898 Chiang Mai J. Sci. 2013; 40(5)

Chiang Mai J. Sci. 2013; 40(5) : 898-908http://it.science.cmu.ac.th/ejournal/Contributed Paper

Overview of Flow Analysis Simulation in ImprovingHeat Treatment ConditionsThanaporn Korad*, Mana Ponboon, Niphon Chumchery and John T.H. PearceNational Metal and Materials Technology Center, Pathumthani, 12120, Thailand.*Author for correspondence; e-mail: thanapk@mtec.or.th

Received: 31 December 2012Accepted: 20 February 2013

ABSTRACT

This paper provides a short overview of the use of flow analysis simulationapplied in 4 cases of heat treatment i.e., 2 furnace models for solution heat treatingaluminium billets and 2 furnace models for gas nitriding H13 extrusion dies. Alsoconsidered is the simulation of 3 different cooling practices applied in Al billet quenching.Towards reducing heat treatment costs and improving efficiency the results obtainedfrom the simulation using COSMOSFloWork® were discussed with plant managementand furnace operators in relation to heat flow behaviour and the likely effective andineffective positions for correct treatment in the furnaces. In each case the too-high andtoo-low temperature zones were identified and the suggested improvements were appliedto the furnace practice. Accordingly, metallurgical checks using microscopy and imageanalysis for microstructural examination and hardness testing, both before and after thestudy using simulation results, were compared in terms of process improvement andenergy savings.

1. INTRODUCTIONHeat treatment process control of

temperature distribution and the atmos-phere inside industrial furnace chamberscan be difficult in practice. Althoughbuilt-in thermocouples and traveling datacollectors for temperature are employedto monitor heat treatment processes, testsamples placed at selected positions in thefurnace are also commonly used in industryas a recognized practice in determining theeffectiveness of treatment. In recent years,manufacturing and the metallurgicalindustries in Thailand [1] have developedrapidly and computer simulation isnow recognized in various kinds of

manufacturing processes as a useful tool inassisting process and design engineers todevelop processes without actual practicaltrials. Computational fluid dynamics [2,3]can model both heat flow, based on heattransfer models in non-homogeneoussituations, and temperature distributionduring the soaking stages of heat treatmentand, as such, can be used to forecast theeffectiveness of the furnace operation [4].

This work summarizes researchperformed by the Metallurgical Laboratory,a research group in the National Metal andMaterials Technology Center (MTEC),on the use of SolidWork COSMOS

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FloWork® in simulating significant factorsin the furnaces used for homogenizationand solution heat treatment of Al billets[5-7], gas nitriding of extrusion dies [8,9],and case hardening surface treatments. Ineach case the work was aimed at applyingthe results to improve the processes toincluding equipment, furnace practice andthe role of operators. Such improvementscan contribute to the solution of problemsin productivity, quality, cost reduction,energy efficiency and environmentalperformance.

2. EXPERIMENTAL WORKExperiments were carried out using

computer simulation models applied to theactual practical operations by controllingthe routine process conditions. The mainaims of the work are to improve theproperties of aluminium billets andextrusion dies via process optimizationand to convey the information that can beobtained from such work to the technicalmanagement of companies involved in theseprocesses.

2.1 Aluminium BilletsDuring work on homogenization

treatments [4] of 6063 aluminium alloy(AA6063; 0.2-0.6Si, 0.45-0.90Mg, 0.35Femax, 0.1Cu max, 0.1Mn max, 0.1 Zn max,0.1Ti max and 0.1Cr max) extrusion billets,there were concerns that the temperaturedistribution recordings from thermocouples,originally positioned when the furnace wasinstalled, were not sufficient. Since thefurnace had been in regular use for sometime it was decided to use additionalthermocouples to obtain a more reliablepicture of temperature distribution.Other important parameters in need ofinvestigation included control of burnershot air temperature and the heat flow

between the combustion and furnacechambers. Heat flow and 3D temperaturedistribution within the furnace weresimulated to optimize thermocouplepositioning, especially at different positionswithin the furnace load of billets. Thestudies were applied to two homogenizingfurnaces having sizes of 2 × 2.5 × 6 m3 and4 × 4 × 12 m3. Although heat treatment ofAA6063 aluminium billet at temperaturesselected to achieve both satisfactoryhomogenization and energy saving appearsrelatively straightforward, fine tuning ofthe process via metallurgical study is stillneeded to obtain consistent results fordifferent batches of billet.

