the effect of stress relief parameters on the mechancial properties of pressure vessel steels and...

8/2/2019 The Effect of Stress Relief Parameters on the Mechancial Properties of Pressure Vessel Steels and Weldments

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By accaptanc* of Khit arficlfr, thapublisher or recipient ncknowlecigasth U.S. Govammant** right toftmln a non#xc(uiiv#, royaltytraaljcm in and to any copyrightcovtring tha articla.

THE EFFECT OF STRESS RELIEF PARAMETERS ON THE MECHANICALPROPERTIES OF PRESSURE VESSEL STEELS AND WELDMENTS*

D. A. Canonico and W. J. StelzmanMetals and Ceramics Division, Oak Ridge National Laboratory

Oak Ridg e, Tennessee 37830

Gentleme n, it is an honor to have been invited to participate in yourSymposium. Before I start my formal lecture, I would like to introduce myself

and the company for whi ch I work. My background is in metal joining (bothbrazing and weld ing ) and mechanical properties (in particular toughness) ofmaterials for pressure vessels and piping. I work at the Oak Ridge NationalLaboratory, an organization that is operated for the Energy Research andDevelopment Administration of the United States Government by the Union CarbideCorporation. The Oak Ridge National Laboratory employs approximately 5000 people

in various research and support divisions. I work in the Matals and Ceramics Division;a group of approximately 300 peo ple, half of whom have at least one degree frojnan accredited university. Our division is divided into three research sections.I am the group leader of the Pressure Vessel Technology Laboratory in theMaterials Engineering Section. The responsibility of my group lies in thecharacterization of materials for pressure vessel and piping applications. Wehave been involved in the investigation of low alloy high strength steels forlight water nuclear pressure vessels since 1966. We are part of a program at

*Research sponsored by Energy Research and Development Administration undercontract with Union Carbide Corporation.

y NOTICETHIS REPORT AM. .IHgnTgT.t; l treproduced from the best availablethe broadest possible avail-

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Oak Ridge ational Laboratory that is laiown as the Heavy Section SteelTechnology program. If anyone is specifically interested in that program,I will be glad to discuss it during our free time.

The lecture that I will present now was undertaken as part of theInvestigations that we have conducted on the Heavy Section Steel Technologyprogram. The HSST program was instrumental in characterizing the propertiesof 300 mm (12 in.) thick SA 533 Grade B Class 1 steel. This steel is one of the twosteels from which light water reactor pressure vessels are fabricated in theUnited States. The pressurized water reactor utilizes plate thicknesses

approaching 300 mm (12 in.) in their fabrication. The pressure vessel manufacturerbuys the plate in accordance with the specification in Section II Part A ofthe American Society of Mechanical Engineers (ASME) Boiler and Pressure VesselCode. More than likely he imposes restrictions on the plate steel supplierthat are beyond those of the Code, but the Code does represent theminimum quality allowed. The vessel fabricator, in most instances, will hotform the plate to the shape desired and then heat treat it by austenitizing[heating the plate to about 871C (1600F)] for about 4 to 6 hoursand then quenching it in water. This treatment is followed by tempering[heating the plate to a subcritical temperature, usually about 677C (1250F)Jfor about one hour per inch of thickness. Following fabrication the Code

that all heavy section welds of ferritic materials be

given a post weld heat treatment. This requirement for nuclear pressure vessels1is given in paragraph NBA620 of Section III Division 1. My first slide is an

view showing the various components of a nuclear pressure vessel.Each component is completely heat treated prior to fabrication. The final

2product is shown in my next slide which is a photograph of the pressure vesselfor the Oyster Creek power plant. You can get a feeling for its size by notingthe men standing near the top of the pressure vessel. This picture was taken


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while the vessel was being hydrostatically tested by the fabricator,Combustion Engineering, at their Chattanooga, Tennessee plant. Section IIIof the ASME Code requires that all vessels be hydrostatically tested priorto their being placed in service. The test must be conducted at a temperaturenot lower than 34C (60F) above the reference nil ductility transitiontemperature as determined by both the drop weight test and Charpy V-notchenergy and lateral expansion requirements. The hydrostatic test is discussedin NB6200. The fracture toughness requirements are discussed in NB2300. Adiscussion of the method for determining reference temperature NDT is provided

in NB2330.The hydrostatic test is a final examination that protects both the buyer

and the public. There have been classic examples of pressure vessels that havefailed during testing. (These failed vessels were not built to the rigid requirementsof Section III Division 1 of the ASME Code.) The Thompson vessel that failed in

