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

ProcessControlAutomationControl ofLow-PressureCarburizingProcessesH. Altena 1 , F. Schrank 1 . S. Heineck 21Aichelin Ges.m.b.H. Mödling, Austria2Stange Elektronik GmbH. Apfelstädt, GermayMeasuringEngineeringServices

Control of low-pressure carburizing processesH. Altena 1 , F. Schrank 1 . S. Heineck 21 Aichelin Ges.m.b.H. Mödling, Austria2 Stange Elektronik GmbH. Apfelstädt, GermayPresent low-pressure carburizing is controlled by computer optimized treatment programs. Up to now nocarburizing process in-situ control was possible. This restricts process monitoring and quality control.Insufficient carburizing by non-optimized gassing or too low gas amounts could not be recognized orcorrected during the process.The development of a low-pressure carburizing sensor that determines the degree of reaction ofacetylene by measurement of H 2 concentration in the furnace hot zone is reported. It is shown that the H 2concentration depends significantly on batch surface area, loading density, treatment temperature andgas flow rate. The developed correlations permit low-pressure carburizing process monitoring as well asprocess control.For the first time for low-pressure carburizing an in-situ monitoring of the pyrolysis reaction is possible bythe introduction of a low-pressure sensor. This represents substantial progress in quality control of lowpressureprocesses. Furthermore through H 2 measurement results can be controlled from the reaction atthe component surface, also carburizing problems can be recognized and/or avoid by process control.The results of this comprehensive study are presented and the actual state of development discussed.1. IntroductionSince ISO 9000 implementation quality awareness and quality control became more important and arenow essential in all fields of heat treatment. Compliance within narrow tolerances throughout the wholeproduction process, beginning with material composition up to heat treatment, is required for modernproduction processes. For case hardening the following customer requirements must be fulfilled:- Narrow heat treatment tolerances(Surface C, C profile, case depth, core strength)- Reproducibility of treatment result- Low surface layer mechanical damage- Low heat treatment distortion- Process control and documentation- Low heat treatment costsLow-pressure carburizing processes can fulfil a lot of these requirements and became an alternativesolution to conventional gas carburizing over the past years.The "vacuum carburizing" disadvantages in the 150 - 600 mbar pressure range was overcome bycarburizing in the mbar range as well as changed system technology [1]. While the new technology wasused rather quickly in France, it was often touched with scepticism in German-speaking countries.page 2 of 11www.stange-elektronik.de

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

To avoid over-carburizing and strong carbide formation, usually a reduced gas flow rate will be used.Alternatively the proportion of carburizing to diffusion time can be reduced with constant gas amount.Because the carbide formation is relatively slow the C-absorption of steel decreases in case of exceedingthe carbide limit.At the end of the carburizing cycle a longer diffusion phase follows, where the surface-C of saturationconcentration is reduced to approximately 0.7 - 0.75 %. As is the case of diffusion over 930°C – evenpossible carbides are dissolved. The diffusion also contributes to uniform carburizing throughout thebatch, so that a high uniformity and quality of the carburizing process can be ensured.Carburizing and diffusion can be simulated by diffusion calculation software. With the offline programoptimal treatment conditions can be selected as well gas flow rates and/or carburizing times can bedetermined.Although this process control works very reliable, this method has a certain restriction for processmonitoring and quality control because it provides no in-situ control of the carburizing process.3. Low-pressure carburizing sensorThe propane pyrolysis first occurs by formation of H 2 CH 4 C 2 H 6 C 2 H 4 C 2 H 2 as well as smaller quantities oflong-chain molecules (Figure 3). C 2 H 2 and C 2 H 4 react with the part surface, also the reaction catalyst, toform H 2 and/or CH 4 and to carburize the part surface [1].Also the acetylene pyrolysis of C 2 H 4 takes place over several intermediate products, but leads in sum todiffusion of C with formation of H 2 and small quantities of CH 4 .Figure 3: Pyrolysis of propane and acetyleneBy measuring of the exhaust gas the concentrations of the reaction products - i.e. predominantly H 2 , CH 4as well the sum of C 2 H x molecules - were determined (Figure 4). It could be shown, that the propanepyrolysis is significantly faster than the pyrolysis of acetylene. Within 3 minutes a constant gascomposition from 54 - 55 % H 2 and 25 % CH 4 was acquired. With acetylene gassing at 930°C an H 2concentration of approx. 35 % was determined, which increased further with longer treatment times orhigher temperatures by pyrolysis of remaining C 2 H x molecules [3].So it seemed to be purposeful to monitor the carburizing reaction by measurement of the H 2concentration. Furthermore it was expected that propane pyrolysis would yeild an in-situ H 2 -concentrationmeasurement that would represent the reaction process and enable process control also.page 4 of 11www.stange-elektronik.de

