J.P. Bricout

Seizure behaviour of manganese phosphate coatings according to the process conditions

Abstract To analyse accurately the performances of some manganese phosphate coatings, it is proposed to use a friction–seizure test procedure to evaluate the effective phosphate bonding and the lubricant effects. The coating seizure resistance is analysed mainly by considering different temperatures, concentration and dipping time values for the main steps of the whole phosphating process, i.e. degreasing, refining and phosphating. The measures, that are found to be easily reproducible and very sensitive to the treatment parameters, have allowed an easy and objective optimization of the manganese phosphating process. Metallurgical variables, which affect the seizure resistance of manganese phosphate coatings, are also studied by considering the influence of carbon content of a set of plain carbon steels.

A new friction test procedure for the improvement of drawing and similar processes

Abstract The development of drawing and similar room-temperature metal-forming processes is strongly related to efficient control of the contact and of the friction factors or coefficients. At this time, it is difficult to achieve accurate management of these factors and consequently there is major interest in new friction tests and analyses in most industrial plants. The ratios between the friction stress and the contact pressure in the contact zones between the tools and the workpieces are usually small in modern processes, say in the range of 0.05 to 0.18: these values increase when coatings and lubricants become ineffective and when wear appears on the tools. It is now important not only to secure the lowest friction stress but also to have accurate descriptions of the contact-zone behaviour throughout all the sequences of the forming process. For the above purpose, a new multi-step testing procedure has been developed, which consists of simulating the cumulative deformation of the workpiece contact-surfaces by the use of an inclined indenter which takes the place of the tools. A testing step is achieved with two indenter movements: in the first movement the indenter—the height of which is chosen so as to produce significant localized deformation—moves at the interface in the tangential direction, after which the distance between the initial surface and the deformed surface of the workpiece is typically in the range of 0.2 to 0.8 mm; the indenter then returns to its starting position without contacting the workpiece and moves again along the trajectory as previously defined. Successive steps, as described, are achieved to simulate at the workpiece surface what actually occurs during each forming sequence in the first movement and to measure updated friction-factors or coefficients in the second movement. Tangential forces, normal loads applied by the indenter to the workpiece surface and the temperature of the indenter are recorded during the movements. Thermo-mechanical models of the plastic flow occurring in the vicinity of the indenter are developed for the calculation of accurate friction factors/coefficients, to be implemented in friction constitutive equations. Experiments have been performed on 1017 steel workpieces with a quenched D4 indenter roughly polished to represent a tool damaged by abrasive wear. To date, phosphate coatings and lubricants have been used. Examples of analyses give a scheme for the determination of the mean isotropic friction factor or coefficient, defined for the former as the ration of the tangential stress to the shear yield stress, and for the latter as the ratio of the tangential stress to the contact pressure. It is shown that the proposed procedures and analyses can be used for most industrial workpieces which are drawn or formed in a similar manner. Friction factors/coefficients and new friction constitutive equations, when more accurate interface models are necessary, can thus be obtained.

New testing procedure of zinc phosphate coatings involved in cold forging of cylindrical steel billets

Abstract Cylindrical cropped billets are often used in the cold forging plants at the beginning of processes and the same cylindrical billets are involved as test specimens in the proposed procedure. On the experimental device, the specimens rotate whilst a semi-cylindrical indenter exerts a constant normal load on it. Tangential force values are measured all along the rotation time. Test adjustable parameters are the normal load exerted, specimen rotation speed and indented radius of curvature. The optimum values of the test parameters are defined in considering the pickup occurrence. Then, the influence of treatments parameters which correspond to different phosphate coatings and lubrication states are analyzed in terms of friction tangential force versus sliding velocities and sliding displacements. The influence of three treatments parameters, acid pickling, phosphating bath and lubricants are enhanced by using the testing procedure for typical 1020 and 4142 carbon steel specimens. Significant tangenital friction force versus sliding length curves are obtained. The proposed test and procedure are able to determine critical friction curves and allow the secure management of both coating treatments and cold forging sequences.

New real-time test for prediction of zinc phosphate/stearate coating breakdown: optimal stearate settling parameters for steel billets in cold forging

Abstract The test discussed here quickly reproduces interface pick-up occurrence and degradation of phosphate coatings wich have been dipped in stearate solutions. It involves a rotating cylindrical specimen directly cropped from bar stock of forging plants and phosphate/stearate processed. A constant load is applied on the specimen coating by means of a profiled indenter. Tangential friction force values are measured throughout the rotation time. The stearate settling parameters are analysed in terms of pick-up sensitivity. It is shown that low friction stress and phosphate/stearate coating reliability are mainly due to zinc stearate formation and stearate settling conditions. Secure rules for stearate settling are proposed from the described reference experimental data. The test is found to be a good support for real-time management of phosphate/stearate baths.