3England in 1965 is a classic example. My next slide is a photograph of thatvessel after it had failed. The failure was due to a faulty post weld heattreatment, the topic of my discussion this morning. The failure originated

i tfrom a flaw similar to the one seen in the next slide. The crack whicl. wasidentified as a stress relief crack would not have been dangerous if it had notbeen presented in a metallurgical structure that had poor toughness. The toughness was

5a result of an incorrect heat treatment. The next slide nicely demonstrates thedamage that can be done to a structure by an incorrect heat treatment. Thisslide shows the results of Charpy tests on material taken froni the failed vessel.The lower curve is the Charpy toughness of the tested vessel. Note that themaximum toughness that the steel absorbed was 28 j (20 ft-lbs) at ~>0C (212F).Compare that value to the greater than 80 j (60 ft-lbs) at the sa:.e temperature when the steel is correctly heated to the required tempering temperature. These last three slides set the stage for my discussion of the effect of post weld


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heat treatment time and temperature on the toughness of pressure vesselsteels and welds.

The aain seams in the vessel that I showed you in slide 2 are fabricatedby one of three joining processes: submerged arc welding, shielded oetal arcwelding and electi'oslag welding. The process* most frequently used in the United

6States is the submerged arc process. The next slide contains a photograph ofthree macro sections from welds that *ere made by the three processes previouslymentioned. The shielded metal arc weld, in the upper left hand corner and thesubmerged arc weld, upper right hand corner, are made by multipass technique,over two hundred passes are employed for each weld. The base metals joined in

these two welds are 300 mm thick. The weld in the bottom area of the slide isan electroslag weld. This weld is completely heat treated, austenitized, quenchedand tempered, after welding and hence the weld fusion line, heat affected zone andweld metal are difficult to differentiate.

After fabrication, the ASKE Code requires chat the vessel be given apost weld heat treatment of 2 hours plus 15 minutes for each inch of thicknessover 2 inches. This requirement is given in Table NB4622.1-1 of the Code.However, during fabrication the manufacturer usually will give the variouscomponents an intermediate stress relief. Consequently, upon completion of thefabrication a vessel may have had over 25 hours of post weld heat treatment. Toassure that the steel studied in the USA/HSST program was being given a representativheat treatment, our 300 mm plate was given a 40 hour treatment at 621*C. Theeffect of this treatment on the Charpy V-notch toughness of the plate at the1/4 thickness location is provided in the next slide. The location and orientationfor specimen removal is described in NB2222.2. Three curves are shown in thisslide. The upper curve represents specimens whose main axis is in the majorrolling (longitudinal) direction and the fracture path is perpendicular to themajor rolling direction. This specimen orientation provides the maximum toughnessvalues. The middle curve is for specimens whose axes are perpendicular to themajor rolling (transverse) direction and the fracture path is in the rolling direction


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5Thl quite often, for place materials, is referral to as the weak direction.The lever curve is for specimens whose euin axis is perpendicular to theplace surface and the fracture path is In the rolling direct, ton. This isoften referred to as the short transverse (through-the-thlcknesa) direction.It I* interesting to note chat specimen orientation has very little effect onChe transition temperature of this very chick plate. Plate thicknesses ofthese magnitudes often have less than a 3:1 reduction from ingot to finishedplate. The transition in the Charpy test is that tetapcrutucc regime where theenergy absorbed increases rapidly with only small increases in temperatures.The *ajor influence of specimen orientation is seen in the maximum toughnessvalues achieved. The longitudinal specimen was able to absorb nearly SOS moveenergy than the short transverse specimen at the sane temperature. Compare thevalues at 66*C ( 1 5 0 * F ) . the upper value is nearly 160 j (115 ft-lbs) whereas thelower is near 110 j (77 f t - l b s ) .

3Ttte next slide shows the Charpy V-notch toughness of the submerged arc

and electroslag welds shown in a previous slide. The transition temperature oithe submersed arc weld i considerably lower than that shown in the previousslide for the base aetal. The maximum (usually referred to as upper shelfenergy) value Xs about the saae as that exhibited by specimens with longitudinalorientations. The electroslag weld has somewhat lower toughness but it toois completely acceptable. Currently, the electroslag technique is not employedin the United States for manufacturing nuclear pressure vessels. It was usedfor a nunber of vessels, however, in the late 1960's.

