Investigation of recrystallization behaviour and mechanical properties of cold drawn C55 and C83 steels

In this thesis, the effects of cold-drawing of C55 and C83 steel wires on their mechanical properties and recrystallization kinetics were investigated. The steel samples were obtained from ÇELİK HALAT ve TEL SANAYİ A.Ş. located in Izmit - Kocaeli. This company produces wire ropes, spring wire, galvanized spring wires and prestressed concrete strands. The one of the plastic forming methods is cold drawing and it is used to reduce the cross section of material under a drawing force through a single or a series of drawing dies. Cold drawing process is used to obtain electrical wires, cables, springs, paper clips, stress-loaded structural components and stringed musical instruments. C55 steel is a medium carbon steel and it has 0.52 – 0.60% carbon. It has also 0.06 – 0.90% Mn, max. 0.045% P, max. 0.045% S, max. 0.4% Si and max. 0.63% Cr+Mo+Ni. C55 steel has high hardness and high strength due to the relatively high carbon. C55 steel is suitable for automotive and engineering materials such as axles, clutch members, shafts, pressed and punched parts, piston rods and gear racks. C83 steel is a high carbon steel and it contains manganese ranging from 0.30 to 0.90%. These steel is used for springs materials and high-strength wires. In the experimental studies of C55 steel wire, the diameter of 5 mm was brought to 3 mm. The amount of cold deformation was determined as 121.2%. In the experimental studies of C83 steel wire, the diameter of 5 mm was brought to 3.2 mm and the amount of cold deformation is determined as 108.3%. The cold drawing processes for C55 and C83 steel wires were carried out in Çelik Halat company. In metallographic examinations made after cold drawing processes of C55 and C83 steel wires, it was observed that the grain structures elongated in the direction of the tensile axis. No crack formation was found. When the tensile tests results of C55 and C83 steel wires are examined, while the tensile strength of C55 steel wire was 859.2 MPa, this value increased to 1358.2 MPa after 121.2% cold deformation. The ductility of C55 steel wire decreased from 65.8% to 24.7% for 121.2 cold deformation. While the tensile strentgh of C83 steel wire was 1216.7 MPa after 108.3% cold deformation, this value increased to 1717.5 MPa after 108.3% cold deformation. The ductility of C83 steel wire decreased from 32.4% to 16.8% after 108.3% cold deformation. Since the strength and ductility of steel depend on the carbon content, it has been determined that the strength of the C83 steel wire was found to be greater than the strength of the C55 steel wire, and its ductility is lower. The increase in strength and decrease in ductility of C55 and C83 steel wires with cold deformation are due to the increase in dislocation density during cold deformation. The shape, microstructure and mechanical properties of the material change during cold drawing. Depending on the dislocation increase, strength and hardness increase, ductility decrases. The workability of the material is decreased. Recrystallization heat treatment should be done to give the material its reworkability property. This heat treatment is an annealing process and it is applied to cold worked metals and alloys to obtain new and stress free grains. Because cell structures of metallic materials are unstable thermodinamically after cold deformation. Recrystallization heat treatment allows recovery process by reduction or removal of stresses on the grains and increases equiaxed ferrite grains formed from the elongated grains, decreases the strength and hardness and increases ductility. There are recovery, recrystallization and grain growth steps in the recrystallization heat treatment and it is carried out at 30 – 60% of the melting temperature of the metal. Recrystallization temperature is affected by initial grain size, composition, impurity content, time and the degree cold deformation. The driving force for recrystallization in cold deformed metals is the stored energy formed during cold deformation. The recovery step takes place at low temperatures and there is no any changing in the microstructure. In this step, dislocations move and rearrange, residual stresses decrease in the structure. Recrystallization step is polygonizing and annihilatizing process of dislocations in the structure. In this step, dislocation density decreases, therefore strength decreases and ductility increases. In the last step of recrystallization heat treatment, as depending on temperature and time, grain growth takes place. Isothermal kinetic analysis is used to explain the kinetics of solid state reactions such as recrystallization of metals and it is obtained kinetic informations needed for modeling the process at an industrial scale. This analysis requires a kinetic function for the evolution the reaction rate with time and the kinetic parameters such as the pre-exponential factor and activation energy can be obtained. The non-isothermal kinetic analysis is the alternative model-free methods based on the isoconversional method. The non-isothermal kinetics is a thermoanalytical method based on differential thermal analysis (DTA), thermogravimetric analysis (TG) and differential scanning calorimetry (DSC). In this thesis, the non-isothermal recrystallization kinetics studies of cold drawn C55 and C83 steel wires were performed by using differential scanning calorimetry (DSC) with four heating rates (5–10–15–20°C/min). Kissenger, Boswell, Ozawa and Starink methods were used to find the activation energy of recrystallization. The recrystallization temperatures obtained from DSC data were between 451.2 °C and 481.3 °C for 121.2% cold deformed C55 steel wire and between 475.4 °C and 504.6 °C for 108.3% cold deformed C83 steel wire, depending on different heating rates. By using recrystallization peak temperatures, the plots of Kissenger method, (ln(β/T2) – 1/T), Boswell method (ln(β/T) – 1/T), Ozawa method (ln(β) – 1/T) and Starink method (ln(β/T1.92) – 1/T) were drawn. In these equations, R is gas constant (kJ/mol.K), T is peak temperature (K) and β is heating rate (K/min). The slopes in the graphics are equal to –E/R, where E is activation energy (kJ/mol). The recrystallization activation energies for 121.2% cold deformed C55 steel wire are 196.5 kJ/mole, 202.6 kJ/mole, 198.7 kJ/mole and 196.8 kJ/mole according to Kissenger, Boswell, Ozawa and Starink methods, respectively. The recrystallization activation energies for 108.3% cold deformed C83 steel wire are 220.5 kJ/mole, 226.8 kJ/mole, 221.8 kJ/mole and 220.8 kJ/mole according to Kissenger, Boswell, Ozawa and Starink methods, respectively. As a result, the recrystallization activation energies are 198.6 ±2 kJ/mole for 121.2% cold deformed C55 steel wire and 222.5 ±2 kJ7mole for 108.3% cold deformed C83 steel wire. It can be explained from these studies that cold deformation of C55 and C83 steel wires increased the strength and decreased their ductilities due to stored energy formed during cold drawing. When C55 and C83 steel wires compared, the recrystallization temperature and the recrystallization activation energy slightly increase because of the increase in carbon content of steel.