Homogenization involves heating theas-cast billets to a temperature between500oC to 620oC (for the 6XXX seriesaluminium alloys), the temperature beingclosely controlled to prevent any incipientmelting in the interdendritic regions. Anappropriate soaking time and subsequentcooling rate [10] must also be selected toprovide optimum workability duringsubsequent extrusion [6,11,12] and correctfinal properties in wrought sections. Ahigh solution heat treatment temperatureincreases solubility [13] of the as-castintermetallic phases, while increasedcooling rate after homogenization reducesthe particle size of any Mg-Si phase[14,15] that may form via precipitationduring cooling.

Although simulation was initiallyaimed at the study of the temperaturedistribution, the means of quenchingbillets with sufficient cooling rate withoutrisks of distortion was also considered [16].Blowing the hot air off from the chamberwas the normal practice but this may notbe effective for all billets in the batch.Billets can be cooled in two ways: forcedair cooling or water spray cooling. In forced

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air cooling, the billet load is forced aircooled within the furnace chamber by fanextraction of the hot furnace atmosphereto allow flow of air at ambient temperatureinto the furnace. In water spray cooling,

the billet load is conveyed to a water spraychamber for cooling. Both type of coolingwere simulated and the results of simulationcompared with actual practical results.

Figure 2. Relation between temperature and Mg-Si phase for solution heat-treatmentof AA6063 [4].

Figure 1. Billets (125 mm in dia.) stacked in the furnace used for study [4].

2.2 Extrusion DiesNitriding is the chemical heat treatment

of machine parts and tools in nitrogenbearing gas media at 500-590oC. Conven-tional processing using ammonia gasnitriding furnaces are commonly used inthe aluminium billet extrusion industry toimprove the wear performance of AISIH13 or JIS SKD61 hot-work tool steel(0.32-0.45C, 0.20-0.50Mn, 0.80-1.20Si, 4.75-5.50Cr, 1.10-1.75Mo, 0.80-1.20V, 0.03P

max and 0.30S max) that is commonly usedfor extrusion dies in producing aluminiumprofiles. This is due to the heat treat-abilityof this steel and its suitability for surfacehardness improvement by nitriding to givesurface hardness levels up to 70 HRC(micro-hardness 1,000 HV) and case depthsup to 200 micron. Nitriding parametersinfluence case hardness levels as shown inFigure 4, hence it is only by matchingtemperature with suitable flow of the

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nitriding atmosphere that optimum valuesfor hardness, case hardness profile anddepth of diffusion layer can be obtained inheat treated dies. To achieve optimumtreatment the dies need to be carefullyarranged within the nitriding chamber since

some dies may require first completenitriding and other dies require re-nitridingonly following repair. This means that eachdie needs to be positioned at a suitable zonein the furnace to receive the correct depthof nitrided layer.

Figure 3. H13 extrusion dies, as routinely arranged for loading into the furnace, andcut hardened H13 samples with size of 1 × 1 × 2 cm that were placed or hung in differentpositions representing each die position [8].

Figure 4. The contour plot of sectional hardness profile on nitriding specimen used forcomparing nitriding results and effects of nitriding variables on the form of the hardnessprofile [8].

3. RESULTS AND DISCUSSIONTemperature-time data obtained from

a thermo-couple, as shown in Figure 5, isan example of the use of installed equipmentfor process parameter monitoring. However,especially for software validation, thefurnace temperature data alone is insufficientand microstructural examination andhardness measurements are necessary.

