SEP 23 '97 12:24PM PROC. SERV. DIV.

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* I/.? / I FINAL REPORT

CONTINUOUS AUSTEMPERZNG FLUIDIZED BED FURNACE

Principal Investigator:

Dr. Malur N. Sriiiivasaii Department of Mechanical Engineering

Lamar University Beaumont, TX 77720

Principal Subcontractor;

Keinp Development Corporation P.O. Box 2 18943

Howton, TX 77218

Industrial Collaborator:

Quality Electric Steel Castings, Inc. 252 McCarty Drive

, Houston, TX 77029

Respectfillty submitted to:

U. S. Department of Energy Idaho Operations Office

850 Energy Drive Idaho Falls, Idaho 83401

Texas Engineering Experiment Station Contracts and Grants Division

College Station, TX 7784313000 T

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CONTlNUOUS AUSTEMPERING FLfflDIZED BED FURNACE

Grant No. EIE-FGM-94ID13323 Amendment MOO3

FJ$AL REPORT

SUMMARY OF PLANNED OBJECTIVllS AND ACTUAL WORK PERFORMED:

Thc iiitcndcd objcctivc of projca was to show thc bcnefits of using a fluidized bcd firrrixct: for mstcnitiziiig and austmpcring of stccl castings in a caiititirrous iiranncr. This pm.$Cr was sponsod by rhc Ofics of Industrid Technologies of tlic Dcpamait of Energy nnd was nmrdcd jointly to Texas h & M Uiiivcrsity at Callcge Station (administcrcd by tlzc Tcxns Enginwing Expcrinicnl Station), Kcnip Tlcvolapiiicnt Corporation, Fioustotl and Quality Elccvric Stccl, Inc., Houston. For admiiiistrativc p~ir l~os~s , Tcxa A&M Uiiiwrsity WBS mind as tlic principal hwmtigator, Kemg Development Corporation as the sub CoRtraCtor tb Texas A&M University aiid Quality Electric Steel as the industriaf collaborator. The division uf reqmmibi1jtitx was as fatlows:

I, Dcsigz oftlie fliiidizcd bcd fiirnace : K m p Developincnt Corporation

2. Fabrimtian af tho fluidized bed furnace: Quality Elcctric Steel, hc.

3. Procedure for austempcrinz of stmi castiiigs, clllaysis of tlic rwults aftor atistempering TIXM A&M University (Texas Engineering Experiment Seaeion).

The Department of Energy provided fiiruting to Texas A&M University and Kemp Development Corporation. The responsibility of Quality Electric Steel was to fhbricate the fluidized bed, makc tcst castings and per;Fom austempdng d t h e steel castings in the fluidizd bed, at their own eqotlse.

The projact gods had M be rcvicwcd several times due to financial constraints and kcchniml difficulties eticountercd during thc course of the project. The modifications imdc mid the %sochrcd events arc lisrcd in the dironological d e r 8s pbllows:

1.. After considerins the cstinutcd cost ofthc cmveyar syshn as dcterniincd by Keinp Dwebjmmt Corporation, it was decided by Quality Electric Steel that it would be appropriate to first ffibricate the fluidized bed funlace chambers and pr?domi inaterial handling imnually rathcr than with a mimyor system. Once the austempering practice was established with tho fluidized bed, the conveyor system m l d be arfdcd. As such, Kemp Developinait Corporation did not pirsuc the plans for the fabrication and instaliation of the ccmvcyor systm a€tcr this dwisim by Quality Electric Stw.1. Tho d&gm for the fluidized bed chambers for aurstmitidiig and austmpering were given by Kemp Development Corporation to Quality Electric Steel, who got the cltanibers fhbricatcd at their awn cost.

2. Duriqg the ccy~lrse of initial trials on the austenitizing chamber, it was found by Quality Electric S t d that the maximum temperature that could be attained in the chamber was d y about 1400 F, Shce the tempt^&^^^ m i r e d for austenitiziiig WBS 1650 F, it was clear that austenitizing of steel casting could not be done in this chamber. A m&ng atkaided by Dr. Malur Srinivasan (TexRs A&M Uiiiversity), A&, Bill Kemp (Kemp Development Carparation), Mr. Ron Ledbetter (Quality

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Ekfric Steel) and the fabricator of the chamber (subcoiltrctcd by Quality Electric steel) ai the premises of Quality Electric Stcd to resolve. this pmblm resulted in the follmving niodificatians. I%@ austmithhg chamber fabricated by Quality Eloctrio Stcel would be used for nustmpering instead of mustmizing, sin= the maximum temperature attainable { 1400 P) was adequate for mwstempt%'hg. For austenitiziirg, a smaller fluidized bed available at R q Llevelopmt Corporation would be used for further work. Since the austmitizing and the ausimnpariug arc required to be done in quick succession I the fluidized bed at Qndity Electric Steel wmId be moved to the p r d a s of Kmp Development Corporation. K m p Development Corporation would then pdonn the nustmitizing and austempering treatments at z charge of $300 per sample.

