August 2023 Volume 5

FORGING RESEARCH

2.3.2 Methods of Heating a Die/ Tool 2.3.2.1 Warming the Die with Billet/ waste material. The Die is heated using the radiation and conduction of heated billet or the waste material. It is seen in Figure 2-5 that the multiple workpieces are heated and placed in the die and the thermocouples in the tooling case measure the temperature of the dies. When the required temperature is attained, the billet material is scraped. In some industries, the billets are just placed in the tooling case and in some cases the forging operation is performed, and the first few forged products are scraped.

properties for the product. As the forging operation progresses, heat exchange occurs between the work piece and the dies, and dies and the environment, resulting in variation in heating conditions. This temperature needs to be maintained to achieve the defect-free final product. Changes in temperature directly affect the punch loads required for the operations, and if loads are increased, surface quality is adversely affected. Vast studies have been conducted to test the effect of temperature in the hot forging operation. Rooks et.al.[20] discusses the physical and metallurgical changes that occur in hot forging dies. The authors also state the effects of temperature cycling and phase transformations on the thermal fatigue of the dies. The authors deduced that the real independent parameters influencing wear is die temperature, dwell time and cycle time [21]. Hence, Temperature is one most important and crucial inherent factor in a forging process that needs to be monitored. • Heat Transfer Coefficient Heat transfer coefficient significantly affects the temperature control phenomena in the forging process. It can lead to a non-uniform heat distribution over the workpiece and dies in forging leading to thermal fatigue causing cracking. Chang et.al.[22] studied the construction of robust thermocouple to measure temperatures accurately and use FEA to determine heat transfer coefficient. Lenard, et al [23] have critically examined the methods available to measure the coefficient of heat transfer; they also discussed its effect on predictive modelling and present experimental results concerning its magnitude in different processes. 2.3 Die Heating Methods in Forging Operation 2.3.1 Introduction In hot forging, the heated workpiece is forged in heated dies. The performance of the dies in an industrial hot forging process is a topic of considerable economic significance. Heating of the punch and die is equally important as of correct heating of the billet. Die heating directly affects the product quality and process efficiency. Improper or non-uniform heating of the dies can lead to the failure or breaking of the dies. It also affects the frequency of maintenance. This increases the cost of the forging operation. Many literatures have been cited indicating the importance of the pre-heating of die. Many new methods have been suggested by the researchers and these methods are applied within specific domains of industries. In these studies, many aspects of tooling are studied, like, the material used, temperature variation during the whole operation, pre-heating techniques already employed, etc. This section below discusses different heating methods used in forging.

Figure 2-5: Heating the tools with a charge or waste material: (a) the photo; (b) the thermogram [24] Benefits: • This process ensures the proper initiation conditions for the forging process. • Although longer inaction intervals are observed in this process, there is no significant cooling of the dies. Limitations: • The amount of scrap generated is high as the workpieces are used to heat the die. • To reach expected temperature, minimum three heating cycles are required which take >50 minutes [24]. • This method enforces the stoppage of the production line, which results in huge number of scraps and huge economic losses. • This method does not ensure long durability or tool operation time. • Human error like excessive heated workpiece is used for operation which may lead to failure. 2.3.2.2 Die heating with Gas Flame Heating In this process, the dies are heated by the heat generated by the combustion gases. The burners are held near the die manually or robotically and the gas flames are directed towards the die. This process continues for some time till the desired temperatures are achieved and the system is kept idle for whole time. Figures 2-6 and 2-7 shows the different die heating techniques used in an industry [25].

FIA MAGAZINE | AUGUST 2023 75