The AFNOR 15CDV6 steel is high strength steel with relatively low level alloy content. Byprocessing the steel through ESR with inoculation a marginal increase in strength and furtherincrease in ductility and notch toughness can be obtained. The strength of the steel is inadequatefor its use in fabrication of rocket motor casing in the Indian Space Programme. By increasingboth carbon and chromium content of the AFNOR 15CDV6 steel strength can be increased,primarily as a cost effective for space launch vehicle applications. Welding is a major step in thefabrication of most of the pressure vessels, structures and equipments. Steels with carbonequivalent in excess of 0.40wt% shows a tendency to form martensite on welding, and thereforeare considered difficult to weld. This ESR modified 15CDV6 material has a carbon equivalentvalue of nearly 1.0 that classified it as a very difficult to weld steel. In the present work, ESRmodified 15CDV6 material welding was carried out successfully by Auto GTAW as well as ManualGTAW, without preheating. This was carried out by modification of welding parameters, cleaningprocess, tacking sequence etc. Mechanical Properties of the Weld are meeting requirements.
INTRODCUTIONAFNOR 15CDV6The AFNOR 15CDV6 steel is high strengthsteel with relatively low alloy content. Theincipient development took place in France.This Ultra high strength steels are becomingincreasingly important in aerospace, defence,power generation and in other applications
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1 ANITS College of Engineering, Sansivalasa, Visakhapatnam, AP, India.2 Ms/Walchandnagar Industries Ltd., Visakhapatnam, AP, India.
industries. Because of its good strength-ductility combination and ease of fabricationthe material has been extensively used inrocket-motor hardware in the Indian SpaceProgramme.
Due to the characteristics of Chromium andMolybdenum, compatibility with highhardenability and high strength emphasizes.
Research Paper
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Int. J. Mech. Eng. & Rob. Res. 2014 M Chandra Sekhar et al., 2014
Vanadium plays vital role in controlling graincoarsening and with the addition of certaininoculants mechanical properties like strengthand ductility increases effectively.
Metallurgical Aspects in AFNOR15CDV6The steel AFNOR 15CDV6 is a high strengthbainitic steel containing low concentrations ofchromium, molybdenum and vanadium asalloying elements. The micro structure of15CDV6 steel in quenched condition consistsof predominantly lower bainite and a smallproportion of lath martensite.
Earlier attempts to improve the propertiesof this steel through Electro Slag Remelting(ESR) resulted in large increase in ductility andtoughness with little or no increase in strength.A small increase in strength and furtherincrease in ductility and notch toughness wereobtained by inoculating the steel with 0.2%addition of niobium or zirconium during ESR.
A recent study has reported that strength ofmixed microstructure containing temperedmartensite and bainite can peak at anintermediate volume fraction of temperedmartensite of around 0.75. Increase fractionof martensite in a mixed martensite-bainitemicrostructure can be achieved by the additionof alloying elements, which retard the bainitereaction.
Alloying elements, particularly carbideforming elements, greatly retard the ferrite-perlite reaction and chromium, in particular, isvery effective in retarding the bainite reaction.This often results in a bay of high relativeaustenite metastability being formed betweenferrite-pearlite and bainite reaction whilstmoving the whole TTT diagram to longer time.
In view of the effectiveness of chromium inretarding the bainite reaction, it was expectedthat increasing the chromium content inmodified 15CDV6 steel would lead toincrease in strength. Moreover addition ofchromium up to 6% can also retard softeningupon tempering. Accordingly, chromiumcontent in the modified 15CDV6 steel wasincreased from 1.5 to 4%.
As the strength level of steel increases, thedefect tolerance level decreases. Therefore,a high degree of chemical and structuralhomogeneity and freedom from inclusionsbecomes important for ultra high strengthsteels. For this reason, subjecting steel toremelting process such as ESR becomesnecessity.
Promoting heterogeneous nucleation duringsolidification by inoculation technique refinesthe grain size and thereby results inimprovement of strength and ductility. Forhomogenous nucleation low contact angle withsubstrate and minimum misfit of lattice isrequired.
Yield strength for ESR Modified 15CDV6steel (0.26C, 4cr, 0.68 Mo), i.e., 1400-1420 Mpais almost 40% higher than yield strength of theTi-inoculated ESR standard 15CDV6 steel(0.15 c, 1.50 Cr.0.8 Mo), i.e., 990-1020 MPa.
Inoculation with titanium during ESR cansignificantly reduce the grain size resulting inimprovement in mechanical properties of thesteel. The increase in toughness and ductilitywith increase in strength at higher chromiumand carbon contents. This due to precipitationof very fine uniformly dispersed chromiumcarbide particles, which resists the crackpropagation.
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EXPERIMENTAL WORKWelding Development ActivitiesWelding is a major step in the fabrication ofmost of the pressure vessels, structures andequipments. Steels with carbon equivalent inexcess of 0.4wt% shows a tendency to formmartensite on welding, and therefore areconsidered difficult to weld. ESR Modified15CDV6 has a carbon equivalent value ofnearly 1.0wt%, that classifies a very difficult toweld steel.
From weldability point of view, it’s verydifficult to weld, this exhibits as follows byCarbon Equivalent (CE).
