Effect of welding current on microstructure and mechanical properties of 15% deformed TWIP steel joined with electrical resistance spot welding

Bu çalışmada, punta kaynağı ile birleştirilen %15 deforme olmuş TWIP levhaların özelliklerine akımın etkisi araştırılmıştır. Mikroyapı karakterizasyonunda optik mikroskopi, Taramalı Elektron Mikroskobu (SEM), SEM/Enerji Dağılım Spektroskopisi (SEM-EDS) ve SEM/Elektron Geri Saçılım Kırınımı (SEM-EBSD) teknikleri kullanıldı. Mekanik özellikleri belirlemek için mikrosertlik ve çekme testleri yapıldı. Külçe boyutu, girinti derinliği ve ısıdan etkilenen bölgenin (HAZ) genişliği artan akımla neredeyse doğrusal olarak artarken kaynak boşlukları artan akımla azaldı. HAZ'da büyük tavlanan ikizler ortaya çıkarken, deformasyon ikizleri ortadan kayboldu. HAZ'da tane irileşmesi ve ikiz kalınlıklar artan akımla birlikte arttı. Ancak kaynak herhangi bir faz dönüşümüne neden olmadı. Bu nedenle kaynak bölgesindeki sertlik ana metal sertliğine göre oldukça düşük çıkmıştır. Genel olarak en düşük sertlik ITAB'da gözlenmiştir. Kaynak bölgesinde akım ile sertlik arasında herhangi bir korelasyon yoktu. Akım arttıkça kırılma yükü arttı: En yüksek kırılma yükü 10 kA'da elde edildi. Daha yüksek akımlarda düğme kopma kırıkları ortaya çıkarken, daha düşük akımlarda arayüzey kırıkları meydana gelmiştir. Kırılma özellikleri genellikle gevrek-sünek karışımıdır. Mukavemeti daha yüksek olan kaynak numunelerinde, gevrek-sünek bölgedeki sünek ve gevrek bölgedeki transgranüler malzemenin kırılma özellikleri artmıştır.The effect of current on properties of 15% deformed TWIP sheets joined with spot welding was investigated in this study. Optical microscopy, Scanning Electron Microscopy (SEM), SEM/Energy Dispersive Spectroscopy (SEM-EDS) and SEM/Electron Backscatter Diffraction (SEM-EBSD) techniques were used in microstructure characterization. Microhardness and tensile tests were performed to determine mechanical properties. The nugget size, indentation depth and heat affected zone (HAZ) width increased almost linearly with increased current, while the weld cavities decreased with increased current. Large annealing twins have appeared in HAZ, while deformation twins have disappeared. Grain coarsening and twin thicknesses in HAZ increased with increased current. However, the welding didn't cause any phase transformation. Therefore, hardness in weld zone was considerably lower than base metal hardness. In general, the lowest hardness was observed in HAZ. There was no correlation between current and hardness in weld zone. Fracture load increased with increased current: The highest fracture load was obtained at 10 kA. Button pullout fractures have emerged with higher currents, whereas interfacial fractures have occurred with lower currents. Fracture characteristics are generally a mixture of brittle-ductile. In weld samples with higher strength, the fracture characteristics of ductile in brittle-ductile zone and the trans-granular in brittle zone have increased

Hardening Behavior Characterization of Dual Phase Steels

The requirements for higher passenger safety, improved fuel economy and weight reduction in automobile industry necessitates the usage of advanced high strength steel AHSS grades. Dual phase DP steels are the most widely used one among AHSS. DP steels become increasingly popular, since they provide a combination of sufficient formability at room temperature and tensile strength over 1000 MPa. The current standards for DP steels only specifies yield and tensile strength. Steels from various producers have considerably different composition and microstructure; however they still have the same grade name. Combined with the inherited heterogeneous microstructure, those steels exhibit different strain hardening behavior. The aim of this study is to evaluate the strain hardening behavior of DP800 steels, obtained from different vendors and thus having different compositions and microstructures. The strain hardening behavior was characterized with tensile tests performed along rolling and transverse directions. The microstructure has been characterized with optical and scanning electron microscopes. The martensite fraction, grain size of ferrite and chemical composition has been correlated to the strain hardening behavior. The results show that the steel with more micro-alloying addition has finer ferritic grain size, which cause higher initial strain hardening rate. The steel with higher Mn and Cr has higher martensite fraction, which cause strain hardening rate to be higher at higher strain levels