3. Wien the fluidizcd bcd chamber ivns tfnnsfcrred from Quality Efectric Stecl to Remp Dwdopmmt Corporation, mic ofthe engineers at Kcmp Developnmt Corporation fclt that the: maximum teirrperxturc zzttainablc was only 1400 F bemuse of a flaw in tho wiring conucction treatment afthe heating clcmmts. Hc climgcd tho connection with intcnt to draw more power from the mains and thmcby increase rhe niaxirniirn attaiilabfc tcnipcmturc. Hvwcver, when triaIs wcre made with the new c o r n d o n , the as the temperature attained being higher than beforc, the chamber (retort) flat sides bulged otttward and made contact with the heater panels and shorted thm. Thus us il~rnxc muild not be used any i%rther as n fluidized bed, but just as a fiimace with a stationary bed serving rn the mpport for the castinp, The project phn was therefore ag& modified to wkwitize in this (nvw stationary) fiirnace and then use the already wtvailnble and operable fluidized bcd at Remp Ilmlopmmt Corporation for austmpering.

4. By this time the termination date ofthe project (which had Wi extmdcd twice before) was approaching d thim were severe finmciel and other constrailits to msolve the problms with thc fluidized bed fabricated at Qudity Elwtric Steel, As w h the project plan bad to be modified tb subject only block one (instead of eight as p1me.d earlier) to the austempering procedure and study the microstructure and rndmnical properties of the black. The project concludad &r the microstmcture and mechanical properties were examined.

Significance of Austem~erinn of Steel in Fluidized Bad:

Austemperhg of staI is its isothmnd transformatioii at a tempcsrature below tlmt for pearjite transfarmatimi aid above that for martensite &rrtarion, so tlmt tlte microstructure consists of bainite. Awtenipxing o f l i s several poknthl advantages swh as increased ductility, @en& and toughness at a given hardness and, redud distortion ofthe product during heat. The major steps in austcmpming are amtwitizing, qumching to the austanpering temperature ad isothermal transformatian to bainite at this temperature. The austmitking temperature ranges from 1450 F to 1675 F and the austemperhg tempemture is usudy iu the range of 500 F to 750 P. +"hc most cdnunm equipment f ir the austempering temperature treatment is the salt bath. Howevm, fluidized bed firnaces have h usod h r austmpring small lots of highly alloyed steels such s1s tool bits, gun components and complex castings, The advantages ofthe fluidized bed inoiude the use of nontoxic mediums, flexibility to have controlled heating and mlixq rates iii a chosen window over a wide range of temperatures asrd dhm. It appears tkerefare apprapiate to examine the hibi l i ty a€ extending the use of fiuidized bed austempering to iow alloy she1 parts as well. Furthetinare, f ir Continuous austernpering, a fluidized bed imy be considered to be a better medium than a salt bath, whose heat trans& coefficients can be imgdarly a f f d by the superposition of the disturbaiice due to the motion of the parts in the molten salt bath.

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In Fig. 1 is showti n schcmntic diagram of the continaous crustenipering equipment as originally designed by the Kmip Development Corporation. In this concept, thc castings hang from a mi~veywr on racks approximately 18” squarc. The conveyor nims the castings approximatdy 2 f@ (vertical) at me the. A piston and ratch assmbfy pmvidas the 2 fmt stroke. The casting .has to pass rapidly from the austenitizing chamber to th ausianperhg chamber and themfore, at the section betwewn the exit: of tho austenitidng chamber and thc entry to thiz austmpdiu chamber, override cylinders are provided to 13 the casting clear of the bed in the aulustenitizing clianiber, pus11 it rapidly forward for about 4 fed and lower the casting into tho austempering chamber. The liftipls, forward movement and lowering opcrntions tnke place in a h i t 5 seconds, to minimize air cooling of the casting and dso to miiiimize the time in which thc castins is out of contact with tlic medium. Thc funlace clmnbcrs are lons, narrow and rccttawghv in cross section. The rctorts in the furnace chamber are made of sheet stcel with a thin wall so that it hwts uniformly and mito t warp. This thin slicet is I V G ~ C ? ~ to a rigid cnrricr at tlic top and a rigid fluidizing fiaiiic at tho bateom.

