Heat Treatment Process Flow

The basis for this investigation is a continuous heat treatment line with a pusher furnace as the primary system component. Since many heat-treated parts are processed in this type of equipment, it should be considered as a relevant example to illustrate possible energy savings. Many of the energy reducing measures highlighted within this narrative may also be applied for batch type heat treatment furnaces, as well as other furnace types for continuous heat treatment processes. In former investigations auxiliary equipment is typically not included [3], although they contribute to the total energy balance of the furnace system. To get the complete representation of the energy consumption, special attention to the washing steps, protective gas generation and the post cooling of the work pieces has been integrated into the comparison. Since the losses of the high temperature furnace, preheating and tempering furnaces are of the same nature, they have been combined to an inner energy balance.

The process performed within the highlighted example system is a carburizing process for gear parts with subsequent hardening and tempering. The capacity of the plant is 1200 lb/hr gross. Although the size of the plant has an influence on the energy consumption, it is not a prime factor. The preheating is performed within a separate furnace, performing at a temperature of 750°F (400°C). Subsequent to the preheating furnace the high temperature furnace heats the charges up to the carburizing temperature of 1700°F (930°C). 

After carburizing, carbon diffusion follows at 1580°F (860°C) to achieve the desired carbon profile. The atmosphere of the high temperature furnace is formed by endothermic gas together with methane as a carburizing gas. Following the diffusion zone, quenching takes place in a 175°F (80°C) oil medium. After a cleaning step, tempering takes place at a level of 390°F (200°C). To process the parts in further manufacturing steps, they have to be cooled down to ambient temperature by air ventilators. 

Basic North American Standard

In the past, the design criteria for the aforementioned heat treatment system in North America did not always focus on energy efficiency as a primary consideration. In typical fashion, attention was paid on low investment, reliability and ease in operation. Therefore, the use of recuperative gas burners was an exception and the discharge of hot flue gases in excess of 1500°F (815°C) was common place. Lower insulation values indicative of thinner wall construction often results in outer wall furnace temperatures in the range of 160°F (70°C). This is especially evident in areas of close proximity to inspection openings, doors, flanges, and burners. For the protective gas generation usually an on/off control of the installed retorts is applied, which means that the excess in endothermic gas is burnt in a torch at the generator exit. The charging and discharging doors are of simple design with a flame curtain to prevent the penetration of ambient air into the furnace.