ON THE MICROCRACKS NUCLEATION AND GROWTH IN THE TECHNICAL CHROMIUM LAYERS COATED ELECTROLYTICALLY OVER THE SURFACE OF HEAVY-DUTY MACHINE PARTS

Transcription

1 ON THE MICROCRACKS NUCLEATION AND GROWTH IN THE TECHNICAL CHROMIUM LAYERS COATED ELECTROLYTICALLY OVER THE SURFACE OF HEAVY-DUTY MACHINE PARTS Tadeusz Hryniewicz Tadeusz Nykiel TECHNICAL UNIVERSITY OF KOSZALIN, Mechanical Engineering Department, Racławicka 15-17, PL Koszalin, Poland Abstract The work is concerned upon the results of metallographic, X-ray diffraction and microhardness studies of a technical chromium layer coated electrolytically on the surface of machine parts of big size and weight. The investigation was performed to find out the reasons of the premature wear and destruction of the technical Cr coating which repeatedly occurred on the heavy-duty parts operating in one of the furniture enterprises. As the result of studies carried out it was stated that the hard technical chromium layer is characteristic with the non-uniform structure both as to the chromium grains size and distribution. The layer is also characteristic with the considerable dispersion of microhardness. Moreover, during the machine part operation the microcracks are originating and due to their growth the network of microcracks may be locally formed. The study results show that the sites of microcracks nucleation are chromium grain boundaries of bigger sizes and microcracks that emerged earlier, i.e. these ones of big stabilized crevice widths. In the first initial phase the microcracks nuclei grow in perpendicular direction to the chromium grain boundary or microcrack, whereas further more advanced growth may proceed in straight line or curvilinearly with a big radius of the curvature. The characteristic phenomenon in the microcracks of very narrow gap, difficult to reveal during etching operation, is the appearance of singular bigger sizes of crevices of the shape being lens-like, circular and even rectangular. 1. INTRODUCTION This work is focused on the experimental study of technical chromium layer coated electrolytically on the surface of steel substrate in the big and long constructional element. The investigation was performed to find out the reasons of the premature wear and destruction of the technical Cr coating which repeatedly occurred on the heavy-duty parts operating in one of the furniture enterprises. Earlier studies have shown [1, 2, 3] that bright and shiny/glittering technical chromium coatings are characteristic with the occurrence of microcracks network and pores and these phenomena originate in the manner of chromium deposition during the process of electrolytic coating. Electrolytic chromium may be deposited on substrate as compact HCP (hexagonal close-packed) or regular BCC (body-centred cubic). Chromium deposited in the HCP lattice is unstable and will tend to change its structure into stable chromium of BCC lattice. This change leads to decrease the volume of layer and that in turn means arising of high internal stresses in the layer that are released through the microcracks formation. Hydrogen present in the layer favours the microcracks originating. There are also other theories presented in the literature attempting to explain the mechanism for microcracks originating. Number of microcracks and their depths are dependent upon the conditions of electrolytic process of chromium deposition. It was shown in the previous authors works [4, 6] that the distribution of microcracks number on the length of 1 mm in the chromium layer of a big thickness is close to symmetric with the decreasing number of microcracks in depth when approaching the - 1 -

2 substrate direction. In the studied hard technical chromium layers the increased number of microcracks at the top of the layer results from the exploitation pressure. The chromium layers coated over big cylindrical parts were operating for some period of time under industrial conditions. In the papers [5, 6] some characteristic systems of microcracks course with intersections were presented. The purpose of this work is to reveal some characteristic features of the chromium layer structure, with nucleation and growth of microcracks as well as the determination of microhardness distribution in the surface zone being directly exposed to the exploitation pressure. 2. MATERIAL AND STUDY METHOD Metallographic microscopic studies were carried out on the longitudinal and transverse crosssections prepared mechanically using abrasive papers of the grades from 150 up to 2500 with very intensive water-cooling. Mechanical polishing was performed using acquaeous Al 2 O 3 suspension. Specimens of the chromium layer were etched using reagents of composition: 3 cm 3 HNO cm3 HCl cm 3 C 2 H 5 OH, and 1 cm 3 HNO cm 3 HCl + 3 cm 3 glycerine with the samples being warmed up to the temperature of 80 C before etching. Different etching times were applied dependent on the required effects. Microscopic photographs were done using Epityp-2 and Neophot 2 microscopes. X-ray diffraction studies of the chromium layer were conducted on the DRON-2 diffractometer. Diffraction patterns were done using cobalt radiation. The lamp filament current was 20 ma. The microhardness studies of the technical chromium layer were performed using the Hanemann microhardness tester of type UPM mph 100 and Epityp-2 microscope by applying load of 40 G (about 400 N). Microhardness was measured/tested on the working chromium layer after etching its surface slightly to reveal places of microcracks occurrence. Microhardness distribution was determined based on 200 measurements. 3. INVESTIGATION RESULTS X-ray diffraction patterns of the studied chromium layer are presented in Fig. 1. As the result of metallographic microscopic studies carried out it was found that the technical chromium layer of about 150 µm thick, electrolytically coated over the steel substrate, revealed nonuniform structure both as to the chromium grain sizes and their distribution (Figs. 2, 5a,b). Fig. 1. Diffraction patterns of the studied technical chromium layer with the reflexes (110) and (211) done with the accuracy of

