The Use of ‘In Process' Monitoring Equipment in Heat Treatment Today

TRANSACTIONS OF MATERIALS AND HEAT TREATMENTVol.25 No.:PROCEEDINGS OF THE 14TH IFHTSE CONGRESSOctober 2004The Use of“In Process' Monitoring Equipment in Heat Treatment TodayDave PlesterDatapaq limited, Cambridge CB4 0GU, UKAbstract: This paper looks at how in. process temperature monitoring systems are being increasingly used in heat treatmentapplications such as surface hardening, annealing, normalizing, tempering etc. We look at the basic equipment required tosuccessfully monitor from within a furmace and how developments such as RF transmission have enhanced the effectivenessof profiling trials to the point where on line adjustments to fumace conditions can be seen and analyzed in real time. Weexarmine how continuous processes can benefit from this type of monitoring and also how routine operations such asfurnace surveying in batch furnaces can benefit greatly from in-process monitoring. Advances in software are alsoconsidered, showing how packages are now designed to speed up the processing of data in day to day heat treatmentoperations.Key words: temperature monitoring, profling, RF tansmission, real time, sofware.HIGH VOLUME PRODUCTION in heat treatmentsafety. But in continuous heat treatment furmaces onlyfacilities today lends itself to continuous heat treatmentthe temperature of the air surrounding the product istechniques. Gear shafts, axles, steel coils, and manymonitored (normally by thermocouples controlling theother products are heat treated in continuous pusher orburmers), and an assumption is made as to how thisroller hearth fumaces or semi-continuous low pressurerelates to product temperature. (see fig. 2 below)carburizing furmnaces, where the products travel throughhe furmace in product baskets. Temperatures in thezones of these furnaces are carefully controlled toimpart the correct properties to the finished product,and a quench and secondary heat treatment will oftenbe incorporated in the production line.The most important elements of the heat treatmentprocess are time and temperature (see Fig. 1 below).All heat treatment specifications call for the product(s)to be heated to specifice temperature levels or thresholds,for specific periods of time, and to be cooled at specificrates. This allows the full transformation of the alloyFqveAto take place before the quench or subsequent coolingFioue A show the Por gaehed o1 anrp monoeino inaprocess. These parameters will vary depending on theecan mesu bo pooo and air imonie hroughout th lodshape and thickness of the part, and the type of alloy,Fig.2 Comparison between the normal monitoringetc.and "In-process" monitoring.This is because it is difficult, if not impossible insome cases, to measure the product temperature in acontinuous furmace due to the problems of trying tofeed long thermocouples through furace doors,atmosphere locks, water quench baths, etc.To effectively monitor the temperature of a productin a continuous furmace it is necessary to place thecomplete monitoring system in the product basket Sothat it can measure the product temperature as it travelsMEwith the product. This is how the in-process'monitoring system works.An 'in-process' monitoring system will generallyFig. 1 important elements in heat treatmentconsist of a data logger that is programmed to recordPrecise measurement of the product temperature isfrom up to 10 thermocouples, which are placed invital to ensuring the product meets the required heatspecified positions around the working area of thetreatment specification. This is especially true_ forfumace.products such as cast alloy road wheels, where the final中国煤化王fom tbe heof.hemechanical properties of the product are critical to- :mal barrier which isYHCNMHG592PROCEEDINGS OF THE 14 IFHTSE CONGRESSOctober 2004selected to withstand the heat and atmosphere of theregarding the maximum process temperature achievedfumace throughout the duration of the process (seeand the time above the specifiedtemperature, but alsoFig.3).valuable data regarding the rate of heating, the .'balance' of heating across the fumnace, any hot andcold spots, and the rate of cooling, i.e.completeevaluation of the applied heat treatment.Fig.3 Components of a Datapaq in-process monitoringFig. 