The microstructure of as-cast Al 6063billet contains a distribution of Mg2Si [5]around the grain and interdendriticboundaries as illustrated in Figure 6 which

compares microstructures before and afterhomogenization treatment. This treatmentis designed to take as much Mg2Si, andother intermetallic phase as possible intosolution. For accurate comparison aquantitative method via image analysis wasused to determine the area fraction ofintermetallic phases (Figure 7). Therelationships between temperature datafrom the simulation and the quantitativemicrostructural analysis for area fractionof intermetallic phase for three differentcooling rates after homogenization are

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Figure 5. An example of therelationship between temperatureof aluminum billets and timeobtained from a thermocouplesituated on a billet surface [16].

Figure 6. Effect ofsolution treatment ondissolving precipitatedphases (a) before solu-tion treatment and (b)after solution treatment(as etched by 10%NaOH) [16].

Figure 7. Optical view of microstructure to determine precipitated phases area fraction.(left). For the same field, the contrast between matrix and precipitated phases is enhancedby Image-Pro® Plus (right) [15].

shown in Figure 8. Accelerated coolingusing forced air or water spray is seen tobe more effective when homogenizationtemperature is below about 570oC. Thespread of results for area fraction ofintermetallic phase after forced air coolingfrom different homogenization temperatures

are shown in Figure 9. Less than 2% ofprecipitated intermetallic phase is normallyacceptable and this can be achieved byforced air cooling. As-cast billet containeda mean area fraction of 2.64% but in someareas the fraction of intermetallics canreach 5% [4].

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Figure 8. Retained precipitated phases after Homogenization at different soakingtemperatures while the as-cast structure contained area fraction of up to 5% [15].

Figure 9. Plot between area fraction (%) of precipitated phases (most are Mg-Si) andhomogenization soaking temperature cooled by forced air cooling. The as-cast structurecontained an average area fraction of 2.46% (not shown in the plot) [4].

Figure 10. Illustration of flow analysis applied forthe homogenization furnaces size of 2 × 2.5 × 6 mshowing temperature distribution in 2D fromcombustion chamber blown down to the soakingchamber for billets[16].

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Figure 11. Illustration of thesimulation showing temperature

distribution in 3D of loading billets forhomogenization furnace size of 4 × 4 × 12 m [15].

Figure 12. Plot of the relationship between hardness and nitriding temperature [8].

From these studies, the use of simula-tion information was used for predicting,checking and evaluating the heat treatmentswith regard to:

3.1 Is the Furnace Loading ArrangementImportant in Nitriding of ExtrusionDies?

The work on heat flow during gasnitriding in a front loading furnace and avertical loading retort furnace for thetreatment of AISI H13 extrusion dies used1 × 1 × 2 cm sized samples that were placedat various positions during loading of thedies into the furnace as shown in Figure 3.

Microstructural and microhardnessinformation obtained to study theeffectiveness of nitriding with respect tosurface hardness, core hardness, nitridedand diffusion layer thicknesses could berelated to simulated temperature data asshown in Figure 12 revealing reducedmaximum surface hardness in dies treatedin the highest temperature zones. In thiscase the combined temperature simulationdata related to microstructural informationenabled the die stacking arrangement in thefurnace to be improved to achieve suitablenitriding on each type of die.

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3.2 Are All Billets in the Furnace LoadCorrectly Treated?

The temperature data obtained fromsimulation for both hot zones and the coolzones can be related to microstructure ofbillet sections to understand the effect ofvariation in process parameters such asblower adjustment, use of furnace wall fins,space between billets and the wall, and thespace between the billets.

The use of heat flow and temperaturedistribution (Figures 10 and 11) indetermining microstructural conditions ofbillets can also be applied in the billetcooling process. In the study of quenchsensitivity [17] the effect of rapid coolingby forced air and by sprayed waterexamined using 125 mm diameter AA6063billets after homogenizing at 560-580oCwere compared with that of the normalpractice forced air (blow off) furnacecooling in Figure 8.

3.3 Do Furnace Thermocouples GiveSufficient Information?

Since the billet homogenizationfurnace had been in regular use for

sometime it was decided to use additionalthermocouples to obtain more reliableinformation of temperature distribution.All billets are normally stackedsymmetrically, but without seriousmonitoring, with the view that onlytemperature control is needed forconsistency. Hence correct thermocouplepositioning must be achieved. Heat flowand 3D temperature distribution withinthe furnace were simulated to optimizethermocouple positioning, especially atdifferent positions within the furnace loadof billets. The simulation highlighted theneed for improvements in the insulatingsystem and in the heating arrangement inthe furnace. In this way energy consump-tion in heat treating the billets could bereduced resulting in cost reduction.

