1998_watkins_development of low cost carbon steel

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DEVELOPMENT OF LOW COST CARBON STEEL TUBEFOR SUGAR MILL EVAPORATORS IN SOUTH AFRICA

1Illovo Sugar Limited, Sezela Sugar Mill, PO Box 28502, Sezela, 42152Department of Mechanical Engineering, University of Natal, King George VAvenue, Durban 4000

AbstractDue to the recent droughts, the retube of Sezela evaporatorswas delayed for a few years, resulting in a need for a low costcarbon steel tube to retube five evaporators. A recent failurehighlighted the importance of the heat treatment and expan-sion characteristics of these tubes. This paper discusses thefailure of a carbon steel tube, the development of the heattreatment of locally manufactured tubing, the expansionprocedure for the tubes and other related problems with largeScale vessel retubing.

IntroductionSezela intended cascading carbon steel tubes from the 6,8 mKestner vessels to lower order evaporators when the Kestnerwas to be retubed with stainless steel tubes for the 1996-97season. However, the Kestner retube was cancelled inNovember 1995 and an alternative supply of carbon steeltubes had to be found. Pongola mill had used an inexpensiveheat treated carbon steel tube (SAE 10110) without problemsbut, due to the long delivery times (on average three months)the best that could be sourced was an SAE 10110 carbon steeltube, without heat treatment. This was considered satisfactoryat the time. (At the time of writing, these heat-treated tubeshad been in service for four years at Pongola mill without afailure.)SAE 10/10 tubing and expansion problemsProblems were experienced when expanding the SAE 10110tubes into the tube plates, and tubes were reportedly harderthan other carbon steel tubes. After repeated re-expansion ofcertain tubes during the season by mill staff and contractors, itwas decided to replace the tubes.Metallurgical examination of SAE IO/IO tubesTube samples of SAE 10110 were sent to the co-author(Bartholemew) at Natal University's Mechanical EngineeringDepartment for analysis to determine the cause of failure.A tensile test indicated that the tube was still in cold fmishedcondition. Microscopic examinations of the electric resistancewelded joint and the parent material were done. The micro-structure of the parent metal of all the specimens was found toconsist of slightly deformed grains of ferrite, with small areasof pearlite at the grain boundaries. The ASTM grain sizenumber was estimated to be nine or 10. The structure of theweld was similar although finer and containing some

widmanstatten ferrite. The microstructures were typicalthis type of tube in the cold finished condition. Vichardness tests, using a 10 kg load, were performed on microspecimens, which yielded the following results:Parent metal : 150-152 HVWeld metal : 202-209 HV.

DiscussionThe results of the tests and examinations performed indicthat the tubes were not in a condition suitable for expandThe tubes were in the cold finished condition and therefexhibited yield point values that were too high and too cto the ultimate tensile strength of the material, i.e. tensileyield ratios of 1,l to 1,O. A more acceptable ratio wouldsay, 1,7 to l ,O (refer to Figure 1).When the yield strength and the tensile strength are closeach other, the control required for tube expansion intotube plate becomes unacceptably tight. This is because tubes need to be expanded sufficiently to put the steel intoplastic range, but without encroaching on the ultimate tenstrength or, in other words, fracturing the tube end.The second point to be discussed here is the effect of electric resistance weld. As can be seen from the results ofhardness tests, the weldment was 50 to 60 HV points higthan the parent metal. A much higher stress would be requto permanently expand this area on the periphery of the tu

STRAIN

Figure 1. Stresslstrain curves for SAE 10110 tube versus Tubecon tube, both cold rolled condition. Curves 1 and 2 are SAE 10110 and c3 and 4 are Tubecon. Apparatus is 500KN Avery Un iversal Testing Mac

Proc S A@ Sug TechnolAss (1998) 72


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Development of low cost carbon steel tube for sugar m ill evap orato rs CS Watkins & JK BartholemeIt is clear from the above that the tubes must be heat-treated Purchase of new tubesas required by the tube specification, i.e. the tube must be Due to the large scale tube failures only becoming evideannealed or normalised, furthermore any weld metal rein- late in the year when there were budget pressures, a tendforcement must be removed prior to expanding. was issued with a wide range of carbon steel specifications,