Microstructural Analysis of Austempered Ductile Iron Castings

A ustempered ductile iron ADI castings have a wide range of application areas in engineering designs due to their promising mechanical properties and lower cost. ADI has very good strength and toughness values at the same time its ductility is relatively high compared to most of the other cast irons. These promising mechanical properties originate from combination of specific graphite and matrix microstructure. The size, shape and fraction of graphite as well as the matrix microstructure influences the mechanical properties. In this paper the efforts regarding to a localization project of ADI is presented. In a more detailed manner, the first locally produced ADI which cannot satisfy the mechanical properties stated in ISO 17804 is compared with the original sample which is conform with the standard. The two pieces are inspected by mechanically and microstructurally by means of which necessary actions are detected for the local production. In other words the relation between the macro mechanical properties and the microstructural conditions are tried to be clarifie

Volume CXIV, Number 4, November 7, 1996

Objective: Turner syndrome (TS) is a chromosomal disorder caused by complete or partial X chromosome monosomy that manifests various clinical features depending on the karyotype and on the genetic background of affected girls. This study aimed to systematically investigate the key clinical features of TS in relationship to karyotype in a large pediatric Turkish patient population.Methods: Our retrospective study included 842 karyotype-proven TS patients aged 0-18 years who were evaluated in 35 different centers in Turkey in the years 2013-2014.Results: The most common karyotype was 45,X (50.7%), followed by 45,X/46,XX (10.8%), 46,X,i(Xq) (10.1%) and 45,X/46,X,i(Xq) (9.5%). Mean age at diagnosis was 10.2±4.4 years. The most common presenting complaints were short stature and delayed puberty. Among patients diagnosed before age one year, the ratio of karyotype 45,X was significantly higher than that of other karyotype groups. Cardiac defects (bicuspid aortic valve, coarctation of the aorta and aortic stenosis) were the most common congenital anomalies, occurring in 25% of the TS cases. This was followed by urinary system anomalies (horseshoe kidney, double collector duct system and renal rotation) detected in 16.3%. Hashimoto's thyroiditis was found in 11.1% of patients, gastrointestinal abnormalities in 8.9%, ear nose and throat problems in 22.6%, dermatologic problems in 21.8% and osteoporosis in 15.3%. Learning difficulties and/or psychosocial problems were encountered in 39.1%. Insulin resistance and impaired fasting glucose were detected in 3.4% and 2.2%, respectively. Dyslipidemia prevalence was 11.4%.Conclusion: This comprehensive study systematically evaluated the largest group of karyotype-proven TS girls to date. The karyotype distribution, congenital anomaly and comorbidity profile closely parallel that from other countries and support the need for close medical surveillance of these complex patients throughout their lifespa

Relation between microstructure and mechanical properties of a low-alloyed TRIP steel

