EFECT OF THERMOMECHANICAL CONDITIONS ON ULTRAFINE GRAINED STRUCTURE FORMATION IN CARBON STEELS BY SEVERE PLASTIC DEFORMATION

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1 EFECT OF THERMOMECHANICAL CONDITIONS ON ULTRAFINE GRAINED STRUCTURE FORMATION IN CARBON STEELS BY SEVERE PLASTIC DEFORMATION Jozef Zrnik a,b Sergei V. Dobatkin c,d Ondrej Stejskal b a COMTES FHT, s.r.o, Průmyslová 995, Dobřany, ČR, jzrnik@comtesfht.cz b West Bohemian University, Universitní 22, Plzen, ČR, zrnik@kmm.zcu.cz c Baikov Institute of Metalurgy and Materiále Science, RAS, Moscow, Russia, d MISiS Technological University, Moscow dobatkin@ultra.imet.ac.ru 4 West Bohemian University, Plzen, Czech Republic,ostejskal@comtesfht.cz Abstract The article focuses on the results from recent experimental of severe plastic deformation of low carbon (LC) steel and medium carbon (MC) steel performed at increased temperatures. The grain refinement of ferrite respectively ferrite-pearlite structure is described. While LC steel was deformed by ECAP die (ε = 3) with a channel angle φ = 90 the ECAP severe deformation of MC steel was conducted with die channel angle of 120 (ε = 2.6-4). The high straining in LC steel resulted in extensively elongated ferrite grains with dense dislocation network and randomly recovered and polygonized structure was observed. The small period of work hardening appeared at tensile deformation. On the other side, the warm ECAP deformation of MC steel in dependence of increased effective strain resulted in more progressive recovery process. In interior of the elongated ferrite grains the subgrain structure prevails with dislocation network. As straining increases the dynamic polygonization and recrystallization became active to form mixture of polygonized subgrain and submicrocrystalline structure. The straining and moderate ECAP temperature caused the cementite lamellae fragmentation and spheroidzation as number of passes increased. The tensile behaviour of the both steels was characterized by strength increase however the absence of strain hardening was found at low carbon steel. The favourable effect of ferritepearlite structure modification due straining was reason for extended work hardening period observed at MC steel. Transmission electron microscoy of thin foils revealed that three executed passes at increased temperature were not enough to form fully fine grained structure with were high angle boundaries dominates regardless the dynamic recrystallization advance was observed. The tensile behaviourof ECAP specimens was noticeblydifferent from that of thermomechanically processed steels. ECAP processing contributed significantly to deformation behaviour of steels and affected the yield stress and ultimate tensile strength of both steels. Unfortunate polygonization process was mostly local and heterogeneous and effected significantly the work hardening for both steels. 1. INTRODUCTION The fabrication of bulk materials with ultrafine grain sizes has attracted a great deal of attention over the past two decades because of the materials enhanced properties [1-3]. In recent years a worldwide effort in manufacturing process to obtain ultrafine grain structures in steels is persiting. 1

2 In the past decade, a number of the various sever plastic deformation (SPD) techniques have been used to refine structure of metals and alloys. To introduce large plastic strain into bulk material, different techniques have been used such as ECAP [1], high pressure torsion (HTP) [4], accumulative roll bonding (ARB) [5, 6], constrained groove pressing (CGP) [7], and others. It is especially the ECAP that generates interest among investigators since it is one of the advanced methods of severe plastic deformation used for metallic materials to produce massive billets with ultrafine grained structure. However, UFG materials manufactured by the SPD processes have the inherent limit for their practical use. Since the SPD accumulates extensive internal energy inside materials, considerable residual stress would still remain even after a large portion of internal energy is dissipated for grain refinement. Very recently, significant interest has shifted to the use of warm and/or even hot severe deformation in order to produce more stable UFG microstructure [8]. With cold ECAP, low and medium carbon steels can only be pressed by two or three passes with channel intersection of 90 before initiation a failure of sample. The two to four passes realized currently with cold ECAP are insufficient and the achievable strain amount is insufficient to produce a completely refined grain structure [9]. To form stable ultrafine grain structure in metals and alloys, ECAP should be carried out at the temperature corresponding to the temperature of cold working [10]. The purpose of this work is to study the formation of submicrocrystalline structure in low and medium carbon steels subjected to large strain during warm ECAP pressing in dependence on varying temperature and effective strain. 2. EXPERIMENTAL PROCEDURES In this work, two grades of commercial carbon steels with different carbon content were used for experimental. The chemical composition of both steels was as follows: low carbon steel AISI 1010, Fe- 0.1C-0.08Si-0.42Mn (in wt pct) and medium carbon steel AISI 1045,Fe0.45C-0.23Si-0.63Mn-0.18Cr-0.043Al. Prior to ECAP pressing, AISI 1010 billets were soaked at temperature of 920 C for 1 hour, and billet of AISI 1045 were soaked at the temperature of 960 C for 2 hours, followed by air cooling. The representative initial structures of both steels after thermal treatment, as SEM micrographs, are presented in Fig. 1 and Fig.2. From the treated plates the cylindrical billets with initial diameter of 9 mm and length of 50 mm were cut off for the ECAP experiment. The warm ECAP pressing of AISI 1010 steel was performed at two temperatures of 250 C and 300 C, respectively, and 10 µm 25 µm Fig. 1. Initial ferrite structure of AISI steel with 0.1%C. Fig. 2. Initial ferite-pearlite structure of AISI each billet was pressed up to a total of N=3 passes. The angle of intersections of the two channels φ was 90. ECA pressing yielded an effective strain ε~3. 2

