In Thermo-chemical Diffusionprocesses elements like carbon,nitrogen or boron are diffused into metal surfaces in order to enhance the surface properties and the strength of metallic components.In modern heat treatment fur-naces, the diffused elements usu- ally originate from gases reacting at high temperatures with the metallic surfaces. This can be a pure thermal and chemical reac- tion as a consequence of the ther- mal dissociation of the gases. An increase of the reaction velocity can be achieved in utilizing an electric field in order to ionize the reaction gas (plasma) resulting in largely increased mass transfer.The industrial thermo-chemicaldiffusion processes existing today are known under the names car- burising, nitriding and boronizing. They exist since many decades

Advances in Thermo-Chemical Diffusion Processes

Abstract

Dr. Bernd EdenhoferIpsen International GmbH, Kleve, Germany Thermo-chemical diffusion pro-cesses like carburising, nitriding and boronizing play an important part in modern manufacturing technologies. They exist in many varieties depending on the type of diffusing element used and the re- spective process procedure. The most important industrial heat treatment process is case-harden- ing, which consists of the thermo- chemical diffusion process car- burising or its variation carboni- triding, followed by a subsequent quench. The latest developments of using different gaseous car- burising agents and increasing the carburising temperature are one main area of this paper.The other area is the evolvement of nitriding and especially the ferritic nitrocar- burising process by improved process control and newly devel- oped process variations using car- bon, nitrogen and oxygen as dif- fusing elements in various process steps.Also boronizing and special thermochemical processes for stainless steels are discussed. and have evolved with time to pre- cisely controlled and reliable processes as part of the total man- ufacturing process of metal, espe- cially steelcomponents.In the last few years, a number ofnew developments and improve- ments in different areas have helped to increase the importance of diffusion processes, leading to metallic components with higher endurance capability. Certain requirements like suffi- ciently high furnace temperature, strong gas circulation, furnace muffle, etc., need to exist in the furnace for a successful utilization of this in-situ gassing technique called Supercarb
® [2]. Therefore,years ago, this process was limit- ed to batch industrial furnaces like pit fur- naces and sealed quench fur- naces. In the meantime, the Su- percarb
® process is used also in alltypes of continuous furnaces like mesh-belt furnaces, rotary hearth furnaces and in the last four years also in specially adapted pusher furnaces [3]. The savings in gas consumption using Supercarb

1. Carburising

The dominating carburising tech- nology today is the gaseous car- burising process using endother- mic gas as carrier gas and a hy- drocarbon gas, like natural gas,propane, lpg or others, as en- richment gas for achieving high carbon potentials. Also methanol diluted with nitrogen can be fed in- to the furnace, creating at elevated temperatures a carrier gas inside the furnace similar to endothermic gas.The most economical gassingprocess is the direct-feed of a fuel (hydrocarbon gas) plus an oxidiz- ing gas (air, carbon-dioxide or wa- ter) into the furnace and creating a CO- and H
® can be very high, as the example of a pusher furnace in figure 1 shows. 1.1 Low-Pressure Carburising Even more process gas can besaved when hydrocarbon gases totally without an oxidizing gas are directly introduced into carburis- ing furnaces. In this case, the car- bon transfer is a direct result of the decomposition of the hydrocarbon into free carbon and hydrogen. Be- cause of the high carbon availabil- ity of hydrocarbon gases, such a process only works with a high di-
2 -containing carburisingatmosphere inside the furnace[1].
Figure 1: Comparison of gas consumption values for a pusher furnace

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