4 In Process thermal barrier together with coilsystem: Thermal barrier, data logger, thermocouples,ready to enter the roller hearth furnaceand softwareAs can be seen from the data resulting from a typicalcoil annealing trial (Fig.5 below), the thermocouplesAfter the system has passed through the furmace with(#2 & #5)embedded deep in the coil reach similarthe product the temperature data is downloaded frommaximum temperatures to all the other thermocouples,the data logger and can be examined using speciallybut the time at the threshold temperature (780°C) atdeveloped software.both these probes can be up to I hour less (3 hr 24 mincompared to 4 hr 24 min).1. Monitoring in Specific ApplicationsThe temperature-time curves resulting from1.1 Coil Annealingsuccessive trials can also be used for predicting andAll thermal processes can benefit from knowing themodelling the strip temperature, and thereby make theprecise product temperature, but with somheat treatment process more effective.applications it is vital. Take for example th1.2 Low Pressure Carburisingre-crystallisation annealing of cold rolled coils oCarburising is more commonly carried out in sealedcarbon and martensitic chromium steel grades. This isquench furmaces than in vacuum furmaces. But today angenerally carried out in a roller hearth furnace under aincreasing number of manufacturers use vacuumnitrogen atmosphere, with atmosphere locks at the exitcarburising (low pressure carburising) because ofand entrance of the furmace (see fig 4).specific advantages it offers over more traditionalAs recrystallisation of the cold rolled steel stripmethods.occurs above a specified temperature, it is veryCritical to the success of the process is establishingimportant that this temperature is reached at the internalhe precise time the component spends above theparts of the coil during annealing. A particular soakingspecified temperature in the carburise and diffusiontime above the specified temperature is also needed incycles. This not only ensures that the components haveorder to ensure that the recrystallisation temperature isthe correct carbon level and depth, but also that thereached at the central areas of the coil which would beprocessing time for the batch is optimised. The controlthe last areas to reach the specified temperature.thermocouples in the vacuum chamber cannot give thisNormally the annealing temperature is controlled byprecise information, only thermocouples attached to thethe thermocouples placed along the heating zone of theload will be able to do this.furnace, but these only measure atmosphereIn single chamber (batch) vacuum furmaces it istemperature above the coil and the true productsometimes possible to attach thermocouples to the loadtemperature is never known. Attempts to run longby passing them through air tight seals in the wall of'trailing lead' thermocouples embedded in the coil arethe fumace. In_ multi chamber (semi-continuous)generally unsuccessful due to the atmosphere locks atvacuum fumaces the load moves automatically betweenthe ends of the furmace. However using an 'in-process'sealed chambers making it impossible to 'feed'monitoring system allows the user to place up to 10thermocouples through the furmace and measuring thethermocouples within, and around the coil (see Fig.6precise temperature of the load can only be achieved byopposite). This provides useful information not only中国煤化工gh the heat treatmentYHCNMHGVol.25 No.STRANSACTIONS OF MATERIALS AND HEAT TREATMENT593process with the load as it progresses from chamber tochamber (See Fig 7).Fig. 7 The data logger inside the thermal barrier sitswithin the product basket monitoring the gear wheeltemperaturesFig.5 Data downloaded from the logger after exitingthe fumace中山一20-Cete bos oto-的A-Crtebece ftixth药C tpoftot0212Fig 6 Thermocouples from data logger inserted in coilIn the graph below (Fig. 8) the precise time at thevarious cycles can clearly be seen. With this data it ispossible to try to reduce the time spent in the variouszones while not sacrificing product quality. This isachieved by making small reductions to the time cycleswhile at the same time closely monitoring the physicalFig. 8 Data from the low pressure carburizing processproperties of the product.1.3 Annealing Laminations for Electric Motors and●Decarburising the steel by carrying out the annealingTransformerscycle in a (typically) exothermic atmosphereHeat treating steel laminations for electric motors●'Blueing' the steel (producing an oxide layer on themay have several specific operations which include:surface), by exposing the product to a super heated，Burning off any residual oil from the stampingsteam atmosphere during the cooling cycle.operation and relieving any stresses from this coldThe heat treatment process must be tightly controlledworking operation中国煤化工with the optimumfYHCNM HG594PROCEEDINGS OF THE 147 IFHTSE CONGRESSOctober 2004magnetic. propeties (low core loss and highpermeability).For high production volumes, the heat treatment oflaminations is generally carried out in rller hearthfurmaces. with the products stacked inside trays orproduct baskets. Furmaces will generally have severalzones; burm off, anneal/decarburise, controlled cooling,blueing, and rapid cooling, which may be separated byatmosphere locks.Fig.10 Process dataMonidoring sytsm (ndicoted)The data in the process shown above (Fig. 10)POwidhin bhe poudat buslcsindicates that at only two thermocouple positions thetime at temperature specification was satisfied, at onethermocouple position the maximum temperature wasTop righa & left Heat reingexeeded, and the rate of contolled cooling was toofast at all thermocouple positionsFig. 