3 The AISI 1045 steel billets were subjected to warm ECA pressing at T= 400 C and to higher number of passes, N = 4, 5, 6 respectively. In this case the intersection angle of channels was φ = 120. Effective strain corresponding to one pass was ε ~ In both ECAP experimental the route Bc was chosen. The sample was rotated 90 around its longitudinal axis between each pass, in the same direction. The heating of sample for prior to pressing was done inside the pre-heated die until sample reached the pressing temperature of 300 and/or 400 C, respectively. The microstructural examination of thermally treated and ECAP samples was carried out by utilizing scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Thin foils for TEM observation were sliced normal to the longitudinal axis of ECAP pressed billets. The SEM and TEM micrographs were obtained by using JEOL JSM 6380 SEM operating at 10 kv and JEOL JEM 200FX TEM operating at 200 kv. Tensile test were carried out using Zwick universal testing machine equipped with Multisens extensometer. Tensile specimens with gauge length of lo= 20 mm were tested at a constant cross-head speed of mm/s until failure. The engineering stress-strain curves were constructed. 1. EXPERIMENTAL RESULTS AND DISCUSSION 1.1 Microstructure of steel AISI The substructure of samples subjected to warm ECAP at temperature of 250 C and 300 C was investigated by TEM. Comparing effect of different ECAP temperatures, exposed to N = 3 passes no substantial difference in microstructure samples was observed. The substructure analysis provided the evidence that at the tine of structure formation not the grain fragmentation modified the newly born ultra fine substructure but also the in-situ recovery process, due to increased ECAP temperature, contributed to development of UFG structure. For the most part, the microstructure consists mainly of parallel bands of elongated, formerly equiaxed, ferrite grains, Fig. 3. The substructure modification across the cross section were detected and varied locally. High dislocation density and dislocation cells inside elongated grains are apparent. At both ECAP temperatures in some elongated ferrite grains, dislocation activities can be related to progress in polygonization and preliminary nucleation of new subgrains. As temperature of ECAP increased, the tendency for development of submicrocrystalline structure becomes stronger, which can be attributed to in-situ dynamic polygonization and recrystallization. The more grown and already equiaxed grains of high angle boundaries, with less dislocations in grains are documented in Fig. 4. This polygonized structure may indicate that formation of well-defined thick boundaries would be attributed to a recovery and recrystallization process. The presence of net pattern in SAED confirms the presence of reasonable portion of boundaries having high angles of misorientation. Fig. 3. TEM microstructure after ECAP at 250 C, N= Microstructure of steel AISI 1045 The TEM microstructures of medium carbon steel after ECAP at 400 C are presented in Fig. 5 and Fig.6. At ECAP channel angle of 120 the structure deformation was found heterogeneous. The areas of 3

