Carbon steel components have for years been routinely carburized and hardened using both endothermic and nitrogen-methanol (N2-CH3OH) atmospheres. It has become more difficult to produce parts at competitive pricing because of increased quality standards and higher pressure on cost reductions.
Current atmosphere-control systems for carburizing processes are well known in the industry and are considered standard equipment. However, variations in the carburizing quality, and therefore, case hardness, continue to be a normal day-to-day struggle when using these control systems. The main reason for these variations is that the control system calculates the carburizing process carbon potential on the basis of a manually set, fixed value of CO level in the furnace atmosphere. Both endothermic atmospheres and manually regulated nitrogen-methanol atmospheres vary in their composition due to inadequate flow regulation of natural gas and air, as well as changes in the natural gas composition and pressure variations or erroneous flow settings in the nitrogen or methanol line [1].
The quality of the carburizing process can be greatly improved by controlling nitrogen-methanol atmospheres using a mass flow controller (MFC). An MFC can regulate flow rate independent of pressure and temperature, can compensate for all variations in line pressure and, thus, can maintain the blend with a constant CO-level in the furnace. In addition, the carburizing cycle can be accelerated by up to 20%, resulting in a significantly increase productivity [2].
Trials showing the feasibility and benefits of using an MFC to accelerate carburizing were carried out at Mubea (Muhr und Bender Maschinenbau GmbH), Attendorn, Germany, in a 25-year old, single-chamber furnace.
Carburizing basics
The theory of the carburizing reactions and carburizing process are well described in the current technical literature. Therefore, only some of the basics are discussed below to explain the role of MFC in controlling and improving the carburizing process.
The carburizing process can be divided into a carbon-deposition phase and a carbon-diffusion phase. Carbon from the carbon-containing species in the atmosphere is transferred to the steel surface during the carbon-deposition phase. The deposition of carbon on the steel surface occurs by thermal decomposition of the carbon-containing species. The main carbon-bearing component in carburizing atmospheres is carbon monoxide (CO) [3]. Carbon deposition is influenced by process temperature and atmosphere composition. The amount of carbon deposited on the steel surface is directly related to the carbon transfer coefficient (beta) and the gradient of the carbon level between atmosphere (Cp) and steel surface (Cs). The amount of carbon (m) available can be described by equation 1.
Figure 1 shows the dependency of the carbon-transfer coefficient of the carburizing atmosphere composition at a temperature of 900 C, or 1650 F [3,4].
In the carbon-diffusion phase, the surface carbon diffuses into the core. Diffusion is influenced by process time, process temperature and carbon gradient (i.e., the carbon content differential between steel surface and core). The process mechanism is expressed by Fick's Second Law of Diffusion in Equation 2 where C is the change in the part carbon concentration, D is the diffusion coefficient, t is time and x is the case depth [6].
Carbon-control systems
To calculate the carbon potential of a carburizing atmosphere, it is necessary to know the oxygen level, temperature and CO levels. A popular system used to monitor and control the carbon potential in carburizing atmospheres is an oxygen probe, which uses the formulas in Equations 3 and 4 where E is the electromotive force of the oxygen probe, F is the Faraday constant, p02 is the oxygen partial pressure of the furnace atmosphere, p'02 is the oxygen partial pressure of air, T is the carburizing temperature and pco is the CO partial pressure in the furnace atmosphere [3,7]. In typical carbon potential (C potential) control systems, p02 and T are measured using an oxygen probe, but the CO level is set manually in the controller; that is, at 20% CO.
To achieve greater control of a carburizing atmosphere, natural gas (or other hydrocarbon) is added to increase carbon potential (or air can be added to decrease carbon potential). If the CO level in the furnace varies from the controller set value, then a difference between the calculated and the regulated C potential will occur, resulting in an inadequate process atmosphere that has a direct impact on part carbon profile. Therefore, it is very important to control the carburizing atmosphere composition to match the controller input value.
Nitrogen-methanol carburizing atmospheres
Endothermic generator atmosphere, produced by partially burning natural gas and air, is composed of approximately 39% H2, 39% N2 and 20% CO, with the balance consisting of residual CO2 moisture and other gases. Several inconveniences such as maintenance costs and the availability and reliability of atmosphere composition led the heat-treating industry to look into alternatives to endothermic generators, and use direct-injection methods like nitrogen and methanol to produce a carburizing atmosphere in Equation 5.
Traditionally, by cracking methanol directly in the furnace, a blend of nitrogen and methanol is used to create 20% CO and 40% H2, although the CO level can be increased up to 33%.
Mass flow-controlled carburizing process
An MFC system for accelerated carburizing process can be installed as a complete atmosphere-control system, including a control of the carbon potential. The MFC system also can be implemented on existing systems, where carbon potential controls already exist.
A simplified explanation of a complete system (Fig. 2) is described here and in [1]. The control unit uses a PC to set parameters and as a visual interface and to document the process. Control hardware for each furnace is used to directly communicate with the PC. The controller independently regulates the process atmosphere, so if the PC shuts down, the controller still regulates the carburizing process. Carbon-potential regulation is based on a signal from the oxygen probe, temperature readings and the CO level in the atmosphere. The MFC enables the calculation of atmosphere CO level from the flow rates of nitrogen and methanol.
During MFC system start up, the theoretical calculated CO level can be adjusted using correction parameters. The corrected CO value is directly linked to the calculation of the carbon potential. Therefore, the real CO level is used to calculate and control the carburizing atmosphere.
This system has the advantage of permitting changes in CO level and carbon potential as required by the various steps of the carburizing cycle. This is especially beneficial when accelerated carburizing is targeted in classic carburizing processes. An example is shown in Fig. 3, where the accelerated carburizing process is implemented directly in the usual carburizing program.
Benefits of accelerated carburizing
Increased productivity resulting from using higher CO levels is extensively covered in the literature [1,2,8]. Benefits of accelerated carburizing directly depend on the required carbon case depth. For example, for a shallow case, the length of the carburizing cycle mainly depends on the deposition phase, which is highly influenced by atmosphere composition as described above. Consequently, shallow-case carburizing cycles will attain a higher productivity increase compared with those requiring higher carburizing depth with required longer diffusion phases.
Figure 4 shows the carbon profile of a typical carbon steel traditionally carburized in 20% CO for 180 min, processed using an MFC system. The carburizing time reduction in this case is 15%. In this example, using an MFC system for accelerated carburizing in a continuous production operation of over 6,250 hours per year allowed running up to 625 additional charges [2,8].
Summary
Manual blending panels were the norm in the past to control set-up appropriate carburizing atmospheres. Therefore, accelerated-carburizing technology based on higher CO levels during the carbon-deposition phase was not widely implemented in practice. Other reasons preventing the use of higher CO levels included the cost of methanol and nitrogen, free production capacities and high investment cost for automatically controlled blend panels.
Today, the situation has changed. Accelerated-carburizing processes are given more attention due to higher production cost and quality requirements, as well as closer consideration to asset use. MFC systems help reduce operating cost and can increase productivity up to 20%. Furthermore, the accurate setting of required flow rates allows producing parts having a very low hardness tolerance and provides reproducible carburizing results.
Tracking and archiving all important process parameters is performed through a data-logging system. The MFC system can be supplied independently by integrating the carbon-control system with the current control system of the furnace. Accelerated carburizing technology is achieved using hardware links to the furnace control panel and includes all required safety features. Air Products also offers process troubleshooting remotely with appropriate connections.
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