J. Oudin

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.

A Finite Element Algorithm for Microvoid Nucleation, Growth and Coalescence:

A radial return algorithm is developed for large strain elasto-plastic struc tures in order to easily compute microvoid volume fraction rates. The basic feature subroutines are described in detail and forwards four typical problems are analysed: Q4 element tensile test, Q4 element compression test, notched cylinder tensile test and pipe bulging. The influence of microvoids volume fraction is clearly enhanced, mainly in regard to hydrostatic stress.

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 simulation of void nucleation growth and coalescence in isotropic standard elasto-plasticity: application to cold forging

Abstract Inside a basic large strain finite element framework, computation of void volume fraction is achieved in taking into account nucleation growth and coalescence in the material matrix. The material constitutive relation involves an elasto-plastic potential and the reference values of matrix parameters may correspond to some forged low carbon steel matrixes. Computations concern patch test of axisymmetric three node element first, collar test proposed in earlier experiments to define limit curves second and pipe bulging process third. It is shown that the three main steps occuring in the most severe processes can be accurately predicted if the initial matrix parameters are well known.

Finite-element analysis of the three-stage cold extrusion of steel cups

Abstract Three-stage cold extrusion of 1017 steel cups involves hobbing, backward extrusion and ironing. Computational finite-element meshes are defined: in the first stage, a unique mesh is used, in the second stage the mesh is R -refined with an adaptive geometric procedure, in the third stage the initial mesh is redesigned. An updated Lagrangian scheme is achieved with isotropic bulk elastic-plastic constitutive equations. A consistent tangent operator is calculated. Effective strain and hydrostatic stress, computed with the non-linear code AstriD , are shown for the hobbing and backward extrusion of 1017 soft annealed and zinc phosphate/sodium stearate coated workpieces. Punch load, loss of contact and height variation are given for the last ironing stage, batch production experiments agreeing very well with these predictions.

Sensitivity of Material Parameters in the Finite Element Computation of Microvoid Nucleation and Growth

A suitable range of microvoided material parameters is defined to perform a set of finite element computations. The sensitivity analysis is focussed on nucleation and growth of microvoids; the related modifications on microvoid volume fraction, effective plastic strain, hydrostatic stress, dimensions, and load are given through two reference models, the tensile test of a single axisymmetric Q4 element and the upset of a clamped cylinder. It is shown that refined analysis requires the determination of four supplementary material parameters, the initial microvoid volume fraction, the effective plastic strain at incipient nucleation, the potential nucleated microvoid volume fraction consistent with the inclusion volume fraction and the Gaussian standard deviation of inclusions distribution. Nevertheless, for large plastic strain occurrences, an accurate identification of the effective plastic strain at incipient nucleation and the standard deviation of inclusions distribution parameters is not necessary.

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.

Finite element analysis of hollow steel parts impact extrusion

Abstract The finite element model involves a three-field, velocity - dilatancy - hydrostatic stress, functional which solution in the discretized from is searched with a Newton Raphson algorithm. Contact and friction conditions are implemented with a penalty function. Visco-plastic damage is represented by void volume fraction and is introduced with a specific potential. Typical bulk and surface constitutive equations available for hot extrusion of carbon steel with solid lubricants are used: exponential isotropic visco-plastic equation for bulk behaviour and plastic isotropic equation for surface behaviour. Secure model with eleven remeshing design of the computational mesh is presented for typical combined forward-backward extrusion of hollow cylindrical parts without flash. Extrusion load, velocity field, effective strain, equivalent stress and void volume fraction are computed. The load results are found to be in good agreement with measurements on batch production hydraulic presses.