There has recently been an increased interest in the United States in theeffect of post weld heat treatment on the toughness of pressure vessel steels.Section III of the ASME Code has requirements that c&tablish the holdingtemperature range for the post weld heat, treatment of welds. This table,NB4622.1-1, is the one that I previously referred to during my discussion ofpost weld heat treatment holding time. The holding range for SA 533 Grade BClass lt a steel that is categorized as P-number 3 in Tat QW-420 of Section IX


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of the ASME Code, ie given us 593-677*C ( 1 1 Q O - 1 2 S O * F ) . The Code does providealternative holding temperatures and times la Table KB4622.4(c)-l, however,this table is designed for post weld heat treatments at lower temperatures.The Code only provides minimum holding tines, There are no maximum timesprovided. I an aware of past weld heat treatment times in excess of 120 hourson vessels built in accordance with the rules of Section VIII of the Code.Therefore, the possibility of excessively long hold tines does exist.

Our study which is still ongoing is concerned with the effect of longhold tine on the Charpy V-notch toughness of SA 533 Grade B Class 1 steel andsubmerged arc welds in the same material. We began with Charpy V-notch blanksfrom the saae plates of steel that I discussed on the previous slides. TheCharpy blanks were canned in a vacuua and heated at various temperatures andfor times up to 160 hours. These treatments were in addition to the original40-hour post veld heat treatment at 621*C (1150*F) that was given to the originalplate. The specimens were all from the 1/4 thickness location. The effect ofpost weld heat treatment time on toughness was evaluated at 621 and 670"C(1150 and 1 2 4 0 * F ) . Both of these temperatures are within the holding rangeallowed in Section III of the Code. The holding times at these temperatures were94 0 , 80 and 160 hours. The results of that: study are shown in the next slide. Itis evident that at the lower temperature, 621*C ( 1 1 5 0 * F ) , time had only a minoreffect on both the transition temperature and the upper shelf temperature. If a CharV-notch energy of 45 j is usM as the energy level at which the shift in transitiontemperature is measured, then the effect of the 40, 80 and 160 hours of additionalpost weld heat treatment time was to increase Che NOT by 14, 24 and 28*C (25,25 and 5 0 * F ) , respectively, at 621#C. Post weld heat treatments at 670C( 1 2 4 0 * F ) , however, had a considerably more drastic effect. Only 40 hours at670*C resulted in a 33*0 (60F) shift in the 45 j temperature and a decrease in


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the upper shelf energy from 160 j (120 ft-lbs) to 125 j (95 ft-lbs).Increasing the time at 671C to 80 hours and 160 hours resulted in ashift in the 45 j temperature of 55 and 97 C (100 and 175F), respectively.Even more important is the fact that the upper shelf energy dropped to below80 j (60 ft-lbs).

10The effect of temperature is even more clearly shown in the next slide.For this series of tests the post weld heat treatment time was held constantat 80 hours. The temperatures investigated were 621*C (1150F), 637C (1180F),654 #C (1210F) and 67 1C (1240F). Increasing the temperature resulted in anincrease in the 45 j temperature of 14, 20, 36 and 58C (25, 35, 65 and 105F),respectively, for the 621, 638, 654 and 671C post weld heat treatment.

In summary, the ASME Code, in particular Section III Division 1, imposesa post weld heat treatment requirement on pressure vessels fabricated from lowalloy high strength steels. The Code permits a holding temperature range,the high side of which could result in poorer toughness properties. Longtimes in excess of 100 hours and/or high temperatures, 649C (1200F) canresult in an increase in the NDT and a decrease in the upper shelf energy.


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A preexist ing weld ingcrack in a hard spot (380 400HV1) of the forging HAZ Areaabout 1 in removed from fractureface A1 (Fig 14) x 9


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Slide 6. Photographs of macro-sections of thick section weld-ments in pressure vessel steels.The left sample is an electro-slag weld joining two 6-in.thick sections of A508 Class 2forging. The center and rightsamples are submerged arc andshielded metal arc welds,respectively. These later twoweldments join 300 mm (12 in.)thick A533 Grade B Class 1 plate.

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the effect of stress relief parameters on the mechancial properties of pressure vessel steels and...

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