3. Forming process: hot forging of  “tripod”

Lubricants studied in the present work are tested according to the conditions of contact encountered during the hot forging of a tripod. Workpieces were made of AISI 4820 steel and tools were made of nitrited AISI H11 steel. Some experimental measurements were performed in the industrial workshop. Just before the forming sequence, the temperature of the workpiece was equaled to 1100 °C, the temperature of the tools was equaled to 200 °C and the workpiece presented an oxide scale layer of 18-20 μm  thick.

The study focusses on the contact between the workpiece along the flank of the dies, near the flash zone. Finite element computations of the process were run using the commercial software Forge® developed by Transvalor in order to identify the mechanical parameters of the contact (Fig. 3). Results lead to a contact pressure equal to 190 MPa and a sliding velocity equal to 60  mm/s.

Fig. 3. (a), (b), (c) Finite element mesh of the process, (d) final form of the tripod, from [7].

In the following parts, the WHUST parameters were adjusted so that the contact pressure and sliding speed simulated produced on the bench were respectively equalled to 160 MPa and 60 mm/s. The temperature  of contactors surface was set to 200 °C by a heating cartridge and the temperature of specimens was set to 1100 °C by induction heating. The heating speed of the specimens was adjusted so that the oxide scale developed at specimen surfaces was equal to 20 μm.

4. Testing of graphite dispersion in water  lubricants

Graphite is widely used in the forging industries. The good lubricity of graphite is due to its lamellar structure. But a lamellar structure is not enough to provide low friction. Graphite has to be contaminated by water or other condensable vapours [8, 9]. So graphite used in hot forging is typically applied as fine suspension of graphite particles in liquid. This suspension is sprayed over the hot dies. The liquid carrier evaporates, leaving behind a thin and uniform layer of solid lubricant. The liquid carrier has then three main functions: it carries graphite onto the surface, it generates vapour that enables graphite layers to slip one on another, and it cools down tool   surfaces.

Graphite suspensions in water are generally composed of 95% of water and 5% of graphite particles. Graphite particle sizes range from 10 to 40 μm. Bonding agents – like Na-Si compounds – are also added to suspensions in order to reinforce the adherence of graphite layer on tool  surfaces.WHUST trials were performed with different graphite in water suspensions and with different conditions of application. Results in terms of coefficient of friction μ and critical length Lc are presented in table 1. The critical length Lc is the sliding distance along the specimen before a scratch appears. It characterises the ability of a lubricant to protect specimen surface. As specimens were 40mm long, an Lc value of 40 mean that no scratch appears during the test and no breakdown of the lubricant layer on tool surface was   observed.

Results highlight that the film thickness of the lubricant layer has to be greater than or equal to 30 μm. For thinner layer, the graphite is quickly “consume” during the sliding contact leading to the occurrence of scratches and an increase of the coefficient of  friction.

Particle sizes of graphite play a role in the structure of the graphite layer. Large particle sizes lead to heterogeneous graphite layers (Fig. 4). Parts of contactor surface are not protected, direct metal-to-metal contacts occur and scratches appear on specimen surface, leading to an increase of the friction   forces.

Bonding agent used in the formulation of the lubricant have a great influence on the structure of the graphite layer on tool surface. For small quantities of bonding agent, the adherence of graphite layer on tool surface is wick and scratches appear very quickly. When an abnormally high quantity of bonding is used, the  protection  of specimen surface is ensured by the graphite layer first, and by the bonding agent when all the graphite have been consumed during the sliding. So scratches finally appear and friction   increases.