Papers by Author: Alicia Mašlejová

Abstract: Material used for screw production was investigated due to its inconvenient properties which caused the screw breaking on threads or in the head during the installation. Chemical composition of analyzed material corresponded with standard STN 17153 according to technological drawing for specific product. The metallographic analysis showed that failure of screws happened due to improper microstructure resulting from unsuitable thermal treatment of material. Fine inclusions based on aluminum nitride (AlN) and chromium carbonitride (Cr(C,N)) were segregated along the ferritic grain boundaries. Coarse aluminum nitride inclusions (AlN) in ferritic matrix affected the character of present fracture surface characterized by cleavage facettes. The fracture was propagated step by step following the planes with the increased concentration of inclusions across the whole cross-section of the screw.

Abstract: This paper deals with rarely occurring defect on the surface of the tinplates produced in the conditions of U. S. Steel Košice, s.r.o. at the cleaning section of the continuous annealing line (CAL), which was caused by burnt. Random occurrence of such surface imperfections was observed within the short period of time (two months) for various continuous annealed tinplate grades, i.e. TH415, TH435 or TH550 with the thickness of the materials in the interval of 0.172 – 0.240 mm. Surface defects caused by burnt (thermal attack) manifested in different ways. Their appearance on the sheet surface was spot with regular or irregular circle shape, as well as line with the length of up to 2 mm oriented not directly in the rolling direction. By observing of the imperfections in scanning electron microscope (SEM) it was found that the surface of the steel substrate was always compact in the defective areas with clearly visible rolling lines. Foreign material, EDS analysis of which revealed that its chemical composition was on the basis of Fe and Cr, was deposited on the surface of the base material. The appearance of the foreign material shape indicated that it was in the liquid state at the time of incidence on the steel substrate surface. Metallographic analysis from the longitudinal metallographic sections through the defective areas revealed that the substrate under the spot defect with regular circle shape had thermally influenced microstructure. Similar manifestations of the microstructure influence were observed for the line defects. In the case of the spot defect with irregular shape, the microstructure of the material was not influenced, what indicated that the particle of liquid metal was solidified at the time of incidence on the plate surface. After revising technical conditions of the cleaning section of CAL, this type of imperfection was suppressed.

Abstract: Introduction of new technologies demands a new access to quality evaluation of products. Joining of steel sheets with laser butt welding requires increased demand on precision of welded sheet ends alignment as well as condition of cutting edge. Moreover, increased requirements are placed on purity of environment, flatness of strips, gases, etc. The paper deals with evaluation of quality of laser butt welds of steel sheets and description of defects which could occur on welds from the metallographic point of view. The influence of relative displacement of welded strips ends alignment, different relative sheet thickness alignment, welding gap size and quality of sheet shear edge on quality of weld will be described. The most significant effect on quality of weld joints and their final mechanical properties has relative position of strip ends and geometry of cutting edge.

Abstract: The paper describes degradation process of ceramic rollers. Rollers behavior under mechanical and thermal load was evaluated using available methods like XRD, instrumented hardness measurement and compressive strength. The influence of residual stresses was also discussed. The first phase during the degradation crystalize from amorphous silica tridymite, cristobalite and quartz, then due to the contact and pressure of the steel strip to the surface roller iron diffuses and forms fayalite. Measured values of compression strength of roller of 55-61 MPa agree with producer documentation declared minimal values. Hardness and Young´s modulus of buildup phases identified from surface into depth were determined. Wüstite was present up to 100 mm, with hardness 6±1 GPa, Young´s modulus 75±6 GPa, fayalite, cristobalite, up to 500 mm, with hardness 5.4±0.6 GPa, Young´s modulus 51±2 GPa, tridymite over 500 mm, with the hardness 7±4 GPa and Young´s modulus 58±25 GPa.

Abstract: The surface line type defects on the can twist-off lids from lacquered tinplate material have been analyzed. The analyzed defects manifested themselves like the lines bulged above the surrounding surface. They were aligned roughly perpendicular to the lid circumference; they were mostly arranged on the lower part of lid circumference edge. The defects surface was fully covered by lacquer layer. After lacquer layer removal it was visible in the defective area that tin layer was damaged on some defect areas. The defect presented itself like filiform corrosion which occurred in conditions slightly above room temperature and high humidity under the originally continuous covering layer. According to the findings, corrosion occurred on the hidden sheet cut edges and then propagated first perpendicular to the circumference into the sheet. Then sometimes its direction changed into the sheet rolling direction along the lines with less but still suitable tin layer thickness. This defect can be controlled by special protecting of lid cut edge against corrosion or decreasing of the temperature and the humidity conditions in the stores.