Planning for 1996-97 off-cropDuring the 1996-97 season, the evaporators were undergoinglarge scale tube failures as a result of corrosion, on the vapourside, that had taken place due to high acetic acid levels in thejuice. The high level of acetic acid was a result of bad canequality during the preceding drought years. Consequently,plans were made to retube six of the 13 evaporators.Stainless steel 439 tubing for the 1 81 Kestner retube had beenpurchased at a cost of R33,OO per metre (3 043 tubes X 6 800mm long).

The 5B2 evaporator with the SAE 10110 tubes requiredreplacing (2 480 tubes X 2 225 mm long).Towards the end of the season, carbon steel tubes in 2Aand 2B evaporators were failing at a rate of about 200 perweek. (2A: 3 613 tubes X 3 505 mm long; 2B: 3 294 tubesX 3 505 mm long).3A and 3B evaporators had 304 stainless steel tubes, 17and 20 years old respectively. These tubes were snappingoff at the lower tube plate and causing significant problemseven though the incidence was relatively low (3A: 3 820tubes X 2 550 mm long; 3B: 2 125 tubes X 2 090 mm long).

Table 1. Vessel and tube lengths.

listed below, that would suit evaporators.Tenders were called for the following tube specifications:

Specification for seamless cold drawn low carbon steheat exchanger and condenser tubes SA- 179lSA 179-M.Specification for seamless medium-carbon steel boiler ansuperheater tubes SA-2 1OISA-2 1OM. Note: hot-finishetubes shall be annealed unless agreed to the contrary writing.Steel boiler and superheater tubes to BS3059 Part 1 1987ERW ST 320.BS3601: 1974 Grade 320 solid drawn normalised.BS3601: 1994 Grade 320 ERW l normalised.ASTM A450: fully annealed material ASTM A214.

Applicable standards and codesBS5500 Unfued pressure vessels.ASTM A370: Methods and definitions for mechanictesting of steel products.

e ASTM A450: Specification for general requirements fcarbon, ferritic alloy and austenitic alloy steel tubes.ASTM E246: Electro-magnetic (eddy current) testing tubes and welded tubular products, austenitic stainless steand similar alloys.ASTM A763: Intercrystalline corrosion testing.

Item12345 Table 2 refers to the schedule of different materials, cost p

metre, delivery and source. Delivery was a critical factor budgets were only approved late in the year.

Number oftubes2 4903 6133 2943 8302 135

15 362

Evaluation of ofers Table 2. Analysis of tender tube prices received and delivery (November 1996 prices).

Vessel5B22A2B3A3B

Total length(m )5 502,9012 663,5711 545,479 766,504 462,15

43 940,59

Tube length(mm)2 2103 5053 5052 5502 090

Proc S Afr Sug Technol As s ( I 998)

Item12345678

Specification : Low carbon steelBS 30 59 PT11 steel 320 wlout heat treatment (2,5 mm W T)(1) with heat treatmentASTM AI 79/A 450 seamless (2 mm )ASME SA 21 0 GRI 2,6 mmBS 3059 PT1 3,25 mmSAE1011ONS94ASTM 179lA450 2 mmASTM 179lA450 stainless steel, 50,s X 1,2 wall thicknessa] 304L ssb] 439 ss

Cost per metreR12,84R17,09(R14,97)R21,81R22,13R33,50R18,78R27,20R22,25

R31,36 to R47,39R33,OO

Deliveryon timeon time

late14 weeks late

l atelatelatelatelatelate

Import/locallocallocal

?import

???

importimportimport


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Development of low cost carbon steel tube for sugar mill evaporators CS Watkins& JK BartholemTubecon was selected as a potential supplier, subject toverification of the material certification and a visit to theirworks to determine their capability of handling 44 000 metresof tubing. Early budget approval was sought to ensure thatTubecon could place an order with ISCOR for the supply ofsufficient quantities of cold rolled plate. Due to large quantityand long delivery times, Tubecon could not undertake theheat treatment so it was decided to handle this in-house. Thetube supplier agent offered ends annealed at Sezela at a costof R68 000. After detailed discussion with them and the co-author, it was decided to heat treat complete tubes at ElginWorks in Durban. Ends only annealed would result in atransition phase between the cold rolled and annealed sectionthat could result in an area highly susceptible to corrosion.