As in the summary, the conveyor system wns not built and incorporated into the fluidid beds fabricated at Quality Elcctric Steel. The retarts of fluidized beds were made of 0,037” thick steel. The hatm panels oftlik furnacc were mounted about 1” away from the outside o f the retort,

of Test Castings and Test Varinbles:

This task was pebrmed by Dr. Mdw N. Shivasan, formerly of Texas A&M University and employed by hmar University at Bcnumcmt at prescnt.

Test Castings:

Steel Composition;

Mer detailed discussions with thi? tcclmiml ntanagemnt personnel of Quality Ebctric Steel, the campition chosen fix the steel to be atistempered was as noted bdow. The considmtions included wstcrnperiiis response and tlia needs of Quality Elmtrio Steel.

Carbon: 0.25%, Manganese: 0.9%, Silicon: OS%, Phosphorus: 0.03% max, SuIfir: 0.025 itlax, Nickel: 0.45%, Chrmimi: 0.7!%% Molybdenum: 0.2%, Vanadium, Alumhimi, Titanium, Copper, Niobiuin, Zinc: traces. This wmsponds to ASTM A 487 Gmdc 4, that is widely used by Quality Electric Steel,

Test Bar Design:

The main cansideration here wag to evaluate the respnse of bars of different thichess to tho awtemperiqg variables. A detailed sketch of the test bar block i s shown in Fig. 2. Four test bm of thickness: 0.9 inch, 0.75 inch, 1.0 inch and 1.5 inch w m connected thmugli R block which also served partly as a rim. With this design, it was possible to mure that tach bar in the block, being from the s m c heat, had ncarIy identical casting conditions.

SEP 23 '97 12:27PM PROC. SERV. DIV. P. 6

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Design of Experiments:

Assuming that the austcnitizins treatmait i s propcrly done, the thm important variables h t affect the prqexties afkr mstanperitis are: tlic austcmpering tmperaturc, the austempciitlg Yime and the casting size (thidcntcss), It is now wdl established that the statistical dclsign of experiments is ant of the best methods to d ~ i a t e effect of the multi-variables with tt iniiiiiiium number of trials. Accordingly it was decidod to study the effect oftliesc three varinbles using a 2' matrix, with two replications. The designd experimental plan was as follows:

Table 1 - Statistical Design of Experimenh

~ I 6 I I 4- 1 ." I

Notes: $. is the upper iml of each variable - is &E lower level of each variable

Table 2- Upper Rnd Lower Levels of the Variables

Austenitizing and Austempsring Furn~ces:

The original plan was to suspend the top hooks of the wstiug blocks from the caiiwyor system designed by K m p Developnmt Carpomtion and move the. blocks at speeds mrresponding to the chosen austempering time. This plan had ta bo replad by mclnual loading, boldins atid unloadin3 for reasdfls already detailed in the sunmary. It WBS decided to austemper the mitire casting Mock instead of bars cut h r ~ the block to en3111-e that each bar wag subjected $0 uniform furnace treatment, garticularly since manual handling was involved. However, studies on the effect of awtexnpering would be restricted only to the. bars cut from the. black after austempering and the riser block would be wluded. The fiunction of the latter was mainly to act as a cmmnon supflbrr for the bars of differerit thickness during austmpsring. The purpose of adding the bars of 0.75 inch and 1.0 heh thick bars to the block, though only the upper (1.5 inch) and the lower (0.5 inch} levels were needed ii: the mperirnmtd matrix w~ls to serve as checks far the prediction$ based 011 the upptr and lower l e d examinations.

