TRIBOMODEL STUDY REGARDING THE BEHAVIOUR OF THE NICKEL COATINGS ON A COPPER SUPPORT

Transcription

1 9 TRIBOMODEL STUDY REGARDING THE BEHAVIOUR OF THE NICKEL COATINGS ON A COPPER SUPPORT Radu Boiciuc S.C. Uzinsider Engineering S.A. Galaţi ABSTRACT This paper contains series of trials at conventional temperature conditions regarding the behaviour of the nickel coating on a copper support, in order to use them for hardening the mould plates surfaces from the continuous casting of steel. It presents the tests for a Hertzian contact between a ball-shaped head and the samples made of copper and copper electrochemically coated with nickel in two different layers. This paper presents the first stage of a more comprehensive study, also including the tribological behaviour of various coatings on other support materials. The paper describes the test stand together with the used methods and the experimental results obtained. These tests aimed to point out the moment of transition from the elastic range to the elasto-plastic range at the contact between the ball-shaped head and the sample. The experimental tests shown that, on the two nickel-coated samples, the coated layer presents almost the same tribological characteristics up to a value of Hertzian pressure σ k of about.00mpa. Above this value, on the sample with a nickel-coated layer of 150µm in thickness, the plastic deformation process is intensifying. KEYWORDS: tribomodel, transition from elastic to the elasto-plastic, nickel coatings on copper support. 1. INTRODUCTION The necessity of increasing the hardness of the mould plate surfaces from the continuous casting of steel, requires performing coats using materials with high values of hardness. The literature contains several data related to the heat transfer inside the mould [1, ], as well as the thermo-mechanical behaviour of the copper plate [3]. The tribomodel featuring of the hard coats is to unfold in three distinct stages: a. The study of the transition from the plastic range to the elasto-plastic range on the contact between the ball-shaped head and the sample; b. The study of the heat transfer in case of using various coats; c. Samples tests performed in a regime close working conditions pattern.. MATERIALS, EQUIPMENT AND EXPERIMENTAL METHOD In view of determination the elasto-plasic transition of the materials intended for manufacturing the mould plates, the following parallelepiped samples (85x45x8 sqm) were executed: of copper and of copper coated with nickel in two different layers of 150 µm and 700 µm. In order to ensure a comparable roughness related to the one of the ball-shaped head in contact, the samples were smoothed to a value R z of 0.8 µm, on etching paper up to a granulation of 800. Before performing the test, the sample surface has been degreased with alcohol in order to obtain suitable conditions of dry friction. A ball-shaped head of 10mm in diameter, made of RUL 1 steel, volumetric hardened and tempered to a hardness of 63.3HRC (HV 49 of 7,800 MPa) was used as a penetration element. After lack test, the ball-shaped head was replaced and degreased. Figure 1 presents the equipment s scheme also used in other research [5]. The contact load is based on the progressive thrust of the ball-shaped penetration element in the plane semi-infinite space of the material, by its forced movement on the inclined plane slope of the sample. The penetration element 16 locked at the lower end of the column 9 is able to perform a vertical motion on the ball bearings in the mounting support 10, stiffened on the horizontal column 7.