9 Laminations in baskets entering the anealing2. Temperature Uniformity SurveysfumaceWe. have seen how in-process temperatureBecause of the atmosphere locks, and the fact thatmonitoring can benefit processes where the actualthe product may progress at varying speeds through theproduct is being monitored. Furnace surveying isdifferent zones, it is difcult to monitor productanother area that can benefit from monitoring fromtemperature throughout the process without the aid ofwithin the fumace.an in-process monitoring system. The productTemperature uniformity surveys are a requirementtemperature data collected from a trial can be vital infor companies operating in the aerospace and someensuring:parts of the automobile industries. in surveying a●All products_ reach the minimum bum offfurmace, thermocouples are placed in the extremities oftemperatures (370°C to 480°C) for a specific time tothe fumnace and, after a period of stabilisation, theremove process oilsmeasured temperatures at these points are compared toThe coldest part of the charge within the productthe set temperature of the fumace. If all the measuredbasket meets the critical time 1 temperaturetemperatures have been within known tolerances for aspecification (e.g. 1 to I% hours at 755°C) in thespecific time period, the set furmace temperature isannealing 1 decarburising zoneraised to the next level and the operation is repeated.， Products do not exceed a maximum specifiedSurveys are generally carried out at set time periods,temperature (e.g. 800°C) in the annealingsay twice per year, or after any major repair work ordecarburising zone, as this may cause them to 'stick'maintenance has been carried out on the furmace.in the product baskets2.1 Surveying in Batch Furnaces●The contolld rate of coolig in the first coolingIn batch heat treatment fumaces, carrying outzone is achieved ( e.g.755°C down to 500°C over 1temperature uniformity surveys is generally easier thanto 1% hours)in continuous furmaces, but it can still be a problem.●The correet time period in the "blueing' zone isThe fumace has to be cooled down before thethermocouples can be set within the workingarea. Thereached (if 'blueing' is part of the cycle)More companies today specify product temperatureends of the thermocouples are then led out through themonitoring on say a weekly basis in every furmace asfumace door where they are connected to a data logger.part of their quality control programme to verify theThe survey is then carried out (Fig. 11), the furmaceabove, and to spot any changes to process conditions,cooled and the thermocouples removed.The main problem for manufacturers, especiallysuch as hot or cold spots, at an early stage, beforethose with multiple furmaces and more than one plant,product quality is affcted.中国煤化工can take up io 24fYHCNMHGVol.25 No.5TRANSACTIONS OF MATERIALS AND HEAT TREATMENT595hours, and there is also the possibility of damage tooperations. For example if a contract heat treatmentrefractory material as the furmace is repeatedly cooledcompany has an average of 10 furmaces per plant, thisand heated. Obviously there is a requirement formeans that they will cary out a minimum of 20 surveysin-process monitoring here as the system could beper year (about 1/2 surveys 1 month). At an average ofmounted in the product basket, and charged an24 hours per survey this means 480 hours of downtimedischarged as a normal load, thus eliminating the needper year. At a cost of say $100 per hour fumace time,to cool and re-heat the furnace before and after thethis equates to $48,000 per year, which could besurvey.reduced by 75% giving a potential savingof $36,000per plant, per year. These figures refer to contract heattreatment plants, but in-house heat treatment operationscan also realize big savings if surveying is arequirement.” A Bypialfoumsceunronilysanre Iinsde ths podust buketsFig.11 Temperature Uniformity Surveying in batchfumacesTempennunes2.2 Use of RF Data Transmission_polerancce 20There has been a downside to using in-processmonitoring systems for uniformity surveys. UntilI Surey rsuls Jrecently it was not possible to see the survey in realtime as the information on the data logger could onlybe examined after the system was discharged from thefumace, and the data logger downloaded to aPC. So itFig 12 real time data shows the convergence point inwas not possible to see the point at which all thethe tolerance zone. Downloaded graph and results alsomeasuring thermocouples converge within the toleranceshown.band (see Fig 12). This is important as the secifiedtime at a survey level begins from this point. With theintroduction of RF transmission however, the data canbe transmitted from inside the furmace to a PC outsidethe furmace, and the whole survey managed from there.2.3 Thermal Barrier Size - A balanceUsing in-process monitoring equipment for surveysrequires a balance between size of thermal barrier andthermal duration. The barrier (Fig. 13) must keep thedata logger at a safe working temperature throughoutthe survey and obviously the bigger the barrier, thelonger it will keep the logger cool. However a thermalbarrier with an over large volume compared to theworking volume of the furmace may affect the spread ofthe survey results.Fig.13 Thermal barrier for temperature uniformity2.4 Potential Savings in SurveyingsurveyingRecent tests have shown that using an in-process2.5 Surveying in Continuous Furnacesmonitoring system for surveying, the time for anIn continuous furnaces the complexity of the surveyaverage survey can be cut to 6 hours from 24 which cannormal methods, themean considerable savings for medium to large中国煤化工fed into the furmace asTYHCNMHG596PROCEEDINGS OF THE 14H IFHTSE CONGRESSOctober 2004the product moves from zone to zone. This has thetemperature reached, etc. Now however softwaredisadvantage that no product can be fed in behind thepackages have become far more advanced and nowsurvey jig and so production losses increase.have multiple functions including:The solution is to place an in-furnace monitoring●Full profile analysis of rate of heating and cooling,system into a survey jig (ig. 14) then load this as youtime at temperature, tolerance settings, and filewould a normal fumace load. This has the advantageoverlay etc.that product can be loaded in front of, and behind the●Full function real time analysis including alarms andsurvey jig. The monitoring system can then beRF signal condition indicatordischarged hot, or in some cases through a water or gas●Full uniformity surveying functions includingquench. This saves valuable production time.thermocouple and instrument correction factors,survey management with warnings of overshoot, fullsurvey report compilation and print out, etc.Although software has become more advanced itmust be stressed that software has to be more 'userfriendly'. Technicians need the full functions of thesoftware but may not use it on a daily basis. If peopleonly use software say once per month then it is easy toforget the basic functions. With this in mind software isbeing designed today that uses 'Wizards' (Fig. 15) toguide the user through the various screens, explainingeach step and allowing them to 'back up’ to theprevious screen when they have made a mistake.So software today is an integral part of the systemand provides a complete solution to peoples monitoringFig. 14 A survey jig complete with thermal barrier andand surveying needs.data logger is removed from a continuous vacuumfumaceo insight2.6 Comparison Product Monitoring vs. UniformityFurmace SurveyingSurveyingIt is difficult to compare and contrast producttemperature monitoring to uniformity surveying. Thepurpose of the latter is to confirm that what is set on thefurnace control is what is achieved in all the workingareas of the furnace. However this does not mean thatthe product will reach the set temperature in all areas ofthe product basket. Surveying is often carried out withan empty or lightly loaded furnace which may be farremoved from actual production conditions.Factors such as size and shape of product, material,Fig 15 Software 'Wizards' guiding through the basicload density in the product basket, atmosphere, etc. allfunctionsaffect the temperature distribution within the load. Forexample non-ferrous products such as aluminium cast4. Conclusionwheels and rolled plate will often correspond with theMonitoring product temperature with in-processset furmace temperature for the majority of the process,monitoring equipment can have major benefits in termswhereas ferrous products tightly packed in the productof improved product quality and process optimization.basket may diverge from set fumace temperatures, as isBy considering the in-fumnace option for temperatureshown on the lamination anneal example above.uniformity surveys it is also possible in someapplications to realize considerable savings. With the3. Advances in Softwaredevelopment of survey specific sofware it is alsoIn the past sofware packages supplied withpossible to automatically produce dedicated surveymonitoring systems have been basic and have mainlyreports from which a hard copy can be produced, or canconcentrated on screens to reset and download the databe stored electronically.logger, plot the graph of time vs. temperature, and showsome of the basic calculations such as maximum中国煤化工YHCNMHG