strength is 70 MPa and yield stress 50 MPa higher than that of conventional process, with elongation to fracture by 2% and area reduction at fracture by 5% higher. Forging on fast action mechanical presses provide convenient conditions to perform controlled forging, as it allows strict control of forging times and temperatures, in comparison with forging on hammers. However, short time of the forging cycle and high strain rate allow keep high temperature of the end of the forging. Thus, the heat may be used to carry out direct controlled cooling, which with application of microalloyed steels may be utilized for precipitation hardening. Satisfactory results obtained for medium-carbon structural steel suggest more detailed investigation, especially as for the determination times and temperature of forging and cooling pattern after forging, as well as, deformation in two-phase region and/or ductility improvement with other transformation during cooling. The evident increase in mechanical properties, suggest also the use of HSLA steels for the manufacture of forgings, which offers much higher benefits. LITERTURE 1. J. Marcisz, B. Garbarz. Hutnik - wiadomości hutnicze, Nr. 3, 2006, 86^ A. Bator, J. Majta. Hutnik - wiadomości hutnicze, Nr. 10, 2003, 402^ A. Barros Cota [et al.]. Material Research Vol. 6, No. 2, 2003, 117-G21 4. J. Adamczyk, M. Opielą. Hutnik - wiadomości hutnicze, Nr. 3, 2005, 162-G70 5. J. Adamczyk. Hutnik - wiadomości hutnicze, Nr. 11, 1999, 529^ C.Garcia-Mateo, B.Lopez, J.M.Rodnguez-Ibabe. Scriptamater.42(2000) 137^ H.Leda. Joum. of Mat. Proc. Techn. 64(1997) H. Karabulut, S. Gunduz. Materials and Design 25 (2004) 521^ R. Dąbrowski, J. Pacyna. Hutnik - wiadomości hutnicze, Nr. 6, 2005, 325U D. K. Matlock, G. Krauss, J.G. Speer. Joum. of Mat. Proc. Techn. 117 (2001) 324^ M. Jahazi, B. Eghbali. Joum. of Mat. Proc. Techn. 113 (2001) 594-^ M.J. Balart, C.L. Davis, M. Strangwood. Materials Science and Engineering A328 (2002) T. Burakowski. Poradnik Inżyniera - obróbka cieplna stopów żelaza, 1977, p M. Dziedzic, P. Skubisz. Materiały konferencyjne - Walcownicywo 2005, III Konferencja Naukowa z udziałem uczestników zagranicznych, Ustroń Października 2005, s N. Biba, S. Stebounov, A. Lishiny. Joum. of Mat. Proc. Techn. 113 (2001) 34-E39