Establishing the heat treatment procedureThe aim of annealing the tubes was to produce a ductile tubefor effective expansion with a uniform grain structure. Thehigher the heat soak temperature the better the result wouldbe, but the more tube distortion and scale build-up on theexternal surface of the tube. This would result in difficulty ininstalling tubes and adversely affect the heat transfer rate.A brief review of the basic metallurgy of hypoeutectoidplain-carbon steels

100%Fe Weigh t pe rcen t carbon

Figure 2. Transformation of a 0,4% carbon hypoeutectoid plain-carbon steelwith slow cooling, with 0,07 arbon shown typical of the tubes used.If a sample of a 0,4% plain-carbon steel (hypoeutectoid steel)is heated to about 900C (point a in Figure 2) for a sufficienttime, its structure will become homogenous austenite. If thissteel is then slowly cooled to the temperature shown at pointb in Figure 2 (about 77SC), proeutectoid ferrite will begin tonucleate heterogeneously at the austenite grain boundaries. Asthe alloy is continuously cooled from the temperature at point

b to that at c in Figure 2, the proeutectoid ferrite will contto grow into the austenite until about 50% of the samptransformed. The excess carbon from the ferrite that is forwill be rejected at the austenite-ferrite interface intoremaining austenite, which becomes richer in carbon.While the alloy is cooled from the temperature at pointthat at c, the carbon content of the remaining austenite wiincreased from 0,4 to 0,8%. At 723"C, if conditions approing equilibrium prevail, the remaining austenite will be verted to pearlite by the eutectoid reaction.Continuous-cooling transformation or a hypoeutectoidplcarbon steel

Time. secFigure 3. (a) continuous-cooling diagram for 0,3% plain-carbon steel (Mn , 0,25% Si). The isothermal diagram for the steel is shown in dashed(6) C-T diagram with selected cooling rates decreasing from left to Diamond pyramid hardness (DPH) values are indicated inside circles focooling curve.Cooling at the slowest rate indicated by the curve withDPH value of 139 produces the softest structure, whichmixture of proeutectoid ferrite and pearlite in almost eamounts. This structure is similar to that obtained by (furnace) cooling this type of steel. Increasing the coolingslightly produces finer pearlite and slightly less ferrite, the hardness increasing. Increasing the cooling ratefurther drastically reduces the amount of proeutectoid fethat now outlines the former austenitic grain boundIncreasing the cooling rate still further causes a split tformation to occur. The rate of cooling is so fast that little proeutectoid ferrite is formed. Instead, pearlite outhe former austenitic grain boundaries. Some bainiformed, and martensite was formed when some ofaustenite remained untransformed. Increasing the coolingeven more increases the amount of martensite formed,still gives a split transformation. Some proeutectoid fand pearlite are formed at the former austenitic grain bdaries. Small amounts of acicular bainite were also forNote that the hardness of the sample increased markedlyto the large percentage of martensite. Finally, the last sture is almost completely martensitic.