3.4 Is the Billet Cooling Process underSufficient Control?

Billet quenching via different coolingconditions such as forced air and waterquenching was also simulated. The resultsfrom microstructural examination wererelated to the simulation data for temperature

Figure 13. Simulation of temperature condition in cut 5-inch diameter billet for 3 differentcooling methods [18].

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distribution in the furnace, and for theeffect of different positions in the load onthe temperature gradients between thebillets and the ambient temperature duringcooling. Examples of temperature profilesthrough billet sections are shown inFigure 13. The temperature distributionin forced air cooled billet was seen to bevery uneven depending on billet position

in relation to the direction of the forced airblow. The simulation data on temperaturegradients can be related to the tendency forbillet distortion and cracking, and thus leadto improvements in cooling practice. Thedata has also been used to understandhardness variation in billets after waterquenching (e.g. Figure 14).

Figure 14. Hardness results after soaking at 555, 580 and 610oC followed by cooling viawater quenching. The average hardness of as-cast sample is 42.78 HB (not shown in theplot) [18].

4. CONCLUSIONSIn the study of homogenization flow

analysis simulation techniques can be usedto determine temperature distributionboth in the furnace chamber and in theloaded billets as well as to predict theeffectiveness of soaking temperature atdifferent furnace load positions in theprocess. The simulation also highlightedthe need for improvements in the insulatingsystem [4] and in the heating arrangementin the furnace.

Homogenizing of Al alloy billetshould produce a microstructure with thelowest quantity of precipitated phases forsubsequent extrusion of complex profiles.This is obtained from homogenizing at the

highest temperature followed by coolingat the fastest rate; however there arepractical concerns that, with theseconditions, the billets may be damaged bydistortion. Hardness testing can be appliedto monitor homogenization, but care isneeded when comparing hardness data forbillets produced by different casts due topossible variations in cast microstructuree.g. dendrite arm spacing. The quantificationof precipitated phases in aluminium billetby image analysis is useful in determiningthe effectiveness of homogenization aswell as predicting the forming ability ofAA6063 billet (or other grades for whichhomogenization is required). Processoptimization using experimental results

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together with application of a suitable flowanalysis simulation technique is useful foraluminium manufacturers who haveconcerns over product quality and controlof energy consumption.

A flow analysis simulation techniquecan be used to determine temperaturedistribution and flow velocity in the gasnitriding process as well as predictingthe relative effectiveness of nitriding atpositions within a component or in theloading arrangement in a batch process.Simulation can provide data that cannot begathered by conventional monitoringequipment. Flow analysis results in thisstudy were simulated only for steadystate conditions, hence, short time soakingand multi-step temperature soaking orfluctuated conditions in practical work maydeviate from this model. The results havebeen used to set up the most effectiveconditions for nitriding in the furnace usedfor the study.

Due to the variation in size and designof extrusion dies, the loading of the furnacefor each batch is not as uniform andsymmetrical as in the case of billethomogenization. The study has shown thatsimulation is useful in predicting flowbehavior and temperature distributionleading to improvements in die stacking forfurnace load arrangement. Some importantparameters were not included in thenitriding study, i.e., process time, percentageof gas dissociation and inlet gas flow rate(for nitriding and other atmospheric heattreatments). These need to be consideredin further work.

Flow analysis simulation could also beapplied for monitoring and processimproving on other routine heattreatments such as

- Air blowing for cooling (or quenching)control in annealing, normalizing and

tempering of plain C and commonlow alloy steels.

- Temperature distribution from heatsources in vacuum heat treatment oftool steels.

- Aging of aluminium alloys for profileproducts and diecastings.

- Annealing and normalizing of ductileiron castings.

- Solution treatment of austeniticstainless steel and high manganesesteel castings.

- Batch and continuous annealing inlow C stainless steel sheet (in bothlong and flat products).

- Hardening and tempering of alloywhite irons.

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