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RESULTS AND DISCUSSION:

As explained in the summary, it was possible to subject only one casting block to an austempehng troatmcnt. For this piirposc, thc casting bldck was auustenitized at about 1650 F for 2.5 hr in the furnace fabricated by Quality Electric Stccl, with t h sand bath king sttttiomzary. It was then quickly mwcd to the austcrnyering fluidized bed firnace available at Kcmp Developincnt Caq~aration whase temperature va&d in tlie mnge of 835 F to 774 F during the period the block was being quenched in the 1Furnw. Tn F i g 3 are shown the details of temperature variation wit11 time on the casting black surface and in the fluidized bed h m m . "lie top curve rcfws to 'the temperature d a t i o n on the surface of the a t steel black and the barkom c u m to the temperature variation in the austempering fluidized bed fiimace available at Kemp Development Corporation.

The four bars in the block were cut away after the heat treamimt and were machined into test bars fbr tensile testing and impact ksting. Piecces cut froin each bar were also palislied using stmdmi mallogmpbic p r d u r e s , All that tasks were pcrkmed in a reputed laboratory specializing in the needs of thc foundry industry. Thc results ofthe laboratory tests are sunmarized in Table 4 to Table 6. For comparattiw? purposes, the results obtained h m a cast: block tIat was directly cabled in air (normatized) from 1650 F are shuvm in Table 5 and Tnblc 6.

Table 4 .I Tensile Test Results on HA~~tempered4' Bars

Lab ID No Size, in

1,5 in. os05 keel bar 663-1 1 in, 0.505

kcel bar

keelbar I 663-3 1 0.5 in. I 0.357

I_

'0.2% offlieu. method

A m , q . i n Yidd Tensile ' %Elong. % Reduct. Briiiell strength, stnngth, 2 in. gaga afarea hdncss

Mi" nd I---

0.200 65,500 106,500 20.5 37.5 228

I 0.200 64,100 101,900 21s 40.5 NA

0.200 B3,900 . 101,900 22.0 44.0 NA -P

- , " , I-... I

0.100 63,000 104,8cK, 23.0 43.0 NA

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Table 5 - Tensile Test Results on Normalized Bars

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Lab ID Size, in -, Yield Tensile O/oEllang. % Reduct. Brinell

psi psi 13 in, 0,SUS 0.200 59.800 97,600 24.0 48.5 197

keel bar

No. sq.ln, strength strength 2 in. gag@ ofawa Hardness

664.1 3 in 0.905 0.200 61,200 97JXM 23.5 48.5 NA

664-2 - ~

-__. __I. .” .. . . L ...

keel bar

0.75 in. 0.505 0.200 60,800 99,400 24.5 44.5 NA keel bar 664-3

c-_ , .

0.S in, 0.357 0. loo 60,000 101,200 23.5 44.5 NA keel bar 6644

Table 6 - Charpy Imp& Tegt Results

1 .. S n s k 1 Sample Lclcarion I Tciiiperature I CVN, itIb I From 1.5 in ked bar +75 F 213-22-19 ____. C75 F I 34-33-32 -

Note: AI! tests in Tnbles 4-6 wre performed ns per ASTM A370.

It is seen that the teiisile strength and the yield strength values we rn~trgimlly better in the “austernpcd” bars t h in tlic i i a d i z d test pieces. Thc BHN ofthe ‘‘aaustempered” test piece is Significantly greator than explained that of the ntsrmalizcd test piece. However, the Chslrpy V=iiotch energy values are significantly lower in the “austemprd” test piece than in the normalized tmt piece.

The cwtmer’s reqtiimeitt for normalized (1650 P) and tempered (1250 F) castings in& from ASTM A487 Gr. 4 at Quality Electric Steel are: Tensile Strength: 90,000 psi and Yield Strength: 60,000 psi. It is evident h r n Table 4 and Table 5 &hat these rquiremenb are? in& both in &e ‘faustempeed” and normalized (not tempered) test pieces. However, in ra truly aus-l;emyered steel casting;, the yield streiigth in only about 10% lower than the tensile strength. The trend slwwii in the present “austcmpered” sample is thus mom in line with a normalized sample than with a truly austempered sample. Tlic tensile strength-to-yield strength ratios am sin2iIa.r at a given hardnew in susteinpercd nnd qumched-and-tmpeEd samples. (A typical ratio achieved at: Quality Electric S-1 for ASTM A487 Gr.4 iii the quenched-and miipcred caiiditian is 1.17). Thus it may be: inferred that thc “aushmpered” saiiple in the pxesent work (with nn iwerage mtia of 1,621 had w microstructure more related to the normalized state (avcrage ratio o f 1.63) than tho austenpred or quen&d-md+mpered states, Irt other words, instead af thc bainitic stnlcture found in ausbnpered state or tempered martensitic structure fmnd in the quenched-and-tempered stnte, the sample would in all likelihood have a predominance dppaarlite. The microsbvcture shown in Fig. ofthe ‘austernp&’ sunple ihows, not mrprisingly, a predominance of pearlite.

