Sae 1008 çelik telin mekanik özellikleri ve yeniden kristallenme kinetiğine soğuk çekmenin etkilerinin incelenmesi = Investigation of the effects of cold drawing on mechanical properties and recrystallization kinetics of sae 1008 steel

Abstract

Bu tez çalışmasında, soğuk çekme işlemi yapılarak soğuk deformasyona uğratılmış SAE 1008 çelik tellerin mekanik özelliklerindeki değişimler ve yeniden kristallenme kinetiği incelenmiştir. Çelik tel numuneler Karabük'de bulunan PELENKOĞLU firmasından temin edilmiştir. SAE 1008 çeliği, düşük karbonlu bir çelik olup maksimum %0,1 karbon içermektedir. Ayrıca %0,30 – 0,50 Mn, maks. %0.03 P, maks. %0.05 S içermektedir. Düşük karbonlu çelikler kolaylıkla soğuk şekil verilebilirler ve genel olarak otomobil parçaları, teller, borular ve konserve kutuların üretiminde kullanılırlar. SAE 1008 çelik telin deneysel çalışmalarında 6,74 mm olan tel soğuk çekme yöntemiyle 5,75 mm çapa, daha sonra da ısıl işlem uygulanmadan soğuk çekme ile 4,24 mm çapa getirilmiştir. Soğuk çekme işlemleri bahsi geçen firmada gerçekleştirilmiştir. Çelik tellerin soğuk çekme işlemleri sonrasında yapılan metalografik incelemelerde, tane yapısının çekme ekseni yönünde uzadığı tespit edilmiştir. SAE 1008 çelik tellerin çekme test sonuçlarına göre, SAE 1008 çelik telin çekme mukavemeti 510 MPa iken, bu değer %31,8 oranında soğuk deformasyon sonrasında 650 MPa değerine, %92,7 oranında soğuk deformasyon sonrasında 810 MPa değerine artmıştır. SAE 1008 çelik telin süneklik değeri %25,6 iken bu değer %31,8 oranında soğuk deformasyon sonrasında %18,7'ye ve %92,7 oranında soğuk deformasyon sonrasında %12,7 değerine düşmüştür. SAE 1008 çelik telin mikrosertlik değeri 243 HV ike bu değer %31,8 oranında soğuk deformasyon sonrasında 287 HV'ye ve %92,7 oranında soğuk deformasyon sonrasında 372 HV değerine artmıştır. Soğuk çekme ile gerçekleştirilen deformasyon sürecinde dislokasyon yoğunluğunun artması nedeni ile SAE 1008 çelik telin çekme mukavemeti ve sertliği artarken süneklik özelliği azalmıştır. Soğuk deformasyon sonrasında malzemenin işlenebilme özelliği azalmaktadır. Malzemeye bu özelliği yeniden sağlayabilmek için yeniden kristallenme ısıl işlemi uygulanmaktadır. Yeniden kristallenme için itici güç, soğuk deformasyon sırasında dislokasyon yoğunluğunun artması ile oluşan depolanmış enerjidir. Yeniden kristallenme ve faz dönüşümleri gibi katı hal dönüşümlerinin kinetik çalışmaları, izotermal olmayan kinetik analiz yöntemleriyle yapılabilmektedir. Bu tez çalışmasında da, diferansiyel tarama kalorimetrisi (DSC), soğuk çekilmiş SAE 1008 çelik telde yeniden kristallenme kinetiğinin incelenmesinde kullanılmıştır. 5-10-15-20°C/dak ısıtma hızlarında alınan termal analiz grafiklerindeki yeniden kristallenme pik sıcaklıkları tespit edilmiş ve Kissenger, Boswell, Ozawa ve Starink gibi izotermal olmayan kinetik modellerle yeniden kristallenme için gerekli aktivasyon enerjisi değerleri tespit edilmiştir. %31,8 oranında soğuk çekilmiş SAE 1008 çelik telde yeniden kristallenme aktivasyon enerjisi Kissenger modeline göre 210,7 kJ/mol, Boswell modeline göre 216,7 kJ/mol, Ozawa modeline göre 212,1 kJ/mol ve Starink modeline göre 211 kJ/mol olarak bulunmuştur. %92,7 oranında soğuk çekilmiş SAE 1008 çelik telde yeniden kristallenme aktivasyon enerjisi Kissenger modeline göre 194,9 kJ/mol, Boswell modeline göre 200,8 kJ/mol, Ozawa modeline göre 196,8 kJ/mol ve Starink modeline göre 195,2 kJ/mol olarak tespit edilmiştir. Kinetik çalışmanın sonucuna göre, soğuk deformasyon miktarının artması ile depolanmış enerji miktarı artmıştır ve bu duruma bağlı olarak yeniden kristallenme için gerekli sıcaklık ve aktivasyon enerjisi bir miktar düşmüştür.

