Control of low-pressure carburizing processes - Stange Elektronik ...

6 Process controlThe target values of temperature-dependent and optimal H 2 concentration makes a process control byadjustment of the acetylene gassing rate possible (Figure 12). In the case of process control withconstant carburizing-diffusion cycles, the gas flow rate is based on C-absorption of the material toconstant H 2 concentration can be controlled within the "process window" (Figure 11). The gassing here isadapted to the decreased carbon absorption of the treatment material.Figure 12: Control loop for low-pressure carburizing processes (diagram)Alternatively the carburizing can be done with increasing diffusion durations between the individualgassing cycles. The duration of carburizing and diffusion times is calculated with FOCOS SimulationCalculation software.The program should result in a rise of surface-C-content to each 1,3 % with a decrease to 0,8 % duringthe diffusion phase. The advantage of this process control is that over-carburizing and carbide formationcan be avoided easier. A disadvantage is the 20% to 30 % extended process time (carburizing anddiffusion phase).Even with such a kind of changed process management, the process control by H 2 concentrationmeasurement appears reasonable, because the gassing quantity is set with respect to the batch surfacearea.Contrary to the described carburizing process, the gas amount during the carbon cycles stays largelyconstant with correct calculation of the diffusion time.7. Conclusions and forecastThe compliance of close tolerances and reproducible heat treatment quality are necessary requirementsfor modern heat treatment systems and processes. In-situ measurement and carburizing control withoxygen sensors allows the fulfilment of these requirements for atmospheric carburizing processes.However, low-pressure carburizing is currently controlled according to given, computer-optimizedtreatment programs.A low-pressure carburizing sensor was developed, which determines the performance grade of theacetylene pyrolysis by measurement of the H 2 concentration in a furnace hot zone. It was shown that thepyrolysis depends largely on batch surface areas, gas flow rates and treatment temperatures. In resultthe H 2 measurement can be used for process monitoring and control.page 10 of 11www.stange-elektronik.de