Abstract: Material used for car sheet wheels production was analyzed due to limited weldability. During the manufacturing of wheel rims, during shaping - cold forming cracks were initiated in the weld joint and in its heat affected zone. The destruction of rims occurred during their shaping. The wheel rims were welded at their manufacturer's site applying the compression technology and using the direct current welding source. Five rims were supplied for the metallographic analysis of which two were with conforming weld joint, and in the case of three rims there was a rupture at the weld joint point; on one them along the whole width of the rim. The rupture of the rim weld joint across its whole width at the weld joint point that was made by diffusionless connection was caused by a low welding input power. The primary cause of the rim cracking initiation on the transverse weld joint was the presence of micro-cracks, micro-notches in the vicinity of the weld joint and the heat affected zone, the notch effect of which was manifested during the rim forming. The occurrence of micro-notches was shown in the heat affected zone as well as on the conforming weld joint of one rim. The carbon equivalent Ce from the measured values of the material chemical composition of all rims was in the range from 0.175 to 0.201. Given the declared weldability guaranteed up to the value of Ce = 0.45, all analyzed materials complied with the requirement. Material cleanliness, grain size and linearity met the requirements of specification for material MW 01. Given the insufficient quality of the material was not the primary cause of the defect, it was recommended to eliminate the occurrence of potential notches in the heat affected zone through slightly planning of the weld collar.

Abstract: Lifting equipment is an important part of metallurgical operations as it ensures continuity of the processes between technology nodes. Ropes are an integral part of the lifting equipment. Their quality and good conditions are a guarantee of the proper functioning of equipment, especially of its security and, therefore, the control of ropes is an important part of the operation of cranes. Malfunction of ropes can be caused by several factors and may occur for several reasons. A common cause of damage to ropes in operation is wear, but sometimes premature damage or failure of rope strands occur. Occasionally, it happens that the rope comes into contact with an electric trolley and consequently, the running short-circuit current melts the rope. This paper is aimed at the damage itself, but also a potential impact on the changes to the structure and possible changes of mechanical properties of ropes in the area struck by ​​electric discharges.

Abstract: Structural steel grades S235JRCCu+N and S355JRCCu+N with 0.25 – 0.35% copper content are intended for products with increased corrosion resistance. They are used for railcar and container bodies. The low copper alloying strongly increases the material life-time in the conditions of atmospheric corrosion. Sheets with fine sharp needle-type bodies on both sheet surfaces were delivered for the analysis. As per the customer information, they were present on almost whole length of on cast strips. Considering the steel copper content, it was assumed that the improper steel alloying was the reason of imperfection. Chemical composition results didn’t confirm this expectation. Metallographic analysis should determine the imperfection origin and decide if copper presence was the primary factor of it. The analyses using SEM coupled with EDS analyzer have shown that the sliver-type defects, few millimeters long bands of ductile iron oxides, were present in very small material depth of about 50 – 150 μm in the analyzed material The bands were formed in more layers parallel to the sheet surface; their thickness was similar to the thickness of surface scales. Oxidic material under the sliver had the same micro-morphology and chemical composition like the scales on the sheet surface. Very fine globular copper particles were randomly and very sporadically present in the layer of ductile oxides in slivers and also in surface scale. Besides this, the material in defect area was chemically homogeneous and no particles were found in the layer boundaries. Copper particles were not detected in the steel basic material; the copper was dissolved in the matrix. The nature of imperfection was typical for V-shaped slivers, caused by overheating sheet surface in reheat furnace. After revising technical conditions of reheat furnace operation, this imperfection type was suppressed.

Abstract: Submerged Entry Nozzle (SEN) are usually made from a combination of carbon bonded alumina, zirconia and magnesia with graphite, which have been found to give the best response to the extreme demands of thermal shock, corrosion and erosion resistance. SEN brittle fracturing and clogging is a serious productivity and quality problem in continuous casting. The SEN critical area of failure is the steel flange where the SEN neck is fixed. Thermal shock or mechanical stress may cause cracking, which can result in premature failure. Preheating on optimal temperature before SEN installation is the standard procedure. Thermal stress effect on fracture behavior was done by analyze of material fractography, morphology, macrostructure and microstructure.