Proc S Afr Sug Technal Ass (1998) 72


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Development of low cost carbon steel tube or sugar mill evaporators CS Watkins & JK BartholemeAnnealing and normalising plain-carbon steels

SPHEROlOlZlNG ANNEAL

0 020 040 060 080 1.00 1.20 1.40 1.60CARBON %

common types of annealing processes that are applied to commercial steel are ull annealing and process annealing.Microstructural changes that occur during annealingDuring annealing the changes in microstructure that occur cabe subdivided into the following major processes:

Recovery. In this process, the cold-worked metal is heateto the required temperature so that dislocations can barranged into lower-energy configurations.Recrystallisation. When a cold-worked metal is heated tohigh enough temperature, termed the 're-crystallisatiotemperature', new strain-fiee grains are formed by thmigration of large-angle boundaries of high mobility.Grain growth. Continued annealing of a recrystallisestructure promotes the formation of a more stable graistructure. In this process, larger grains grow at the expensof the smaller ones.

Selection ofmos t appropriate heat treatment spec$cationThe co-author (Bartholemew) ~rovoseda number of he, ' L~ i g u r e . comm only used t empera tu re r a nges fo r annea ling p la in - treatment procedures for tubes to be supplied to Se~ela.Acarbon s teel s (af ter Digges et al., 1966).can be seen from the brief discussion on the heat treatment o

Most useful engineering alloys must possess an appropriatecombination of strength and ductility. Ductility in metals andalloys allows them to be deformed plastically by various fab-rication processes into the desired shape, without fracturing.During plastic deformation or cold working, the main reasonfor the increase in strength is due to the increased generationand rearrangement of dislocations. In order to make cold-worked metals ductile, they are annealed at appropriate tem-peratures. During annealing, the highly distorted cold-workedstructure is partly or completely returned to a softer, moreductile structure containing fewer dislocations. The two most

low carbon steels there was a need to do actual furnace triaat Natal University to determine the most effective procedurSample lengths of the tube were obtained and subjected tpredetermined heat-treatment cycles as follows:

Normalised : 900 - 920CAnnealed : 910-930CTempered : 590 - 610CSub-critical annealed (Spheroidised) : 680 - 720C.

On completion of heat-treatment, tensile test specimens wermachined and tested. In addition to these, one specimen in thas-received (cold-finished) condition was prepared.

Table 3. Analysis o f different heat treatments.

Note: Elongation ( O h ) describes the extent to which the specimen stretches before fracture.

170

DescriptionAt Univ ersitv of NatalAs received - o heat treatment, i.e. cold finishedNormalisedAnnealedTemperedSub-critical anneal (spheroidised)Sub-critical anneal (spheroidised)At Elgin WorksSAE 10110Network, not heat treatedST320-Tubecon, not heat treated

Proc S Aj? Sug Technol Ass (1998) 7

Heat "*temp("C)

-900-920920600700700

710-730690-710

-

H O I ~ime(hours)

-onetw oon etwo

threethreetw o--

Cooling

-air

furnaceair

furnaceairAi rAi r--

Yield stress(Mpa)

352,4249,2235,7323,l308,9241,3292,O308,s365,O324,l

'ltimatetensile stress( M P 4

412,7335,4309,s381,5374,O354,6371,3375,O400,O374,7

Elongation(W

17,432,230,O33,O39,424,427,s25,318,s31,2


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Development of low cost carbon steel tube or sugar mill evaporators CS Watkins& JK BartholemResults obtained from these tests indicated that normalising at920C would be the most acceptable treatment. However,primarily because of the economic implications and thedanger of distortion, it was decided to modify the subcriticalanneal cycle to determine whether a workable result could beobtained at a lower temperature. Two further tests wereprocessed:

Sub-critical anneal (spheroidised): 690C for three hours,followed by air coolingSub-critical anneal (spheroidised): 690-710C for threehours, followed by furnace cooling.