Steels with transformation induced plasticity (TRIP) offer a good balance of ductility and strength, which is a result of gradual transformation of metastable austenite into martensite during straining. A thorough understanding of the relation between microstructure and martensitic transformation and the related austenite stability is the key for a successful improvement of the mechanical properties. The present low-alloyed TRIP steel has a rather heterogeneous texture and microstructure on the microscopic level due to the intense austenite banding. This makes microstructure characterization particularly important and difficult. As EBSD is used as the main microstructure characterization tool particular attention is given to the statistical reliability of EBSD-based results. In addition, a new and fully automated mapping technique, that helps to cover larger areas and probe more grains while keeping the particular advantages of a small step size, is presented. Lastly, the comparison of results obtained by EBSD and XRD revealed that the EBSD technique is reliable and representative provided that sampling and sample preparation are adequately done. After identifying the optimum sampling conditions the deformation and transformation of austenite during interrupted ex-situ bending tests were studied by EBSD investigations. The effects of size and shape of austenite grains on the strain-hardening of the present TRIP steel have been interpreted by a simple rule of mixture for stress-partitioning and a short fiber reinforced composite model. The composite model of Cox was used to investigate the effect of grain shape on load partitioning. Moreover, the grain average misorientation (GAM) analysis of the EBSD results revealed that at the initial stage of deformation mainly larger grains deform. These larger grains, however, do not reach the same strain level as the smaller grains because they transform into martensite at an early stage of deformation. The texture development during tension and compression of the present TRIP steel is studied by analysing the results of bending experiments. Here, the change in the retained austenite texture was due to both deformation and transformation into martensite, whereas the ferrite texture changed due to deformation only. A theoretical relation between the deformation of FCC and BCC crystals was derived by a phenomenological constitutive deformation model. By comparing the experimental observations to the theoretical relation, the changes in the austenite texture due to slip were distinguished from the changes due to transformation. These comparisons have shown that the strong decrease in the Brass and Goss components of austenite are mainly due to the martensitic transformation. Lastly, a detailed analysis of the effects of grain size, grain shape and crystallographic orientation on austenite stability is presented. The experiments revealed that the retained austenite is not transforming continuously, but rather in a distinct 3-stage behaviour. Accordingly, the grains can be grouped into 3 categories; grains that are transforming with a high rate at low strain, those that transform at high strain with a low rate and lastly a strain gap between these two types where the austenite does not transform. Grains that are transforming at low strains tend to be large, have low Taylor factor and a strongly elongated shape, while the remaining stable grains are small, more spherical in shape and have high Taylor factor. The grain size effect is quantitatively explained by a diffusion model that shows that larger grains are not fully stabilized due to insufficient carbon diffusion. The grain shape effect is related to the strain distribution that is found in short-fiber composite materials. The Taylor factor indicates the ease of plastic deformation and a low Taylor factor, therefore indicates a large abundance of possible nuclei for martensitic transformation according to strain-induced transformation mode. The strain-gap between the early and late transforming grains is explained by the lack of grains with medium Taylor factor. Finally, a logistic regression model was used to quantify the relative contribution of each of the microstructure-related parameter. The results of this model showed that the grain size is the most important parameter as its significance is about an order better than the second most important parameter, the Taylor factor. This thesis shows, firstly, how to obtain a reliable and representative description of rather heterogeneous microstructures using the EBSD technique. The presented concepts can be used to improve the statistical reliability of all possible types of EBSD-based microstructure analysis. Secondly, a comprehensive analysis of the relation between microstructure and macroscopic mechanical properties of a low alloyed TRIP steel is presented. These results provide a useful basis for further development and improvement of TRIP-assisted steels

Çelik içyapılarının manyetik barkhausen gürültüsü yöntemi ile karakterizasyonu.

This aim of this thesis is to examine the possibility of using Magnetic Barkhausen Noise (MBN) technique in characterizing the microstructures of quenched and tempered low alloy steels as well as annealed low carbon steels. To determine the average grain size by MBN, SAE 1010 steel consisting of dominantly ferrite was used. The specimens were slowly cooled in the furnace after austenitizing at different time and temperature variations. By metallographic examination the average ferrite grain size of specimens was determined. The magnetic parameters were measured by a commercial MBN system. With increasing ferrite grain size, the magnetic Barkhausen jumps caused by the microstructure were decreased due to the reduction in grain boundary density per unit volume. A clear relationship has been observed between average grain size and the magnetic Barkhausen noise signals. SAE 4140, 5140 and 1040 steels were used to characterize the microstructures of quenched and tempered specimens. After austenitizing and quenching identically, the specimens were tempered at various temperatures between 200oC and 600oC. Formation of the desired microstructures was ensured by metallographic examinations and hardness measurements. The results show that as tempering temperature increases the Barkhausen activity increases due to the enhancement of domain wall displacement with softening of the martensite. It has been shown that MBN is a powerful tool for evaluating the microstructures of martensitic and annealed steels.M.S. - Master of Scienc

Monitoring the microstructural changes during tempering of quenched SAE 5140 steel by magnetic barkhausen noise

The aim of this work is to characterize the microstructures of quenched and tempered steels non-destructively by a diverse set of parameters of the Magnetic Barkhausen Noise method (MBN fingerprint, frequency spectra, pulse height distribution, root-mean-square, and total number of pulses). Identical specimens from a SAE 5140 steel bar were prepared. All specimens were austenitized at 860 degrees C for 30 minutes and water-quenched identically. The quenched specimens were then tempered at various temperatures between 200 degrees C and 600 degrees C. The microstructures were characterized by metallographic examinations and hardness measurements. Pulse height distributions, noise signal envelopes and frequency spectra were used to evaluate Barkhausen activity. The results show that as the tempering temperature increases, the Barkhausen activity increases due to the enhancement of domain wall displacement with softening of the martensite. An excellent correlation was found between Barkhausen parameters and hardness values