2. Warm and hot upsetting-sliding  test

The Warm and Hot Upsetting Sliding Test (WHUST) is an experimental apparatus designed to  simulate conditions of contact taking place in hot forging operations [3,4]. The WHUST involves a contactor which comes in contact with a specimen and slides along its surface with constant sliding speed. The WHUST parameters are the geometry of the contactor, its velocity, its penetration within the specimen, and the contactor and specimen temperatures (Figure 1).

Coefficients of friction are strongly related to mechanical, physical and chemical parameters of the surfaces in contact [5]. The use of a laboratory friction test to perform a reliable identification of tribological data requires the simulation of these parameters. It is impossible for a laboratory test to simulate each and every parameter acting on friction. To overcome these difficulties, the WHUST uses parts of the industrial workpiece as specimens, parts of actual tools as contactors, and lubricants taken from the process tanks. This peculiarity allows the WHUST to respect the chemical and physical characteristics of the contact (surface reactivity, adsorbed gas, material structure, hardness, roughness, oxide scale, wear particles, etc.). Contactor and specimen temperatures are respectively adjustable from room temperature to 300 and 1200 °C. The mechanical parameters of the contact (mainly contact pressure, strain, strain rate and sliding velocity) are adjusted using the test parameters (penetration, geometry of the contactor).

Fig. 1. Schematic view of the WHUST (a), unused contactor (b) and worn contactor (c).

Fig. 2. Methodology to analyse tribological performances of a

lubricant according to real conditions of contact.

The methodology to select test parameters in order to simulate realistic mechanical process parameters operates in four steps (Fig. 2):

x Step 1: process mechanical analysis. A finite element simulation of the industrial process is performed. Mechanical data such as strains, strain rates and contact pressure at the tool/workpiece interface  are computed. They become the target to be reproduced on the WHUST. Direct measurements on the industrial process are also performed when possible (tool temperature, lubricant film thickness before   forming…).

x Step 2: identification of optimum WHUST parameters. Test parameters are identified so that mechanical data simulated by the WHUST are closed to the process ones. Finite element computations of the WHUST are then performed with various contactor penetrations and geometries. An inverse methodology of identification is used to compare mechanical data at the contactor/specimen interface to the process ones, obtained in step one.

x Step 3: testing. First, an induction heating is used to bring the specimen  to  the  desired  temperature. Specimen temperature is controlled by three thermocouples. Then, the contactor is heated by a regulated heating cartridge up to 300 °C. Lubricant is applied on the contactor surface, respecting industrial lubrication conditions. Finally, the penetration of the contactor is adjusted, and the contactor creates a local plastic strain by sliding along specimen surface. Tests are performed three times, with unused contactors and specimens, to ensure their repeatability.

x Step 4: lubrication evaluation. Normal and tangential forces on contactor are recorded during the tests. Contactor and specimen surfaces are analysed before and after testing. “Performance markers” are derived from these data in order to obtain quantitative or qualitative information on the ability of the lubricant to reduce friction and to protect both specimen and contactor surfaces: a coefficient of friction, a critical length before defect occurrence and surface analyses [6]. Material transfer from specimen  to  contactor  and contactor roughness evolution are also considered to evaluate lubricant  efficiency.