DiscussionThe results of the tensile test performed on the as-receivedspecimen confirmed that the tube was in the cold-finishedcondition. Of the first four treatments, the normalisingtreatment yielded the best set of results. However, the heattreatment advisor at Elgin Works expressed concern that thetubes would distort during such a cycle, since this treatmenttook place at the high temperature of 910C.If the ends were not fully annealed it would take longer tomechanically expand the tubes into the tube plate; this wouldincrease the cost of installing the tubes in the evaporators.This being the case, the soaking time of the sub-criticalanneal treatment was increased so as to ensure completespheroidisation, thus rendering the steel in a soft, ductilecondition. It was also decided to try the two forms of cooling,i.e. still-air cooling and cooling in the furnace, in the hopethat the desired effect could be achieved with the quicker aircool. The difference between yield and tensile was the same,i.e. 113 MPa for the final two tests, with the tensile to yieldratios being near enough to 1,5 to 1,O. It was thereforedecided to heat-treat the tubes using the following cycle:

Heat to 7 10-730CHold for three hoursRemove from furnace and cool in still air.It was expected that the results from production heat treat-ment would differ from those achieved under laboratoryconditions. Steps were therefore taken to monitor the heattreatment results after the first production batch wascompleted.It should be pointed out here that spheroidisation of thestructure, which is the transformation effect that renders thesteel softer and more ductile, is not as predicatable a treat-ment as is normalising, which refines the grain size andhomogenises the mucture . In the case of sub-critical anneal,the degree of spheroidisation achieved is dependent on theholding temperature and rate of cooling, i.e. the longer thetime at temperature and the slower the cooling rate, the softerand more ductile the steel. However, furnace cooling, whichis part of the sub-critical annealing cycle, can often result inyield and tensile strengths which are below the minimumrequirements specified for the steel and thus great care had tobe taken when determining the actual cycle to be used. Inaddition to the acceptable tensile properties obtained, the

desired reduction in the hardness of the weld was achieved. Samples were taken after the first batch had heat treated in the Elgin furnace and tested to monitoreffectiveness of the heat treatment. The results were inwith the laboratory test and Elgin were instructed to proas follows:Heat to 720C (710-730C).Hold for three hoursRemove from furnace and cool in still air.

Heat treatment at Elgin WorksElgin Works has an International Standards Organisprocedure for heat treatment, which was relied uponconsistency. To limit distortion, the hmace had to be loto ensure no direct flame impingement on tubes. Tubes hbe placed to get even heat distribution. Each heat treatmbatch had thermocouples installed to monitor the furtemperature of the heating and cooling processes. Aftefirst heat treatment, a tube was sent for analysis andresults were as follows:

Yield stress : 292 MPaUltimate tensile stress : 37 1 MPaElongation : 27,8%.The difference between the ultimate tensile stress andyield stress was sufficiently large to allow the mechaexpansion of the tubes into the tube plates without posingproblems (refer also to Table 3). Since the results acceptable the tubes were delivered direct from the tubeto Elgin Works for heat treatment. To straighten bent tubsimple two-man press was used. To remove the scatrommel was designed using an old 800 mm steam mounted on four trolley wheels and driven by a mgearbox and rubber wheel.Essential aspects of quality control

Full chemical analysis and tensile test should be donfinal tubes delivered to site.Each bundle of tubes to be traceable from the tube msite with the relevant manufacturers' certificates, tubetest certificates and heat treatment certificates.Test samples to be supplied attached to relevant bundles, i.e. flare and flattening tests.Establish a claims procedure or penalty system incontract with the heat treatment company and tube miout of specification tubes, i.e. bent due to heat treatmeweld failures, or oversize tubes.Check the required tube length in the vessel at least tbefore final order is placed as tube plates could moved since the original drawing.Support large tube plates in the centre when installingtubes to prevent sagging.Establish a tube expansion procedure and a quality plensure the tube expansion procedure is followed.