2 30 The feed motion of the penetration element is performed by the horizontal column 7, the ball guide 8 and is carried off with the aid of the screw 5-nut 6 system, by a motor-generator set of alternative current 1-reductor 3. The feed rate is adjusted by means of the change gears 4 within the limits from 0.15 to 0.6 mm/s (a feed rate of 0.15 mm/s was used). The slow movement of the penetration element produces suitable conditions for a quasi-statistical stress of a contact. The flat sample 17 is fitted under an angle of as against the movement direction of the penetration element through a support interlocked with the slide 18, which is able to glide on the ball guides 1 related to the working bench. At the gliding of the penetration element, the conventional load to the contact surface is progressively increasing and it determines an initial elastic deformation of the material, followed-up by a plastic deformation, locking in the end. The vertical thrust strain is measured with horseshoe dynamometer 11 in contact by means of a ball joint fitted with a vertical column holding the penetration element. The tribomodel deformation is possible to be measured with the dial gauge 1 using a strain gauge bridge bonded to a dynamometer, on the two plane outer surfaces placed on 45 related to the strain direction. The elastic deformation of the dynamometer is changed over into an electric signal, picked-up and processed on the channel of the electronic tensometer 14 of N30 type and recorded on the X-Y autoplotter (13) of H303 type. The mechanical strength impeded to the advance of the penetration element shell be measured with the aid of the lamellar dynamometer 0 being in contact with the sample holding slide. The dynamometer is fitted with a dial gauge 19 and a strain gauge bridge whose signal is processed by the electronic tensometer and recorded by means of the X-Y autoplotter. 3. EXPERIMENTAL RESULTS The experiments performed consist in a study of certain penetration element impressions in the direction of the longitudinal axis of the samples in the sliding conditions without lubrication, with the feed rate of the penetration element of 0.15 mm/s and the value of the vertical thrust strain within the limits of P from 10 to N. For each and every sample, three tests in a row have been performed, the penetration element impressions being codified as follows: - copper samples: Cu-1, Cu-, ; - copper samples with a nickel coated layer of 700 µm in thickness: Ni-1, Ni-, ; - copper samples with a nickel coated layer of 150 µm in thickness: Ni-1*, Ni-*, *. During each test, within constant period of time (10 s) the following values were experimentally determined: h V - the horseshoe dynamometer deformation in the vertical plane; h H - the lamellar dynamometer deformation in the horizontal plane; ε V the specific material deformation in the vertical plane; ε H the specific material deformation in the horizontal plane. As a result of conversion these values, the P (vertical thrust strain) and Q (horizontal advance strength force) forces have been determined. Fig. 1 Equipment used for studying the material behaviour at plastic deformation 1-engine; -belt gearing; 3-reductor; 4-change gears; 5-screw; 6-nut; 7-horizontal column; 8-linear ball guidance; 9-vertical column; 10-support; 11-horseshoe dynamo-meter; 1-dial gauge; 13-adjuster; 14-electronic tensometer; 15-X-Y autoplotter; 16-penetration element; 17-sample; 18-slide; 19-dial gauge; 0-lamallar dynamometer; 1-linear ball-bearing; -working bench; 3-bed frame.

3 In order to evaluate the supporting power in conditions of a contact between a ball-shaped head and an inclined plane slope of the tested material, depending on the P and Q forces obtained, the normal (F n ), respectively tangential (F t ) forces related to the movement direction of the penetration element, in distinct moments of the test were determined [4]: F n =P cos +Q sin, (1) F t = P sin + Q cos () The value of Hertzian pressure and total deformation depending on the F n force, in various moments of the test, were calculated. The friction coefficient in the elasto-plastic stress range for different contact loads (F n ) has been determined. The plastic deformation of the plane semiinfinite space has been measured on a Surtronic3+ (Taylor Hobson ) electronic roughness tester with a TalyProfile soft. 4. INTERPRETATION OF RESULTS Using an excel programme for processing the experimental data, the variation of the friction coefficient f depending on the normal force F n value (fig. ), as well as the variation of the vertical thrust strain P with the time (fig. 3) was outlined. In order to point out the moment of the elastoplastic transition of these two analysed materials (copper and layer of nickel), the influence of the Hertzian pressure (σ K ) upon the total depth h of the impression profile (figure no. 4) was plotted. Comparing the variation curves pictured in figure no. 3 P=f(t) for the three tested samples, it comes out that, in case of the copper-made part is outlined a more rapid increase of the advance strength, i.e. a more rapid penetration of the material in the plastic ranges emphasised. The macroscopic analysis of the penetration element impressions as well as of the transverse surface analysing graphs point out that, in the applied strain conditions, the copper sample presents a plastic deformation visible at a minimum contact load, while the nickel coated samples have, initially, an elastic behaviour. In case of copper, the aspect of the transverse surface analysing graphs indicates a flow along the penetration element of the compressed material, as early as the plastic deformation begins. On a maximum load, emphasised, sharpen and increased heights were created and placed near the penetration element. 0.7 Cu Cu- 0.5 f Ni-1 Ni- Ni-1* Ni-* * Fn [N] Fig. Variation of the friction coefficient depending on the normal force.

