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

This scepticism principally consists of:- memories of negative experiences with process engineering problems using older "vacuumcarburizing", like soot formation in the furnace and low loading density as well,- failed attempts for control of the carburizing process. High demands on quality control andprocess control were contrary to low-pressure carburizing distribution in start-up period.Low-pressure carburizing processes are currently controlled by preset, computer optimized treatmentrecipes [1-3]. This is a kind of restriction for process monitoring, because no in-situ control is possible.According to this, insufficient carburizing due to incorrect gas flow rates can not be either recognized aswell corrected during the process.2. Control of low-pressure carburizing processesDuring low-pressure carburizing the C-transition occurs by pyrolysis reaction of thecarbon carriers (mostly propane oracetylene) at the hot component surface. Nobalance reaction occurs by the pyrolysis sono c-level is definable which guides to acarbon-balance-concentration in the workpiece that is already established fromatmosphere carburizing. (Figure 1).When low-pressure carburizing a very fastrise of surface C-content to the absorptionlimit happens. In order to improve theuniformity of carburizing, the carburizingprocess is interrupted after a few minutesand a diffusion phase is attached, where theprocess gas is evacuated from the furnaceand carbon diffuses into the base material[2,3]. This interruption is followed by a newcarburizing cycle (Figure 2)Figure 1: Equilibrium condition and C oversaturation atatmosphere gas carburizing gas and LP-carburizing(according to D. Grassl)AtmospheregascarburizingFigure 3:Low pressure carburizing / Processdiagram (schematic)page 3 of 11www.stange-elektronik.de