Conclusions

1. T b) a) Introduction The one year miniproject within the project - ‘Cesta k vede’ was realized at FNSPE CTU.The main motivation was to get new experience with the materials science and engineering. The first semester consisted of the introductory courses of metal physics, stereology and engineering mechanics. The theoretical skill were then directly applied during laboratory sessions where tensile tests, Charpy impact toughness tests (with the evaluation of transition temperature) and Vickers hardness tests were carried out. The metallographic specimens for the complex study via optical microscopy were also prepared. The aim of the minoproject was to create entire metallographic descriptionof the steel CSN - 11 373. The consultations as well as the lectures were given by experienced PhD students and other academic stuff. All experiments were carried out at the Department of Materials, FNSPE, the Czech Technical University in Prague. The Applied Heat Treatment 1) The Preparation of the Metallographic Specimens 1) a) 2) Preparation of the Cross-sections b) • a) Putting into Resin • The next step is to put the specimens into the resin (cold as well as hot moulding resins are used). In this case, hot moulding was employed (6 minutes / 180°C / 20KPa + 2 minutes water cooling). • b) Grinding • Grinding was performed on grinding wheels with the roughness from 200 to 2000, under the water cooling. • c) Polishing • Polishing with textile polishing wheels using suspensions from 6µ to 1µ was employed. • d) Etching • The specimens were etched by 5% Nitalfrom 3s to 5s to evoke the microstructure (by different etching of grains and boundaries). After the etching, the specimens were cleaned with water and ethanol and dried. c) The Study of the Microstructure, Stereology Charpy Notch Impact Test c) d) e) The transposition of the circle from ruler to micrograph Transition Curve Vickers Hardness Ductile fracture Brittle fracture Conclusions Acknowledgement The authors would like to express their thanks to the project-Cesta k vědě, Gymnázium Karla Sladkovského and Gymnáziu Jaroslava Seiferta. The authors are also grateful to European Social Fund and Prague – The Capital for financial help and FNSPE, especially Department of Materials for labs provided. The heartfelt thank goes to Ing. Vladimír Pospíšil and supervisors Ing. Štěpán Válek a Ing. Tomáš Skiba. The Characterization of Structural Materials Marek Kovář*,1), Tomáš Peták*,2), Jiří Švancara*, Školitelé: Tomáš Skiba**, Štěpán Válek** *High school Karla Sladkovského, Sladkovského náměstí 8, Praha 3, 130 00 **Department of Materials, Faculty of Nuclear Sciences and Physical Engineering, CTU, Trojanova 13, Praha 2, 120 00 Corresponding authors: 1) kovar.ma@seznam.cz; 2)t.petak@seznam.cz Abstract The miniproject ‘The Characterization of Structural Materials’ consists of complete metallographic study of the steel 11 373 (according to the CSN). The different heat treatments were applied. The metallographic samples were prepared and these studied by means of optical microscopy with subsequent application of the quantitative stereologic analysis.The hardness tests as well as the Charpy notch impact toughness tests were performed.The transition temperature was then evaluated. • 7 steel 11 373 specimens with different heat treatment applied were prepared: • Combined heat treatment: 30 minutes / 900°C + air cooled at room temperature + given to the furnace heated to 900°C and then very slow cooled in the furnace down to the room temperature. • Annealed: 4 hours / 900°C + air cooled at room temperature. • Water quenched: 30 minutes / 900°C + water quenched. • Oil quenched: 30 minutes / 900°C + oil quenched. 1) Taking of Samples The specimens were taken from the all three directions (x, y, z ) to ensure having the 3D representation of the material. Very often the grains are not completely symmetrical but deformed in some direction caused by different phenomenon. • At 800°C: 30 minutes / 900°C. Charpy impact toughness test performed at approximately 800°C. (100°Closs was estimated during transport from the furnace to the Charpy test equipment) • Air cooled: 30 minutes / 900°C + air cooled at room temperature. • As cast: without any heat treatment applied. The experimental set-up consists of the Charpy punch (see figure on the right). The punch has defined potential energy at its starting position E1 (300J in this case).The specimen, having the dimensions of 10x10x55 mm according to the CSN EN 10 045 – 1 (420381), has a 2 mm notch. The specimen is then put on a support, having the notch directly at the place where the punch hit the sample. The tests were carried out on 7 specimens subjected to different heat treatments:: The cross-sections of the specimens were studied by means of optical microscopy (using NEOPHOT 32). The microstructure evaluation was then carried out. Three circles of known radius were put on each micrograph. All micrographs exhibit the same nominal magnification 300x. Along the circle, the distances between the grain boundaries was measured (for each phase). Overall distances were then summed and divided by the number of the grains. After the calculation according to the magnification, the result show the average grain size. Combined heat treatment→ 202 J Annealed→ 168 J Water quenched → 188 J Oil quenched → 198 J At 800°C → 135 J Air cooled → 228 J As cast → 20 J From the results it can be clearly seen that the heat treatment applied has a fundamental impact on mechanical properties. Note:Specimen 5‘At 800°C’ was taken from the furnace having 900°C but during the manipulation, the loss of 100°C was taken into account. Few specimens were heated in the furnace up to the different temperatures where the Charpy notch impact toughness test was performed. At certain temperature region, the rapid change of energy (needed for breaking the specimen) is observed. This region is called transition region and its ‘mean’ temperature the transition temperature. In this region the brittle fracture changes into the ductile fracture. The Vickers hardness test is a static test. The experimental set-up is prescribed by the Czech State Norm ČSN 42 0374. It is the most often used hardness test. The test is based on the indentation of the diamond tip into the material and subsequent evaluation of the deformation of the material (measurements of the sizes of indents) The Vickers hardness test were performed on 7 steel specimens using the loading of 30 kg. On each specimen 10 indents were carried out. These average values were evaluated: Combined heat treatment HV 30 = 127,9 AnnealedHV 30 = 110,2 Water quenchedHV 30 = 192,9 Oil quenchedHV 30 = 164,5 At 800°CHV 30 = 140,9 Air cooled HV 30 = 132,6 As castHV 30 = 195,7 Energy (J) It can be seen that the highest value of the Vickers hardness HV30 were measured at the as cast condition of the material. Very high values were also measured on water and oil quenched specimens.The smallest hardnesses were found on annealed material. The standard deviations has also be taken into account. Temperature (°C) T0 = 50°C Using the least square method, the function (1) was smoothed by experimental data.The parameter evaluated were A0 = 78J; B0 = 70J; C0 = 11,56°C; T0 = 48,01°C. The transition temperature is then approximately 50°C. In fact, it is an inflex point of the curve, the boundary between the lower and the higher plateau. The values of the lower and higher plateau were estimated as 10 and 150J, respectively. The work within the miniproject ‘Characterization of the Structural Materials’involved complex metallographic investigation of the steel CSN 11 373. The work was carried out at the Department of Materials, FNSPE, CTU in Prague. The authors had the opportunity to get experience with an applied research and after being trained perform mechanical tests by themselves (namely Vickers hardness tests, tensile tests or Charpy impact toughness tests). The heat treatment of the material was applied, the cross-sections were prepared for metallographic study using optical microscope. The results from the Charpy impact toughness tests were than used for evaluation of the transition temperature. It was investigated, that the impact of the heat treatment on a material is of great importance. For example, due to the different heat treatment, the values of the notch impact toughness were increased ten times. The huge differences were observed in the microstructure where the impact on the grain size was reported. All measured and evaluated values were in a good agreement with the literature.