4 P 600 [N] t [s] Cu-1 Cu- Ni-1 Ni- Ni-1* Ni-* * Fig. 3 Variation of the vertical thrust strain P with the time. h (microns) Ni-* σ k [M Pa] Fig. 4 Influence of Hertzian pressure (σ k ) upon the total depth h of the impression profile. On the other hand, it comes out that, on the nickel coated samples, the emission of the compressed material is much more diminished, the increased height aspect being maintained (this being connected and extended towards the contact adjacent areas). In the same time, notice shell be paid to the fact that, the diameter of the contact spot depends on the type of the studied material, as follows: - in case of copper, the profile height h and the a radius are of great values; - for nickel coated surfaces, the impression of the contact spot is characterised by a small depth and a small width, as well, mainly for the sample with a layer of 700 µm in thickness. The graphical plotting of total impression depth h versus Hertzian pressure (fig. 4) outlined the supporting power of the analysed materials, namely: - at the copper made sample a maximum Hertzian pressure σ Kmax of 389.7MPa, results, value being obtained in an early stage of the test (for l of 1.3mm); - at the copper-made sample with a nickel coa-ted 700µm in thickness, results a maximum Hertzian pressure σ Kmax of 0MPa, value achieved for a depth l of 10.4mm from the moment the test has began; - at the copper-made sample with a nickel coated 150 µm in thickness, results a maximum Hertzian pressure

5 of.15 MPa, value achieved for a depth l of 10. mm from the moment the test has began. 5. CONCLUSIONS Further to an attentive analysis of the two nickel coated samples in discussion, notice shell be paid to the fact that, the coated layer presents almost the same tribological characteristics up to a value of the Hertzian pressure of about.00 MPa. Above this value, on the sample with a nickelcoated layer of 150 µm in thickness, the plastic deformation process has been intensified. After a thorough study of the longitudinal surface analysing graphs, it comes out that, on the nickel coated samples, the plastic deformation process has been delayed, the elasto-plastic transition being achieved only for Hertzian pressure values much higher than in copper case were. Analysing the figure no., it is established that, all samples shown a common behaviour, namely, the period of flattening the plane semiinfinite space asperities causes an increase of the friction coefficients with the normal force, followed-up by a further stabilisation of them. Therefore, comparing these three tribological systems, it comes to the conclusion that the tribological property set (the impression dimensional characteristics, supporting power of the material, thrust strain value and friction coefficient) has much higher values in case of the nickel coated samples than of those made only of copper. Many thanks I owe to professor dr. eng. Ion Crudu for all the scientific support and high professionalism guidance he has given to me in order to materialise this work. REFERENCES 1. Huang, X., B. G. Thomas, and others, 199, Modeling Superheat Removal during Continuous, Casting of Steel Slabs. Metall. Tran B 3B(6), pp Thomas, B. G., B. Ho, and others, 1998, Heat Flow Model of the Continuous Slab Casting Mold, Interface, and Shell, Alex McLean Symposium Proceedings, Toronto. Warrendale, PA, Iron and Steel Society, :pp Thomas, B. G., Parkman J. T., 1997, "Simulation of Thermal Mechanical Behavior during Initial Solidification." Thermec 97 Internat. Conf. on Thermomechanical Processing of Steel and Other Materials, Wollongong, Australia, TMS. 4. Popinceanu N., Gafiţeanu C., Diaconescu E., Creţu S., Mocanu D. R, 1985, Probleme fundamentale ale contactului cu rostogolire, Editura Tehnică, Bucureşti. 5. Spânu C., 00, PhD, University Dunărea de Jos of Galaţi.