In this thesis, the effects of cold-drawing of SAE 1008 steel wire on its mechanical properties and recrystallization kinetics were investigated. The steel samples were obtained from PELENKOĞLU located in Karabük. Cold drawing is one of the plastic forming methods and it is used to reduce the cross section of metallic materials under a drawing force through a single or a series of drawing dies. Electrical wires, cables, springs, paper clips, stress-loaded structural components and stringed musical instruments were produced by cold drawing of metallic materials. SAE 1008 steel is a low carbon steel and it has maximum 0.10% carbon. It has also 0.30 – 0.50% Mn, max. 0.04% P, max. 0.05% S, max. 0.1% Si. It has good weldability and good ductility. Additionally, adding certain elements such as aluminum helps improve the hot rolled finish during operation. This type of steel is used in construction, automotive, aerospace and other industries where strength and durability are essential. In additionally, it can be used for various applications, including cold headed fasteners, screws, nails and drilling components. In the experimental studies of SAE 1008 steel wire, the initial diameter of 6,74 mm was brought to 5,75 mm in first cold drawing step. The amount of cold deformation in first step was determined as 31,8%. In the second cold drawing step, the diameter of 5,75 mm was brought to 4,24 mm and the amount of cold deformation is determined as 92,7%. The cold drawing processes for SAE 1008 steel wires were carried out in Pelenkoğlu company. As a result of metallographic examinations on structures of cold drawn SAE 1008 steel wires, the grain structures were elongated in the direction of the tensile axis and no crack formation was found. When the tensile tests results of SAE 1008 steel wires are examined, while the tensile strength of initial SAE 1008 steel wire was 510 MPa, this value increased to 650 MPa after 31,8% cold deformation and increased to 810 MPa after 92,7% cold deformation. The ductility of SAE 1008 steel wire decreased from 25,6% to 18,7% for 31,8% cold deformation and then decreased to 12,7% for 92,7% cold deformation. The micro hardness of initial SAE 1008 steel wire was 243 HV. This value increased to 287 HV after 31,8% cold deformation and increased to 372 HV after 92,7% cold deformatiom. The strength and hardness of SAE 1008 steel wire increased and the ductility of SAE 1008 steel wire decreased during cold drawing due to the increase in dislocation density in structure. Because dislocation density increases during plastic deformation leading to work hardening, also named as strain hardening. The microstructure and mechanical properties of the metallic materials change during cold plastic deformation. As the dislocation density in cold deformed metallic material increase, their strength and hardness increase and their ductility decrases. The workability property of the metallic material is decreased. The heat treatment named as recrystallization should be done to give back the material its reworkability property. This is an annealing process and it is applied to cold worked metallic materials to obtain new and stress free grains. The structures of metallic materials are unstable thermodinamically after cold deformation, therefore recrystallization heat treatment allows recovery process by reduction or removal of stresses on the grains and increases equiaxed grains formed from the elongated grains. This heat treatment decreases the strength and hardness and increases ductility. Recrystallization of cold deformed metallic materials is carried out at 30 – 60% of the melting temperature of the material. This temperature depends on the initial grain size, composition, impurity content and degree of cold deformation. The three steps in recrystallization are recovery, recrystallization and grain growth. The driving force in the recrystallization of deformed metalllic materials is the stored energy. The recovery ocuurs at low temperatures and no change in the microstructure is observed. Dislocations move and rearrange, residual stresses decrease in the structure during recovery. Recrystallization is polygonizing and annihilatizing process of dislocations. Dislocation density decreases during recrystallization and therefore strength decreases and ductility increases. Finally grain growth takes place in the structure, as depending on temperature and time. Kinetic studies of solid state transformations such as recrystallization of deformed materials and phase transformations can be done by non-isothermal kinetic analysis methods. Isothermal kinetic analysis requires a kinetic function for the evolution the reaction rate with time and the kinetic parameters such as the pre-exponential factor. The non-isothermal kinetic analysis is the alternative model-free methods based on the isoconversional method. The thermal analysis, such as differential thermal analysis (DTA, differential scanning calorimetry (DSC and thermogravimetry (TG), are used to study the non-isothermal kinetics. In this thesis, differential scanning calorimetry (DSC) was used to investigate the recrystallization kinetics of cold drawn SAE 1008 steel wire. The recrystallization peak temperatures were determined in the thermal analysis graphs taken at 5 – 10 – 15 – 20°C/min heating rates (β) and the activation energy values (E) required for recrystallization were determined with non-isothermal kinetics modelssuch as Kissenger, Boswell, Ozawa and Starink. The recrystallization peak temperatures obtained from differential scanning calorimetry data were between 447,6 °C and 475,4 °C for 31,8% cold deformed SAE 1008 steel wire and between 434,3 °C and 462,7 °C for 92,7% cold deformed SAE 1008 steel wire, depending on different heating rates. The graphs of Kissenger (ln(β/T2) – 1/T), Boswell (ln(β/T) – 1/T), Ozawa (ln(β) – 1/T) and Starink (ln(β/T1.92) – 1/T) methods were drawn by using recrystallization peak temperatures. The slopes in the graphics are equal to –E/R, so the activation energies can be determined from the slopes. The recrystallization activation energies for 31,8% cold deformed SAE 1008 steel wire are determined as 210,7 kJ/mole, 216,7 kJ/mole, 212,1 kJ/mole and 211 kJ/mole according to Kissenger, Boswell, Ozawa and Starink methods, respectively. Considering these four models, the recrystallization activation energy for 31,8% cold deformed SAE 1008 steel wire is 212,6 ±2 kJ/mole. The recrystallization activation energies for 92,7% cold deformed SAE 1008 steel wire are determined as 194,9 kJ/mole, 200,8 kJ/mole, 196,8 kJ/mole and 195,2 kJ/mole according to Kissenger, Boswell, Ozawa and Starink methods, respectively. Considering these four models, the recrystallization activation energy for 92,7% cold deformed SAE 1008 steel wire is 196,7 ±2 kJ/mole. It can be explained from these studies that cold deformation of SAE 1008 steel wires increased their strength and hardness, decreased their ductilities due to stored energy formed during cold drawing. As the amount of cold deformation on SAE 1008 steel wires increased, the recrystallization temperature and the recrystallization activation energy decreased slightly.