3 The metallographic microscopic studies of the transverse cross-sections reveal that the process of electrolytical deposition of chromium varies and depends on the substrate surface roughness. The smoother the substrate surface is, the more uniform chromium deposition (Fig. 2a). On the rough surface the process proceeds perpendicularly to the projection surface with a distinct tendency to form spruce-like/fir structure (Fig. 2b). Fig. 2. Chromium layer structure, transverse cross-section; etching reagent HNO 3 + HCl + glycerine: (a) smooth substrate surface, (b) substrate of uneven surface Besides, another characteristic feature is the presence of small number of individual microcracks appearing at the layer s working surface (Fig. 3a) and the areas adjoining to the substrate (Fig. 3b). These microcracks proceed along the stepwise line whereas in case of Fig. 3. The view of longitudinal microcracks in the technical Cr layer. Transverse crosssection, sample surface etched using the reagent of HNO 3 + HCl + glycerine - 3 -

4 microcracks proceeding at the substrate areas the following steps are parallel to the substrate surface. The presence of microcracks at the working surface of the chromium layer results from the specific mechanism of wear of the chromium layer whereas the second type of microcracks probably results from the improper dehydration of the Cr layer and the substrate itself. It is known that the presence of hydrogen in the layer and substrate leads to arising the high stresses/tensions and they are the reason of microcracks formation. Also the stresses arising in the chromium layer being the result of action of the high external forces (service, exploitation conditions) and the big weight of the construction element itself favour the origination of microcracks. As the result of conducted measurements of microhardness it was found that the studied technical chromium layer in the area of direct exposure to abrasion/wear is characterized with a high discrepancy/dispersion of hardness of 582 µhv 0.04 what correlates with the previously stated big inhomogeneity of the structure. Besides, the consolidation/hardening phenomenon effected from a relatively high hardness of the zone occurs, resulting from the exploitational conditions/pressure. It results from the computed relative frequencies that the biggest and the most frequent group is microhardness of 1051 µhv The diagonals length distribution indicates the left-side asymmetry what means the domination/majority of diagonals above the arithmetic average, and in relation to the counted microhardnesses means the prevailing of lesser microhardness values than the average ones (Fig. 4). 40 µhv 0.04, min = 772 Σmp = 200 points Number of chords µhv 0.04, max = 1354 µhv 0.04av = Lengths of chords, µm Fig. 4. Empirical distribution of the chord lengths of indentation marked in the obtained chromium layer using a Hanemann microhardness tester The metallographic microscopic studies of the technical chromium layer carried out on the longitudinal samples indicate the microcracks in the form of lattice/net to occur. Based on the microscopic observations of many samples one may conclude that the places/spots of microcracks nucleations are: - the area boundaries of the shape close to polygonal (Fig. 5a), - the areas of the shape close to polygonal in which very fine chromium grains occur (Fig. 5b), - the primary developed/expanded microcracks of a stabilized crevice width (Fig. 5a,b)

5 Fig. 5. View of microcracks in the technical chromium layer; longitudinal cross-section, sample surface etched in HNO 3 + HCl + glycerine In the initial stage the microcracks nuclei develop in the direction nearly perpendicular to the area boundary of polygonal shape or to microckack whereas the further growth/expansion is curvilinear of big value of the curvature radius. The primary microcracks are not heterogeneous in character but with the characteristic striae(?)/stripes of the shape resembling the fatigue stripes (Fig. 5a,b). These striae may be used for the analysis of microcracks development. The phenomenon characteristic to the microcracks of very narrow gap, revealed after longtime etching, is the occurrence in them some individual gaps of greater dimensions. Their shapes may be lens-like, squared and/or rectangular (Fig. 5b). Fig. 6. The concentration areas of intersecting microcracks in the technical Cr layer The places of concentration of the microcracks intersection are the areas in the chromium layer of the shape nearing polygonal in which very fine chromium grains occur (Fig. 6a,b). Multiple studies of microhardness testing of these areas have shown that their microhardness essentially do not differ from the microhardnesses of the neighbouring areas. An example of the measurements area is presented in Fig. 6b

6 4. CONCLUSIONS Based on the investigation carried out on the technical chromium layer coated electrolytically over the steel substrate of a huge big-dimensional cylindrical constructional element the following conclusions may be formulated: 1. Microcracks occurring in the chromium layer are perpendicular to the substrate, with very few of them situated longitudinally close to the substrate of stepwise character. Microcracks at the substrate result probably from inadequate dehydriding. 2. The microhardness distribution of the chromium layer directly exposed to the abrasion is approaching to symmetric with the zone characteristic of high dicrepancy in microhardness ranging from 772 up to 1354 µhv In the primary microcracks there are striae/stripes similar to the fatigue stripes whereas in the microcracks of very narrow gap, hardly appearing after etching, some individual of bigger dimensions gaps appear of the lens-like, rectangular, or squared shapes. 4. Microhardness of the areas with concentration of microcracks intersections does not essentially differ from the microhardnesses of the neighbouring areas. LITERATURE 1. BOCKRIS J.O M., KHAN S.U.M., Surface Electrochemistry A Molecular Level Approach, Plenum Press, New York-London, FLIS J., Corrosion of Metals and Hydrogen-Related Phenomena, PWN Polish Scientific Publishers, Warszawa, KUHN A.T., Industrial Electrochemical Processes, Elsevier Publishing Co., London-New York, HRYNIEWICZ T., NYKIEL T., Acta Mechanica Slovaka, Košice, 1999, Ročnik 3, s HRYNIEWICZ T., NYKIEL T., Proc. of the International Conference on Environmental Degradation of Engineering Materials, Gdańsk-Jurata, Poland, September, 1999, EDEM 99, vol. II, pp HRYNIEWICZ T., NYKIEL T., Proc. of International Conference on Fracture and Damage Mechanics, ed. by M.H. Aliabadi, Queen Mary & Westfield College, University of London, July, 1999, pp