Mirentxu Dubar

Determination of a hardening behaviour law for a cold forging TiN-coated tool steel

Abstract Damage occurrence has been industrially observed in cold forging tooling even though stresses remain lower than the yield point derived from a conventional rigid perfectly plastic law. A hardening stress–strain law has therefore been determined for the TiN-coated AISI M2 tool steel to improve the knowledge of the tool behaviour. The methodology used to establish the hardening law is based on the Brinell indentation test, which is simulated under increasing load. For each increment of the load, the parameters of the behaviour law are adjusted in order to make the numerical diameter and the experimental results of the Brinell test converge. In parallel, scanning electron microscope observations on the Brinell indentations show the presence of cracks at high loads, justifying the slight decay of the behaviour law towards the non-coated specimen behaviour one. Finally, this behaviour law is applied to an industrial process showing plastic deformation on the surface of the tool.

Tribological Performances of Two White Lubricants in Hot Steel Forging

The tasks of a lubricant used in the hot forging of steel is to reduce friction, cool down tool surfaces, and limit tool surfaces pollution by wear debris and oxides scales transferred from work piece surfaces. The most widely used lubricants in hot forging are dispersions of graphite particles in water or in oil. The graphite is involved to reduce friction. The liquid is used, first, to carry the graphite solid particles to the tool surfaces and, second, to reduce tool surface temperature. But graphite and vapours resulting from the lubricant deposition on hot surfaces lead to dirty workspaces. Some new lubricants are then developed in order to reduce friction in a cleaner way. When they are graphite free, those new lubricants are called “white lubricants”. The aim of the present work is to test two different white lubricants. The first one is a mineral salt; the second one is an organic salt. Lubricant performances are classified using the Warm and Hot Upsetting Sliding Test (WHUST). This friction bench simulates tests with contact pressure, sliding velocity, contactor and specimen temperatures similar to industrial ones. Before performing friction tests, work piece are heated up to 1200°C, contactors are heated up to 200°C, and lubricants are sprayed on contactor surfaces. Then the contactor slides against the specimen with a constant penetration, leaving a residual deformed track on its surface. Direct WHUST results are tangential and normal loads measured on contactor, surface roughness and chemical compositions on specimen and contactor surfaces. “Wear markers” are derived from those direct results, and provide useful information on the ability of the tested lubricants to reduce friction and protect tool surfaces. In the present study, tests are performed with different sliding speed and different contact pressure. The two white lubricants are compared to a generic graphite in water dispersion. Results show the tested white lubricants lead to coefficient of friction in the same range of the graphite lubricant one, but white lubricants lose their ability to reduce friction as soon as the sliding lengths becomes greater to 10 mm, where graphite lubricants can undergo sliding length greater than 30 mm.

Quantitative Evolution of WC-Co Cemented Carbide Tool Surface during the Cold Forming of Steel Parts

The present paper investigates the evolution of the WC grain size and morphology with the number of produced parts. Four reduction dies used in cold forward extrusion of steel are taken from the production line: a brand new die and three dies which have respectively produced 100.000, 150.000 and 220.000 parts. 3D roughness measurements and SEM micrographs are performed on the contact surface of each die. Results of these analyses highlight that WC grains are subject to plastic strain and are removed from the surface as the number of produced parts increase, leading to a growth of WC free areas where steel adhesion may occur. When analyzing the size of the WC grains, it appears that the population of small grains increases with the number of produced parts until 150.000. Then the population of small grains decreases. A wear mechanism is proposed to explain this variation of WC grains size with the number of extruded parts.

Towards the Numerical Prediction of Galling Onset in Cold Forming

Aluminum alloys are materials that have a strong tendency to galling when they are cold formed. Caused by a breakdown of the lubricant film between the part and the tool, galling can have dramatic consequences on the forming operation: scratches and cracks in the surface of the piece, clogging and deterioration of tools, etc. The present work studies the galling mechanisms of the aluminum alloy 6082 during its cold forming. Trials involving the Upsetting-Sliding Test (UST) are performed first. The UST is a test bench able to simulate in laboratory conditions the contact encountered at the part/tool interface of industrial processes. Trials are achieved under varying contact pressure and lubrication. UST results show that galling is strongly influenced by tool roughness and is not accompanied by a significant increase of friction. Three sets of finite element computation of the UST are then run to predict galling onset. Lubricant and adhesion forces are not modelled in this simplified approach: only the mechanical aspects are taken into account, the chemical ones are implicitly taken into account by coefficients of friction. The Lemaitre’s and the Xue’s damage models are compared. Results show that the Lemaitre model needs the tool roughness to be modeled to detect the galling onset. The Xue model is able to detect the occurrence of galling without modelling roughness. This result is due to the used of the Lode angle with enable the calculation of damage under low stress triaxiality.