Materials Science Forum Vol. 782

Abstract: In this work a failed rotor shaft of φ 7.2 mm diameter manufactured from constructional steel 44SMn28 (1.0762) was examined. Fracture splitting into 2 pieces in the threaded part of the shaft in a direction parallel to the longitudinal axis occurred. The failure was induced by mechanical forces in the threading process. The failed shaft was examined on a metallographic and scanning microscope. It was found that the failure was caused by excessive accumulation of MnS inclusions in an unfavourable banded formation.

Abstract: The final quality of flat products is directly related to surface and internal quality of continuously cast slabs. This aspect is very important especially in case of IF and deep-drawing tin grades, in which even the smallest defects can cause problem in the final processing of the material, especially during forming by stamping, when the material fails. This work is devoted to the analysis of the surface defect of star crack type. The presence of defect was visually detected after scarfing process during removal of surface layer on the small radius side of the slab. This type of defect occurs particularly in cases, when steel is contaminated by copper in the process of continuous slab casting. In view of the copper melting point (1083°C) and the surface temperature of the cast slab shell at the mold exit (about 1200°C) in the process of steel cooling at continuous casting, Cu penetrates along the boundaries of primary austenitic grain, or separate particles are precipitated at the grain boundaries. Initial cracks are formed in the area of primary cooling in the mold. Thermal and mechanical stresses acting in the surface area of slab shell when the slab passes thorough the rollers of secondary cooling cause gradual expansion of crack defects along boundaries of grains contaminated by copper.

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: The paper describes the rail failures occurring repeatedly in the same distance from the rail butt weldment. The results of this investigation, especially fractographic analysis of the fracture surface of rail, evaluation of its macrostructure and microstructure, EDS analysis and hardness measurements revealed that transverse crack initiated in the foot of rail gauge and propagated by fatigue mechanism until the final cleavage fracture.

Abstract: The failure of a spherical-roller bearing consisting of spherical rollers, outer and inner rings made of 100CrMn6 steel was studied. After a trial test, in which several spherical rollers were completely destroyed and a long continuous radial crack appeared in the outer ring, pitting degradation was observed on all bearing parts. The bearing components were studied (separately) by means of light and electron microscopy. The chemical composition was estimated by means of glow discharge optical emission spectroscopy. X-ray diffraction was utilized for qualitative and quantitative phase analyses. As a representative of mechanical properties, the hardness was also measured.

Abstract: This paper presents the cavitation erosion research through the stationary specimen method of the X20Cr13 martensitic stainless steel. Actual research involves a number of 4 different tests for this material and the comparison between these experimental tests is highlighted by the images and graphs forwarded in the paper. All research was done using vibratory apparatus composed from electro-acoustic piezo-converter, acoustic transformer, a titanium sonotrode and the ultrasonic generator DG-2000. The mass loss of the martensitic stainless steel samples was measure with special digital balance.

Abstract: This paper presents the cavitation erosion research of the antifriction alloy YSn83 regarding on its behavior in laboratory. The antifriction alloy YSn83 is not subject to cavitation erosion in its practical applications, as is happening in reality at the materials used in hydraulic turbines, valves, piping; but the experimental tests for this material, highlight the laboratory research on its behavior by means of images and graphs in this paper. The laboratory tests were performed in accordance with the standards G32-92 (Standard Method of Vibratory Cavitation Erosion Test) and G32-10 (Standard Test Method for Cavitation Erosion Using Vibratory Apparatus).

Abstract: The paper presents ample researches on cavitation erosion of two charges of stainless steel type X3CrNi13-4 on a cavitation stand, through the stationary specimen method. The results of the experimental trials are highlighted in tables and in graphics regarding the reproduction of mass losses and the cavitation erosion rate according to time. The images of the microstructures before the cavitation test and the macrostructures after the cavitation test of the two charges of stainless steel type X3CrNi13-4 are also presented in this paper.

Abstract: The paper presents the cavitation stand used, the work methodology, the trials realized on specimens of gray cast irons, with flake graphite and spheroidal-graphite, which were tested for cavitation erosion during different periods of time of the cavitation attack. Finally, the conclusions were interpreted on the basis of comparative graphics realized at the macro and microstructures level obtained for these two types of cast iron. Of the two types of gray cast iron, the nodular cast iron presents a better cavitation erosion resistance.

Abstract: A nodular cast iron (German designation GGG) sewer pipe leaked within a short time of use (13 years). The pipe was destroyed by sulphuric acid induced corrosion. Microbes form hydrogen sulphide and further oxidation leads to H₂SO₄. Sewer pipes are coated inside with a protective alumina cement layer, which was attacked firstly, followed by a corrosion of cast iron. Spongiose is a corrosion of cast iron, thereby ferrite is destroyed and graphite and cementite are remaining unattacked. Spongiose is observed commonly in grey cast iron, but is unusual for nodular cast iron. The alumina cement coated GGG tube, the corrosion products and their different graduations were investigated. In the corroded spongiose-layer the crystalline phases graphite, cementite, hematite and magnetite were identified. In the rust-layer deposits of lepodocrocite and goethite are the main phases but also sulphur, gypsum and other sulphuric compounds were observed.