August 2023 Volume 5

FORGING RESEARCH

2.3.3 Measurement of Temperature of Die During hot die forging, the tools undergo very high cyclic thermal load (from 80 to 600°C) as well as mechanical (0–800MPa) loads. As a result of such disadvantageous working conditions, the tools are characterized by instability and a relatively low life. That is why dies, stamps, pads and pushers are especially closely controlled before the forging process, as it is their precision, quality and initial surface state which largely determine the shape and quality of the forging. Measurements of forging tools are also more and more often performed during and after their operation with the purpose to analyze the proceeding effects of tool wear. Thus, the accurate measurement of temperature of dies is very important [28]. The die temperatures are measured mainly using two devices. • Pyrometers: It is a device for measuring relatively high temperatures, such as those that are encountered in furnaces. Most pyrometers work by measuring radiation from the body whose temperature is to be measured. Radiation devices have the advantage of not having to touch the material being measured. In an elementary radiation pyrometer, the radiation from the hot object is focused onto a thermopile, a collection of thermocouples, which generates an electrical voltage that depends on the intercepted radiation. Proper calibration permits this electrical voltage to be converted to the temperature of the hot object [29]. • Thermocouples: Thermocouples are the thermoelectric device for measuring temperature, consisting of two wires of different metals connected at two points, a voltage being developed between the two junctions in proportion to the temperature difference. PID controller needs to be connected to get the digital outputs of the values of temperatures corresponding to the voltage developed [30]. Thermocouples can be placed directly in the furnace to get very accurate temperatures. 2.4 Lubrication Methods in Forging Process 2.4.2 Lubricants Used for Forging • Water based graphite lubricants: It’s a suspension of ultra-fine graphite powder with water add on with special additives. It’s a most popular and highly consumed type of die lubricant. It can be sprayed, brushed or can be used by manual swabbing. This is a best type of lubricant for heavy and critical press forging operations. It gives higher dilution ratios with water, provides low per tonnage lubrication cost and gives excellent die life.

• Water based non-graphite lubricants: These are special water soluble polymer-based lubricants. They contain no graphite thus they keep working area clean. These lubricants are best for shallow cavity dies. These lubricants are having excellent cooling and lubrication properties and proven cost-effective solution. Figure 2-10 shows the effect of dilution of lubricant on heat transfer co-efficient of the process. • Oil based lubricants: These are basically suspensions made with base oil or vegetable oil add on with graphite and special additives as per forging process lubrication demand. Some grades come without graphite also. These lubricants are having excellent release properties and provide better material flow. These lubricants can be used in deep cavity hammer forgings, heavy stainless-steel forgings, critical brass and aluminum forgings, and deep extrusions.

Figure 2-10: Predicted heat transfer co-efficient for lubricant sprays with different dilution ratios

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FIA MAGAZINE | AUGUST 2023 77