Finite-element analysis of the initial stage of the indentation-rotation test for phosphate and stearate coatings

Abstract Well fitted to determine the behaviour and seizure of thin coatings, the test involves a rotating cylindrical specimen on which a small profiled indentor exerts a non-uniform normal pressure. The normal load remains constant as the time of rotation increases. The friction-force values are related to the behaviour of the coatings and when the interface become damaged generally, the friction force increases strongly. The rotating time required for these observations is related to the initial stress and strain distribution in the vicinity of the contact surface when the normal load is applied on the indenter, the spcimen being motionless. For this identatition phase, the distribution of stress and strain are obtained by means of a refined localised finite-element model which takes into account the mean thickness and the mechanical properties of a typical zinc-phosphate coating first, and then of a typical bilayered zinc-phosphate and zinc-stearate coating second. It is found that the slider-specimen contact area and the stress and strain maps in the contact vicinity change significantly from the virgin steel specimens to the bi-layered specimens. The effective stress decreases whilst the effective strain increases, the strain maps being more perturbed by the coatings than the stress map. Particular attention is focused on the local shear stress which is related to the performance of the adhesion coating.

A Comparative Study of Plasma Sprayed Coatings on Railway Brake Discs

Abstract In the railway context, increasing the number of trains in service relies in the development of new equipment. This objective requires increased peak speeds at the expense of stronger motorisations. The axle loads are thus heavier, especially for the new double-decker trains. These parameters have significant effects on the brake system in terms of size, non-suspended masses, and therefore energies to be dissipated. The constituent materials of the friction brakes of some railway equipment (full non-ventilated steel alloy discs for the TGV, for example) have thus reached their technological limits in terms of energy to be dissipated for fixed mass and size. It is therefore necessary to find more powerful braking devices using new materials. The Laboratoire d’Automatique et de Mécanique Industrielles et Humaines (LAMIH) has therefore decided to focus its research towards high energy full discs. This approach consists of coating the steel discs with low thermal diffusivity friction materials that have very strong and stable mechanical and tribological characteristics at high temperature. Using blown thermal plasma, the LAMIH has already tested reduced scale C38 discs coated with NiCr–Cr3 C2 cermet facing an Al2 TiO5 pad. Henceforth, studies relate to cobalt or nickel based superalloys obtained by the transferred plasma process with a view to making a reliable coating–substrate bond. Performance is evaluated in terms of friction factor, wear, contact temperature, and reliability.

Influence of cleaning pretreatments on tribological properties of zinc phosphated steels

Among lubrication processes for cold forging of steel, phosphating is the most common. Depending on contact between piece and phosphating solution, the success of this treatment depends on steel surface preparation. Despite this obvious fact, the influence of cleaning pretreatments such as degreasing and acid pickling are seldom considered from a mechanical point of view and specially from a tribological one. Through a tribological test, the preparation of the surface of steel billets to be zinc phosphated and soaped was studied. Having shown the importance of degreasing, this study tries to connect the parameters of warm acid pickling, such as bath composition and dipping time to the tribological performances of the coated part.

A new tensile test on in situ solidified notched specimens: hot ductility analysis of continuous casting steels

Abstract In order to securely predict the hot ductility and prevent hot-cracking propensity of continuous casting steels, a new tensile test device available for in situ solidified notched specimens is proposed. The test analysis makes it possible to define the thermomechanical properties directly after solidification, without the usual cooling step down to room temperature and the following one to the test temperature. One of the original feature of the test consists in the shifting of the shrinkage cavity toward the unstrained region of the specimen and this allows accurate measurements in the notch non-segregated zone as well as secure observations of fracture types. Tests of two reference steels differing by their solidification mode show that the temperature path is a very significant parameter of the hot ductility.

A new device for upsetting tests of steels at high temperatures

Abstract Comprehensive analysis of forming processes requires reliable and rapid determination of stress-strain relationships. For that purpose, the upsetting of steels is achieved on a new device with cylindrical specimens of diameters in the range from 6 mm to 10mm. The upsetting dies are made of two cylinders of alumina on which are placed 3 mm thick carbide plates. The specimen and the dies are inside a transparent and almost airtight box connected to an argon tap. The specimen is heated by a three-spire inductor and the temperature is measured with an infra-red sensor. Two typical procedure are presented: the first involves heating the specimen to the test temperature and then test achievement; the second involves heating to a temperature higher than that of the test and cooling to the test temperature. Upsetting loads, cylinder current-height, and the ratio between equatorial diameter and contact surface diameter are saved via a RS 232 line. Specific software, written in Basic, has been developed for the computation of the flow stress, taking into account friction and the contact conditions between the specimen and dies, assessed from the barrelling of the cylinder. Experiments have been performed on 1015 continuous-casting steel with the heating-only procedure and on 1010 also continuous-cast steel with the heating-cooling procedure. The stress-strain curves are given and the flow-stress equations are defined for typical problems; large effective strain from 0.2 to 0.8 in the first case, moderate effective strain from 0 to 0.15 in the second case. The accuracy is found to be very good: the variation of measurements for the same test conditions never exceed 4%. The testing procedure and the device are very convenient for daily analysis: each test may be carried out in less than three minutes.

Analytical and finite-element analysis of insitu solidified specimen upsetting

Abstract The behaviour of solidified steel in the continuous casting process is determined by means of a laboratory-scale upsetting test of specimens that are obtained by fusion and in-situ solidification in the test machine. From the curves of upsetting load-cylinder height decrease and the initial and final specimen dimensions, the stress-strain-strain rate relationships and the related equatorial stress-strain paths are calculated using an analytical model, the performance of which is estimated by comparing the results with those given by non-linear finite-element models. Reference experiments are achieved on in-situ solidified A 283 steel cylinders at 1050°C; stress-strain-strain rate relationships, equatorial strain and stress paths being determined. The analytical results give rather good estimates, especially for strain and strain rate.