4 severe deformation where cementite fragmentation and dislocation network in ferrite is evident are next to polygonized structure in deformed ferrite grains. Investigating the substructure, also the cementite lamellae spheroidization was apparent. The dislocation substructure in ferrite grains was modified upon dynamic polygonization, however the low angle boundaries are still in ferrite grains. Submicrocrystalline structure is formed within ferrite grains. As ECAP straining increases the progress in dynamic polygonization proceeded and formation of submicron size grains can be observed in ferrite and also between cementite plates. This observation on Fig. 4. TEM microstructure substructure development indicates that formation of more homogeneous submicrograin structure was after ECAP at 300 C, N=3. postponed in medium carbon steel due to lower strain introduced to specimen after six resulting from ECAP angle of Tensile properties of AISI 1010 steel Fig. 5. TEM microstructure of ferrite - pearlite resulted 500 nm 500 nm Fig. 6. TEM microstructure of ferrite - pearlite resulted after ECAP at 400 C, N=4. The results of tensile testing at room temperature are shown in Fig. 7a, for fully annealed condition, in Fig. 7b for ECAP specimens. In case of the fully annealed condition, there is an extensive period of strain hardening and a high elongation to failure. The deformation behaviour of ECAP specimens is very similar for both specimens and the after ECAP at 400 C, N=4. tensile strength is decreasing as ECAP temperature increases. As similar to other UFG materials, the tensile deformation behaviour of the UFG low carbon steel is characterized by strength increase and absence of strain hardening. The region of strain hardening prior the softening period is quiet short but detectable and the period of uniform elongation is moderately increasing with increasing temperature of ECAP. The decrease of the UTS can be attributed to the effective dynamic recrystallization process and formation of submicrocrystalline microstructure. 1.4 Tensile properties of AISI 1045 steel. As concerns the deformation behaviour of the MC steel subjected to ECAP the tensile tests records are shown in Fig. 7c. For all specimens there is quiet extensive region of work hardening, after discontinuous yielding, and a large elongation to failure. 4

5 1.5 Tensile properties of steel AISI Fig. 7. Engineering stress-strain curves of AISI 1010 steel after solutioning treatment. As concerns the deformation behaviour of the MC steel subjected to ECAP the tensile tests records are shown in Fig. 7c. For all specimens there is quiet extensive region of work hardening, after discontinuous yielding, and a large elongation to failure as the introduced strain was increased in dependence of ECAP passes involved. The plastic behaviour of this steel is noticeable different from that of the LC steel. At this time such deformation behaviour can be attributed to existence of larger volume fraction of submicrostalline grains in the structure. The more advanced Fig. 7. Engineering stress-strain curves: a) ECAP AISI 1010 steel; b) ECAP AISI 1045 steel. dynamic recovery and dynamic recrystallization are the processes, which actually participated in structure transformation process. CONCLUSIONS Microstructural evolution during warm ECAP was studied in low carbon steel AISI 1010 and medium carbon steel AISI The major results can be summarised as follows: 1. Warm ECAP of low carbon steel leads to formation of heavily deformed substructure consisting of dislocation cells and subgrains. In the substructure, due to local recovery and polygonization process, areas with submicrocrystalline structure are formed. Partly recovered plastic ability caused the decrease of strength properties. 2. Microstructural observation of ECAP AISI 1045 medium carbon steel revealed the appearance of polygonized submicrocrystalline structure of high angle boundaries in large extent, which were the formed due to effective activation of dynamical recovery and polygonizational process during warm ECAP. Formation of ultrafined polygonized structure recovered the plastic ability of ECAP steel without strength reduction. Acknowledgement This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic through the Research Proposal under the contract No. MSM

6 REFERENCES [1] SEGAL, V.M., Mater. Sci. Eng. A, 197, 1995, p [2] VALIEV, R.Z. R.K. ISLAMGALIEV, R.K., ALEXANDROV, I.V.,,Progr. Mater. Sci., 45, 2000, p [3] VALIEV,R.Z., KORZNIKOV, V., MULYUKOV, R.R., Fiz. Met. Metalloved. 4, 1992, p. 70. [4] VALIEV, R.Z., ISLAMGALIEV, R.K., ALEXANDOV, I.V., Progr. Materials Science, 45, 2000, p [5] SAITO, Y. UTSUNOMIYA, H., TSUJI, N., SAKAI, T., Acta Mater., 47, 1999, p [6] TSUJI, N., UEJI, R., MINAMINO, Y., Scripta Mater. 47, 2002,p. 69. [7] SHIN, D.H., PARK, J.J., KIM, Y.S., PARK, K.T., Mater.Sci. Eng. A, 375, 2002, p [8 ] DE HODGSON, P., HICKSON, M.R., GIBBS, R.K., Mater. Sci.Forum, 63-72,1998, p [9] S.V. DOBATKIN, S.V., ODESSKI, P.D., PIPPAN, R., RAAB, G.I., KRASILNIKOV, N.A., ARSENKIN, A.M., Russian Metallurgy (Metally), 1, 2004, p. 94. [10] DOBATKIN, S.V. Severe Plastic Deformation of Steels: Structure, Properties and Techniques, in Investigations and Applications of Severe Plastic Deformation, Ed. by T.C. Love and R.Z. Valiev (Kluwer, Netherlands, 2000), Vol. 3/80, p