5
%%%6
%%% VMoCrSiMnCCE
15
%% NiCu
If CE > 0.40 prone to cracking tendency inHAZ.
Initial trials were carried out to establish thewelding parameters for Welding ProcedureSpecification (WPS) activity. Although, withoutpre-heating it was difficult to weld the plates,but trials were taken and found successful.
Plate size used: 500 mm (L) x 150 mm (W)x 7.8 mm (thick)
For WPS qualification it was necessary toweld continuously 10 weld coupons. AfterWelding Heat treatment and subsequentlyMechanical testing activity was carried.
Parameter Settings: Welding parametersettings took place, by welding almost 9-10plates. Preliminarily problem faced likepenetration issues, porosities, LF, etc. whichwas overcome after taking proper action. Bythis Phase-0 was completed.
For Phase-I further WPS activity was carriedout by welding continuously 10 weld couponsby Automatic GTAW process. Radiographyand UT Test were satisfactory.
HEAT TREATMENTIn addition of niobium content of about 0.10%and vanadium content of 0.25% this hasdeleterious effect on the toughness propertiesof low carbon welds, because of this ESRmodified 15CDV6 Welded Plates were takenfor heat treatment activities, i.e., weldingfollowed by hardening and tempering Heattreatments.
Hardening
Loading < 600 °C – –
Hardening 920 + 10 °C 1 hour water
Quench Delay – < 45 sec –
Stress Reliving: To be carried out within 2 hrs ofhardening
Loading < 200 °C – –
Stress Relieving 300 + 10 °C 1 hour Air
Tempering
Loading < 300 °C – –
Tempering 505 + 5 °C 2 hour water
Quench Delay – < 45 sec –
Table 1: Heat Treatment Cycle
Process Temperature Time QuenchMedium
Note: The Heating rate shall not be more than 100 °C/hour).
WELDING PROCEDURESPECIFICATIONWelding Procedure Specification Qualificationincludes following documentation:
1. Weld Data sheets
2. Radiographic Test report
3. Ultrasonic Test report
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4. Heat treatment details
5. Transverse Tensile Test report
6. Fracture Toughness Test reports
RESULTS AND DISCUSSIONThe welding of ESR Modified 15CDV6 Platesby Automatic GTAW process withoutpreheating, was difficult because of CarbonEquivalent (C E ) > 0.9. As increasing carbonand chromium content in this steel, there hasbeen an increasing amount of martensite inthe mixed microstructure of martensite-bainitebeing (0.26 C, 4.0 Cr and 0.68 Mo) ascompared to the microstructure of standard
Table 4: Strength Specifications for Various Quench Medium
Type UTS ValueMin.
UTS ValueMax.
0.2PSMin.
0.2PSMax. Remark
Initial ProblemsPorosities: porosities were observed in initialtrials, but later on by proper cleaningmethodology it was eliminated. Wire cleaningby means of scrapper and followed by threetimes wire cleaning with acetone was carried
out. Entire welding carried out in clean dustfree hall, by taking extra precautions not to getany contamination.
After welding of 10 nos of Coupon platesfollowing tests were carried out:
Wil JOB NO - 78371 CONSOLIDATED RESULTS OF TEST PLATES - ESR Modified 15CDV6 ( Oil Quenched)
2 TP 20 OK OK
3 TP 21 OK OK
4 TP 22 OK OK
5 TP 23 OK OK
OK OK
6 TP 24 OK OK
7 TP 25 OK OK
10 TP 30 OK OK
8 TP 26 OK OK
9 TP 27
WI JOB No - 78371Consolidated Results of Test Plates - ESR Modified 15CDV6 (Water Quenched)
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Visual Examination: The weld was free fromvisual defects like Lack of Fusion, Undercuts,excess penetration, visual pores, etc.
Dye Penetrant Test: No DP indicationobserved on weld.
Radiographic Test and Ultrasonic Test: NoRadiographic defects and NO UT defectsobserved in the weld. Weld was absolutelysound.
Mechanical Testing: Transverse Tensile testand Fracture Test carried out on Heat treatedWeld specimen.
CONCLUSION• ESR Modified 15CDV6 Welding by Auto
GTAW process can be possible by properparameter setting and precautions withoutpreheating technique.
• Oil Quenching Treatment to the WeldCoupon gives success rate 100% than theWater Quenched Weld Coupon.Improvement in mechanical properties ascompared to Water Quenching treatment.
• Oil Quenched Weld Coupon gives 12.4%more UTS and 14% (maximum) more 0.2PS value than Water Quenched WeldCoupon.
• Not much improvement in % elongationobserved either by Water Quenching or byOil quenching.
REFERENCES1. Bandyopadhyay T R, Rao P K and Prabhu
N (2012), “Improvement in MechanicalProperties of Standard 15CDV6 Steel byIncreasing Carbon and ChromiumContent and Inoculation with TitaniumDuring ESR”, ISRN Materials Science.
2. Journal of Material Science, Vol. 42,No. 14.
3. Suresh M R, Sinha P P, Sarma D S, BallalN B and Krishna Rao P (2007), “Study ofWelding Characteristics of 0.3C-CrMoV(ESR) Ultrahigh Strength Steel”, Journalof Materials Science, Vol. 42 , No. 14,pp. 5602-5612.
