In modern manufacturing technologies, thermochemical diffusion processes play a critical role similar to boronizing, carburizing and nitriding processes. Depending on the type of diffusing element utilized and the particular process, thermo-chemical diffusion processes occur in a variety of forms.
Case-hardening happens to the most critical heat treatment process in industries and includes the carburizing or carbonitriding process, followed by a successive quench. This application note focuses on the latest developments of different gaseous carburizing agents and the optimization of carburizing temperature.
The other area of interest is the evolution of the ferritic nitrocarburizing process through enhanced process control and latest process variations utilizing oxygen, nitrogen and carbon as diffusing elements in several process steps. In addition, thermochemical and boronizing processes for stainless steels are also explored in detail.
Present thermochemical diffusion processes are called nitriding, boronizing and carburizing. Over the years, these processes have evolved as accurately controlled and consistent processes as part of the total production process of metal. Conversely, in the last few years, several enhancements in different areas have underscored the importance of diffusion processes, which resulted in metallic parts with high endurance capability.
In the thermochemical diffusion processes boron, carbon, nitrogen and other elements are diffused into metal surfaces so as to improve the strength and surface properties of metallic components. In current heat treatment furnaces, the diffused elements mostly come from gases reacting with the metallic surfaces at increased temperatures.
As a result of the thermal dissociation of gases, this can be a pure chemical and thermal reaction. The reaction velocity can be increased by using an electric field to ionize the reaction gas, thus leading to increased mass transfer.
The gaseous carburizing process uses hydrocarbon gas and endothermic gas carrier gas to achieve high carbon potentials. Direct-feed of a fuel is a low-cost gassing process.
Certain factors like strong gas circulation, adequate high furnace temperature, furnace muffle, etc. should exist in the furnace for optimum use of the in-situ gassing method known as Supercarb.
Earlier, the Supercarb process was restricted to batch furnaces such as sealed quench furnaces and pit furnaces. Now, this process is employed in various types of continuous furnaces such as rotary hearth furnaces, mesh-belt furnaces as well as in uniquely adapted pusher furnaces. The Supercarb process considerably saves gas consumption. An example of a pusher furnace in illustrated in the figure given below.
Figure 1. Comparison of gas consumption values for a pusher.
Low-Pressure Carburizing and Low-Pressure Carbonitriding
When pure hydrocarbon gases are introduced into carburizing furnaces, more amount of process gas can be saved. In this process, the transfer of carbon transfer is the result of the decomposition of the hydrocarbon into hydrogen and carbon.
Owing to the high availability of hydrocarbon gases, this process only works with highly diluted hydrocarbon gases, if not hydrocarbon gases can be used at low pressures.
Figure 2. Mean carbon flux values (g/m2h) for different carburizing processes.
Figure 3. Carbon and nitrogen profiles of a steel 30CrMo4 after low-pressure carbonitriding at 880ÉC.
Earlier, propane was used as the main hydrocarbon gas for low-pressure carburizing process, in spite of its inherent shortcomings of furnace sooting and uneven carburizing. In recent years, the acetylene has been used in low-pressure carburizing process.
Low-pressure carburizing offers a number of benefits such as high mass transfer, enhanced layer uniformity, reduced process times, better stress resistance and surface quality, and no internal oxidation.
Prior to low low-pressure carbonitriding, a deficiency existed because of the inability to perform a carbonitriding process at low pressure. In the earlier years, carbonitriding was done by utilizing nitrogen gas in the diffuse phases, and propane or methane in the boost phases. However, this process is not possible with low-pressure carburizing, since nitrogen gas begins to dissociate thermally above 1000°C.
The benefit of carbonitriding against carburizing is that a carburised microstructure with high nitrogen content has better hardenability, higher temperature resistance, and enhanced wear resistance.
Figure 4. Industrial applications of high-temperature carburising furnaces.
High temperature carburizing is widely used to cut down cycle times and thus save costs. Using vacuum furnaces for low-pressure carburizing are more optimized for higher temperatures, since graphite is used for the furnace heating elements and furnace lining and exhibits high temperature resistance.
On the other hand, high temperature carburizing has a major drawback. It causes grain growth in current case-hardened steel and reduces lifespan of baskets and grids.
Boronizing and Nitriding
Plasma boronizing and gas boronizing are much cleaner and more economical procedures. However, their industrial application is virtually non-existent owing to the hazardous character of the gaseous donor media, such as borontrifluoride, borontrichloride or diborane.
For controlling the nitriding potential, one needs to determine the hydrogen content or the ammonia content of the atmosphere. This can be performed using infrared or other gas analyzers.
In the last few years, carbonitriding, carburizing and boronizing processes specify only a limited amount of the development work on thermochemical diffusion processes.
On the other hand, these thermochemical diffusion processes have undergone rapid advancements and these can be seen the development of higher stressed car suspension parts, motor engine components, gear components and drive shafts along with low-friction surface coating created in pvd- installations.
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