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An cxc?mination of t1ic tcmpsi-;tturc-tintc cwvc (Fig, 3) indicates that thc soaking conditioii for austcnitizing \vns according to norm1 practicc. Howcvcr, wlslwn t b casting was traiisfcrr-cd EO the fluidizing furnace, the surface tempemtiire dropped contiiiuously in rle fluidized bed, wm though the fluidized bed temperahire WBS remonably stcady. This suggcsu~ that thc cooling rate of tilie casting was toto0 fast for any arrest at the bainitic transformation teniperature, While the cooling rate was not €ast enough for martensitic trnsformation, the contho~is coolbg of tlio cadng hi the fluidizcd bcd, instcad of having m arrest of its tcniperatnre at around the fluidizd bed temperature resultcd ia the prcdoniinant Formation of pearlitc instcad of: bainitc. In what Wllo~vs, the likcly xwsm For this undcsirabtc rcsult wilt bc discussed.

Iit zt fluidizcd bed, unliko in a salt bath, both condirctioii aiid convwtion play cqunlly doiiiinant rcrics in dcetcnniniog the liwt exctiangc between rhc fluidizcd bcci incdiuiii and thc product being hoar trcatd. If t lx hcat cxchange proccss is such that mox hcnt Flirx is lost by tho prodtict than gained by it from rhhe iixdiuiii, the product would continuously cool, ssswiiiing thnt it was at a higher tanperratwe than the fluidized bed at the time of immersion, This seems to have ow~irrcd in the present experiment. Tlic obvious solution is to equalize the heat loss snd heat gain. This, jn principle, w be scliicvcd in tbe prescnt case by incrmsing the heat flux into the casting and rcduchg the heat loss from it. In the absence of sufficient data on the thcmml cltsractcrktics of the systcnis used, it is not possiblc to makc dcfi~iite recommendations for remedy. Howevcr, it appcars that thc fluidizing agent velocity in thc bcd was too high in thc prcscnt cxperimmt. contributing to hcrmed coiivcctive heat loss from the casting and restricting the time for condirction hcat trsursfcr to occur bctweeii thc hot mcdiuin Nid thc casting. Also tho use of a mdiurn with lowcr conductivity in &c medium inay have hdped to decrease the heat loss from the casting.

h the positive side, however, was the obsorvaticm that the casting “austempred” in the fluidized bed showed a much better siuface quality b i the sir cooled casting, Iwhg m h i m ~ l swle on the surface, Since both fie castings were austeiiitized wider similar conditions, it may therefarc bc inkred that the fluidizcd bed treatinent resuh in better surface finish IS a result of the mild “shot peening” action ofthc fluidized bod.

YUGGESTIONS FOR I!WTURE W+’

1, Develq a process madd using the finite clunmt mn&d to study the teniperature profifes in the fluidized bed as s hiiction of the medium varia4les such as tlic? type of medium, type and rate of flow of the fluidizing age116 pressure drop amss and dag the bed and &race variables like pbwer input, nozzle shape and size (membrane design) and others. Next it is ne~cssary to model the heat mchmgc between the fluidized bed medium aid the tmt costing. For this purpose, it is necessw to determiiic the heat transfer wfficient in tllo fluidizcd bed at tho castinghncdium inkdace zaffected by thc above variabbs. It would therefore be strongly advisabla to swk supplemwtnry fiiiding to make thc fluidized bad fabricated by Quality Electric Stcd &lfy fiwcticrml ta facilitate the colloctim of dmt noted above.

2. Couple tbe nwdel with the t c m p e r s l t u r e - t i i i i e - ~ ~ ~ ~ t ~ ~ n (TTT) diagram for the steel under consideration and sd& a good mibination of the variables fix the austempering miidition to be whieved, k, cquilibriurn af lieat flow between the medim and the casting during austmpering at a given temperature,

3, Perform expcrimcnts in the h iace and adjustments in the model interactively to get tlic dcsired results with actual castings subjected to austeiiipcring.

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Fig. 3. Cssting Hesting and Coaling Cunve (top) and Airstempering Flaidized Bed Tempcrnturc Curve (bettom)

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