Figure 4: Low-pressure carburizing with high pressure gas quench (exhaust gas analysis)Hydrogen sensors are already successfully used for monitoring and control of nitriding process, for whichthe H 2 -measurement takes place at atmospheric pressure. An existing sensor must be adapted in suchway that it can be used in vacuum. The decision was made to use the Stange Elektronik H 2 sensor, whichdetermines the H 2 concentration based on a heat conductivity measurement principle and fulfils followingrequirements:- Low dead time- High response sensitivity- No cross sensitivity e.g. compared to CH4 and other process gases- No pressure dependence of measurement signal- Vacuum solidThe sensor had to be adjusted to the specific requirements, where especially high response sensitivity inthe mbar range as well as a sufficient accuracy of measurement was the greatest challenge.3.1. H 2 sensor mode of operationThe measuring principle is based on the fact that hydrogen’s heat conductivity is much greater than thoseof all other gases (Figure 5).Figure 5: Relative heat conductivity of gases compared with air (at 100°C) (Documentation of Hartmann & Braun AG,Frankfurt)page 5 of 11www.stange-elektronik.de

The device is based on a Wheatstone bridge which consists of four measuring curvettes. Two parallelcurvettes are located in the gas flow. The other two are surrounded by a reference gas. The producedelectric signal is a measurement for the hydrogen concentration of the analysed gas (Figure 6).Figure 6: Measuring principle (H 2 -Sensor)It is essential that the measuring cell is surrounded by a measuring gas according to the hydrogendiffusion principle and achieves a temperature which depends on the heat conductivity of the measuringgas. The bridge connected curvettes are characterized, that the measuring cells are non-catalytic and theheats of reaction are measured. An external pressure sensor is used for compensation of the H 2 sensormeasurement signal. The different reaction times of the signals were to be considered.The sensor was connected to a flange located in a water-cooled double jacket (Figure 7).Figure 7: Measurement setup (H 2 -Sensor)page 6 of 11www.stange-elektronik.de

4. Required gassing quantitiesDue to the high carbon availability the rise of surface C-content up to saturation happens within a fewminutes during low-pressure carburizing processes. The ideal C mass flow rate which is needed formaintaining the saturation concentration decreases with preceding treatment duration (Figure 8) based onthe √ t-law.Gas flow rate m c [g/m 2 h]Figure 8: C evacuation by diffusion (C = C 5 )The intent of H 2 sensor carburizing control is to set the gas flow rates by H 2 measurement signal. So thatinequalities in carburizing can be prevented and on the other hand over-carburizing and/or carbideformation will be prevented.This should be possible at constant pulse-pause ratio with decreasing gas flow rates, as well as variablepulse-pause ratio and largely constant gassing quantities.5. Results of measurementTo achieve the desired sensor responsivity, the measurement setup had to be optimized as well themeasurement signal needed to be electrically amplified. The calibration of the measurement signal wasdone with reference gases which were made by defined compositions. This optimization now permits anin-situ measurement of the pyrolysis of propane and acetylene by measurement of H 2 concentration withsufficient accuracy.The analysis takes place in today's usual temperature range from approx. 860 - 1000°C, whereby most ofthe tests were made at 930 °C. The batch surface areas were varied between 1 and 8 m 2 , the batch sizewas approx. 600 X 600 X 600 mm (single batch). The gas flow rates were varied in test programsaccording to requirements.5.1 Gassing with propaneThe Carburizing took place at 930 °C; the proportion of gas flow rates to batch surface areas was variedwithin one dimension. In Figure 9 the H 2 signal is compared at 1 and 4 m 2 . The H 2 content was in the firstcycle approx. 54 % and increased with further gassing up to 55 %. The build-up of sensor signal tookplace faster naturally at greater gas flow rates (900 I/h).page 7 of 11www.stange-elektronik.de