Proc -SAfr Sug Technol Ass (1998) 72


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Development of low cost carbon steel tube for sugar mill evaporators CS Watkins& JK Bartholemec Make sure copies of the standards to which tubes are to be ensure that the steel grade complies with specification anmanufactured, heat treated and installed are available, read that the mill's quality control is adequate to manufacture and understood by all parties. the desired specifications. The heat-treatment procedure mu

Do not start tube removal until tubes are manufactured and be carefully worked out and tested both in the laboratory anaccepted with respect to quality, or tubes have landed in in production.the country. If, however, there is sufficient budget and time is availabthe correct grade of stainless steel should be considerAnalysis of tube cost because in the long term, stainless steel tubes are cheaper thThe final tube cost was R14,97 per metre delivered to site. carbon steel tubes.Sezela had budgeted R21,OO per metre; the next acceptable Acknowledgementsprice was R22,13; this gave a saving of R7,16 Per metre or he author would like to thank John Field for his support aR3 13 3 18,OO on the retube programme. This saving allowed Sezela mill staff fo r their effort in this pro,ect.one additional retube, which was not budgeted for.

REFERENCETable 4. Summ ary of final tube costs. Digges, TG et a1 (1966). Heat treatment and properties of iron asteel. p 10 In: NBS Monograph 88.Tube analyses -University

Cooperheat inspection at Tubecon M ill I 10 808,OO7 350,OO

Heat treatment at Elgin I 55 200,ooTransport from Elgin to site I 10 000,OOTest plates for expansion I 6 900,OO

APPENDIX 1Cost comparison between carbon steel andstainless steel evaporator tubes.Sezela has 13 evaporator vessels with an average of 3 4tubes per vessel and an average tube length of 3 915 mm.

Assume a life of 20 years for stainless steel.Tube manufacture

Total tube co st564 688,3 1

R654 946,3 1-Total tube length (m)

between R32,OO and R49,Oo per tube, depending on the Stainless steel tubes over 20 yearscontractor's rate. All the above are at January 1997 prices. Tube cost 3 425 tubes X 3,915 mltube X R33,OOlm R442 493,O

43 756,49 Stainless steel tubes cost R33,OOlm.Cost to fit a tube was R45,OO.Cost per metre

Cost comparison between carbon steel and stainless steelevaporator tubes

R1497 Assume an average life of carbon steel of 6,7 years.

Because of the large number of retubes required, and due to atight budget, carbon steel tubes were used. A cost comparisonover the long term showed that stainless steel tubes are atleast 45% less expensive, and at best 78% less expensive thancarbon steel tubes, depending on whether the carbon steeltubes are re-used in shorter tube length vessels (Appendix 1).Stainless steel tubes can have a life of about 17 to 21 yearsdepending on damage done by skato skalo mechanicalcleaning. Carbon steel tubes have a life of about four to sevenyears at Sezela, and if tubes are re-used in other shorter tubelength vessels, tube life can be extended to 14 to 15 years.

Three retubes in carbon steel would give an average life Had 439 stainless steel been used at R33,OO per meter, tube 20 years.cost would have been R1 443 964,17. The labour cost to Cost to remove and replace a tube was R45,OO.retube the vessels (excluding the cost of the tube itself) was

Conclusion

Labour cost 3 425 tubes X R45,OOltube R154 125.Total for stainless steel R596 617.Carbon steel tubes over 20 years(re-using tubes only once)Tube cost 3 425 tubes ~3,915mltube X R14,97/m x2(one set re-used mls + 2 sets new mls tubes) R401 462,OLabour cost 3 425 tubes X R45,00/tubeX 3 R462 375.Total for carbon steel R863 837.Difference (carbon steel - stainless steel totals) R267 220.% difference (carbon steel- stainless steel totals) 45Carbon steel tubes over 20 years(using tubes only once)

Carbon steel tubes in the cold finished condition can be heat- Tube cost 3 425 tubes x3.915 mltube xR14.97lm x3 R602 193.treated to a condition such that they are suitable for expansion Labour cost 425 tubes R45,001tube R462 375,Ointo the tube plate. This course of action is usually onlyrequired if, for example, there is a late season failure of Total for carbon steel R1 064 568,Oevaporator tubes, or perhaps a limited budget. For the whole Difference carbon steel- R467 95 1process to be successful the tube mill must be inspected to % difference (carbon steel- stainless steel totals) 78172 Proc S Aj? Sug Technol Ass (1998)

1998_watkins_development of low cost carbon steel

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