Batch surface areaGas cycleFigure 9: H 2 measuring values with propane gassing (influence of batch surface)The H 2 concentrations of the in-situ measurement were the same value as the exhaust gasmeasurement, which was measured in earlier tests. Contrary to expectations the considerably differentbatch surface areas (1 to 4 m 2 ) resulted in a very small change of H 2 concentration or H 2 measurementprogress.As well changes of gas flow rates did not lead to a clear change of the measurement signal. The resultsof these tests were very reproducible. Also after improvement of the sensor sensitivity no significantinfluence of gas fow rates and batch surface areas on measurement signal could be realized. Becausethe measurement signal gave no conclusions to batch carbon requirements and the process gas carboneffect, it seems not to be suited for in-situ measurement or control.5.2. Gassing with acetyleneAcetylene gassing takes place at first analogue to propane test at 930 °C with 1 and/or 4 m 2 batchsurface areas and variable gas flow rates. The H 2 measurement now shows a clear batch surfacedependency (Figure 10). The first cycle leads to a very high crack level at 4 m 2 batch surface arearegardless of a relatively huge amount of gas, which refers to a lack of process gas. On the other hand at1 m 2 surface the crack level was just under 50 % in the first cycle.Batch surface areaGas cycleFigure 10: H 2 measurement values at acetylene gassing (influence of batch surface)page 8 of 11www.stange-elektronik.de

Changed carburizing temperatures of 860 °C as well as 1000 °C result in slightly different H 2 measuringvalues as expected, which document the temperature dependency of acetylene pyrolysis as well ascarbon absorption of the steel. The batch surface area was enlarged up to 8 m 2 in these tests. A rise ofthe H 2 measurement signal was noticed with increasing treatment temperatures. In order to implementthe H 2 concentration as a measurement signal for the process control, a correlation between crack leveland a satisfying carburizing cycle must be made, where no inequalities at carbon absorption or overcarburizingand carbide formation occur.Low-pressure carburizing processes are relatively "good-natured", regarding the required gas flow ratesin dependence of the batch surface [3]. With acetylene an under-run of the "minimal gassing" does notlead to serious inequalities of carburizing, like in case of propane. In result of this there should be arelatively wide range, where a satisfying carburizing cycle can be done.Figure 11: Range of optimal H 2 concentration with acetylene gassingIn scope of the extensive tests, the range was determined with respect to the treatment temperature(Figure 11). A largely linear dependence of an "optimal" H 2 concentration from treatment temperaturefollows.5.3 ReproducibilityA pre-cursor to use a measurement signal for control purposes is a sufficient reproducibility of themeasurement signal. It could be verified, that repeat of the tests with identical conditions lead to verygood reproducible H 2 signals; the maximum deviation amount was 2%. Furthermore it was detected thatthe measurement signal showed no drift over a period of several months.page 9 of 11www.stange-elektronik.de

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

Compared to that, propane pyrolysis takes place much faster and leads to a result of constant H 2concentration. This shows no significant dependence of process parameters and currently allows nocontrol possibility using the H 2 sensor.The introduced low-pressure sensor makes an in-situ measurement and process control of acetylenecarburizing possible for the first time. This represents an important improvement for low pressurecarburizing processes.A changed measurement setup as well an increased H 2 measurement accuracy could be therequirements for process control with propane gas. Even here, corresponding approaches showdevelopment possibilities.8. Literature[I] Altena, H.: HTM 49 (1994) 1, S. 58 63.[2] Löser, K.: Tagungsband der Europ.Tagung; „Das Einsatzhärten", Zürich, CH, 3.-4.4.2003; S. 115 122.[3] Altena, H.; Schrank, F.: HTM 53 (1998) 2, S. 93 101.[4] Altena, H.; Schrank, F., Heineck, S.: HTM 61 (2006) 4, S. 195 206.page 11 of 11www.stange-elektronik.de